Electrooxidation of Glycerol on an Electrodeposited Pd-Ni Catalyst on Ni Foam

Abstract

An increased global demand for clean energy and a sustainable future has led to the intense investigation of hydrogen as a low-carbon energy source in fuel cells and chemical reductant in vital industries such as low-carbon steel production. However, the current status of H2 production through water electrolysis in terms of global H2 availability only makes up around 5%, with the overwhelming bulk of that being formed through the chlor-alkali process. The remaining 95% of H2 production is through steam reforming with coal and natural gas. With the climate crisis expected to be the challenge of the century, it is vital that H2 production be uncoupled from fossil fuels if it is to be a green energy carrier and fuel of a low-carbon global economy.The implementation of direct water electrolysis on a large scale has the dominating issue of the oxygen evolution reaction (OER) on the anode. This reaction is limiting due to it being energy intensive as it proceeds through a four-electron transfer with a high equilibrium potential that exhibits sluggish kinetics, which are only overcome by operating at overpotentials requiring high energy input. The electrochemical oxidation of glycerol, a tri-hydroxyl functionalised alcohol, offers a more thermodynamically favourable anodic reaction to support the concurrent hydrogen evolution reaction at the cathode.Glycerol, a low-cost by-product of biodiesel refineries, has shown to be an excellent platform chemical. Through oxidation it can provide valuable chemical precursors in the food, cosmetic and pharmaceutical industries. These glycerol oxidation products typically include glyceric, tartronic, glycolic, lactic, oxalic and formic acid, to name a few. The specific formation of these products is highly dependent on the experimental conditions. In aqueous electrochemical setups, the chemistry and structure of the catalyst, pH of the electrolyte and the anode potential can all have significant effects on the selectivity of the glycerol oxidation products.The electrodeposition of bimetallic PdNi electrocatalysts on a porous Ni foam substrate (PdNi/NiFoam) provides a facile synthesis route for highly active catalysts for the glycerol electrooxidation reaction (GEOR) at high temperature and alkaline conditions. From previous work on PdNi electrodeposited on an RDE, it was established that a ratio of 2:1, NaOH:glycerol with moderate convection at 80 °C achieved the highest GEOR current density.1 This study, conducted in 2 M NaOH and 1 M glycerol in a divided 3-electrode cell using a Luggin capillary, builds upon the aforementioned previous work, using the optimised experimental conditions, for a porous electrode with a higher Ni composition in the electrocatalyst, approximately 60:40, Pd:Ni. Here the focus is on the achievable anodic current density and glycerol oxidation product selectivity for puriss glycerol in aerated and deaerated atmospheres. It is found through IR-corrected polarisation curves and steady state LSVs, that current densities of 1000 mA cm-2 can be reached at potentials lower than -0.1 V vs. Hg/HgO (~0.9 V vs. RHE), without deactivation. Furthermore, it is seen that in galvanostatic conditions at a current density of 500 mA cm-2 in a deaerated batch solution for 2 h, the PdNi/NiRDE electrocatalyst did not deactivate and provides a glycerol oxidation product selectivity of > 70% for glycerate.The present results emphasise strongly the possibility for the GEOR to be a contributor to not only the production of H2 for the future hydrogen economy but also as a means to convert a waste product into valuable chemical products for further utilisation. Moreover, the results show that even at industrially relevant current densities, good product selectivity is possible. J. White et al., Electrochimica Acta, 139714 (2021) https://doi.org/10.1016/j.electacta.2021.139714. Figure 1. IR-corrected polarisation curve of glycerol oxidation between 1 and 1000 mA cm-2 (from low to high current) on a PdNi catalyst electrodeposited on Ni Foam, in 2 M NaOH and 1M glycerol at 80 oC. No deactivation of the electrocatalyst at 1000 mA cm-2. Figure 1