Electrochemical measurement on the photoelectrochemical reduction of aqueous carbon dioxide on p-Gallium phosphide and p-Gallium arsenide semiconductor electrodes

Solar Energy Materials 8 (1983)425~,40 North-Holland Publishing Company

425

ELECTROCHEMICAL MEASUREMENTS ON THE PHOTOELECTROCHEMICAL REDUCTION OF AQUEOUS CARBON D I O X I D E O N p - G A L L I U M P H O S P H I D E AND p - G A L L I U M ARSENIDE SEMICONDUCTOR ELECTRODES B. A U R I A N - B L A J E N I , M. H A L M A N N and J. M A N A S S E N Weizmann Institute of Science, Rehovot, Israel Received 9 October 1982; in revised form 26 January 1983 The photoelectrochemical reduction of aqueous carbon dioxide was carried out using single crystal p-gallium phosphide and p-gallium arsenide as photocathodes. The organic products measured were mainly formic acid, but also formaldehyde and methanol. The effect of carbon dioxide pressure on the current-potential relationships and on yieldsof organic reduction products was studied using a photoelectrochemicalautoclave, fitted with a quartz window. Highest Faradaic yield of reduction products, 80y., was obtained with a p-GaP cathode at a cathodicbias of - 1.00 V (vs. a standard silver electrode), in a medium initially 0.5 M Na2CO 3, under 8.5 atm pressure of CO2, a cation exchange diaphragm and a platinum counterelectrode, under illumination with a 150 W Xe lamp.

1. Introduction 1.t. Background

Photochemical conversion of solar energy involves endergonic reactions with light as their 6nergy source. The principles and advantages of chemical conversion of solar energy by using semiconductor materials have been extensively reviewed, e.g. by Manassen et al. [1], Nozik [2], Bolton [2] and Bard [4]. The photoelectrochemical splitting of water to hydrogen and oxygen on illuminated semiconductor materials has been a subject of considerable research effort during the last decade such as by Fujishima and Honda [5, 6], Mavroides et al. [7], Gerischer [8], Nozik [9], Bockris and Uosaki [10], Mavroides [11], Tomkiewicz and Fay [12]. There would, however, be great attraction in alternative systems of energy storage, such as by production of reduced carbon or nitrogen compounds. The system water-carbon dioxide-semiconductor could be such an alternative photosynthetic system, in which water and carbon dioxide are the reactants and the semiconductor is the device which, by absorption of light, provides the charges necessary for the redox process. The chief attraction of a direct conversion of carbon dioxide into an organic fuel by sunlight is to eliminate the costly intermediate steps of generating, separating and storing hydrogen. The reduction products are easy to handle and represent no environmental hazard. The formation of methanol from carbon dioxide and water C0 2 +2HzO~CH3OH

+3/202

0165-1633/83/0000-0000/$03.00(~) 1983 North-Holland

426

B. Aurian-Blajeni et al. / Photoelectrochemical reduction ofaqueous (-'02

requires an energy of 713 kJ/mol and a transfer of six electrons. This means that for each electron transferred an energy of 713:6=119 kJ is necessary. This energy corresponds to a wavelength of about 900 nm, i.e. 70~ of the solar energy could be used [ 13]. Although most of the interest towards synthetic fuels has been focused on methanol, formic acid has also a certain potential as fuel. Formic acid was used as fuel in alkaline fuel cells [14], and has been proposed as a convenient means for hydrogen storage [ 15].

1.2. Semiconductor-electrolyte interactions The pioneering work in the field of semiconductor-electrolyte interface was done by Myamlin and Pleskov [16] and by Gerischer [17]. Since then numerous reviews have been published [4, 18, 19]. Semiconductors differ from metals and insulators in that a moderate energy gap exists between the highest electron energy levels in the valence band and the lowest energy levels in the conduction band. The energy gap in a semiconductor is relatively large so relatively few electrons from the valence band have sufficient energy to bridge the energy gap to give rise to electronic conductivity. The electronic conductivity of a semiconductor can be improved greatly by doping the material with a suitable impurity that either donates electrons to the conduction band or accepts electrons from the valence band. Yoneyama et al. [20] reported a catalytic effect for hydrogen evolution obtained by deposition of minute amounts of metal atoms (Pd, Ni, Cu, Pt) on p-type GaP or p-Si. The photocurrents that they obtained decreased gradually with time, so that the i-V curves showed hysteresis. In a mechanistic study of photoelectrochemical reactions at p-GaP, Uosaki and Kita [21] report the measurement of anodic photocurrents at potentials cathodic to the flatband potential, originating in the difference in the number of majority carriers in the dark and under illumination. By examination of thermodynamic data, they calculated the anodic decomposition potentials for a wide pH range. Their conclusion is that at pH < 2.56 and pH > 11.7 anodic dissolution takes place, while at intermediate pH passivation occurs. Uosaki and Kita define also the photocurrent onset potential as the potential at which the sign of the photocurrent changes. The differences obtained between the flatband potential and the photocurrent onset potential are explained by the differences in the diffusion length of the two types of charge carriers. Their conclusion is that at medium and small bias potentials the rate determining step is the electrochemical process (i.e. the surface process). At large bias potentials, the rate determing step is the supply of photoexcited electrons.

1.3. Electrochemistry of carbon dioxide The study of the electrochemical reduction of carbon dioxide started in the last century with the pioneering work of Royer [22]. Fischer and Prziza [23] found that the Faradaic yield increases with CO2 pressure for amalgamated zinc electrodes

B. Aurian-Blajeni et al. / Photoelectrochemical reduction of aqueous C02

427

and does not change much for amalgamated copper electrodes. Udupa et al. [24] found that for carbon dioxide reduction on amalgamated copper electrodes in aqueous media, both bicarbonate ions and CO2 species are necessary, and that the Faradaic yield decreases with increasing current density. In the reduction of carbon dioxide on platinum electrodes in acid media, carbon dioxide reacts with chemisorbed hydrogen to form a chemisorption product in the potential range 0-250 mV vs. NHE [25]. The rate increases with temperature. The reoxidation of the chemisorbed product is highly irreversible, with an activation energy of 22 kcal/mol on bright platinum. The appearance of an adsorption peak at 600 mV vs. SHE at 25°C in acid solutions containing methanol and formic acid was ascribed to the "reduced carbon dioxide" reported previously [26]. The same species supposedly appeared by oxidation either of formic acid or methanol. The reducible species on a dropping mercury electrode is CO2 (or possibly H2CO3) but not the bicarbonate or carbonate ions [27]. It was concluded that the primary stage is adsorption on the electrode, and that the mechanism of reduction of carbon dioxide is, COz + e- ~CO2 followed by CO2- + H 2 0 ~ H C O 2 - + O H

OH + e - ~ O H -

or

CO2 + 2 e - ~ C O 2-

CO22- + H E O ~ O H - + H C O 2 -

The kinetics of the cathodic reduction of carbon dioxide in neutral media at mercury electrodes was studied in detail by Paik et al. [28], by Ryu et al. [29] and recently by Hori and Suzuki 1-30]. They found that the most probable path is: CO 2 + e - .~-~CO 2(ads) CO2(ads)- + H20 ~HCO2(ads) +OH HCO2(ads) + e - ~ H C O 2 Russel et al. [31] confirmed the results ofPaik et al. [28] for various metal electrodes and suggested a pH dependent mechanism for the reduction of formaldehyde: HCHO + H20~H2C(OH)2 H2CO - + H + ~-H2COH H2COH + H + + e - -*CH3OH pH 6.8-9

HCHO + e - ~ H 2 C O H2CO- + e- ~ - H 2 C O 2 H 2 C O 2 - +2H + -*CH3OH pH 11.1-13.0

Zakharyan et al. [32] suggest for carbon dioxide reduction on noble metal electrodes the mechanism : C O 2 --*CO2(ads)

CO2 (ads) + e - ~-CO2 (ads) COz(ads)- + H 2 0 + e - -'* H C O 2 - + O H Ito et al. [33] found that electrochemical reduction of aqueous carbon dioxide

428

B. Aurian-Blajeni et al. / Photoelectrochemical reduction of aqueous C02

on metals takes place best on indium electrodes, with production of formic acid. Electrochemical reduction of aqueous carbon dioxide at high pressure (0-20 atm) was studied by Ito et al. [34] on metal electrodes (Zn, In, Sn and Pb). The reaction product was formic acid. The current efficiency increased with pressure up to 5 arm, at higher pressure remaining constant. They found that there is an optimum voltage for maximal current efficiency. After more than 300 C had passed, the concentration of formic acid decreased. Ito et al. [35, 36] studied the reduction of carbon dioxide in aqueous solutions of tetra-alkylammonium salts on various metal electrodes with high overpotential for hydrogen evolution. The products obtained were formic acid and traces of oxalic acid, propionic acid and n-butyric acid. Halmann [37] reported the photoelectrochemical reduction of aqueous carbon dioxide on p-type GaP, with optical conversion efficiencies at 365 nm of 3 to 5~o. The mechanism suggested is CO2+e

~CO2- +H+~HCO2

HCO 2 -t-e -+HCO 2

Inoue et al. [38] carried out similar photoelectrosynthetic reduction of CO2 on p-GaP, and also the cathodic reduction of CO2 in the dark on n-TiO2. Taniguchi et al. [39] investigated the reduction of carbon dioxide at p-gallium phosphide photocathodes in lithium carbonate electrolytes. They observed that the reduction occurs in competition with hydrogen evolution, and that the current efficiency is enhanced in the presence of crown ethers such as 15-crown-5 ether in the electrolyte. Monnier et al. [40] reported the cathodic reduction of carbon dioxide at titanium dioxide and ruthenium doped titanium dioxide electrodes yielding products up to methanol. In another publication, Monnier et al. [41] reported that carbon dioxide reduction occurs on these electrodes before the hydrogen evolution. By XPS they found a strong interaction of the dissolved carbon dioxide and the surface.

2. Experimental 2.1. Electrochemical methods

Galvanostatic experiments were performed in a two electrode cell. The current source was a dc current supply. The anode was bright platinum, glassy carbon or carbon rod and the cathode was a p-GaP single crystal. Potentiostatic experiments were performed in a two compartment, three electrode cell. The cathode was p-GaP or p-GaAs. The anode was glassy carbon, a carbon rod or bright platinum. 2.2. Irradiation cells

Experiments at atmospheric pressure were carried out in conventional reaction flasks of borosilicate glass, while bubbling through CO2 or argon.

B. Aurian-Blajeni et al. / Photoelectrochemical reduction of aqueous

CO 2

429

C02 4

12 Fig. 1. High pressure irradiation cell. 1 - Photoetectrode; 2 referenceelectrode ; 3 - counterelectrode; 4 samplingport with septum; 5 - pressure regulator ; 6 pressure gauge; 7 O-rings; 8 reaction cell: 9 separator; 10 - quartz window; 11 - insulated connection; 12 bolts : 13 - connectionsto potentiostat. The high pressure unit for studying the effects of pressure on carbon dioxide reduction was built from cylindrical stainless steel (fig. 1). The volume of the cell is about 500 ml ( I D = 6 6 mm, height =140 mm, O D = 7 9 mm). Laterally at 3 cm from the bottom, there is a circular window holder ( I D = l l mm). The window is made from optical quartz, 2 mm thick. It is sealed to the cell by means of an O-ring. Another O-ring is placed between the cover and the main body of the cell. The cover is fastened to the cell by 8 hexagonal screws. The cover is one of the main components of the cell. It has five electrical connections passing through insulating material (Sn soldered pressure tight glass disks). Three of the electrical connections are intended for the potentiostat. The other two serve alternatively for connecting inside a small light bulb for the alignment of the electrode, or for connecting a thermistor for temperature control inside the cell. The cover has also ports for sampling using a silicone rubber septum, reactant introduction and pressure monitoring and regulation. The pressure inside the cell also drives the liquid from the glass reaction cell towards the sampling port. The reference electrode used is ajunctionless one because of adsorption-desorption problems we had with liquid junctions (a saturated calomel electrode cannot be used, since gas is adsorbed in the saturated KC1). The absence of a junction between the solution and the reference electrode

430

B. Aurian-Blajeni et al. / Photoelectrochemical reduction ol aqueous (-:02

(silver/silver chloride) excluded the use of chloride and/or silver containing solutions. A silver/silver chloride electrode (SSE) was thus used as reference electrode which was prepared according to a method cited by Bailey [42]. The cathodic compartment was separated from the anodic one by a cation exchanger membrane (BDH). The apparatus was tested at the Israel Institute of Standards for pressures up to 35 kg/cm z 2.3. Irradiation methods

The radiation sources were either high-pressure mercury lamps (Hanau Q81 or Hanau Q718) or xenon lamps (Osram XBO 150). The light source during the electrochemical experiments was either a tungsten halogen lamp or a 150 W Xe lamp, with a maximum output of 7 suns. The light intensity was measured with a Yellow Spring Instruments Co. Radiometer (model

65A). 2.4. Materials

The purification of commercial carbon dioxide from organic impurities was carried out by passing the gas through a succession of two wash-bottles with chromic acid-sulfuric acid cleaning solution, then through two wash-bottles with distilled water, and finally an empty wash-bottle, before entering the irradiation flask. Samples from the second wash-bottle (which was cooled at 0°C) were analyzed as a blank, together with the samples from the irradiation flask and from the distillate traps. These "blank" analyses gave a check on the purity of the carbon dioxide used. Collection o f the distillate consisting of water and of the organic products was carried out in glass traps at 0°C. For the electrochemical experiments, several single crystals were used: Three p-type G a P single crystals, all of them zinc doped" a. one of 0.19 fl cm resistivity ; b. a second cut parallel to the (111 ) face, 1.312 f~ cm resistivity, 5.8 x 1016 cm-3 carrier concentration; c. a third, donated by Prof. W. P. Gomes from Gent, also (111) face, 0.11 f~ cm resistivity and 1018 cm -3 carrier concentration (MCP Electronics Ltd.). The area was variable due to breaking of crystals either when electrodes were manufactured or when subject to high pressure. Etching of p-GaP was done either with a 5 ~ solution of Br2 in methanol, or with a HC1 :HNO3 :CH3 C O O H (1:1:1) mixture [43]. The ohmic contacts were made by sputtering with gold. Subsequently a copper wire was attached with silver epoxy. Insulation of the back and sides of the electrode was made with an inert masking lacquer. A p-GaAs crystal came from Prof. Gerischer's laboratory, by the kindness of Dr. Norbert Miiller. Ohmic contacts were made with gallium indium alloy, and a copper wire was indium soldered to the contact. The electrodes were encapsulated in epoxy resin. Etching of the titanium dioxide crystals was made in a HNO3 :HF: H202 (3:1:4) solution (A. J. Bard, personal communication). As in the case of gallium phosphide, the area varied due to mechanical damage during the preparation of electrodes, or during high pressure experiments.

B. Aurian-Blajeni et al. / Photoelectrochemical reduction of aqueous C02

431

2.5. Analytical methods Methanol, acetaldehyde and ethanol were determined by gas chromatography on Porapak Q, using flame ionization detection. Formaldehyde was analyzed by spectrophotometry, using the color reaction with acetylacetone [44]. Formic acid was determined by its reduction with magnesium to formaldehyde, followed by the acetylacetone reaction.

3. Results

3.1. Current-potential relationships The reports about p-GaP photoelectrodes differ from laboratory to laboratory. Thus, good stability was reported for P-type gallium phosphide electrodes by Madou et al. [45], Bourasse and Horowitz [46], and Dare-Edwards et al. [47] while instability was reported by Yoneyama et al. [20] and Butler and Ginley [48]. In previous work on the photoassisted reduction of carbon dioxide on p-GaP, the electrode showed a marked decrease in photoresponse within a few hours of operation 1-37]. The variation of the open-circuit voltage of the p-GaP(a) electrode with the light intensity showed a saturation starting at about 5000 W/m 2. The same behavior was observed for all electrodes (fig. 2).

300 200 E

o

kOl

~

I Light

I

,

3 intensity

I

5

~ 1 ,

7 ( KW/m 2)

Fig. 2. Variation of the photopotential with the light intensity, p-GaP(a) working electrode; glassy carbon counterelectrode; medium, phosphate buffer 0.2 M, pH=6.2; illumination, tungsten halogen lamp; carbon dioxide atmosphere.

The full output of the xenon lamp used subsequently was about 7000 W / m 2. In spite of transmission losses, the light incident on the electrode had always an energy exceeding the saturation value of 6000 W/m 2. This was the intensity used in most experiments, the purpose being to work under saturation conditions, independently of the redox couple in solution. When the potential was kept constant, the current decreased with time in a quadratic manner (fig. 3). It seems that we are faced with the cathodic decomposition,

432

B. Aurian-Blajeni et al. / Photoelectrochemical reduction oJaqueous CO,

~- I

%

'

I

'

I

'

I

1.0

v

2

6 I0 time (h)

14

Fig. 3. Time evolution of the photocurrent with p-GaP electrode (b) at constant potential q-1.00 V vs. SCE): CO2 at atmospheric pressure: pH = 3: Pt counterelectrode.

observed by Butler and Ginley [48], but not by Madou et al. [45], nor by Bockris and Uosaki [10], but thermodynamically possible [45, 21]. This could be due to formation of an oxidic layer at the surface ( A H ( G a P ) = - 2 9 2 . 5 kcal/mol, AH(GazO3) = - 2 5 7 kcal/mol), passing.through a step of reduction of Ga 3 + ions at the surface (E°[Ga 3 +/Ga] = - 0 . 5 6 V vs. NHE)[49]. The influence of pH on the behavior of the p-GaP electrodes was studied in a one compartment, three electrode cell. Surprisingly, the i - V curve at various pH values showed a displacement of the onset potential towards more positive values with the increase in pH. The slope was of about 0.19 V/pH unit (fig. 4). Butler and Ginley [48] and Horowitz [50] reported displacement of the flatband potential by - 0 . 0 5 9 V/pH unit. For the time being we have no valid explanation for the above phenomenon, but it may be connected with the dependence of the CO2 solubility on pH. A strong photoeffect was seen for all pH values. The higher the pH the faster the initial decrease of the photocurrent. In the range of pH 6-7, the replacement of carbon dioxide by argon had no noticeable effect on the photocurrent (table 1 ). Measurements in table 1 were made at constant potential of either - 800 or - 200 mV. In the third and fourth rows (pH =2), the voltage was set at a lower value in order to maintain the same current density.

t

i

I

-8OO -600 :~ -400 0

-200

%

2

4 pH

6

Fig. 4. Influence of pH on the onset potential, p-GaPla) electrode :,Pt counterelectrode: SCE reference: carbon dioxide atmosphere.

B. Aurian-Blajeni et al. / Photoelectrochemical reduction of aqueous C02

433

Table 1 Effect of pH on the photocurrent, 1, of p-GaP(b) electrode. Counterelectrode Pt foil, 150 W Xe lamp Potential (mV vs. SCE)

pH

Gas

l(init) (mA)

1(3 h) (mA)

1(5 h) (mA)

-800 -800 -200 - 200

6 7 2 2

CO2 Ar

3.6 4.0 4.8 4.2

0.7 0.6 3.0 3.1

0.01 0.01 2.7 2.6

CO 2

Ar

3.2. Carbon dioxide pressure effects on current-potential curves The effect o f c a r b o n dioxide pressure on the behavior o f the electrode was checked in a specially designed cell (fig. 1). U n d e r pressure the current was fairly stable in time, as well as the onset potential. The photoeffect showed some hysteresis when the potential was swept between + 9 0 t o - 5 0 0 mV vs. SCE. Scans were performed between + 1 0 0 a n d - 3 0 0 mV vs. SCE and between the atmospheric pressure and 9 atm. In these experiments, the p h o t o c u r r e n t varied with pressure (fig. 5). The variation o f the onset potential with pressure is illustrated in fig. 6. The rest potential o f the electrode changed rapidly with pressure (fig. 7) and showed a strong hysteresis ; it started saturating at a b o u t 8 atm. I

I

160

1bias-260-~ Jmv,vsSCE

,/

,20

I

i.~

/

-

ine

40

,i=-200 /

.

v,S C E L ~

. ~

'

t

I

2

8 C 0 z pressure (arm)

Fig. 5. Variation of the photocurrent of p-GaP(b) with the C O 2 pressure. Pt counterelectrode ; electrolyte, HCIO4 0.1 M; illumination, 150 W Xe lamp.

3.3. Products o f the photoreduction o f carbon dioxide During the reduction o f c a r b o n dioxide with illuminated electrodes, the m a j o r p r o d u c t s were one c a r b o n c o m p o u n d s . A l t h o u g h t w o - c a r b o n c o m p o u n d s were detected they were not taken into a c c o u n t when calculating the F a r a d a i c yields

434

B. Aurian-Blajeni et al. / Photoelectrochemical reduction o f aqueous (?02 I

-.300

1

I

I

I

I

I

I

v

x

hi rJ GO

>

--.200

-

I

l

I

2

3

4

5 6 Pco2(alm)

I

I

7

8

9

Fig. 6. Variation of the onset potential with CO2 pressure, p-GaP electrode ; Pt counterelectrode ; SSE reference electrode. Electrolyte, 0.1 M Na2CO3. Illumination, 150 W Xe lamp.

(defined as the percentage of current passed involved in the reduction of carbon dioxide). The preparative experiments (for yield determination), were performed in two compartment cells. Fig. 8 shows that for illuminated p-GaP electrodes, carbon dioxide reduction takes place at more anodic potential than proton reduction. The same was reported by Monnier et ai. [40, 41] for titanium dioxide electrodes. Nevertheless, bubbles were seen evolving at the electrode surface during photoelectrolysis, presumably due to release of hydrogen. The values of the yield are always less than 100~ and the evolution of gas suggests that water electrolysis takes place in parallel. Therefore carbon dioxide may also be reduced by hydrogen formed by photoelectrolysis. The influence of the counterelectrode material on the Faradaic yield was checked for the p-GaP(a) electrode (table 2), in a two electrode system. The higher yield was obtained with a platinum counterelectrode. After this experiment, bright platinum was used as counterelectrode. The low yields obtained with carbon electrodes are due to the total absence of formic acid. Table 3 shows that the higher the current, the lower the yield. The lower inital voltage for the last two experiments is due to etching of the electrode before the experiment. A plot of the Faradaic yield as a function of current density shows that the higher the current density, the lower the yield (fig. 9). The same had been observed for copper amalgamated electrodes [24]. Experiments were performed in order to check the influence of the photoelectrode

B. Aurian-Blajeni et al. / Photoelectrochemical reduction of aqueous COz I

-50

I

1

I

I

I

I

I

~' increosingpressure ~ decreasingpressure

•t~ , ~ ,

435

_

0 O3

g

>

-4(

-5(

I 2

I 3

I 4

I 5

1 6

I 7

I 8

I 9

I0

Pco 2 (otm) Fig. 7. Variation of the rest potential o f p-GaP(b') in the dark with CO2 pressure. Electrolyte HCIO4 0.1 M ; counterelectrode Pt ; SSE reference.

i ( mA/cn~- ) -I.5 -I.0

dork • ".2

Ar,,~"/ / /"

.:

/'coa

--a5

v(vs SCE)

2 I

--I.0

.-I.5

Fig. 8. i-V characteristic o f p - G a P ( b ) electrode in a r g o n a n d carbon dioxide a t m o s p h e r e , in the dark a n d u n d e r illumination. Electrolyte, N a H C O a 0.5 M : SCE reference ; sweep rate 10 mV/s.

material. GaAs seems to be a better electrode than GaP(a), the respective Faradaic yields being 39.3 and 12.6% in Li2CO3 0.1 M medium, at a potential of - 1 . 0 V vs. SCE. The use of p-GaAs electrodes was discontinued due to their poor stability. The influence of the applied voltage on the p-GaAs electrode upon the Faradaic

B. Aurian-Blajeni et al. / Photoelectrochemical reductWn o/aqueous (5"0.

436

Table 2 The influence of the counterelectrode material on the Faradaic yield. Cathode p-GaP ; medium phosphate buffer: pH =6.3-6,5: carbon dioxide at atmospheric pressure: bias 2.1 V for carbon electrodes, 1.5 V for platinum : current density about 0.8 m A / c m z : illumination 6000 W/m 2 with Xe lamp: all the counterelectrodes had approximately the same area Anode

Electric charge passed

material

HCOOtt

l(')

carbon rod glassy carbon bright platinum

HCHO

Yield

tl~mol)

963 684 536

i~/o)

0.0 0.0 71.0

5.3 6.5 0.4

2.2 3.8 26.8

Table 3 Effect of quantity of current passed in galvanostatic regime, for a GaP(a) electrode. Counterelectrode Pt: CO2 at atmospheric pressure: 150 W Xe lamp; electrolyte. 0.5 M KC1; pH =5.5 Potential (mY)

Electric charge passed (C)

CH3OH

HCHO t/~mol)

360 540 721 1800

10 8 0 0

9 10 13 9

- 7 8 0 to - 1 0 5 0 - 7 8 0 to 980 - 2 0 0 t o - 900 - 2 0 0 t o - 1000

r5

-o

I

I

HCOOH

640 670 340 550

Yield I~) 36.9 25.5 9.8 6.t

I

I0

.o

o

£_ 5

5(30 Current density (/./,A/era 2)

i 8OO

Fig. 9. Variation of the Faradaic yield of CO2 reduction on p-GaP with the current density. Electrolyte N a H C O 3 0.5 M : system p-GaP(b)/0.5 M N a H C O f f P t . Illumination, 150 W Xe lamp.

B. Aurian-Blajeni et al. / Photoelectrochemical reduction of aqueous C02

437

Table 4 The influence of applied voltage on the Faradaic yield and product distribution for p-GaAs electrode. Counterelectrode Pt; reference electrode, sat. calomel; 150 W Xe lamp: carbon dioxide at atmospheric pressure ; electrolyte, 0.5 M KCI Potential (mV) -

1840 1440 1290 1140

Electric charge passed (C)

CH3OH

HCHO (/~mol)

HCOOH

Yield I~o)

216 720 555 724

0.0 0.0 0.0 0.0

0.0 0.0 5.0 0.0

170 320 200 60

15.2 8.6 7.3 1.6

yield and product distribution is that the more negative the potential, the higher the yield (table 4). This trend is evident, in spite of the fact that the quantity of the current passed varied. Under elevated carbon dioxide pressure it was possible to reach Faradaic yields of 80~o. Chemical results of experiments with p-gallium phosphide under pressure are shown in table 5.

4. Discussion

The results presented in table 5 show that by illuminating p-gallium phosphide in a potentiostatic regime under moderate carbon dioxide pressured in a neutral or weakly acid medium, good Faradaic yields can be obtained for the reduction of CO2. The main product is formic acid, which was also found in the dark electrochemical reduction of CO2 [30, 31]. At the more positive potentials given in table 1, no difference in current is observed in the presence as well as absence of CO2. In both cases the current decreases steeply with time. This decrease is accompanied by formation of a greyish-black deposit on the p-GaP electrode. Fig. 3 shows that at more negative potentials, a decrease in current is also found, but it is less steep. The peculiar results of table 2 show that the counterelectrode material is of prime importance, and that with carbon electrodes which demand high overpotentials no formic acid is formed at all. Presumably at their high positive potentials formic acid is reoxidized. That this also occurs, although to a lesser degree, on platinum anodes, is apparent from table 3, where in spite of large differences in current passed, the concentration of formic acid remained more or less constant, and in any case did not increase measurably with the current passed. From the results shown in table 5 it follows that with p-GaP the Faradaic yield decreases when the reaction is carried out at a more negative potential. The contrary is seen in experiments with a p-GaAs electrode, as shown in table 4. The different behavior of the two electrodes is probably connected with the greater stability of G a P towards anodic decomposition, while in the case of the unstable GaAs the more negative potentials provide a degree af cathodic protection.

438

B. Aurian-Blajeni et al. / Photoelectrochemical

5&

reduction o f aqueous CO~

© ©

0

~,~ e~ e~,oO~e~O

~×

[]

I

I

I I I

0

£ ~ = Z Z Z

I

B. Aurian-Blajeni et al. / Photoelectrochemical reduction o f aqueous CO 2

439

Fig. 6 shows how in weakly acid solutions (initially NaxCO3, but becoming under CO2 pressure essentially NaHCO3) the photocurrent onset changes by about 120 mV with a ten times increase in CO2 pressure, while the rest potential even changes by 200 mV over the same pressure range (fig. 7). In this range, the measured photocurrent is enhanced by a factor o f two (fig. 5). These effects indicate a rather strong interaction between dissolved carbon dioxide and the semiconductor surface, and a consequent shift o f the flatband potential to more negative potentials with increasing CO2 pressures. This may explain the strong beneficial effects of CO2 pressure on the Faradaic yields, which have also been observed with metal cathodes [33, 36]. Fig. 8 suggests that at too negative potentials hydrogen evolution will compete seriously with CO2 reduction, but that with a moderate bias CO2 reduction is the preferred mode. Therefore, several trends can be observed in the above measurements: Carbon dioxide reduction at an illuminated negatively biased p-gallium phosphide semiconductor electrode is possible with high Faradaic yields, provided a buffer is present (such as LiHCO3 or NaHCO3) and the CO2 is held at a pressure of several atmospheres. Adsorption of the CO2 on the semiconductor surface clearly plays an important role in the process. The electrodes are not stable, and deteriorate markedly within several hours. With longer electrolysis runs, a steady state of H C O O H seems to be formed, because of its reoxidation at the counterelectrode. When p-gallium arsenide is used as the photoelectrode, more negative potentials give higher yields, presumably because of more cathodic protection at these potentials [511. Carbon dioxide reduction to formic acid and other organic products is clearly possible on negatively biased p-type semiconductor electrodes which absorb in the visible range, but some serious drawbacks will have to be overcome in order to make this into a practical process.

Acknowledgements We wish to thank Ms. V. Katzir and Ms. K. Zuckerman for performing the chemical analyses. This research was supported in part by a grant from the United States-Israel Binational Science Foundation (BSF), Jerusalem, Israel.

References [i] J. Manassen, D. Cahen, G. Hodes and A. Sofer, Nature 263 (1976)97. [2] A.J. Nozik, Ann. Rev. Phys. Chem. 29 (1978) 189. [3] J. R. Bolton, Science202 (1978)705. [4] A. J. Bard, J. Photochem. 10 (1979)59. [5] A. Fujishima and K. Honda, Bull. Chem. Soc. Japan 44 (1971) 1148. [6] A. Fujishima and K. Honda, Nature 238 (1972)37. [7] J. G. Mavroides,J. A. Kafalas and D. F. Kolesar, Appl. Phys. Lett. 28 (1976)241.

440 [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]

B. Aurian-Blajeni et al. / Photoelectrochemical reduction oJ aqueous CO z

H. Gerischer, Solar Power and Fuels, ed. J. R. Bolton (Academic Press, New York. 1977) p. 77 A. J. Nozik, Appl. Phys. Lett. 30 (1977) 567. J. O'M. Bockris and K. Uosaki, J. Electrochem. Soc. 125 (1978) 223. J. G. Mavroides. Mater. Res. Bull. 13 (1978) 1379. M. Tomkiewicz and H. Fay, Appl. Phys. 18 !1979) 1. M. D. Archer, J. Appl. Electrochem. 5 (1975) 17. C. Hamann and P. Schmode, J. Power Sources 1 (197617) 141. R. Williams, R. S. Crandall and A. Bloom, Appl. Phys. Lett. 33 (1978) 381. V. A. Myamlin and Yu. V. Pleskov, Electrochemistry of Semiconductors (Nauka, Moscow, 1965L H. Gerischer, in: Physical Chemistry, IXA, ed. H. Eyring (Academic Press, New York, 1970) p. 463, H. Gerischer, Topics in Applied Physics. vol. 31, ed. B.O. Seraphin (Springer Verlag, Berlin, 1979) p. 115. [19] R. Memming, Electroanalytical Chemistry, vol. 11, ed. A. J. Bard (Marcel Dekker, New York, 1979) p. 1. [20] H. Yoneyama, H. Sakamoto and H. Tamura, Electrochim. Acta 20 (1975) 341. [21] K. Uosaki and H. Kita, J. Electrochem. Soc. 128 (1981) 215Y [22] M. E. Royer, Compt. Rend. Acad. Sci. 70 (1870) 731. [23] F. Fischer and O. Prziza, Ber. Deutsch. Chem. Ges. 47 (1914) 256. [24] K. S. Udupa, G. S. Subramanian and H. V. K. Udupa, Electrochim. Acta 16 (1971) 1593. [25] J. Giner, Electrochim. Acta 8 (1963) 857. [26] J. Giner, Electrochim. Acta 9 (1964) 63. [27] T. E. Teeter and P. Van Rysselberghe, Int. Committee of Electrochem. Thermod and Kinetics, Proc. 6th Meeting, Poiters. 1954 (Butterworth, London, 1955) p. 538. [28] W. Paik. J. N. Andersen and H. Eyring, Electrochim. Acta 14 (1969) 1217. [29] J. Ryu, T. N. Andersen and H. Eyring, J. Phys. Chem. 76 (1972) 3278. [30] Y. Hori and S. Suzuki, Bull. Chem. Soc. Japan 55 (1982) 660. [31] P. G. Russek N. Kovac, S. Srinivasan and M. Steinberg, J. Electrochem. Soc. 124 (1977) 1329. [32] A. V. Zakharyan, Z. A. Rotenberg, N. V. Osetrova and Yu. B. Vasilev, Elektrokhimiya 14 (1978) 1520. [33] K. Ito, T. Murata and S. Ikeda, Bull. Nagoya Inst. Technol. 27 ~1975) 209. [34] K. Ito, S. lkeda and M. Okabe. Denki Kagaku 48 (1980) 247. [35] K, Ito. S. lkeda, T. lida and H. Niwa. Denki Kagaku 49 (1981) 106. [36] K lto, S. Ikeda, T. lida and A. Nomura, Denki Kagaku 50 (1982)463. [37] M. Halmann, Nature275 H978)115. [38] T. lnoue, A. Fujishima, S. Konishi and K. Honda, Nature 277 (1979) 637 [39] Y. Taniguchi, H. Yoneyama and H. Tamura, Bull. Chem. Soc. Japan 55 11982) 2034. [40] A. Monnier, J. Augustynski and C. Stalder, Electroanal. Chem. 112 11980) 383. [41] A. Monnier, J. Augustynski and C. Stalder, Boulder Meeting on Photochemical Solar Energy Conversion (1980) p. 423. [42] P. L. Bailey, Analysis with Ion-Selective Electrodes. 2nd ed. (Heyden, London, 1980) p. 20 [43] E. Kaminska, A. Piotrowska, A. Kaminska and M. Klimkiewicz, Surface Technol. 12 (1981) 205. [44] H G. Wood and H. Gest, in: Methods in Enzymology, vol. lII, eds. S. P. Colowick and N. O. Kaplan (Academic Press, New York, 1957) p. 288. 1145] M. J. Madou, F. Cardon and W. P. Gomes, Ber. Bunsenges. Phys. Chem. 82 (1978) 819. [46] A. Bourasse and G. Horowitz, J. de Phys. 38 (1977) L291. [47] M. P. Dare-Edwards Hammett and J. B. Goodenough, J. Electroanal. Chem. 119 (1981) 109. [48] M. A. Butler and D. S. Ginley, J. Electrochem. Soc. 127 (1980) 1273. [49] N. N. Sirota. Chemical Bonds in Semiconductors and Solids (Consultants Bureau, New York, 1965) Translated from Russian. [50] G. Horowitz, J. Appl. Phys. 49 (1978) 3571. [51] H. Gerischer. N. Miiller and O. Haas, J. Electroanal. Chem. 119 (1981)41.