Catalytic Activation of Carbon Dioxide - American Chemical Society

Chapter

14

Electrochemical Activation of Carbon Dioxide K. Chandrasekaran

and J. O'Μ. Bockris

Downloaded by UNIV OF ARIZONA on December 12, 2012 | http://pubs.acs.org Publication Date: December 17, 1988 | doi: 10.1021/bk-1988-0363.ch014

Department of Chemistry, Texas A&M University, College Station, TX

77843-3255

The surface states at the semiconductor electrolyte interface under illumination for the electrochemical reduction of carbon dioxide has been determined to be 10 cm . Surface states are induced by adsorbed ions and act as faradaic mediators for the photoelectrochemical reduction of carbon dioxide. It is shown that CO is adsorbed on platinum and adsorbed CO 2 is the intermediate radical. The rate determ ining step involves further reduction ofCO 2to give the final products. Adsorption ofNH 4ions on p-GaP has been studied using FTIRRAS. At cathodic poten tials adsorbed ammonium ions are reduced and the reduced ammonium radical desorbs. The structure of adsorbed ammonium is investigated. 14

-2

2

-

-

+

Electrochemical reduction of carbon dioxide provides one method of converting this p l e n t i f u l l y available substance to useful fuels. It can be carried out b i o l o g i c a l l y (1-2^ as i n photosynthesis; i n the gas phase (3-4), heterogeneously (5-7); electrochemically (8-15) or photoelectrochemically (18-20). The e f f i c i e n c i e s of the b i o l o g i c a l and heterogeneous processes are impractically small. E l e c t r o chemical reduction of carbon dioxide has been carried out on several metal electrodes (21-25), although a large overvoltage i s required. Electrocatalysts (26-27) can be used to decrease this overvoltage. It has been proposed that the slow r a t e _ i s due to the formation of a one-electron reduction intermediate, C02> which i s involved i n the rate determining steps (28).

0097-6156/88/0363-0179$07.50/0 © 1988 American Chemical Society

In Catalytic Activation of Carbon Dioxide; Ayers, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

180

CATALYTIC ACTIVATION OF CARBON DIOXIDE

The proposed mechanism involves both adsorbed CO^ and CCL r a d i c a l s . However, there_has been hitherto no d i r e c t evidence for the presence of adsorbed C0^ r a d i c a l . Light energy may be used to reduce the necessary e l e c t r i c a l potential i n photoelectrochemical reactions. The overpotential i s decreased by 700 mV for the photoelectrochemical reduction of CO^ on p-CdTe, compared to that on indium - the best metal electrode for (X> reduction. For these semiconductors which involve a high concentration of surface states, the double layer at the semiconductore l e c t r o l y t e interface plays an important role i n the k i n e t i c s of photoelectrochemical reactions. In this paper, we report spectroscopic and impedance aspects of the electrode-electrolyte interface as affected by reactants and radicals involved i n C0 reduction. 2


2

EXPERIMENTAL Preparation of Electrode Single c r y s t a l CdTe (100) and GaP (110) was cut into 1mm thin wafers. The wafers were wiped clean and sonicated i n absolute ethanol for 30 minutes. One side of the wafer was etched with aqua-regia for 30 seconds and washed. An ohmic contact was made with Ga-In a l l o y . The l a t t e r was prepared by mixing equal amounts of Ga and In (wt/wt) at 120°C for 10 minutes under a nitrogen atmosphere to avoid oxide formation. The molten a l l o y was cooled to room temperature i n this atmosphere. Ohmic contact was made at two d i f f e r e n t positions on the etched face and the resistance between these two contacts was measured. The p o l a r i t y of the terminals were changed and the resistance was measured again. Etching and rubbing of the Ga-In a l l o y was continued u n t i l the same resistance for current passage i n each d i r e c tion was obtained. After t h i s , an ohmic contact was made on the entire face. The electrode holder was made of copper rod. The rod was covered with teflon to avoid contact with solution. The front side of the electrode was etched for 30 seconds i n aqua regia, rinsed with water and immersed into the e l e c t r o l y t e immediately. Before carrying out experiments, the electrode was cycled between -0.56 and -2.24 NHE for c. 20 minutes. Electrolytes Tetraalkylammonium perchlorate (TBAP)(Fluka) was r e c r y s t a l l i z e d from ethanol. Dimethylformamide was used without further p u r i f i c a t i o n . T r i p l y d i s t i l l e d and pyrolyzed water was used. 0.1 M t e t r a a l k y l ammonium perchlorate i n dimethylformamide - 5% water mixtures were used as e l e c t r o l y t e for the photoelectrochemical reduction of C0~ to CO. C e l l Compartment The c e l l was made up of PYREX glass with an o p t i c a l quartz window i n the front (Fig. 1). Reference and working electrode compartments were fixed on the sides. The working electrode was mounted on the c e l l by means of a teflon screw for ease of position adjustment. CO^ was bubbled through chromic acid and DMF to remove impurities, e.g., methanol vapor. A C0 blanket was maintained during the 9


14.

CHANDRASEKARAN AND BOCKRIS

Electrochemical

181

Activation

e x p e r i m e n t s . CdTe e l e c t r o d e s were i l l u m i n a t e d w i t h 555 nm l i g h t w h i l e GaP e l e c t r o d e s were i l l u m i n a t e d w i t h 436 nm l i g h t .


Impedance Measurements Experiments were c a r r i e d out under p o t e n t i o s t a t i c c o n d i t i o n s u s i n g an 1172 S o l a r t r o n Frequency Response A n a l y z e r and 1186 S o l a r t r o n E l e c t r o c h e m i c a l I n t e r f a c e . A s m a l l ( i n p u t ) a m p l i t u d e (10 mV) s i n e wave ( P s i n u)t) was a p p l i e d t o the system under s t u d y . The response of the system t o the a p p l i e d p e r t u r b a t i o n was m o n i t o r e d as a s i n e wave c u r r e n t [Y s i n (u)t + θ )] and a s i n e wave p o t e n t i a l [X s i n ( u t + θ ) ] . These were transformed i n t o the complex form A + i Β and A* + i Β , r e s p e c t i v e l y . The r e a l and imaginary parts' o f the* imped ance were computed u s i n g the r e l a t i o n t = (A + i Β )/(A + i Β ) where the phase s h i f t θ i s θ - θ . y χ The AC p o t e n t i a l o u t p u t was measured between the w o r k i n g e l e c t r o d e and the r e f e r e n c e e l e c t r o d e , and the AC c u r r e n t measured between the w o r k i n g e l e c t r o d e and the c o u n t e r e l e c t r o d e . Thus, t h e impedance between the w o r k i n g e l e c t r o d e and the L u g g i n c a p i l l a r y was measured. The DC p o t e n t i a l o f the w o r k i n g e l e c t r o d e was c o n t r o l l e d e i t h e r by means o f the p o t e n t i o s t a t o r the Frequency Response A n a l y z e r . A 1000 ohm s t a n d a r d r e s i s t o r was used to measure the DC c u r r e n t . Ten r e a d i n g s were averaged a t each f r e q u e n c y . The f r e q u e n c y range used was from 0.1 Hz t o 9999 Hz. Ten r e a d i n g s were r e c o r d e d per decade of f r e q u e n c y w i t h a d e l a y time o f 10 sec between r e a d i n g s t a k e n a t each f r e q u e n c y . y

FTIR

v

x

x

Spectra

A p l a t i n u m f o i l was used as a w o r k i n g e l e c t r o d e . I t was 10 mm i n d i a m e t e r , f i x e d on a p o l y e t h y l e n e r o d . The t i p o f the rod was melted and c o o l e d t o p r o v i d e a l e a k p r o o f s e a l i n g . The e l e c t r o d e was p o l i s h e d w i t h 0.05pm a l u m i n a p a s t e . L i C l O ^ (0.4M) d i s s o l v e d i n HPLC grade a c e t o n i t r i l e ( F i s c h e r S c i e n t i f i c ) was the s o l v e n t . The s o l u t i o n was p r e - e l e c t r o l y z e d i n a n i t r o g e n atmosphere f o r 2 hours t o remove r e s i d u a l w a t e r . The f i n a l water c o n t e n t o f the s o l u t i o n was e s t i m a t e d by means o f c y c l i c voltammetry t o be 0.01%. Carbon d i o x i d e was bubbled through the s o l u t i o n f o r 40 minutes and the c e l l was s e a l e d . S i m i l a r r e s u l t s were observed when the gas was bubbled through the s o l u t i o n c o n t i n u o u s l y d u r i n g the e x p e r i m e n t . A p l a t i n u m c o i l was used as the c o u n t e r e l e c t r o d e and Ag/Ag ( a s i l v e r w i r e i n a c e t o n i t r i l e c o n t a i n i n g 0.1M s i l v e r n i t r a t e ) was the r e f e r e n c e e l e c t r o d e . The e l e c t r o d e was p o t e n t i o s t a t i c a l l y p o l a r i z e d i n the r e g i o n o f 0.0 to -2.2V NHE (-0.8 to -3.0V v s Ag/Ag ) and IR s p e c t r a were r e c o r d e d a t e i g h t d i f f e r e n t p o t e n t i a l s i n t h i s r e g i o n . A D i g i l a b FTS-20E s p e c t r o m e t e r w i t h Nova 4 computer was used t o r e c o r d the s p e c t r a o f the adsorbed s p e c i e s u s i n g p o l a r i z a t i o n m o d u l a t i o n approach ( 3 0 ) . D e t a i l e d d i s c u s s i o n o f the i n s t r u m e n t a t i o n i s g i v e n elsewhere ( 3 0 ) .


182


RESULTS


E f f e c t o f S u r f a c e Treatment The onset p o t e n t i a l f o r t h e f r e s h l y etched p-CdTe f o r the photo e l e c t r o c h e m i c a l r e d u c t i o n o f c a r b o n d i o x i d e i s -0.76V NHE. When t h e e l e c t r o d e was c y c l e d between -0.56 t o -2.24V under i l l u m i n a t i o n , t h e onset p o t e n t i a l f o r the photocurrent s h i f t s to l e s s cathodic poten t i a l s and remains c o n s t a n t a t -0.66V a f t e r about c. 20 minutes. When the e l e c t r o d e i s p o t e n t i o s t a t e d a t -2.0V the p h o t o c u r r e n t remained c o n s t a n t f o r about 24 h o u r s . These r e s u l t s a r e r e p r o d u c i b l e and con s i s t e n t w i t h the p u b l i s h e d d a t a . S u r f a c e a n a l y s i s of t h e etched s u r f a c e o f p-CdTe u s i n g XPS and SEM showed o n l y t r a c e amounts o f c a r bon and oxygen. S e v e r a l e t c h i n g procedures were attempted f o r p-CdTe f o r t h e p h o t o e l e c t r o c h e m i c a l r e d u c t i o n of c a r b o n d i o x i d e . E t c h i n g w i t h d i l u t e t h i o s u l f i t e o r bromine i n methanol d i d n o t r e s u l t i n b e t t e r p h o t o c u r r e n t - p o t e n t i a l r e l a t i o n s h i p . Hence, i t was concluded t h a t e t c h i n g w i t h aqua r e g i a f o l l o w e d by r i n s i n g w i t h water i s t h e b e s t s u r f a c e treatment f o r the p h o t o e l e c t r o c h e m i c a l r e d u c t i o n of carbon d i o x i d e . A l l t h e impedance r e s u l t s d e s c r i b e d below were recorded using t h i s surface i n contact with e l e c t r o l y t e . Photoelectrochemical

Reduction

o f CO^ on CdTe

P h o t o c u r r e n t - P o t e n t i a l R e l a t i o n s h i p . The p h o t o c u r r e n t - p o t e n t i a l curve under monochromatic l i g h t (555 nm) i n a DMF ( 5 % H O ) s o l u t i o n c o n t a i n i n g 0.1M TBAP i s shown i n F i g . 2. I n the absence of (Χ> , t h e p h o t o c u r r e n t s t a r t s t o i n c r e a s e a t -1.4V NHE due t o hydrogen e v o l u t i o n . When C 0 gas i s bubbled through t h e s o l u t i o n , the onset p o t e n t i a l f o r t h e p h o t o c u r r e n t i s s h i f t e d t o l e s s cathode p o t e n t i a l s by about 700 mV. The r e d u c t i o n product was found t o be carbon monoxide. A t low c a t h o d i c p o t e n t i a l s an a n o d i c c u r r e n t i s observed. 2

2

Impedance Spectrum o f CdTe-DMF ( 5 % H 0 ) C o n t a i n i n g 0.1 TBAP. When the r e a l ( Z * ) and imaginary (Z") impedances a r e r e c o r d e d under i l l u m i n a t i o n as a f u n c t i o n o f frequency from 0.1 t o 9999 Hz a t -0.76V NHE, the r e a l p a r t o f t h e impedance i s found t o d e c r e a s e w i t h i n c r e a s i n g frequency ( F i g . 3 ) , w h i l e t h e imaginary p a r t o f t h e imped ance passes through a pronounced maximum a t i n t e r m e d i a t e f r e q u e n c i e s and a s u b s i d i a r y maximum a t h i g h f r e q u e n c i e s . The complex plane p l o t measured a t -1.8V v s NHE i s shown i n F i g . 4. P a r t s of t h e p l o t can be r e p r e s e n t e d by s e m i c i r c l e s . I n most c a s e s , t h e p l o t s c a n be d i v i d e d i n t o t h r e e s e m i c i r c l e s w h i c h , as shown l a t e r , correspond t o t h e dominance o f d i f f e r e n t p a r t s o f t h e e q u i v a l e n t c i r c u i t i n d i f f e r e n t frequency ranges. 2

P o t e n t i a l Dependence o f t h e Impedance S p e c t r a . I n F i g . 5, t h e maximum of the Z" - frequency p l o t i s g i v e n as a f u n c t i o n of p o t e n tial. The maximum v a l u e o f Z" decreases as the e l e c t r o d e p o t e n t i a l i s made more c a t h o d i c . F i g . 6 shows t h e dependence o f frequency ( f ) , a t which t h e maximum i n t h e Z"-frequency p l o t o c c u r s , as a f u n c t i o n o f e l e c t r o d e p o t e n t i a l . As shown, t h e p o t e n t i a l a t which f i s observed s h i f t s m

a

x



Electrochemical Activation

183


14.

Figure 2. Photocurrent potential curve for the photoelectrochemical reduction of carbon dioxide on p-CdTe i n DMF (5% HÔ) solution containing 0.1M TBAP.



184


Log Frequency

Figure 3. Bode plot for CdTe - DMF (5% HO) interface containing 0.1M TBAP under illuminations (555nm, 2.5mw/cm ) at -0.76 NHE. C0 1 atmosphere. ?

Z71000

Figure 4. containing

Complex plane plot for CdTe - DMF (5% H 0) interface 0.1M TBAP. Conditions as i n F i g . 2. 2



14.


Electrochemical

185

Activation

-0.56

Figure 5. Imaginary impedance as a function of bias potential for CdTe - DMF (5% H 0) interface. Conditions as i n Fig. 2. 2


186


to higher frequencies with increase i n electrode potential i n the cathode direction. Impedance Spectra i n the Presence of Tetraalkylammonium Salts. The observation of the impedance spectra obtained at the same electrode potential, -0.96V (NHE), and i n the presence of a series of t e t r a alkylammonium cation shows that the maximum which i s observed i n the low frequency region is shifted to lower frequencies as the carbon chain length i s decreased, while the frequency at which the maximum is observed at higher frequencies remains constant. Z' i s observed to decrease with decrease i n the carbon chain length.


Photoelectrochemical

Reduction of CO^ on GaP

Impedance Spectrum of GaP-aqueous DMF Containing 0.1M TEAP. F i g . 7 shows Z" as a function of frequency for p-GaP during the reduction of C0 i n aqueous DMF containing 0.1M TEAP. Z" passes through a broad maximum between 1-100 Hz and a shoulder i s observed on the high f r e quency side of the maximum. Z decreases with an increase i n the frequency with an i n f l e c tion corresponding to the Z" maximum. 2

1

Potential Dependence of Impedance Spectra. F i g . 8 shows the frequency at which Z" maximum occurs as a function of electrode potential. Contrary to the behavior i n CdTe, where Z" decreases with increasing cathodic potential, Z" passes through a minimum at -1.16V (NHE). Adsorption of CO^ and CO^ on Platinum The spectra reported here were obtained by subtracting from the spectra at various potentials, the reference spectrum at 0.0V NHE; the spectra reported here are thus termed difference spectra. The difference spectrum of surface adsorbed species on platinum i n a c e t o n i t r i l e at -1.2V NHE i s shown i n F i g . 9. The peaks pointing downward show a decrease i n surface concentration with respect to the reference potential, 0.0V NHE, and the peaks pointing upwards i n d i cate an increase i n surface concentration. Three peaks pointing _^ downwards and one peak pointing upwards i n the region 2400-1500 cm are seen i n F i g . 9. _^ There i s a broad maximum centered around 1680 cm , the intensity o| which increases i n the cathodic d i r e c t i o n . Absorbance at 1680 cm as a function of electrode potential i s shown i n F i g . 10. F u l l width at half maximum for this peak i s 40 cm . The r e l a t i v e area under the peak has been taken as proportional to the concentration of adsorbed species on the surface of the electrode, an assumption which i s acceptable at l e ^ s t up to θ = 0.5. The area of the peak i n the region 1650-1700 cm i s shown i n F i g . 11 as a func tion of electrode potential. Adsorption increases i n the cathodic d i r e c t i o n and reaches a saturation value around -2.0V NHE. A sharp peak centered at 2342 cm i s also observed and may be i d e n t i f i e d (31) with adsorbed C0 ( F i g . 12). Absorbance corresponding to the C=N stretching vibrations of adsorbed a c e t o n i t r i l e (32) i s observed at 2250 cm . The spectra of adsorbed C0 and adsorbed CH CN at various electrode potentials are 2

9

Q



Electrochemical

187

Activation


14.

Figure 8. Frequency of imaginary impedance maximum as a function of bias potential of GaP e l e c t r o l y t e interface containing CC> under illumination. 2



188


Figure 9. The d i f f e r e n t i a l spectrum of surface adsorbed species on platinum i n a c e t o n i t r i l e at -1.2V NHE.




14. CHANDRASEKARAN AND BOCKRIS


189

190


shown i n F i g . 12. The area of the peak centered at 2250 cm decreases with an increasingly cathodic p o t e n t i a l .


Adsorption of NH*

on

GaP

Ammoniumîons i n a c e t o n i t r i l e solution show a broad maximum around 3400 cm corresponding to the symmetric N-H stretching and a sharp band around 1600 cm for N-H asymmetric stretching (33). Ammonium ions adsorbed on GaP shows a broad maximum centered at 3475 cm and 3000 cm (Fig. 13). There are two maxima corresponding to N-H deformation i n the region 1700 cm" and 1560 cm" ( F i g . 13). The potential dependence of a l l these four peaks follow the same trend (Fig. 14). Absorbance increases i n the beginning and then decreases at more cathodic potentials. DISCUSSION The Equivalent C i r c u i t An appropriate equivalent c i r c u i t can be created i f the sequence of events between the creation of hole-electron pairs and (e.g.), the acceptance of electrons i n solution i s c l e a r l y known. The p r i n c i p a l a r b i t e r of an equivalent c i r c u i t i s the degree to which i t represents trends i n impedance as a function of frequency. An equivalent c i r c u i t for a photoelectrochemical system has to take into account: 1. The generation of electron-hole pairs; 2. Passage of c a r r i e r s through the space charge region; 3. Passage of c a r r i e r s through the surface states; 4. Passage of c a r r i e r s through the double layer; 5. The c i r c u i t must allow for the fact that both holes and electrons are generated but move i n opposite d i r e c t i o n s . Considering the sequence of events at the semiconductorsolution interface, the four c i r c u i t s shown i n F i g . 15 were a l l used to simulate the r e s u l t s . I t i s seen that the c i r c u i t 15d f i t s the results to a greater degree than do other c i r c u i t s . I t i s reason able, therefore, to conclude that the appropriate c i r c u i t for the evaluation of Ν (the surface state concentration per square cm) i s 15d. s s

Evaluation of Parameters The t o t a l impedance of the c i r c u i t given i n F i g . 15 for the c i r c u i t 15d i s given by the following equations.

PL 2 z2

1+to C R

2z

R sc sc

2

C sc sc 2 2 2 l+o) C R sc sc z

z

2 2 2 l+u> C R ss ss R

2 2

+ R + R

(4) so'

SL DL

1 + Ω

R

DL DL

2

C ss ss 2 2 2 l+o) C R ss ss

2

R

C

2 2

z

1 + Ω

(5)

2

SL DL R

where C i s the space charge capacitance, C i s the surface state capacitance, C ^ i s the double layer capacitance, R i s the space charge resistance, R i s the surface state resistance, R - i s the gc

D



14.


191

Electrochemicat Activation

CO _ Ζ D >-

DC < OC

1 8.75 x ΙΟ -0.8

-1.6

-2.4

2400

V/N Η Ε

Figure 11. Relative_concentration of adsorbed CO2 on platinum (see text).

2350

-4

I 2300

2250

2200

Wavenumbers

Figure 12. Adsorption spectra of adsorbed C0 (2340 cm"") and CH3CN (2250 cm ^ on platinum in a c e t o n i t r i l e containing 0.4M LiC10 . 1

2

4


192

CATALYTIC ACTIVATION OF CARBON DIOXIDE N-H STRETCHING


1

I

,

4000

ι

3500

3000

I

2500

N-H DEFORMATION

I

2000

Wavenumbers

•

1800

•

1600

•

1400

. wnm

1200

1000

Wavenumbers

Figure 13. Absorption spectrum of ammonium ion (N-H stretching and N-H deformation) on GaP i n a c e t o n i t r i l e .

160

0

-0.16

-0.56

-0.96

-1.36

-1.76

-2.16

V/NHE

Figure 14. Potential dependence of ammonium ion adsorption on GaP. (.) 3475 c m , (o) 1700 cm and (x) 1560 c m .




193

double l a y e r r e s i s t a n c e , R i s t h e s o l u t i o n r e s i s t a n c e and ω i s the so frequency. The Z - f r e q u e n c y p l o t s h o u l d pass through t h r e e maxima. I n f a c t , i n a m a j o r i t y o f t h e e x p e r i m e n t s , o n l y two maxima were ob s e r v e d , a l t h o u g h i n a m i n o r i t y , t h r e e maxima o c c u r r e d . S i n c e t h e Z" maxima i n t h e Bode p l o t s a r e w e l l s e p a r a t e d , t h e impedance c o n t r i b u t i o n o f one t o another c a n be taken as n e g l i g i b l e . At Ζ" , coCR = 1, hence Z" = R/2 and C = l/u)R. C a p a c i t a n c e s and r e s i s t a n c e s c a l c u l a e d f r o m c o m p l e x plane p l o t s ( F i g . 4) g i v e n u m e r i c a l v a l u e s o f C&R which d i f f e r by l e s s than 20% w i t h those from the Z&Z'-frequency r e l a t i o n s . C a p a c i t a n c e s and r e s i s t a n c e s c a l c u l a t e d f o r t h e two maxima a t v a r i o u s b i a s p o t e n t i a l s a r e shown i n T a b l e 1. C a p a c i t a n c e s c a l c u l a t e ^ from t h e h i g h frequency maximum a r e o f t h e o r d e r o f 0.1 pF cm and i n c r e a s e w i t h i n c r e a s i n g c a t h o d i c p o t e n t i a l . The time c o n s t a n t remains c o n s t a n t i n t h e p o t e n t i a l r e g i o n s t u d i e d . The v a l u e o f the c a p a c i t a n c e and the p o t e n t i a l dependences a r e c l o s e t o those expected f o r the space charge r e g i o n and hence a r e a s s i g n e d t o this. T a b l e I . C a p a c i t a n c e s and R e s i s t a n c e s f o r CdTe i n DMF ( 5 % H O ) . C o n t a i n i n g 0.1 M TBAP M


a

V (NHE) -0.56 -0.76 -0.96 -1.16 -1.36 -1.56

R

C

C

R

ss ohms

ss UF

sc ohms

sc yF

2940 2880 1440 360 156 84

421.6 421.6 341.6 133.3 30.0 18.33

240 300 288 204 132 85

0.331 0.265 0.276 0.390 0.693 0.948

R

DL ohms

C

500 250 150 63 28 23

100 100 100 100 100 100

DL UF

The c a p a c i t a n c e s c o r r e s p o n d i n g t o t h e low f r e q u e n c y maximum a r e of t h e o r d e r o f 10-400 μ F cm and decrease i n magnitude w i t h i n c r e a s i n g l y c a t h o d i c p o t e n t i a l . R e s i s t a n c e s c o r r e s p o n d i n g t o these maxima a r e o f t h e o r d e r o f 1000 ohms and decrease w i t h i n c r e a s i n g c a t h o d i c p o t e n t i a l . S i n c e most o f t h e c a p a c i t a n c e v a l u e s a r e h i g h e r than those c h a r a c t e r i s t i c o f the double l a y e r , and v a r y w i t h poten t i a l , they may be a t t r i b u t e d t o s u r f a c e s t a t e s . The s u r f a c e s t a t e r e s i s t a n c e i s l a r g e r than t h e space charge r e s i s t a n c e i n the T a f e l r e g i o n . Were t h e r a t e - d e t e r m i n i n g s t e p f o r the p h o t o e l e c t r o n s l i e i n t h e space charge r e g i o n and n o t a t t h e i n t e r f a c e , t h e r e s i s t a n c e o f t h e space charge r e g i o n would be g r e a t e r than t h e v a l u e f o r the s u r f a c e s t a t e s . Hence, t h e r a t e d e t e r m i n i n g s t e p l i e s n o t i n t h e space charge r e g i o n , b u t a t t h e i n t e r f a c e . A t s u f f i c i e n t l y c a t h c d i c p o t e n t i a l s , t h e space charge r e s i s t a n c e does become r e l a t i v e l y g r e a t e r than o t h e r s e r i e s r e s i s t a n c e s , c o n s i s t e n t w i t h t h e c o n c l u s i o n t h a t t h i s r e g i o n becomes r a t e d e t e r m i n i n g a t high current densities ( i . e . , a t s u f f i c i e n t l y high current densi t i e s , a t r a n s p o r t c o n t r o l l e d l i m i t i n g c u r r e n t i s observed ( 3 4 ) .


194


Calculation of Sruface State Capacitance

at a Fixed Potential

The number of surface states at a given electrode potential can be calculated without model assumptions from the d i f f e r e n t i a l surface capacitance, using the r e l a t i o n (35). 1


e Ν = Q = \ C dV ο ss ss ss ο where Ν i s the number of surface states per unit area at the potential V , C i s the d i f f e r e n t i a l surface state capacitance and V i s the potential at which the surface state capacitance i s zero (?n practice, the minimum value on the C potential p l o t ) . The surface state capacitance for tee CdTe-electrolyte i n t e r face i s plotted as a function of electrode potential i n F i g . 16 (the minimum was taken as the value at 0.2V NHE). The surface state capacitance decreases i n the cathodic d i r e c t i o n i n the region -0.56 to -2.26V (NHE). Capacitance measurements at cathodic potentials less negative than -0.56V could not be carried out because of the onset of a CO^-independent anodic dark current. Assuming ( i n con sistence with other examples of pseudo capacitance behavior) that the capacitance-potential curve i s symmetrical with respect to a maximum at -0.66V, the number of surface states was calculaed using the above equation. The number of surface states as a function of electrode potential, on the basis of this assumption, i s shown i n Fig. 17. Geometric area of the electrode was used to calculate the surface state density. Real surface area may be larger. As to the nature of the surface states represented by F i g . 17, these are not l i k e l y to be c a l s s i c a l surface states provided, e.g., by dangling bonds. Such bonds ( c h a r a c t e r i s t i c of the semiconductorvacuum interface) are l i k e l y to have been removed by the adsorption of water from the solution. They would not have been expected to vary with potential (cf. F i g . 17). The surface states being measured here may result from adsorption of ions from solution. These kinds of surface states are expected to vary with potential, solvent, electrode, e l e c t r o l y t e and current density. Adsorption of the tetraalkylammonium ion i n a c e t o n i t r i l e on p - s i l i c o n has been studied by FTIR relection-absorption spectroscopy (30). The ad sorption isotherm resulting from such measurements i s similar i n nature to the surface state density data for this ion on CdTe. The fact that the measured surface state density on CdTe varies with the nature of the cation (Table 2) i s consistent with the concept that the states a r i s e from ionic adsorption. Table I I .

Surface State Capacitance for Several E l e c t r o l y t e s at the CdTe-DMF (5% H 0) Interface 2

Electrolyte Et.NCIO, 4 4 Pr.NCIO 4 r Br.NCIO 4 r Ot.NCIO, 4 4

C

ss

uF cm

-2

r

32 30.4 16.0 4.6




Electrochemical

Log Frequency

Activation

195

Log Frequency

Figure 15. Simulated imaginary impedance - frequency plot ( C

g c

0.276 F, R = 0.288 k , C = 341 F, R = 1.44k , C = 100 F, ' sc ' ss ' ss ' DL = 250 ) (X) and experimental results (*). m

R

500 h

+0.24

+0.04

-0.16 -0.36 -0.56 -0.76 -0.96 -1.16 -1.36

-1.56

V(NHE)

Figure 16. Surface state capacitance as a function of bias potential f o r the CdTe e l e c t r o l y t e interface.



196


Thus, i n this model, the surface states formed by the t e t r a alkylammonium cation would be acting as faradaic mediators. Photogenerated electron passes through the space charge region to surface states, and then to acceptor species i n solution. Thus, for the region of potential below the transport controlled l i m i t i n g currenj (Table 1), e.g., at -0.96V NHE, the surface state r e s i s t i v i t y (cm ) is some f i v e times greater than the correspondingly resistance i n the space charge region. A possible model consistent with these facts would have adsorption of tetraalkylammonium ion ( i . e . , the formation of surface states) as the rate determining step. The subsequent electron transfer to CO i s evidently rapid i n this case. Taniguchi et a l . (36) have found that addition of ammonium ion to the solution caused s i g n i f i c a n t c a t a l y s i s of the photoelectrochemical reduction of carbon dioxide and suggested the mechanism. +

NH. 4

+ e~

NH; + co

0

+

+

NH;

4

NH* + co~

4

2 4 2 Such a mechanism would be consistent with the present r e s u l t s . The absence of the third maximum can be shown to be consistent with the g i s e n t picture.. Thus, i f the exchange current density i s 10~ A cm , C = 60.10 cm , then at w = 0.1 HZ, Z" = 20 ohms cm , assuming R = RT/i F, the equilibrium value. The value would in fact be less because a? -0.96V (NHE), the electrode i s 600 mV negative to equilibrium and R^ would become n e g l i g i b l e compared with the surface state resistance. It follows that the r e c i p r o c a l of R would be proportional to the measured photocurrent. Such a r e l a t i o n i s shown in F i g . 18 and is consistent with the model suggested. 2

E f f e c t of Electrolytes Perchlorate s a l t s of tetraalkylammonium ions were chosen as electrolytes for this study, because they reduce hydrogen evolution. The chain length of the a l k y l group was varied, i . e . , ethyl, propyl, butyl, and o c t y l . A decrease i n photocurrent was observed when the carbon chain length was increased. The surface state resistance was found to increase with increase of chain length. Correspondingly, a decrease i n surface state capacitance was observed. These results indicate that tetraethylammonium ions are adsorbed stronger than tetrabutylammonium ions. The anamolous behavior of tetraalkylammonium ions can be explained as follows. Hydration of these organic ions may be weak. In DMF (5% HO) solution, the s o l u b i l i t y of tetraalkylammonium ion increases with increase of a l k y l chain length and hence, the adsorption of cation decreases with increase of chain length. E f f e c t of Solvent When the water concentration i n the DMF water mixture i s increased from l%-25%, the surface state resistance decreases and there i s a concurrent increase of surface state capacitance. Such a change would be expected to occur i f there were adsorption of ions when the water concentration i s increased, and this i s indeed the case (37).


14.


Electrochemical

Activation

3

2 -

Ο X


1r

+0.24 +0.04 -0.16 -0.36 -0.56 -0.76 -0.96 -1.16 -1.36 -1.56 -1.76 -1.96 V(NHE)

Figure 17. Surface state density as a function of bias poten t i a l calculated from the capacitance data. 90

80

70

< 60

C ξ 50

ο ο ο £ 40

30 20

10

-0.56

-0.76

-0.96

-1.16 -1.36 V(NHE)

-1.56

-1.76 -1.96

Figure 18. Relative rate constant f o r the photoelectron trans fer across the surface state as a function of bias potential. Photocurrent measured under i d e n t i c a l conditions i s shown i n dotted l i n e s .


198


GaP-electrolyte Interface The impedance data for the GaP-electrolyte interface can be repre sented by the equivalent c i r c u i t discussed for the CdTe electrode. The surface state capacitance calculated by a similar procedure i s shown i n Fig. 19 as a function of bias p o t e n t i a l . F i g . 20 shows the surface states density as a function of bias p o t e n t i a l . Surface states density i s an order of magnitude less than that at the CdTe interface under similar conditions. This i s consistent with the fact that the photocurrent for GaP i s less^compared to CdTe for the photoelectrochemical reduction of C0 ( c f . the model suggested of mediator surface states). The surface state density increases from i t s minimum at 0.2V NHE, a potential which l i e s close to the pzc value determined for the system (0.17V NHE). Correspondingly, the form of the surface state density as a function of bias potential resembles an adsorption isotherm. These results support the concept that surface dates are induced by adsorbed ions at the interface.


2

Adsorption of the Carbon Dioxide Radical The absorption maximum at 1680 cm ^ can be attributed to the adsorbed CO r a d i c a l . The IR spectrum of this radicalJjias been recorded at -190°C. I t has a shargâximum at 1671 cm (38). The f u l l width at half maximum i s 3 cm . The C0 r a d i c a l i n the l a t t e r case was generated by the r a d i o l y s i s of sodium formate i n potassium bromide matrix^ The broadening of the spectrum ( f u l l width at half maximum 40 cm ) i s consistent with the model of a r a d i c a l adsorbed on the electrode surface. The integrated peak areas between 1650-1700 cm , at several bias potentials, are shown i n F i g . 11. The adsorption of the_anion, C0 , increases i n the cathodic d i r e c t i o n . I f the adsorbed C0 r a d i a l were i n equilibrium with C0 i n solution a decrease i n adsorption coverage would be expected when the potential i s moved i n the cathodic d i r e c t i o n . However, increase i n anion concentration at cathodic potentials i s consistent with C0 as an intermediate r a d i c a l in the electrochemical reduction of C0 . Thus, from ( l ) - ( 3 ) , 2

2

2

?

2

2

•

H

kp

co2

and with (2) i n equilibrium θ - = k k Ρ C0 2 l C0 9

k

k

P

2

6

e"

V F / R T

2

where V i s the electrode p o t e n t i a l , k.. and k are equilibrium constants for (1) and (2), respectively. Thus, θ - increases with 2 increasing negative value of V (Fig. 10), with (3) rate determining, the current density, i , -VF/RT 2

OT?1

1

=

2 F

Vco2 "

e

= 2Fk k k Ρ l 2 3*C0 Z i ? K

K

+

-(l ">VF/RT

e e

K

2



14.


Electrochemical

Activation

V(NHE) Figure 19. Surface state capacitance as a function of applied bias potential f o r GaP-DMF (5% Hy)) interface f o r the photoelectrochemical reduction of C0 . o

20 h

V(NHE)

Figure 20. Surface state density as a function of bias potential for GaP-electrolyte interface.


200


Thus, i t i s possible to obtain information on the ratec o n t r o l l i n g step by spectroscopically i d e n t i f y i n g an intermediate r a d i c a l and following the r e l a t i o n of the surface coverage with potential. Adsorption of Carbon Dioxide The IR spectrum of CO^ i n gas phase has a maximum of 2349 cm (30). Carbon djoxide adsorbed on platinum i n a c e t o n i t r i l e has a maximum at 2342 cm . Since the frequency s h i f t s (compared to gas phase CO^) are very small, C0 i s probably physisorbed. I t has been reported that adsorption of C0 on platinum at anodic potentials involves chemisorbed CO (39). However, such a chemisorbed CO was not observed at cathodic potentials on platinum i n a c e t o n i t r i l e . If C0 i s adsorbed p a r a l l e l to the surface of the electrode, the asymmetric stretching vibrations are not IR active, for the e l e c t r i c f i e l d vector of the p a r a l l e l y polarized l i g h t i s zero i n the plane p a r a l l e l to tljie surface of the electrode. Since adsorbed C0 absorbs at 2340 cm , the adsorption occurs through one oxygen atom and the other oxygen atom must project towards the solution (Fig. 21). Symmetric stretching vibrations of Œ > are not IR active as the t r a n s i t i o n dipole moment for this symmetriestretching is zero. The bending vibrations of C0 occurs at 667 cm , which i s beyond the s e n s i t i v i t y of the instrument. The r e l a t i v e concentration of C0 decreases i n the cathodic d i r e c t i o n (Fig. 11). The decreasing concentration of C0 may_£e due to the reduction of C0 to give C0~, which absorbs at 1680 cm . Adsorption of neutral molecules as a function of electrode potential generally passes through a maximum near the potential of zero charge. The potential of zero charge for platinum a c e t o n i t r i l e interface has been determined to be 0.3V NHE (40). The concentration of adsorbed C0 w i l l decrease on either side of potential of zero charge, which is consistent with the present experimental r e s u l t s . 1

2


2

?

2

2

2

2

2

2

2

Adsorption of A c e t o n i t r i l e 1

The absorption maximum at 2250 cm" i s attributed to the stretching vibrations of C^N group of a c e t o n i t r i l e (31). Since -C=N groups p a r a l l e l to the surface of the electrode are not IR active and C-C-N is l i n e a r , the a c e t o n i t r i l e must be adsorbed through the Ν atom and the methyl group projects towards the solution. Another p o s s i b i l i t y is that the -C^N group adsorbs p a r a l l e l to the surface of the electrode and the stretching vibrations of -C=N group observed i s due to electrochemical Stark e f f e c t . But, according to Pons et a l . (41), thegelectjochemical Stark e f f e c t i s important only at high potentials (10 V cm ) whereas the e l e c t r i c f i e l d under the present experimental conditions i s less than 4 χ 10 V cm" . The e l e c t r i c f i e l d increases toward more cathodic potentials. If the electrochemical Stark e f f e c t were responsible for the -C=N vibrations, the intensity would be expected to increase towards more cathodic potentials as the e l e c t r i c f i e l d increases, wherreas a decrease i n absorbance i s observed under these conditions. Hence, the electrochemical Stark e f f e c t i s unlikely under the present experimental conditions. 1




201

The r e l a t i v e c o n c e n t r a t i o n o f CH-CN on the s u r f a c e o f p l a t i n u m d e c r e a s e s w i t h i n c r e a s i n g c a t h o d i c p o t e n t i a l s , because a d s o r p t i o n o f n e u t r a l m o l e c u l e decreases away from t h e p o t e n t i a l o f zero charge. The a c e t o n i t r i l e c o n c e n t r a t i o n w i l l be f u r t h e r decreased a t h i g h c a t h o d i c p o t e n t i a l s by i n c r e a s i n g θ ^ - . S i n c e C0^ and CH^CN a r e l i n e a r , the a r e a o c c u p i e d p e r m o l e c u l e on the s u r f a c e o f t h e _ e l e c t r o d e i s s m a l l . However, once CCL i s reduced t o g i v e CO^, t h e r a d i c a l i s no l o n g e r l i n e a r ( 4 2 ) ( t h e 0-C-0 angle i s 134°), and t h e bond l e n g t h f o r C-0 i s i n c r e a s e d from 1.8 A f o r CO2 t o 2.1 X f o r CO! ( 4 2 ) . The CC>2 r a d i c a l may be adsorbed through one oxygen atom o r through two oxygen atoms. I n e i t h e r c a s e , t h e a r e a o c c u p i e d p e r CO^ i o n i s l a r g e r than t h a t o f CO^. Hence, t h e adsorbed a c e t o n i t r i l e m o l e c u l e i s e x p e l l e d i n c r e a s i n g l y from the s u r f a c e d u r i n g p r o c e s s (2) t o accommodate the l a r g e r CO^ r a d i c a l .


o

A d s o r p t i o n o f Ammonium Ions Ammonium i o n s have been shown t o a c t as a c a t a l y s t f o r the photo e l e c t r o c h e m i c a l r e d u c t i o n o f carbon d i o x i d e t o c a r b o n monoxide ( 3 6 ) . I t has been proposed t h a t they a r e adsorbed on t h e s u r f a c e and a c t as e l e c t r o n m e d i a t o r s f o r the p h o t o e l e c t r o c h e m c i a l r e d u c t i o n o f C 0 . Ammonium i o n s i n solution, b e i n g s y m m e t r i c a l , shows a broad maximum around 3400 cm , c o r r e s p o n d i n g t o N-H symmetric s t r e t c h i n g and a sharp maximum around 1600 cm due t o N-H d e f o r m a t i o n v i b r a t i o n s ( 3 2 ) . Ammonium ioijts adsorbed on__ÇaP show two broad peaks, c e n t e r e d around 3475 cm , and 3000 cm , c o r r e s p o n d i n g t o t h e N-H symmetric s t r e t c h i n g ( F i g . 13). Two peaks o f e q u a l intensity were observed f o j N-H d e f o r m a t i o n v i b r a t i o n : one a t 1700 cm and another a t 1560 cm ( F i g . 1 2 ) . Two d i f f e r e n t N-H groups a r e i n v o l v e d . From t h e e q u a l i n t e n s i t i e s o f t h e two peaks, i t i s e v i d e n t t h a t each peak c o r r e s p o n d s t o two d i f f e r e n t N-H d e f o r m a t i o n v i b r a t i o n s . Three o r i e n t a t i o n s o f adsorbed ammonium i o n s a r e p o s s i b l e as shown i n F i g . 22. U n l i k e ammonium i o n s i n homogeneous s o l u t i o n , t h e adsorbed ammonium i o n i s n o t a s y m m e t r i c a l t e t r a h e d r o n . Two d i f f e r e n t N-H groups a r c p r e s e n t for_£he adsorbed ammonium i o n . The peaks a t 3000 cm and 1560 cm a r e a t t r i b u t e d t o s t r e t c h i n g and d e f o r m a t i o n v i b r a t i o n s o f N-H group p r o j e c t i n g towards the s o l u t i o n as i t liçs c l o s e t o thç ammonium i o n s i n s o l u t i o n . Absorbance o f 3400 cm and 1700 cm may be due t o adsorbed N-H group. Since the i n t e n s i t i e s o f these two peaks a r e e q u a l a t a l l b i a s p o t e n t i a l s , i t i s proposed t h a t the ammonium i o n a d s o r p t i o n o c c u r s through two hydrogen atoms as shown i n F i g . 21b. F o r the o t h e r two o r i e n t a t i o n s ( F i g . 22), t h e i n t e n s i t y r a t i o s s h o u l d be 3:1 and 1:3. Symmetric s t r e t c h i n g v i b r a t i o n s o f the adsorbed ammonium i o n s a l s o i n d i c a t e a s i m i l a r behavior. The p o t e n t i a l dependence o f t h e ammonium i o n i s anamolous i n t h a t i t begins a t 0.3V n e g a t i v e t o the PZC, passes through a maximum, and then decreases w i t h i n c r e a s i n g c a t h o d i c p o t e n t i a l . T h i s b e h a v i o r may r e f l e c t t h e l a r g e d i p o l e moment (3.92 D) o f t h e a c e t o n i t r i l e molecule. I t may be n e c e s s a r y t o invoke a p o s s i b l e c h e m i c a l bonding of CHXN to GaP. Adsorbed a c e t o n i t r i l e may be reduced a t about -0.3V NHE, d e c r e a s i n g t h e s u r f a c e c o n c e n t r a t i o n , and a l l o w i n g ammonium i o n a d s o r p t i o n . However, a t s u f f i c i e n t l y c a t h o d i c p o t e n t i a l s ?



202 H Η |/Η χ

Ο II

c I

i

ni

Ο

Ν

/

/-A \o

0/ Θ

\

Η

Η

Electrode


Figure 21. Possible structures of adsorbed molecules on p l a t i num.

V /

H

Η

H

\ V "

Η

kmmmmmw

Η

immmmmw

Η

Η

Η

Ltmmmmmt

Figure 22. Possible structures of adsorbed NH+" ions on GaP.

> -0.7V, ammonium ion i s reduced (36), and the reduced r a d i c a l decomposes. +

NH. + e " ^ NH. » NH + Η 4 4 3 0

Thus, the ammonium ion concentration decreases at more negative potentials. Since N-H vibrations of NH^ are not observed under our experimental conditions, i t i s proposed that the decompositi reaction of ammonium ion occurs i n the diffuse layer. Conclusions 1.

2. 3. 4. 5. 6. 7.

Surfce states at the semiconductor-electrolyte interface under illumination can be calculated from the impedance measurements using the new equivalent c i r c u i t proposed. Surface states density at a given bias potential can be calcu lated from integral surface state capacitance. Surface states are induced by adsorption of ions at the semiconductor-electrolyte interface. Surface states act as faradaic mediators for the photoelectrochemical reduction of CO^. Adsorbed C0~ amd CO are involved i n the electrochemical reduction of CO-. The rate determining step has been determined to be further reduction of CO to give products. Ammonium ions are adsorbed at the semiconductor e l e c t r o l y t e interface and the reduced ammonium ion r a d i c a l acts as mediator for the photoelectrochemical reduction of C0 . 9




203

Ackno wledgments The a u t h o r s would l i k e t o thank the Gas R e s e a r c h I n s t i t u t e f o r s u p p o r t i n g t h i s p r o j e c t and Dr. K e v i n K r i s t , Dr. M. A. Habib and Dr. B. S c h a r i f k e r f o r d i s c u s s i o n s .


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