Reaction engineering for high yield electrosynthesis: Unraveling the impact of reaction conditions and conversion on methanol oxidation to formate

Abstract

Electrochemical processes offer defossilized alternatives to conventional routes. Key processes, such as hydrogen evolution and electrochemical CO2 reduction, are typically paired with the anodic oxygen evolution reaction (OER). However, the generated oxygen holds little value and the electrical costs associated with energy-intensive OER pose a significant economic barrier. The methanol oxidation reaction (MOR) to formate is a promising alternative to OER, requiring less energy and providing a value-added product. Extensive research focuses on MOR regarding high Faraday efficiencies, but conversion and product yields are mostly neglected. However, high conversion with sufficient yield is a prerequisite to transition from lab-scale catalysis towards feasible industrial applications. In this work, we investigated the selectivity of MOR to formate with progressing conversion at high current densities of up to 200mA/cm2 on hierarchically structured copper(II) oxide electrodes. We assessed the impact of the reaction conditions, including current density, temperature, flow rate, electrolyte composition, and membrane type. We found a positive influence of low current density and high temperature on the FE. Through tailored reaction conditions, we achieved a formate yield of 70% at 100mA/cm2 with an anodic potential of 1.33V vs. RHE. The anodic potential remained below typical OER potentials even at high conversion. For the first time, we demonstrated that MOR can achieve significant formate yields at high current density. Our results reveal the impact of conversion, reaction conditions and ion balance on selectivity and provide valuable insights for operating MOR at high yield in paired processes, e.g., with hydrogen evolution or CO2 reduction.