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Conditions for Rapid Photocorrosion at Strontium Titanate Photoanodes R. E. SCHWERZEL, E. W. BROOMAN, H . J. BYKER, E. J. DRAUGLIS, D. D. L E V Y , L. E. V A A L E R , and V. E. WOOD Battelle Columbus Laboratories, 505 King Avenue, Columbus, OH 43201
In 1912, the great Italian photochemist, Giacomo Ciamician, published a remarkable paper entitled "The Photochemistry of the Future" (1) in which he considered the wealth of benefits which might be gained by the photochemical utilization of solar energy for the production of useful chemical materials. In discussing the role of plant crops (or biomass, as we would say now) as solar energy transducers, he suggested that: "The harvest, dried by the sun ought to be converted, in the most economical way, entirely into gaseous fuel, taking care during this operation to fix the ammonia (by the Mond process, for instance) which should be returned to the soil as nitrogen fertilizer together with all the mineral substances contained in the ashes". This elusive goal of efficient, economical fuel production from renewable biomass resources has stimulated research efforts around the world since Ciamician's time. While much progress has been made, the problems involved are far from solved, and the production of gaseous fuels from biomass is s t i l l too expensive to be economically feasible on a large scale. Nonetheless, recent developments i n s e v e r a l aspects of photoelectrochemistry have o f f e r e d renewed promise that the production of u s e f u l f u e l s with s o l a r energy might indeed become a v i a b l e process. Of p a r t i c u l a r i n t e r e s t i n t h i s context has been the f i n d i n g that the Kolbe r e a c t i o n , the anodic o x i d a t i o n of c a r b o x y l i c acids (Equation 1) ( 2 ) , can be made to occur at n-type oxide semiconductor photoanodes to the v i r t u a l exc l u s i o n of oxygen formation ( 3 , 4 , 5 ) .
2 C H 3 C O 2 H AG
0
C H6 +2 C 0 +H 2
2
2
= - 2 2 . 3 kJ/mole ( - 5 . 3 kcal/mole)
0097-6156/81/0146-0191$05.00/0 © 1981 American Chemical Society
(1)
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192
PHOTOEFFECTS
AT
SEMICONDUCTOR-ELECTROLYTE
INTERFACES
While t h i s r e a c t i o n i s s u b s t a n t i a l l y exothermic (6), i t provides an i n t r i g u i n g approach to the production of f u e l s from renewable resources, as the required acids ( i n c l u d i n g a c e t i c a c i d , b u t y r i c a c i d , and a v a r i e t y of other simple a l i p h a t i c c a r b o x y l i c a c i d s ) can be produced i n abundant y i e l d s by the enzymatic fermentation of simple sugars which are, i n t u r n , a v a i l a b l e from the m i c r o b i o l o g i c a l h y d r o l y s i s of c e l l u l o s i c biomass materials (7). These considerations have l e d us to suggest the concept of a "tandem" p h o t o e l e c t r o l y s i s system, i n which a s o l a r p h o t o e l e c t r o l y s i s device f o r the production of f u e l s v i a the photo-Kolbe r e a c t i o n might derive i t s a c i d - r i c h aqueous feedstock from a biomass conversion plant for the h y d r o l y s i s and fermentation of crop wastes or other c e l l u l o s i c materials (4.). As one aspect of our recent studies i n t h i s f i e l d , we have sought to extend the range of c o n d i t i o n s under which the photo-Kolbe r e a c t i o n could be conducted, so as to explore the s e n s i t i v i t y of the process to such v a r i a b l e s as l i g h t i n t e n s i t y , pH, concentration of c a r b o x y l i c a c i d , and so on. It has gradually become apparent that under at l e a s t some of our experimental c o n d i t i o n s , the strontium t i t a n a t e photoanodes can undergo severe photodegradation. When t h i s occurs, v i s i b l e p i t s or c r a t e r s are formed i n the i l l u m i n a t e d p o r t i o n of the e l e c t r o d e a f t e r s e v e r a l hours or days of exposure to focused l i g h t from an Eimac 150W xenon arc lamp. This obs e r v a t i o n i s t o t a l l y unprecedented; a f t e r a l l , strontium t i t a n a t e i s one of the few m a t e r i a l s that has been unanimously reported i n the l i t e r a t u r e to be a robust, s t a b l e photoanode (8-16). We have conducted numerous r e p l i c a t e c o n t r o l experiments to determine whether a procedural e r r o r or an equipment malf u n c t i o n (such as, f o r example, the leakage of a l t e r n a t i n g current from the l i n e c i r c u i t s i n t o the dc c i r c u i t s of the p h o t o e l e c t r o l y s i s apparatus) could have been responsible f o r the observed e f f e c t s . U l t i m a t e l y , these c o n t r o l experiments led us to add an o s c i l l o s c o p e to the d i a g n o s t i c equipment (to check f o r ac leakage), to replace each of the major e l e c t r o n i c components ( i n c l u d i n g the p o t e n t i o s t a t , voltage scan u n i t , and electrometer) with a l t e r n a t i v e components of comparable q u a l i t y , and to replace the simple, one-compartment c e l l we had been using with a newly designed two-compartment c e l l , so as to minimize the p o s s i b i l i t y that cathodic products could somehow a f f e c t the s t a b i l i t y of the strontium t i t a n a t e photoanodes. In a d d i t i o n , the electrodes used i n the new c e l l were mounted to t h e i r Pyrex support tubes using heat-shrinkable T e f l o n tubing rather than epoxy cement. As before, the new c e l l had a Pyrex window through which the semiconductor electrode could be i r r a d i a t e d , a Luggins c a p i l l a r y p o s i t i o n e d c l o s e to the surface of the i l l u m i n a t e d e l e c t r o d e , and i n t e g r a l gas burets above each e l e c t r o d e f o r the c o l l e c t i o n of gas samples.
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12.
SCHWERZEL
ET
SrTiOs Photoanodes
AL.
Despite these precautions, marked c o r r o s i o n was s t i l l observed on some, but not a l l , of the n - S r T i 0 3 photoanodes obtained from four d i f f e r e n t sources. The c o r r o s i o n appeared to be most severe a f t e r s e v e r a l experiments ( t o t a l l i n g t y p i c a l l y 20 hours or more of use as an e l e c t r o d e ) had been conducted under photo-Kolbe r e a c t i o n c o n d i t i o n s . A f i n e white f i l m was a l s o observed to form g r a d u a l l y on the i r r a d i a t e d areas of n-SrTi03 when the a c i d e l e c t r o l y t e ( t y p i c a l l y 2N H 2 S O 4 ) was used i n the absence of added a c e t i c a c i d . C h a r a c t e r i s t i c s of the Photocorrosion
Process
The magnitude of the problem can be appreciated by comparing the c r y s t a l s marked A and B i n Figure 1. C r y s t a l A i s t y p i c a l of the c o n d i t i o n of our n - S r T i 0 3 e l e c t r o d e s j u s t p r i o r to etching and mounting; i t i s smooth, shiny ( a f t e r p o l i s h i n g with a 1.5y diamond paste c l o t h ) , and somewhat transparent. C r y s t a l B i l l u s t r a t e s the degree of photocorr o s i o n which occurred i n a s i m i l a r n-SrTi03 e l e c t r o d e a f t e r approximately 50 hours of i r r a d i a t i o n (during s e v e r a l experiments) under t y p i c a l photo-Kolbe c o n d i t i o n s , i n t h i s case aqueous s u l f u r i c a c i d c o n t a i n i n g a c e t i c a c i d . The s e v e r e l y eroded area i s l o c a t e d where the l i g h t beam was focused on the e l e c t r o d e ; the r e s i d u a l p i t t i n g on the surface i s probably due to s c a t t e r e d l i g h t s t r i k i n g the e l e c t r o d e . V i r t u a l l y no photo-Kolbe products have been observed by mass spectrometry i n the gas evolved from the decomposing e l e c trodes; the primary gaseous product i s oxygen, although v a r i a b l e amounts of C 0 have been observed at times. Thus, there appears to be a competition between e l e c t r o d e decomp o s i t i o n and the photo-Kolbe r e a c t i o n under these c o n d i t i o n s . While we have not yet c a r r i e d out d e t a i l e d k i n e t i c measurements on the r a t e of photocorrosion, our impression i s that the process i s r e l a t i v e l y i n s e n s i t i v e to the s p e c i f i c composition of the strontium t i t a n a t e . Trace element comp o s i t i o n s , obtained by spark-source mass spectrometry, are presented i n Table I f o r the four boules of n - S r T i 0 3 from which electrodes have been cut. Photocorrosion has been observed i n samples from a l l four boules. In a l l cases, the electrodes were cut to a thickness of 1-2 mm using a diamond saw, reduced under H at 800-1000 C f o r up to 16 hours, p o l i s h e d with a diamond paste c l o t h , and etched with e i t h e r hot concentrated n i t r i c a c i d or hot aqua r e g i a . Ohmic cont a c t s were then made with gallium-indium e u t e c t i c a l l o y , and a wire was attached using e l e c t r i c a l l y conductive s i l v e r epoxy p r i o r to mounting the e l e c t r o d e on a Pyrex support tube with e i t h e r epoxy cement or heat-shrinkable T e f l o n tubing. Some information about the nature of the photocorrosion process i s provided by a comparison of the U V / v i s i b l e ab2
2
194
PHOTOEFFECTS
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TABLE I .
AT SEMICONDUCTOR-ELECTROLYTE
TRACE ELEMENT COMPOSITION OF n-SrTiO
Q
INTERFACES
BOULES
Sample Element
x
(b)
2
3(d)
(c)
4
(e)
j-e.
0.3
1.0
0.3
