Glycerol is considered an important platform chemical for value-added organic chemical synthesis. The electrooxidation of glycerol in aqueous media provides a facile synthesis method to generate chemicals important for pharmaceutical and medical industries. Therefore, critical knowledge regarding the most active catalysts for the glycerol electrooxidation reaction (GEOR) is necessary for such a process to be viable. The GEOR on noble metals is well studied but there are significant gaps in the literature regarding the activity of specific crystal facets, particularly in the case of Pd. Through SECCM electrochemical mapping correlated with EBSD mapping to identify grain orientations, a much larger picture of the GEOR on Pd catalysts can be formed. Here, the facet dependent activity for the GEOR on Pd, evaluated through cyclic voltammetry using SECCM with co-located EBSD, is reported for the first time in alkaline solutions of KOH, NaOH and LiOH at pH 13. NaOH is shown to result in the highest performance for the GEOR and for all three electrolytes, Pd (111) provides the highest activity, Pd (001) provides intermediate to high activity and that Pd (101) is the least active. The facet dependent activity for the GEOR is directly mirrored for Pd oxidation and the subsequent PdO reduction process. Suggesting these two processes are signifiers towards catalytic activity for the GEOR.

Electrochemical conversion of glycerol offers a promising route to synthesise value-added glycerol oxidation products (GOPs) from an abundant biomass-based resource. While noble metals provide a low overpotential for the glycerol electrooxidation reaction (GEOR) and high selectivity towards three-carbon (C3) GOPs, their efficiency and cost can be improved by incorporating non-noble metals. Here, we introduce an effective strategy to enhance the performance of Pd nanoparticles for the GEOR by alloying them with Ni. The resulting PdNi nanoparticles show a significant increase in both specific activity (by almost 60%) and mass activity (by almost 25%) during the GEOR at 40 °C. Additionally, they exhibit higher resistance to deactivation compared to pure Pd. Analysis of the GOPs reveals that the addition of Ni into Pd does not compromise the selectivity, with glycerate remaining at around 60% of the product fraction, and the other major product being lactate at around 30%. Density functional theory calculations confirm the reaction pathways and the basis for the higher activity of PdNi. This study demonstrates a significant increase in the GEOR catalytic performance, while maintaining the selectivity for C3 GOPs, using a more cost-effective nanocatalyst.

The development of alcohol-based electrolysis to enable the concurrent production of hydrogen with low electricity consumption still faces major challenges in terms of the maximum anodic current density achievable. Whilst noble metals enable a low electrode potential to facilitate alcohol oxidation, the deactivation of the catalyst at higher potentials makes it difficult for the obtained anodic current density to compete with water electrolysis. In this work the effect of significant parameters such as mass transport, glycerol and OH- concentration and electrolyte temperature on the glycerol electrooxidation reaction (GEOR) in alkaline conditions on a bimetallic catalyst PdNi/Ni-RDE (Pd0.9Ni0.1) has been studied to discern experimental conditions which maximise achievable anodic current density before deactivation occurs. The ratio of NaOH:glycerol in the electrolyte highly affects the rate of the GEOR. A maximum current density of 793 mA cm(-2) at-0.125 V vs. Hg/HgO through steady state polarisation curves was achieved at a moderate and intermediate rotation rate of 500 RPM in a 2 M NaOH and 1 M glycerol (ratio of 2) electrolyte at 80 & DEG;C. Shown here is a method of catalyst reactivation for enabling the longterm use of the PdNi/Ni-RDE for electrolysis at optimal conditions for extended periods of time (3 h at 300 mA cm(-2) and 10 h at 100 mA cm(-2)). Through scanning electron microscopy (SEM), X-ray photon electron spectroscopy (XPS) and X-ray diffraction (XRD) it is shown that the electrodeposition of Pd and Ni forms an alloy and that after 10 h of electrolysis the catalyst has chemical and structural stability. This study provides details on parameters significant to the maximising of the GEOR current density and the minimising of the debilitating effect that deactivation has on noble metal based electrocatalysts for the GEOR.

The glycerol electrooxidation reaction (GEOR) has been increasingly studied for providing value-added chemical products whilst also facilitating a concurrent reduction process such as hydrogen evolution. Noble metals have been shown to be highly active for the GEOR in alkaline media. Here, to assess the effects of mass transport, catalyst layer thickness and pH on the GEOR, three thicknesses of Pd are electrodeposited onto a Ni rotating disk electrode and studied for a constant glycerol concentration of 0.50 M with NaOH to glycerol ratios of 1:2, 1:1 and 2:1. The electrodeposited catalysts are found to be morphologically similar with similar crystallographic structures. The activity, evaluated from the peak current density at the point of deactivation, shows that for every pH, the thinnest catalyst has the highest specific activity, whereas the thickest catalyst has the lowest. Therefore, there is a significant underutilisation of the thicker porous Pd electrodes for the GEOR. The thinnest catalyst layer is furthermore investigated in a solution of 1.0 M NaOH and 1.0 M glycerol. The doubling of the glycerol concentration in this case did not provide a significant increase in current density. Therefore, we propose that there is an optimal ratio of OHˉ to glycerol ratio in solution of around 2:1 due to the stoichiometry of the GEOR with the diffusion layer thickness and flux at higher glycerol concentrations considered.

Through glycerol electrooxidation, we demonstrate the viability of using a PdNi catalyst electrodeposited on Ni foam to facilitate industrially relevant rates of hydrogen generation while concurrently providing valuable organic chemicals as glycerol oxidation products. This electrocatalyst, in a solution of 2 M NaOH and 1 M glycerol at 80 °C, enabled current densities above 2000 mA cm⁻² (in a voltammetric sweep) to be obtained in atmospheres of both air and N₂. Repeated potential cycling under an aerated atmosphere to these exceptional current densities indicated a high stability of the catalyst. Through steady state polarisation curves, 1000 mA cm⁻² was reached below an anodic potential of 0.8 V vs RHE. Chronoamperometry showed glycerate and lactate being the major oxidation products, with increased selectivity for lactate at the expense of glycerate in aerated systems. Aerated atmospheres were demonstrated to consistently increase the apparent Faradaic efficiency to >100%, as determined by the concentration of oxidation products in solution. The excellent performance of PdNi/Ni in aerated solutions suggests that O₂ removal from the electrolyte is not needed for an industrial glycerol electrooxidation process, and that combining electrochemical and chemical glycerol oxidation, in the presence of dissolved O₂, presents an important process advantage.