The sluggish kinetic of oxygen evolution reaction (OER) consistently reduces the efficiency of solar water splitting and therefore its competition with the fossil-based technologies widely available on the market. To overcome this limitation, the focus of the scientific community is shifting towards alternative oxidation reactions¹, characterized by lower energy requirements and inexpensive starting compounds. In this work, the oxidation of biomass derivatives into useful chemicals was investigated at the anodic compartment of a photoelectrochemical (PEC) cell. The photoanode was selected, evaluating the material’s stability, performance, and sustainability, in addition to the specific selectivity towards the desired products. The following PEC systems were explored:  i) Titanium doped hematite (Ti: Fe₂O₃) photoanodes, modified with cobalt- or nickel-based co-catalysts, for the conversion of 5-hydroxymethil furfural (HMF) into 2,5- furan dicarboxylic acid (FDCA); ii) bismuth vanadate (BiVO₄) photoanodes for glycerol oxidation to dihydroxyacetone (DHA).

Ti: Fe₂O₃ photoanodes were employed to oxidize the biomass derivative HMF to FDCA, a valuable building-block chemical for the synthesis of the PET-alternative, polyethylene furanoate (PEF). At first, a borate buffer solution was employed (pH 9) and the (2,2,6,6-Tetramethylpiperidin-1-yl) oxyl (TEMPO) mediator was introduced to accelerate the process. To improve selectivity over OER, cobalt-based cocatalysts were deposited on the photoanode’s surface, and the one modified with cobalt phosphate (CoPi) showed the highest efficiency and selectivity for FDCA². The source of this enhancement was correlated to the effect of the cocatalyst on the charge carrier dynamics, investigated by electrochemical impedance spectroscopy (EIS) and Intensity Modulated Photocurrent Spectroscopy (IMPS). To avoid the use of TEMPO, nickel-based electrocatalysts were deposited on the electrode’s surface. The Ni(OH)₂-electrodeposited (Ti: Fe₂O₃-Ni)³ and the NiMo-sputtered Ti: Fe₂O₃ photoanodes (Ti: Fe₂O₃-NiMo) were tested for the direct HMF oxidation in 0.1 M NaOH (pH 13) electrolyte. Partial HMF photoelectrochemical conversion to FDCA was achieved, pointing out the beneficial effect of Ni-based co-catalyst in shifting the selectivity. Operando X-ray Absorption Spectroscopy (XAS) measurements were also performed to explore the interaction between HMF and the two deposited electrocatalysts, helping to achieve some insights into the oxidation mechanism.

Glycerol oxidation to DHA was studied using nanoporous BiVO₄ photoanodes under acidic conditions, in a flow PEC cell. This time, no cocatalysts nor electron mediator were required, as glycerol proved to be an effective hole scavenger for this photoanode. The stability of the semiconductor was evaluated through long-term chronopotentiometries, both by fixing and modulating the current over time. After the conversion, a photoelectrochemical characterization was performed to assess the photoanode’s performance and SEM images were acquired for structural analysis.

Overall, a deep understanding of the favourable reaction conditions was essential not only to enhance process efficiency, but also to elucidate the underlying oxidation mechanism. This knowledge may also facilitate the successful coupling of valuable cathodic reactions, such as the hydrogen evolution reaction (HER) or CO₂ reduction, thereby substantially improving the overall utility of the PEC device.