Influence of electrolyte and pH value on the photoelectrochemical oxidation of 5-hydroxymethylfurfural using mesoporous tungsten oxide photoanodes

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Abstract
Photoelectrochemical (PEC) water splitting is a promising strategy for providing clean, sustainable fuel. However, its efficiency is limited by the high overpotential and sluggish kinetics of the oxygen evolution reaction. To improve oxidation kinetics and produce a higher-value product, it is possible to oxidize organic molecules instead. One such molecule is 5-hydroxymethylfurfural (HMF), which can be converted into the high-value platform chemical 2,5-furandicarboxylic acid (FDCA). In this study, we used porous tungsten oxide (WO3) photoanodes to oxidize HMF photoelectrochemically. Our work focuses on electrolyte selection for this conversion reaction, and we therefore conducted a detailed reproducible study. Furthermore, we carried out a mechanistic investigation and studied the capability of oxidizing the reaction intermediates to support our electrolyte choice. Using the most suitable electrolytes in a self-designed flow-cell setup enabled us to produce FDCA (0.02 mM) for the first time using WO3 photoanodes under AM 1.5 G illumination. Due to the identification of the side products maleic acid and formic acid, this work also provides knowledge for further optimization of PEC HMF oxidation.

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