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Synergetic molecular approaches towards artificial and photosynthetic water photoelectrolysis
JO~I~X~L OF
ELSEVIER
Journal of Electroanalytical Chemistry 396 (1995) 53-61
Synergetic molecular approaches towards artificial and photosynthetic water photoelectrolysis 1 Helmut Tributsch, Ludwig Pohlmann Hahn-Meitner-lnstitut, Dept. Solare Energetik, 14109 Berlin, Germany
Received 6 April 1995
Abstract 25 years after the discovery of UV photoelectmlysis of water with T i t 2 by Fujishima and Honda, which triggered high expectations with respect to water splitting using visible light, an evaluation of progress is attempted. On the basis of an analysis of existing electrocatalysts for oxygen evolution alternative reaction possibilities are compared: one-electron transfer, four-electron transfer and the role of intermediates chemically bonded to a catalyst. For the manganese complex of photosynthetic oxygen evolution the puzzling situation is that electrocatalytically poorly active Mn metal centres in a fragile complex are highly catalytic for oxygen evolution. The conclusion is reached that the complex organizes itself into a favourable transition state through an autocatalytic process (export of entropy) that facilities a real (intermediate-free) four-electron transfer. An experimental model system for oscillating hydrogen evolution is discussed to demonstrate the feasibility of synergetic reactions for fuel generation. The energetic and kinetic advantages of a synergetic mechanism of water splitting are outlined and a kinetic model is calculated which simulates oscillations observed in photosynthetic oxygen evolution. Keywords: Water; Photoelectrolysis;Synergy
1. The challenge of water photoelectrolysis Scientists have been puzzling for decades over photoinduced oxygen evolution in photosynthesis without having a realistic molecular clue, when after 1970 it became known that Fujishima and Honda succeeded in splitting water using illuminated T i t 2 electrodes [1]. Fig. 1 shows a photocurrent voltage curve typical for T i t 2 and oxygen evolution detected with differential electrochemical mass spectroscopy (DEMS). At that time it was not so important that UV light was needed for photoelectrolysis as well as a small supporting potential. This remarkable photoelectrochemical process stimulated the imagination of scientists and their hope that splitting water using visible light would just be a matter of systematic research. An incredible scientific dynamic developed in the field of photoelectrochemistry, which was carded along by the first energy crisis and an honest interest in energy matters. In the decade which followed some of the most important progress in photoelectrochemistry was witnessed. The fundamental
energetic, thermodynamic and kinetic concepts were developed, hundreds of photoactive materials were investigated and dedicated efforts were developed to extend the sensitivity of T i t 2 into the visible spectral region. Now, 25 years after the discovery of T i t 2 photoelectrolysis, this UV-absorbing material is still fascinating to scientists. In particular, in the laboratory of Fujishima
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H. Tributsch, L Pohlmann/ Journal of Electroanalytical Chemistry 396 (1995) 53-61
54
several practical applications for its properties have been developed. In quite a few international laboratories TiO 2 particle suspensions are applied successfully for oxidizing and neutralizing toxic chemicals. However, what happened to the hopes for energy-efficient solar photoelectrolysis of water? The problem is not yet solved. Most research groups that had once concentrated their efforts on this fascinating problem have chosen new subjects. What is the scientific background for the complications encountered?
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2. Options for oxygen evolution mechanisms Reversible thermodynamics opens basically two pathways for water photoelectrolysis determined by electrochemical reaction mechanisms requiring less than 3 V of electrode potential. Since oxygen is a gas and water a liquid the conditions basically require that the electrodes involved are inert, only exchanging electrons. The most favourable pathway is the well-known reaction (in acid solution, h = positive hole) 2H:O + 4h ~ O 2 + 4H + E 0 = 1.229 V(1 M acid solution)
(1)
involving the simultaneous transfer of four holes (equivalent to the transfer of four electrons into the opposite direction). The second possibility is a succession of onehole-transfer reactions of the type H20+h~OH'+H
+
E0 = 2.85 V
(2)
where OH • radicals continue to react to liberate molecular oxygen. Photoelectrochemical theory requires that illumination in a semiconductor interface creates a Gibbs energy level (quasi-Fermi energy level) for minority charge carders sufficient to run one of the above reactions (Fig. 2). Significant complications arise since we do not have ideally inert oxygen electrodes, since classical electron transfer theory does not consider possible the simultaneous transfer of several electrons, and since intermediate products formed will temporarily bind to the electrode interface thereby affecting the reaction potentials and the electrode material properties involved. In fact we know now that oxygen evolution via TiO 2 is possible because this material with an energy gap of 3 eV facilitates reaction path (2) with the limitation that the generated radical immediately binds to the interface. The reason why TiO 2 photoelectrolysis of water did not succeed in pioneering visible light photoelectrolysis of water is simply that the very different reaction path (1) would have had to be activated for that purpose. As amply discussed in photoelectrochemical literature (e.g. Refs. [3-5]), in order to drive the most favourable reaction (1) and to overcome the expected overpotentials a semiconductor with an energy gap of 2.1 eV would be required. A significant overpotential for
J (~'~
nu
[ ~OH
Fig. 2. Energy scheme, based on the materials TiO 2 and RuS2, indicating
the four-electronwater oxidation reaction, the one-electronwater oxidation reaction and successive electron transfer steps via intermediates in which speciesof water oxidationare bound to an interfacialcatalyst.
oxygen evolution has to be considered since a real multielectron transfer is not expected to occur in classical electrochemistry. In fact, it is assumed that intermediate products will be formed which bind to the electrode surface and the foremost aim in improving efficiency of catalysis should be to find a catalyst which can bind these intermediates of oxygen evolution in such a way as to provide an overall reaction potential as close as possible to the ideal reaction potential of mechanism (1). The reaction potential of such a stepwise-catalysed reaction will typically be more positive than the potential for the theoretical four-electron oxygen evolution reaction from water. Many efforts have been undertaken to obtain a visiblelight-induced splitting of water into oxygen and hydrogen. Numerous inorganic and organic photoactive materials were tested, often additionally modified with catalysts. We would especially like to call attention to the efforts developed by Calzaferri and collaborators with zeolites encaging silver species, which are able to liberate oxygen [6], We also should mention the work of Graetzel and coworkers (for a review see Ref. [7]) who claimed to have accomplished the reaction with a sensitization of TiO 2 via a ruthenium complex and RuO 2 as catalyst. Their results have, however, apparently never been confirmed by groups who have tried to verify their data. Photoinduced hydrogen evolution itself has, on the contrary, never been a major problem. Classical p-type semiconductors, covered with islands of noble metals, can quite easily be induced to liberate hydrogen under illumination [8,9]. Hydrogen evolution alone (and using a sacrificial electron donor) does not solve the problem of water splitring. The real problem is the catalysis of water oxidation near the potential of the multielectron transfer process (1) of oxygen evolution. This requires that water species are
H. Tributsch, L. Pohlmann / Journal of Electroanalytical Chemistry 396 (1995) 53-61 2.0
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bound to electrode interfaces and charge carriers transferred to interfacial complexes. The semiconductor material RuS 2 (energy gap Eg = 1.35 eV) has been developed [10,11] to take advantage of the low overpotential of the metallic oxide R u t 2 which at present is the most efficient electrode for oxygen evolution in the dark [12]. As a precondition it is required that photogenerated holes are transferred via ruthenium d states to initiate the reaction with water. The stepwise oxidation of attaching O H - ions leads to the formation of an interfacial ruthenium peroxo complex which eventually releases molecular oxygen. Even though high quantum efficiencies (50%-70%) could be obtained [13], the energy conversion efficiency is quite low. This is due to significant energy losses which occur when photogenerated holes are captured in the interfacial complex formed by Ru surface states interacting with water species. These surface states which actually lead to oxygen evolution are positioned quite high in the forbidden energy region, which leads to energy losses during capture of holes. Nevertheless it is remarkable that potential-assisted oxygen evolution can be obtained with near-IR light with high quantum efficiency (Fig. 3). Experience with RuS 2 (and related semiconducting transition metal sulfides) has shown that metal-centred electron transfer mechanisms leading to interfacial coordination chemical mechanisms can indeed lead to efficient oxygen evolution from water in terms of quantum efficiency. However, it appears to be very difficult to optimize energy efficiency since the energetic position of intermediates for oxygen evolution cannot easily be controlled, because it is determined by the nature of the transition metal involved as well as by the structure of the materials and their interface. It has been demonstrated that the capability for oxygen liberation from water is correlated with the density of d states that can mediate photoreactivity of holes with water [15]. Also, a semiconducting layer compound with a d band, which is conducting with photoreactive holes, PtS2, is able to photoevolve oxygen from
55
water under bias [16], as well as chalcogenides of Ir and Os. Recently, dz2 states were identified with tunnelling electron microscopy as protruding to the van der Waals interface in layered transition metal compounds [17]. This may explain why also freshly prepared MoS 2 interfaces temporarily liberate oxygen under illumination [18]. There may be some chance to find a semiconducting transition metal compound with still improved catalytic ability for photoinduced oxygen evolution. Our own research strategy is aimed at identifying semiconducting transition metal cluster compounds (e.g. Mo6_xMxSe 8 with M = Ru, x = 2) for oxygen evolution from water and other demanding multielectron transfer reactions [19]. Clusters of mixed transition metals can, first, accommodate several electrons on states which are energetically very close and, second, they could provide bimetallic adsorption and reaction sites. These cluster materials turned out to be excellent catalysts for oxygen reduction but are unfortunately unstable towards oxidation in aqueous media owing to the reactivity of molybdenum. Present research aimed at identifying clusters with other elements shows that this strategy is not easy to pursue owing to the complicated material research involved. However, without some delocalization of d electrons it will not be easy to have the potentials of all intermediates aligned close above the valence band for energetically efficient hole capture and transfer during water oxidation. It is interesting to mention that semiconductor materials which have until now been found to be active for oxygen evolution (e.g. T i t 2, RuS 2) are either thermodynamically or kinetically quite stable. Water-oxidizing materials sensitive for potential-assisted oxygen evolution must contain transition metals which reach sufficiently high oxidation states. They react photoelectrochemically with water and restore their interface subsequently by releasing oxygen. The order required for creating a reasonably favourable activation complex for oxygen evolution comes from the specific interfacial reactivity and the interfacial structure of these materials. In other words, the activation complexes are a consequence of interfacial reactivity of transition metal centres. They increase their oxidation state as a consequence of hole capture which leads to the attachment and oxidation of water species.
3. O x y g e n evolution in p h o t o s y n t h e s i s
Photosynthetic oxygen evolution [20,21], which has always been the inspiring example for all efforts towards artificial solar fuel production, is equally described by the electrochemical reaction (1) where the oxidizing redox equivalent is provided by the photosynthetic reaction centre PS 1I. There is evidence that photosynthetic oxygen evolution is extremely well catalysed with nearly negligible overpotential [22]. A large amount of work has led to
56
H. Tributsch, L. Pohlmann/ Journal of Electroanalytical Chemistry396 (1995) 53-61
PROTEN I
J
Ill
Z gl O 2
4
6
FLASH
8
16
NUMBER
Fig. 4. Simplifiedmodel of the oxygenevolvingphotosyntheticreaction centre (PSII) complex.The relativepositions of Mn atoms are indicated as well as oscillativeoxygenevolution.
the present concept of the function of the photosystem H reaction centre complex, which consists of the D], D 2, Cyt b559 and the ' T ' protein as shown in Fig. 4. The reaction centre P 680 donates electrons into a sequence of electron transferring systems (Pheo, QA, QB,) and receives electrons via a tyrosine residue (Z) from the manganese complex (mainly associated with D l) which is supported by a manganese-stabilizing protein which, by its presence, also improves oxygen evolution catalysis. A real puzzle for electrochemical catalysis is the structure and function of the manganese centre which contains four Mn atoms. Up to 12 ligands may be required for binding the active manganese complex. Remarkably, manganese can easily be dissolved from the complex and restituted, a process which can be followed with electron spin resonance techniques. Addition of 1 mM Mn 2÷ to manganese-deficient mutant stabilized oxygen evolution to a significant extent, while other divalent ions were inactive [23]. The oxygen evolution centre is sensitive to heat stress and only active in a pH range between 6 and 8. Extended X-ray absorption fine structure outlined the relative positions of the manganese atoms to each other and indicated O or N as the most likely candidates for both bridges or terminal ligands. It is assumed that two pairs of manganese atoms are bridged by p~-oxo bridges and linked via carboxylate groups. In spite of significant effort, no evidence for a complexed peroxide species could be found. The molecular
function of the manganese centre is today still unexplained. Four oxidation states, S~-S 4, have been identified. Only one manganese pair is active for catalysis; the other pair may play a role in electron conduction. Only part of the oxidation is metal centred. Also, amino acid ligands of the protein matrix participate in storing positive charge. The possible valence states of manganese have been discussed [24]. From the electrochemical point of view, it is remarkable that nature has adapted for oxygen evolution catalysis a transition metal, Mn, which has in no case proved a reasonable electrocatalytic ability in artificial systems. For more than two decades model systems of manganese complexes have been synthesized to simulate oxygen evolution (e.g. Ref. [25]). In no case was significant catalytic activity found. Theoretical calculations of manganese complexes even predict that no currently known or imaginable stable complex of Mn can react with water with an activation energy less than 1 eV [26]. Apparently nature did not take advantage of a transition metal with good (artificial) catalytic activity for the interaction with water species but of a transition metal which can easily change its coordination state. A highly labile, that is reactive, complex was chosen and not a stable complex as characteristic for synthetic model systems investigated for artificial oxygen evolution.
4. On the real significance of multielectron transfer There is no doubt that photosynthetic oxygen evolution is occurring near the thermodynamic potential for a fourelectron transfer reaction (at neutral pH) [22]. The process takes place during the transition from oxidation step J4 to Jl in the manganese complex. This reaction is very fast and may not easily be resolved. During this reaction step four electrons are extracted from water. They are transferred to the positive holes localized on Mn centres and amino acid ligands. However, what is providing the order for the complicated series of transition complexes required for the transfer of four electrons near the thermodynamic equilibrium potential? Alternatively, is it, in contradiction to notions of classical electron transfer theory, possible to imagine a real four-electron transfer reaction without identifiable intermediates as suggested by reaction (I)? We have investigated this question in some detail [27-30] and came to the conclusion that multielectron transfer is possible as a non-linear autocatalytic process occurring far from equilibrium. Theoretically this implies that some potential (the value may be rather small) is lost to dissipation as compared with the equilibrium situation described in relation (1). Two types of reaction mechanisms have been distinguished, which have been called "stimulated" and "cooperative" electron transfer [27]. In the case of stimulated electron transfer, the first electron transfer exerts a positive feedback on subsequent electron
H. Tributsch, L. Pohlmann/ Journal of Electroanalytical Chemistry 396 (1995) 53-61
A
> Xl .--~ X 2 - ~ X3 ---> ""Xn Q
V(M)
>B
+M
30
. . . . . . . . . . . . . . . . . . . . . . .
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Fig. 5. Synergetic roultistep electron transfer achieved by non-linear positive feedback steps within the catalytic complex (above). A synergetic four-electron transfer process can be understood in analogy to a ball rolling into a potendal dip while the electronic structure of the catalystreactant complex changes dynamically.
transfer steps to generate a largely accelerated, synchronized stepwise transfer of several electrons. However a cooperative electron transfer in the true sense of the word, that means a real multielectron transfer where the electrons have lost their independence (are enslaved, in synergetic terminology), occurs only when a sufficiently non-linear feedback between electrons is present (Fig. 5). Possible molecular mechanisms which may allow feedback between electron transfer steps have been discussed [30]. We consider experimental model systems recently presented by Anson and collaborators [31-33] as cases, which we described and predicted as stimulated electron transfer [27]. Coordination of Ru(NH3) 2+ to unsaturated ligand sites on the periphery of cobalt porphyrin rings converts two-electron to four-electron catalysts for the reduction of 02. Interestingly, the extent of d-,rr backbonding of the Ru complexes to the unsaturated ligands has been argued to be more essential for the catalytic activity than the reducing strength of the coordinated Ru(NH3)g + groups. This is supported by the finding that more weakly backbonded donor groups do not produce a four-electron reduction catalyst. This is essentially what has to be fulfilled during stimulated electron transfer. The electrons of the different coordination groups "communicate" with each other through the bonding system of the porphyrin ring. When one electron is transferred, there is an immediate feedback on the second electron transfer and so on. If this feedback is positive, the calculated stimulated electron transfer will occur as a very fast sequence of individual electron transfer steps. When a sufficiently non-linear feedback is provided, cooperative multielectron transfer may be expected. The highly catalytic transition metal cluster compounds of the composition Mo4Ru2Se 8 have also been suggested to provide possible examples for non-linear, autocatalytic electron transfer, since the transi-
57
tion metal cluster size changes with the number of electrons contained [28-30]. We propose that the catalytic complex for oxygen evolution in photosynthesis has evolved in such a way that transfer of the first electron exerts a positive non-linear feedback on the transfer of the next electron and so on. The capacity to change the coordination state easily may be a key role of the manganese centres rather than to act as efficient catalysts in the classical sense of the word. In the calculated model [29] an autocatalytically generated state M is released during a final step of electron transfer and this state by itself acts as an accelerator of all preceding steps of electron transfer (Fig. 5). If the positive feedback is sufficiently non-linear, electron transfer steps become dependent on each other; the first electron is "enslaving" the others and the transfer of charge may be a real cooperative four-electron transfer step. It is clear that this described synergetic electron transfer mechanism does not fall within the scope of Marcus theory which is a quasi-equilibrium approach based on Boltzmann statistics. Non-linear cooperative electron transfer is a synergetic process which is able to export entropy, that is to decrease entropy locally (or to increase order) at the expense of overall entropy production by the system.
5. Water photoelectrolysis as a synergetic process
The set of non-linear differential equations corresponding to the model discussed describing multielectron transfer has one eigenvalue which changes its sign if the feedback parameter exceeds a critical value. Around this point it is possible to apply the centre manifold theorem (which in the context of synergetic processes is also called the enslaving principle) leading to a reduction of the system to only one non-linear differential equation [29]. This is a synergetic order parameter equation qualitatively analogous to Hakens's laser equation. A simple mathematical transformation of this order parameter equation allows one to understand the catalytic system as a ball rolling into a potential dip (Fig. 5) [29]. The system is behaving in such a way, that while energy is flowing through, an electronic-structural rearrangement occurs while electron transfer reactions proceed in well-synchronized steps. In this way the entropy of the system is reduced locally (order is being built up) for creation of a highly structured and reactive transition complex. The mechanism can be understood as taking advantage of a self-organization process which facilitates coordinated electron transfer from water into the oxidized manganese complex. The visualization of the model depicted in Fig. 5 expresses the relaxation of the multielectron transfer process (of the photosynthetic manganese complex) into a far from equilibrium dynamic state with higher order (less entropy), which facilitates oxygen evolution.
58
H. Tributsch, L. Pohlmann/ Journal of Electroanalytical Chemistry 396 (1995) 53-61
Reinterpreting the knowledge on structure and function accumulated for the manganese complex in photosynthesis relevant for the proposed synergetic four-electron transfer from water, we can list the following arguments: - - Poorly catalytic metal centres must be applied to avoid the formation of unwanted oxidation species with water by the positive charges stored in the complex before the synergetic mechanism can set in after positive charges are being stored in the complex. Manganese is such a poor electrocatalyst. In addition, part of the stored positive charges are associated with organic ligands. - - The complex can be highly reactive and labile since order (entropy reduction) is accomplished in a dynamic way during the synergetic reaction explained above (transition from J4 to J0) (Fig. 5). - - A highly dynamic structure is required in which various coordination states are involved. Manganese can indeed assume a series of different coordination states. - - No complexed peroxide species is to be expected since we are dealing with a real four-electron transfer process (experimentally confirmed by many failed attempts to detect them).
6. O s c i l l a t i o n s in w a t e r p h o t o e l e c t r o l y s i s
Synergetic mechanisms for water splitting have until now not been developed but our group has made some efforts towards model systems which may suggest that such non-linear mechanisms are feasible. With far from equilibrium mechanisms one cannot expect to approach the theoretical energy efficiency calculated for reversible energy conversion systems. However, the kinetics should be much more favourable and an indirect energetic benefit may be obtained by taking advantage of the energetically favourable reaction potential of real multielectron transfer processes. Secondary energy dissipation may also be reduced by oscillating energy conversion behaviour, in which thermodynamic forces and fluxes are out of phase (the product of which defines energy dissipation). Recently we succeeded in demonstrating, with the semiconductor electrode CulnSe 2 in contact with an H202-containing electrolyte, that hydrogen can be evolved in a periodic reaction (Fig. 6) [34]. Periodic hydrogen evolution was clearly detectable with DEMS. With a p-type CulnSe 2 electrode the reaction can be driven as a photoassisted process. These findings show, for the first time, that hydrogen evolution from water, which in classical electrochemistry has always been treated as a reversible electrode reaction, can be obtained far from equilibrium through an autocatalytic mechanism leading to a synergetic process. The research opportunities for non-linear catalytic mechanisms are significant, since the conditions for catalysis, the conditions for forming suitable activation complexes, are very different from those in catalysis under reversible
0.0
234 <
-0.5
i
i
t/s
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i
L
i
r
223
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Fig. 6. Periodic hydrogen evolution observed with CulnSe z in contact with an H202-containing electrolyte. Shown are galvanostatic oscillations (above) and synchronous hydrogen signals (DEMS)(below) [34].
conditions. However, the theoretical challenge involved in understanding or designing such mechanisms is much larger. After suggesting a synergetic mechanism of oxygen evolution in photosynthesis we should ask whether there is any evidence for a non-linear cooperative process in thylakoid membranes. Since it is well known that non-linear systems may develop oscillations, it is reasonable to search for such phenomena. The fact is that oscillations in photosynthetic membranes have long been known. Damped oscillations in carbon dioxide assimilation, in chlorophyl a fluorescence and scattered light generation as well as in oxygen evolution (Fig. 7) are observed in whole leaves of higher plants, when a leaf exposed to an atmosphere with high CO 2 concentration is subjected suddenly to strong illumination. Similar damped oscillations are seen when strongly illuminated leaves are exposed suddenly to an atmosphere containing excess carbon dioxide [35-42]. The damped oscillations in photosynthetic systems have been explained by non-linear reactions occurring with the Calvin cycle [43,44] or by assuming a negative resistance for the proton current through the ATPase [45] in the thylakoid membrane. While this kinetic mechanism developed by Kocks and Ross is able to explain oscillations of transmembrane pH differences it does not, as the authors themselves confirm, explain oscillating changes in photochemical efficiency related to the rate of photoinduced electron transfer in photosynthesis (e.g. oscillating oxygen evolution). An interesting additional fact is that not only an external change of a parameter but also sequences of short light pulses after a dark adaptation period (Fig. 4) may induce oscillations in oxygen evolution. This phenomenon, first explored by the groups of Joliot and Kok [46] has been interpreted and is still explained as the result of a four-step linear oxidation of the manganese complex en-
H. Tributsch, L. Pohlmann / Journal of Electroanalytical Chemistry 396 (1995) 53-61 LIGHT ....
IT
59
ter change (period of approximately 1 min) and numbers of saturating flashes may be explained to arise as a consequence of synergetic oxygen catalysis in combination with reasonable boundary conditions. Considering the linear four-step model of charge accumulation by Kok et al. [46], So
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one may criticize that a fully linear model is not able to describe the genuine non-linear phenomena occurring in photosynthetic systems under certain conditions. Therefore it seems to be reasonable to assume an autocatalytic process after the absorption of the fourth photon. Here it would be possible to consider a synergetic cooperative electron (or hole) transfer process as published elsewhere [27-30] and explained in Fig. 5. However, in the photosynthetic system transfer of holes is equivalent to the liberation of protons as explained in relation (1). It is therefore possible to presuppose a non-linear kinetic mechanism involving the proton concentration during the water oxidation itself. Therefore an autocatalytic proton reaction will be assumed for synergetic oxygen evolution rather than an autocatalytic electron extraction from water (Fig. 5). In principle, however, both representations are qualitatively equivalent: hv
- > [ B +]
[B+]+p
hv
,
[C2+
]
lO
Fig. 7. Scheme of a thylakoid membrane showing photoinduced water oxidation with proton liberation as well as the proton flax through the ATPase system. Oscillations in oxygen evolution are shown.
[C2+]+p
[D3+]+p+2H20+2H I
gaged in oxygen evolution. Four positive charges are needed for the liberation of one molecule of oxygen and this is believed to be reflected in damped oscillations, separated from each other by four saturating flashes. In a sequence of light flashes a large fluctuation in the relative concentration of the intermediate oxidation states is assumed. However, oscillations in oxygen evolution would also be expected, if a synergetic mechanism of catalysis was involved, so further experimental research is necessary. The mechanism which attributes the oscillation behaviour to a negative resistance in the ATPase complex [45] could be tested using a selective toxic inhibitor. A synergetic mechanism involving oxygen evolution should cause a distinct reduction in entropy, possibly detectable with pbotocalorimetric measurements of the oxygen evolution complex. Such experiments will be attempted.
7. A photoelectrochemicai model yielding oscillations in photosynthetic oxygen evolution
The origin of oscillations in photosynthetic oxygen evolution, with respect to both time after a sudden parame-
hv ,[D3+]
~ , H +, H+
k2 , F
+ *.hV,[A]+O 2+6H + (4)
Here the species [A]... [D] denote the oxidation states of the manganese complex and p is a defect electron induced by a light quantum in reaction centre II. Here the set of linear reactions (3) is completed by an autocatalytic mechanism of water splitting, where the protons (liberated by the capture of positive electronic charges) are assumed to be the autocatalytic active species. This assumption is plausible because of the important role the protons are playing in the photosynthetic process. They close the loop of electrical current powered by photoinduced photosynthetic electron transport (Fig. 7). This means that the water oxidation (equivalent to a proton release) is functioning with a considerable velocity only in the presence of a sufficient concentration of reacting protons which may interact with the manganese centre. Consequently it is also necessary to consider a pair of processes maintaining a constant level of proton concentration in the absence of illumination, i.e. a proton recovery (e.g. through the plastocyanine) and a proton sink (e.g. through membrane transport to the outside via ATPase). It should be noted here that as a result of the water dissociation it is in principle equivalent to use the OH-ions instead of the protons.
H. Tributsch, L. Pohlmann / Journal of Electroanalytical Chemistry 396 (1995) 53-61
60 0.4
--Ra,oo, O oo. Evo,o;,o. i - -- Concentration of Protons
///- \x\ iii
/~ \
"
[
can be remarkably large (describing in principle the experimental results of Walker and coworkers [36-37]), whereas under other conditions the simulations of Kok et al. [46] using a series of short light flashes can also be reproduced by our model. In comparison with the model of Kocks and Ross [45] the model presented here is also able to describe the oscillations in the electron activity of the photosynthetic electron transfer chain itself. However, it is possible that in addition the hypothetical negative resistance in the ATPase complex proposed by Kocks and Ross also exists and contributes to the whole mechanism of water splitting. This would not destroy the oscillations of the photosystem described above, but under certain conditions it could convert damped oscillations into sustained limit cycle oscillations.
] (a)
/ -
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---
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0
20 Time in a.u.
1.0
(b)
// ¢1I
8. Conclusions and outlook
tB"l' 02
~ / _
I ! / ~ ~ ~ 0
Time in a.u,
2O
Fig, 8. Numerical solmion of the equation system (5) with a Runge-Kuua method of fourth order usint the following initial conclitions and parameter value: [A]o = 1, [B] o ---[C]o = 0, k = 5, k~ = 5, j = 0.5, [tt] o = j / k 2 = 0.1, p(t)-= 1. (a) Time dependence of the nutocmal~ic active proton
concentrationand of the rate of oxygenevolution;(b) time dependenceof the concentrationof states [A]... [B] of the manganesecomplex.
The reaction set (4) leads to the following set of kinetic equations (for the sake of simplicity here the charges at the symbols are omitted): dA --dt
pA + kpDH 2
dB -
-
=
-pB
+ pA
dt dC
dt dD -dt
= -pC+pB
(5)
= - kpDH 2 + pC
dH _ _ = 4 k p D H 2 - k E H + ¢p dt
Under certain initial conditions (if the system is in the dark-adapted state, i.e. only A or B is not equal to zero, and the illumination is switched on abruptly) this kinetic system is able to exhibit damped oscillations of the intermediate states and consequently of the rate of oxygen evolution (Fig. 8). It should be noted that the variety of time-dependent behaviour in this reaction sequence is much broader than in the usual linear model (3): depending on the parameter values the amplitude of damped oscillations
Research on artificial photocatalysts for water oxidation and research on photosynthetic oxygen evolution from water are not evolving towards a consistent understanding of photoelectrolysis of water. While artificial model systems are very slowly improving on the basis of transition metal centred semiconductor electrochemical mechanisms of very stable compounds, the photosynthetic manganese catalyst turns out to be a highly fragile complex, arranged around electrocatalytically largely inert (in artificial systems) manganese centres. The solution to the contradiction may be that technical-scientific development has been persuing a multistep photo-oxidation of water in an interfacial reaction while nature has evolved a synergetic real four-electron transfer mechanism, where temporary order is generated through autocatalysis in a transient synergetic process. We succeeded in proposing the first synergetic model of photosynthetic oxygen evolution. It is interesting to note that the features of the manganese complex discussed are also characteristic for the elements which are responsible for the occurrence of the well-known examples of homogeneous chemical oscillations (Br, J and S in the Belousov-Zhabotinsky reaction, the iodate-iodine reaction and the thiosulphate reaction respectively): these elements are very reactive and exhibit a broad range of oxidation states as also known for the manganese chemistry. It was shown in this article that synergetic or cooperative electron transfer is theoretically possible and that the catalytic manganese centre involved would transform itself into a state of decreased entropy to catalyse the complicated process. The manganese thus plays the role of holding back the oxidative reaction with water, until, when the autocatalytic reaction starts, the synergetic four-electron transfer proceeds. Here, the manganese may again play a significant role in mediating the conformational changes. A process of oscillating hydrogen evolution was discussed as evidence that synergetic photoinduced fuel gen-
H. Tributsch, L Pohlraann/ Journal of Electroanalytical Chemistry 396 (1995) 53-61
eration may in principle be possible. The presence of oscillations in photosynthetic membranes, with respect to both time and numbers of saturating flashes, may prove to be the result of a synergetic process of oxygen evolution and not the consequence of a side phenomenon (e.g. a negative resistance across the ATPase system). The proposed autocatalytic reaction of the manganese complex during oxygen evolution is qualitatively able to explain the oscillating properties of the photosynthetic electron transfer chain. Experiments will now have to be made to test and improve the outlined hypothesis. Asking the right questions is important for progress in understanding oxygen evolution in photosynthesis. Research in the direction of artificial water splitting will benefit in the long term from the understanding of the biological mechanism. It will be especially important to find out whether the oxygen evolution strategy applied in nature is very different from that pursued during the last two decades in photoelectrochemical research. If our hypothesis is right, it is not surprising that photosynthetic oxygen evolution, the basis of higher life on earth, is unique and so strongly resists elucidation. We are dealing with non-linear synergetic photoelectrochemistry, a field which we still have to develop, for which we only have few examples (e.g. experiment of Fig. 6). The questions that have been asked after the experimental demonstration of UV photoelectrolysis with TiO 2 two and a half decades ago have led us to many answers relevant for the problem of artificial water electrolysis using solar light, but we have to continue asking many questions to learn how to handle such a process of fundamental significance. References [1] A. Fujishima and K. Honda, Nature (London), 238 (1972) 37. [2] P. Bogdanoff and N. Alonso-Vante, Ber.Bunsenges. Phys. Chem., 97 (1993) 940. [3] A.J. Nozic, Philos. Trans. R.Soc.London, Set. A, 295 (1980) 453. [4] R. Memming, Electrochim. Acta, 25 (1980) 77. [5] H. Gerischer, in R. van Overstraaten and W. Palz (Eds.), Proc. 2nd EC Photovoltaic Solar Energy Conf., Reidel, Dordrecht, 1979 p. 408. [6] G. Calzaferri, N. Gfeller and K. Pfanner, J. Photochem. Photobiol. A.: Chem., 87 (1995) 81. [7] M. Graetzel, in R.E. White, J.O'M. Bockris and B.E. Conway (Eds.), Modern Aspects of Electrochemistry, Vol. 15, Plenum New York, 1983, p. 83. [8] A. Heller, Acc. Chem. Res., 14 (1981) 154. [9] Y. Nakato, S. Tonomura and H. Tsubomura, Ber. Bunsenges. Phys. Chem., 80 (1976) 1289. [10] H. Ezzaounia, R. Heindl, R. Parsons and H. Tributsch, J. Electroanal. Chem., 145 (1983) 279.
6l
[11] H.-M. Kuehne and H. Tributsch, Ber. Bunsenges. Phys. Chem., 83 (1979) 655. [12] S. Trasatti, in Electrochemistry of Novel Materials, VCH, New York, 1994, p. 207. [13] H. Colell, N. Alonso-Vante and H. Tributsch, J. Electroanal Chem., 324 (1992) 127. [14] P. Bogdanoff, Hahn-Meitner-lnstitut, unpublished results. [15] M. Kuehne, W. Jaegermann and H. Tributsch, Chem. Phys. Lett., 112 (1984) 160. [16] H. Tributsch and O. Gorochov, Electrochim. Acta, 27 (1981) 215. [17] D. Haneman and H. Tributsch, Chem. Phys. Lett., 216 (1993) 81. [18] H. Tributsch, Z. Naturforsch., 32 (1977) 972. [19] N. Alonso-Vante and H. Tributsch, in J. Lipkowsky and Ph. N. Ross (Eds.), Electrochemistry of New Materials; Frontiers of Electrochemistry, VCH, New York, 1994 p. I. [20] A. Boussac, L. Zimmermann and A.W. Rutherford, in J. Barber (Ed.), The Photosystems. Structure, Function and Molecular Biology, Elsevier, Amsterdam, 1992, p. 187. [21] G. Renger, Chemie Unserer Zeit, 28 (1994) 118. [22] T. Watanabe, M. Kobayashi and T. Sagara, 8th Int. Cong. on Photosynthesis, Stockholm 1989, abstract and poster; Physiol. plant., 76 (3, part 2) (1990) A147. [23] W.F.J. Vermaas, J. Charite and G. Shen, Biochemistry, 29 (1990) 5325. [24] G.T. Babcock, in J. Amesz, (Ed.) Photosynthesis, Elsevier, Amsterdam, 1987, p. 125. [25] K. Wieghardt, Angew. Chem., 101 (1989) 11790. [26] D.A. Proserpio, personal communication; 10th Int. Conf. on Photochemical Conversion and Storage of Solar Energy (IPS-10), Interlaken, August 1994. [27] L. Pohlmann and H. Tributsch, J. Theor. Biol., 155 (1992) 443; 156 (1992) 63. [28] H. Tributsch and L. Pohlmann, Chem. Phys. Lett., 188 (1992) 63. [29] L. Pohlmann and H. Tributsch, in R.K. Mishra, D. MaaB and E. Zwierlein (Eds.) On Self-Organization, Vol. 61, Springer, Berlin, 1994, p. 133. [30] H. Tributsch, J. Electrochim. Acta, 39 (1993) 1495. [31] B. Steiger and F.C. Anson, lnorg. Chem., 33 (1994) 5767. [32] C. Shi and F.C. Anson, Inorg. Chim. Acta, 225 (1994) 215. [33] B. Steiger, C. Shi and F.C. Anson, Inorg. Chem., 32 (1993) 2107. [34] L. Pohlmann, G. Neher and H. Tributsch, J. Phys. Chem., 98 (1994) 11007. [35] H. Kautsky and A. Hirsch, Naturwissenschaften, 19 (1931) 964. [36] D.A. Walker, Photosynth. Res, 34 (1992) 387. [37] D.A. Walker, New Phytol., 121 (1992) 325, [38] D.R. Keller and D.A. Walker, Proc. R. Soc. London, Ser. B, 241 (1990) 59. [39] M.N. Sivak and D.A. Walker, Proc. R. Soc. London, Ser. B, 217 (1983) 377. [40] T. Ogawa, Biochim. Biophys. Acta, 681 (1982) 103. [41] A. Laisk, K. Siebke, U.M. Gerst, H. Eichelmann, V. Oja and U. Heber, Planta, 185 (1991) 554. [42] R.T. Furbank and C.H. Foyer, Arch. Biochem. Biophys., 246 (1986) 240. [43] A. Laisk and D.A. Walker, Proc. R. Soc. London, Ser. B, 227 (1986) 281. [44] C. Giersch, Arch. Biochem. Biophys., 245 (1986) 263. [45] P. Kocks and J. Ross, Biochim. Biophys. Acta, (1994). [46] B. Kok, B. Forbush and M. McGIoin, Photochem. Photobiol., 11 (1970) 457.
ELSEVIER
Journal of Electroanalytical Chemistry 396 (1995) 53-61
Synergetic molecular approaches towards artificial and photosynthetic water photoelectrolysis 1 Helmut Tributsch, Ludwig Pohlmann Hahn-Meitner-lnstitut, Dept. Solare Energetik, 14109 Berlin, Germany
Received 6 April 1995
Abstract 25 years after the discovery of UV photoelectmlysis of water with T i t 2 by Fujishima and Honda, which triggered high expectations with respect to water splitting using visible light, an evaluation of progress is attempted. On the basis of an analysis of existing electrocatalysts for oxygen evolution alternative reaction possibilities are compared: one-electron transfer, four-electron transfer and the role of intermediates chemically bonded to a catalyst. For the manganese complex of photosynthetic oxygen evolution the puzzling situation is that electrocatalytically poorly active Mn metal centres in a fragile complex are highly catalytic for oxygen evolution. The conclusion is reached that the complex organizes itself into a favourable transition state through an autocatalytic process (export of entropy) that facilities a real (intermediate-free) four-electron transfer. An experimental model system for oscillating hydrogen evolution is discussed to demonstrate the feasibility of synergetic reactions for fuel generation. The energetic and kinetic advantages of a synergetic mechanism of water splitting are outlined and a kinetic model is calculated which simulates oscillations observed in photosynthetic oxygen evolution. Keywords: Water; Photoelectrolysis;Synergy
1. The challenge of water photoelectrolysis Scientists have been puzzling for decades over photoinduced oxygen evolution in photosynthesis without having a realistic molecular clue, when after 1970 it became known that Fujishima and Honda succeeded in splitting water using illuminated T i t 2 electrodes [1]. Fig. 1 shows a photocurrent voltage curve typical for T i t 2 and oxygen evolution detected with differential electrochemical mass spectroscopy (DEMS). At that time it was not so important that UV light was needed for photoelectrolysis as well as a small supporting potential. This remarkable photoelectrochemical process stimulated the imagination of scientists and their hope that splitting water using visible light would just be a matter of systematic research. An incredible scientific dynamic developed in the field of photoelectrochemistry, which was carded along by the first energy crisis and an honest interest in energy matters. In the decade which followed some of the most important progress in photoelectrochemistry was witnessed. The fundamental
energetic, thermodynamic and kinetic concepts were developed, hundreds of photoactive materials were investigated and dedicated efforts were developed to extend the sensitivity of T i t 2 into the visible spectral region. Now, 25 years after the discovery of T i t 2 photoelectrolysis, this UV-absorbing material is still fascinating to scientists. In particular, in the laboratory of Fujishima
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H. Tributsch, L Pohlmann/ Journal of Electroanalytical Chemistry 396 (1995) 53-61
54
several practical applications for its properties have been developed. In quite a few international laboratories TiO 2 particle suspensions are applied successfully for oxidizing and neutralizing toxic chemicals. However, what happened to the hopes for energy-efficient solar photoelectrolysis of water? The problem is not yet solved. Most research groups that had once concentrated their efforts on this fascinating problem have chosen new subjects. What is the scientific background for the complications encountered?
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2. Options for oxygen evolution mechanisms Reversible thermodynamics opens basically two pathways for water photoelectrolysis determined by electrochemical reaction mechanisms requiring less than 3 V of electrode potential. Since oxygen is a gas and water a liquid the conditions basically require that the electrodes involved are inert, only exchanging electrons. The most favourable pathway is the well-known reaction (in acid solution, h = positive hole) 2H:O + 4h ~ O 2 + 4H + E 0 = 1.229 V(1 M acid solution)
(1)
involving the simultaneous transfer of four holes (equivalent to the transfer of four electrons into the opposite direction). The second possibility is a succession of onehole-transfer reactions of the type H20+h~OH'+H
+
E0 = 2.85 V
(2)
where OH • radicals continue to react to liberate molecular oxygen. Photoelectrochemical theory requires that illumination in a semiconductor interface creates a Gibbs energy level (quasi-Fermi energy level) for minority charge carders sufficient to run one of the above reactions (Fig. 2). Significant complications arise since we do not have ideally inert oxygen electrodes, since classical electron transfer theory does not consider possible the simultaneous transfer of several electrons, and since intermediate products formed will temporarily bind to the electrode interface thereby affecting the reaction potentials and the electrode material properties involved. In fact we know now that oxygen evolution via TiO 2 is possible because this material with an energy gap of 3 eV facilitates reaction path (2) with the limitation that the generated radical immediately binds to the interface. The reason why TiO 2 photoelectrolysis of water did not succeed in pioneering visible light photoelectrolysis of water is simply that the very different reaction path (1) would have had to be activated for that purpose. As amply discussed in photoelectrochemical literature (e.g. Refs. [3-5]), in order to drive the most favourable reaction (1) and to overcome the expected overpotentials a semiconductor with an energy gap of 2.1 eV would be required. A significant overpotential for
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Fig. 2. Energy scheme, based on the materials TiO 2 and RuS2, indicating
the four-electronwater oxidation reaction, the one-electronwater oxidation reaction and successive electron transfer steps via intermediates in which speciesof water oxidationare bound to an interfacialcatalyst.
oxygen evolution has to be considered since a real multielectron transfer is not expected to occur in classical electrochemistry. In fact, it is assumed that intermediate products will be formed which bind to the electrode surface and the foremost aim in improving efficiency of catalysis should be to find a catalyst which can bind these intermediates of oxygen evolution in such a way as to provide an overall reaction potential as close as possible to the ideal reaction potential of mechanism (1). The reaction potential of such a stepwise-catalysed reaction will typically be more positive than the potential for the theoretical four-electron oxygen evolution reaction from water. Many efforts have been undertaken to obtain a visiblelight-induced splitting of water into oxygen and hydrogen. Numerous inorganic and organic photoactive materials were tested, often additionally modified with catalysts. We would especially like to call attention to the efforts developed by Calzaferri and collaborators with zeolites encaging silver species, which are able to liberate oxygen [6], We also should mention the work of Graetzel and coworkers (for a review see Ref. [7]) who claimed to have accomplished the reaction with a sensitization of TiO 2 via a ruthenium complex and RuO 2 as catalyst. Their results have, however, apparently never been confirmed by groups who have tried to verify their data. Photoinduced hydrogen evolution itself has, on the contrary, never been a major problem. Classical p-type semiconductors, covered with islands of noble metals, can quite easily be induced to liberate hydrogen under illumination [8,9]. Hydrogen evolution alone (and using a sacrificial electron donor) does not solve the problem of water splitring. The real problem is the catalysis of water oxidation near the potential of the multielectron transfer process (1) of oxygen evolution. This requires that water species are
H. Tributsch, L. Pohlmann / Journal of Electroanalytical Chemistry 396 (1995) 53-61 2.0
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bound to electrode interfaces and charge carriers transferred to interfacial complexes. The semiconductor material RuS 2 (energy gap Eg = 1.35 eV) has been developed [10,11] to take advantage of the low overpotential of the metallic oxide R u t 2 which at present is the most efficient electrode for oxygen evolution in the dark [12]. As a precondition it is required that photogenerated holes are transferred via ruthenium d states to initiate the reaction with water. The stepwise oxidation of attaching O H - ions leads to the formation of an interfacial ruthenium peroxo complex which eventually releases molecular oxygen. Even though high quantum efficiencies (50%-70%) could be obtained [13], the energy conversion efficiency is quite low. This is due to significant energy losses which occur when photogenerated holes are captured in the interfacial complex formed by Ru surface states interacting with water species. These surface states which actually lead to oxygen evolution are positioned quite high in the forbidden energy region, which leads to energy losses during capture of holes. Nevertheless it is remarkable that potential-assisted oxygen evolution can be obtained with near-IR light with high quantum efficiency (Fig. 3). Experience with RuS 2 (and related semiconducting transition metal sulfides) has shown that metal-centred electron transfer mechanisms leading to interfacial coordination chemical mechanisms can indeed lead to efficient oxygen evolution from water in terms of quantum efficiency. However, it appears to be very difficult to optimize energy efficiency since the energetic position of intermediates for oxygen evolution cannot easily be controlled, because it is determined by the nature of the transition metal involved as well as by the structure of the materials and their interface. It has been demonstrated that the capability for oxygen liberation from water is correlated with the density of d states that can mediate photoreactivity of holes with water [15]. Also, a semiconducting layer compound with a d band, which is conducting with photoreactive holes, PtS2, is able to photoevolve oxygen from
55
water under bias [16], as well as chalcogenides of Ir and Os. Recently, dz2 states were identified with tunnelling electron microscopy as protruding to the van der Waals interface in layered transition metal compounds [17]. This may explain why also freshly prepared MoS 2 interfaces temporarily liberate oxygen under illumination [18]. There may be some chance to find a semiconducting transition metal compound with still improved catalytic ability for photoinduced oxygen evolution. Our own research strategy is aimed at identifying semiconducting transition metal cluster compounds (e.g. Mo6_xMxSe 8 with M = Ru, x = 2) for oxygen evolution from water and other demanding multielectron transfer reactions [19]. Clusters of mixed transition metals can, first, accommodate several electrons on states which are energetically very close and, second, they could provide bimetallic adsorption and reaction sites. These cluster materials turned out to be excellent catalysts for oxygen reduction but are unfortunately unstable towards oxidation in aqueous media owing to the reactivity of molybdenum. Present research aimed at identifying clusters with other elements shows that this strategy is not easy to pursue owing to the complicated material research involved. However, without some delocalization of d electrons it will not be easy to have the potentials of all intermediates aligned close above the valence band for energetically efficient hole capture and transfer during water oxidation. It is interesting to mention that semiconductor materials which have until now been found to be active for oxygen evolution (e.g. T i t 2, RuS 2) are either thermodynamically or kinetically quite stable. Water-oxidizing materials sensitive for potential-assisted oxygen evolution must contain transition metals which reach sufficiently high oxidation states. They react photoelectrochemically with water and restore their interface subsequently by releasing oxygen. The order required for creating a reasonably favourable activation complex for oxygen evolution comes from the specific interfacial reactivity and the interfacial structure of these materials. In other words, the activation complexes are a consequence of interfacial reactivity of transition metal centres. They increase their oxidation state as a consequence of hole capture which leads to the attachment and oxidation of water species.
3. O x y g e n evolution in p h o t o s y n t h e s i s
Photosynthetic oxygen evolution [20,21], which has always been the inspiring example for all efforts towards artificial solar fuel production, is equally described by the electrochemical reaction (1) where the oxidizing redox equivalent is provided by the photosynthetic reaction centre PS 1I. There is evidence that photosynthetic oxygen evolution is extremely well catalysed with nearly negligible overpotential [22]. A large amount of work has led to
56
H. Tributsch, L. Pohlmann/ Journal of Electroanalytical Chemistry396 (1995) 53-61
PROTEN I
J
Ill
Z gl O 2
4
6
FLASH
8
16
NUMBER
Fig. 4. Simplifiedmodel of the oxygenevolvingphotosyntheticreaction centre (PSII) complex.The relativepositions of Mn atoms are indicated as well as oscillativeoxygenevolution.
the present concept of the function of the photosystem H reaction centre complex, which consists of the D], D 2, Cyt b559 and the ' T ' protein as shown in Fig. 4. The reaction centre P 680 donates electrons into a sequence of electron transferring systems (Pheo, QA, QB,) and receives electrons via a tyrosine residue (Z) from the manganese complex (mainly associated with D l) which is supported by a manganese-stabilizing protein which, by its presence, also improves oxygen evolution catalysis. A real puzzle for electrochemical catalysis is the structure and function of the manganese centre which contains four Mn atoms. Up to 12 ligands may be required for binding the active manganese complex. Remarkably, manganese can easily be dissolved from the complex and restituted, a process which can be followed with electron spin resonance techniques. Addition of 1 mM Mn 2÷ to manganese-deficient mutant stabilized oxygen evolution to a significant extent, while other divalent ions were inactive [23]. The oxygen evolution centre is sensitive to heat stress and only active in a pH range between 6 and 8. Extended X-ray absorption fine structure outlined the relative positions of the manganese atoms to each other and indicated O or N as the most likely candidates for both bridges or terminal ligands. It is assumed that two pairs of manganese atoms are bridged by p~-oxo bridges and linked via carboxylate groups. In spite of significant effort, no evidence for a complexed peroxide species could be found. The molecular
function of the manganese centre is today still unexplained. Four oxidation states, S~-S 4, have been identified. Only one manganese pair is active for catalysis; the other pair may play a role in electron conduction. Only part of the oxidation is metal centred. Also, amino acid ligands of the protein matrix participate in storing positive charge. The possible valence states of manganese have been discussed [24]. From the electrochemical point of view, it is remarkable that nature has adapted for oxygen evolution catalysis a transition metal, Mn, which has in no case proved a reasonable electrocatalytic ability in artificial systems. For more than two decades model systems of manganese complexes have been synthesized to simulate oxygen evolution (e.g. Ref. [25]). In no case was significant catalytic activity found. Theoretical calculations of manganese complexes even predict that no currently known or imaginable stable complex of Mn can react with water with an activation energy less than 1 eV [26]. Apparently nature did not take advantage of a transition metal with good (artificial) catalytic activity for the interaction with water species but of a transition metal which can easily change its coordination state. A highly labile, that is reactive, complex was chosen and not a stable complex as characteristic for synthetic model systems investigated for artificial oxygen evolution.
4. On the real significance of multielectron transfer There is no doubt that photosynthetic oxygen evolution is occurring near the thermodynamic potential for a fourelectron transfer reaction (at neutral pH) [22]. The process takes place during the transition from oxidation step J4 to Jl in the manganese complex. This reaction is very fast and may not easily be resolved. During this reaction step four electrons are extracted from water. They are transferred to the positive holes localized on Mn centres and amino acid ligands. However, what is providing the order for the complicated series of transition complexes required for the transfer of four electrons near the thermodynamic equilibrium potential? Alternatively, is it, in contradiction to notions of classical electron transfer theory, possible to imagine a real four-electron transfer reaction without identifiable intermediates as suggested by reaction (I)? We have investigated this question in some detail [27-30] and came to the conclusion that multielectron transfer is possible as a non-linear autocatalytic process occurring far from equilibrium. Theoretically this implies that some potential (the value may be rather small) is lost to dissipation as compared with the equilibrium situation described in relation (1). Two types of reaction mechanisms have been distinguished, which have been called "stimulated" and "cooperative" electron transfer [27]. In the case of stimulated electron transfer, the first electron transfer exerts a positive feedback on subsequent electron
H. Tributsch, L. Pohlmann/ Journal of Electroanalytical Chemistry 396 (1995) 53-61
A
> Xl .--~ X 2 - ~ X3 ---> ""Xn Q
V(M)
>B
+M
30
. . . . . . . . . . . . . . . . . . . . . . .
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20
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0
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.
.
.
.
.
.
.
.
.
.
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.
.
.
.
.
.
.
.
.
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Fig. 5. Synergetic roultistep electron transfer achieved by non-linear positive feedback steps within the catalytic complex (above). A synergetic four-electron transfer process can be understood in analogy to a ball rolling into a potendal dip while the electronic structure of the catalystreactant complex changes dynamically.
transfer steps to generate a largely accelerated, synchronized stepwise transfer of several electrons. However a cooperative electron transfer in the true sense of the word, that means a real multielectron transfer where the electrons have lost their independence (are enslaved, in synergetic terminology), occurs only when a sufficiently non-linear feedback between electrons is present (Fig. 5). Possible molecular mechanisms which may allow feedback between electron transfer steps have been discussed [30]. We consider experimental model systems recently presented by Anson and collaborators [31-33] as cases, which we described and predicted as stimulated electron transfer [27]. Coordination of Ru(NH3) 2+ to unsaturated ligand sites on the periphery of cobalt porphyrin rings converts two-electron to four-electron catalysts for the reduction of 02. Interestingly, the extent of d-,rr backbonding of the Ru complexes to the unsaturated ligands has been argued to be more essential for the catalytic activity than the reducing strength of the coordinated Ru(NH3)g + groups. This is supported by the finding that more weakly backbonded donor groups do not produce a four-electron reduction catalyst. This is essentially what has to be fulfilled during stimulated electron transfer. The electrons of the different coordination groups "communicate" with each other through the bonding system of the porphyrin ring. When one electron is transferred, there is an immediate feedback on the second electron transfer and so on. If this feedback is positive, the calculated stimulated electron transfer will occur as a very fast sequence of individual electron transfer steps. When a sufficiently non-linear feedback is provided, cooperative multielectron transfer may be expected. The highly catalytic transition metal cluster compounds of the composition Mo4Ru2Se 8 have also been suggested to provide possible examples for non-linear, autocatalytic electron transfer, since the transi-
57
tion metal cluster size changes with the number of electrons contained [28-30]. We propose that the catalytic complex for oxygen evolution in photosynthesis has evolved in such a way that transfer of the first electron exerts a positive non-linear feedback on the transfer of the next electron and so on. The capacity to change the coordination state easily may be a key role of the manganese centres rather than to act as efficient catalysts in the classical sense of the word. In the calculated model [29] an autocatalytically generated state M is released during a final step of electron transfer and this state by itself acts as an accelerator of all preceding steps of electron transfer (Fig. 5). If the positive feedback is sufficiently non-linear, electron transfer steps become dependent on each other; the first electron is "enslaving" the others and the transfer of charge may be a real cooperative four-electron transfer step. It is clear that this described synergetic electron transfer mechanism does not fall within the scope of Marcus theory which is a quasi-equilibrium approach based on Boltzmann statistics. Non-linear cooperative electron transfer is a synergetic process which is able to export entropy, that is to decrease entropy locally (or to increase order) at the expense of overall entropy production by the system.
5. Water photoelectrolysis as a synergetic process
The set of non-linear differential equations corresponding to the model discussed describing multielectron transfer has one eigenvalue which changes its sign if the feedback parameter exceeds a critical value. Around this point it is possible to apply the centre manifold theorem (which in the context of synergetic processes is also called the enslaving principle) leading to a reduction of the system to only one non-linear differential equation [29]. This is a synergetic order parameter equation qualitatively analogous to Hakens's laser equation. A simple mathematical transformation of this order parameter equation allows one to understand the catalytic system as a ball rolling into a potential dip (Fig. 5) [29]. The system is behaving in such a way, that while energy is flowing through, an electronic-structural rearrangement occurs while electron transfer reactions proceed in well-synchronized steps. In this way the entropy of the system is reduced locally (order is being built up) for creation of a highly structured and reactive transition complex. The mechanism can be understood as taking advantage of a self-organization process which facilitates coordinated electron transfer from water into the oxidized manganese complex. The visualization of the model depicted in Fig. 5 expresses the relaxation of the multielectron transfer process (of the photosynthetic manganese complex) into a far from equilibrium dynamic state with higher order (less entropy), which facilitates oxygen evolution.
58
H. Tributsch, L. Pohlmann/ Journal of Electroanalytical Chemistry 396 (1995) 53-61
Reinterpreting the knowledge on structure and function accumulated for the manganese complex in photosynthesis relevant for the proposed synergetic four-electron transfer from water, we can list the following arguments: - - Poorly catalytic metal centres must be applied to avoid the formation of unwanted oxidation species with water by the positive charges stored in the complex before the synergetic mechanism can set in after positive charges are being stored in the complex. Manganese is such a poor electrocatalyst. In addition, part of the stored positive charges are associated with organic ligands. - - The complex can be highly reactive and labile since order (entropy reduction) is accomplished in a dynamic way during the synergetic reaction explained above (transition from J4 to J0) (Fig. 5). - - A highly dynamic structure is required in which various coordination states are involved. Manganese can indeed assume a series of different coordination states. - - No complexed peroxide species is to be expected since we are dealing with a real four-electron transfer process (experimentally confirmed by many failed attempts to detect them).
6. O s c i l l a t i o n s in w a t e r p h o t o e l e c t r o l y s i s
Synergetic mechanisms for water splitting have until now not been developed but our group has made some efforts towards model systems which may suggest that such non-linear mechanisms are feasible. With far from equilibrium mechanisms one cannot expect to approach the theoretical energy efficiency calculated for reversible energy conversion systems. However, the kinetics should be much more favourable and an indirect energetic benefit may be obtained by taking advantage of the energetically favourable reaction potential of real multielectron transfer processes. Secondary energy dissipation may also be reduced by oscillating energy conversion behaviour, in which thermodynamic forces and fluxes are out of phase (the product of which defines energy dissipation). Recently we succeeded in demonstrating, with the semiconductor electrode CulnSe 2 in contact with an H202-containing electrolyte, that hydrogen can be evolved in a periodic reaction (Fig. 6) [34]. Periodic hydrogen evolution was clearly detectable with DEMS. With a p-type CulnSe 2 electrode the reaction can be driven as a photoassisted process. These findings show, for the first time, that hydrogen evolution from water, which in classical electrochemistry has always been treated as a reversible electrode reaction, can be obtained far from equilibrium through an autocatalytic mechanism leading to a synergetic process. The research opportunities for non-linear catalytic mechanisms are significant, since the conditions for catalysis, the conditions for forming suitable activation complexes, are very different from those in catalysis under reversible
0.0
234 <
-0.5
i
i
t/s
J
i
L
i
r
223
--,.-
Fig. 6. Periodic hydrogen evolution observed with CulnSe z in contact with an H202-containing electrolyte. Shown are galvanostatic oscillations (above) and synchronous hydrogen signals (DEMS)(below) [34].
conditions. However, the theoretical challenge involved in understanding or designing such mechanisms is much larger. After suggesting a synergetic mechanism of oxygen evolution in photosynthesis we should ask whether there is any evidence for a non-linear cooperative process in thylakoid membranes. Since it is well known that non-linear systems may develop oscillations, it is reasonable to search for such phenomena. The fact is that oscillations in photosynthetic membranes have long been known. Damped oscillations in carbon dioxide assimilation, in chlorophyl a fluorescence and scattered light generation as well as in oxygen evolution (Fig. 7) are observed in whole leaves of higher plants, when a leaf exposed to an atmosphere with high CO 2 concentration is subjected suddenly to strong illumination. Similar damped oscillations are seen when strongly illuminated leaves are exposed suddenly to an atmosphere containing excess carbon dioxide [35-42]. The damped oscillations in photosynthetic systems have been explained by non-linear reactions occurring with the Calvin cycle [43,44] or by assuming a negative resistance for the proton current through the ATPase [45] in the thylakoid membrane. While this kinetic mechanism developed by Kocks and Ross is able to explain oscillations of transmembrane pH differences it does not, as the authors themselves confirm, explain oscillating changes in photochemical efficiency related to the rate of photoinduced electron transfer in photosynthesis (e.g. oscillating oxygen evolution). An interesting additional fact is that not only an external change of a parameter but also sequences of short light pulses after a dark adaptation period (Fig. 4) may induce oscillations in oxygen evolution. This phenomenon, first explored by the groups of Joliot and Kok [46] has been interpreted and is still explained as the result of a four-step linear oxidation of the manganese complex en-
H. Tributsch, L. Pohlmann / Journal of Electroanalytical Chemistry 396 (1995) 53-61 LIGHT ....
IT
59
ter change (period of approximately 1 min) and numbers of saturating flashes may be explained to arise as a consequence of synergetic oxygen catalysis in combination with reasonable boundary conditions. Considering the linear four-step model of charge accumulation by Kok et al. [46], So
hv
hv , S ~ +
,S~
hv ' 533 +
hv
'
S44+
'->50+02
(3)
IG
oZ
%.//t.1 I,,1[11
RI gg
~ V I,,/I,.1~1 I Ildi~l
[A]+p
i TIME I MIN
5
one may criticize that a fully linear model is not able to describe the genuine non-linear phenomena occurring in photosynthetic systems under certain conditions. Therefore it seems to be reasonable to assume an autocatalytic process after the absorption of the fourth photon. Here it would be possible to consider a synergetic cooperative electron (or hole) transfer process as published elsewhere [27-30] and explained in Fig. 5. However, in the photosynthetic system transfer of holes is equivalent to the liberation of protons as explained in relation (1). It is therefore possible to presuppose a non-linear kinetic mechanism involving the proton concentration during the water oxidation itself. Therefore an autocatalytic proton reaction will be assumed for synergetic oxygen evolution rather than an autocatalytic electron extraction from water (Fig. 5). In principle, however, both representations are qualitatively equivalent: hv
- > [ B +]
[B+]+p
hv
,
[C2+
]
lO
Fig. 7. Scheme of a thylakoid membrane showing photoinduced water oxidation with proton liberation as well as the proton flax through the ATPase system. Oscillations in oxygen evolution are shown.
[C2+]+p
[D3+]+p+2H20+2H I
gaged in oxygen evolution. Four positive charges are needed for the liberation of one molecule of oxygen and this is believed to be reflected in damped oscillations, separated from each other by four saturating flashes. In a sequence of light flashes a large fluctuation in the relative concentration of the intermediate oxidation states is assumed. However, oscillations in oxygen evolution would also be expected, if a synergetic mechanism of catalysis was involved, so further experimental research is necessary. The mechanism which attributes the oscillation behaviour to a negative resistance in the ATPase complex [45] could be tested using a selective toxic inhibitor. A synergetic mechanism involving oxygen evolution should cause a distinct reduction in entropy, possibly detectable with pbotocalorimetric measurements of the oxygen evolution complex. Such experiments will be attempted.
7. A photoelectrochemicai model yielding oscillations in photosynthetic oxygen evolution
The origin of oscillations in photosynthetic oxygen evolution, with respect to both time after a sudden parame-
hv ,[D3+]
~ , H +, H+
k2 , F
+ *.hV,[A]+O 2+6H + (4)
Here the species [A]... [D] denote the oxidation states of the manganese complex and p is a defect electron induced by a light quantum in reaction centre II. Here the set of linear reactions (3) is completed by an autocatalytic mechanism of water splitting, where the protons (liberated by the capture of positive electronic charges) are assumed to be the autocatalytic active species. This assumption is plausible because of the important role the protons are playing in the photosynthetic process. They close the loop of electrical current powered by photoinduced photosynthetic electron transport (Fig. 7). This means that the water oxidation (equivalent to a proton release) is functioning with a considerable velocity only in the presence of a sufficient concentration of reacting protons which may interact with the manganese centre. Consequently it is also necessary to consider a pair of processes maintaining a constant level of proton concentration in the absence of illumination, i.e. a proton recovery (e.g. through the plastocyanine) and a proton sink (e.g. through membrane transport to the outside via ATPase). It should be noted here that as a result of the water dissociation it is in principle equivalent to use the OH-ions instead of the protons.
H. Tributsch, L. Pohlmann / Journal of Electroanalytical Chemistry 396 (1995) 53-61
60 0.4
--Ra,oo, O oo. Evo,o;,o. i - -- Concentration of Protons
///- \x\ iii
/~ \
"
[
can be remarkably large (describing in principle the experimental results of Walker and coworkers [36-37]), whereas under other conditions the simulations of Kok et al. [46] using a series of short light flashes can also be reproduced by our model. In comparison with the model of Kocks and Ross [45] the model presented here is also able to describe the oscillations in the electron activity of the photosynthetic electron transfer chain itself. However, it is possible that in addition the hypothetical negative resistance in the ATPase complex proposed by Kocks and Ross also exists and contributes to the whole mechanism of water splitting. This would not destroy the oscillations of the photosystem described above, but under certain conditions it could convert damped oscillations into sustained limit cycle oscillations.
] (a)
/ -
i I
---
0.1
0
20 Time in a.u.
1.0
(b)
// ¢1I
8. Conclusions and outlook
tB"l' 02
~ / _
I ! / ~ ~ ~ 0
Time in a.u,
2O
Fig, 8. Numerical solmion of the equation system (5) with a Runge-Kuua method of fourth order usint the following initial conclitions and parameter value: [A]o = 1, [B] o ---[C]o = 0, k = 5, k~ = 5, j = 0.5, [tt] o = j / k 2 = 0.1, p(t)-= 1. (a) Time dependence of the nutocmal~ic active proton
concentrationand of the rate of oxygenevolution;(b) time dependenceof the concentrationof states [A]... [B] of the manganesecomplex.
The reaction set (4) leads to the following set of kinetic equations (for the sake of simplicity here the charges at the symbols are omitted): dA --dt
pA + kpDH 2
dB -
-
=
-pB
+ pA
dt dC
dt dD -dt
= -pC+pB
(5)
= - kpDH 2 + pC
dH _ _ = 4 k p D H 2 - k E H + ¢p dt
Under certain initial conditions (if the system is in the dark-adapted state, i.e. only A or B is not equal to zero, and the illumination is switched on abruptly) this kinetic system is able to exhibit damped oscillations of the intermediate states and consequently of the rate of oxygen evolution (Fig. 8). It should be noted that the variety of time-dependent behaviour in this reaction sequence is much broader than in the usual linear model (3): depending on the parameter values the amplitude of damped oscillations
Research on artificial photocatalysts for water oxidation and research on photosynthetic oxygen evolution from water are not evolving towards a consistent understanding of photoelectrolysis of water. While artificial model systems are very slowly improving on the basis of transition metal centred semiconductor electrochemical mechanisms of very stable compounds, the photosynthetic manganese catalyst turns out to be a highly fragile complex, arranged around electrocatalytically largely inert (in artificial systems) manganese centres. The solution to the contradiction may be that technical-scientific development has been persuing a multistep photo-oxidation of water in an interfacial reaction while nature has evolved a synergetic real four-electron transfer mechanism, where temporary order is generated through autocatalysis in a transient synergetic process. We succeeded in proposing the first synergetic model of photosynthetic oxygen evolution. It is interesting to note that the features of the manganese complex discussed are also characteristic for the elements which are responsible for the occurrence of the well-known examples of homogeneous chemical oscillations (Br, J and S in the Belousov-Zhabotinsky reaction, the iodate-iodine reaction and the thiosulphate reaction respectively): these elements are very reactive and exhibit a broad range of oxidation states as also known for the manganese chemistry. It was shown in this article that synergetic or cooperative electron transfer is theoretically possible and that the catalytic manganese centre involved would transform itself into a state of decreased entropy to catalyse the complicated process. The manganese thus plays the role of holding back the oxidative reaction with water, until, when the autocatalytic reaction starts, the synergetic four-electron transfer proceeds. Here, the manganese may again play a significant role in mediating the conformational changes. A process of oscillating hydrogen evolution was discussed as evidence that synergetic photoinduced fuel gen-
H. Tributsch, L Pohlraann/ Journal of Electroanalytical Chemistry 396 (1995) 53-61
eration may in principle be possible. The presence of oscillations in photosynthetic membranes, with respect to both time and numbers of saturating flashes, may prove to be the result of a synergetic process of oxygen evolution and not the consequence of a side phenomenon (e.g. a negative resistance across the ATPase system). The proposed autocatalytic reaction of the manganese complex during oxygen evolution is qualitatively able to explain the oscillating properties of the photosynthetic electron transfer chain. Experiments will now have to be made to test and improve the outlined hypothesis. Asking the right questions is important for progress in understanding oxygen evolution in photosynthesis. Research in the direction of artificial water splitting will benefit in the long term from the understanding of the biological mechanism. It will be especially important to find out whether the oxygen evolution strategy applied in nature is very different from that pursued during the last two decades in photoelectrochemical research. If our hypothesis is right, it is not surprising that photosynthetic oxygen evolution, the basis of higher life on earth, is unique and so strongly resists elucidation. We are dealing with non-linear synergetic photoelectrochemistry, a field which we still have to develop, for which we only have few examples (e.g. experiment of Fig. 6). The questions that have been asked after the experimental demonstration of UV photoelectrolysis with TiO 2 two and a half decades ago have led us to many answers relevant for the problem of artificial water electrolysis using solar light, but we have to continue asking many questions to learn how to handle such a process of fundamental significance. References [1] A. Fujishima and K. Honda, Nature (London), 238 (1972) 37. [2] P. Bogdanoff and N. Alonso-Vante, Ber.Bunsenges. Phys. Chem., 97 (1993) 940. [3] A.J. Nozic, Philos. Trans. R.Soc.London, Set. A, 295 (1980) 453. [4] R. Memming, Electrochim. Acta, 25 (1980) 77. [5] H. Gerischer, in R. van Overstraaten and W. Palz (Eds.), Proc. 2nd EC Photovoltaic Solar Energy Conf., Reidel, Dordrecht, 1979 p. 408. [6] G. Calzaferri, N. Gfeller and K. Pfanner, J. Photochem. Photobiol. A.: Chem., 87 (1995) 81. [7] M. Graetzel, in R.E. White, J.O'M. Bockris and B.E. Conway (Eds.), Modern Aspects of Electrochemistry, Vol. 15, Plenum New York, 1983, p. 83. [8] A. Heller, Acc. Chem. Res., 14 (1981) 154. [9] Y. Nakato, S. Tonomura and H. Tsubomura, Ber. Bunsenges. Phys. Chem., 80 (1976) 1289. [10] H. Ezzaounia, R. Heindl, R. Parsons and H. Tributsch, J. Electroanal. Chem., 145 (1983) 279.
6l
[11] H.-M. Kuehne and H. Tributsch, Ber. Bunsenges. Phys. Chem., 83 (1979) 655. [12] S. Trasatti, in Electrochemistry of Novel Materials, VCH, New York, 1994, p. 207. [13] H. Colell, N. Alonso-Vante and H. Tributsch, J. Electroanal Chem., 324 (1992) 127. [14] P. Bogdanoff, Hahn-Meitner-lnstitut, unpublished results. [15] M. Kuehne, W. Jaegermann and H. Tributsch, Chem. Phys. Lett., 112 (1984) 160. [16] H. Tributsch and O. Gorochov, Electrochim. Acta, 27 (1981) 215. [17] D. Haneman and H. Tributsch, Chem. Phys. Lett., 216 (1993) 81. [18] H. Tributsch, Z. Naturforsch., 32 (1977) 972. [19] N. Alonso-Vante and H. Tributsch, in J. Lipkowsky and Ph. N. Ross (Eds.), Electrochemistry of New Materials; Frontiers of Electrochemistry, VCH, New York, 1994 p. I. [20] A. Boussac, L. Zimmermann and A.W. Rutherford, in J. Barber (Ed.), The Photosystems. Structure, Function and Molecular Biology, Elsevier, Amsterdam, 1992, p. 187. [21] G. Renger, Chemie Unserer Zeit, 28 (1994) 118. [22] T. Watanabe, M. Kobayashi and T. Sagara, 8th Int. Cong. on Photosynthesis, Stockholm 1989, abstract and poster; Physiol. plant., 76 (3, part 2) (1990) A147. [23] W.F.J. Vermaas, J. Charite and G. Shen, Biochemistry, 29 (1990) 5325. [24] G.T. Babcock, in J. Amesz, (Ed.) Photosynthesis, Elsevier, Amsterdam, 1987, p. 125. [25] K. Wieghardt, Angew. Chem., 101 (1989) 11790. [26] D.A. Proserpio, personal communication; 10th Int. Conf. on Photochemical Conversion and Storage of Solar Energy (IPS-10), Interlaken, August 1994. [27] L. Pohlmann and H. Tributsch, J. Theor. Biol., 155 (1992) 443; 156 (1992) 63. [28] H. Tributsch and L. Pohlmann, Chem. Phys. Lett., 188 (1992) 63. [29] L. Pohlmann and H. Tributsch, in R.K. Mishra, D. MaaB and E. Zwierlein (Eds.) On Self-Organization, Vol. 61, Springer, Berlin, 1994, p. 133. [30] H. Tributsch, J. Electrochim. Acta, 39 (1993) 1495. [31] B. Steiger and F.C. Anson, lnorg. Chem., 33 (1994) 5767. [32] C. Shi and F.C. Anson, Inorg. Chim. Acta, 225 (1994) 215. [33] B. Steiger, C. Shi and F.C. Anson, Inorg. Chem., 32 (1993) 2107. [34] L. Pohlmann, G. Neher and H. Tributsch, J. Phys. Chem., 98 (1994) 11007. [35] H. Kautsky and A. Hirsch, Naturwissenschaften, 19 (1931) 964. [36] D.A. Walker, Photosynth. Res, 34 (1992) 387. [37] D.A. Walker, New Phytol., 121 (1992) 325, [38] D.R. Keller and D.A. Walker, Proc. R. Soc. London, Ser. B, 241 (1990) 59. [39] M.N. Sivak and D.A. Walker, Proc. R. Soc. London, Ser. B, 217 (1983) 377. [40] T. Ogawa, Biochim. Biophys. Acta, 681 (1982) 103. [41] A. Laisk, K. Siebke, U.M. Gerst, H. Eichelmann, V. Oja and U. Heber, Planta, 185 (1991) 554. [42] R.T. Furbank and C.H. Foyer, Arch. Biochem. Biophys., 246 (1986) 240. [43] A. Laisk and D.A. Walker, Proc. R. Soc. London, Ser. B, 227 (1986) 281. [44] C. Giersch, Arch. Biochem. Biophys., 245 (1986) 263. [45] P. Kocks and J. Ross, Biochim. Biophys. Acta, (1994). [46] B. Kok, B. Forbush and M. McGIoin, Photochem. Photobiol., 11 (1970) 457.