Electrochemical Oxidation of Methanol to Formate with High Yield and Conversion for Paired Electrosynthesis Processes
Authors: Vollmert Niklas , Bäßler Jonas , Vehrenberg Jan , Mineur Miriam , Schenke Sophia , Griesberg Lukas , Montero Pineda Paulina , Keller Robert , Wessling Matthias 
Published: December 2024

Anthropogenic climate change driven by greenhouse gas emissions from fossil-based processes highlights the importance for alternative synthesis pathways for platform chemicals. Electrochemical CO2 reduction and hydrogen evolution offer sustainable options for platform chemical synthesis. However, pairing of the reduction reactions is often done with the oxygen evolution reaction (OER) at the anode, which consumes a major share of the total energy demand without contributing to value creation. In contrast, the partial methanol oxidation reaction (MOR) is an energy-efficient alternative to OER, yielding formate as a valuable product.

While MOR is often studied in terms of high Faraday efficiencies, little attention is paid to conversion and yield regarding paired processes. In this work, we investigate the oxidation of methanol to formate and examine the influence of different reaction parameters including current density, temperature, flow rate, and electrolyte to increase the yield and conversion towards industrially relevant levels.

Hierarchically structured CuO foam electrodes (5x5cm²) were employed as the MOR anode. To this end, electrochemically deposited dendrites on a fine-pored copper foam were subsequently oxidized to provide a large surface area for the MOR. The electrode was positioned within an electrochemical cell to ensure continuous flow through the foam, thereby utilizing the high electrode surface area. A defined electrolyte volume was recirculated through the cell and product analysis was conducted via Fourier Transformation Infrared Spectroscopy (FTIR) for calculation of methanol conversion and Faradaic efficiency (FE) to formate.

Increasing the current density from 50 to 200 mA/cm² led to a decrease in the initial FE and final conversion from 92% to 76% and 88% to 85%, respectively. Conversely, elevating temperature from 25 °C to 55 °C increased FE from 83% to 93% and conversion from 74% to 83%. A raise of the electrolyte flow rate from 25 to 100 mL/min showed no impact on FE and conversion. Change in electrolyte composition revealed a negative correlation between KOH concentration and FE, decreasing from 91% to 64% with a rise from 1 M to 4 M KOH. Conversely, increasing MeOH concentration from 0.5 M to 2 M enhanced FE from 71% to 92%.

Finally, MOR was paired with alkaline hydrogen evolution reaction (HER) using two different membrane types, either a bipolar membrane (BPM) or an anion exchange membrane (AEM). Operating at optimized conditions resulting from the previous investigations (100 mA/cm², 55 °C, 50 mL/min, 2 M KOH, and 1 M MeOH), both setups achieved over 90% methanol conversion before the FE dropped to zero. The BPM approach exhibited slightly higher initial FE (over 95%) compared to the AEM approach (over 86%), with respective yields of 70% and 68% at anodic potentials of 1.49 V and 1.33 V.

This study provides valuable insights into the influence of reaction conditions on methanol oxidation to formate regarding industrially relevant conversions. Furthermore, the study successfully demonstrates two concepts of pairing HER as the value-adding cathode reaction with MOR as an additional value-adding anode reaction for future applications.