Nickel-Based Cocatalysts on Titanium-Doped Hematite Empower Direct Photoelectrochemical Valorisation of 5-Hydroxymethylfurfural.

The photoelectrochemical oxidation of 5-hydroxymethylfurfural (HMF), a biomass-derived intermediate, to 2,5-furandicarboxylic acid (FDCA), a key building block for industrial applications, is a well-studied anodic reaction. This photoelectrochemical (PEC) conversion typically requires an electron mediator, such as TEMPO, regardless of the semiconductor used. Various electrocatalysts can also perform this reaction electrochemically, without additional organic species in the electrolyte. In this study, Ti-doped hematite (Ti:FeO) photoanodes were employed for the HMF photoelectrochemical conversion at the anodic side of a two-compartments PEC cell. To avoid the need of an electron mediator, nickel-based electrocatalysts were deposited on the electrode's surface. The Ni(OH)-electrodeposited (Ti:FeO-Ni) and the NiMo-sputtered Ti:FeO photoanodes (Ti:FeO-NiMo) were characterised and tested for the 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 cocatalyst in shifting the selectivity towards the di-carboxylic acid. Fixed Energy X-ray Absorption Voltammetry (FEXRAV) and X-ray Absorption Near-Edge Structure (XANES) measurements were conducted to investigate the interaction between HMF and the two deposited electrocatalysts. These techniques offered valuable insights into the oxidation mechanism, which were further validated using a rate deconvolution procedure.