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Effects of electrolytes on the photoelectrochemical reduction of carbon dioxide at illuminated p-type cadmium telluride and p-type indium phosphide electrodes in aqueous solutions
143
J. Electroan&. Chem., 249 (1988) 143-153 Elsevier Sequoia ,%A., Lausanne - Printed in The Netherlands
EFFECTS OF ELECTROLYTES ON THE PHOTOELECTROCHEMICAL REDUCI-ION OF CARBON DIOXIDE AT ILLUMINATED p-TYPE CADMIUM TELLURIDE AND p-TYPE INDIUM PHOSPHIDE ELECTRODES IN AQUEOUS SOLUTIONS
HIROSHI YONEYAMA,
KENJI SUGIMURA
and SUSUMU KUWABATA
Department of Applied Chernrslry, Faculty of Engineering, Osaka 565 (Japan)
Osaka Universrty, Yamada-oka 2-1, Suita,
(Received 11th August 1987; in revised form 10th March 1988)
ABSTRACT Photoelectrochemical reduction of carbon dioxide on illuminated p-type CdTe and p-type InP electrodes were studied in aqueous solutions containing a variety of supporting electrolytes such as carbonates, sulfates, phosphates, and perchlorates of alkali salts, and tetraalkylammonium salts. During the electrolysis, the electrodes were corroded a little, but reliable results were obtained at both electrodes that the carbonates favored formic acid production, while the other electrolytes favored carbon monoxide production. Furthermore, the highest current efficiency, more than 80%, was achieved for carbon dioxide reduction at the p-type CdTe electrode in the presence of tetraalkylammonium salts. It is concluded that the difference in hydrophobicity or hydrophilicity of the electrode surfaces between the two kinds of semiconductors results in the difference in reducibility of carbon dioxide as observed.
INTRODUCTION
The photoelectrochemical reduction of carbon dioxide on semiconductor electrodes has gained popularity in recent years, and studies have been extended to a variety of subjects including photoelectrocatalysis of semiconductor electrodes [l-4], modification of the photoelectrocatalysis by depositing a small amount of metals [5] and metal complexes [6], and the effects of the use of mediators on enhancing the rate of carbon dioxide reduction [7-lo]. The electroreduction of carbon dioxide at GaAs electrodes in the dark has also been studied as an example of electrocatalysis by semiconductor surfaces [2,11,12]. It is conceivable that besides the electrocatalytic activities of the electrodes, several factors such as supporting electrolytes, solvents, and electrode potentials influence the reduction behavior of carbon dioxide. Taniguchi et al. [3,4] found in organic solvents with 5% water that carbon monoxide was produced selectively at 0022-0728/88,‘$03.50
0 1988 Elsevier Sequoia S.A.
144
p-CdTe electrodes with more than 80% current efficiency in the presence of tetraalkylammonium ions. In the present study, we have investigated in detail the effects of supporting electrolytes on the reduction behavior of carbon dioxide in aqueous solutions. Two kinds of semiconductor electrodes (p-CdTe and p-InP) were used to obtain common electrolyte effects on the phot~l~~oche~~ reduction of carbon dioxide. As will be shown below, the supporting electrolytes in aqueous solutions have a great effect on the reduction behavior of carbon dioxide. EXPERIMENTAL
The semiconductor materials used in the present study were p-CdTe and p-InP, both provided by Japan Mining Co., Ltd. The electrode surface was the (111) plane for the p-CdTe and the (100) plane for the p-InP, and their surfaces were polished to a mirror finish by using 0.06 pm alumina, followed by washing in an ultrasonic bath. Ohmic contacts to the p-CdTe and p-InP were made according to the literature [13,14], and these semiconductors were mounted in glass tubes with epoxy resin to make electrodes. Prior to use in electrolysis experiments, the p-CdTe electrode was etched in HCl + HNO, (3 : 1) for 10 s [4], and the p-InP electrode in 1% bromine dissolved in methanol for 1 min [14]. After etching, the electrodes were washed in a stream of deionized water for 15 min. The electrolysis experiments were performed in an H-type cell whose compartments were separated by a cation exchange membrane (Nafion@ 417). A saturated calomel electrode (SCE) served as the reference electrode. The constant potential electrolysis of CO, was conducted under air-tight conditions with 1 atm CO, above the electrolyte solutions. The volume of the CO, and the electrolyte solution in the cathode comp~tm~t were 15 ml and 25 ml, respectively. A 500 W xenon lamp (Ushio Electric Co., DSB-SOlA) was used as a light source, and light of wavelengths shorter than 450 nm was cut off by passing through a glass filter (Toshiba, W-45). The light intensity was 0.8 W cm-* as determined by a laser power meter (Coherent Radiation, model 201). The electroreduction of CO, under the experimental conditions employed in the present study gave CO and formic acid together with simult~~us evolution of hydrogen. The CO and H, produced were determined by gas c~omato~aphy using a 5A molecular sieve column at 100°C with Ar as the carrier gas. Formic acid was determined first by reducing it to formaldehyde with the use of Mg and then by applying calorimetric analysis using chromotropic acid. The current efficiency was determined by assuming that two electrons were involved in the production of a CO, formic acid or H, molecule. To determine the flat-band potential of the electrodes, the capacitance was measured as a function of electrode potential. For this purpose, a potentiostat (Hokuto Denko, model HA-104) was used to control the electrode potential, and the capacitance was measured as the 90” quadrature signal from a lock-in amplifier (NF Circuit Design Co., Ltd., LI-574A) with excitation by an external oscillator (NF Circuit Design Co., Ltd., EI-1011). Photocurrent-potential curves were mea-
145
sured using a potentiostat (Nikko Keisoku NPOT-2501), a potential sweeper (Nikko Keisoku, NPS-2A) and an X-Y recorder (Yokogawa Electric Co., Ltd., Type 3077). In the constant potential electrolysis, a coulometer (Hokuto Denko, model CLM-2) was used. RESULTS
Photocurrent-potential curves of the p-CdTe electrode taken in four kinds of electrolyte solution in the presence and absence of dissolved CO, are shown in Fig. 1. The current-potential curves shown in Fig. 1 were taken on the initial potential sweep>and did not change greatly with potential cycling. The onset potential of the cathodic photocurrents in the presence of carbon dioxide was slightly more positive than that in its absence. By dissolving CO,, the solution pH changed to acidic. 0.1 M Na,C03 changed its pH from 11.0 to 6.9, 0.1 M Na,SO, from 6.5 to 4.2,0.1 M LiClO, from 6.9 to 3.9, and 0.1 M te~aethyl~o~um perchlorate (TEAP) from 7.0 to 4.0. Thus, it is probable that the difference in the onset potential of the cathodic photocurrents between the presence and absence of dissolved CO* is caused by the difference in pH of the electrolyte solutions. To make clear this
I- -7.5 3
-1.0
-0.5
E/vvs,sCE
Fig. 1. Cnrrent-potential curves af a p-type CdTe electrode in four kinds of electrolyte solutions. (a) 0.1. M Na,CO,, (b) 0.1 M Na,S04, (c) 0.1 M LiCQ, and (d) 0.1 M TFAF. dfi/dt-100 mV s-l. ) Under i&mination with CO,-saturated solution; (- - -) under illumination with C02-free (solution; (- - - - - -) in the dark in the presence and absence of CO,.
N
m. .
-1.0 E/V VSSCE
E/V
j ‘
vsSCE
1 -1.5
-1.o
E/V
-05
vs.SCE
~
0
E/V
vs.SCE
Fig. 2. As in Fig. 1, but for a p-type InP electrode.
possibility, photocurrent-potential curves were obtained again in CO,-free electrolyte solutions having the same pH as that of the CO,-saturated solutions. The pH was adjusted in those cases by using acids of the same anions as the supporting electrolytes. It was found that the photocurrent-potential curves obtained were not very different from those shown in Fig. 1 when an N, atmosphere was employed; adjustment of the pH in the acidic direction did not result in marked positive shifts of the photoc~ent-potenti~ curves, and the onset potential of the cathodic photocurrents was different in solutions of the same pH in the presence and absence of dissolved CO,. In Fig. 2, photocurrent-potential curves of the p&P electrode are shown for the four kinds of electrolyte solution chosen in Fig. 1. By comparing this figure with Fig. 1, it can be seen that the onset potential of the cathodic phot~u~ents at the InP electrode is very close to that at the p-CdTe electrode, but that an increase in negative potential at the former electrode caused a greater increase in cathodic photocurrent than at the latter. The effect of the electrode potential on the electrolysis results was investigated at the p-CdTe electrode for two kinds of electrolyte solution: 0.1 M Na,CO, and 0.1 M TEAP, and the results are given in Fig. 3. Formic acid and carbon monoxide were produced as the reduction products of CO, by the electrolysis at potentials between - 1.0 and - 1.4 V vs. SCE, and the relative yields of the photoelectrolysis products were influenced slightly by the electrode potential. However, in the potential region investigated one can recognize that the electrolyte influences the reduction behavior of CO, greatly; TEAP favors the production of CO, while Na,C03 favors the production of formic acid. Then the electrolysis was performed
147
E/Vvs.SCE
E Iv
VS.SCE
HCOOH (m -@, and H, Fig. 3. Current efficiencies for the production of CO (O -o), A) as a function of the electrode potential at a p-type CdTe electrode. Electrolyte: (a) 0.1 M (b) 0.1 M Na,C03.
(A----TEAR
in a variety of electrolyte solutions at a fixed potential of -1.2 V vs. SCE, the results obtained are summarized in Table 1. In this table the results obtained in duplicate experiments are also included for several electrolyte solutions, and from these one can recognize that fairly good reproducibilities were achieved in the electrolysis experiments.
TABLE 1 ~ot~lec~~he~~~
reduction of CO, on a p-type CdTe electrode a
supporting electrolyte
Temp./ o C
0.1 M Na,CO,
25 25 0 25 25 25 25 25 25 25 25 25 25 25 0 0
0.1 M Li,CO, 0.1 M K&O3 0.1 0.1 0.1 0.1 0.1 0.1
M M M M M M
Na,SO, K2S04 Na,PO, Na,HPO, LiClO, TE!.AP
Current effici~cy/% Hz
co
HCOOH
Rdn. of COz in total
37.6 40.3 31.4 46.3 47.2 38.2 39.1 45.1 51.5 56.1 57.2 41.5 9.4 10.9 5.5 6.1
6.6 15.6 28.7 6.4 10.7 7.9 19.1 43.3 32.7 25.4 27.2 40.5 65.6 60.7 77.8 70.6
48.4 41.8 27.5 45.5 37.8 47.1 35.5 5.4 3.4 2.6 1.7 18.9 20.2 23.8 18.0 20.7
55.0 51.4 56.2 51.9 48.5 55.0 54.6 48.7 36.1 28.0 28.9 59.4 85.8 84.5 95.8 91.3
a The electrolysis was conducted at - 1.2 V vs. SCE up to 10 C. The electrode area was 0.74 cm2.
148 TABLE 2 Photoelectrochemical reduction of CO, on a p-type InP electrode a supporting electrolyte
Temp./ ’ C
0.1 M Na,CO,
25 0 25 25 25 25 0 0
0.1 0.1 0.1 0.1
M M M M
Na,SO, NasPO., LiClO, TEAP
a The electrolysis was conducted at -1.2
Current efficiency/% HZ
co
HCOOH
Rdn. of CO, in total
71.0 47.5 70.7 80.3 61.8 25.5 30.0 28.8
8.6 11.4 16.1 10.7 13.7 26.0 31.2 27.5
17.5 32.7 4.6 2.3 24.2 36.2 42.3 40.6
26.1 44.1 20.7 13.0 37.9 62.2 73.5 68.1
V vs. SCE up to 10 C. The electrode area was 1.15 cm’.
According to this table, alkali carbonates favor the production of formic acid, while the other electrolytes favor the production of CO. This trend of the product selectivity is also recognized with the p-Inp electrodes, though rather weakly, as shown by Table 2. Furthermore, it should be noted that, at both electrodes, TEAP had a marked effect in suppressing the hydrogen evolution which occurred in competition with the reduction of CO*, as Taniguchi et al. [4] found in DMF + 5% water solutions at p-CdTe electrodes. They obtained a current efficiency of 76.1% for the production of CO at - 1.6 V vs. SCE. In contrast with their results, formic acid was produced in the present aqueous solution systems, but if the current
TABLE 3 Photoelectrochemical reduction of CO, in the presense of tetraakyl-onium Semiconductor
Supporting electrolyte b
M M M M M M M M
p-CdTe
0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
TEAP TBABr TBABr TMABr TBACl TEACl Th4ACl TMAI
p-InP
0.1 M TEAP 0.1 M TEABr 0.1 M TEACl
ions a
Current efficiency/% HZ
co
HCOOH
rdn. of COa in total
9.4 11.2 26.0 19.3 31.8 42.8 39.2 22.4
65.6 78.8 61.1 65.6 51.3 43.6 50.1 64.1
20.2 7.6 10.8 15.0 8.8 8.2 12.3 16.7
85.8 86.4 71.9 80.6 60.1 51.8 63.0 80.8
25.5 34.8 37.9
26.0 31.1 28.8
36.2 28.0 30.1
62.2 59.1 58.9
’ Electrolysis was conducted at - 1.2 V vs. SCE up to 10 C. b TEA: tetraethylammonium; TBA: tetrabutylamrnonium; Th4A: tetramethylammonium.
149
-:rL!TI 1 P-CdTe
P-lnP
_ . . . . __. -
0
/
?
I’ I
E ”
4
’
1
0
/’
E
4
I
I
v
i’
:/
E
1
$
:
\
:s.._
&......,‘CL_, ‘.,.
:
:2
:
2
1
1.5
I
I
1.0
0.5
E/V
vs.SCE
3
0
1.5
1.0
E/V
05
0
vs.SCE
Fig. 4. Current-potential curves of p-type CdTe and p-type InP before and after the photoelectrolysis in CO,-saturated 0.1 M Na,SO, at -1.2 V vs. SCE up to 10 C. The electrode area was 0.74 cm* for Under illumination before the electrolysis; (- - -) under p-CdTe and 1.15 cm’ for p-InP. ( -) illumination after the electrolysis, (- - - - - -) in the dark after the electrolysis.
efficiency for the production of this substance is included, a current efficiency of more than 80% was achieved for the reduction of CO, at the p-CdTe electrode in totally aqueous solutions in the presence of TEAP, and of more than 60% at the p-InP electrode. Results obtained in other tetraalkylammonium salts are collected in Table 3, which shows that all the tetraalkylammonium cations are effective in suppressing hydrogen evolution. In Tables 1 and 2, results obtained at 0 “C are included for two kinds of electrolyte solution: 0.1 M Na,CO, and 0.1 M TEAP. Lowering the electrolysis temperature is found to be effective in supressing hydrogen evolution, possibly due to enhancing the solubility of CO,. Semiconductor electrodes used in the present study are not totally stable in aqueous solutions, though naked p-CdTe [W-17] and noble metal-loaded p-InP [18,19] have been used as hydrogen-evolving photocathodes. p-CdTe suffers cathodic decomposition to leave metallic Cd on the electrode surface [20,21], and similarly p-InP is decomposed into metallic In and phosphine [22]. Our CO2 reduction experiments were carried out with an electrolysis charge of 10 C, which seemed enough to cause electrode deterioration. Then photocurrent-potential curves were taken in CO,-saturated 0.1 M Na,SO, after the electrolysis in this solution; they are shown in Fig. 4 for both kinds of electrode. According to this figure, the photoelectrode behavior of the p-CdTe electrode was not changed greatly by the electrolysis. The onset potential of the cathodic photocurrents was not changed appreciably and the currents in the dark did not become large. In contrast, the effect of the electrolysis was remarkable at the p-InP electrode, where currents in the dark became large and a pair of redox waves was generated in cyclic voltammograms of the electrode. The redox waves seem to be related to the redox reactions involving In/In(OH), due to the metallic In which was produced on the electrode surface during the course of electrolysis up to 10 C. However, the photosensitivity was not lost appreciably and the onset potential of the cathodic photocurrents was not shifted markedly from that of a fresh electrode.
150
Observations with a scanning electron microscope (SEM) revealed that both kinds of electrode were corroded, as expected. In the case of the p-CdTe electrode, many craters like deformed asteroids of ca. 1 pm or less were produced by the electrolysis up to 10 C and according to the results obtained by energy dispersive electron probe X-ray microanalysis, the electrode surface becomes enriched with Cd. In the case of the p-InP electrode, metallic In dispersed on the electrode surface with an aggregated flake-like structure. By comparing SEM pictures of the p-CdTe and p-InP electrodes, the latter electrode was judged to be more corroded. However, the electrolysis up to 10 C was not enough to cover the electrode surface with a metallic layer and the photosensitivity was thus retained at these electrodes. If the electrolysis were continued further for a time long enough to produce a thick metal layer on the electrode surface, the photosensitivity of the electrode would be lost. DISCUSSION
The purpose of the present study was to examine the effects of electrolytes on the photoelectrochemical reduction of CO* at semiconductor photocathodes. The use of the two kinds of semiconductor was intended to elucidate common features of the electrolyte effects which have not yet been clarified experimentally. According to the results shown in Tables 1-3, the effect of the electrolytes seems not to be simple: one can recognize that carbonates favor the production of formic acid, but the other electrolytes used favor the production of CO. This finding seems to be related to the reduction mechanisms of CO, as judged from those already proposed by other investigators. An initial step in the reduction of CO, is believed to be the production of its anion radical [23], and this reaction may be followed by a CEC reaction [23,24], the net reaction of which is given by eqn. (2). CO, + e- = CO;-
(I)
CO;-
(2)
+ CO, + e- = CO + CO;-
On the other hand, the net reaction for formic acid production [25,26] is given by CO;-
+ H,O + e- = HCOO- + OH-
(3)
In carbonate solutions, reaction (2) will be less likely to occur because of the abundance of carbonate as one of the product components, and then reaction (3) predominates. On the other hand, if electrolytes other than carbonates are used, reaction (2) will allow the production of CO. This implies that in the absence of carbonates the apparent rate constant of reaction (2) is larger than that of reaction (3). Amatore and Saveant [23] reported the rate constant of the rate-determining step of reactions (2) and (3) to be 3.2 X lo3 M s-* and 7.7 X lo2 M s-‘, respectively, when obtained in DMF containing TEAP. Although the rate constants may depend on the kind of solvent used, the above values suggest the possibility of preferential production of CO in aqueous solutions except for carbonate solutions.
151
The main difference between the results obtained with the p-CdTe and p-InP electrodes seems to be that the current efficiencies for CO, reduction were higher at the former electrode. If the reduction of CO, proceeds under mass transport control but hydrogen evolution occurs under activation control, higher energies in the conduction band of the semiconductor electrode will result in higher current efficiencies for hydrogen evolution. To examine this possibility, the potentials of the conduction band edge of the electrodes were estimated on the basis of measured flat-band potential values (&,J in CO,-saturated 0.1 M Na,SO, (pH 4.2). The determined E,s of the p-CdTe and p-InP electrode were 0.9 V and 0.6 V vs. SCE, respectively, and the potentials of the conduction band edges of the semiconductor electrodes are estimated to be roughly -0.4 V and -0.55 V vs. SCE for the former and the latter electrode, respectively, by subtracting the bandgap values from the E, values. Since the conduction band energies are not very different, the difference in the degree of hydrogen evolution between the two kinds of electrodes does not seem attributable to the conduction band edge energy. The finding that the onset potentials of the cathodic photocurrents are very close to each other on the two kinds of electrodes also supports the closeness of the flat-band potentials of the two kinds of semiconductor electrode. As described already, both kinds of electrode are corroded during the eleetroreduction of CO, to leave Cd or In on the electrode surface, depending on the kind of semiconductor electrode. Accordingly, the metal covering may control the competing reactions of hydrogen evolution and CO* reduction. At p-GaP electrodes, Ikeda et al. [S] found that the deposition of metals of high hydrogen overvoltage such as Pb was effective in enhancing the rate of CO, reduction as a result of suppressing hydrogen evolution. The present results cannot be explaind in terms of hydrogen overvoltage of the metal covering, however. It has been established well that the hydrogen overvoltage is slightly larger at In than at Cd [27]. As described already, the onset potential of the cathodic photocurrents in the presence of CO, was more positive than that in its absence, even if the solution pH was adjusted to the same value. However, this difference does not necessarily allow the prediction of the reducib~ty of CO2 in competition with hydrogen evolution. Owing to the lack of buffering action of the pH-adjusted supporting electrolyte solutions which have very low proton concentrations, the pH in the environment of the electrode surface supposedly increases at the instant of the electrolysis, resulting in apparent negative shifts in the onset potential. Accordingly, one cannot predict the competing ratio from the difference in the photocurrent-potential curves in the presence and absence of CO,. Phenomenolo~cally, the high rate of the competing hydrogen evolution at the p-InP electrode can be explained in terms of differences in the hydrophilic nature of the electrode surface. By comparing the photocurrent-potential curves in Fig. 1 with those in Fig. 2, it can be noted that the cathodic photocurrents at the p-InP electrode are larger than those at the p-CdTe electrode. If it is assumed that the reducibility of CO, does not differ much between the two kinds of electrode, the magnitude of the cathodic photocurrents should cause greater hydrogen evolution at p-InP as the reaction competing with CO2 reduction.
152
According to Fig. 4, the deterioration of the electrode was more marked at the p&P electrode than at the p-CdTe electrode. Since the corrosion of the electrode occurs with production of phospbine at p-I@ and hydrogen telluride at CdTe, the greater corrosion rate should be expected if the electrode surface possesses higber hydrophilicity. This view is consistent with the finding that the production of formic acid, in which water is involved as shown by eqn. (3), was more marked at the p-InP electrode than at the p-CdTe electrode even in the presence of tetraalkylamrnonium cations, which assist in making the environments of the electrode surface hydrophobic. The interaction of water molecules with the semiconductor electrode surface will take place in such a way that the oxygen atom of a water molecule coordinates to a metal site of the semiconductors. Accordingly, the ~s~ption of the higher hydrop~i~ity of p-InP will be supported by the greater Gibbs energy of formation of In@, (- 830.7 kI mol-i) than that of Cd0 (-228 kJ mol-‘). The results obtained in this study do not conflict with those expected from the difference in hydrophilicity between the two kinds of the electrode. ACKNOWLEDGEMENTS
This research was supported by Grant-m-Aid for Energy Research No. 61040040 and by the General Sekiyu Research & Development Encouragement & Assistance Foundation. REFERENCES 1 M. Halmamt, Nature (London), 275 (1978) 115. 2 D. Canfield and K.W. Fresa, Jr., J. Electrochem. Sot., 130 (1983) 1772. 3 I. Tan&hi, B. Aurian-Blajeni and J.O’M. Bockris, J. Electroanal Chem., 157 (1983) 179; 161 (1984) 385. 4 I. Tan&u&i, B. Aurian-Blajeni and J.G’M. Bockris, EIectrochim, Acta, 29 (1984) 923. 5 S. Ikeda, M. Yoshida and K. Itoh, Bull. Chem. Sot. Jpn., 58 (1985) 1353. 6 M.G. BradIey, T. Tysak, D.J. Graves and N.A. VIachoponIos, J. Chem. Sot. Chem. Commun., (1983) 349. 7 Y. Taniguchi, H. Yoneyama and H. Tamura, BuII Chem. Sot. Jpn., 55 (1982) 2034. 8 M. Zafrir, M. Ulman, Y. Zucketman and M. Halman, J. Electroand. Chem., 159 (1983) 373. 9 B.A. Parkinson and P.F. Waver, Nature (London), 309 (1984) 148. 10 M. Beley, J.-P. CoIIin, J.-P. Sauvage, J.-P. Petit and P. Chartier, J. Electroanal Chem., 206 (1986) 333. 11 K.W. Frese, Jr. and D. Canfield, J. EIectrochem. Sec., 131 (1984) 2519. 12 W.M. Scars and S.R. Morrison, J. Phys. Chem., 89 (1985) 3295. 13 J. Gu, T. Kitahara, K. Kawakami and T. Sakaguchi, J. Appl. Phys., 46 (1975) 1184. 14 H. Lewerenz, D.E. Aspens, B. MiIIer, D.L. Maim and A. Heller, 3. Am. Chem. Sot., 104 (1982) 3325. 15 K. Ohishi, K. Uosaki and J.G’M. Bockris, Energy Res., 1 (1977) 25. 16 Y. Hashimoto, S. Fuyuki, T. Akutagawa and S. Hayakawa, Jpn. J. Appl. Phys., 20 (1981) 565. 17 M. Takahashi, K. Uosaki and H. Kita, J. EIectrochem. Sec., 131 (1984) 2304. 18 A. HeIIer, E. Aharon-Shalom, W.A. Bonner and B. Miller, J. Am Chem. Sot., 104 (1982) 6942. 19 P.G. Ang and A.F. Sammells, J. Electrochem. Sot., 131 (1984) 1462. 20 H. Get&her and F. Beck, Z. Phys. N. F., 24 (1960) 378. 21 J.S. Curran, J. Electrochem. Sot., 127 (1980) 2063. 22 S. Mayumi, C. Iwakura, H. Yoneyama and H. ‘&mum, Denki Kagaku, 44 (1976) 339.
153 23 24 25 26 27
C. Amatore and J.M. Savkmt, J. Am. Chem. Sot., 103 (1981) 5021. J.C. Cressin, D. Michelin, L. Nadjo and J.M. Saw&ant,Now. J. Chim., 3 (1979) 545. W. Pa&, T.N. Andersen and H. Eyring, Electrochim. Acta, 14 (1969) 1217. J. Ryu, T.N. Andersen and H. Eyring, J. Phys. Chem., 76 (1972) 3278. S. Trasatti, J. Electroanal. Chem., 39 (1972) 163.
J. Electroan&. Chem., 249 (1988) 143-153 Elsevier Sequoia ,%A., Lausanne - Printed in The Netherlands
EFFECTS OF ELECTROLYTES ON THE PHOTOELECTROCHEMICAL REDUCI-ION OF CARBON DIOXIDE AT ILLUMINATED p-TYPE CADMIUM TELLURIDE AND p-TYPE INDIUM PHOSPHIDE ELECTRODES IN AQUEOUS SOLUTIONS
HIROSHI YONEYAMA,
KENJI SUGIMURA
and SUSUMU KUWABATA
Department of Applied Chernrslry, Faculty of Engineering, Osaka 565 (Japan)
Osaka Universrty, Yamada-oka 2-1, Suita,
(Received 11th August 1987; in revised form 10th March 1988)
ABSTRACT Photoelectrochemical reduction of carbon dioxide on illuminated p-type CdTe and p-type InP electrodes were studied in aqueous solutions containing a variety of supporting electrolytes such as carbonates, sulfates, phosphates, and perchlorates of alkali salts, and tetraalkylammonium salts. During the electrolysis, the electrodes were corroded a little, but reliable results were obtained at both electrodes that the carbonates favored formic acid production, while the other electrolytes favored carbon monoxide production. Furthermore, the highest current efficiency, more than 80%, was achieved for carbon dioxide reduction at the p-type CdTe electrode in the presence of tetraalkylammonium salts. It is concluded that the difference in hydrophobicity or hydrophilicity of the electrode surfaces between the two kinds of semiconductors results in the difference in reducibility of carbon dioxide as observed.
INTRODUCTION
The photoelectrochemical reduction of carbon dioxide on semiconductor electrodes has gained popularity in recent years, and studies have been extended to a variety of subjects including photoelectrocatalysis of semiconductor electrodes [l-4], modification of the photoelectrocatalysis by depositing a small amount of metals [5] and metal complexes [6], and the effects of the use of mediators on enhancing the rate of carbon dioxide reduction [7-lo]. The electroreduction of carbon dioxide at GaAs electrodes in the dark has also been studied as an example of electrocatalysis by semiconductor surfaces [2,11,12]. It is conceivable that besides the electrocatalytic activities of the electrodes, several factors such as supporting electrolytes, solvents, and electrode potentials influence the reduction behavior of carbon dioxide. Taniguchi et al. [3,4] found in organic solvents with 5% water that carbon monoxide was produced selectively at 0022-0728/88,‘$03.50
0 1988 Elsevier Sequoia S.A.
144
p-CdTe electrodes with more than 80% current efficiency in the presence of tetraalkylammonium ions. In the present study, we have investigated in detail the effects of supporting electrolytes on the reduction behavior of carbon dioxide in aqueous solutions. Two kinds of semiconductor electrodes (p-CdTe and p-InP) were used to obtain common electrolyte effects on the phot~l~~oche~~ reduction of carbon dioxide. As will be shown below, the supporting electrolytes in aqueous solutions have a great effect on the reduction behavior of carbon dioxide. EXPERIMENTAL
The semiconductor materials used in the present study were p-CdTe and p-InP, both provided by Japan Mining Co., Ltd. The electrode surface was the (111) plane for the p-CdTe and the (100) plane for the p-InP, and their surfaces were polished to a mirror finish by using 0.06 pm alumina, followed by washing in an ultrasonic bath. Ohmic contacts to the p-CdTe and p-InP were made according to the literature [13,14], and these semiconductors were mounted in glass tubes with epoxy resin to make electrodes. Prior to use in electrolysis experiments, the p-CdTe electrode was etched in HCl + HNO, (3 : 1) for 10 s [4], and the p-InP electrode in 1% bromine dissolved in methanol for 1 min [14]. After etching, the electrodes were washed in a stream of deionized water for 15 min. The electrolysis experiments were performed in an H-type cell whose compartments were separated by a cation exchange membrane (Nafion@ 417). A saturated calomel electrode (SCE) served as the reference electrode. The constant potential electrolysis of CO, was conducted under air-tight conditions with 1 atm CO, above the electrolyte solutions. The volume of the CO, and the electrolyte solution in the cathode comp~tm~t were 15 ml and 25 ml, respectively. A 500 W xenon lamp (Ushio Electric Co., DSB-SOlA) was used as a light source, and light of wavelengths shorter than 450 nm was cut off by passing through a glass filter (Toshiba, W-45). The light intensity was 0.8 W cm-* as determined by a laser power meter (Coherent Radiation, model 201). The electroreduction of CO, under the experimental conditions employed in the present study gave CO and formic acid together with simult~~us evolution of hydrogen. The CO and H, produced were determined by gas c~omato~aphy using a 5A molecular sieve column at 100°C with Ar as the carrier gas. Formic acid was determined first by reducing it to formaldehyde with the use of Mg and then by applying calorimetric analysis using chromotropic acid. The current efficiency was determined by assuming that two electrons were involved in the production of a CO, formic acid or H, molecule. To determine the flat-band potential of the electrodes, the capacitance was measured as a function of electrode potential. For this purpose, a potentiostat (Hokuto Denko, model HA-104) was used to control the electrode potential, and the capacitance was measured as the 90” quadrature signal from a lock-in amplifier (NF Circuit Design Co., Ltd., LI-574A) with excitation by an external oscillator (NF Circuit Design Co., Ltd., EI-1011). Photocurrent-potential curves were mea-
145
sured using a potentiostat (Nikko Keisoku NPOT-2501), a potential sweeper (Nikko Keisoku, NPS-2A) and an X-Y recorder (Yokogawa Electric Co., Ltd., Type 3077). In the constant potential electrolysis, a coulometer (Hokuto Denko, model CLM-2) was used. RESULTS
Photocurrent-potential curves of the p-CdTe electrode taken in four kinds of electrolyte solution in the presence and absence of dissolved CO, are shown in Fig. 1. The current-potential curves shown in Fig. 1 were taken on the initial potential sweep>and did not change greatly with potential cycling. The onset potential of the cathodic photocurrents in the presence of carbon dioxide was slightly more positive than that in its absence. By dissolving CO,, the solution pH changed to acidic. 0.1 M Na,C03 changed its pH from 11.0 to 6.9, 0.1 M Na,SO, from 6.5 to 4.2,0.1 M LiClO, from 6.9 to 3.9, and 0.1 M te~aethyl~o~um perchlorate (TEAP) from 7.0 to 4.0. Thus, it is probable that the difference in the onset potential of the cathodic photocurrents between the presence and absence of dissolved CO* is caused by the difference in pH of the electrolyte solutions. To make clear this
I- -7.5 3
-1.0
-0.5
E/vvs,sCE
Fig. 1. Cnrrent-potential curves af a p-type CdTe electrode in four kinds of electrolyte solutions. (a) 0.1. M Na,CO,, (b) 0.1 M Na,S04, (c) 0.1 M LiCQ, and (d) 0.1 M TFAF. dfi/dt-100 mV s-l. ) Under i&mination with CO,-saturated solution; (- - -) under illumination with C02-free (solution; (- - - - - -) in the dark in the presence and absence of CO,.
N
m. .
-1.0 E/V VSSCE
E/V
j ‘
vsSCE
1 -1.5
-1.o
E/V
-05
vs.SCE
~
0
E/V
vs.SCE
Fig. 2. As in Fig. 1, but for a p-type InP electrode.
possibility, photocurrent-potential curves were obtained again in CO,-free electrolyte solutions having the same pH as that of the CO,-saturated solutions. The pH was adjusted in those cases by using acids of the same anions as the supporting electrolytes. It was found that the photocurrent-potential curves obtained were not very different from those shown in Fig. 1 when an N, atmosphere was employed; adjustment of the pH in the acidic direction did not result in marked positive shifts of the photoc~ent-potenti~ curves, and the onset potential of the cathodic photocurrents was different in solutions of the same pH in the presence and absence of dissolved CO,. In Fig. 2, photocurrent-potential curves of the p&P electrode are shown for the four kinds of electrolyte solution chosen in Fig. 1. By comparing this figure with Fig. 1, it can be seen that the onset potential of the cathodic phot~u~ents at the InP electrode is very close to that at the p-CdTe electrode, but that an increase in negative potential at the former electrode caused a greater increase in cathodic photocurrent than at the latter. The effect of the electrode potential on the electrolysis results was investigated at the p-CdTe electrode for two kinds of electrolyte solution: 0.1 M Na,CO, and 0.1 M TEAP, and the results are given in Fig. 3. Formic acid and carbon monoxide were produced as the reduction products of CO, by the electrolysis at potentials between - 1.0 and - 1.4 V vs. SCE, and the relative yields of the photoelectrolysis products were influenced slightly by the electrode potential. However, in the potential region investigated one can recognize that the electrolyte influences the reduction behavior of CO, greatly; TEAP favors the production of CO, while Na,C03 favors the production of formic acid. Then the electrolysis was performed
147
E/Vvs.SCE
E Iv
VS.SCE
HCOOH (m -@, and H, Fig. 3. Current efficiencies for the production of CO (O -o), A) as a function of the electrode potential at a p-type CdTe electrode. Electrolyte: (a) 0.1 M (b) 0.1 M Na,C03.
(A----TEAR
in a variety of electrolyte solutions at a fixed potential of -1.2 V vs. SCE, the results obtained are summarized in Table 1. In this table the results obtained in duplicate experiments are also included for several electrolyte solutions, and from these one can recognize that fairly good reproducibilities were achieved in the electrolysis experiments.
TABLE 1 ~ot~lec~~he~~~
reduction of CO, on a p-type CdTe electrode a
supporting electrolyte
Temp./ o C
0.1 M Na,CO,
25 25 0 25 25 25 25 25 25 25 25 25 25 25 0 0
0.1 M Li,CO, 0.1 M K&O3 0.1 0.1 0.1 0.1 0.1 0.1
M M M M M M
Na,SO, K2S04 Na,PO, Na,HPO, LiClO, TE!.AP
Current effici~cy/% Hz
co
HCOOH
Rdn. of COz in total
37.6 40.3 31.4 46.3 47.2 38.2 39.1 45.1 51.5 56.1 57.2 41.5 9.4 10.9 5.5 6.1
6.6 15.6 28.7 6.4 10.7 7.9 19.1 43.3 32.7 25.4 27.2 40.5 65.6 60.7 77.8 70.6
48.4 41.8 27.5 45.5 37.8 47.1 35.5 5.4 3.4 2.6 1.7 18.9 20.2 23.8 18.0 20.7
55.0 51.4 56.2 51.9 48.5 55.0 54.6 48.7 36.1 28.0 28.9 59.4 85.8 84.5 95.8 91.3
a The electrolysis was conducted at - 1.2 V vs. SCE up to 10 C. The electrode area was 0.74 cm2.
148 TABLE 2 Photoelectrochemical reduction of CO, on a p-type InP electrode a supporting electrolyte
Temp./ ’ C
0.1 M Na,CO,
25 0 25 25 25 25 0 0
0.1 0.1 0.1 0.1
M M M M
Na,SO, NasPO., LiClO, TEAP
a The electrolysis was conducted at -1.2
Current efficiency/% HZ
co
HCOOH
Rdn. of CO, in total
71.0 47.5 70.7 80.3 61.8 25.5 30.0 28.8
8.6 11.4 16.1 10.7 13.7 26.0 31.2 27.5
17.5 32.7 4.6 2.3 24.2 36.2 42.3 40.6
26.1 44.1 20.7 13.0 37.9 62.2 73.5 68.1
V vs. SCE up to 10 C. The electrode area was 1.15 cm’.
According to this table, alkali carbonates favor the production of formic acid, while the other electrolytes favor the production of CO. This trend of the product selectivity is also recognized with the p-Inp electrodes, though rather weakly, as shown by Table 2. Furthermore, it should be noted that, at both electrodes, TEAP had a marked effect in suppressing the hydrogen evolution which occurred in competition with the reduction of CO*, as Taniguchi et al. [4] found in DMF + 5% water solutions at p-CdTe electrodes. They obtained a current efficiency of 76.1% for the production of CO at - 1.6 V vs. SCE. In contrast with their results, formic acid was produced in the present aqueous solution systems, but if the current
TABLE 3 Photoelectrochemical reduction of CO, in the presense of tetraakyl-onium Semiconductor
Supporting electrolyte b
M M M M M M M M
p-CdTe
0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
TEAP TBABr TBABr TMABr TBACl TEACl Th4ACl TMAI
p-InP
0.1 M TEAP 0.1 M TEABr 0.1 M TEACl
ions a
Current efficiency/% HZ
co
HCOOH
rdn. of COa in total
9.4 11.2 26.0 19.3 31.8 42.8 39.2 22.4
65.6 78.8 61.1 65.6 51.3 43.6 50.1 64.1
20.2 7.6 10.8 15.0 8.8 8.2 12.3 16.7
85.8 86.4 71.9 80.6 60.1 51.8 63.0 80.8
25.5 34.8 37.9
26.0 31.1 28.8
36.2 28.0 30.1
62.2 59.1 58.9
’ Electrolysis was conducted at - 1.2 V vs. SCE up to 10 C. b TEA: tetraethylammonium; TBA: tetrabutylamrnonium; Th4A: tetramethylammonium.
149
-:rL!TI 1 P-CdTe
P-lnP
_ . . . . __. -
0
/
?
I’ I
E ”
4
’
1
0
/’
E
4
I
I
v
i’
:/
E
1
$
:
\
:s.._
&......,‘CL_, ‘.,.
:
:2
:
2
1
1.5
I
I
1.0
0.5
E/V
vs.SCE
3
0
1.5
1.0
E/V
05
0
vs.SCE
Fig. 4. Current-potential curves of p-type CdTe and p-type InP before and after the photoelectrolysis in CO,-saturated 0.1 M Na,SO, at -1.2 V vs. SCE up to 10 C. The electrode area was 0.74 cm* for Under illumination before the electrolysis; (- - -) under p-CdTe and 1.15 cm’ for p-InP. ( -) illumination after the electrolysis, (- - - - - -) in the dark after the electrolysis.
efficiency for the production of this substance is included, a current efficiency of more than 80% was achieved for the reduction of CO, at the p-CdTe electrode in totally aqueous solutions in the presence of TEAP, and of more than 60% at the p-InP electrode. Results obtained in other tetraalkylammonium salts are collected in Table 3, which shows that all the tetraalkylammonium cations are effective in suppressing hydrogen evolution. In Tables 1 and 2, results obtained at 0 “C are included for two kinds of electrolyte solution: 0.1 M Na,CO, and 0.1 M TEAP. Lowering the electrolysis temperature is found to be effective in supressing hydrogen evolution, possibly due to enhancing the solubility of CO,. Semiconductor electrodes used in the present study are not totally stable in aqueous solutions, though naked p-CdTe [W-17] and noble metal-loaded p-InP [18,19] have been used as hydrogen-evolving photocathodes. p-CdTe suffers cathodic decomposition to leave metallic Cd on the electrode surface [20,21], and similarly p-InP is decomposed into metallic In and phosphine [22]. Our CO2 reduction experiments were carried out with an electrolysis charge of 10 C, which seemed enough to cause electrode deterioration. Then photocurrent-potential curves were taken in CO,-saturated 0.1 M Na,SO, after the electrolysis in this solution; they are shown in Fig. 4 for both kinds of electrode. According to this figure, the photoelectrode behavior of the p-CdTe electrode was not changed greatly by the electrolysis. The onset potential of the cathodic photocurrents was not changed appreciably and the currents in the dark did not become large. In contrast, the effect of the electrolysis was remarkable at the p-InP electrode, where currents in the dark became large and a pair of redox waves was generated in cyclic voltammograms of the electrode. The redox waves seem to be related to the redox reactions involving In/In(OH), due to the metallic In which was produced on the electrode surface during the course of electrolysis up to 10 C. However, the photosensitivity was not lost appreciably and the onset potential of the cathodic photocurrents was not shifted markedly from that of a fresh electrode.
150
Observations with a scanning electron microscope (SEM) revealed that both kinds of electrode were corroded, as expected. In the case of the p-CdTe electrode, many craters like deformed asteroids of ca. 1 pm or less were produced by the electrolysis up to 10 C and according to the results obtained by energy dispersive electron probe X-ray microanalysis, the electrode surface becomes enriched with Cd. In the case of the p-InP electrode, metallic In dispersed on the electrode surface with an aggregated flake-like structure. By comparing SEM pictures of the p-CdTe and p-InP electrodes, the latter electrode was judged to be more corroded. However, the electrolysis up to 10 C was not enough to cover the electrode surface with a metallic layer and the photosensitivity was thus retained at these electrodes. If the electrolysis were continued further for a time long enough to produce a thick metal layer on the electrode surface, the photosensitivity of the electrode would be lost. DISCUSSION
The purpose of the present study was to examine the effects of electrolytes on the photoelectrochemical reduction of CO* at semiconductor photocathodes. The use of the two kinds of semiconductor was intended to elucidate common features of the electrolyte effects which have not yet been clarified experimentally. According to the results shown in Tables 1-3, the effect of the electrolytes seems not to be simple: one can recognize that carbonates favor the production of formic acid, but the other electrolytes used favor the production of CO. This finding seems to be related to the reduction mechanisms of CO, as judged from those already proposed by other investigators. An initial step in the reduction of CO, is believed to be the production of its anion radical [23], and this reaction may be followed by a CEC reaction [23,24], the net reaction of which is given by eqn. (2). CO, + e- = CO;-
(I)
CO;-
(2)
+ CO, + e- = CO + CO;-
On the other hand, the net reaction for formic acid production [25,26] is given by CO;-
+ H,O + e- = HCOO- + OH-
(3)
In carbonate solutions, reaction (2) will be less likely to occur because of the abundance of carbonate as one of the product components, and then reaction (3) predominates. On the other hand, if electrolytes other than carbonates are used, reaction (2) will allow the production of CO. This implies that in the absence of carbonates the apparent rate constant of reaction (2) is larger than that of reaction (3). Amatore and Saveant [23] reported the rate constant of the rate-determining step of reactions (2) and (3) to be 3.2 X lo3 M s-* and 7.7 X lo2 M s-‘, respectively, when obtained in DMF containing TEAP. Although the rate constants may depend on the kind of solvent used, the above values suggest the possibility of preferential production of CO in aqueous solutions except for carbonate solutions.
151
The main difference between the results obtained with the p-CdTe and p-InP electrodes seems to be that the current efficiencies for CO, reduction were higher at the former electrode. If the reduction of CO, proceeds under mass transport control but hydrogen evolution occurs under activation control, higher energies in the conduction band of the semiconductor electrode will result in higher current efficiencies for hydrogen evolution. To examine this possibility, the potentials of the conduction band edge of the electrodes were estimated on the basis of measured flat-band potential values (&,J in CO,-saturated 0.1 M Na,SO, (pH 4.2). The determined E,s of the p-CdTe and p-InP electrode were 0.9 V and 0.6 V vs. SCE, respectively, and the potentials of the conduction band edges of the semiconductor electrodes are estimated to be roughly -0.4 V and -0.55 V vs. SCE for the former and the latter electrode, respectively, by subtracting the bandgap values from the E, values. Since the conduction band energies are not very different, the difference in the degree of hydrogen evolution between the two kinds of electrodes does not seem attributable to the conduction band edge energy. The finding that the onset potentials of the cathodic photocurrents are very close to each other on the two kinds of electrodes also supports the closeness of the flat-band potentials of the two kinds of semiconductor electrode. As described already, both kinds of electrode are corroded during the eleetroreduction of CO, to leave Cd or In on the electrode surface, depending on the kind of semiconductor electrode. Accordingly, the metal covering may control the competing reactions of hydrogen evolution and CO* reduction. At p-GaP electrodes, Ikeda et al. [S] found that the deposition of metals of high hydrogen overvoltage such as Pb was effective in enhancing the rate of CO, reduction as a result of suppressing hydrogen evolution. The present results cannot be explaind in terms of hydrogen overvoltage of the metal covering, however. It has been established well that the hydrogen overvoltage is slightly larger at In than at Cd [27]. As described already, the onset potential of the cathodic photocurrents in the presence of CO, was more positive than that in its absence, even if the solution pH was adjusted to the same value. However, this difference does not necessarily allow the prediction of the reducib~ty of CO2 in competition with hydrogen evolution. Owing to the lack of buffering action of the pH-adjusted supporting electrolyte solutions which have very low proton concentrations, the pH in the environment of the electrode surface supposedly increases at the instant of the electrolysis, resulting in apparent negative shifts in the onset potential. Accordingly, one cannot predict the competing ratio from the difference in the photocurrent-potential curves in the presence and absence of CO,. Phenomenolo~cally, the high rate of the competing hydrogen evolution at the p-InP electrode can be explained in terms of differences in the hydrophilic nature of the electrode surface. By comparing the photocurrent-potential curves in Fig. 1 with those in Fig. 2, it can be noted that the cathodic photocurrents at the p-InP electrode are larger than those at the p-CdTe electrode. If it is assumed that the reducibility of CO, does not differ much between the two kinds of electrode, the magnitude of the cathodic photocurrents should cause greater hydrogen evolution at p-InP as the reaction competing with CO2 reduction.
152
According to Fig. 4, the deterioration of the electrode was more marked at the p&P electrode than at the p-CdTe electrode. Since the corrosion of the electrode occurs with production of phospbine at p-I@ and hydrogen telluride at CdTe, the greater corrosion rate should be expected if the electrode surface possesses higber hydrophilicity. This view is consistent with the finding that the production of formic acid, in which water is involved as shown by eqn. (3), was more marked at the p-InP electrode than at the p-CdTe electrode even in the presence of tetraalkylamrnonium cations, which assist in making the environments of the electrode surface hydrophobic. The interaction of water molecules with the semiconductor electrode surface will take place in such a way that the oxygen atom of a water molecule coordinates to a metal site of the semiconductors. Accordingly, the ~s~ption of the higher hydrop~i~ity of p-InP will be supported by the greater Gibbs energy of formation of In@, (- 830.7 kI mol-i) than that of Cd0 (-228 kJ mol-‘). The results obtained in this study do not conflict with those expected from the difference in hydrophilicity between the two kinds of the electrode. ACKNOWLEDGEMENTS
This research was supported by Grant-m-Aid for Energy Research No. 61040040 and by the General Sekiyu Research & Development Encouragement & Assistance Foundation. REFERENCES 1 M. Halmamt, Nature (London), 275 (1978) 115. 2 D. Canfield and K.W. Fresa, Jr., J. Electrochem. Sot., 130 (1983) 1772. 3 I. Tan&hi, B. Aurian-Blajeni and J.O’M. Bockris, J. Electroanal Chem., 157 (1983) 179; 161 (1984) 385. 4 I. Tan&u&i, B. Aurian-Blajeni and J.G’M. Bockris, EIectrochim, Acta, 29 (1984) 923. 5 S. Ikeda, M. Yoshida and K. Itoh, Bull. Chem. Sot. Jpn., 58 (1985) 1353. 6 M.G. BradIey, T. Tysak, D.J. Graves and N.A. VIachoponIos, J. Chem. Sot. Chem. Commun., (1983) 349. 7 Y. Taniguchi, H. Yoneyama and H. Tamura, BuII Chem. Sot. Jpn., 55 (1982) 2034. 8 M. Zafrir, M. Ulman, Y. Zucketman and M. Halman, J. Electroand. Chem., 159 (1983) 373. 9 B.A. Parkinson and P.F. Waver, Nature (London), 309 (1984) 148. 10 M. Beley, J.-P. CoIIin, J.-P. Sauvage, J.-P. Petit and P. Chartier, J. Electroanal Chem., 206 (1986) 333. 11 K.W. Frese, Jr. and D. Canfield, J. EIectrochem. Sec., 131 (1984) 2519. 12 W.M. Scars and S.R. Morrison, J. Phys. Chem., 89 (1985) 3295. 13 J. Gu, T. Kitahara, K. Kawakami and T. Sakaguchi, J. Appl. Phys., 46 (1975) 1184. 14 H. Lewerenz, D.E. Aspens, B. MiIIer, D.L. Maim and A. Heller, 3. Am. Chem. Sot., 104 (1982) 3325. 15 K. Ohishi, K. Uosaki and J.G’M. Bockris, Energy Res., 1 (1977) 25. 16 Y. Hashimoto, S. Fuyuki, T. Akutagawa and S. Hayakawa, Jpn. J. Appl. Phys., 20 (1981) 565. 17 M. Takahashi, K. Uosaki and H. Kita, J. EIectrochem. Sec., 131 (1984) 2304. 18 A. HeIIer, E. Aharon-Shalom, W.A. Bonner and B. Miller, J. Am Chem. Sot., 104 (1982) 6942. 19 P.G. Ang and A.F. Sammells, J. Electrochem. Sot., 131 (1984) 1462. 20 H. Get&her and F. Beck, Z. Phys. N. F., 24 (1960) 378. 21 J.S. Curran, J. Electrochem. Sot., 127 (1980) 2063. 22 S. Mayumi, C. Iwakura, H. Yoneyama and H. ‘&mum, Denki Kagaku, 44 (1976) 339.
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