Energy conversion and storage using insertion materials

Prog. Solid St. Chem. Vol. 16, pp. 195-290, 1985 Printed in Great Britain. All rights reserved.

0079--678~/g5$0.00 + .50 Copyright~) 1986PergamonJournals Ltd.

E N E R G Y CONVERSION A N D STORAGE USING INSERTION MATERIALS G.

Betz

Hahn-Meitner-Institut,

and

H.

Tributsch*

1000 B e r l i n 39, G l i e n i c k e r s t r a s s e , F . R . G .

LIST OF SYMBOLS

a .................... a c t i v i t y a ................... a b s o r p t i o n c o n s t a n t A , B ............... h o s l m a t e r i a l s

Ce, Ch, C i

. . . . . . . .

electron, hole and ion concentration

D ................... chemical diffusion coefficient of the species n Dgi .................. self-diffusion coefficient

Ev, E ................ valence b a n d ,conduction b a n d energies Ef ................... Fermi-level nE~, pE~ ......... quasi Fermi-level for electrons and holes r/..................... electrochemical potential E °.................... redox level Sg .................. energy of the b a n d gap EMF

........:.....eleclromotjve force

F ................... Faraday-constant G .................. Gibbs free energy l..................... currenl j..................... particle current density k ................... B o l t z m a n n constant Ln, Lp ........... diffusion l e n g t h s for e l e c t r o n s a n d holes M ................... g u e s t species /~ ..................... c h e m i c a l p o t e n t i a l p ..................... p r e s s u r e ¢ ...................... p h o t o n flux q ..................... e l e m e n t a r y c h a r g e Q .................. h e a t R ................... g a s c o n s t a n t a .................... c o n d u c t i v i t y T ................... t e m p e r a t u r e

PARTLY

JPSS¢ 16:4-A

WRITTEN

DURING A STAY AT STANFORD UNIVERSITY~ DEPARTMENT S C I E N C E , AS W A L T E R SCHOTTKY VISITING P R O F E S S O R

195

OF

MATERIALS

196

G. Betz and H. Tributsch t . ...................transference number of the species n ui ...................electrical mobility of the ions V ...................electrode potential V .................. open circuit voltage of a galvanic cell Vph................ open circuit photovoltage Vp .................photovoltage under current flow W ...................enhancement-factor Ws ...............xtension of the space charge layer x,y ................stoichiometries z ...................distance

1.INTRODUCTION

Materials with framework structures which are able to intercalate or insert guest species become increasingly important for high energy density batteries as well as for electr0chromic devices. The range of known host lattices extends from inorganic materials of highly distinct structures to amorphous systems and organic polymers. In a similiar diversity, guest species can be hydrogen or alkali atoms as well as larger inorganic or organic molecules. Insertion reactions allow to store energy in the form of chemical and electrochemical energy. The energy can be laken from electrical sources or converted from heat. In addition, photon energy conversion is feasible with semiconducting insertion compounds. Thus, solar energy storage and photo-induced pumping of ions through membranes becomes possible as well as optical information storage. The investigation of insertable energy conversion and storage systems is an interdisciplinary field comprising solid state chemistry, solid state physics, electrochemistry, photoelectrochemistry and interfacial science. Most concepts on energy converting inlerfaces have still to be elaborated. Theoretical approaches on ion transfer are complicated by the presence of strong molecular interactions. The interplay of ionic and electronic processes also poses many problems on the treatment of electrically conducting insertable materials. This review aims at a systematic discussion of possible mechanisms and applications of energy converting and storing insertion processes. Relatively simple theoretical concepts are introduced to pinpoint underlying principles and complications. We also try to elaborate the energy efficiency limitations of the discussed mechanisms. The research area discussed is still in a very early stage of development. Its future will essentially depend on the ability to tailor insertion materials with improved stability and increased ionic mobility. Selected examples of working insertion systems, such as the LixTiS~- intercalation battery and the Lix 'HxWOs - electrochromic displays, give optimistic prospects for the future of this class of materials. The relevance of these mechanisms of energy conversion is emphasized by biological membranes operating via insertion reactions both in the dark and during illumination (e.g. solar energy conversion in halobacteria).

2.PHYSICAL CHEMISTRY OF INSERTION REACTIONS

2.1. Introduction and Definition of Topotactic Reactions A topotactic chemical reaction can be defined as a solid state reaction of a structured species A (host) with a species M (the guest), during which the host material conserves certain parameters of its original structure:

~M ÷ A -. MzA

(1)

Energy Conversion and Storage

197

In other words, a topotactic reaction is an insertion reaction, where the guest species M is inserted into the host material A. In case that the host material is layered-type, the term 'intercalation' is used instead of the more general term 'insertion'. As far as the reaction type of insertion reactions is considered, two main cases may be distinguished. In zeolites 1, layered type silicates 2 and polyphosphates 3 we are dealing with electrical insulators and the effective charge of the host structure is not changing during the insertion reactions. On the other hand, compounds like graphite, layered - type transition metal dichalcogenides 4 and metal oxides s are examples of electrically conducting host lattices, which allow the insertion reaction (1) to proceed electrochemically as a simultaneous, but separate insertion of electrons and ions. In most cases the effective charge of the host lattice is changed during such a topotactical redox reaction. The spectrum of ions, which can be inserted according to this mechanism ranges from protons and other small cations like the alkali metal cations to metal complexes, heterocyclic compounds and dyes. There is an abundance of host materials, where electronic, chemical and optical properties can be controlled by the insertion of suitable guest species. This prospect of tailoring compounds with a desired behaviour, like superconductivity or catalytic reactivity, has stimulated the research activities during the past. Since we will focus our work on the aspect of inserted materials in energy converting and related devices, we only want to direct the attention of the reader to publications, where the general physics and chemistry of insertion compounds is extensively reviewed 6,7.s A well-studied example of an insertion compound, known since 1841. is graphite, which can intercalate a wide variety of guest species, ranging from Li and Br2 to IrF0 . The insertion involves the penetration of guest species into the carbon layers accompanied with a parallel expansion of the graphite lattice in the direction of the c-axis. The molecular interactions between the guest species and the graphite host lattice provide the free energy change, including the energy necessary to overcome the binding force of the graphite layers which is approximately 5 kJ/mol, as well as the decrease of entropy during insertion. Graphite intercalation compounds which meanwhile have been intensively studied show a peculiar kind of orderinglO- 13.

~CX:

host

3rd stage

2nd stage

1st stage

Fig.1. Staging phenomenon1 ill intercalated graphite.

As shown in Fig.l, ordered superlattices appear in which one or several contiguous carbon layers exist between the intercalated layers . The numbers of such layers is defined as ~the stage" and the phenomenon ilself as "staging".

Simitiar ordering phenomena which give rise to superlattice reflections in electron

diffraction patterns have also been detected in alkali metal intercalates of transition metal chalcogenides 14

|~

198

G. Betz and H. Tributsch

An additional peculiarity of topotactic chemical mechanisnls was detected,when the end basal planes or the edges of a graphite specimen were encapsulated, before they were exposed to bromine vpor. It was found,that bromine enters from the edges as expected, however, only when the basal planes are also exposed to bromine 1o. Thus, bromine intercalation, even in regions far away from the basal, planes, needs the adsorption of bromine at the basal planes. The intercalation starts from the basal planes with the degree of staging obtained depending on the partial pressure of the intercalant. These studies show that mechanisms of insertion reactions can involve cooperative guest-host interactions leading to an ordering of the inserted material. This aspect will be considered again in section 2.4.

2 2. Structural Properties of Host Lattices Independent of their chemical composition host materials can be classified according to their structural properties. Five different classes can be distinguished• Class I,II and III comprise one- two-, and three-dimensional crystalline compounds (Fig.2.) and are the best investigated materials 20. In class IV we can summarize solid state compounds with molecular crystal structures (e.g. metal chelates) of varying crystalline perfection. To class V, finally, we count host species which are macromolecules (e.g. DNA) or form membranes of macromolecules. Each class of insertahle host'materials has'peculiar prop.erties~in mechanisms of energy conversion and should therefore be shortly analyzed in the following: Three-dimensional host materials (class III) usually have crystal lattices which are perforated with isolated or cross-linked channels of crystal vacancies. It is clear that these materials can only accomodate guest atoms, which fit into these channels and are sufficiently small to be inserted and transported. Most compounds identified up to now were transition metal oxides and -chalcogenides. Molybdenum cluster compounds of the ~omPosition MosXs and M x M o s X s with X--S,Se and M=metal atom are tYPical examPles. Their empty lattice sites are partially occupied in the ternary phase as shown in Fig.3 for M = Ni with x=0.8. The position of guest atoms turns out to be strongly dependent on the stoichiometry, the radius and the electrical charge of the guest species. Their size is limited to approximately 120 pm in MosXs • This has the advantage that intercalation can be very selective. So Mn 2 " - , C o 2+-, or Ni 2+- ions can be reversibly intercalated from an aqueous electrolyte without danger of cointercalating fractions of the solvate shell. As another advantage of these threedimensional syslems the expansion of the lattice during intercalation turns out to be very small• The existence of empty lattice channels parallel to the tetragonal c-axis of rutile-structured MeO2 -compounds allows the formation of metastable LixMeO2 -phases. This has been confirmed for the metals Me = Ti, V, Cr, Mn, W, Ru, Os, and Ir 21. Another group of compounds with parallel lattice channels has the matrix composition MexXz • Typical examples are the compounds Nb3S4 and NbsSe4 22. Analogous host materials with reasonably good ionic conductivity are formed by vanadium- and titanium chalcogenides. Layered-type materials of the composition MeX2 , with X = S, Se or Te are among the best studied host compounds for intercalation and are characteristic of class II compounds. They are built up by layers of transition metals Me sandwiched between chalcogen layers. These sandwiches are stacked and hold together merely by van der Waals forces.

Energy Conversion and Storage

199

M

li :

A

,_

AI ,,_

M

one-dimensional

M

M

M M

M~I

""'1 M "~"h

two -dimensional

three-dimensional

M

M

I / I // I / I / I / I / ,t ,L. .... 2--/-/.- S / I ,.01" I / C C 4 /"-, 0 t;,i C I / / ~. I R\ / // ~....-o ,," 1 / /

/

/

u /M~ I / ¢ v_....y, M

"

O' _~.,, ,vI

M.

"r"

J/

/~'e"~L j . ~

M _~_~_ M

/

/ / /

/

/ /

/

r"--I/

/ /

; q

/

,/,--k..-d q

/

//I I I I I I I -)

/'

/

,,"

/

J,' membrane systems

Fig.2. Schematic representation of structural types of host materials. /

200

G. Bctz and H. Tributsch

N

N

•

•

0

0

0

0

0

0

¢.--R R

0 0 0 0 0 0 0 0

TiS2.Li N

1

TiS z. coballocen¢

57

Ta S2 • OCTADECYLAMINE

Fig.3. Structure of MxMo~Ss-cluster compound inserted with M=Ni at x=0.8.

Man), metals can be intercalated into the so-called van der Waals gaps between the layers. The distance between the layers increases thereby in an elastic way (Fig.4). However, a remarkable variety of molecular species can equally be intercalated. Structural changes may occur which can result in a damage of the crystalline order . This happens especially in aqueous or in another polar environment, where the cointercalation of solvate molecules is entailed. Intercalation batteries based on layered-type transition metal dichalcogenides have therefore to be operated with nonpolar electrolytes to reduce irreversible degradation. Layered-type transition metal dichalcogenides can undergo a large variety of topotactic redox reactions. However, the chemical stability of ternary phases strongly depends on the electronic structure of the host materials. Semiconducting Group VI transition metal dichalcogenides ( e.g. MoS2, WS2 ) which have a full d~, -valence band show only a moderate stability (e.g.MexMoS2). Group V compounds , like the metallic niobium or tantalum dichalcogenides, with a semi-filled d,: -energy band or Group IV compounds with all d energy bands empty are on the other hand quite stable. This provides the basis for the use of LixTiS2 in intercalation batteries 2~

Energy Conversion and Storage

201

Nio. 8 1'406 S 8 ,, o . , /

L

,Oo

--.' ~ "

14C-,

~-"o

~',

",,

•

O'i)~r" 0 S M° •

Ni

Fig.4. Structural organization of layered-type TIS2, TaS.: intercalated with Li, cobaltocene and octadecyclamine. The planes N are the van der Waals planes, the planes R are the R-faces (perpendicular to the van der Waals planes) across which the intercalation reaction proceeds. A number of additional binary and ternary compounds form layered structures and are able to undergo topotactic redox reactions. Among these we can list : MePXs 24, CrPS4 , MoO3 2.'.,V20.~ ,TaS2 and FeOCl 26. Into the latter compound metallocenes and pyridine can be intercalated like in Group V transition metal dichalcogenides, however with a topochemica] change of the crystal structure. In class l - crystalline host materials with one-dimensional MexXy chains - not many compounds have been investigated. Characteristic materials are MeX3 compounds with Me = Ti,Zr,Hf, Ta, or Nb, and X=S,Se. They consist of MeS¢ prisms linked together at their bases to form infinite chains. The arrangement of chains relative to each other which is mainly dominated by van der Waals forces, can vary in the different MeX3 compounds. This could be the reason for different degrees of stabilities encountered in these materials. As a host for the intercalation of Li the compound TiS3 is much less stable than NbSe3 . Host materials with organic lattices ( class IV) have very complex properties. A typical example is polyacetylene, (CH)x which can be reversibly doped (change of Fermi-level) upon insertion of guest species. Polyacetylene is therefore technically interesting as an electrode material 27 ( see reference (28) for a review of the properties of conducting polyacety]ene). Additional materials belonging to this class are other polymers, such as heterocyclic or polyaromatic compounds as well as phthalocyanines and a variety of metal chelates. To describe the introduction of specific molecules into biological macromolecules (e.g.drugs into DNA) in terms of intercalation is at present very uncommon. However, it seems that an understanding of the mechanisms and dynamics of such intercalation reactions is perrequisite for a detailed knowledge of the cellular chemistry. It may have great practical importance in drug design , where the transport and storage of a chemical is determined by how it interacts with the phospholipid membrane 29 2.3. Classification of Electrical Properties Considering the electrical conductivity the host materials can be classified into insulators, pure ionic conductors and mixed electronically and ionically conducting solids. The third class can be divided into metals and semiconductors. a)Insulating host materials. Although the insulating host materials like the sheet silicates will not be considered in detail in this review, some of their special properties should be mentioned here. Their technical applications range from petrochemical catalysis, where they are used as hydrocarbon crack catalysts to applications as ion exchangers for the treatment of waste water a0,zl. In the field of energy conversion chemical heat pumps for the transformation and storage of heat can be realized with zeolite/H20 -systems (see chapter 8.1) z2'33.

202

G. Betz and H. Tributsch

b)Purely ionic conductors If the electrically insulating framework of host materials like zeolites or /3 -alumina contains mobile ions these materials are ion conductors. As an example the Na-/3 -alumina or the/3 "-alumina can be mentioned. Its potential use as a solid electrolyte in the sodium sulfur battery has given rise to a great deal of work s~. Also the zeolites can be prepared by ion-exchange to become proton conductors (NH~ -montmorillite). Kreuer et al introduced a vehicle mechanism to describe proton conduction in these framework materials, where the transport of the proton is coupled to the diffusion of water or NH3 -molecules3s .

c)Mixed ionically and electronically conducting solids in order to perform insertion reactions the host lattice must have both mobile ions and electrons (or holes). Thus, the uptake of an atom M is carried out as an simultaneous uptake of an ion M "~ e"

and m electrons

. In principle the exchange of anions is also possible. It is observed with carbon containing materials

like graphite or polymers like (CH)~ a6.37. The mostly studied examples of mixed conducting host materials are graphite and TiS~ . For battery applications the host lattices investigated are metals or metallically conducting degenerate semiconductors like TiS~, because a high total electrical conductivity of the electrodes reduces the internal resistance of the cell.

d) Distinction between metallic and semiconducting properties The characterisation of electronic properties of host materials did not play a significant role in the older literature of studies on insertion mechanisms. For topotactic redox reactions to proceed it is necessary to provide a high electronic and ionic conductivity. In case the host material is not a metal but a semiconductor, sufficient doping can help to overcome the problem. When, however, energy conversion with insertion materials is discussed in a broader sense, the electronic structure of the host species is of fundamental interest. A clear distinction has especially to be made betweeen metallic and semiconducting materials. Information concerning the density of energy states available for electron occupation, their variation with energy and their occupation probability is frequently visualized in an energy band scheme. It depicts the energy per electron versus the distance z from the geometrical interface (Fig.5). Metals have their Fermi-level which corresponds to the electrochemical potential of electrons, within a partly filled energy band of the electrons. Semiconductors on the other hand, have their Fermi-level situated in a (forbidden) energy region, an energy gap Eg, which separates an occupied lower valence band (Ev) from a nearly empty higher conduction band (Ec). In an undoped (intrinsic) semiconductor the Fermi-level is situated in the middle of the forbidden energy gap. If electron donors are added to the material, the activity of electrons increases and the Fermi-level approaches the conduction band ( .n-type semiconductor ). In the case of a large excess of electrons in the conduction band we talk of a degenerate semiconductor. A well-known example of a degenerate material is the intercalation host material TiS2 . An electron acceptor, on the other h a n d , lowers the electrochemical potential of the electrons (Fermi-level) to the valence band of the semiconductor. There is one additional significant thermodynamic difference between metals and semiconductors: the electrostatic contribution to the electrochemical potential of an electron , qV(z), is generally not the same throughout the solid. There are significant variations near the surface of a material (e.g. used in Schottky barriers ) and at locations where the composition varies (e.g. used in pn- junctions ). In energy schemes these spacia] variations result in the well-known energy band bending. This is due to the comparatively low

Energy Conversion and Storage

203

empty states c

wA Ef

Ef

occupied states

~D(E)

X

metal

o

WA

Ec

Ev Ef

Eg

Ef

~DIE)

X

-i -° ±

q

semiconductor Fig.5. Density of states D(E) as a function of the energy E (energy band scheme) for a) metals and b) semiconductors. Ef is the Fermi-level and WA the work function of the material.

concentration of charge carriers in semiconductors which cannot effectively shield electric fields. In metals however, the high number of charge carriers present allows electric fields to penetrate only a few Angstroms. With respect to energy conversion properties there are some additional, fundamental differences between metallic and semiconducting insertable,materials, In,semicondqctors small amounts of electron donors or acceptors can significantly dhange the electrochemical potential without any phase transition. Depending on the magnitude of the energy g a p t h i s change can amount up to 3eV per electron. When semiconductors are used in insertion batteries the shift of the Fermi-level accross the forbidden energy region of the material corresponds to a significant change of the electronic conductivity (charge carrier concentration). A p-type conducting material may gradually convert into an n-type conductor passing through a minimal conductivity in the intrinsic conduction state (Fermi-level in the middle of the forbidden energy region.). Another significant difference between metals and semiconductors concerns their suitability for the conversion of photon energy (solar energy ) into electrical or electrochemical energy, which we will discuss below: For the conversion of photon energy i1 is necessary to store excitation energy in an intermediate state for the purpose of charge separation. In metals excited states do not live long enough to accomplish this ( l0 -is to 10-Is sec ). Excitation energy is rapidly converted into thermal energy. In semiconductor materials excited states have much longer lifetimes due to the existence of the energy gap which necessitates energy release in multi-phonon processes. Excited states have typical lifetimes ranging between 10 -]°

and l0 -7 sec,

sufficient to separate charge carriers. Only semiconducting insertion compounds can therefore be used for ]ight-drlven insertion devices. In addition, semiconducting materials have a colour which is determined by

204

G. Bctz and H. Tributsch

the distribution of electronic states in the energy gap. Any insertion of guest species which are producing electronic levels within the forbidden energy region, or which are increasing the concentration of mobile electrons (free carrier absorption ) can drastically change the optical properties of the insertion material. A well-known example is WO3 which is used in electrochromic devices. In addition, there are many electronically insulating insertion compounds. They can, for example, be used to store chemical energy and release it in the form of heat, as demonstrated with zeolites 32. In this review semiconducting insertion compounds will be discussed in more detail than metallic and insulating ones. The reason is t h a t semiconducting materials offer new possibilities in electrochromic devices and as optical information and solar energy storage materials. They have also not been discussed so extensively as the other classes of insertion materials. Table I shows a list of semiconducting host materials for insertion reactions, which is by far not complete and will be extended as research continues. Many well-known materials are among them, such as transition metal dichalcogenides of Group IV and VI, metal phosphorous chalcogenides and many oxides. Only semiconductors with energy gaps between 1 and 3 eV are listed, since lower energy gaps would not be useful for optically induced insertion processes.

Eg/eV

Compound

n/p Insert.Species

Reaction

p n p p

Li Cu Li Cu

Photoins. -deinsert -deinsert -insert

(46) (140) (212) (141)

i Cu

-deinsert

i unpubl.

ip

ICu

-insert

[(144)

I_

Ref.

Remarks

ZrSe2 HfS2 FePS3 InSe

1.05 -1.22 1.96 1.5 1.3

InSe

1.3

Cu3PS4

2.3

CuaPSSe4 CuaPSe4 CuaVS4

1.8

ip

Cu Li

-insert

unpubl. (207)

small Vph

1.3

~p I ~p

Cu0PS7 Cu~PSsI

ip IP

Cu Cu

-insert -insert

unpubl. (180)

small Vph

2.05

AgaSbS4 TiO2(B)

p n

Ag H

-insert -deinsert

i unpubl. I(157)

small Vph

3

V205

2.5

In

IH

-deinsert

! unpubl.

WO3

2.5

In

IH

-deinsert

i unpubl.

(CH)~ PFV

1.5 1.6

IP rP

I ClO tH

-insert -insert

i unpubl.

gets metallic small Vph small at

i(161)

Table I. List of semiconducting insertion compounds

2.4. Thermodynamics of Insertion Reactions The treatment of energy conversion and storage with insertablematerials involves not only equilibrium thermodynamics limiting the maximum efficiency obtainable from ideal devices, but also non-equilibrium thermodynamics. This is evident, since the insertion reactions in reality are mainly limited by transport phenomena like diffusion of the guest species in the host. The methods of linear equilibrium and nonequilibrium thermodynamics have been applied to insertion reactions in several cases, where thermodynamic data have also been linked to kinetic parameters 38. Above this, we believe that insertion reactions uniquely

Energy Conversion and Storage provide means to study p h e n o m e n a in the range of non-linear, non-equilibrium t h e r m o d y n a m i c s .

205 Such

p h e n o m e n a can be encountered, when feedback-mechanisms are present.

a) conditions of equilibrium t h e r m o d y n a m i c s fulfilled. If the insertion reactions are carried out electrochemically in a galvanic cell, the t h e r m o d y n a m i c d a t a such as the free energy change, A G , are ready available as easy measurable electrical quantities. Written as an electrochemical insertion reaction Eq. (1) then reads:

x M -~ + x e

+ A ~

M~:A

(2)

where it is assumed, without any loss of generality, that the exchanged cations are monovalent. The cation M + from the electrolyte is reduced upon insertion. T h e reversed reaction is then :

M~A

- xe-

----* x M *

+ A

(3)

The free energy change AG of the reaction (1) is related to the open circuit voltage Vo¢ of a galvanic cell with MxA as the insertable cathode and a M - electrode as the anode by: AG Voc --

r

(4)

where F is the Faraday constant. Vo¢ is electromotive force, EMF, of the cell. Essential for the function of energy converting and storing devices based on insertion reactions is the relation between the open circuit voltage,Vo¢ ,and the composition x of the host electrode, MxA . M . A r m a n d first introduced a model to analyse the composition-voltage behaviour of an insertable electrode material in a galvanic cell 39. The formula he derived quite well describes the composition-voltage curves found with systems such as the LixTIS2 40 In the following consideration a redox equilibrium is assumed to have established within the electrode. This internal equilibrium can be written:

[M ÷ + e- ~ M],,,

(5)

{M + h ~ ~ M+]i.~

(6)

Eq.(6) is valid, if holes, h ÷ , are the electronic charge carriers. In the ideal case the chemical potential of M, #M , can be derived from (6) as:

~M : #, ~ ~,0

(7)

where IZ, denotes the chemical potential per particle of the ions M + and IZ~ the chemical potential of the electrons. The electrochemical potential rh of the ions, M ÷ , is:

,1,(z) = ~,(~) + qV(z)

(s)

q denotes the elementary charge. V(z) is the locally varying electrostatic potential in the electrode and in the electrolyte. Since at each of the electrode/electrolyte interfaces only ions are exchanged, rh is at

206

G. Betz and H. Tributsch

equilibrium constant across the junctions of the galvanic cell. Thus. the open circuit voltage Vo¢ of the cell is determined by the chemical potential ~M in the hosl electrode and the counter-electrode, b t M ( C E )

voc =

-

1_(

~,u

q

-

#M(CE))

=

1

- -#~

(9)

+ con,t

q

:

This can be verified by inserting Eq.9 into Eq.7 using that the E M F of the cell is the difference of the electrochemical potentials of the electrons, rh = p , - q V , between the electrodes. Following the model of A r m a n d the chemical potential pi can be written as a s t a n d a r d potential, #i0, and a configurational entropy term:

Pi := /~io + k T In c°ccupi'd

(10)

Cempty

Here c,,¢¢,p~,d and Cempty denote the concentration of occupied and e m p t y lattice sites for the ions,respectively. With the ionic lattice site occupancy

Oi - -

Ci Cimax

where ci = actual concentrations of the ions in the electrode and Cimax

-~

m a x i m u m concentration of the

ions, gti can be expressed as:

#i = P,o + k T l n

Oi

1 -

o,

(11)

The chemical potential of the electrons is determined by the total energy band occupancy,0, : fff,,.+L D ( E ) P ( E ) d E 0e

~

#.-

f~;.;.+L D ( E ) d E

(12)

where P(E) is the Fermi distribution ,D(E) the density of states and L the energetic width of the band filled during insertion of M. Thus p, can be expressed in analogy to (10) as :

~ , = p,o + k T l n

O,

1 - O,

(13)

According to the basic model which is valid for perfectly non-stoichiometric compounds, one obtains for the electrode potential V as a function of the degree of insertion 0 :

V(O) = Vo + rO -

nkT q

In 0

O -1

(14)

where Vo is a s t a n d a r d potential. Here O can be related with the stoichiometry or composition x of the electrode by: .T Xmaz

where Xm~ denotes the stoichiometry of the maximally inserted compound. It is n = l , if only one species, either electrons or ions. is limiting the degree of insertion; n = 2 , if both species are simultaneously limiting. T h e second case is for example valid, when the inserted electron is strongly localized at the M + -ion. The interaction term,r 0 , describes the interaction between the inserted ions. It is r < 0 in case of an repulsive interaction. Examples of nearly perfect non-stoichiometric insertion c o m p o u n d s are LixTiS~ in the range 0 < x < 1 and the HxMnO2 -electrode in the Leclanche-cell 41.42

Energy Conversion and Storage

207

The non-stochiometric domains are limited, when strong guest-guest or specific guest-host interactions occur. As a result the insertion proceeds following one (treshold compound) or several (adjacent domains) reaction fronts. The guest species then tends to form an ordered sublattice within the host phase MxA with a limited deviation from stoichiometry. The equilibrium between these phases is resulting in a plateau in the composition-voltage diagram, V(0). Such a behaviour is for example typical for the LixVSe2 -system 43 The term in equation (14) which is proportional to 0 yields an enthalpy change, AH . Although it cannot be always clearly decided, wether this enthalpy term is due to an ionic or an electronic interaction, there are experimental indications from the LixTiS~ -system that ion-ion interactions determine the magnitude of AH 44. LixMo6Ses

is an example of a system with a long-range attractive interaction between the

intercalated atoms (r>0) 45. On the other hand, it is often found that the electronic interaction most strongly influences the composition-voltage curve of insertion electrodes. As an example, smaller voltage changes occur during insertion of transition metal oxide systems compared to those found with transition metal sulfides and selenides. The reason is that conduction bands in transition metal dichalcogenides are broader than those in transition metal oxides.

~

--

a) ¢-

~~;)

D(E) Nb-d

t2 9

S-p

~

Mo-d

M0 - dz2 S-p

P

Se-p Ti S 2

Nb S 2 Se 2

trigona[ prismatic

octahedra[

Fig.6.

Mo S 2

Distribution of electronic state

and

the

filling of orbitals

during

Li-intercalation in

TiS:, NbS: and MoS:.

The electronic configuration is also crucial for the stability of the insertion compound. In TiS~ with an octahedral coordination of the transition metal all the d-orbitals are empty. During insertion the electrons occupy the low-lying t2g -level. For NbSe~ with prismatic coordination of Nb, the inserted electron completes the filling of the d,= -level. Both Li,TiS: and LixNbSe~ are stable. On the other hand, MoS2 with a filled d,= -band can only accept electrons in the higher d- and p-level. Thus, the Mo-S bonds become unstable at this electron activity and the intercalated material disproportionates. The electronic configuration of these compounds is shown in Fig.6. Equation (14) can be extended to include photoprocesses which are observed at interfaces of semiconducting materials 46

In this case illumination changes the electrochemical potential of the electronic charge car-

riers (Fermi-level) in the inserted material. Thus, the following relation is obtained between the electrode potential V

, the insertion site occupancy 0 and the photopotential Vvh which is also dependent on the

degree of insertion :

V'(P) = Vt, + rO - nkTln 0 _ Vvh(O) q 1-O

(15)

208

G. Betz and H. Tributsch

The photopotential is generated by illumination of the host electrode/electrolyte junction. It is proportional to t h e logarithm of the light intensity, ~0 :

kT 0 v ' ( o ) = V o + r O - nkZ q ln ~ - o + .y(o) -ql n , o

(16)

where "~ is the diode quality factor of the electrode/electrolyte junction ( "~ ~ 1 ). For an n-type semiconductor it is Vph < 0 ; for an p-type semiconductor it is Vph > 0 . This forms the base of energy conversion with p h o t o n powered insertion reactions.

b) irreversible t h e r m o d y n a m i c s (linear range) Insertion and deinsertion processes as they for example occur in b a t t e r y systems under flow of current and mass have to be treated as p h e n o m e n a of irreversible thermodynamics.

Technically interesting systems

should operate close to equilibrium and are thus subject to the linear range of irreversible thermodynamics. From the work of Gibbs, Onsager, Prigogine and others it can be deduced that in this situation the rate of entropy production e plays a fundamental role, since it approaches a m i n i m u m in a steady nonequilibrium state( for a s u m m a r y compare reference (47)).

Under such conditions the entropy production can fie

separated into two parts:

o = Och + od

(1 7)

The first contribution, Oeh , is due to a scalar p h e n o m e n o n such as a chemical reaction. It can be written as:

o~ = ~ : ~

>o

(is)

k where wk is the reaction rate per unit volume and Ak the chemical affinity of the k-th reaction, which measures the deviation of the chemical reaction from the state of equilibrium:

Ak = - ~ ~i"i~

(19)

J where tt~

is the chemical potential of the j-th reactant and v~k the stoichiometric factors of the j-th

reactancts in the k-th reaction. The chemical affinity is equivalent to the Gibbs free energy G which relates this t r e a t m e n t to the previous chapter. T h e second contribution, ira , is due to a vector p h e n o m e n o n such as diffusion and is described by:

o~ = - ~

j~ 9cad ( T )

> 0

(20)

k where Jk are diffusion fluxes and grad ( ~ ) the gradient of the chemical potentials of the k species divided by t h e absolute t e m p e r a t u r e T. An exergonic insertion reaction according to relation (17) causes an oriented diffusion flux of t h e guest species, while the entire entropy production of the insertion reaction approaches a minimum. An insertion reaction with a high reaction rate wk and a high chemical affinity Ak will give rise to a correspondingly higher mass t r a n s p o r t rate. It is obvious that a for a technical system the entropy production should be minimal although it can not be avoided totally as Eq.(20) tells especially in the case when high current densities are needed.

Energy Conversion and Storage

209

When a chemical or electrochemical insertion reaction creates an inhomogenity in an initially equilibrated host material/solution- or host material/gas- interface the diffusional motion of atoms or molecules tends to neutralise it and a transport of matter results. Starting from relation (17) and assuming dilute systems the diffusion laws well-known as Fick's first and second law can be derived which relate the transport of an inserted species n with the corresponding concentration gradient grad p . . The first law reads:

with the diffusion coefficient D ,

j, = -D, gradp,

(21)

D,=

(22)

:

L~ (~)

r.p

and the phenomenological transport coefficients, L , , . The flux of matter is always directed to the region of low concentration presumed that the state before the appearence of the inhomogenity is homogenous and isotropic. In anisotropic media, such as in membranes. the diffusion flux can be inverted, when nonlinear chemical reactions are present involving a scalar entropy production ach • Then active transport results and matter is transported against a concentration gradient. When ions are inserted from the electrolyte into the surface of the host material they are subject to internally existing or externally applied potentials. With metallic host materials the potential drop will be limited to the Helmholtz layer in the electrolyte. In semiconducting host materials there will be a space charge layer within the electrode which depending on the charge carrier concentration of the material may extend 102 to 103 nm into the bulk of the electrode. The inserted ions on the other hand will themselves affect the local potential. We therefore have to write:

(24)

~7 = Iz- FV(p.)

Substituting in Eq.(22) the chemical potential #~ by the electrochemical potential rt of Eq.(24) yields for the diffusion coefficient : L..,6#.,

FL..

6V

m.(v) : ~-tTy0 ~T,p- - y - ( V ) r p

= D. + Dv

(25) (26)

it is clear from this relation that ion diffusion which is accompanying the insertion or deinsertion reaction can be accelerated or slowed down according to the potential distribution in the space charge layer and the sign of the ionic charge. Since illumination of a semiconductor interface will change the prevailing potential distribution, it will therefore also affect the diffusion in the surface region. Light does not only influence the diffusion coefficient, it also can drive the entire insertion and deinsertion process. This effect can be expressed with the help of Eq.(17)..(20), where a change of ach is connected with a change of ad. If Oa is equal to zero (no transported of inserted atoms) an increase of the chemical affinity of the electrode by illumination will increase ach (the affinity of all other species is assumed to remain unchanged). This increase of ach is the origin of a vectorial transport with the entropy production rate, od which balances the total entropy production in order to minimize it (see Eq.17). Most important, the potential dependent term Dv

in the diffusion coefficient is expected to be different for insertion and

deinsertion mechanisms, since the ions will diffuse with or against the field, respectively.

c) Irreversible thermodynamics,nonlinear range; Cooperative phenomena

210

G. Betz and H, Tributsch

Additional phenomena can be expected, when the insertion reaction is occurring further away from thermodynamic equilibrium. According to Prigogine and others the excess entropy production at constant fluxes, 6za , becomes a quantity of crucial significance 47:

~,o =

Ak

E6J. 69rae ( ~ ) + ~6.,~ T n

(27)

k

When 6,o becomes negative, new structures (cooperative phenomena,chemical oscillations) can appear at the expense of energy dissipation. Critical is the behaviour of the second term of the excess entropy production. It becomes negative in the case of autocatalysis, i.e. when the insertion product has a feedback on the rate of insertion. Feedback mechanisms, as already mentioned, can lead to cooperative or autocatalytic reations, e.g. chemical oscillations. Such feedback mechanisms can be easily described with rate equations of the chemical species involved in the reactions. In order to destabilize the thermodynamic branch one needs at least a cubic nonlinearity in the rate equations. Although it is somewhat problematic to describe a heterogenous reaction in terms of homogenous kinetics, we will note such a nonlinearity for an insertion reaction with M,d, as the adsorbed guest atom and IM} as the insertion product:

2IM] + M,d, = 3[M]

(28)

With M as the unbound guest species and B,C,D as different intermediate reaction states of the host material this reaction scheme can be enlarged to

M = IM]

(20)

{M] + B ~-~ M,a, + C

(30)

2[M] + M,d~ ~ M,,~, + C

(31)

[M! ~ D

(32)

Such a trimolecular model was first introduced by Prigogine and Lefever so. It is mentioned here to encourage the search for autocatalytic mechanisms of insertion reactions as the origin of some still little understood phenomena encountered with topolactic reactions. One is the phenomenon of staging which we have already mentioned. It had been discovered by Rouxel and co-workers with Group IVB transition metal dichalcogenides 51 and with intercalation compounds of graphite s2. It still leaves a number of questions open, which are difficult to solve in terms of detailed models of the electronic interaction between host lattice and guest atoms but could be understood in terms of irreversible thermodynamics on the base of autocatalytic mechanisms. Questions concerning for example the long range interaction in inserted materials with high-numbered staging that causes a release of an entire layer of guest species with subsequent reinsertion, could appear in a different view. The feedback can be provided by strains induced by elastic dipoles as in the domain model of staging in graphite ss, or it can be exerted by an electric field in the surface region. Also charge density waves (CDW) which appear as a consequence of a distortion of the host lattice on intercalation s4,ss, could be accesssible to such considerations of irreversible thermodynamics. Such an approach would, of course, not be in contradiction with microscopic treatments, but could be very helpful for understanding how the system behaves, when external parameters, such as the temperature or

Energy Conversion and Storage

211

the concentration of a guest species, are gradually changed. Thus. analogous behaviour in different systems could be more easily recognized and correlated,which would make the complicated interplay between guest species and host material (such as for example encountered in LixTiSe2 s(~ and NaxVS2 sT) more transparent. The interaction of the DNA with medically important planar aromatic cations such a.~ ethidium or proflavine can be understood in terms of an intercalation model 29. Intercalation can affect both the local secondary structure of the double helix and the longe - range coiling of the molecule . It leads to dramatic changes in vmible absorption and flourescence spectra of the DNA. From measuring moles of ligand bound per base pair of the DNA and relating this quantity to the free ligand concentration, effects of ligand cooperativity are found with these cations intercalated into the helix of the DNA 59. The model that accounts best for all experimerltal findings has been called "neighbour exclusion binding" where the interaction is provided either by a physically blocking of potential binding sites or by an induced perturbation of the DNA conformation so

This effect of negative cooperativity is paralleled by a positive cooperativity, when carcinogens are

intercalated into the DNA at low concentrations6°. Many membrane bound oscillating reactions are kown in living cells. A simple model of an oscillatory photoinserting membrane system is the following: a light sensitive host membrane, the optical absorption constant of which is temporarily decreased when inserted, is absorbing photons inducing the insertion reaction. Since after the insertion process light absorption is temporarily reduced, the photoreaction is stopped until the inserted ion is released on the opposite membrane surface. With such a thin photochromic membrane light-induced ion insertion is functioning as an oscillatory mechanism. A working biological example of such a system is the photon powered proton pump in the purple membrane of the Halobacteria halobium 61 In this photobiological system the light absorbing chromophor (retinal) is activating a photochemical cycle, which takes several milliseconds to complete before the pumped proton is released and a new photon can be absorbed for energy conversion. Recent studies on oscillatory energy conversion by J.Ross have clearly shown that nonlinear systems have an increased efficiency as compared to stationary mechanisms 62,63

Although we are still far away from technical devices based on such oscillatory mechanisms, these considerations show that synthetic insertable host materials are suitable candidates to study regulatory and recognition processes on a molecular level. The outcome of this research with simple systems may have significant value for the understanding of the more complex processes in living systems. 2.5. Potential Composition Analysis and Phase diagrams In reversible equilibrium thermodynamics of insertion reactions a solid scientific and experimental basis has been gained due to some fundamental studies performed 64.6s. With respect to energy conversion processes it is therefore helpful to review some widely used experimental and theoretical investigation techniques and to extend them to hitherto neglected photoreactions. The knowledge of the phase diagram and the value of the Gibbs free energy of formation of individual phases is essential in order to understand the reaction behaviour of energy converting inserted materials. Thermodynamic parameters can be derived from composition voltage diagrams which are measured by reversibly varying the concentration of a guest species in a host material. This can be done in a galvanic cell using the cou]ometric titration technique. A characteristic potential - composition curve is obtained for each system which may exhibit well defined plateaus, when there are two phases in coexistence (Fig.7). For ternary systems a so-called Gibbs triangle can be developed in which the three pure phases correspond to the corners. Stable binary phases correspond -according to their stoichiometry - to points on the tie lines between the corners, and stable ternary phases to points within the triangle (Fig.8).

JPSSC 16:4-B

212

G. Betz and H. Tributsch

with tight / p - M x A

+2

~-~-_-,-. _ / \

dark

+1

Vc \,

J---- -- ph0toinsertion I/1

!

G)

I

0~5

o .o +1

/

o o.

~

III

1

dark with tight/n-Mx A

/

Vc

.

\

-----

photodeinsertion----t

-1

\\

i

o,5 insertion

i occupancyX/Xmox

site

Fig.7: Schematic potential-composition curves and the effect of illlumination on a p-type semiconductor(positive photopotential) and on an n-type semiconductor (negative photopotential). VcE is the potential imposed by the counter-electrode.

C

//~BC3 -

M

/

MB

Vph2

B

Vl +Vphl Fig.8: Hypothetical Gibbs triangle for a ternary system. The dotted line shows the reaction paths with M as the inserted guest species. The potential V] and V2 define the stability window which can be changed by illumination generating photopotentials Vphl, Vph2

Energy Conversion and Storage

213

Two coexisting phases can be connected with tie lines, which separate subtriangles. Within these subtriangles three phases are coexisting at a constant thermodynamic potential. The reason is t h a t , according to the Gibbs phase rule, displacement reactions within these subtriangles leave the intensive thermodynamic variables constant while certain phases nucleate and grow or others are consumed. When studied with electrochemical techniques , a ternary system crossing one of these subtriang]es shows a constant voltage plateau. If, however, the material shows solid solution behaviour as in the case of LixTiS2 , the thermodynamic potentials will vary with composition (compare Eq.(14)). Electrochemical measurements will not show a potential plateau, but a gradually varying electrode potential. By evaluating coulometric titration curves and supplementing them with thermodynamic calculations for the construction of Gibbs triangles a profound understanding of the reversible behaviour of inserted energy conversion materials can be reached. There are, however, some supplementary details to be considered when semiconducting materials are involved which can be used for a conversion of photon energy. According to relation (14) a photoinduced potential has to be added to the equilibrium potential of the inserted or intercalated material. It has in general a positive sign in the case of a p-type semiconductor (Fig.7a) and a negative sign in the case of a a n-type semiconductor (Fig.7b). In Fig.7 the photopotentials ( at constant light intensity ) have been assumed to be independent of the composition which is generally not the case due to variations of the energY gaP and the Fermi-level, resPectivelY (see later). The difference between the dark and the illuminated host electrode should now be closer examined using Fig.7a. When the potential of the host electrode is shifted from the equilibrium value to a more positive potential by applying an external voltage in the dark, its composition will change towards a lower degree of insertion according to the composition-voltage curve. There will be a release of cations accompanied by an anodic current flow through the external circuit. In other words, a deinsertion reaction proceeds. The situation will be different, when the host electrode is illuminated. Let us assume in the following an illuminated p-type semiconductor. Generation of electron/hole pairs and the subsequent charge separation increases the electron activity at the surface of the electrode. The dark bulk of the electrode assumes therefore a more positive potential. Thus, although the photovoltage Vph has a positive sign, insertion of cations may still occur due to this increased electron activity at the surface. If the potential in the bulk of the electrode is kept fixed (e.g. by a potentiostat or by the counter-electrode), the potential at the surface will shift upon illumination more negatively by the amount of the photovoltage. Under current flow conditions in the external circuit photoinsertion will proceed according to the composition voltage curve until the activity of the M-atoms in the illuminated surface regions corresponds to the new potential in that region. In analogy, an n-type semiconductor will, when potential more negative than the equilibrium potential is applied at the electrode in the dark, start inserting cations. Upon illumination of an n-type semiconductor a negative photovoltage is generated which means that the bulk of the electrode is at a more negative value than the illuminated surface. So, if equilibrium with the counter-electrode is established in the dark, a deinsertion reaction will start under illumination when closing the external circuit. The reason is that. the potential of the bulk is alligned with the potential of the counter-electrode (neglecting potential drops in the external circuit). So the surface potential will become more positive compared to its equilibrium value in the dark, and ions are released from the surfcace region. When anions are the exchanged species, the potential-composition curve of the electrode material will be inverted. Then illumination of an n-type material will lead to an insertion of anions and illumination of a p-type material to a deinsertion of anions 66. We conclude: the thermodynamic basis for photoinsertion and photodeinsertion reactions is that for a given degree of insertion the cell potential and the corresponding driving force of the insertion reaction is different in the dark and during illumination. At an externally imposed electrode potential the equilibrium composition of a combined electronic- ionic semiconductor under illumination will therefore be different from the same material in the dark.

214

G. Betz and H. Tributsch

It is obvious t h a t photoinduced potential changes can also be considered in the Gibbs triangles (together with possible potential changes during transitions between n- and p-type materials in the course of composition changes). As a consequence, there will be changes during illumination such as, for example, changes of stability ranges of ternary phases. This is illustrated in Fig.8, which shows a Gibbs triangle for a hypothetical ternary system (M,B,C). With respect to a reaction with the guest species M the ternary phase MBC 2 is stable between the potential Vj corresponding to the subtriangle MBC~ - M2C - MB and the potential V2 corresponding to the subtriangle MBC2 - BC - BCs . Let us assume now t h a t the first subtriangle corresponds to a semiconducting material with p-type character, the second to a semiconductor with n-type behaviour. ]n this case the potential within the first triangle will increase by a positive photopotential, VphI and the potential within the second triangle decrease by a negative photopotential, --Vph: .Since photovoltages of materials that absorb solar light (energy gap Eg = 1-3 eV ) typically reach values between several hundred mV and one Volt, the stability range of the hypothetical material MBC 2 with respect to the component M will have narrowed by Vphl + Vph2 , which is of the order of 1 Volt. Consequences with respect to the stability of phases have to be taken into account. Thus, light can alter the solid state properties of an host compound providing the energy for solid state reactions.

2.6. Thermodynamic Limitations of Solar Energy Conversion The capability of energy conversion is a basic property of most existing and technically feasible devices operating with insertable materials. In intercalation batteries chemical energy is interconverted with electrical energy. ]n electrochromic devices electrical energy is used to induce chemical changes resulting in a change of the optical absorption of the electrode.

J

60"

combined

quantum thermal

.......j J"

50"

~.-'""'" """ o,,'°'°

c Q)

~. ~.oQ,I

/o°

• '/" 30-

10

// f

_L

/% //

20

I energyloss due J to storage

/ /

solar thermal

/// ~olar quantum iIII

0

3oo

5oo

700

9oo

temperature / K Fig.9. Solar energy conversion efficiency as a function of collector temperature for quantum, thermal and combined quantum-thermal converters.

It will be now discussed in more detail how solar energy can be converted into chemical energy of inserted materials or into electricity. We consider it therefore reasonable to analyze briefly the thermodynamic li-

Energy Conversion and Storage

215

mitations valid for these mechanisms. It is evident that interconversion of chemical and electrical enersy in intercalation batteries according to relation (4) is subject to the same limitations as applicable to ordinary batteries: if energy conversion is performed sufficiently close to equilibrium, the energy is fully interconvertible. This is of course a condition never fulfilled in practice. Heat generation during the operation of an intercalation battery is inevitable. Most inserted materials can, in addition, not be fully recycled during the insertion/ deinsertion process. Energy recovery is therefore only partial and subject to the degree of reversibility obtained. For the conversion of solar energy into chemical or electrochemical energy of an inserted system the thermodynamic limitations are very different. Most important is the distinction between a primary conversion of q u a n t u m energy or of thermal energy. In a quantum collector photons are directly harvested and converted into electrical energy or chemical energy. This situation is encountered in photoinsertion batteries and photon powered-ion pumps as discussed below. In these cases it is not the Carnot factor which is limiting the energy conversion efficiency, because the temperature of the light emitting photosphere of the sun (6000 K) is far above the temperature of the collector system. The Carnot limitation does apply, however, when solar energy is converted into heat before being used to drive an insertion device, e.g. an chemical absorber heat pump. Examples of theses devices will also be discussed below. They basically operate as Carnot machines between two heat reservoirs, one maintained by the solar heat collector, the other by the environment. For a quantum converter it is necessary to consider the forbidden energy gap, across which absorbed photons excite electrons, when calculating the energy conversion efficiency, which is the ratio of generated electrical power to the incident solar power. An energy gap of approximately 1.5 eV is found to be optimal for a solar energy converting semiconductor. Light absorption is still high ( visible and near infrared region ) and the photopotential, which can be generated ( up to ca.55% of the energy gap} is sufficiently high for a reasonable electric power output. With such an optimal energy gap the thermodynamic energy conversion efficiency can be calculated to reach 31 ~ at ambient temperature GT. It decreases with increasing temperature (Fig.9), mainly because more radiation energy is being reemitted from the collector. In the case of a thermal converter of solar energy the collector temperature must be increased above the ambient temperature to convert the incident radiant energy into another energy form. When a temperature difference of approximately 250°C

is reached, a solar thermal converter could operate with an energy

conversion efficiency of 54% which is higher than t h a t of a quantum collector.

But such temperature

differences can, in practice, only be reached by complicated concentrating solar collectors. Fig.9 also shows the thermodynamic efficiency limit of a combined quantum thermal converter. Such a combined system converts photon energy by a quantum process, and in addition utilizes thermal energy through processes, t h a t are limited by the Carnot factor . The thermodynamic limits indicated in Fig.9 reflect idealized conditions which cannot easily be met in practice. Only in special cases (e.g.GaAs photovoltaic cells) efficiencies have been approached more than 20%. Photoinsertion batteries and photon powered ion pumps act,in addition, as storage devices. The energy loss due to storage is also indicated in Fig.9. A series of additional factors (e.g recombination processes, internal resistances, heat production and losses ) can further decrease the energy conversion efficiency in real systems. When developing photon powered insertion devices it has to be considered t h a t the above discussed thermodynamic limitations only apply to single light absorbing and energy converting systems. When two ore more semiconductors are properly arranged in series, so that each one utilizes the photon energy not absorbed by the preceding one, a higher energy output can be obtained. In the limit of infinite graduated absorbers, which in practice is approached by 3-5 systems in series (cascade arrangement), theoretical energy conversion efficiencies up to 60% for the quantum converter can be expected. Such arrangements could ,for example, be technically imaginable for membrane systems (see later). Two photosensitive materials (p- and n-type) as photocathode and -anode in a photoinsertion battery and absorbing complementarily ( tandem arrangement ) can equally yield a correspondingly higher energy conversion efficiency. Semiconducting insertable materials offer the possibility of developing combined quan-

216

G. Betz and H. Tributsch

tum thermal devices,when the guest atoms are released (desorbed) from the host material with thermal and photon energy (see 8.4). However, a series of practical limitations makes it improbable that, even with such technologically complicated strategies more than 30 % of the incident solar energy can be converted into stored electrochemical energy of an insertion battery. It will be seen below that our technological understanding is still far away from these scientifically possible goals. In any case, it is necessary to know the limitations of the systems in order to properly estimate their development capacity.

3. SOLID STATE IONIC MECHANISMS

3.1. C o n d i t i o n s / o r High Ionic Conductivity A reasonably high ionic conductivity at ambient temperature is a crucial condition for the functioning of energy converting devices based on insertable materials. It is therefore essential to examine under what structural and chemical conditions high ionic conductivities can be expected. A number of suitable methods for determining kinetic properties of solids is available 6s When dealing with energy converting insertable materials one has to be aware of the fact, that the mechanism of ion transport can be very different in purely ionic and mixed ionically and electronically conducting materials. The ionic conductivity, oi , is defined by the concentration of mobile ions ci , their charge q and their electrical mobility ui :

(33)

oi = qciui

Purely ionic conductors have been investigated relatively well, both experimentally and theoretically6°,7°. Combined electronic-ionic conductors (electrodes), which are especially interesting for energy conversion have, on the other hand only received major attention during the last decade n.Tz

Material ~-alumina with

oi(ohm cm) -1 at T

-Na +

1.4.10 -z (25°C) 2'°

-Li *

1.9.10-4 (25oc) 211

_H+

RbAg41s HUO2(IO6) 4 H20

1'10 -11 (25 -

Dki(cmZ/sec)

r 8.10-9 zll

100°C) 212 I

0.25 (25~'C) 213

W

10-7 0.18 10 -s z14

0.2(25oc) ss 5.10-8 21s

E Li~TiS2 Cu2S

3.3.10-4 (60oC) 146

1.2.10 -9

Li3Sb

1.5'10 -4 (360"C) sl

2 . 1 0 -5

5'10

4

1.3.10 4

Table II: Examples of investigated ion conductors and mixed conducting materials with conductivities, self-diffusion coefficients Dki and enhancement-factors W

Energy Conversion and Storage

217

A list of some of the investigated ion conductors and mixed-conducting materials is given in Tab.II with ionic conductivities and diffusion coefficients. Experimental work mostly concentrated on the diffusion of alkali metal and copper and silver ions 7~,74. The diffusion mechanism can strongly vary depending on whether ions diffuse in a van der Waals gap between transition metal dichalcogenide sandwiches or along structure specific plains or channels made up of vacancies. Atoms can, for example, leave their regular sites to form Schottky defects (transferred to the crystal surface) or Frenkel defects ( transferred to interstitial sites ). The concentration, c, , of mobile ions then writes: -Ed

c, = B co exp(~-~)

(34)

where Ed describes the activation energy for the formation of a defect pair. co is the concentration of regular lattice sites and B an entropy factor which strongly depends on the crystal structure and on the type of defects involved rs. Since coulombic interaction is involved, polyvalent cations have usually significantly higher activation energies. X-ray structure analysis of compounds with high ionic conductivity shows high concentrations of unoccupied sites available for thermally activated ion transport T6. In superionic conductors a very large fraction of cations participates in ionic conduction. The thermally activated motion has been described by a model in which ions oscillate within a potential well before being transferred to a vacant site over an activation barrier. The temperature dependent mobility,ui ,is then determined by:

A

- Em

u, = ~ e ~ p ( - ~ - ) where A = Ca2qu"k

. u

(35)

is the vibrational frequency of ions within the potential well, a the distance

between neighbouring potential wells in direction of ion transfer and C a constant containing an entropy factor and a factor dependent on the crystal structure involved. E,, is the activation energy for the ionic mobility. It is an advantage for the ion transport, if the activation state passed during an ion transfer from one lattice site to another one is energetically compatible with the initial state. This is, for example, the case when the coordination of the ion is maintained in both states. It seems to be for this reason that ions in good ion conductors usually have tetrahedric or even lower coordination. It is difficult to maintain a constant coordination with higher coordination numbers. Ionic mobility in layered-type transition metal dichalcogenides has to be described in a somewhat different way. The activation energy for the conduction of alkali metal ions in MxMeX2 compounds depends on the stoichiometry, the structural position of the cation M + and the bonding situation in the host lattice 'r. It is clear that small ions like H * , L i ' , N a * and K ÷ are the most mobile species. However, also Cu + and Ag* with relatively large ionic radii are reasonably well conducted. The reason is that their nuclear charge is incompletely shielded by d 1° - electrons. As a consequence they can easily polarize their environment and partially undergo covalent bonding which stabilizes a tetrahedric or lower coordination in compounds which they form . However, easily polarizable anions in these compounds are a precondition 7s

3.2.Ionic Conductivity in Mized Conducting Solids In mixed conductors electronic charge carriers are interacting with the ionic species during the transport via an internal electrical field. The electronic species interacting with ions in semiconducting compounds may generate high internal electrical fields which can significantly accellerate ion transport. This opens the possibility of controlling mass transport by light in semiconducting electrodes, especially in membranes. The theoretical description for this interaction is the following : In linear transport theory current densities j , are described in terms of generalized potentials X,,~ :

£

=

Ln,, grad Xnm

(36)

with the sum taken over m. The matrix L.m of the Onsager transport coefficients has in general nondiagonal terms describing the coupling between the currents of different species such as ions and electrons.

218

G. Betz and H. Tributsch

In the case of the diffusion of monovalent cations M +

the generalized potential is the electrochemical

potential ~/i of the ions. The ionic current density, 3; , therefore is:

3i = -°--2i grad rh

q

(37)

with rh = lz i + qV (z) , and using relation (33) we obtain :

3i = - c i u i g r a d ( # i + qV (z))

(38)

With the the self-diffusion coefficient kTui q

Oki -

Einstein relation

(39)

we obtain in the one-dimensional ease:

J,

,~tn., ~c, ~ - Dkil Slnci

6--z-

qc,6v] k T ~z

(40)

The self-diffusion coefficient Dki which is via the Einstein-relation directly connected with the mechanical mobility of the species has to be compared with the chemical diffusion coefficient, Di , defined by the first Fick's law :

Ji = - D i grad ci

(41)

These two diffusion coefficients can be related by

Di = W Dki

(42)

where W is the so-called enhancement factor, named in honour of C. Wagner, who first studied the influence of the transport of different species upon each other 79.80 This factor W is frequently used in the literature in preference to the equivalent description with nondiagonal transport coefficients L. . . .

The values of enhancement factors under various solid state ionic

and electronic conditions have been discussed by Weppner and Huggins sl. In case only one monovalent cationic and electronic species has to be considered neglecting the transference number of all other species the enhancement factor for the ions becomes:

61nai blna, 1 W = t, ~ - ¢Slnc,;

(43)

Large enhancement factors can be present when highly mobile electronic species are present such as in Li3Sb where an enhancement factor of up to 70 000 was found 72. These large enhancement factors can be explained in terms of a microscopic model. When charged species with different mobilities are transported an internal electric field is created. The slower species are accellerated and the faster ones retarded in order to obtain local charge flux neutrality. If the electronic charge carrier concentration is too large, these internal fields are shielded so that enhancement effects are not possible. As a consequence there is an optimal electronic charge carrier concentration for a given ion concentration (activity) at which the enhancement factor has a maximum value. 61na The enhancement or thermodynamic factor ~ can be determined from the slope of the composition -voltage

curve, because the voltage is proportional to the activity. It remains a major challenge for materials research of the future to take advantage of the large enhancement factors which can be achieved in some combined electronic-ionic conductors. This will especially be true for

Energy Conversion and Storage

219

solid state ionic devices, which operate with semiconducting materials. Electrochromic and photoelectrochromic devices could be made much faster this way. Electrode materials with large enhancement factors are also desired for photoinsertion batteries as well as for photon powered ion pumps, the efficiency of which is essentially limited by the rate of ion transport. When dealing with semiconductors we have also to take into consideration that electrical potential differences at phase boundaries (junctions) extend into the materials resulting in electrical fields. Gradients of electron and ion concentrations will thus occur at the interface even in thermodynamic equilibrium. Illumination will , in addition , change the concentration of electronic charge carriers . All these conditions will affect the enhancement factor according to relation (42) and (43). We will come back to the influence of light on ion transport in semiconductors in section 10.2.

4. ELECTROCHEMICAL INSERTION REACTIONS

4.1. Electrochemical Theory and Techniques Electrochemistry provides easily accesible techniques of studying the thermodynamics and kinetics of insertion reactions. We will not review these electrochemical techniques, but point to some basic features that appear, when semiconducting electrode materials are studied in energy converting cells. The electrochemical behaviour of semiconductor electrodes is essentially different from metal electrodes.

Ef

I ..,I .a_ I E°

.>_

;I

E°

Ef E,

~y~.~,EC .t E°

Ev

Ec

.c_ (rl

Ef

E@

Ev

e-

metal

electrolyte

semicon ductor

electrotyte

Fig.10: Changes in the a) metal/electrolyte interface and b) semiconductor/electrolyte interface by lowering the applied electrode potential

As shown in Fig.10a) the Fermi-level in a metal is situated within a continuum of occupied and unoccupied electronic states. Due to the high concentration of mobile charge carriers the width of the space charge layer of the metal is reduced to a few Angstrom at the metal/electrolyte interface. Since the Fermi-level of a metal electrode lies within a partly filled energy band there are always charge carriers present which can be exchanged with the spcies in the electrolyte. When the applied electrode potential is changed, the current is therefore following the potential changes principally without large overvoltages.

220

G. Betz and H. Tributsch

In a semiconductor on the other hand, the Fermi-level which corresponds to the electrochemical potential of the electrons, is located within the forbidden energy region. The width of the space charge layer is typically between several 10 nm and several 100 nm wide depending on the charge carrier concentration. It is visualized in an energy scheme ( Fig. 10b ) as a bending of energy bands at the semlconductor/electrolyte interface. The changes which occur when an external potential is applied, depend largely on the quality of the semiconductor surface. In absence of chargeable surface states the energetical position of the band edges of the conduction band and of the valence band can be considered fixed. An applied electrode potential moves the Fermi-level within the semiconductor, but keeps its distance from the energy bands constant in the bulk and thus causes a change of energy band bending. When a sufficiently positive electrode potential is applied, the energy bands are bent down, so that positive charge carriers (holes) are drifting towards the semiconductor/electrolyte interface. When a negative potential is applied the energy bands are bent upwards and electrons are accumulated near the surface (Fig.10b). With semiconductors that do not exchange ions with the electrolyte there are, however, two situations, in which the position of the energy band edges does not stay constant. In one situation they are unpinned because of surface states which accumulate charges thus producing a potential drop in the electrolyte/semiconductor interface {Helmholtz-layer). In the other situation the potential drop occurs as a consequence of inversion, that is, when the Fermi-level near the surface is approaching the energy bands so close that additional charge carriers are created. Furthermore, there are remarkable differences between charge transfer processes at metal and semiconductor surfaces. The energy gap does not permit electron transfer processes to energy levels in the electrolyte that correspond to energy levels in the forbidden region. In addition, light-induced reactions can be initiated at semiconductor electrodes, whenever photon absorption increases the concentration of minority carriers in the interface. Semiconductor electrochemistry has during the past twenty years developed into a quite complicated and wide ranged discipline. Originally and still inspired by solid state descriptions of p-n and semiconductormetal junctions, the chemical nature of interactions at the semiconductor electrolyte interface is recently receiving increased attention. A series of reviews and books have appeared on that subject 82-86 The established electrochemistry of metals and photoelectrochemistry of semiconductors is an important basis also for electrochemical insertion studies. However, work on inserted electrodes has to go beyond. When guest species are inserted into host materials, these electrode materials may entirely change their thermodynamic properties as well as their physical and interfacial chemical behaviour. Semiconducting materials may become metallic or change the width of their space charge layer. They may change their energy gap, Eg ,or assume a new electrochemical potential. For the present time electrochemistry of insertion reactions is still in an initial stage of development, where thermodynamic aspects are more important than kinetic ones. 4.2. Electrochemical Insertion Batteries If an insertion reaction is carried out electrochemically in a galvanic cell, the free energy change AG of the insertion reaction is stored in form of electrochemical energy of separated anode and cathode materials. The charging and discharging reactions of an insertion cell are shown in Fig.ll. The open circuit voltage Voc , the electromotive force of the cell, is given by (4). The cell is discharged by inserting atoms from the anode into the cathode host material. If the system allows only an irreversible discharge reaction to be performed it is at least possible to build a primary battery . Secondary batteries that can be recharged by deinsertion of the host cathode demand reversible insertion reactions. Other criteria an insertion cathode material MxA has to meet are: -

wide range of x ,since the energy density of the cell is determined by xm,~x ,the maximum stoichiometry

of the inserted compound.

Energy Conversion and Storage

e"

221

RL

VL 0

I

0

e-"

h-

leI

e~

e-i w

Mt

M+

,

i

MI

" --__M÷ p

M÷---- ~

I,A

MxA

charging

MI

discharging

Fig.11 : Charging and discharging reactions of an insertion cell

-little change of free energy AG

over the composition range, yielding a constant cell voltage. However, a

higher slope of the voltage-composition curve corresponds an increased t h e r m o d y n a m i c factor,

(~q~),

~ln~ and

thus a higher chemical diffusion coefficient of the inserted atoms. - little structural change during the reaction, which is decisive for the reversibility of the insertion reaction. - high diffusitivity of the inserted ion - good electronic conductivity - no solubility in the electrolyte - no coinsertion of the electrolyte A decisive criterion for the construction of practical cells is the chemical and electrochemical stability of the electrolyte in the presence of the anode and cathode materials. The energy W,i , stored in the cell, is given by W¢l = f V ( l ) I d t

. where the cell potential V is dependent on the discharging current, I which

is d e p e n d e n t on the discharge time, t. This is due to internal resistances, which limit the output power. Because of the same reason the energy input must also exceed always the stored free energy. Most of the technologically investigated systems use alkali metal atoms as inserted species due to their highly negative redox potentials. Transition metal oxides or sulfides are mostly used as cathode materials, since the transition metal atoms can be found in higher oxidation states compared to other metal atoms. T h e technologically most advanced intercalation battery system is the Li/TiS~ -system,

x L i + TiS2 ~

Li,TiS2

(44)

which is a perfectly non-stoichiometric system for the entire composition range 0 _< x <_ l . The intercalation p r o d u c t represents a homogeneous single phase s~. It is stable with respect to both Li2S

and TisS 8s

No energy e x p e n d i n g new phase has to be nucleated upon insertion as shown both by measurements of t h e electromotive force and determinations of lattice parameters. The lithium-titanium disulfide reaction provides a free energy of 206 kJ 'mole which corresponds to a theoretical energy density of 480 W h / k g ( m e a s u r e d during the first discharge at a current density of 1 0 m A / c m 2 )8~. The lithium ions occupy the octahedral sites in the Van der Waals gap. A voltage-composition curve of a Li/TiS2 - electrode with the potential of lithium as the reference is shown in Fig.12 a).

G. Betz and H. Tributsch

222

o

,._1 ÷

2.5

LixTiS2

2.4 :~ 2.3 2.2 2.1 2.0 1.9 181.?

0'.2 63 0'./, 0'.5 016 0'.7 0'.8 0'.9 110 composition x

I}.1

o

:l~

3.0

l

LixNiPS 3 .1J L/)

2.0-

-~ ¢J

a

"6

1.0.

composition

x

o Ol

._1

÷

3.5" Lix V6 03

-~ 3.0,Q tO

o ¢3.

o

2.s.

2.(; composition x

Fig.12. Composition-voltage diagrams for a) the LixTiS2 sr b) the LixV~,Ol~z°° and c) the LixNiPSs 9° system.

Energy Conversion and Storage

223

The free energy change of the intercalation reaction can be split into three contributions:

-AG

37

(45)

= E1 + zE~ - 2 R T l n - x-]

The first term, El , is due to the change in the chemical potential of the electrons ( change of the Fermilevel). It describes the filling of the t2g -levels of TiS2 . Each intercalated electron donates one electron to the TiS2 -host. E2 represents the Li + - Li ÷ - interaction energy; its value is approximately 33 k J/tool. The last term is the configurational entropy with both the electrons and the Li-ions determining the maximum degree of intercalation ss. In practical systems the titanium disulfide cathode material is typically mixed with 10 wt% Teflon and pressed at 300°C into a metal grid. It is surrounded by a polypropylene separator and a sheet of lithium metal or a lithium alloy.

The whole cathode structure is then immersed into an organic electrolyte (

propylene carbonate, dioxolane ) containing lithium perchlorate. Such a practical system has an energy density of approximately 100 Wh/kg, roughly 20 % of the theoretical value r Another very promising cathode material for lithium insertion is V60]3 of this system is 800 Wh/kg 89. V60]z

.The theoretical energy density

has a non- close packed structure. The simplest class of these

structures is the ReO3 structure and shear structures derived from it. The problem of these metal oxide shear structures is to keep them open during insertion of Li, since they contract by cation-cation interactions. The crystal structure of the metal oxides is much more heavily influenced by these interactions as in the case of the other metal dichalcogenides, since the oxygen atoms are smaller and less polarizable than the heavier homologes. The V6013 -structure consists of alternating double and zig-zag chains of distorted VO(~ octahedra interconnected by corner shared oxygen and infinite channels along (010). These channels provide paths for the diffusion of the lithium ions. The Li/V60]s -cell has an average voltage of 2.5 V and shows excellent reversibility (see the composition-voltage curve in Fig.12) 2°9 . Similar high energy densities (1000 Wh/kg) can be expected from the systems Li/FePS3 and Li/NiPSz . The electrodes are layered-type and show practically no volume change during intercalation 90. An interesting fact is that both cathode materials are semiconductors(for a composition voltage curve see Fig.12). Additional transition metal dichalcogenides have also been tested, especially the system Na/TaS2 . Generally, sodium cells have a lower voltage as expected from the less negative redox potential compared with lithium. In diagrams showing the dependence of the cell EMF on the degree of intercalation breaks are observed in the system Na/TiS2 . They indicate irreversible structural changes which will gradually cause a degradation of the electrode material 9]. The only secondary lithium cell which is available on the market is based on the system Li//MoS2 92

4.3. Organic Comp ounds as Electrode Materials

Organic electrode materials are being developed with the hope of obtaining lightweight inexpensive battery systems. The discovery of rechargeable batteries based on conducting polymers such as polyacetylene (CH)x has stimulated this research area. Due to the simplicity of its structure polyacetylene is often considered the prototype conducting polymer 9s,94. Polyacetylene films are prepared by polymerization of acetylene with a Ziegler-Natta-catalyst9r' or a Luttinger catalyst 96 onto a suitable substrate. The product obtained is a highly crystalline material that occurs in the cis- or trans-form as shown in Fig.13. At room temperature the cis-form slowly transforms into the trans-form. The one-dimensional structure is held together by two of the three sp 2 -electrons per C-atom forming o -bonds, while the third one forms the bond with the hydrogen atom.

The 7r -

electrons that strengthen the tr (C-C)-bonds are responsible for the electronic conduction in a partially filled energy band. Low-lying completely filled energy bands arise from a -bonds. Polyacetylene can be doped chemically9r'gs or electrochemically 99 with a variety of donor (D) and acceptor (Ac) species. In

224

G. Betz and H. Tributsch

Cis -(CH)x:

/

C=C

\

-----C

/

C=C

\

C-----C

C----

--C

,//

C-C

%

C --C

J

C-C

\\

C--

trans -(CH )x :

/

C

C

\\/ C

%,/

C

"%/

C

C

~

..--,.. f

C

C

C

\J

C

\~

C

C

C

\f

C

\

Fig.13: Ground states of cis- and trans-(CH)x. connection with (semi)conducting polymers the term doping is equivalent to the previously used term insertion, especially when the doping is carried out electrochemically. With doping levels up to 1% nand p-type semiconductors ((CH-)rDF+)X, ((CH+)yAc~-)x are obtained. Doping to higher levels results in a semiconductor-metal transition. The conductivity that is obtained by doping ranges from 10 5 to

10 3

(ohm cm) -3 . Organic compounds such as polyacetylene working as battery electrodes have the polymer chains in different oxidation states which are controlled by the inserted dopants. There are different ways to use polymers in energy storing cells. They can act as an anode, as a cathode or both as anode and cathode materials. Discharge can be performed either by doping or by undoping the polymer electrodes.

RL

( CH )x+ yLi++Ye" ((CH- Li+)y)x

(CH)x+YC[O ~ -ye(( CH÷ CtO~.)y )x

conduction band ft*

I

Li"

//C\~/C%

~

-1"__--

I

C\ ~C\~/C c

!"~

vatence bend

CIOZ

c

%

-,~C\ ~ / C % c

It=

E*

4411,

It

Fig.14: Upper part: Discharge of a (CH)x-insertion cell with Li" and C]O~ as the inserted species. Lower part: Creation of negatively and positively charged solitons from a neutral soliton upon insertion (doping).

Energy Conversion and Storage

225

The species inserted into the cathode is frequently lithium, the one inserted into the anode are CIO4 -ions. The discharge reaction of a cell with polyacetylene forming both the anode and cathode is shown in Fig.14. The charging reaction is the reverse of the reaction depicted in Fig.14. This kind of tandem cell represents a new approach in battery technology, since no reactive akali metal electrodes are needed. When a Li metal strip is used as an anode, the polyacetylene cell has an open circuit voltage of 3.7 V and a short circuit current density of 100 m A / c m 2 9a. This large current density reported is due to the fibril structure of the polymer that increases the effective surface by a factor of 1000 (with a film of 100p, thickness the effective surface area is 60 m 2 /g ). The energy density of a

(CH)x/LiCIO4/Li -cell reported

is 58 Wh/kg at a discharge current density of 100

m A / c m 2 . The authors conclude that the polyacetylene battery will have an energy density comparable to that of a lead acid battery taking into account a factor 7 for packaging 100. But a Pb acid battery of the same energy density per weight will still have an energy density per volume of a factor 5 higher. The reported power densities are an order of magnitude larger. The major disadvantage for the applications of po]yacetylene as a battery or solar cell material is its low stability in the presence of oxygene or water which destroys the conjugated 7r-electron system by oxidizing double bonds in (CH)x • From a more fundamental point of view the question is interesting what the charge carriers are that transport and store the electronic charge in these conducting polymers. In an ideal infinite chain of (CH)x one expects the ~r -electrons to form a half-filled band leading to a metallic conductivity as observed, e.g. in graphite. However, as was found in optical studies on finite chains, the frequency of maximum absorption decreases first with increasing chain length x (as expected) and then tends to saturate at a finite value of about 2 eV. An alternating bond length geometry of the carbon backbone is considered as the origin of the gap 101 This finding is closely related to the Peierls theorem which states that a one-dimensional metal is unstable with respect to a periodic lattice distortion. Together with the occurence of bond length alternation, a gap in the electronic spectrum at the Fermi surface is opened separating the highest occupied from the lowest unoccupied molecular orbital 1°2. From optical absorption measurements the energy gap is determined to 1.5 eV l°a. In trans-(CH)x double and single bonds can be interchanged without energy expanditure. This twofold ground state degeneracy leads to the existence of nonlinear topological excitations,solitons,that are bond-alternation domain walls shown in Fig.14. This ground state degeneracy is not present in cis(CH)× , which has an energy gap of 2.0 eV. Here the solitons are confined into a polaron-like entity. As a further consequence of this confinement the light-induced charge carriers rapidly recombine and no photoconductivity is observed in cis-(CH)x, whereas photoelectric properties are found in trans-(CH)x 104 The degenerate ground state of polyacetylene is quite unique although there are many conjugated polymers with a nearly degenerate ground state,for example conducting polyparaphenylene, where it was found that any contribution to Pauli paramagnetic susceptibility is neglectably small 1°'~. The energy of creating a soliton is ~-, 24 where 2A

is the energy of the bandgap Eg . Therefore, solitons

in polymers, such as polyacetylene, are more stable than band electrons or polarons. When electrons or polarons are injected from the external circuit they combine to form pairs of solitons l°e. The neutral soliton exists in undoped polyacetylene due to locally incomplete cis-trans isomerization (see Fig.14). The neutral soliton having a spin of 1/2 corresponds to a midgap state. Upon doping charged soliton states are created that are spinless as shown in Fig.14. The injected electronic charge is stored in the midgap soliton states and is compensated by the inserted ions. The resulting midgap absorption was verified by an in-situ optoelectrochemica] spectroscopy 1°6. Although there is a lot of evidence for the soliton-hypothesis, it has to be added here that the conduction mechanism in polyacetylene is still controversial. Organic polymers such as for example polyethylene oxide(PEO) may also be applied in future battery technology as solid electrolytes. Complexed with NaI, PEO conducts Na ~ - and iodide ions. The electrical conductivity is 104 (ohmcm) 1 at 358 K 1°7.The material can be pressed into self-sustaining films of 0.10.2 mm thickness. These films can be sandwiched between polyacetylene films to give an all-solid state polymeric cell

226

O. Betz and H. Tributsch ((CH)y(Na+)y)~/PEO- N a l / ( ( C H ) ; I ; ) ~

This cell is charged by undoping the polyacetylene los. The EMF of the cell ranged from 2.8 V to 3.5 V and the short circuit current from 1 to 12 m A / c m 2 . For a (CH)x/PEO - NaI/Na -battery the estimated power density is 250 W/kg and the energy density reported is 20 Wh/kg 10s.

4.4. lnterJaeial Electrochemistry o] Topochemical Mechanisms Interfacial mechanisms are of fundamental importance for topotactic redox reactions, since they determine the catalytic aspects of insertion reactions. The field is still too young and too concentrated on thermodynamic aspects as to yield relevant experimental and theoretical informations on how ions are crossing the interface between an electrolyte and an ionic material, while simultaneously exchanging electrons with the energy band system of the solid. The first problem is that the reactive interface cannot unambiguously be defined. In the case of an exergonic intercalation reaction such as in the Li/TiS2 - intercalation battery the host material assumes a positive potential and attracts both the electrons (through the external circuit where electrical energy can be extracted) and the Li + -ions (through the electrolyte) into the host material, where the electronic equilibrium is established. During this interracial redox reaction an equilibrium between the electrons e - and the holes h ~ on the one hand and the inserted guest species M on the other hand can be formulated:

M ÷ + e- ~ Mira = [M + + e-lint

(46)

Mint + h + ~- M +

(47)

Here M + denotes an ion in the electrolyte, that means: the equilibrium is established across the electrode/electrolyte interface. This equilibrium is different from the internal redox equilibrium described in Eqs.(5,6). But the redox reaction does not necessarily take place at the geometrical interface between the combined ionic and electronic conductor and the electrolyte. Because of the expected better overlapping of electronic orbitals it is more probable that the ions penetrate some distance into the space charge layer of the material before exchanging electrons with the solid. It is clear that inserted atoms in an environment such as the crystalline framework of the host lattice will have a chemical potential that differs from that in the metal. Apart from a different redox potential, kinetic parameters depend upon the interaction of the host with the species. This means thai we are generally dealing with a situation of electron transfer involving strong interactions between the interacting species 109 The strong interaction already begins with the stripping of the solvate shell of the guest ion. No simplified classical or semi-quantum mechanical approach for the transition of electrons between donor and aceeptor species assuming weak interaction can be applied. All major consequences concerning electron transfer involving strong interaction have to be taken into account: a) The electronic states of reactants are transformed to and have to be treated like surface states, b) Electrical potentials applied to insertion electrodes are affecting the energetic position of such electronic states, c) The presence of topotactic redox species will significantly modify the structure of the electrochemical double layer. These complications are not the only ones. The facl that the int.erfacia] potential difference adjusting at the host material electrolyte interface depends on the concentration of inserted guest ions and on the ionic concentration in the electrolyte can easily be derived from basic electrochemical formula. This treatment is parallel to that for a metal electrode in a metal ion solution with the difference that in the case of an insertion electrode the potential will depend on the degree of insertion. When equating the electrochemical potentials of the exchanged ions interacting in (46,47):

~i(electrolyte) = th(electrode)

(48)

Energy Conversion and Storage

227

and resolving the equation with respect to the difference between the electrostatic potential of the host electrode, V, and that of the electrolyte, rmV m, we obtain:

q(V - V m) : # m _ Pi

(49)

By expressing the chemical potentials in terms of activities and introducing s t a n d a r d potentials (label o) this relation is changed to :

q(I/ - V E`) = ( # m _ #,o) - kT In

a?

(50)

where the tzi0 are the s t a n d a r d chemical potentials, a, the activity of the ions in the insertion electrode and a,m the activity of the ions in the electrolyte. The term in parenthesis can be related with the negative s t a n d a r d free energy gain, AGo, of the redox reactions (46.47):

v - vE'=

_aGo

F

+

R:tn='¢°~' F

(5,)

ai

where the guest species If the pure phase M is taken as the reference, AGo has a negative value for exergonic insertion reactions and makes the potential difference the bigger the more energy is turned over during insertion. This AGo should not be mistaken for the AG defined by eq.(4), which refers to the total insertion reaction, whereas AGo only refers to the redox reactions (46),(47) in one of the half-cells. Thus, eq.(51) defines a redox potential E°(M"/Mi,,)for the insertion reaction at the host electrode (see Fig.15). The electrochemical potential for an insertion reaction with energy gain is therefore a]ways more positive than the potential for the deposition of the element involved and will depend on the electronic and crystal structure of the host material. ]n p- and n-type electrodes the activity of the M-atom is different because of the difference in the electron activity (different Fermi-levels). Therefore p- and n-type electrodes of the same composition show a different cell potential. Also the coulometric titration curve is different for n- and p-type electrodes of the same material. But for an p-type semiconductor the ho]es induced by doping it will be compensated by the inserted electrons. ]n order to visualize part of the resulting complicated situation during interracial insertion reactions we suggest the following schematic representations in energy diagrams, depicted in Fig.15. On the left side an insertion battery consisting of a metal anode (M) and a metallic homogenous host electrode, separated by an electrolyte, is shown. The insertion reaction is exergonic which means that the insertion redox potential is situated positive from the Fermi-level of the metal anode. The horizontal bar, indicating the redox potential of the insertion reaction is drawn across the host material electrolyte interface to indicate strong surface interactions and a surface state type distribution of electronic redox levels. In the case of the depicted metallic host structure the Fermi-level adjusts to the redox potential and assures the supply of electrons (ultimately arriving from the M-anode) needed to intercalate M~ . ]t is also important where the electronic levels of the inserted guest species wil] finally he situated within the inserted host compound. ]n Fig.15a) they are depicted below the Fermi-level. They could as we]] be situated above. ]n both cases the electrons would be delocafized within the conduction band of the metal The situation is different in the case depicted in Fig.15b), where a semiconductor is considered to be the host material undergoing an endergonic insertion reaction (AG > 0 which is only possible under imput of external energy. An interfacia] potential difference described by eq.(5]) will occur within the host electrode. The related e]ectric fie]d can he used to separate light-induced charge carriers. The Fermi-level of the metal anode (pure phase M) and that of the semiconducting host cathode prior to insertion are now supposed to be on the same level. Without input of external energy which provides electrons at an elevated energy level for insertion, the guest species cannot be inserted into the host cathode. Light generating electrons and holes can provide the necessary energy. It is depicted in Fig.15b), how the excited electrons can in an interracial process recombine with the guest ions to incorporate them as a neutral species. JPSSC

16:4-C

G. Betz and H. Tributsch

228

a)

'AG
~

==<

Mint electrode

m eta [

oV=_-~G

meta

o

F

in the dark

Ec

b)

Ef

~

upon

*IMin,

.......

7

p-

.°.°° .......

22;:;2::

~

M÷

. . . . .-. - . . . . . . . . .

eteetr01yte metal

illumination

I

o

Vph

o

Fig.15: Representation of the thermodynamic potential of the insertion reaction in a galvanic cell for a) a metal electrode and b) a semiconductor, illuminated (dotted line) and in the dark (filled line). As the reference the potential of the M-electrode is taken. The electrons inserted with the guest species M are asssumed to occupy states EM. The light-induced charge carriers have an excess energy and therefore an electrochemical energy which is different from that in the bulk. This is particularly the case for minority carriers (e.g. electrons in a p-type semiconductor). It is therefore necessary to attribute to them a specific Fermi-level, the so called quasi Fermi-level (see below). The quasi Fermi-level for electrons,nEi , which is the thermodynamically relevant quantity for the photoinsertion of cations into p-type host materials is indicated in Fig.15b). The quasi Fermi-level is a helpful concept to describe the photoreactions at semiconductor surfaces and will be also used to describe the photoinsertion reactions. It is difficult to define, where the interfacial process will take place, within the space charge layer or at the geometrical interface. The inserted guest species can introduce new electronic levels into the bulk of the semiconductor. In Fig.15b) they have been placed above the original Fermi-level of the host material. Since available unoccupied electronic levels are very limited in a p-type semiconductor their position will determine how the Fermi-level shifts as a result of insertion. If the electronic levels of the guest species

Energy Conversion and Storage

229

are situated low in an electronic energy band scheme, the semiconductor will remain p-type conducting. The higher they are situated the more n-type conducting the material will become. As a consequence the sign of charge in the space charge layer and that of the photopotential can change, so that photoinduced deinsertion starts. In this analysis we could just list several effects which have a strong influence on the nature of the interfacial processes during insertion without yet being in a position to quantify them. The guest species moving through the space charge layer will additionally distort its field distribution ( see later ). All these factors will have a significant influence on kinetics and catalysis and will gradually have to be elaborated for a better understanding of insertion processes.

5. APPLICATIONS IN OPTICS, SENSORING, CATALYSIS

5.1. Electrochromic Devices and M e c h a n i s m s A technologically advanced development of insertion mechanisms is found with electrochromic displays. These displays work with an externally applied potential to insert small atoms like lithium or hydrogen into host elctrodes (e.g.WOs, MoOs H0, ir(OH) ~1~) accompanied with a colour change. As compared with liquid crystal displays, electrochromic systems have the advantage of permitting a much wider viewing angle. They also provide a much better contrast, since the contrast is created by optical absorption as in most objects of every day life. It is obvious that the energy consumption involved in electrochromic systems has to be minimized for optimal technical performance. However, a small amount is always needed for information storage. A high information content is equivalent to a high degree of order, which can according to thermodynamics only be obtained with a non-vanishing energy input.. So it is helpful to consider the el~trochromic host electrodes as materials, where electrical energy is converted and stored as chemical energy of the inserted, coloured host electrode. Typically, the empty semiconducting host electrode is white and gets colored as a consequence of the insertion. The only exception is iridium oxide which gets bleached, when it is reduced and colored, when it is oxidised. At lower concentrations x of the guest species new energy levels in the forbidden energy region of the semiconducting host electrode are formed. Electronic excitation from these levels gives rise to an additional optical absorption in a spectral region, where the empty host electrode is largely transparent. At higher concentrations x the Fermi-level is shifted so close to the conduction band that the new levels created upon insertion donate mobile electrons into the conduction band at room temperature.

These carriers

produce a coloration due to free carrier absorption. The minimum electrochemical energy involved in the colouration process corresponds to this change of the Fermi-level of the host electrode. A large number of electrochromic systems have already been studied and reviewed n2. The tungsten trioxide systems are the most advanced examples of electrochromic displays . The underlying reaction is:

W 0 3 ( w h i t e ) + x M + + x e - ~ M x W O s (blue)

(52)

with M = H,Li. The maximum value of x reached, is x=0.1 in the case of M = hydrogen. A decisive criterion for the use of electrochromic devices is the time constant of the colouration. With tungsten trioxide typical time responses are 0.5 see for lithium insertion and 0.1 sec for hydrogen insertion. The charge consumption reported for an change in the optical density of 0.3 is approximately 5 m C / c m 2 113. These 5 m C / c m ~ correspond approximately to a turnover of 15 monolayers only, but since hydrogen or lithium are inserted at concentrations of up to 10% only, the insertion is not limited to this surface region, but extends into the host electrode at least ten times as deep. A layer of 0.3/~m thickness of the electrode material is sufficient for electrochromic performance. With an suitable electrolyte and a counter-electrode over 100 cycles can be run without a significant degradation of the electrochromic display.

230

G. Betz and H. Tributsch

Since tungsten trioxide is a semiconductor, photoelectrochromic effects (see later) should also be possible. But since the material is intrinsic or n-type, illumination produces anodic photocurrents and does not aid the photoinsertion of cations. Higher concentrations of inserted hydrogen or lithium furthermore lead to metallic conduction and to a breakdown of the photovoltage, because the extension of the space charge region reduces to a few Angstroms preventing efficient charge separation.

5.2. Chemical Sensors The technology of chemical sensors has during recent years experienced a very fast development,both in research and application (for a review see M.Kleitz in reference (114)). The principle of operation of all electrochemical sensors is to measure the change of the electrochemical potential or the conductivity of an host material due to the insertion of the species that is to be detected. As in the case of electrochromic devices, we also have to pay attention to the energy involved in the sensoring reaction, in order to understand the underlying principle and for the purpose of developing new mechanisms for sensor devices. T h e most sucessful application of chemical sensors to date has been the measurement of oxygen concentration using zirconia solid electrolytes 115. The electrochemical reaction concerned, which is strongly temperature dependent, can be written as:

02 + 4e- --* 202-

(53)

The oxygen ions are formed within the zirconium oxide layer adjacent to the metal contact providing electrons. According to the Nernst equation an output voltage V will be created, which is proportional to the partial pressures of oxygen RT

Po~

Y = ~-~ i n - -

(54)

Po~.

,where po~. is the oxygen partial pressure that has to be measured and Poi the oxygen partial pressure at the reference electrode. Many additional oxygen electrolyte materials have been explored, such as C e O 2 - Y 2 0 3 - B i 2 0 3 ; Bi203 - Er2Os 116 and 8c203 - ZrO2 11~. It has been discovered that an externally applied electric field can favourably influence the motion of the ions and enhance the interracial reaction of oxygen with the solid electrolyte. Thus, a higher sensitivity is obtained even at lower temperatures 11~ Ionically conducting sensor materials integrated as active parts into semiconductor devices such as metal/oxide/semiconductor (MOS) structures have now become a widely investigated approach in sensor technology (ion - selective FET) 119. It has been shown for example that hydrogen can be monitored with a MOS device,the metal of which is palladium. Palladium, like a number of other metals (e.g.Ni,V,Nb,Ta,Ti) can uptake hydrogen. Due to this insertion of hydrogen the Fermi-level shifts towards negative potentials.Thus, the junction properties of the MOS- device are changed, although details of the mechanism are not yet completely understood 12°. During this review of insertion materials we have already made a series of comments on the energetics of photoinduced insertion reactions. We conclude that sensor devices based on photoinsertion mechanisms will be available in the future,integrated in optoelectronical circuits. Two pecularities speak for such a development. One is ,that photon energies (typically 0.8 - 3 eV) by far exceed thermal energies (typically 0.025 eV (300 K) - 0.08 eV (1000 K)) and can therefore used to overcome much more easily activation barriers and drive endergonic reactions. The second concerns the selectivity of sensoring. Insertion products may have a photoelectric response which is typical for a special guest species (e.g.hydrogen in TiO2(B) 121). The spectral sensitivity could therefore be directly related to the chemical nature of the guest species.

Energy Conversion and Storage

231

5.3. Insertion Compounds in Catalysis Insertion mechanisms and insertion compounds in catalysis have remained a neglected research area although their possible importance has repeatedly been emphasized (e.g.SchSllhorn 20). It is reasonable to distinguish between two different cases when dealing with insertion in catalysis. One concerns mechanisms in which the catalyst simply happens to be an insertion compound.

The second includes all reactions, in which

one or more reactants act as guest species of the catalyst during the reaction. In technical catalysis, which frequently involves complex oxides in presence of noble metals at elevated temperatures formation of insertion compounds is probable and frequently even unevitable. However, little scientifically useful information is available, except for specific systems. So,. for example the activity of HxMoO3 in the reduction of NO by H2 has been established 1:2. Many catalytically active metals such as Ni,Pd,Ru,Fe,Ta,Nb,V,Cu,Ti can uptake hydrogen 12,. Also oxides such as ReOs, TiO2, IrOs, VO2, RuOs, MnOs, V2Os and PbOs insert hydrogen as well as many sulfides 2o. It is sufficient to mention TiS2, VSs, NbSs, TaSs, CrS2 and MoSs . Let us have a closer look at MoSs which is the best kown and commonly used catalyst for the hydrodesulfurization reaction ~2s. It can also be used for the catalysis of hydrodenitrogenation and methanation. In the hydrodesulfurization reaction hydrogen and sulfur containing organic compounds are reacting at elevated temperature and pressure to form hydrogen sulfide. A number of reaction steps must be involved: adsorption and bonding of the sulfur containing species, adsorption of hydrogen molecules and subsequent separation into adsorbed atoms, reaction to hydrogen sulfide and desorption of the products. It is the role of hydrogen in this catalytic reaction which evokes the possible participation of insertion mechanisms. MoSs can form an insertion compound of the composition HxMoSs (x < 0.3). The concept of hydrodesulfurization catalysis would be very different, if inserted hydrogen is involved. This is evident for the steric conditions of the reaction, which would allow hydrogen atoms to react from within the bulk of the catalyst rather than from neighbouring surface sites. Hydrogen bound to the catalyst's host lattice also forms a much larger reservoir of activated reactants. It is also possible, that the activation barrier for the reaction from within the catalyst is lower. The electronic effect of hydrogen on MoSs would be different for a surface reaction compared to an insertion reaction. In a surface reaction only comparatively few hydrogen molecules could react with the catalyst and would rapidly produce a negative charging of its interface by donating electrons. During an insertion reaction a much higher amount of hydrogen can be accomodated within the catalyst without charging it electrically, but nevertheless shifting its Fermi-level towards negative potentials. A ptype semiconductor with its Fermi-level located near the valence band edge can uptake more hydrogen than an n-type material of the same structure and composition. Coincidentally, cobalt-activation of MoSs which is a common technical practice in hydrodesulfurization catalysis, makes the material p-type conducting ~ss Under these circumstances, hydrogen insertion would be favoured as compared to n-type MoSs which is catalytically less active. However, pure surface electrochemical reasoning can in principle also explain the effect of doping in terms of differently charged space layers, so that further research will be necessary to clarify this hypothesis. Another example of a possible involvement of insertion during a catalytical reaction is the CO methanation on Ru,Ni,Fe or Pd catalysts ls4 which all are able to insert hydrogen. Catalysis is an important aspect of energetics in chemical technology.

Inserted catalysts and insertion

mechanisms may play a much more fundamental role than commonly imagined. One interesting example of a system, which has been thoroughly studied in the past, is the well- kown lead acid battery. Not long ago it was discovered,that its high initial current density is due to the formation of a HxPbOs -hydrogen bronce 125

6. PHOTOINSERTION REACTIONS

In this section we want to outline the mechanisms of light-induced ion transfer at semiconductor electrode/electrolyte-interfaces.Some of the characteristics of insertion electrodes under illumination were

232

G. Betz and H. Tributsch

already mentioned in the previous chapters, but will be treated systematically in detail here. This treatment will be directed towards possible applications in photon energy conversion and storage. 6.1. Experimental Techniques and Materials The equipment neccessary to investigate photoelectrochemical insertion reactions is the standard equipment needed for photoelectrochemical measurements. Standard electrochemical techniques to analyse electrode reactions and kinetics are described elsewhere in detail (see e.g.126).

salt

Ag-wire- ~b~'bridge

\\\\

Py e -screw\\\\

brass cylinder

~

I I iI./.co-.,,.

/ crystal vespel cylinder

Fig.16. Cell arrangement for the investigation of photoelectrochemica] insertion reactions.

In general photoelectrochemica] measurements are carried out with transparent liquid electrolytes,since they can be handled much easier than solid electrolytes. An electrochemical cell for photoinsertion studies is shown in Fig 16. ]t has an optical window for photoelectrochemical studies and the determination of the energy gap of the semiconductor electrodes. The electrode can be displaced towards the optical window in presence of highly absorbing electrolytes and for the microscopic observation of the surface.

ramp generator

plotter

light' • source

focussing system

cell

Fig.17: Basic photoelectrochemical set-up

~ :l:;'f:rl

Energy Conversion and Storage

233

Stepmotor Fig.18: Experimental set-up for the determination of photocurrent spectra. The set-up to obtain potentiostatic current-voltage -curves in the dark and under illumination is shown in Fig.17. In a potentiostatic measurement with a three-electrode cell,the potentiostat keeps the electrode potential constant in relation to the reference electrode independent of the current flowing through the working electrode. For the determination of composition-voltage curves and the measurement of diffusion coefficients the potentiostat with the ramp generator is replaced by an galvanostat or a constant current source. As light sources xenon and tungsten iodine lamps or lasers (Ar ÷ ,He-Ne) are commonly used depending on the spectral sensivity of the semiconductor material studied. Small photocurrents are measured with the lock-in technique. This is usually necessary for the determination of photocurrent spectra, if monochromatized light originating from a white light source is used instead of a tunable laser system. A computerized set-up to measure photocurrent-spectra is depicted schematically in Fig.18. The monochromatized light is periodically intermitted by a chopper giving the reference signal for the lock-in amplifier.

Igatvanostat, I

Ipotentiostat [ WE IRE CE

7

/

electrotyte

hv

I °' -

--M--

electrode

Fig.19: Membrane cell to determine diffusion coefficients and to study changes of photoelectrochemical electrode properties while performing an insertion reaction.

234

G. Betz and H. Tributsch

An experimental technique which is especially suitable for the study of (light-induced) transport phenomena is the one reported by Devanathan and Stachurski ]27. The electrochemical cell is divided into two compartments by the electrode forming a membrane. One side is normally polarized at a constant current density to insert guest species, while the other side is held at a constant potential to detect the transported species by a corresponding deinsertion current. Such an arrangement is shown in Fig.19. From the time dependence of the deinsertion current the diffusion coefficient in the electrode can be determined ]~8. It is also possible to study the change of the photoelectric response due to the controlled insertion of a guest species on the dark side of the membrane. The spectroscopic techniques for the study of solid state materials such as optical absorption, IRspectroscopy, NMR-spectroscopy can be applied to investigate the electrochemical insertion recation in-situ. One published example is the in-situ optoelectrochemical spectroscopy of CIO4 -doped polyacetylene 10s Time resolved photoconductivity measurements using a contactless microwave technique may give informations about the influence of the guest species on the charge carrier dynamics ]Zg(see also 9.3). Impedance spectroscopy is a commonly used technique to investigate electrode/electrolyte interfaces although it is very intricate to find the appropriate equivalent circuits and to correlatee them with real physicochemical processes. Since set-ups for impedance spectroscopy are commercially available, we only shortly mention this interracial technique and refer to one of the many reviews that have been published recently 130

Except for experiments dealing with the insertion of hydrogen from water, organic aprotic solventssuch as aeetonitrile or propylene carbonate are used to prepare the electrolytes for the electrochemical measurements. One reason for the choice of organic electrolytes is that water molecules tend to be coinserted. The other is the low stability of most of the semiconductor electrodes against photocorrosion in aqueous electrolytes. Additionally, at higher activities of the alkali metal atoms hydrogen is evolved in aqeous electrolytes. Cu* ions in acetonitrile are, for example, stable against oxidation to Cu 2.

up to a potential of 0.744 V vs

Cu ÷ / C u ° in acetonitrile 131,132. To avoid any contamination of the electrolyte with water or oxygen, it is necessary to work in a glove box under nitrogen or argon atmosphere. It is further unevitable to dry the solvent and the electrolytes (for details see (133)).

6.2. Electronic Properties of the Energy Converting Interface

P,,=i~ V, I ] R, _ E=.... IqV=-~-("E= e- °"~dhv Re 0 ¢.. .4,¢u

Eqvf~~.-.Lfit :

_"E* --.-E ..... E~.~/ ..._.(~,~',..,,.~ l~w¢. ,,,.-,..

distance

z

semiconductor

counterelectrode

Fig.20.Representation of the electron energy distribution in an regenerative photoelectrochemical cell in the dark (solid line ) and under illumination (dotted line).

Energy Conversion and Storage

235

For a better understanding of the photoprocesses at semiconducting insertion electrodes it is quite illustrative to recapitulate briefly the energy conversion reactions in the regenerative photoelectrochemical cells. These cells, which produce electricity from light without the possibility of storage, contain a suitable semiconductor electrode (such as WSe2 ), a redox couple (such as 12/I~ ) and a metallic counter-electrode ls4. An energy band scheme of a regenerative photoelectrochemical cell is shown in Fig.20. The electrolyte with a redox couple acts on the semiconductor like a metal contact. Therefore, if one attributes a work function to the normal hydrogen electrode lss, the semiconductor /electrolyte junction can be treated in analogy to a metal/semiconductor junction (Schottky barrier). Since electrons are exchanged at both interfaces, the electrochemical potentials of the electrons (Fermi-levels Ef ) in the semiconductor and the metal counter-electrode are in equilibrium with the redox potential E ° of the redox couple Ox/Red.

By equilibrating the Fermi-level of the semiconductor electrode with the

redox level E ° a space charge region is formed within the electrode. With a work function of 4.48 eV for the hydrogen electrode (NHE) which serves as a standard reference, the resulting band bending or barrier height, V~, , is related to the work function WA of the semiconductor by:

V~o = W A

-

E ° +

4.48eV

(55)

where the redox potential of the couple refers to NHE la8 The extension W~ of the space charge region is given in the Schottky model of an abrupt junction as:

w.:~/ •

./~

V ¢oNAc

(56)

where ~ is the dielectric constant of the semiconductor material, e0 is the permittivity of the free space and

NAc

the concentration of acceptor species, which dopes the semiconductor to a p-type conductivity.

An externally applied potential adds in the case of an ideal junction to the barrier height in (55). At an externally applied potential,

Vfb = E ° -

V~o

,

the band bending is reduced to zero (flat band situation).

In the dark charge transfer with the redox couple is carried out with the holes in the valence band, since only a negligibly small quantity of thermally excited minority electrons are present. Because of the energy barrier (energy bands bent upwards) this charge transfer is blocked until an externally applied positive potential inverts the band bending, so that holes can reach the electrode surface (accumulation layer) giving an anodic dark curent. In the other direction of the applied potential a cathodic dark current is observed which is low under depletion or inversion condition (perfect barrier)• It increases rapidly when the Fermilevel crosses the conduction band edge and an largely extended layer of electrons is formed at the surface (breakthrough voltage). This ideal diode characteristics is generally disturbed by surface states or electronic states in the energy gap of the semiconductor, exchanging electrons at potential values more positive than necessary for breakthrough. These electrons can be transferred into the electrolyte resulting in a cathodic dark current under reverse bias. When charge is accumulated in the surface states a potential drop occurs in the Helmholtz layer. In that case not the band bending is changing, but the band edges are shirr according to the potential drop in the Helmholtz layer. When the semiconductor electrode is illuminated with photons of an energy of ht, > Eg , electron/hole pairs are generated. The charge carriers which are created within the space charge region will be separated by the electrical field (band bending). Those minority charge carriers that are generated outside the space charge layer have to diffuse into this region to be collected by the electrical field. The determining quantity for this diffusion process are the diffusion lengths Ln, Lp of the minority carriers (electrons in a p-type,holes in an n-type material). This quantity is strongly influenced by recombination processes at defects or impurities in the semiconductor electrode. A high absorption coefficient has the advantageous effect that only a few photons are absorbed outside the space charge region so that a smaller diffusion length does not limit the efficiency of the photoelectric conversion.

236

G. Betz and H. Tributsch

The energetic situation under illumination can be described by introducing quasi Fermi-levels for holes and electrons pE~, nE~ • The quasi Fermi-level concept is an steady-state approximation of the non-equilibrium situation of a semiconductor interface under illumination. It describes the electrochemical potential of holes and electrons, when for instance their concentration is changed due the absorption of light and th subsequent charge separation by the electric field of the space charge layer. Photogenerated holes or electrons do not have electrochemical potentials equal to the edges of valence and conduction band respectively, from where they are reacting.

The electronic energy band scheme for semiconductors (Fig.19) only considers the

enthalpy contribution to the free energy, but not the entropy term which is dependent on the concentration of electronic carriers. It is included in the quasi Fermi-level which usually varies in a complicated way with the distance z from the interface due to locally different recombination and charge transfer rates. For simplicity, we only write down the relation for the quasi Fermi-level of electrons in a p-type semiconductor:

.E~ = E~ + k r t, c;

(SS)

Ce

where c~ is the concentration of the electrons under illumination and ce the electron concentration in the dark. For a p-type semiconductor it is always c~ ~ ca • So the quasi Fermi-level of the holes is approximately identical with the Fermi-level, Ey , of the electrode in the dark. So it is sufficient to consider the quasi Fermi-level of the electrons,the minority carriers x36. Upon illumination the separated charge carriers reduce the electrical field in the space charge region (visualized as a reduced band bending in Fig.19 or Fig.15). This reduced band bending corresponds to the photovoltage Vp, which is maximal in the case of flat tened energy bands, where charge separation is no more possible. Thus, the maximum obtainable photovoltage, Vph, is equal to the barrier height, V,o . Surface states and states in the energy gap pin the quasi Fermi-level during illumination thus reducing the maximum obtainable photovoltage. Additionally, these states can act as efficient recombination centers lowering the conversion efficiency of the cell. The minority carriers having reached the electrode surface reduce the oxidized species Ox in the electrolyte. At the ohmic back contact holes are transferred into the external circuit.

6.3. Mechanisms of Light-induced Ion Transfer Reactions Since the mass of the lightest ion, the proton , is approximately 2000 times larger than that of the electron, it can be assumed that the absorbed photons are only interacting with the mobile ions via the excitation of electronic charge carriers. This includes also polaron- or soliton-like excitations, where the excited electronic charge carriers are strongly coupled to lattice deformations which also influence the motion of ions. It is often found in biological systems that the excitation is localized on molecular units resulting in a lightinduced structural change such as photoisomerization that opens or closes an ion gate. An example of that mechanism is the cis/trans photoisomerization of retinal in the purple membrane or the visual pigments 6a. It can in principle not be completely excluded that light excites highly mobile ions such as protons directly, although only across a very small energy gap. In connection with this it is interesting to mention that an energy band scheme had been proposed for the description of proton conduction in solids with hydrogen bonds, which could be used to describe the excitations of protons with light. In the case of water the energy gap between the ground state and the first excited state is 0.01 eV a3~ As already mentioned the photoelectrodes for light induced ion transfer reactions have to be semiconductors with mobile ions that can be exchanged with the electrolyte. A survey of semiconducting insertion electrodes is shown in Tab.1. The mixed ionic and electronic conductivity of the electrodes considerably complicates the conditions at semiconductor/electrolyte interfaces as compared to the conventional purely electronic scheme of Fig.20. Such a scheme describes only the contribution of electrons to the free energy of an electrode. However, it is

Energy Conversion and Storage

237

well known that both the electrons and the ions contribute to the free energy of inserted electrode materials ( see equations (7) and (9)). Since the electrolyte contains no mobile electrons and since ions may penetrate into the space charge region, there has to be a transition from a mixed conductivity of the electrode to the purely ionic conductivity of the electrolyte somewhere within the interface. This transition can be located at the geometric electrode/electrolyte interface or it can be situated within the electrode at a varying distance away from the surface. In the first case of the so-called localized electrode 1~8, the Fermi-level and the electron energy bands are well defined up to the actual interface as in the situation depicted in Fig.20. For a so-called delocalized electrode in contrast, the energetic relations cannot be anymore described with simple pictures such as that of Fig.20, since it is not clear up to which point within the space charge region the Fermi-level for electrons is still defined. For the further discussion the simpler case of a localized electrode is assumed. The electrochemical equilibrium at such an interface then follows equation (51). Since here the ions are the species that is exchanged across the semiconductor/electrolyte junction the electrochemical potential of the ions has to be equilibrated. This can be considered in analogy to the equilibrium of the Fermi-level of the semiconductor with the redox potential of an ordinary redox couple (e.g. ]2/1~ ) in a regenerative photoelectrochemical cell (Fig.20). The electric field corresponding to the potential drop of eq.(51) is in the case of an ideal interface screened within the semiconductor electrode by both the mobile ions and the electronic charge carriers. Thus, the space charge layer is formed by both, electronic and ionic charges. An increase of the ion concentration in the electrode has the same effect on the width of the space charge layer as an increase of the dopant concentration (for a detailed consideration see reference (133)). Within the electrode the following redox equilibrium is established either with electrons (n-type material) or with holes h* (p-type material) as the electronic species:

[M ~ M * -~ e-]~,t

(59)

M + :-

(60)

M - h ~ i,~,

When ionized the guest species M acts as donor which shifts the Fermi-level of the electrode towards the conduction band edge. A guest atom M. which stays neutral, introduces an electron which is localized and does not change the conductivity and the position of the Fermi-level. During illumination this internal redox equilibrium will, of course, be changed according to the distribution of the quasi Fermi-level. The work function of the inserted guest species should not be too low, when cations are involved. Otherwise the equilibrium (59,60) is on the side of the dissociated M-species and the semiconductor electrode will become degenerate as a consequence of the insertion of only a small amount of guest atoms. The photoinsertion of cations into a p-type semiconductor MxA has already been mentioned. The reaction can be formulated as following:

M x A ÷ y M + + ye~,, ---* M~_uA

(61)

where M + are ions in the electrolyte and eh~, the photogenerated electrons. In Fig.21 the photoprocess is illustrated in detail. As in the case of Fig.20, photons generate electron/hole pairs, which are subsequently separated in the electric field of the space charge region. The holes are transported to the back contact, where they are exchanged with electrons from the external circuit. The electrons are driven towards the electrolyte interface. The actual reduction of guest ions may happen directly at the geometric phase bounda.ry or somewhere within the electrode as in Fig.20. Here the neutral guest species is assumed to form an occupied electronic state EM within the energy gap.

238

G. Betz and H. Tributsch

E•~

h

~E,

E, E,

....

~

h~ M+

~,:~._ ~--~

.................................. :--:

"

M"

M

E;,

M+

/

~1/

Ev

M

Z

n- Mx A

electrolyte

Fig.21. Photoinsertion of cations in an electronic energy band scheme,

h~

f

M+

E=

M+

E,

M+

M+

E~ // E, Z

n- MxA

electrolyte

Fig.22. Photodeinsertion of cations represented in an electronic energy band s c h e m e .

The reverse reaction of the photoinsertion of cations (61) is the photodeinsertion of cations at an n-type MxA ,shown in Fig.22 :

M , A + y h ~h,,---* M, u A - yM"

(62)

Energy Conversion and Storage where h ~

239

are the photogenerated holes. The holes oxidise M within the semiconductor to M + which are

subsequently released into the electrolyte. This corresponds to breaking a bond between the guest species and the host lattice. The reaction can proceed via holes excited in the valence band or via an excitation of electrons from states EM in the energy gap. Because of the position of the Fermi- level in the n-type M~A these states in the energy gap are occupied in the dark.

cation

n-type p-type photodeinsertion photoinsertion

amon

photoinsertion

photodeinsertion

Table lI: Classes of topotactic photoreactions

If we take into consideration that anions may be exchanged as well as cations, four different types of light-induced ion transfer reactions are possible which are summarized in Tab.II. Thus, for example, a photogenerated electron in a p-type insertion electrode may reduce a cation from the electrolyte or oxidize an anion within the electrode. 6.4. Photon Energy Conversion and Storage In the last section it has been shown, how the illumination of an insertable semiconductor /electrolyte junction can generate a photovoltage and an ionic photocurrent. In this section it will be analysed, how the photon energy can be stored and discharged in the dark. We limit the discussion to the case, where cations are the exchanged species. Then, according to Tab.3, there are two different ways of converting and storing photon energy: by photoinsertion or by photodeinsertion reactions.

e-

E=

t-

'"~.,,. %

T

q.V.

E:

M"

M÷ e-

F hv M+ M÷

q.V(M~A)

f ...............

Me

'Vf(x) E, q.~/'(M~A),

L

E:

.-/-._ J - ,

.../

..~_~xx .

M÷ M+

M÷

•~i

I q.EHF

M+

p -I~A

Fig.23. Charging of a photoinsertion cell (PIC) with the energetic relations for electrons in the dark (solid lines) and under illumination (dotled lines).

240

G. Betz and H. Tributsch

In the first case the photoinsertion - cell (PIC), which is shown in Fig.23, consists of a p-type insertion electrode MxA , a M + -conducting electrolyte and a counter-electrode MyB that releases M + -ions at a suitable potential VcE . The counter-electrode can be an alloy of the guest species M which is usually a metal, or it can be another insertion electrode. It should have a metallic conductivity to lower internal resistances in the PIC. In Fig.23 the photocharging reaction is visualized. In order to use the generated photovoltage internally to drive the photoinsertion reaction, the external circuit is short-circuited.

.................

V:!M, AI

>o o

VcE

\

VlM~

v IM=.~

i

x

x'

composition

Fig.24. Charging/discharging cycle of a photoinsertion cell in a composition-voltage diagram.

During the insertion reaction the Fermi-level of the electrode is shifted towards negative potential values. This is shown in Fig.24, where a charging/discharging cycle is drawn into a composition-voltage diagram. For the sake of simplicity it is assumed here that the counter-electrode remains at a constant potential VcE. In general,it will shift towards positive values, because it is deinserted during the charging reaction. The charging reaction will stop at a composition x', when the potential of the illuminated electrode equals VcE

V'(x') =

VcE

(63)

When the light is now switched off, the potential of the semiconductor electrode is reduced by the amount of the photovoltage (see Fig.

24). Thus,

the maximum obtainable EMF of a PIC is the photovoltage Vph(X')

obtained at the final composition x' of the semiconductor electrode. Thus the condition for an optimal performance is that the change of the cell voltage due to the stoichiometry variation of the electrodes is in the range of the obtained photovoltage. If it is smaller, the photon energy cannot be converted efficiently into electrochemical energy but dissipates mostly. If the cell voltage in the dark is too large, the photovoltage is not sufficient to drive the charging reaction. Additionally, the cell voltage should not vary too drastically with composition otherwise only a small amount of energy can be converted and stored. These conditions determine the selection of suitable semiconductors and inserted species.

Energy Conversion and Storage

241

Dischorging

I

(D

¢z tO

e_

Ec

T qV=

M+

M+

0

M+

m

o

~Ef

M+

I_ . . . . . . . . . . . . . . . . . . . . . . . . . .

M + M +

EMF.q

M +

=M +

M+

z

p- M,A

semiconductor

electrolyte

counterelectrode

Fig.25. Discharge of a PIC

The cell of Fig.23 can now be discharged in the dark transferring the guest ions through the electrolyte back inlo the counter-electrode while the electrons are performing work in the external circuit (see Fig. 25). The energy efficiency, )cPic, of a PIC can be defined as the relation between the electric energy, We~, taken out of the cell during discharge and the incident photon energy during the charging reaction:

w°,

X P W = P, TI

(64)

where W~l is the electrical energy extracted from the cell at an external load ( l ~ 0 ) and Pl the incident photon power (assumed to be constant) and T~ the charging time. As in the case of a conventional insertion electrode, the ideal composition-voltage curve of a pbotoinsertion electrode is typically that of a two-phase system with a plateau both in the dark and during illumination. To obtain a large energy capacity that can be converted and stored in a cycle, it is also necessary that the semlconductlng properties are maintained over a wide range x of inserted guest species (compare Fig.7). To fulfillthis condition most of the inserted electrons have to be placed into localized orbitals, where they do not contribute to a shift of the Fermi-level relative to the energy bands. These levels can be created by the inserted species as it is observed at HxTiO2(B) ~I. The maximal shift should be just as high as the obtained photovoltage and should be caused by a large amount of inserted atoms.

242

G. Betz and H. Tributsch

E

variation of

t

0

EO:

the doping

E,

E; f

Ef

electron energy

E,,

a}

I

Z

EOI

E°

Ef .......

E,

b)

_•

the band I edges

E

E, ...... ,,,-~,~-,-~....... "'~I

0¸

\

"........

I

z

z

x

E 0

EO:

t

c)

V(x}

the band bending

E, Z

z

due to insertion

Fig.26. Variations of the electron energy distribution in the interface due to an insertion reaction. On the left these variations are visualized in a three-dimensional energy band diagram with the composition x as the third axis.

In Fig.26 the three extreme cases are depicted, how an electrode potential change can alter the interracial situation. An increase of the chemical potential of the electrons corresponds to a change of the location of the Fermi-level relatively to the band edges. In other words in this case the doping and the conductivity of the electrode will be changed. An increase in the chemical potential of the inserted ions leads to an increase of the potential difference present in the semiconductor/electrolyte interface (eq.(51)).

This additional

potential difference can change the band bending in the case of an ideal junction or it can lead to a shift of the band edges in the case when the potential drop occurs in the Helmholtz layer. All three variations may in the general case appear in parallel . This can be visualized in a three-dimensional energy band scheme with the composition x as the third axis (insert on the right of Fig.26). It cannot be excluded ,in principle, that the whole energy band structure including the energy gap, E~, is changed during insertion. Up to now such a phenomenon has not been observed at inorganic semiconductors (see below). The energetic relations of a cell that is based on a photodeinsertion reaction is shown in Fig.2? 1a9. Upon illumintion of the semiconductor anode, n - MxA, a negative photopotentia] is generated at the semiconductorelectrolyte interface. When the external circuit is closed, this photopotential will drive a deinsertion of M-atoms at the semiconductor while the M+-ions are reduced at the counter-electrode. This electrode can be a metal M electrode, because the potential of the semiconductor electrode in the dark is always more positive. A photodeinsertion cell is discharged like a conventional insertion battery. An in-situ conversion and storage of photon energy is also possible, when a metallic insertion electrode is incorporated into a regenerative photoelectrochemic~l cell as a passive counter-electrode, which inserts the /

reduced species of ,the redox couple. In such a passive photoinsertion cell a conventional n-type semiconductor electrode is working as the photoanode which drives the insertion reaction at the counter-electrode. As a redox couple M2+/M * the Cu2+/Cu ~ -couple can be used, for example.

Energy Conversion and Storage

= e-

243

Ip~)~..~

Sc

T _

e"

ff-

e_

qVphiX) LEf/

re

~'~'-

.

VCE

;

ohmic back contact

Fig.27.

.

n-

Mx A semiconductor

electrolyte

metat M

M+-ion-conductor

Photocharging of a photodeinsertion cell With an n-type semiconductor anode and a metal M

counterelectrode. The photocharging reactions are the following: semiconductor electrode: y M - ÷ yh~, ~ y M2+

(65)

M=A + y M ~ ~ ye- - , M=+vA

(66)

counter-electrode:

The cell is divided by a semipermeable membrane, which passes only M + -ions to prevent self-discharge of the cell according to the reaction:

y M 2÷ + M=+vA ~ 2 y M ÷ + M~A

(67)

In the dark, the cell is discharged. The discharge reactions are: semiconductor electrode: y M 2+ + ye- ~ y M -

(6s)

M=.vA --~ y M + + ye- + M=A

(69)

counter-electrode: J PSSC 16:4-D

244

G. Betz and H. Tributsch

A working example of an passive photoinsertion cell is reported in x40. The authors used n-GaAs as the photoanode and TiS2 as the insertable counterelectrode. Mono- and divalent copper ions served as the redox couple. As a cation-exchange membrane the author,s used a transparent perfluorosulfonic acid membrane (Dupont,Nafion N-125). They obtained a EMF of 0.4 V and a current density of 0.1 mA/cm 2 , but the n-GaAs is not stable in the presence of this redox couple. Since the Cu + -ions can have high mobilities in insertion electrodes the further development of such a cell will be determined by the search for semiconductor electrodes that can be stabilized in the presence of the copper redox couple. Another problem reported is the physical deterioration of the TiS2 -electrode durihg the intercalation and deintercalation of copper. This may be avoided by the choice of another more stable, insertable counter-electrode with a three-dimensional framework structure.

6.5. Experimental Investigations of Photoinsertion Cells In Tab.I we have already listed the semiconducting insertion materials that have been studied up to now, mostly in our laboratory. Research on photoinsertionrprocessesfirst concentrated on semiconducting layeredtype dichalcogenides. At ZrSe2 -electrodes in a solution of LiCIO4 in acetonitrile a cathodic photocurrent due to the insertion of lithium could be found. But due to the low quality of the crystals used in the experiment, no photon energy storage was possible4~ Rauh et al attempted to build an active photoinse~tion cell based on the photodeintercalation of copper from layered type HfS2 -electrodes ~41. The authors reported semiconducting behaviour of CuxHfS2 up to x = 0.4. But they obtained at an illumination of 80mW/cm 2 only a photovoltage of 30 mV. During discharge in the dark the current density ( 2#A/cm ~ ) was strongly limited by the low diffusion coefficient of copper in HfS2 . At higher discharge current densities copl~er was plated on the surface of the electrode. As another promising material for photoinsertion , the layered - type InSe (Eg = 1.2 eV ) has been investigated 141. With p-type electrodes the photoinsertiorl of copper could be obtained. Upon illumination with a 6.8 mW He-Ne-laser a maximum photovoltage of 400 mV was found at the InSe/Cu + - junction. The short circuit current density had been in the range of 2 to 3 m A / c m ~ . The photoinserted copper was subsequently detected by measuring a SIMS-profile. The copper had migrated to a depth of 55 # beneath the surface. A considerable amount of photoinserted copper could only be found, when the Van der Waals gaps had been exposed to the electrolyte. After 3h pf photointercalation of copper, the authors measured n-conducting domains near the surface of the electrode. When the copper ions had been reduced at a surface parallel to the layers, plating of copper occured. The cell could not work as an energy storing cell, because a copper wire was used as the counter-electrode, the potential of which is too negative to be a suitable counter-electrode for photon energy conversion (see Fig.24). Since upon intercalation many of the layered semicor~ductors showed degradation effects due to a irreversible expanding of the layers, semiconducting materials with three-dimensional framework structures seemed to be more interesting candidates for photoinsertion reactions. Because of the unique properties of copper. as already mentioned,we screened the semiconductirlg copper compounds for a possible application as photoinsertion electrodes. Cu3PS4 turned out to be a reasonably good model system to study the mechanism of photoinsertion . It has a wurtzit-related structure providing open channels for the diffusion of copper atoms 142. The copper diffusion coefficient had been determined to 9.10-5cm2/sec by a method introduced by Rickert and Wiemh6fer 143.133. Yellow brownish crystals were grown by chemical vapour transport using bromine or iodine as transport agents ~44. The energy gap had been determined both by measurement of the photocurrent and the absorption spectrum to be E s = 2.3 eV. The transition is direct. In contact with a Cu* -containing electrolyte additional copper atoms can be photoinserted into the Cu3PS4 lattice. Fig.28 shows the correspondingphotocurrent-voltage curves with a copper wire as the counterelectrode. After having inserted additional copper in the lattice during the first voltage scan (Fig.28a) the photocurrent density and the photovoltage improves remarkably. The additional ions in the electrode surface increase the ionic conductivity and the barrier height by forming a new equilibrium with the electrolyte. The current-voltage behaviour has a hysteresis which is typical for such insertion electrodes.

Energy Conversion and Storage

245

a)

u

50

100

I

I

.>, -1

/ /

u~

-5

"o

V~c200 2

//

¢-

~

300

.==....,,/P

electr0de potential/mVvs Cu+/Cu =

I I

/

/

-10

/

o 0 o

-15 -20

b} 0.5100

200

I

I

300 r

E

o <:

E

/

-0,5

>.,, el c

-1

/

"o

/

/

/

//

/ /

I--

o o ,4--,

I

electrode potential/mVvs Cu÷/Cu o

'7

-2

20

-3

/

/

/

/

/

/

/

/

/ /~h~,

I"ig.28. a) and b)

246

G. Betz and H. Tributsch

/

1.5

/

/

//'-"~

I

/ v.

--: I

=-

I

l~h~'

i

I \

/ 0.5. ,_

c)

i

\

I

100 '

E u ,<

//

E -0.5

/

° ~

co - 1 "o

/

/

/

2~o

""300 '

electrode potential I mV vs Cu~/Cu°

/

/

/

P -1.5 u

/

o ~-2

//

/~hp

-2.5

/ -3

/

/

/1

// .t//

Fig.28. Typical (photo)current-voltage curves of CusPS4-electrodes in a 0.01 M CuCI solution in CH3CN a) recorded immediately after immersing the electrode in the electrolyte b) scan in direction to negative values of the electrode potential c) reversed scan . A copper wire served as the counter-electrode. The electrode was illuminated at an intensity of 300mW/cm2(Xenon-lamp).

When the scan direction of the potential is reversed, the reduced copper ions are oxidized again giving an anodic discharge current. This discharge current is increased under illumination due to a photoconductivity effect.

At potential values where the deinsertlon of ions is thermodynamically possibly the delnsertion

reaction cannot proceed because of the high resistance formed by the depletion layer. When electronic charge carriers are generated this resistance will decrease. Upon electron excitation from levels EM corresponding to copper atoms diffusing through the space charge region the concentration of mobile ions will also increase leading to a further decrease of the resistance. A photoinsertion current spectrum is shown in Fig.29. ]t equals both the photocurrent spectrum measured with a non-insertable redox couple in the electrolyte and the absorption spectrum.

Energy Conversion and Storage

247

photo0.2 current quantum efficiency 0.1

2.0

1.7

2,3 Eo

2.6

2.9 3,0 photon energy/eV

Fig.2g. Photoinsertion current spectrum of a Cu,~PS4-e ectrode at equilibrium (no diffusion of previously inserted copper atoms, compare Fig.59).

with light

Q

0

÷~I00

Cu 3.a, PS

>

E

Vph c

~ 50 o

2

in the dark ~ {

u

Ax =I I

100

~ A x =2 I.

Ax=3 i

compositionchange Ax charge/m C

-50

-100-

Fig.30: Composition voltage diagram of CuxPS4 in the dark and under illumination ~4r

248

G. Betz and H. Tributsch

To study the possibility of photon energy conversion a composition-voltage diagram has been determined in the dark and under illumination. The result is shown in Fig.31. When inserted up to x=6, the maximum photovoltage of 170 mV is reached. The photovoltage decreases, when copper atoms are then removed. This phenomenon which is also observed with other photoinsertion electrodes may be explained using equation (51), from which it can be derived that an increased activity of the ions in the electrode increases the interfacial potential difference. The negative potential values referring to C u + / C u ° can be explained on the base of an oversaturation of copper within the CuxPS4 -electrode.

This composition-voltage curve

demonstrates that a photon energy converting and storing cell can be realized with such a light-induced insertion mechanism provided the counter-electrode is suitably chosen. The first energy converting PIC was built by the authors with a Cu2S insertion electrode as the counter-electrode z47 etectrode pofentio[ V/mY vs Cu'/Cu"

120 110

tight

100 90

%%% % %% X

eO 70, 60

50 4O, 30"

1'2 I~

I'~

I"5 I~

I"/

I~

1'9

-10

photocharge / m Ccm "z

-2O, -30

x

-40"

\

dark

-SO. -60-70. -II0. -90-100.

- 111O-120

EMF

~/mV

120 110 100

~ht on

off

Cu x PS, ICuCI I Cu2.yS

9O 80 70

~ . . . ~

1

60 S0 4O 30 2O

charging photocurrent density ~ A c m -2

cyc(e 1

Vph

10

.' ..... ,o,

1001 " : :

IL = 117mWlcm z

60 50 t*0

w i t h light

lOmin ~min 3'Omin ~ i g

30 20 t~°--= • °

~min" th

~h

¢

;;h':' : 1,o, '

,

i q i

-20

10 20S

30S

L0S

S0t

60s

70t

80S

time --10 --20

-60

......

voltage

- -

current

"-]0

in the d a r k -100

Fig.3]. a) and b)

discharging current density pAcm-~

--40

Energy Conversion and Storage

249

charging

cycte 2 "Jr uJ 100

b)

~" "--dightoff

photocurrent

density/ ~Acm "z

•

,Ii

110

"100 • 90

a0

• 80

i i J i

"70

6O

"60

V~

50 60 ¸

50

: hv 30

charging

/0.

-

20

- 10

~s

% ~s;"

20,

"

3~05

500|

t

I

10, 201 301 &0l 50s u "chorge

"-__

2001. ~ time

?,35'

01 02 (13

30-

It

/,0,

flh

50.

(15

Fig.31. a) Photocharging curve of a Cu~PS4/CuCI/CuyS-PIC and b) two chargingdischarging cycles of that cell with the photocurrent density, the discharge current density and the cell potential.

Fig.31 shows a charging curve (Fig.31a) and a charging/discharging cycle of such a cell with the EMF,the photopotential and the current densities. To avoid plating of copper in these long time experiments the light intensity was chosen such,that the charging current density was not. larger than 50 # A / c m 2 . Yet during short periods current densities of several m A / c m 2 could be obtained without plating of copper (see Fig.28). The efficiency (ca 0,5%) of these cells was mostly limited by the small discharge current density due to large internal losses in the cell. These internal losses are partly due to the non-ideal electrode material such as a too low p-type electronic conductivity and non-ideal ohmic back contacts. But there are additional losses which will be discussed in the next section.

6.6. Efficiency Considerations Since photon energy conversion with photoinsertion reactions includes energy storage, the photoelectric material parameters relevant for photon energy conversion as well as the parameters relevant for energy storage have to be optimized simultaneously. This is much more difficult than to optimize an energy converting and an energy storing material separately, because both tasks may result in contradictory conditions. Thus, for example a high degree of structural disorder is suitable to reach high ionic mobilities, but may generate a large density of states in the energy gap, which reduces the photovoltage. It will be discussed in the following section, how these contradictory conditions may be resolved to improve the low efficiency of the PIC investigated to date. At first the photo-charging reaction will be considered. It is evident that the chemical diffusion coefficient of the guesl species is the rate determining quantity for that reaction.

In the ideal case the reduced

ions have to be transported away from the interface as fast as new minority carriers reach this region. Although it is not correct to calculate an ionic mobility from the chemical diffusion coefficient, we will do it just for an estimation in case of CusPS4

and compare it to the hole mobility of this electrode

material. This "ionic mobility" calculated via the Einstein-relation from the measured diffusion coefficient is 5-10-3cm2/Vsec , whereas the hole mobility is 30 cm2/Vsec 13z. The chemical diffusion coefficient can be improved by either increasing the self-diffusion coefficienl Dki or the enhancement factor W. Dki is essentially determined by structural properties of the lattice incorporating the mobile ions. The enhancemenl

250

G. Betz and H. Tributsch

factor W can be influenced by the electronic properties of the electrode material 145 It describes the interaction of mobile electronic and ionic species during the diffusion process via an internal electrical field. In semiconductors,where internal fields are not screened by a large density of charge carriers, W can reach values of up to 104 14~. For this reason semiconductor materials with high electronic mobilities should be preferentially selected as electrode materials. High mobilities of electronic charge carriers will in addition reduce internal electronic resistances without influencing too much the position of the Fermi-level relative to the band edges. The photocharging reaction is further complicated by the influence of the incident light on the diffusion of the ions within the semiconductor electrode. As can be seen from the current-voltage curves (Fig.28) light facilitates the transfer of ions from the electrolyte into the semiconductor electrode. The ions that are inserted in the electrode are driven both by the concentration gradient and the electric field into the bulk of the electrode. But, as the separation of charge carriers reduces the electric field in the space charge region (reduced band bending), the electric part of the driving force for the ionic motion is reduced under illumination. In other words photon-generated electrons attempt to pin the ions in the electrode near the surface. The distribution of reduced species is determined by the quasi Fermi-level varying with the distance z. Furthermore, the inserted positively charged ions may invert the direction of the electrical field in the space charge region (see section 10.2). Therefore, the highest photocurrent densities with photoinsertion systems such as the Cu/CusPS~ -system had been found, when the incident light was periodically intermitted with a chopper. So the inserted atoms were allowed to redistribute in the dark to empty insertion sites in the interracial region. These experiments indicate that oscillatory photoinsertion reactions such as occuring in ion pumps might be more efficient than steady state reactions. As already outlined, these oscillatory reactions have to proceed far away from thermodynamical equilibrium. The necessary influx of energy is in the case of photoinsertion recations provided by the light.

h~

Cu ÷

P

microheterogenous semiconductor

Cu÷

electrolyte

Fig.32: (Photo)insertion reaction at the interface o f a microheterogenous semiconductor electrode.

The current density, corresponding to the transfer of ions through the electrode'/electrolyte interface, can be lowered by increasing the effective surface area. Instead of a plane electrode surface, where the area available for the ion transfer is identical to the geometrical area exposed to light, highly structured electrode surfaces should be used, where part of the ion transfer can be carried out on the shaded parts of the electrode

Energy Conversion and Storage

251

surface. This would be also advantageous for obtaining higher discharge currents in the dark. But the discharge currents are further limited by a principle difficulty: positive ions coming from the interior of the electrode have to do work against the electrical field in the space charge region. This electrical field which is needed for charge separation is a barrier for the ions as well as for the holes. This difficulty can only be overcome, when the guest species diffuses as a neutral atom through the space charge region and being oxidized immediately at the electrode surface. Or the electrode surface has to be split heterogenously into normal semiconducting regions and degenerated regions across which the ions can be transferred into the electrolyte (Fig.32). In the degenerated regions no space charge layer is formed, so that they do not produce large internal resistances. The problem can be generalized for all devices that are in- situ converting and storing photon energy. To avoid recombination energy storage is necessary in an intermediate or permanent state. Charge separation is the more effective the bigger the energy difference between this state and the excited state is. But on the other hand, this energy difference will reduce the amount of energy stored. Charging and discharging reactions can also be separated, if one uses membrane structures which are discussed in the next section.

7. ION PUMPS WITH'PHOTOSENSITIVE MEMBRANES

Active ion transport reactions through biological and artificial membranes can be considered as topotactic insertion /deinsertion reactions, since many characteristic structural parameters of the membrane remain intact during such an ion transfer process. The structural and dynamic details of membrane processes are usually very complicated.

However . the thermodynamic considerations on insertion / deinsertion

mechanisms apply, and energetic schemes as they were discussed for electrode materials can at least be an intuitive help to imagine the interaction between electronic excitations and ion transport.

7.1. The Light - Driven Proton-Pump of the Halobacteria Since Oesterhelt and Stockenius showed that the purple membrane of the Halobacterium halobium acts as a light-driven proton pump, m a n y details of this biological energy conversion mechanism have been discovered (for a review see e.g. reference (61)). For the lack of space, we can only roughly outline the basic features of this light-induced proton-pumping. The purple m e m b r a n e contains bacteriorhodopsin as the photoactive molecule.

In this molecule retinal

(vitamin A-aldehyd) as the chromophor group is linked to a protein (lysine) via a Schiff base.

Upon

illumination the bacteriorhodopsin undergoes a cyclic reaction in which the Schiff base is protonated and deprotonated, resulting in a proton transport through the membrane.

During the cycle the 13-cis retinal

is photoisomerized to the all-trans retinal. As already mentioned the bacteriorhodopsin is bleached and unbleached in one photocycle.

The absorption peak is shifted from 412 n m to 570 nm. U p to now it is

not completely clear, how these structural changes are involved in the translocation of protons through the membrane. Free energy is partly stored in the proton gradient between the inner and the outer side of the cell membrane that is established during illumination (additional energy storage occurs in the generated potential difference). In a dark reaction of the cell membrane the energy stored in the proton gradient is transformed into ATP by an inflow of protons into the cell. The dark- and photoprocesses at the halobacterium halobium are schematically outlined in Fig.33. The proton gradient can for example used to pull sodium and chloride ions through the cell membrane to balance the ion concentration in the interior of the cell. A proton gradient may also be generated in the dark at the expense of energy, which is provided by the consumption of ATP.

252

G. Betz and H. Tributsch

-.

Purplemembrane~

ADP*PI ATP ~

H~

0

B rn

B_ ~ Na÷

Q

8 g

Q

Fig.33.Solar energy conversion by the proton pumping purple membrane of Halobacterium halobium 146

7.2. Artificial Light - Driven Ion Pumps Several attempts have been made to use directly the light-driven proton pump of the Halobacteria for the conversion of solar energy . In all these experiments the purple membrane was extracted from the Halobacteria and integrated into an artificial solar collector device. Water desalination 147 and the conversion of solar into electrical energy 14~ and chemical energy (hydrogen) 149 were the proposed applications. But these semi - biological systems had limited success 150 Murphy et al reported a cell, where a photoelectric converter (n-CdSe with a sulfide/polysulfide solution as a redox couple) was used to drive H + - and OH- -currents through a bipolar membrane. Thus, the converted photon energy was stored producing acid and base 151. The energy could be taken out of the system letting the acid and the base react back to give a neutral salt solution. Another approach is to synthesize molecules that are themselves capable of performing light-induced transport of ions. Shinkai et al. demonstrated, that photoresponsive crown ethers can accelerate the transport of alkali ions across a liquid membrane upon illumination 25s. The macrocyclic polyether photoisomerise to a structure, where the positively charged ions are enclosed between the oxygen atoms of two macrocycles. At the dark side, the two cycles open the enclosure by thermal isomerization. The very stable complexes, which may rapidly extract ions into the membrane phase, cannot release the ion efficiently on the other side of the membrane 154,155. This is in parallel to the kinetic difficulties that occur in PIC, when the charging and the discharging reactions are carried out at the same homogeneous interface. A carrier mediated photodiffusion membrane was also coupled to a fuel cell to convert and store solar energy 156 Photoinsertion reactions with semiconducting membrane-like structures provide another approach to artificial light driven ion pumps. Some kinetic difficulties to be expected with such combined electronic/ionic systems have been discussed in chapter 6.6. Due to the feed-back of the insertion product on the electronic-

Energy Conversion and Storage

253

ionic insertion mechanism, the energetically most favourable mechanism would be a periodic one, such as it can be found in the purple membrane of Halobacteria. In artificial systems a cyclic ion pumping reaction is possible, since most materials show reversible changes of the optoelectronic properties during the transport of the guest ions.

¢1z

M÷

¢11

M

=

M÷

Fig.34: Schematlcal representation of light-induced ion pumping through a photosensitive membrane with light-induced topotactical redox reactions. The different electric fields at the interfaces on the right and on the left side are assumed to be due to different ion activities al,a~ in the electrolyte.

Fig.34 shows such an membrane, where the ion transport is facilitated by a photoexcitation inducing a successive insertion/deinsertion reaction on the two different sides of the membrane . Since the basic features of photoinsertion reactions have been outlined in the previous chapters, we will focus the attention on one of the main problems that have to be faced, when developing such membrane structures. According to the Curie principle asymmetric or vectorial processes cannot occur without pre-existing asymmetry. There are.several ways of introducing an asymmetry into a membrane.A potential difference can be built in (Donnan potential), which generates an electric field within the membrane. The question is, how to generate such an electric field. An asymmetric situation can be caused by different interfacial ionic equilibria ( for a semiconducting membrane this would result in different energy band bending} . Then, as shown in Fig.34 ,the absorption of light will induce the photoinsertion and , subsequently the deinsertion reaction ( or vice versa ). The band model used in Fig.34 is not a correct model for the description of a nearly two-dimensional molecular array such as a membrane, but it helps to visualize the photoprocesses and can give some ideas on how to find a suitable material. It will be the more correct and applicable, the thicker the membrane is and the more semiconducting character it has. According to equation (51) a different interfacial potential drop might be only achieved in a homogenous material with a constant ion activity a, . when there are different ion concentrations present on both sides of the membrane. In case that this mechanism is valid, one needs a small ion gradient across the membrane to start the PumPing Process. which had been evaporated on a conducting glass substrate (]TO-glass). This electrode arrangement was put into an electrochemical cell with an aqueous electrolyte. At a potential value, where the photocurrent and dark current had opposite signs (cathodic photocurrent, anodic dark current}, the WO3 -layer turned blue upon illumination and bleached in the dark. The blue colour of the WOs -layer is due to hydrogen insertion (electrochromlc effect}. Since the tungsten trioxide is an n-type semiconductor yielding only anodic

254

G. Betz and H. Tributsch

photocurrents, it could be concluded that the blue color is caused only by a photoinsertion of hydrogen into the polymer layer and subsequent transfer into the tungsten trioxide. This gradient increases then during the ion pumping process. The membrane material which should have a high ionic conductivity, must be electronically an intrinsic material, so that both types of band bending can occur simultaneously. Upon illumination the membrane behaves like a photoconductor, separating electron and holes and transferring them to the different surfaces. The electron triggers an insertion reaction and the hole a deinsertion reaction. The electron density of the diffusing guest species can be localized around the guest ion.

RE

..e-

l

i

Sc

~e-

e• ..-

,'.:

I

7

M÷ '.'."

::?

electrolyte

Fig.35.

-,,-~-

BP

electrolyte

Photon energy converting and storing cell with a light-driven ion pumping membrane M. BP

represents a by-pass valve that is closed during the charging of the cell (ion pump is active) and is opened during discharge of cell.

An energy converter with a light-driven ion pump M is shown in Fig.35. Upon illumination of the membrane M the generated photovoltage drives ions M ÷ fom the left half-cell into the right. Energy storage is performed at the anode A and at the cathode C, which are insertion electrodes. A by-pass BP bridges during discharge the large internal resistance of the membrane in the dark. This by-pass can in practice be a valve which connects the electolytes in both cells. In the field of solar energy conversion the development of membranes capable of light-induced proton pumping especially aim at solar sea water desalination involving no production of electricity. Such a photoelectrodialytic system is schematically shown in Fig.36. It consists of the proton - pumping membrane Lp and of suitable anion-selective (A) and cation-selective (C) membranes, which are already used for technical sea water desalination. Artificial membranes exchanging protons with other cations and anions are being developed 15r

Energy Conversion and Storage

NoOH Fig.36.

H=O

255

HCI

Photoelectrodialytic system for solar sea water desalination with photoactive proton pumping

membranes.

7.3. Investigations of Materials for Photosensitive Membranes Our investigations concentrated on the development of proton conducting semiconductors for light-driven proton pumps. To learn about the mechanisms, we first studied oxide electrodes, although it is clear that they are not easily fabricable into suitable flexible thin film membrane structures.

u

200 ~t

e-

..

100

eQ; b..

u o

o

0

-1.5 -1.0 -0.5 0 etectrode potential / V vs Hg2SO4

Fig.37. Photocurrent-voltage curve of HxTiO2(B) in aqueous electrolyte (0.1 M K2SO4 as the conducting salt).

In "l'iO~(B) hydrogen atoms can be electrochemically inserted 121. Its perovskite type structure, which can be set up by units of the ReOs -type, contains cubooctahedral vacancies providing channels for the diffusion of hydrogen. The n-type material has an energy gap of 3.0 eV. With polycrystalline electrodes

256

G. Betz and H. Tributsch

a photodeinsertion current of protons could be found after the electrochemical insertion of hydrogen. A photocurrent-voltage curve obtained in 1 N H2SO4 as an electrolyte that only conducts protons is shown in Fig.37.

u~ 1.0 ¢.=_-

P •-~ 0 . 5 E

,

-4'

tO

~

-1.3V

,

,

+05V ~

0"

0

,

350

"

/,00

:

7

h50 500 550 wavelength I nm

r-

600

Fig.38. Photocurrent spectra of HxTiO~(B) in aqueous electrolyte equilibrated at different electrode potentials (measured against a Hg2SO4-electrode as the reference). Each spectrum is individually normalized 121

There is a large hysteresis due to the shift of the Fermi-level after the insertion and deinsertion of hydrogen. This is also reflected by the spectral distribution of the photocurrent measured at differnt equilibrium potentials as depicted in Fig.38. At an electrode potential of + 0.5V vs Hg2SO4 all hydrogen is removed from the electrode. The photoprocess corresponding to the anodic photocurrent at this potential is the oxidation of O H - -

ions (pH of the electrolyte in this experiment was 5). During electrochemical insertion

of hydrogen new occupied states are created within up to 0.6 eV above the valence band. Upon illumination electrons are excited from these states releasing protons into the electrolyte. This gives rise to a photocurrent response in the visible range. At an electrode potential of 0.5 V, when no hydrogen is inserted into the electrode, the electronic excitation was possible only with light of an energy hv >Eg = 3 eV (=413 nm). The spectra shown in Fig.38 are normalized setting the maximum quantum efficiency for each spectrum as unit. At 0.5 V the maximum quantum efficiency was 1.4-10 -3 ,whereas it was 3. l0 -2 at -1.5 V 121 As amore promising class of materials organic polymers are now under investigation. They combine semiconducting properties with the possibility of being inserted with relatively high insertion current densities. Among the wide class of conducting polymers only those can seriously be taken into account that are stable against oxidation in air and stable in aqueous solution. To reach these conditions it seemed reasonable for us to search for conducting polymers, that contain already oxygen atoms. In order to make possible the release or uptake of protons under illumination, the polymer units have to change their basicity, when excited by light. From photochemistry it is well-known that at molecules A with heterocycles the protonation equilibrium in the dark:

A + H +~

AH +

(70)

(AH+)"

(71)

and the equilibrium under illumination :

A" + H + ~

has different pK - values (asterisk means the excited state) 157.

Energy Conversion and Storage

L\o/

257

C

H-C--

\o /

A

\H

-n

_/ \

/--\

\o/

\o/

Fig.39. Poly(2,5-furylenvinylene) with the two possible ground state configurations, which are not degenerate due to the loss of aromaticity in the left case.

Polymers derived from the furane ring meet the two conditions. Poly(2,5-furylenvinylene) can be obtained by a catalytic aldol condensation reaction from 5-methylfurane-carbaldehyd (methylfurfural) or by a thermal condensation of methylfurfural lsg. The structure of that polymer in the two possible ground states is shown in Fig.39. The polymer has the advantage of being soluble in organic solvents~ like N,N-dimethylformamid or N,Ndimethylacetamid, with chain lenghts of up to 10 monomeric units. Thus, electrodes can be easily fabricated either by coating the back contact metal with the polymer containing solution or by thermally polymerizing a film on to the metallic back contact. The undoped poly(2,5-furylenvinylene) has an electronic conductivity in the order of 10-S(ohm cm) -~ . If it is doped with iodine, conductivities of 20 (ohmcm) -1 have been obtained. The activation energy for the electronic conductivity in the undoped polymer is 0.41 eV. This high activation energy might be due to the fact that, unlike in polyacetylene, the charge carriers are not solitons, but polarons or bipolarons, because the ground state is not energetically degenerate as it is in polyacetylene. This is a consequence of the loss of the aromaticity as shown in Fig.39. For such a system one expects an energy gap of 2.0 ev, which is the theoretical limit for an r - 7r" - transition of a polyene with n ---* c~ carbon atoms 2s. However, the energy gain due to the aromatlcity is the lowest compared to all other heterocyclic systems such as polythiophene. So the question of which kind the charge carriers in this conducting polymer are, remains open. Fig.40 shows the absorption spectrum of a polyfurylenvinylene film yielding an energy gap of 1.6 eV lS~ However. there is also a considerable optical absorption found below 1.0 eV.

258

G. Betz and H. Tributsch

o¢. lOSlcrn"~ H

0.5

!

t

I

0.5

1.0

1.5

t

I

I

I

2.0

2.5

3,0

photon energy

Fig.40. Absorption spectrum of a thermally polymerized poly(2,5 - furylenvinylene).

The photoelectrochemical experiments have been concentrated on the light induced proton insertion from aqueous electrolytes. Electrodes were obtained by thermally polymerizing polyfurylenvinylene onto a brass electrode holder coated with a 50 nm gold layer. Fig.41 shows current-voltage curves with different conducting salts to prove that the photoelectrochemical behaviour is independent of the species dissolved in the electrolyte but only determined by the proton or the OH- -ions of the solvent water. When we speak of the uptake of protons in this section,we want also to include the possibility of a release of OH- -ions without mentioning it. Which of the reaction actually takes place could not be determined. Yet experiments, where polyfurylenvinylene had been polymerized on Nation foils that only conduct protons indicate that indeed protons are the exchanged species. At a light intensity of 250 m W / c m 2 an open circuit photovoltage of up to 200 mV was reached. The current-voltage characteristics is that of a Schottky barrier, but here not electrons but protons are the exchanged species. To prove this further a polyfurylenvinylene layer was polymerized onto a WO 3 -layer, which had been evaporated on a conducting glads substrate (ITO-glass). This electrode arrangement was The equilibrium at the polyfurylenvinylene/aqueous electrolyte interface is determined by the pH-value of the electrolyte as it is expected, when protons are the exchanged species (Fig.42). The maximum open circuit photovoltage, Vph , increases with increasing pH-value, because according to equation (51) a decrease of the concentration of protons in the electrolyte causes an outflow of protons from the polymer electrode. This outflow of protons is compensated by an increased potential drop across the interface yielding a higher photovoltage 1~1 The photocurrent spectra are very dependent on the electrode preparation and on the applied potential. Fig.43a) shows a photocurrent spectrum of a polymer film prepared with a catalyst (KOH) for the aldol condensation and one prepared only by thermal polymerization. When charged with hydrogen at 0.3 V vs SCE a cathodic photocurrens is observed. This turns over to an anodic discharge photocurrent at 0.5 V vs SCE (Fig.43b). The small cathodic contribution to the photocurrent spectrum is an effect of diffusion and the absorption length of the light, in this spectral region. This effect which also occurs at other insertion electrodes, will be explained in detail in chapter 10.

Energy Conversion and Storage

TBAPC(ges.)

/ /z

/,//

_-~'.~/-" ~"So

t-

.--J

//,

~...,,"

hv

"E

ss

.,.s

s S

,~..''"

D U

f

s"*

/

/"

Inv .......-'"

259

IL= 250 m W l c m

z

-u_T

10,1mAcm "2

// I KOH pH=10 //,

I

//t

hv /-',/ h~ ." /"/"/"/'"'ff'"'/"

0,1mAcm -z

;./..'" pH=I HzSO~

I

i

i

i

I

-800 -700 -600 -500 -400 -300 -200

Fig.41.

-100 ' 100 200 electrode potential/mVvs SCE

(Photo)current-voltage curves of polyfurylenvinylene electrodes in different aqeous electrolytes

(thickness of the polymer layer 2-5#m).

JPSSC 16:4-E

260

G. Betz and H. Tributsch

,;,o //

electrode potentiat V0c

>

E

\

-200

E

o

-100

"6

150

i +o~

2 o -2ffl-

0

+

+

0 e" Q.

/

0

/

100

o /~

PhotovoltageYpb

/

-:t;10,

/ .50

7/

-/~00.

/ /o -500

/

62 ~V/pH

I

I

I

I

I

I

1

2

3

~

5

6

I 7

I 8

I 9

I 10

I ~

I 12

I 13

1¢

pH Fig.42:

pH-dependence of the electrode potential and photopotential of a poly(2,5-furylenvinylene)-

electrode. 160

Doping with iodine introduces defects from where electrons can be excited to insert protons from the electrolyte. Upon hydrogen insertion these states vanish. M e m b r a n e cells for the light induced proton t r a n s p o r t made of polyfurylenvinylene are under investigation. Since the polymer tends to crystallize self-sustaining dense films are not easy to fabricate. Several a t t e m p t s are now undertaken in our laboratory to overcome these problems. We polymerized polyfurylenvinylene on teflon nets and on commercially available Nation-foils. With the layers on teflon nets photovoltages up to g00 m V were obtained, but due to the large thickness of the membranes a current densities of only several n A / c m 2 could be observed. Nation as the supporting material has the advantage that it allows to form thin films of polymer on its surface. Additionally, it acts as separator, which only transfers protons. The current densities obtained with such membranes were in the order of # A / c m ~ 1G1

8. INSERTABLE T H E R M A L E N E R G Y C O N V E R T E R S

8.1. Heat Transformation and Storage In the previous sections photon energy converting systems were considered, where the guest species is inserted from the electrolyte. These systems were all q u a n t u m converters. When the host material exchanges guest species with the gas phase, i.e absorbs or desorbs guest atoms, thermal energy can be converted or

Energy Conversion and Storage

261

u'l .1 t-

poiymerizedwith catatyst

a,$ ¢,,_

(D

"G

0.5.

=

I

~

2.5

3.0

o_merizedty

oJ

E rto .-1 E3r"

1.35 1.5

2.0

photonenergy h~/eV quantum efficiency cath. current /

photo

ret. units

-0.2y ~

~k

0.5.

2

hv/eV

0.5

quantum efficiency ¢ anod. current / reL units

Fig.43. (a) Photoinserlion current spectra of a polyfurylenvinylene electrode at equilibrium (b) photocurrent spectra at different applied electrode potentials. stored. First, we will consider chemical heat p u m p s with solid absorbers, where heat is transformed between different t e m p e r a t u r e levels. Secondly, it will be discussed how these a b s o r p t i o n / d e s o r p t i o n reactions performed with electrically insulating materials can be applied to conducting absorbers to perform the conversion of thermal energy into electricity. T h e r e are different modes to operate a chemical heat transforming system consisting of the absorber, an e v a p o r a t o r and a condenser, as shown in Fig.44 : a) as a heat storage system, b) as an heat p u m p , c) as a heat transformer. A technologically promising example of an absorber system is the zeolite/H20 -system 51c,. where heat is stored as the heat of absorption of water within the cages of the zeolite framework. In the storage mode heat has to be put into the absorber by desorbing for example water from the zeolite. The heat should be released from it ideally at the same temperature. In the heat p u m p mode water is evaporated in the evaporator at a t e m p e r a t u r e , T o , where it takes up the heat of evaporation ,Q~ , from the surrounding, which is cooled this way.

262

G. Betz and H. Tributsch

temperature

1

T

Qz -

Q_

Q

charge

discharge

T

T2

_

T,

-

%-

-

T2

Q;

a;_

a;

a;-

Q2

Tl

To

TO

-Q,

I heat

heat

storage

Fig.44.

Operation

modes

of a heat

T, -1OO“C

pump

heat

transforming

t

chemical

absorber

transformer

system.

a;

condenser

watertank l

zeolite

Fig.45:

Chemical

consists

of cages,

heat

wherein

Due to the comparatively wat.er that with cages

pump

with

the water

large

has to be evaporated

any other

technical

(see Fig.45)

working

a zeolite

(faujasite)

molecules

fluid.

leads to an reduction

The framework

structure

of this zeolite

are absorbed.

heat of evaporation to convert

absorber.

absorber

of water

and store

Adsorption of water

(2500 kJ,‘kg)

a certain OF water

vapor

amount molecules

pressure.

the amount

of thermal

of the working

energy

at the inner

Q is smaller

surfaces

fluid than

of the zeolite

263

Energy Conversion and Storage

zeolite inserted with H20 at different concentrQtions a.

H2(~

Pl

condensation

PO ~ , . ~ m m ,1~

vaporat ior absorption

i

TO

i

T1

T2

-lIT

Fig.46. S c h e m a t i c water v a p o u r p r e s s u r e d i a g r a m (In p = f(-1/T)) of t h e water/zeolite s y s t e m .

T h i s is s h o w n s c h e m a t i c a l l y in a In p - ( - l / T ) - d i a g r a m (Fig.46). Because of t h e reduction of t h e water v a p o u r p r e s s u r e t h e h e a t of a b s o r p t i o n , Q b is released at higher t e m p e r a t u r e s , T] . T e m p e r a t u r e u p g r a d i n g TI - To of u p to 100°C can be achieved. T h e r m a l energy Q2 h a s to be supplied at a high t e m p e r a t u r e T~ to t h e zeolite a b s o r b e r to desorb t h e water. ]n case of a reversible h e a t t r a n s f o r m a t i o n Qi is equal to Q2- W h e n t h e water v a p o u r is c o n d e n s e d , t h e heat of c o n d e n s a t i o n Q~ which is a p p r o x i m a t e l y equal to t h e h e a t of e v a p o r a t i o n , Q0 , is released at a t e m p e r a t u r e T] • In total, Q0 is p u m p e d in one cycle from To to T] • T h e coefficient of power (COP) is therefore:

COP -

Qo + Q~

Q2

(72)

]n t h e h e a t t r a n s f o r m e r m o d e , waste h e a t or heat from a solar collector can be u p g r a d e d u s i n g solely t h e exergy of t h e w a s t e h e a t which h a s to be supplied therefore at t h e m e d i u m t e m p e r a t u r e level T] of t h e w a s t e h e a t Q]

. Part.

is used to desorb water from t h e absorber activating the zeolite. T h e water v a p o u r

is c o n d e n s e d at t h e low t e m p e r a t u r e level To , where the heat of c o n d e n s a t i o n is released as t h e final, useless waste h e a t . T h e energy stored in t h e activated zeolite a b s o r b e r can be discharged, when water is e v a p o r a t e d u s i n g again p a r t of the waste heat Ql to provide t h e h e a t of evaporation. T h e h e a t of a b s o r p t i o n is released at a useful t e m p e r a t u r e Tz • Typical values for t h e t e m p e r a t u r e at which such a zeolite/water heat t r a n s f o r m e r can operate are 80°C for T] and l l 0 ° C for T.~ . Since t h e water v a p o u r p r e s s u r e at a m b i e n t t e m p e r a t u r e s is below a t m o s p h e r i c p r e s s u r e t h e zeolite/water s y s t e m s are u s u a l l y o p e r a t e d in a closed v a c u u m s y s t e m . Yet, t h e s e s y s t e m s m a y o p e r a t e u n d e r a t m o s p h e r i c p r e s s u r e w h e n nitrogen or air is used as a t r a n s p o r t gas into which t h e water is e v a p o r a t e d . For t h e conversion of solar energy also open s y s t e m s have been proposed u s i n g t h e a m b i e n t air as t h e working m e d i u m }6~ T h e efficiency of these s y s t e m s is s t r o n g l y limited by pressure drops in t h e a b s o r b e r requiring additional electrical energy to o p e r a t e fans. Since all t h e s e s y s t e m s with solid a b s o r b e r s are working discontinuosly, they are suitable for applications, where e n e r g y storage h a s to be included, such as for load leveling purposes. A large h e a t of absorption increases t h e energy s t o r a g e capacity, b u t lowers t h e efficiency of energy conversion which can be seen from Eq.(72).

T h e m a x i m u m conversion efficiency ( C O P = 2 ) is o b t a i n e d , w h e n Q2 is equal to Q0. T h i s

264

G. Betz and H. Tributsch

condition is for example fulfilled,when the vapour pressure line of the water over the zeolite in Fig.46 is parallel to the line of pure water. Here we again have to optimize between the parameters responsible for high energy conversion efficencies and those, respbnsible for a high energy storage density. For solar energy conversion the heat of absorption of the zeolites investigated to date is too high to give reasonably good conversion efficiencies in simple flat plate collector systems. Therefore, research has to develop stable materials like the sheet silicates with an optimal heat of absorption ranging between that of silica gel and that of X- and Y-type zeolites (faujasite). Since the water vapour pressure reduction of the zeolite is very large a zeolite/water system can operate with the evaporation temperature below the freezing point of water (down to -30°C). Thus, a new method of solar sea water desalination is proposed which combines the freeze-separation technique with an evaporation/condensation process. The whole system is an open system based on a zeolite/water absorption cooling machine where the working fluid water provides simultaneously the fresh water. In the freeze-separation step ice is formed by the evaporation of water from the sea water. The molten ice is yielding pure water. The heat of absorption can be taken to melt the ice produced before. ]n the desorption step pure water is again obtained as the condensation product. Such an open system works most favourably under ambient pressure using a suitable carrier gas (N~) 163. Metal/hydrogen systems such as LaNis-hydride can be similarly used for the combined heat transformation and storage. In such a single-stage heat pump two metal hydrides are operating each at different temperature and pressure levels for the desorption and absorption reactions respectively. Thus, it is not necessary to liquify the hydrogen (resorber-principle). Prototypes of those metal/hydrogen systems had already been tested for large-scale energy storage in connection with utility load leveling (for a review of those hydrogen metal systems see K.H.J.Buschow and H.H. van Mal in reference (7)). Since the metal hydrides are electrical conductors the conversion and storage of electricity can be realized . As an example LaNis-hydrides have been used as electrodes in a nickel/hydrogen battery 164. The combination of metal/hydrogen systems providing both heat and electricity have also been proposed ~5

8.2. Thermoionic Energy Converters As discussed, conversion of solar thermal and photon energy obeys different thermodynamic limitations. Conversion of thermal energy always involves the Carnot factor and works only,if one of the thermal reservoirs is at sufficiently high temperature.

However, there are additional constraints according to the special

mechanism of thermal energy conversion.

T.AT / T

~

~'~-

J

/

/

/

Ag2,,x S

~

VTh Fig.47. Thermoionic solid state cell with the temperature profile T(z). Vt h i8 the thermal voltage generated due to the temperature difference. AT.

Energy Conversion and Storage

265

The thermoionic power generators are functioning like ordinary thermoelements, but involve instead of purely electronically conducting materials mixed ionically and electronically conducting solids. One contact of such a junction is held at an elevated temperature, the other at ambient temperature. An example of such a thermoionic solid state cell is the cell (see also Fig.46)

/Ag/Agl/Ag~+xS or Ag2_.S/AgI/Ag/ 166 Other thermoionic devices have been operated on the base of intercalation compounds such as Ag.TiS2 ,Li,TiS2 and AgxNiPS3 167 The relation for the generation of thermoionic power can be derived from the phenomenological equations of irreversible thermodynamics (linear range) correlating linearly the fluxes j , of the transported species n with the generalized forces X,,~ . In a thermoionic system only one ionic species Ag + ( n = l ) and electrons (n=2) contribute to the fluxes. Neglecting any coupling between the ions and electrons their fluxes jl,j2 can be calculated. Due to the temperature difference a concentration (activity) gradient is build up across the device leading to a voltage Vth . This is commonly referred to as the Soret-effect. The Seebeck-coefficient ~ , which relates the voltage V across the junction to the temperature change, can be calculated by setting Jl and j~ equal to zero:

d(rh + ~ ) dV - F(~) = re. dT

(73)

Thus,we obtain:

Vth : ~.AT

(74)

where AT is the temperature difference across the junction. The electronic and ionic contributions to the Seebeck-coefficient are very different in magnitude. Typically, due to the much higher heat of transport., the ionic contribution is one order of magnitude larger than the electronic one. For Ag,TiS2

it ranges

depending on x between -400 and -530 # V K -1 compared to between 10 and 70#VK -~ for the electronic species 167 Thermoionic devices are therefore producing considerably higher thermal voltages from a temperature gradient than conventional thermoelements. But their conversion efficencies are strongly limited by the thermal conductivity of the materials. Such thermoionic potentials will also have to be taken into consideration in experiments with illuminated insertion electrodes. However, their magnitude will typically be more than an order of magnitude lower than the generated photovoltages. They will also show a slow time response. In connection with combined electronic and ionic processes at interfaces it is worthwhile to mention that relation (73) can be differentiated with respect to z (the distance from the interface) to yield:

dT

T

d z = S, + S2 + Q, + q~

d(rh + ~2) d~

(7s)

This equation can be interpreted in the following: in the presence of an electrical or chemical gradient a temperature gradient is produced. It is proportional to the absolute temperature and inversely proportional to the partial molar entropies and heats of transport of the participating species (combined electronic and ionic Peltier effect).

8.3. Evaporative Thermoelectric Energy Converters The thermoionic devices discussed so far have inherently a relatively low efficiency of only a few percent, because of the modest activity ratio established. Thus only small voltage outputs are achievable. In addition,

266

G. Betz and H. Tributsch

these devices characteristically involve a temperature gradient across their constituents, which results in an irreversible heat transfer against the temperature gradient. To overcome this difficulty evaporation and condensation reactions were incorporated into the thermoionic cells to separate the cold and the hot regions. This way thermal energy is converted into a pressure difference of the working medium which corresponds to an activity differnce and thus to an EMF (see Eq.(76))

pump

i'/ i

T1 Pl

Na

\\

1,

\

~i ?

H ',

o|

V~I I",

I~

I

~.~.~__... . t / . /

condensation

sodium

solid electrolyte 13-aluminium

/ evaporation h-t

porous electrode Fig.48. Sodium evaporative generator (explanation see text below)

In the early regenerative cells a mobile species is recycled from a molten salt in contact with a liquid metal at T1 to T2 by evaporation and condensation, with a driving force provided by the free energy of solutions16S, 100. The improved generator shown in Fig.48, is based on the evaporation of a guest species (Na) from a solid into the gas phase. As a result(heat of evaporation involved) higher activity ratios could be acchieved combined with smaller losses due to irreversible heat transfer 170-m. A closed sodium circuit is separated into two regions,by a pumping unit and a septum consisting of a solid electrolyte (~ -alumina) and two porous electrodes. A heat reservoir maintains the sodium near the electrolyte membrane at an elevated temperature Tz , while the pump keeps the pressure of the sodium up to P2 • The pressure difference is converted across the Na t conductor into electrical energy. At one porous metal electrode the sodium atoms are oxidised to Na t -ions before they enter the solid electrolyte. While the Na t -ions pass through the electrolyte, work is extracted by transferring the electrons through the external circuit. At the opposite porous electrode they recombine with the sodium ions and neutral sodium atoms are desorbed into the gas phse. At the low temperature T] sodium vapour is condensed as liquid sodium, which is then recycled. In principle this process is an isothermal expansion of sodium from P2 to p~ at the temperature T2 • The only moving part is the pump, which consumes only a small amount of energy, since it compresses a liquid with a small volume. The EMF generated by this thermoelectric device is :

EMF

= ~RT~ -

tnp~ P]

(76)

From this equation it can be seen that the EMF is the bigger the larger the vapour pressure reduction by the absorber material is. This is in analogy to the thermal energy converting absorber systems.

Energy Conversion and Storage

267

N.Weber reports an energy conversion efficiency of 23.9 percent at a power output of 0.5 W per cm 2 of the heated electroyte area 172. This high efficiency is confirmed by calculations of Fluggare and Huggins 173 A similiar cell which also is based on the evaporation of inserted species has been proposed by Lalancette and Roussel 174. They construct a t a n d e m cell consisting of two graphite electrodes intercalated with bromine each in contact on one side with a common electrolyte, while the other side is exposed to the gas phase. Heating of one electrode causes the bromine to evaporate. ]t is absorbed at the colder electrode and reduced at the graphite/electrolyte interface to bromide, which is cycled back to the hot electrode, when the external circuit is closed. An improved version of this cell based on bromine intercalation in graphite fibers was constructed by M.Endo et al 17s. The authors report an open circuit thermal voltage of 200 mV p u t t i n g the hot electrode at a temperature of 80°C , while the cold electrode is held at 15°C . These results show t h a t the concept of thermal energy conversion with absorption/desorption processes can be extended to the conversion of thermal into electrical energy. In the next section we will discuss how this concept may be further extended to allow the combined conversion of q u a n t u m and thermal energy into electricity.

8.4 Photon Powered Evaporative Generators T h e possible use of q u a n t u m processes for energy conversion driving desorption reactions of guest species from semiconducting insertion compounds has first been pointed out by one of the authors 1r6 Such a q u a n t u m process would operate without the need of an temperature gradient obeying therefore the efficiency t e m p e r a t u r e relation of the q u a n t u m generator in Fig.9. Additionally, a temperature gradient induced by the supply of thermal energy would facilitate the evaporation process, so t h a t the total system works as an combined q u a n t u m - t h e r m a l generator.

b4

m

"Y////~ a)

b}

~.~

c)

.oVpo

~ h'l

semiconductormetatgas

Fig.49. Photon powered evaporative processes with (a) anions M - in a p-type electrode and (b) cations in an n-type semiconductor. Schottky-type evaporative cell (c) with an insertable front electrode. The transfer of M into t h a t front material is a q u a n t u m process, the evaporation from the front contact a thermal process. The M atoms are reinserted at the back of the semiconductor electrode.

268

G. Betz and H, Tributsch

Photoinduced evaporation can be linked to the process of photodesorption of molecules from illuminated semiconductors such as the photodesorption of oxygen from ZnO 177. But with the photoinduced evaporation of guest species continuous flow and replenishment of the evaporated species is maintained by the diffusion of the guest species from within the bulk of the electrode. One suitable class of materials would be n-type semiconducting insertion compounds, in which the guest species are present within the host lattice as negatively charged ions (see Fig.49a). Upon illumination the negative charge could be trapped by a photoinduced hole at the surface. The guest species is then neutralized and can escape from the surface of the host electrode. The negative charge trapped by an hole correponds in a chemical view to a chemical bond of the guest atom with the host lattice that is broken by a photon. Another possible class of materials are p-type semiconductors inserted with positively charged guest species, which could be photoreduced at the surface and released into the gas phase as neutral atoms (Fig.49b). It is evident that only very mobile guest species can be used in such systems at ambient temperature. Hydrogen atoms are the most attractive one, because they are small enough to achieve high diffusitivity and undergo practically no corrosion reactions with commom materials (neglecting hydrogen embrittlement). Additionally, storage concepts for hydrogen are the most elaborate ones (hydrogen storage in metals or metal alloys). Also neutral guest species could become involved in photoinduced evaporation reactions. In this case an n-type semiconducting host electrode has to be covered by a thin transparent metal layer (porous electrode) through which the guest species can rapidly diffuse (Fig.49c). Photogenerated holes oxidize the guest species within the semiconductor electrode to a positive ion which drifts towards the porous metal electrode. The depletion layer with its low electronic conductivity is assumd to behave as an electrolyte conducting only ions. The electronc leakage currents may be totally avoided by introducing a thin-film separator. At the metal electrode the ion is reduced by an electron from the external circuit and gets inserted into the metal film as a neutral atom. The guest atom is evaporated from the porous electrode surface into the gas phase and reinserted at the semiconductor electrode. The transfer of the guest species from the semiconductor into the metal electrode is the quantum step, whereas the evaporation from the metal electrode is a step involving only thermal energy. In principal, this system resembles a Schottky-cell with two circuits, one for the electrons and a second for the atoms. Energy storage can be achieved as soon as the atoms, separated from the host electrode, are stored in another container ( e.g. hydrogen in metal:storage).:

RL

- H

• .

• .'.'.'.

Tz

2W

H

H+'~

""' • "-I~

-I:-I

Fig.50. Combined quantum thermal generator for the conversion of solar energy. The working medium is hydrogen.

Energy Conversion and Storage

269

An anologous example of such a combined quantum thermal generator with a p-type electrode is shown in Fig.50 with hydrogen as the working medium. This generator is based on an photoinduced electrochemical insertion of hydrogen into a p-type semiconducting host electrode, from where it is subsequently evaporated by thermal energy at the other interface. At the anode the atoms are oxidized to ions which are then reinserted by the action of light. 1~3

9. OPTICAL INFORMATION PROCESSING AND STORAGE Optical signals will play a dominant role in future data processing systems. In compact discs or other data disks information is already stored as optical information and can be read with an laser beam. These commercially available systems up to now are read-only memories, where the information is burned in with a laser beam. Thus, the information is encoded in a sequence of holes (pits) with different light scattering properties than the starting polymeric disk material [polycarbonate or polymethylmethacrylate) 178. There are several attempts to develop reversible optical memories such as magneto-optical systems . The information can also be for example erased by a thermally induced phase change of the storage material 179 or electrically by applying an external voltage. The latter concepts which are still in the experimental stage are mostly based on reversible light-induced redox reactions in thin electrolyte films brought onto semiconductor electrodes. The reduced and the oxidized species differ in their colour, so that a concentration change of either the oxidized or reduced species changes the colour of the surface film 1~0-183. Additionally, there are attempts to bring thin films of electrochromic HxWO3 on amorphous silicon photoelectrodes to drive an electrochromic insertion of hydrogen into the HxWO3 -film according to Equation (35} 184. A direct coupling of an insertion mechanism to a photoprocess is therefore also technologically interesting. It is well-known that with many host materials insertion reactions are accompanied by pronounced changes of solid state properties. This includes optical, magnetical and electrical properties ~ When considering optical excitations of insertion related changes, it is unevitable to start with an evaluation of the time constants involved.

Electrochromic devices such as those based on the insertion of WO3

with hydrogen or lithium have a disappointingly slow time response of 0.1 to 0.5 sec. The question is whether there is any chance to reach time responses with this kind of mechanisms that are technologically interesting. The self-diffusion coefficient of the mobile ionic species is limited by the velocity of sound, the propagation speed of phonons that assist the hopping of the ions between the lattice sites and does not exceed Dki w. 10-Scm2/sec

. Table I shows experimentally found self-diffusion coefficients and ionic

mobilities of several ion conducting materials. As explained before, the chemical diffusion coefficient D of inserted guest species in mixed conducting electrodes can be much higher due to the enhancement factor W (see Table I). This expresses the influence of internal electric fields accelerating the hopping ions. Since values as high as 104 - 106 have been reported 14~ an optimal chemical diffusion coefficient D : W Dki between l0 -* and 10 cm~/sec can be expected. For integrated optoelectronic materials we can typically assume an active thickness d of 100 nm. Provided they have an high enough absorption constant (10 s cm -1 ) they can absorb most of the incident light. To change photoelectrochemically such a layer guest species must penetrate it. The time r necessary for this process would then be: 42

D which ranges according to the above estimated value of D between 10 -9

and 10 -11 sec, sufficient for

information processing. These considerations show the principal feasability of optical information storage based on insertion reactions, but fast thin film materials have still to be developed.

270

G. Betz and H. Tributsch

9.1. Photoeleetroehromie Information Storage if the optical absorption of the host electrode is changed as a consequence of a photoinsertion reaction optical information storage can be performed with such a photoelectrochromic reaction is6

reading beam M+-ion-conductor

Wr be

(h .

I'1'~ I f.I,L

H

p-semiconductorI

write

J -tl ÷ erase

Fig.51. Optical information storage with a photoinsertion reaction, which is changing the optical absorption of the electrode MxA.

Fig.51 shows an element for the reversible storage of optical information which is storing one bit.

A

transparent metal electrode and M ~ -ion conductor is evaporated onto a photosensitive insertion electrode M=A .

Upon illumination of the electrode with light of an energy hv >Eg (writing light beam) the optical absorption of the electrode is changed (e.g. lowered), because of the insertion of M-atoms into the MxA -electrode. The time constant T of the reaction can be lowered by applying an external voltage increasing the electricfield in the interface. We can attribute a logical ] to the transparent and a logical 0 to the opaque electrode. This information can be read with a probe laser beam of the energy hu

< Eg (reading beam) which is

only absorbed, if the electrode is opaque,i.e, if a logical 0 is stored in the element. By applying a reversed bias M atoms are deinserted from the MxA -electrode, which consequently increases the optical absorption in the spectral range of the reading beam. This mechanism was first demonstrated with Cu6PS,sl -electrodes 186. The crystalline electrodes grown with chemical vapour transport in presence of liquid Cul have a cubic high temperature structure (F43m;Z=4) with a disordered copper ion sublattice 187. The insertion photocurrent density as a function of the applied voltage is given in Fig.52. After chemical or electrochemical deinsertion of copper the red transparent Cu6PSs] becomes opaque Cu~_xPSs] . The change of the optical absorption is shown in Fig.53 together with the corresponding photocurrent spectra. The co]our change, which is not paralleled by a change of the photocurrent spectra, is due to the creation of I~ -centers in analogy to the Vk -centers in LiF 18s Yoshino et al constructed a photoelectrochromic cell according to this principle where CIO~

- ions were

photoinserted into polythiophene films 189. It will be much easier to realize such thin film devices with polymer electrodes than with inorganic compounds.

Energy Conversion and Storage

271

Vph

/

/

-0.2~

E

I=l

-0./, ~c

E -0.6 \ hv

/

/

.Ira

-0.8 ~

J

/

-1.o %

f

aJ ¢_

Cu6 PSs I

/

/

-12 ~

CuTCu' I

I

i

-0.8

1

- 0 .'6

I

I

i

-OA.

electrode potential /

I

i

-0.2

V vs Ag / Ag ÷

Fig.52. (Photo)current voltage curve of a Cu6_xPS,d-electrode. Curve 2 was obtained in contact with a 0.02 M CuCl solution in acetonitrile. Curve 1 was recorded in the absence of copper ions in the electrolyte. The results indicate that copper ions are the species that are reduced under illumination.

1.4 Ii,

......

a) 0.75 .~

1.2 1.0

0.50

0.8

~ o.6 0.4

t=r"

0.2 0

i Jllllll|el

500

lilt

600

illllllllllll|

700

0

800

wavelength k /nm Fig.53.

Optical absorption (a,b) and photocurrent spectra (1,2) of a Cu6PSsI-electrode (1,b) and a

Cu6_ ~PSsl-electrode (2,a). Mixed electronic and ion conducting semiconductors offer the perspective that information storage can be incorporated into the data processing switches or into the data uptaking sensors. Image sensors for example could be realized which are capable of learning, i.e which sum up the photons in form of atoms inserted into a photosensitive host matrix, as shown in Fig.54. Upon insertion of a guest species a physical parameter of the host, such as the conductivity or the optical absorption, is changed and encodes the incoming optical information. This information can be read as a light or a current signal. Since these physical parameters are gradually changed upon insertion, all only occasionally occuring optical information can be filtered out by introducing a threshold value for the information storing parameter, when the stored picture is digitalized.

272

G. Betz and H. Tributsch

--0 Vext 0-~ |??

I::~

99 output of the ments (resistance etc.)

; ~

~'--,~i

/,# i MI

.

back

photosensitive ' - ~MxA-host matrix

I

transparent metal M electrode

Fig.54.

Integrating image sensor based on a photoinsertion reaction into a matrix of a photosensitive

host electrode. Upon insertion of the guest species a physical parameter such as the conductivity changes gradually integrating the absorbed photons.

9.2. Microwave Studies of Photoinduced Ionic Mechanisms Electrical measurements of insertion compounds are frequently limited by polarization phenomena, interfacial barriers and RC-constants of the measuring circuits (R = resistance and C = capacity of the circuit). For the study of combined photoinduced electronic and ionic processes especially in optical information processing and storage, where microstructures and thin layers are used, a contactless method is desirable in order to avoid all those difficulties. It should also be able to detect electric and ionic charges with a high time resolution in order to give access to fast kinetic mechanisms. Suitable methods are recently developed microwave detection methods, which are basing on the reflection or absorption of microwave power P from a material, whose conductivity is changed by an external perturbation (e.g.light pulse) 190. The experimental conditions have to be chosen such that the relative change of the absorbed microwave power ~£ is proportional to the conductivity change Ao Ap

(77)

= AAo

where A is a proportionality factor.

The change A a

of the microwave conductivity can be split into

contributions from the electrons and holes,ions and dipoles:

k=e,h

{

d

The principle of such measurements is schematically shown in Fig.55. A microwave source (e.g. gunn diode) generates microwave power in the X- or Ka-band, which is transmitted through a wave guide system to the sample. It is importanl that in the set-up of Fig.55 the sample is on an open waveguide which reduces the time constant of the measuring system compared to measurements in cavities. The reflected microwave power is guided through the circulator to the detector. The time resolution is in the order of 0.I ns. A

Energy Conversion and Storage 4

273

91

~[ lightsource~ v ~

v

trigger

I x-Y t a b l e

)

I

_~waveguidesystem

'~ [

digitizer ~

~._J (x,y) stepmotorl cantrot ,

time resolved measurement

spaceresolved measurement

ImiCoru° rW~Ve circulator detector I

I

~

I

~

=

---• voltage generatorJ I

function

4

L stationary measurement

Fig.55. Overview of the experimental set-ups and possibilities of the microwave photoconductivity technique 193

laser beam can scan the surface of the material to get an spacially resolved information of the charge carrier dynamics in the material 19~. The spacial resolution is depending on the used wavelength of the laser light and is approximately one micron. Additionally,an external voltage can be applied to study the influence of the electric field on the photoconductive behaviour at junctions. The microwave photoconductivity technique has been sucessfully applied to study fast photoelectron kinetics in silver halides 19z,194. Since the electron mobility in silver halides is much higher than the hole mobility, the observed photoconductivity is due to the excited electrons. Theoretical considerations show that the rate determining step for the photoconductive decay of the electrons is the capture of the electrons by mobile silver ions 193. The microwave technique therefore allows to study the correlation of fast electronic ionic interactions in mixed conducting semiconductors. Another example of a photoconductive decay in a mixed conducting material is shown in Fig.56 with a Cu~PS,~I - electrode and a copper poor CUe-xPSsl - electrode. Extraction of copper from the CuePSsI lattice increases the lifetime of the light-induced charge carriers. This is also reflected by the photocurrent behaviour of the electrodes. Complex microwave susceptibility of Agl - type solid electrolytes has been studied by Funke in a broad frequency range giving informations about the hopping frequency of the ions jumping between the lattice sites and about their residual time constants 19s. The influence of electronic conductivity on the ionic transport processes could be studied by generating electronic charge carriers under illumination.

9.3. Optoelectronic Control of Chemical Mechanisms Molecular electronics and interfaces between semiconducting electronic elements and biological molecular units are a very young research area 196. Since during information processing in nerve cells signals are converted into ion gradients at ion gates which are controlled by neurotransmitters such as acetylcholine,insertion compounds may provide a link between electronic circuits and molecular mechanisms. Insertion compounds are like the synapses able to release molecules upon an optical or electrical excitation. These can subsequently catalyze certain reactions. Membranes, for example inserted with hydrogen, could start spatially resolved protonation reactions, when the protons are deinserted by illumination with a scanning laser system. In such a way an optoelectronic system can directly communicate with a chemical enviroment.

274

G. Betz and H. Tributsch

,4-' t"

Cu6 -x PS51 {J ¢#

'r=

50 160 150 200 250 300 time'' /ns' t--

.=

Cu6 PSsI

lJ

O~

~J

r= 50 100 150 200 250 300 time/ns Fig.56. Decay of t h e electronic p h o t o c o n d u c t i v i t y in Cu6PSsI a n d Cu6_xPSsl after an laser pulse excitation (A = 530nm).

In t h e field of molecular electronics concepts of how to realize molecular switches,gates a n d s t o r a g e u n i t s already exist 1~7, b u t there is no idea how to p u t t h e i n f o r m a t i o n into t h e molecular s y s t e m a n d how to read it out. If one does n o t want to go back to a macroscopic level t h e r e s e e m s no o t h e r way possible t h a n to search for molecular arrays which can be synthesized u p o n illumination at m a t r i c e s like t h e aforementioned p r o t o n releasing m e m b r a n e . At a given m o m e n t a certain p a t t e r n involving one layer of molecules is written at t h e surface, where a species such as p r o t o n s is deinserted u p o n a b s o r p t i o n of an p h o t o n . T h e n this p a t t e r n is removed from t h e surface so t h a t a new p a t t e r n can be written in t h e next layer of molecules. Such a molecular t h r e e d i m e n s i o n a l memory, which a p p e a r s now r a t h e r speculative .would have in two d i m e n s i o n s a s t o r a g e density, which is finally limited by t h e wavelength of t h e used light, b u t would have in t h e o t h e r d i m e n s i o n a s t o r a g e density d e t e r m i n e d by t h e distance of t h e molecules.

10. T H E O R E T I C A L C O N C E P T S

10.1. Theoretical Model for Photoinsertion Reactions T h e m a t h e m a t i c a l analysis of s e m i c 6 n d u c t o r / e l e c t r o l y t e interfaces in contact with a redox couple in t h e electrolyte t u r n s o u t to be very complex. Some of t h e e x p e r i m e n t a l l y observed relations such as t h e logarithmic d e p e n d e n c e of t h e photopotential on the light intensity can only be obtained u n d e r drastic simplifications 201. A complete a n d exact m a t h e m a t i c a l description of t h e p h o t o c u r r e n t - v o l t a g e b e h a v i o u r at an interface of an insertable s e m i c o n d u c t o r - electrode a n d an ion - c o n d u c t o r is n o t possible, because of several additional complications. T h e charge carriers t h a t c o n t r i b u t e to t h e p h o t o c u r r e n t in t h e elctrode are no m o r e only electrons and holes, b u t also ions (M ÷ ). Additionally, t h e diffusion of n e u t r a l a t o m s h a s also to be taken into consideration when calculating t h e p h o t o c u r r e n t density. T h e t r a n s p o r t processes in t h e s e m i c o n d u c t o r

Energy Conversion and Storage

275

electrode are accompanied by reactions between the charge carriers, which can be described by the Eqs.(5,6). As a consequence decisive parameters of the electrode such as the position of the Fermi-level change during insertion. Normally the current-voltage curves at insertion electrodes are recorded far away from equilibrium so that they are explicitly time-dependent showing pronounced hysteresis effects ( see Fig.28). Stationary current densities can only be obtained assuming a membrane-like arrangement where the activity of the inserted atom is fixed on both sides. The photoinsertion reaction with a p-type semiconductor can be split into several steps which are practically going on in parallel. The whole process is shown in Fig.57. Fig.57a) visualizes the energetic relations of the electrons, while Fig.57b) shows ionic energies and concentrations. l) Generation of an electron/hole pair by the incident photon, as the elementary step. The rate of generation is:

g(z) where ¢0 is the incident photon flux and a photon energy.

-- ~o ~ e x p ( o ~ )

(79)

the absorption coefficient for the photons depending on the

We consider here only the one-dimensional case with z as the distance from the elec-

trode/electrolyte boundary. 2) Separation of electron and holes due to the electric field in the interface (barrier height V,o ). This electric field can be caused by the exchange of M + -ions as discussed in section 4.4 (equilibrating the electrochemical potentials of the ions M + ) 3) Light-induced internal redox reactions. The minority carriers (electrons) react according to Eq.(5), if we neglect recombination reactions with other centers than the mobile M ÷ -ions. In terms of semiconductor physics eq.(79) is a trapping of photoinduced electrons by mobile trapping centers. The reduction can occur at the geometrical interface so that the electrons have to drift a certain distance z from the place where they had been excited towards the electrode/electrolyte boundary. This is most probable in semiconductors with relatively high electron mobility #

and lifetime r

. In materials with a small pv -product, such as

the sere/conducting polymers, the reduction is localized to the place of electron generation. Trapping of the electron at a localized inserted atom may help to avoid recombination, because the excitation energy is immediately stored upon creation of a chemical bond between the inserted species and the host matrix. The current carrying charge carriers are the mobile ions in that case. The ions have to be re-supplied from the electrolyte after the reduction. The holes are drifting to the back contact, where they recombine with electrons from the external circuit. Part of the holes can induce the back reaction according to (6). Also the holes as majority carriers can lake part in this reaction. The reduction (5) takes place with a rate

R,c,(z)c~(~_)

(80)

where c e(z) is the electron concentration at z, ci(z) the ion concentration and Re the rate constant of the process. Correspondingly the rate for the reoxidation of the neutral species is:

RheM(z)c~(z)

(81)

There is no photoreaction with M" -ions. if

RhcM(z)ch(z)dz =

R~cdz)c~(z)dz

since there is no light-induced change of the ion - concentration within the electrode. JPSSC 16:4-F

(82)

276

G. Bctz and H. Tributsch

a)

i

M÷ qV.

M+

/

1-"=;:"-~F J~

M+

/

E,

;:I/.,z_, ......

M÷

M÷

•

I

W

)

1/o(

L bl

concentration of the reduced species c.(o}

electronenergy I I

I

M'k °,, ,,O,,o... . i /CM

I

[ L (o)

(Z ) ",,° "°',. ...

e"

o.

':'....

." I_ C,. , M++ e . . . . . . . ?, C,

M+

M+

...

,0 "M

"" ~ h + + M

d'~.

I

I

I

W

i

L

Fig.57. Representation of a model for the elementary steps of a photoinsertion reaction (a) in an electronic energy band scheme. Lp, Ln are the diffusion lengths for holes and electrons respectively. At the end of the electrode (z=L) the shaded region visualizes the backcontact. (b) Energy and concentration distribution c~(z) of the ions.r+, r_ are the ion transfer rates into and out of the electrode, z=0 is at the dotted line. For z
Energy Conversion and Storage

277

The electron concentration in the surface is higher than the hole concentration, if the quasi Fermi level is situated more negative than intrinsic level (middle of the energy gap). This can be achieved by a high enough photon flux and subsequent charge separation which drives the electrons to the interface. In this case the right integral exceeds that on the left side of Eq.(82). Consequently the concentration ci is lowered leading to an inflow of M ÷ -ions into the electrode. The degree of insertion 0 which is assumed upon illumination, corresponds according to Eq.(15) to the photovoltage Vph obtained. From the experimental results of the photoinsertion of copper into CuxPS4 -electrodes, it can be concluded that the electrons eh~ in the interracial region occupy localized states EM , which diffuse with the ions from the surface into the bulk of the electrode. The rate constants Re, Rh depend on the energetic distance of this state from the band edges. If EM is close to the valence band the electrons coupled to M + relax very fast to the valence band. The M atom acts as a mobile donor compensating holes in the valence band. If the electron has an higher mobility than the ions M ~ , internal fields Ed are created, as alrady mentioned in section 3.2, when introducing the enhancement factor W.

quantum efficiency~ 1 current / reL units

coh t.

/~

ioo $

~

o q b e iv ~ irli /i ~---___ . ~ / ,,r,, d....... /I ---"../ /

/.

2b--~:~-...'l 2;,s/ '~.~, 11

i-

/\

quantum efficiency ~, [ an0d. current/ rel. units 1

E o~ o -4,"

>o o o eQ.

" 3:o '.;[" \~.~,'1"~" \

.... 3.s

-

photonenergy

hv/eV

//.

",,..~'.../

1/. 12 10

I equilibrium

8 6 l, 2 I

-2 -6 -8 -10 -12 -1/.

2,

I

e.

'

h0f0n

I

during

energy hvleV

diffusion

Fig.58. (a) Photocurrent spectra of a Cu3PS4-electrode at equilibrium and .after an insertion pulse, (b) photovoltage spectrum of a CusPS4-electrode at equilibrium and after an insertion pulse. Each spectrum is individually normalized.

278

G. Betz and H. Tributsch

These internal fields are to be superposed to the existing fields and can invert the direction of the field present in equilibrium. This is shown in Fig.58, where the spectral distribution of the photocurrent and the photovoltage at a Cu3PS4 -electrode in the equilibrium and during copper diffusion is given. Since the potential at which the photocurrent spectra were recorded was lower than the equilibrium potential a deinsertion reaction started at the surface. Therefore a spacially limited copper front is diffusing away from the surface into the bulk. With the help of such a diffusion front the influence of light on the diffusion process could be studied. Due to the diffusion field Ed anodic p hotocurrents and a negative photovoltage can be observed. These anodic effects could only be seen, if the incident photons are absorbed in the region of the diffusion field.

(

Ec

Cu ÷

m-,~'N

2.35 eV < hv < 2.6eV /hP<2.35eV

E9 =2.35eV

fhv

> 2,6eV

_Cu ÷

Ev

Cu +

)-,,,\

Cu +

N

Cu3 PS~ ----

Ed

e-)

Fig,59. Representation of the electron energies in a Cu3PS4-electrode at equilibrium and during the migration of a front of copper atoms (Cu t -- e-), where the diffusion field Ed is imposed on the electrical field in the interface.

As visualized in an energy band scheme (Fig.59) the photons absorbed immediately at the surface induce a photoinsertion reaction as well as those which are absorbed deeper in the bulk. The electrons generated in front of the diffusing front accelerate the diffusion, those generated behind it retard it. Similiar effects have been observed with photoinsertion reactions at other electrodes, e.g. the polyfurylenvinylene electrodes. The influence of light on the diffusion of mobile ions in semiconductors will be discussed more in detail in the next section. No diffusion field is built up, when the electrons are strongly localized at the diffusing M" -ions. i.e. in case that the guest species diffuses as a neutral species. 4) Ion transport in the interface. If the reduction of the ions occurs far away from the geometrical electrode interface, the transport of the ions in the electrode is determing the rate of ion insertion. This diffusion is assisted by the electrical field in the interface (band bending). Under current flow condition the quasi Fermi-level of the illuminated electrode determines the distribution of reduced species. It stabilizes the difference in the activity of the reduced species at the surface and in the bulk. So during illumination practically no inserted and reduced species is transported into the bulk. Only under open circuit conditions in the dark a redistribution of the reduced species takes place during which the electrode assumes a new equilibrium potential. An equivalent explanation is possible using the fact that the electrical field which drives the ions into the bulk is reduced under illumination.

Energy Conversion and Storage

279

5) T h e t r a n s p o r t of the inserted species in the bulk, where no electrical fields are present, besides the internal fields Ed due to diffusion. If the absorption length, a - t , is greater than the extension, W~ , of the space charge layer plus the diffusion length of minority carriers, Ed is influenced by the light-induced charge carriers. T h e resulting particle flux densities for the ions M + , electrons, holes and neutral atoms M are obtained from a drift and a diffusion term : ions, M + • q c dV,. + ~ i~z )

(83)

dc¢ q dl~,, j~(z) = - D ~ ( ~ z - ~ n ~ z )

(84)

, dc h q dt~ jh(z) = -vh(~z "- ~ C h ~ z )

(85)

ji(z) = --Dk,(~z

electrons, e-

:

holes, h + :

reduced species, M : (86)

3M = - D d C M dz

If the electrons are localized at the diffusing ion M + , the diffusion coefficient D of the neutral atom is approximately the diffusion coefficient Dki of the ion M + . The enhancement factor is then W ~

1. The

influence of the electronic charge carriers on the motion of the ions is incorporated in the Eq.(82)..eq.(85). All particle flux densities obey continuity equations, where the generation of electron/hole pairs and the reactions with the mobile species have to be regarded:

(87)

- d--z - R,e,c~ + RhcMeh = 0

dJ•

- d-~ ÷ Ooae . . . .

_dj~

(88)

R~c,c~ = 0

+ ¢0ae- ~z _ R h c ~ c ^ : 0

(89)

_ diM 4- Recice - RhCMCh = 0 dz

(90)

dz

T h e time derivatives such as ~~t disappear in the steady state situation. The electrical field (- ~ interface is influenced by the electrons, holes and ions. The Poisson equation yields:

d2Y" dz 2

) in the

q (c, - ch - c, - N A c )

~{o

(9a)

T h e interaction of electronic and ionic charge carriers is provided by this electrical field. For z --~ a p p r o p r i a t e concentrations for the electronic and ionic charge carriers have to be assumed. At z = 0 the concentration CM can be calculated from A r m a n d s relation (15) according to:

V(0) = V ( ~ )

= I~)

(92)

where V! is the photovoltage which is related to the barrier height and the current-voltage curve V.,(1) :

280

G. Betz and H. Tributsch

The boundary conditions for electrons,holes and ions at z = 0 can be obtained from the rate of ion transfer through the semiconductor/electrolyte boundary, with transfer rates r " , r -

as shown in Fig.57b). For

these ion transfer rates a Butler-Volmer-like relation can be derived (details see in reference (133)). With the simplification that the redox reactions with the M + -ions occur at the geometrical interface z=0, the photoinsertion reaction can be treated in a similiar way as it is done in the model of Reiss assuming a redox couple to be present in the electrolyte. Numeric calculations of energy conversion efficiency of a polymeric semiconductor barrier with mobile donor and acceptor species reported by Z.W.Tian show only a reduction of the efficiency of only 5% compared to that obtained when the donor and acceptor species are assumed to be fixed 199

10.2

Photodiffusion

The influence of light on diffusion processes in semiconductors is a frequently observed phenomenon, but still only little understood. In amorphous semiconductors such as a - AszSe3 the diffusion of silver and copper atoms being previously evaporated onto these semiconductor layers is enhanced by illumination 200 This effect can be applied for microlithographic purposes ~0~. Photodiffusion has also been observed at ionic conductors such as /~ - RbAg4J5 with excitation by UV-light 20~. Photodoping is reported with (CH)x in presence of AsF5 20~. It is obvious that the photoinsertion reactions as discussed in section 6 imply photodiffusion mechanisms as well. Although these examples document the influence of light on the transport of ions in semiconductors, they may imply different mechanisms. We have to distinguish between phenomena occurring at interfaces, where electrical fields are present and those which can also be found in the bulk of the material. Because of the large effective mass of the ions photodiffusion effects are mediated by the excitation of electronic charge carriers. First we have the already discussed field effect, where the generation and separation of electronic charge carriers reduces an existing electric field in the interface (band bending) that accelerated the motion of ions from the phase boundary into the bulk. The transfer of ions across the phase boundary is caused by re-establishing a new equilibrium under illumination, where the light-induced electrons have lowered the ion activity by reducing the ions internally. In a homogeneous material different mechanisms are responsible for an increase of the ionic conductivity upon illumination. Electrons can be excited from localized states EM corresponding to neutral atoms M. This is equivalent to saying that the atoms are ionized internally increasing the number of mobile ions. Such an internal photoionisation effect is observed at n-type or intrinsic materials, where light is inducing photodeinsertion reactions. It is most probably also the reason of the photodiffusion observed in/3 - RbAg4Is . In homogenous semiconductors another mechanism can be described in terms of an increase of the enhancement factor which depends on the electronic charge carrier concentration. The light-induced generation of highly mobile electrons which drift ahead of the mobile ions will cause an internal field E a , that accelerates the ion and retards the electron. This effect is in full analogy to the occurrence of the dember-potential in conventional semiconductors, which is due to different mobilities of electrons and holes. This potential is determined by the mobility difference and is in the order of 10 to 50 mV. With ions and electrons the mobility difference is much larger, so that these internal fields play a dominant role especially in intrinsic materials, where the fields are not shielded by a high charge carrier concentration. Let us consider the diffusion of atoms M in a semiconductor p - MxA , which is homogenously illuminated with light of an energy ht, > Eg . If no external field is applied and the diffusing atoms are in the bulk, the electric field which is in this situation equivalent to Ed can be calculated from the condition of charge flux balance

dz 81

.

Energy Conversion and Storage

281

j,,~z,n = 0

(94)

m=i,e,h

z= : sign of the charge of the charge carrier m T h e electrical field - dr. can be eliminated by inserting Eq.(83),(84) and (85) into (94). This yields equations for the current densities j ~ , which are of the form of Ficks first law: .61na~ jm :

-Din((1

-

¢-.,

z, ~lna~ .dc~

-

j

~z

(95)

where fm = o_~ is the transference number of the m-th charge carrier and:

o = oi+o.

(96)

+ op

is the total conductivity. From (95) the enhancement - f a c t o r W for the ions (m=i) is obtained according to Eq.(42):

w : (1 -

t "61na~ S " z, 61naj ')6~, - ~ t~z~ blnc~

Upon illumination an electronic excess conductivity A e

(97)

is generated:

Ao" = q ( A n ' # , + A p # r )

(99)

Thus the activities a~ and the concentrations cj of the charge carriers will in general assume new values which are marked with an asterisk. Consequently also the transference numbers i of the carriers are changed to t~ and t~, ,where: t~-

o~ _ o . + e"

Ao"

o + A o " 'tP -

o~ o"

(100)

This yields an e n h a n c e m e n t factor W for the ions : • 6lna~ ¢.-., 6lnaj W" = (1 - t i ) ~ - 2 . . t j zi ~ , zj 61no i

(101)

In general this expression has to be evaluted to calculated the influence of light on the enhancement factor. But in order to roughly outline the basic features we can introduce some simplifying assumptions. First we can assume that the electrons have a mobility that is some orders of magnitude larger than that of the holes as it is for example the case in a-Si:H. Then we can negleglect the change in the transference number due the light-induced holes. If we assume additionally that Henry's law is valid, i.e. the activity constants for both the ionic and electronic species is a constant, we can obtain W" from the electroneutrality conditions:

dce= dci

(102)

dCh = - d c i

(103)

to:

W

= t;'l ,- (c, c,+ c~'i

(104)

In case we have in the dark an ion-conductor with a vanishing electronic conductivity ( solid electrolyte) the e n h a n c e m e n t factor tends toward zero. If now electronic charge carriers are induced upon illumination,

282

G. Betz and H. Tributsch

t~ will approach unity and the value of W" strongly depends on the relation ci/c~ + c~. In case charge carriers with a high mobility are generated in a small concentration W can assume large values. This is especially true for mixed conducting semiconductors as already pointed out. Since c~, c~ include the carrier concentrations in the dar,k B'" is the bigger the smaller the dark conductivity is. This explains, why the photodiffusion effects appear in photo-conductors, such as a - As~S3 and a-Si:H with extremely small dark conductivities. Finally, we want to mention the theoretically predicted effect of ion entrainment by electrons in semiconductors which is due to a momentum transfer from the electron quasi-particle to the diffusing atom 20s. This effect is especially large in semiconductors, where the electron - scattering cross-sections are larger than in metals. It has been shown that with an increase of temperature and ,consequently, electronic charge carrier density the effective charge of the ions can change the sign, i.e. the direction of ion transport reverses. We conclude that this effect can also be caused by an light-induced increase of the electronic charge carrier concentration. We have now roughly outlined the basic features of how to understand the experimentally observed photodiffusion effects. These considerations have to be elaborated further in next future to provide a model on which photosensitive mixed-conducting devices can be designed. The experimental verification of the model will depend on the availability of large mixed-conducting semiconductor crystals that can be illuminated far away from contacting interfaces.

11. SUMMARY AND OUTLOOK

We have attempted to guide the reader through the large research field of energy conversion based on insertion materials. Classical areas of investigations, such as intercalation batteries or electrochromic displays have been discussed only briefly in a more general scientific context. We did emphasize new and little explored research areas which include nonlinear mechanisms and photoinduced processes. Investigations of photon-powered reactions seem to be especially interesting, because they open new areas of applications. The possibility of using photon energy to modify the thermodynamic, physical or chemical properties of materials is scientifically appealing. In this publication we have discussed the feasibility of three new devices, light-driven insertion batteries, photon powered proton pumps and photoelectrochromic systems, based on photoinsertion/photodeinsertion mechanisms. We hope that other laboratories will join our efforts to place this research area on a broader scientific base and identify improved materials for photoindueed insertion reactions. It is encouraging that similiar mechanisms are used for biological solar energy conversion. Insertion materials with their versatile properties will have an even broader range of applications, if large efforts in materials s~ience are being made. Acknowledgement: The authors would like to express their thanks for stimulating discussion to their colleagues at the Bereich Strahlenchemie of the Hahn-Meitner-Institut and to the collaborators of the Department of Materials Science (group Prof.R.A.Huggins) at Stanford University. The authors especially thank Dr.H.-M.Kfihne for his helpful comments and for critically reading the manuscript. Dr.J.Lilie from Bereich Strahlenchemie of the Hahn-Meitner-Institut is thanked for his assistance in editing the manuscript on a computerized text-system.

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