Electrocatalytic Urea and Ammonia Oxidation for Cell Voltage Optimization in CO2 Electrolyzers

Abstract

Considering the continuous growth of carbon dioxide emissions and resulting climate change, the electrochemical reduction reaction of CO2 (CO2RR) to fuels and other valuable compounds is a promising strategy for utilizing the emitted greenhouse gas. The most current CO2RR approaches rely on the oxygen evolution reaction (OER) as the anodic process, which requires high potentials (Eo = 1.23V vs SHE) and produces low-value O2 as a product. The electrochemical CO2 conversion to CO or valuable carboxylic acids coupled with an alternative useful anodic reaction with a lower energy requirement than that for OER, could be a good strategy for lowering the cell potential. Based on the thermodynamic calculations of standard reaction Gibbs free energies and standard cell voltages for various anodic reactions, we demonstrate that the anodic oxidation of urea (UOR) and ammonia (AOR) are excellent alternatives to the OER in the CO2 electrolyzers for reducing its energy consumption. At the same time, performing UOR and AOR along with CO2RR introduces additional environmental value to the overall process, as these anodic reactions can be used for wastewater treatment. We evaluate the performance of divided batch and flow electrochemical cells, with CO2RR at the Ag and Au cathodes coupled with UOR and AOR at Ni-based and Pt-based anodes. The replacement of OER with Ni3+-catalyzed UOR decreases the operating cell potential by ≈250mV at current densities of up to 100mA cm-2, while enabling high faradaic efficiencies of CO formation at the cathode. Further decrease in cell potential of [CO2RR | UOR] electrolyzer is challenging due to the relatively high thermodynamic potential of Ni2+ to Ni3+ oxidation (Eo = 0.49V vs SHE). However, we show that the replacement of OER with AOR at Pt/C can decrease the operating cell potential by ≈700mV at current densities of up to 2mA cm-2 limited by the poor activity of current state-of-the-art catalysts for AOR. Finally, we establish an electrochemical system for the efficient CO2 conversion to CO coupled with UOR and AOR that operates at lower cell potentials, than that of OER.