The Importance of Catalyst Choice in Photoelectrochemical Glycerol Oxidation

Abstract

Photoelectrochemistry is a promising method for the direct conversion of sunlight into valuable chemicals by combining the functions of solar panels and electrolyzers in one technology. In most studies, semiconductor/catalyst photoelectrode assemblies are used to achieve reasonable efficiencies. At the same time, unlike in dark electrochemical processes, the role of the catalyst is not straightforward in photoelectrochemistry, where the onset potential of the redox process should be mostly determined by the flatband potential of the semiconductor. In addition, the energy of holes (i.e., the surface potential) is independent of the applied bias; it is defined by the valence band (VB) position. In this study, we compared PdAu, Au, and Ni on Si photoanodes in the photoelectrochemical (PEC) oxidation of glycerol at record high current densities (> 180 mA cm^‒2), coupled to H₂ evolution at the cathode. We successfully decreased the energy requirement (i.e., the cell voltage) of the paired conversion of glycerol and water by 0.7 V by exchanging the widely studied Ni catalyst with PdAu. The catalyst choice also dictates the product distribution, resulting mainly in C3 products on PdAu, glycolate (C2 product) on Au, and formate (C1 product) on Ni, without complete mineralization of glycerol (CO₂ formation) that is difficult to rule out in dark electrochemical processes (as demonstrated by comparative measurements). Finally, we achieved a bias-free (standalone) operation with PdAu/Si and Au/Si photoanodes by combining the PEC oxidation of glycerol with oxygen reduction reaction (ORR).

Keywords

bias-free operation / concentrated sunlight / continuous flow cell / glycerol oxidation / photoelectrocatalysis / semiconductor

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