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Studies on photoelectrochemical storage cells formed with chemically deposited CdSe and Ag2S electrodes
Solar Energy Materials and Solar Cells 29 (1993) 183-194 North-Holland
Solar Energy Materials and Solar Cells
Studies on photoelectrochemical storage cells formed with chemically deposited CdSe and Ag2S electrodes S.S. D h u m u r e a n d C.D. L o k h a n d e Department of Physics, Shivaji University, Kolhapur 416 004, India Received 12 October 1992 AgeS and CdSe films were prepared by the simple and inexpensive chemical deposition method. These films were used in various charge storage configurations, such as the three-electrode storage cell, the septum cell and the redox storage cell. Their charging and discharging characteristics are discussed. It was found that storage of chemical energy in the above three configurations is possible by using chemically deposited AgeS and CdSe electrodes.
I. Introduction Photoelectrochemical (PEC) devices are attracting much interest and activity in solar energy research. The PEC devices are very similar to the Schottky type solid state solar cells and have many advantages [1,2]. PEC devices are classified into five groups: (1) photoelectrolytic cells, (2) electrochemical photovoltaic cells, (3) rechargeable photoelectro-chemical cells, (4) photogalvanic cells and (5) photoelectrocatalysis cells. In the photoelectrolytic cell, the optical energy is converted into chemical energy. The photogenerated hole is used to oxidise one species and the electron is used to reduce another species. In the electrochemical photovoltaic cell, the photoproduced hole-electron pair is used to generate electricity directly in the same way as in the case of solid state solar cells. In this case, the reaction that occurs at the counter electrode is simply the reverse of the photo-assisted process at the semiconductor electrode. In the photogalvanic cell, the incident light is absorbed by molecular species in solution, and electrical power is generated by charge transfer from excited molecular species to the electrodes in contact with light absorbing system. In photoelectrocatalysis cell, when a semiconductor is illuminated the electrons and holes move to the surface, the electrons will reduce species at the surface and the holes will oxidize species at the surface. If not, however, permanent chemical changes will occur in the medium in which the semiconductor is immersed; such photoinduced chemical changes are termed Correspondence to: C.D. Lokhande, Department of Physics, Shivaji University, Kolhapur 416 004, India. 0927-0248/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
184
S.S. Dhumure, C.D. Lokhande / Photoelectrochemical storage cells
photocatalytic effects. The important concept in photocatalysis is the flow of electrons and holes to the surface must be equal at steady state. If they are not either (a) the surface becomes electrostatically charged and the process stops, or (b) another electrode must be present to complete the electrical circuit and the process is termed photoelectrocatalysis. A number of review articles on photoelectrochemical (PEC) storage have been appeared in the literature [3,4]. PEC cells have been used in rechargeable electrochemical storage, septum cells and redox couple storage devices. In the rechargeable electrochemical storage cells, the storage electrode capable of undergoing reversible chemical change is used. PEC cells using a third electrode as a storage electrode have been reported in the literature [5,6]. Two general configurations of such a cell have been reported by Hodes et al. [6]. In the septum a different approach of the photoelectrochemical cells has been carried out, where the semiconductor electrode acts as a separator of two aqueous compartments and larger photovoltages and photocurrents with greater stabilities are obtained. The idea for constructing such a type of redox storage cell was due to the work reported by Tien et al. [7] which was based on the modeling of natural photosynthetic systems with a pigmented bilayer liquid membrane [8]. Specific configurations which have been studied are: (1) CdSe Ipolysulphide ]Ct IAgNO 3 lAg, (2) F e 2 0 3 IK3Fe(CN) 6 Iptl ICrC13 Ipt, (3) CdS Ipolysulphide Ipt [ ICu(NO 3) If u In recent years redox battery systems have generated interest as a means for energy storage. A redox flow cell [9] is a cell in which the chemical species participate in storing electrical energy and then regenerate the energy when needed. The oxidized species are produced in one half cell called the anodic compartment and the reduced species are formed in another half cell called the cathodic compartment. A number of redox couples have been studied for such systems. These include Fe(III)/Fe(II)(HC1), Cr(III)/Cr(II)(HCI), Ti(IV)/Ti(III) and Br2/Br. Out of these couples, F e - C r redox system [10] seems to promising due to its high electrochemical potential ( E = 1.1.8 V). In the present work we report on the preparation of CdSe and Ag2S films by a simple chemical deposition method. The PEC cells are formed by using CdSe and Ag2S films as photoelectrode and carbon as a counter electrode and their current-voltage characteristics are studied. These films have been used in two types of configurations of rechargeable liquid junction cells: as rechargeable liquid junction cell three-electrode system and semiconductor septum. In the first configuration, a three-electrode system is employed with CdSe as the photoanode, graphite as the counter electrode and silver sulphide on stainless-steel substrate (St/Ag2S) as a storage electrode. In the second configuration the CdSe film itself is used as a separator of the two different redox electrolytes in the two compartments. The redox electrolyte in first compartment is kept fixed, while those in the second compartment are changed. The redox storage cell is formed using different electrolytes.
S.S. Dhumure, C.D. Lokhande / Photoelectrochemical storage cells
185
2. Experimental
2.1. Preparation of electrodes The Ag2S and CdSe electrodes have been prepared in the form of thin films on stainless-steel substrates with the chemical deposition method. This is one of the most simple, inexpensive and convenient methods for large area deposition.
2.1.1. Preparation of Ag2S films Preparation of Ag2S films has been reported elsewhere [11]. In short, an aqueous solution of 0.1 M silver nitrate and 0.5 M thiourea are mixed in a beaker. The stainles-steel substrates were immersed after being attached to a holder and stirred continuously during deposition. The p H was adjusted to be around 8 - 1 0 using ammonia. The deposition was carried out at room temperature (25°C). The films are adhesive, uniform and matt-gray in appearance. The thickness of the film was between 0.3 and 0.6 I~m. The surface area of the film was 4 cm 2. The physico-chemical characterization of films is reported elsewhere.
2.1.2. Preparation of CdSe films Chemical deposition of CdSe is based on the reaction between complexed C d 2+ ions and sodium selenosulphate. 0.1 M CdSO 4 and 0.13 M Na2SeSO 3 were mixed and an ammonia solution was added to obtain p H = 10. The film thickness is 0.8 ixm and the surface area 4 cm 2. The electrical and optical properties of the films are reported elsewhere.
2.2. PEC cell fabrication 2.2.1. Photoelectrochemical cell Photoelectrochemical cells were fabricated using n-type CdSe o r Ag2S as the photoelectrode and a carbon rod of area 2.5 × 2.5 cm 2 as the counter electrode to study the I - V characteristics. The electrolyte is 1 M ( N a O H - N a 2 S - S ) solution.
2.2.2, Three-electrode storage cells The cell consisted of three electrodes, namely CdSe as the photoelectrode, graphite as the counter electrode and AgzS as the storage electrode. The storage electrode was separated from the photoelectrode by an agar gel membrane. The electrolyte used in photoelectrode compartments was 1 M ( N a O H - N a z S - S ) and in the storage electrode compartment was 1 M ( N a O H - K C I ) .
2.2.3. Semiconductor septum cell Thin films of CdSe of 1 cm 2 area deposited on stainless steel substrates were inserted in a rectangular box of glass to form two compartments. All edges of the rectangular cell were fixed with araldite. The photoelectrode c o m p a r t m e n t was filled with 1 M polysulphide ( N a O H - N a 2 S - S ) solution. To create the proper
186
S.S. Dhumure, C.D. Lokhande / Photoelectroehemicalstorage cells 50
VOLTAGE(mV)
I00
150
0.]!)
0.14
<,
0.18
0.2, L
Fig. 1. Power output characteristics of the cells. (a) CdSeL1 M KI solutionlC, (b) CdSell M polysulphide with NaC1+ KC1IC, (c) Ag zS I1 M polysulphide solution IC. potential drop across the septum, different redox couples such as AgNO3, CuC12, Fe 3+/a+, KI and K4Fe(CN) 6 were used in the second compartment, 2.2. 4. Redox storage cell Two beakers are connected by a conducting bridge of 2.0 cm length formed with agar gel. The reduction c o m p a r t m e n t of the cell consists of a polysulphide solution with AgzS as the photoelectrode and graphite as the counter electrode. The oxidation c o m p a r t m e n t consists of CrCI 3 solution with graphite as the electrode.
3. Results and discussion 3.1. I - V characteristics o f CdSe and Ag2S based PEC cells
The I - V characteristics of CdSe and Ag2S based P E C cells were studied using a 200 W tungsten filament lamp. The power output characteristics of the above
S.S. Dhumure, C.D. Lokhande / Photoelectrochemicalstorage cells
187
Table 1 The conversion efficiency, fill factor, Voc and Isc of PEC cells Semiconductor/electrolyte CdSe/1 M polysulphide with NaC1+ KC1IC Ag 2S [1 M polysulphide/C CdSell M KI IC
Voc (mV)
I~ (mA)
Efficiency %
FF
100 95 150
0.25 0.256 0.19
0.098 0.084 0.0165
0.28 0.25 0.41
cells are given in fig. 1. The power efficiency, fill factor, Voc and Isc of the cell are listed in table 1. The low efficiency of the cell could be due to the high series resistance of the films. 3.2. Stability of P E C cells formed with CdSe electrodes In case of the C d S e ] l M ( N a O H - N a 2 S - S ) I C reactions take place:
configuration the following
2 h+ $2--9S,
(1)
S + $2---* S~-
(2)
and at the CdSe surface, 2 e - + S~- ~ 2 $22-
(3)
at the cathode. A competition between CdSe self-oxidation and oxidation of S z- decides on the stability. The latter reaction is desired for a stable regenerative PEC cell. The first reaction produces zero-valent sulphur or selenium and Cd 2+ ions. The rapid reprecipitation of these ions leads to the formation of a finely polycrystalline top layer of CdS on the CdSe photoelectrodes. The formation of this layer on CdSe leads to deactivation which may be due to an increase in grain-boundary-induced resistance to current flow or to the formation of a porous dead layer which blocks the mass transport of reactants to and form the active underlying surface. It was recently reported that addition of KCI and NaCI in this polysulphide electrolyte improved the stability in the case of CdS electrodes [12,13]. Licht et al. [14] have reported on the choice of cations to be used in aqueous polysulphide and the effects of large concentrations of O H - ions and the addition of a copper sait to the aqueous polysulphide. In the present investigation, a mixture of I M KC1 and 1 M NaCI (50 : 50%) was added to a 1 M polysulphide solution and a P E C cell was formed with a CdSe electrode. The variation of Is~ with time is shown in fig. 2. It is seen that the stability is improved with the addition of a 50% mixture of KCI and NaCI in the electrolyte.
188
S.S. Dhumure, C.D. Lokhande / Photoelectrochemical storage cells
o.! 0.2
0
I
L
20
40
LI 60
~ 80 TIME (M]N.)
~ 100
L 120
Fig. 2. Variation of short circuit current (ISc) with time for (a) C d S e l l M polysulphide without NaCI + KCI IC and (b) CdSe I1 M polysulphide with NaCI + KC1 IC.
3.3. Three-electrode storage cell The three-electrode storage system C d S e l l M polysulphide I C I I 1 M N a O H IAgzS was formed and illuminated with 200 m W / c m 2 light intensity. The cell was stabilized by adding KCI and NaCI and charged photoelectrochemically for about 2 h. In the absence of any resistance in the external circuit the photon energy will be converted into chemical energy equivalent to the difference in the redox potentials of the redox electrolytes. The photoreactions occurring at the two electrodes can be explained as follows: CdSe + h v ~ e - + h +
(hv>Eg)
(4)
due to the electric field, e - + h + ~ e - ( b u l k ) + h+(surface)
(5)
(e.g. near the interface of the semiconductor electrolyte), 2 h + + $ 2 - ~ S(at CdSe)
(6)
(e.g. oxidation o f the electrolyte would occur which is present near the interface), e - ( b u l k ) --* e (counter electrode)
(7)
189
S.S. Dhumure, C.D. Lokhande / Photoelectrochemical storage cells
150
1.0
0.8
130
0.6
110
= o
9O
0.4
Dischargin 9
0.2
50 0
20
40
60
80 TIME (MIN.)
100
12O
Fig. 3. Charging and discharging of storage cells. CdSell M polysulphide with NaCI+ KCIIC111 M NaOH [Ag2S. (transfers through the back of the semiconductor to the counter electrode), Ag2S+ 2 e-~2
A g + S 2-
(8)
(e.g. reduction of the storage electrode). During discharge S + 2 e ~ S 2-
(9)
(at the n-CdSe c o m p a r t m e n t I), 2 Ag + $ 2 - ~ AgeS + 2 e -
(10)
(at inert electrode of half cell II). The charging and discharging cycle is shown in fig. 3. During charging photocurrent was increased from 0.47 to 1 mA, and the cell voltage from 60 to inV. Discharge of the cell using a 200 f~ load resistance between the storage counter electrode resulted into a initial current of 0.68 mA. After 60 min
the 133 and this
190
S.S. Dhumure, C.D. Lokhande / Photoelectrochemical storage cells
Table 2 Effect of different electrolytes in storage compartment on charging and discharging of Ag2S based cell No.
1 2 3
Compartment I
CdSe II M polysulphide CdSe[1 M polysulphide CdSe [1 M polysulphide
Compartment II
Charging
(storage)
1<: max (mA)
Voc max (mV)
Discharging isc min (mA)
Voc rain (mV)
AgeS INaOH Ag2S IKCI Ag2S IkI
0.515 0.387 0.265
70 43 47
0.276 0.074 0.151
30 17 24
current dropped to 0.41 mA. The cell voltage was decreased from 80 to 52 mV. The result for different electrolytes used in the storage compartment are similar and are shown in table 2. 3.4. Semiconductor septum cell
Tien et al. [15] have reported on a septum cell for photoelectrolysis of seawater. This cell consists of CdSe deposited over Ni foil and a Pb counter electrode. The CdSe side contains f e r r o / f e r r i c cyanide and the Ni side of the electrode contains seawater. Tien and Jockowska [16] have developed a septum cell in which the PEC cell consists of two chambers with the configuration Cds Ipolysulphide [ptl ICuNO3ICu. Pawar and Patil [17] have reported the use of Fe203 as a septum electrode. Santhanam et al. [18] have developed a circular configuration with an electrodeposited CdS [Ni and CdS ITi bipolar junction with the configuration pt I[electrolyte ICdS INi ICoS Ielectrolyte]n Ipt, where n represents the number of bipolar junctions. Murali et al. [19] have reported on an electrodeposited CdSe spectrum of 25 cm 2 area. In the present study we have used chemically deposited CdSe films as a septum in the redox storage battery. The electrolyte 1 M ( N a O H - N a z S - S ) as a supporting electrolyte was kept fixed in the first compartment throughout the experiment. The second compartment contained different electrolyte redox couples. The metallic electrodes (Pt, stainless steel or Ag) were immersed in the compartments. The illumination intensity at the cell was 100 m W / c m 2. Open circuit voltages and short circuit currents of the cells were measured between the metallic electrodes and are shown in table 3. Fig. 3 shows the charge and discharge cycle of the CdSe [polysulphide [C lAgNO3 lAg cell. The cell was charged for 2 h. During the period of charging the cell voltage was increased from 65 to 92 mV and the current from 0.15 to 0.92 mA. Discharge of the cell using a 100 f~ load resistance across the two metallic electrodes resulted in an initial current of 0.92 mA. After 2 h the current had dropped to 0.22 mA and the voltage had dropped from 92 to 20 mA. Similar results were obtained for charging and discharging CdSe-polysulphide; these are also shown in table 3.
S.S. Dhumure, C.D. Lokhande / Photoelectrochemical storage cells
191
Table 3 Isc and Vo~ for different types of septum cells Compartment I
1 2 3 4 5
CdSell M CdSe I1 M CdSell M CdSe I 1 M CdSe I 1 M
Compartment II
polysulphide polysulphide polysulphide polysulphide polysulphide
AgNo 3 lAg KI ISt. FeCI 3 ]St. CuC12 ICu KaFe(CN)6 Ipt
Charging
Discharging
/~c
Voc
~c
Voc
max (mA)
max (mV)
min (mA)
min (mV)
0.155 0.061 0.641 0.041 0.073
14 5 65 3 13
0.739 0.024 0.157 - 0.373 0.212
78 3 31 - 45 26
3.5. Redox storage cells
Redox storage cells have been studied by a number of workers using the external battery to drive the overall cell reaction. In the present study we have developed a redox storage cell with the following configuration:
Ag2S 10.01 M (NaOH-Na2S-S) ICI [0.01 M CrC13 IC. compartment I
compartment II
In this comfiguration, without using and external battery, electrons generated at the Ag2S-polysulphide junction are used for the cell reaction. The cell was illuminated with an intensity of 100 m W / c m 2. The charging and discharging of the redox electrolyte were studied (table 4). The variation of current with time in the charging mode of the CrC13 solution is shown in fig. 4. It is seen that as the charging time proceeds the current increases and attains a steady state value upto two hours. The open circuit voltage of the system is also found to increase from 28 to 72 mA. Electrons from compartment I are transfered to compartment II and the associated electrode reactions during charging are
$2-+ 2 h+---> S (compartment I)
(11)
Cr3++ e - ~ Cr 2+
(12)
(compartment II).
Table 4 Isc and Voc for different redox couples (polysulphide in compartment I was fixed) No.
1 2 3
Compartment I
A g 2 S / 1 M polysulphide A g z S / 1 M polysulphide Ag2S/1 M polysulphide
Compartment II
K4Fe(CN) 6 / C CrCI 3 / C CrC13/C
Charging
Discharging
/~
Voc
~
voc
max (mA)
max (mV)
min (mA)
rain (mV)
0.129 0.377 0.293
15 36 27
0.044 0.346 0.320
05 68 29
192
S.S. Dhumure, C.D. Lokhande / Photoelectrochemical storage cells
1.0
100 F
O. 80
80
O. 60
60
>
0.40
40
0.20L
Disc har gmcJ
201
(1
I
40
60
I
R0
[
100
1120
TIME(MIN)
Fig. 4. Charging and dischargingof septum cells of the configuration CdSe II M polysulphideIC [ 10.1 M AgNO 3 Ag.
Thus electrical energy is stored in the form of a chemical species (Cr 2 +). From the nature of the plots shown in fig. 5 it is found that the rate of charging of the redox storage cell is not constant, but it is increasing exponentially for 50 rain followed by saturation. Discharge of the cell was studied through a load resistance of 100 1). It is seen that both the voltage and the current decrease with time and attain constant values after two hours discharge. In the mode of discharging the following reactions take place: Cr 2+---~ Cr 3 + - e S + 2 e-~
S 2-
(compartment II), (compartment I).
(13) (14)
Similarly changing and discharging of the cell Ag 25 [ polysulphide [C J IFe(CN)6 JC were also studied.
S.S. Dhumure, C.D. Lokhande / Photoelectrochemical storage cells
193
100
80
0.80
Charging 60
0.6()
< v
>o 40
0.40
Disc harging 20
O. 20
0
I 20
40
I
r
I
I
60
80
100
120
TIblE(KIN. )
Fig. 5. Charging and discharging of redox storage cells of the configuration AgeS J0.01 M polysulphide IC110.01 M CrCI 3 IC.
4. C o n c l u s i o n s
In this investigation, A g 2 S and CdSe films were prepared by a simple and inexpensive chemical deposition technique and were employed in the following PEC storage configurations: (1) three-electrode storage cell, (2) septum cell and (3) redox storage cell. It is concluded that the storage of chemical energy in the above three configurations is possible by using chemically deposited Ag2S and and CdSe electrodes.
References [1] A. Heller and B. Miller, Electrochim. Acta 25 (1980) 29. [2] A. Aruchamy, G. Aravmudam and G.V. Subbarao, Bull. Mater. Sci. 4 (1982) 483. [3] R. Memming, Electrochim. Acta 25 (1980) 77.
194 [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
S.S. Dhumure, C.D. Lokhande / Photoelectrochemical storage cells
M. Sharon and R.K. Pandey, Phys. Stat. Sol. (a) 72 (1982) 415. J. Manaseen, G. Hodes and D. Cahen, J. Electrochem. Soc. 124 (1977) 532. G. Hodes, J. Manassen and D. Cahen, Nature 102 (1976) 403. H.T. Tien, Z.C. Bi and A.K. Tripathy, Photochem, Photobiol. 44 (1986) 779. H.T. Tien, Nature 227 (1970) 1232. R.F. Savinelli, C.C. Liu, R.T. Galasco S.H. Chiang and J.F. Coetzee, J. Electrochem. Soc. 126 (1979) 357. L.H. Thaller, NASA TM-79186, 14th Intersociety Energy Conversion Engineering Conf. Boston, MA, August 5-10 (1979). S.S. Dhumure and C.D. Lokhande, Mater. Chem. Phys. 28 (1991) 141. C.D. Lokhande and S.H. Pawar, Solid State Commun. 49 (1984) 765. R. Tenne, J. Electrochem. Soc. 129 (1982) 143. S. Licht, R. Tenne, G. Degan, G. Hodes, J. Manassen, R. Triboulet, J. Rioux and C. Levy-clement, Appl. Phys. Lett. 46 (1985) 608. H.T. Tien and J.W. Chem, Photochem. Photobiol. 49 (1989) 327. H.T. Tien an K. Jackowska, Sol. Cells 23 (1988) 147. S.H. Pawar and P.S. Patil, Bull. Electrochem. 6 (1990) 618. K.S.V. Santhanam, V. Sundaram and S. Aravamuthan, Indian J. Tech. 25 (1987) 613. K.R. Murali, V. Subramanian, N. Rangarajan, A.S. Lakshmanan and S.K. Rangarajan, Bull. Electrochem. 7 (1991) 230.
Solar Energy Materials and Solar Cells
Studies on photoelectrochemical storage cells formed with chemically deposited CdSe and Ag2S electrodes S.S. D h u m u r e a n d C.D. L o k h a n d e Department of Physics, Shivaji University, Kolhapur 416 004, India Received 12 October 1992 AgeS and CdSe films were prepared by the simple and inexpensive chemical deposition method. These films were used in various charge storage configurations, such as the three-electrode storage cell, the septum cell and the redox storage cell. Their charging and discharging characteristics are discussed. It was found that storage of chemical energy in the above three configurations is possible by using chemically deposited AgeS and CdSe electrodes.
I. Introduction Photoelectrochemical (PEC) devices are attracting much interest and activity in solar energy research. The PEC devices are very similar to the Schottky type solid state solar cells and have many advantages [1,2]. PEC devices are classified into five groups: (1) photoelectrolytic cells, (2) electrochemical photovoltaic cells, (3) rechargeable photoelectro-chemical cells, (4) photogalvanic cells and (5) photoelectrocatalysis cells. In the photoelectrolytic cell, the optical energy is converted into chemical energy. The photogenerated hole is used to oxidise one species and the electron is used to reduce another species. In the electrochemical photovoltaic cell, the photoproduced hole-electron pair is used to generate electricity directly in the same way as in the case of solid state solar cells. In this case, the reaction that occurs at the counter electrode is simply the reverse of the photo-assisted process at the semiconductor electrode. In the photogalvanic cell, the incident light is absorbed by molecular species in solution, and electrical power is generated by charge transfer from excited molecular species to the electrodes in contact with light absorbing system. In photoelectrocatalysis cell, when a semiconductor is illuminated the electrons and holes move to the surface, the electrons will reduce species at the surface and the holes will oxidize species at the surface. If not, however, permanent chemical changes will occur in the medium in which the semiconductor is immersed; such photoinduced chemical changes are termed Correspondence to: C.D. Lokhande, Department of Physics, Shivaji University, Kolhapur 416 004, India. 0927-0248/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
184
S.S. Dhumure, C.D. Lokhande / Photoelectrochemical storage cells
photocatalytic effects. The important concept in photocatalysis is the flow of electrons and holes to the surface must be equal at steady state. If they are not either (a) the surface becomes electrostatically charged and the process stops, or (b) another electrode must be present to complete the electrical circuit and the process is termed photoelectrocatalysis. A number of review articles on photoelectrochemical (PEC) storage have been appeared in the literature [3,4]. PEC cells have been used in rechargeable electrochemical storage, septum cells and redox couple storage devices. In the rechargeable electrochemical storage cells, the storage electrode capable of undergoing reversible chemical change is used. PEC cells using a third electrode as a storage electrode have been reported in the literature [5,6]. Two general configurations of such a cell have been reported by Hodes et al. [6]. In the septum a different approach of the photoelectrochemical cells has been carried out, where the semiconductor electrode acts as a separator of two aqueous compartments and larger photovoltages and photocurrents with greater stabilities are obtained. The idea for constructing such a type of redox storage cell was due to the work reported by Tien et al. [7] which was based on the modeling of natural photosynthetic systems with a pigmented bilayer liquid membrane [8]. Specific configurations which have been studied are: (1) CdSe Ipolysulphide ]Ct IAgNO 3 lAg, (2) F e 2 0 3 IK3Fe(CN) 6 Iptl ICrC13 Ipt, (3) CdS Ipolysulphide Ipt [ ICu(NO 3) If u In recent years redox battery systems have generated interest as a means for energy storage. A redox flow cell [9] is a cell in which the chemical species participate in storing electrical energy and then regenerate the energy when needed. The oxidized species are produced in one half cell called the anodic compartment and the reduced species are formed in another half cell called the cathodic compartment. A number of redox couples have been studied for such systems. These include Fe(III)/Fe(II)(HC1), Cr(III)/Cr(II)(HCI), Ti(IV)/Ti(III) and Br2/Br. Out of these couples, F e - C r redox system [10] seems to promising due to its high electrochemical potential ( E = 1.1.8 V). In the present work we report on the preparation of CdSe and Ag2S films by a simple chemical deposition method. The PEC cells are formed by using CdSe and Ag2S films as photoelectrode and carbon as a counter electrode and their current-voltage characteristics are studied. These films have been used in two types of configurations of rechargeable liquid junction cells: as rechargeable liquid junction cell three-electrode system and semiconductor septum. In the first configuration, a three-electrode system is employed with CdSe as the photoanode, graphite as the counter electrode and silver sulphide on stainless-steel substrate (St/Ag2S) as a storage electrode. In the second configuration the CdSe film itself is used as a separator of the two different redox electrolytes in the two compartments. The redox electrolyte in first compartment is kept fixed, while those in the second compartment are changed. The redox storage cell is formed using different electrolytes.
S.S. Dhumure, C.D. Lokhande / Photoelectrochemical storage cells
185
2. Experimental
2.1. Preparation of electrodes The Ag2S and CdSe electrodes have been prepared in the form of thin films on stainless-steel substrates with the chemical deposition method. This is one of the most simple, inexpensive and convenient methods for large area deposition.
2.1.1. Preparation of Ag2S films Preparation of Ag2S films has been reported elsewhere [11]. In short, an aqueous solution of 0.1 M silver nitrate and 0.5 M thiourea are mixed in a beaker. The stainles-steel substrates were immersed after being attached to a holder and stirred continuously during deposition. The p H was adjusted to be around 8 - 1 0 using ammonia. The deposition was carried out at room temperature (25°C). The films are adhesive, uniform and matt-gray in appearance. The thickness of the film was between 0.3 and 0.6 I~m. The surface area of the film was 4 cm 2. The physico-chemical characterization of films is reported elsewhere.
2.1.2. Preparation of CdSe films Chemical deposition of CdSe is based on the reaction between complexed C d 2+ ions and sodium selenosulphate. 0.1 M CdSO 4 and 0.13 M Na2SeSO 3 were mixed and an ammonia solution was added to obtain p H = 10. The film thickness is 0.8 ixm and the surface area 4 cm 2. The electrical and optical properties of the films are reported elsewhere.
2.2. PEC cell fabrication 2.2.1. Photoelectrochemical cell Photoelectrochemical cells were fabricated using n-type CdSe o r Ag2S as the photoelectrode and a carbon rod of area 2.5 × 2.5 cm 2 as the counter electrode to study the I - V characteristics. The electrolyte is 1 M ( N a O H - N a 2 S - S ) solution.
2.2.2, Three-electrode storage cells The cell consisted of three electrodes, namely CdSe as the photoelectrode, graphite as the counter electrode and AgzS as the storage electrode. The storage electrode was separated from the photoelectrode by an agar gel membrane. The electrolyte used in photoelectrode compartments was 1 M ( N a O H - N a z S - S ) and in the storage electrode compartment was 1 M ( N a O H - K C I ) .
2.2.3. Semiconductor septum cell Thin films of CdSe of 1 cm 2 area deposited on stainless steel substrates were inserted in a rectangular box of glass to form two compartments. All edges of the rectangular cell were fixed with araldite. The photoelectrode c o m p a r t m e n t was filled with 1 M polysulphide ( N a O H - N a 2 S - S ) solution. To create the proper
186
S.S. Dhumure, C.D. Lokhande / Photoelectroehemicalstorage cells 50
VOLTAGE(mV)
I00
150
0.]!)
0.14
<,
0.18
0.2, L
Fig. 1. Power output characteristics of the cells. (a) CdSeL1 M KI solutionlC, (b) CdSell M polysulphide with NaC1+ KC1IC, (c) Ag zS I1 M polysulphide solution IC. potential drop across the septum, different redox couples such as AgNO3, CuC12, Fe 3+/a+, KI and K4Fe(CN) 6 were used in the second compartment, 2.2. 4. Redox storage cell Two beakers are connected by a conducting bridge of 2.0 cm length formed with agar gel. The reduction c o m p a r t m e n t of the cell consists of a polysulphide solution with AgzS as the photoelectrode and graphite as the counter electrode. The oxidation c o m p a r t m e n t consists of CrCI 3 solution with graphite as the electrode.
3. Results and discussion 3.1. I - V characteristics o f CdSe and Ag2S based PEC cells
The I - V characteristics of CdSe and Ag2S based P E C cells were studied using a 200 W tungsten filament lamp. The power output characteristics of the above
S.S. Dhumure, C.D. Lokhande / Photoelectrochemicalstorage cells
187
Table 1 The conversion efficiency, fill factor, Voc and Isc of PEC cells Semiconductor/electrolyte CdSe/1 M polysulphide with NaC1+ KC1IC Ag 2S [1 M polysulphide/C CdSell M KI IC
Voc (mV)
I~ (mA)
Efficiency %
FF
100 95 150
0.25 0.256 0.19
0.098 0.084 0.0165
0.28 0.25 0.41
cells are given in fig. 1. The power efficiency, fill factor, Voc and Isc of the cell are listed in table 1. The low efficiency of the cell could be due to the high series resistance of the films. 3.2. Stability of P E C cells formed with CdSe electrodes In case of the C d S e ] l M ( N a O H - N a 2 S - S ) I C reactions take place:
configuration the following
2 h+ $2--9S,
(1)
S + $2---* S~-
(2)
and at the CdSe surface, 2 e - + S~- ~ 2 $22-
(3)
at the cathode. A competition between CdSe self-oxidation and oxidation of S z- decides on the stability. The latter reaction is desired for a stable regenerative PEC cell. The first reaction produces zero-valent sulphur or selenium and Cd 2+ ions. The rapid reprecipitation of these ions leads to the formation of a finely polycrystalline top layer of CdS on the CdSe photoelectrodes. The formation of this layer on CdSe leads to deactivation which may be due to an increase in grain-boundary-induced resistance to current flow or to the formation of a porous dead layer which blocks the mass transport of reactants to and form the active underlying surface. It was recently reported that addition of KCI and NaCI in this polysulphide electrolyte improved the stability in the case of CdS electrodes [12,13]. Licht et al. [14] have reported on the choice of cations to be used in aqueous polysulphide and the effects of large concentrations of O H - ions and the addition of a copper sait to the aqueous polysulphide. In the present investigation, a mixture of I M KC1 and 1 M NaCI (50 : 50%) was added to a 1 M polysulphide solution and a P E C cell was formed with a CdSe electrode. The variation of Is~ with time is shown in fig. 2. It is seen that the stability is improved with the addition of a 50% mixture of KCI and NaCI in the electrolyte.
188
S.S. Dhumure, C.D. Lokhande / Photoelectrochemical storage cells
o.! 0.2
0
I
L
20
40
LI 60
~ 80 TIME (M]N.)
~ 100
L 120
Fig. 2. Variation of short circuit current (ISc) with time for (a) C d S e l l M polysulphide without NaCI + KCI IC and (b) CdSe I1 M polysulphide with NaCI + KC1 IC.
3.3. Three-electrode storage cell The three-electrode storage system C d S e l l M polysulphide I C I I 1 M N a O H IAgzS was formed and illuminated with 200 m W / c m 2 light intensity. The cell was stabilized by adding KCI and NaCI and charged photoelectrochemically for about 2 h. In the absence of any resistance in the external circuit the photon energy will be converted into chemical energy equivalent to the difference in the redox potentials of the redox electrolytes. The photoreactions occurring at the two electrodes can be explained as follows: CdSe + h v ~ e - + h +
(hv>Eg)
(4)
due to the electric field, e - + h + ~ e - ( b u l k ) + h+(surface)
(5)
(e.g. near the interface of the semiconductor electrolyte), 2 h + + $ 2 - ~ S(at CdSe)
(6)
(e.g. oxidation o f the electrolyte would occur which is present near the interface), e - ( b u l k ) --* e (counter electrode)
(7)
189
S.S. Dhumure, C.D. Lokhande / Photoelectrochemical storage cells
150
1.0
0.8
130
0.6
110
= o
9O
0.4
Dischargin 9
0.2
50 0
20
40
60
80 TIME (MIN.)
100
12O
Fig. 3. Charging and discharging of storage cells. CdSell M polysulphide with NaCI+ KCIIC111 M NaOH [Ag2S. (transfers through the back of the semiconductor to the counter electrode), Ag2S+ 2 e-~2
A g + S 2-
(8)
(e.g. reduction of the storage electrode). During discharge S + 2 e ~ S 2-
(9)
(at the n-CdSe c o m p a r t m e n t I), 2 Ag + $ 2 - ~ AgeS + 2 e -
(10)
(at inert electrode of half cell II). The charging and discharging cycle is shown in fig. 3. During charging photocurrent was increased from 0.47 to 1 mA, and the cell voltage from 60 to inV. Discharge of the cell using a 200 f~ load resistance between the storage counter electrode resulted into a initial current of 0.68 mA. After 60 min
the 133 and this
190
S.S. Dhumure, C.D. Lokhande / Photoelectrochemical storage cells
Table 2 Effect of different electrolytes in storage compartment on charging and discharging of Ag2S based cell No.
1 2 3
Compartment I
CdSe II M polysulphide CdSe[1 M polysulphide CdSe [1 M polysulphide
Compartment II
Charging
(storage)
1<: max (mA)
Voc max (mV)
Discharging isc min (mA)
Voc rain (mV)
AgeS INaOH Ag2S IKCI Ag2S IkI
0.515 0.387 0.265
70 43 47
0.276 0.074 0.151
30 17 24
current dropped to 0.41 mA. The cell voltage was decreased from 80 to 52 mV. The result for different electrolytes used in the storage compartment are similar and are shown in table 2. 3.4. Semiconductor septum cell
Tien et al. [15] have reported on a septum cell for photoelectrolysis of seawater. This cell consists of CdSe deposited over Ni foil and a Pb counter electrode. The CdSe side contains f e r r o / f e r r i c cyanide and the Ni side of the electrode contains seawater. Tien and Jockowska [16] have developed a septum cell in which the PEC cell consists of two chambers with the configuration Cds Ipolysulphide [ptl ICuNO3ICu. Pawar and Patil [17] have reported the use of Fe203 as a septum electrode. Santhanam et al. [18] have developed a circular configuration with an electrodeposited CdS [Ni and CdS ITi bipolar junction with the configuration pt I[electrolyte ICdS INi ICoS Ielectrolyte]n Ipt, where n represents the number of bipolar junctions. Murali et al. [19] have reported on an electrodeposited CdSe spectrum of 25 cm 2 area. In the present study we have used chemically deposited CdSe films as a septum in the redox storage battery. The electrolyte 1 M ( N a O H - N a z S - S ) as a supporting electrolyte was kept fixed in the first compartment throughout the experiment. The second compartment contained different electrolyte redox couples. The metallic electrodes (Pt, stainless steel or Ag) were immersed in the compartments. The illumination intensity at the cell was 100 m W / c m 2. Open circuit voltages and short circuit currents of the cells were measured between the metallic electrodes and are shown in table 3. Fig. 3 shows the charge and discharge cycle of the CdSe [polysulphide [C lAgNO3 lAg cell. The cell was charged for 2 h. During the period of charging the cell voltage was increased from 65 to 92 mV and the current from 0.15 to 0.92 mA. Discharge of the cell using a 100 f~ load resistance across the two metallic electrodes resulted in an initial current of 0.92 mA. After 2 h the current had dropped to 0.22 mA and the voltage had dropped from 92 to 20 mA. Similar results were obtained for charging and discharging CdSe-polysulphide; these are also shown in table 3.
S.S. Dhumure, C.D. Lokhande / Photoelectrochemical storage cells
191
Table 3 Isc and Vo~ for different types of septum cells Compartment I
1 2 3 4 5
CdSell M CdSe I1 M CdSell M CdSe I 1 M CdSe I 1 M
Compartment II
polysulphide polysulphide polysulphide polysulphide polysulphide
AgNo 3 lAg KI ISt. FeCI 3 ]St. CuC12 ICu KaFe(CN)6 Ipt
Charging
Discharging
/~c
Voc
~c
Voc
max (mA)
max (mV)
min (mA)
min (mV)
0.155 0.061 0.641 0.041 0.073
14 5 65 3 13
0.739 0.024 0.157 - 0.373 0.212
78 3 31 - 45 26
3.5. Redox storage cells
Redox storage cells have been studied by a number of workers using the external battery to drive the overall cell reaction. In the present study we have developed a redox storage cell with the following configuration:
Ag2S 10.01 M (NaOH-Na2S-S) ICI [0.01 M CrC13 IC. compartment I
compartment II
In this comfiguration, without using and external battery, electrons generated at the Ag2S-polysulphide junction are used for the cell reaction. The cell was illuminated with an intensity of 100 m W / c m 2. The charging and discharging of the redox electrolyte were studied (table 4). The variation of current with time in the charging mode of the CrC13 solution is shown in fig. 4. It is seen that as the charging time proceeds the current increases and attains a steady state value upto two hours. The open circuit voltage of the system is also found to increase from 28 to 72 mA. Electrons from compartment I are transfered to compartment II and the associated electrode reactions during charging are
$2-+ 2 h+---> S (compartment I)
(11)
Cr3++ e - ~ Cr 2+
(12)
(compartment II).
Table 4 Isc and Voc for different redox couples (polysulphide in compartment I was fixed) No.
1 2 3
Compartment I
A g 2 S / 1 M polysulphide A g z S / 1 M polysulphide Ag2S/1 M polysulphide
Compartment II
K4Fe(CN) 6 / C CrCI 3 / C CrC13/C
Charging
Discharging
/~
Voc
~
voc
max (mA)
max (mV)
min (mA)
rain (mV)
0.129 0.377 0.293
15 36 27
0.044 0.346 0.320
05 68 29
192
S.S. Dhumure, C.D. Lokhande / Photoelectrochemical storage cells
1.0
100 F
O. 80
80
O. 60
60
>
0.40
40
0.20L
Disc har gmcJ
201
(1
I
40
60
I
R0
[
100
1120
TIME(MIN)
Fig. 4. Charging and dischargingof septum cells of the configuration CdSe II M polysulphideIC [ 10.1 M AgNO 3 Ag.
Thus electrical energy is stored in the form of a chemical species (Cr 2 +). From the nature of the plots shown in fig. 5 it is found that the rate of charging of the redox storage cell is not constant, but it is increasing exponentially for 50 rain followed by saturation. Discharge of the cell was studied through a load resistance of 100 1). It is seen that both the voltage and the current decrease with time and attain constant values after two hours discharge. In the mode of discharging the following reactions take place: Cr 2+---~ Cr 3 + - e S + 2 e-~
S 2-
(compartment II), (compartment I).
(13) (14)
Similarly changing and discharging of the cell Ag 25 [ polysulphide [C J IFe(CN)6 JC were also studied.
S.S. Dhumure, C.D. Lokhande / Photoelectrochemical storage cells
193
100
80
0.80
Charging 60
0.6()
< v
>o 40
0.40
Disc harging 20
O. 20
0
I 20
40
I
r
I
I
60
80
100
120
TIblE(KIN. )
Fig. 5. Charging and discharging of redox storage cells of the configuration AgeS J0.01 M polysulphide IC110.01 M CrCI 3 IC.
4. C o n c l u s i o n s
In this investigation, A g 2 S and CdSe films were prepared by a simple and inexpensive chemical deposition technique and were employed in the following PEC storage configurations: (1) three-electrode storage cell, (2) septum cell and (3) redox storage cell. It is concluded that the storage of chemical energy in the above three configurations is possible by using chemically deposited Ag2S and and CdSe electrodes.
References [1] A. Heller and B. Miller, Electrochim. Acta 25 (1980) 29. [2] A. Aruchamy, G. Aravmudam and G.V. Subbarao, Bull. Mater. Sci. 4 (1982) 483. [3] R. Memming, Electrochim. Acta 25 (1980) 77.
194 [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
S.S. Dhumure, C.D. Lokhande / Photoelectrochemical storage cells
M. Sharon and R.K. Pandey, Phys. Stat. Sol. (a) 72 (1982) 415. J. Manaseen, G. Hodes and D. Cahen, J. Electrochem. Soc. 124 (1977) 532. G. Hodes, J. Manassen and D. Cahen, Nature 102 (1976) 403. H.T. Tien, Z.C. Bi and A.K. Tripathy, Photochem, Photobiol. 44 (1986) 779. H.T. Tien, Nature 227 (1970) 1232. R.F. Savinelli, C.C. Liu, R.T. Galasco S.H. Chiang and J.F. Coetzee, J. Electrochem. Soc. 126 (1979) 357. L.H. Thaller, NASA TM-79186, 14th Intersociety Energy Conversion Engineering Conf. Boston, MA, August 5-10 (1979). S.S. Dhumure and C.D. Lokhande, Mater. Chem. Phys. 28 (1991) 141. C.D. Lokhande and S.H. Pawar, Solid State Commun. 49 (1984) 765. R. Tenne, J. Electrochem. Soc. 129 (1982) 143. S. Licht, R. Tenne, G. Degan, G. Hodes, J. Manassen, R. Triboulet, J. Rioux and C. Levy-clement, Appl. Phys. Lett. 46 (1985) 608. H.T. Tien and J.W. Chem, Photochem. Photobiol. 49 (1989) 327. H.T. Tien an K. Jackowska, Sol. Cells 23 (1988) 147. S.H. Pawar and P.S. Patil, Bull. Electrochem. 6 (1990) 618. K.S.V. Santhanam, V. Sundaram and S. Aravamuthan, Indian J. Tech. 25 (1987) 613. K.R. Murali, V. Subramanian, N. Rangarajan, A.S. Lakshmanan and S.K. Rangarajan, Bull. Electrochem. 7 (1991) 230.