Proceedings of September Meeting 2016 (NFM16)
Publication date: 14th June 2016
An effective coupling of light absorbers to redox cocatalysts is of crucial importance for the activity, selectivity, and stability of photo(electro)catalysts for solar fuel production. This is in particular true in case of photoanodes for water splitting in which the light absorber must typically be modified by a water oxidation catalyst catalyzing the highly complex multi-electron transfer reactions required for water oxidation to dioxygen. However, our understanding of the catalysis of water oxidation at absorber/cocatalyst interfaces is still limited. This can be exemplified by several recent studies on cocatalyst-modified metal oxides (e.g., α-Fe2O3, BiVO4) which have, rather surprisingly, shown a substantial evidence that cobalt oxide-based cocatalysts (e.g., Co-Pi)[1] deposited on the light absorber surface do not act as “true” catalysts enhancing the rate of water oxidation but function rather as “surface passivation agents” which diminish surface recombination,[2-3] and/or modify the band bending in the semiconductor.[4] Therefore, in order to develop our understanding of various factors influencing the operation of catalysts in photoanodes for water splitting, there is a need for studies of photoanodes in which the cocatalyst acts as a genuine oxygen evolution catalyst.
We have recently investigated incorporation of various cocatalysts into porous inorganic-organic hybrid photoanodes. Importantly, in this type of photoanodes the cocatalyst unambiguously acts as a water oxidation catalyst since no dioxygen evolution is observed in the absence of cocatalyst. In this contribution I will focus on our investigations of porous photoanodes loaded with ultrasmall (1-2 nm) CoO(OH)x nanoparticles which outperform photoanodes comprising conventional cobalt-based cocatalysts (Co-Pi).[5] In particular, the effects of optical and structural properties of cocatalysts, the activity and stability in different electrolytes, and cocatalyst transformations during photoelectrocatalysis (studied by ex-situ XANES/EXAFS) will be discussed.
[1] M. W. Kanan, D. G. Nocera, Science 2008, 321, 1072.
[2] C. Y. Cummings, F. Marken, L. M. Peter, A. A. Tahir, K. G. U. Wijayantha, Chem. Commun. 2012, 48, 2027.
[3] Y. Ma, F. Le Formal, A. Kafizas, S. R. Pendlebury, J. R. Durrant, J. Mater. Chem. A 2015, 3, 20649.
[4] M. Barroso, A. J. Cowan, S. R. Pendlebury, M. Grätzel, D. R. Klug, J. R. Durrant, J. Am. Chem. Soc. 2011, 133, 14868.
[5] L. Wang, D. Mitoraj, R.K. Hocking, R. Beranek, in preparation.
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