Nanostructure-performance relationships in titania-only electrodes for the selective electrocatalytic hydrogenation of oxalic acid

The preparation of robust and selective hydrogenation electrodes not based on critical raw materials is a relevant goal for the development of sustainable electrocatalytic processes. Oxalic acid is an emerging intermediate produced from CO2 that can be used to produce valuable, added-value monomers for high-performance polymers by selective electrocatalytic reduction. We demonstrated that with the rapid breakdown anodization (RBA) method, superior-performance TiO2-only electrodes can be directly prepared with a fast and low-cost procedure. The electrodes were based on a conductive Ti substrate, used to generate a roughened titania layer by RBA characterized by an ordered array of vertically aligned TiO2 nanotubes (NTs) on top of them and partially reduced TiO2-x nanoparticles stabilized by the interaction with the NTs layer. In addition, the preparation method leads to highly nanoporous NTs. The electrodes obtained by RBA at 60 V led to the best Faradaic efficiency (FE) to glycolic acid (86 %) and oxalic conversion (48 %) at −0.8 V vs. RHE, superior to the existing literature on TiO2-only electrodes at room temperature. The NTs structural features, such as length and surface roughness, were calculated by AFM and linearly correlated with the oxalic acid conversion and the electrochemical active surface area (ECSA). Additionally, the ECSA was found to be correlated with selectivity, indicating the complex interrelations between the NT nanostructure and performance. This finding emphasizes the importance of controlling the nanostructure as a critical factor in tuning the selectivity toward glyoxylic acid or glycolic acid.