Insights into the Carbon Balance for CO2 Electroreduction on Cu using Gas Diffusion Electrode Reactor Designs

In this work, the carbon balance during high-rate CO2 reduction in flow electrolyzers is rigorously analyzed. The CO2 consumption at gas-diffusion electrodes due to electrochemical reduction and reaction with OH- at the electrode-electrolyte interface leads to a substantial reduction in the volumetric flowrate of gas flow out of the electrolyzer, especially when highly alkaline electrolytes and elevated current densities are utilized, mainly owing to elevated pH at cathode/electrolyte interface. Without considering the CO2 consumption, the Faradaic efficiencies for major gas products could be significantly overestimated during high current density CO2 reduction conditions, particularly in the case of high pH electrolyte. In addition, a detailed carbon balance path is elucidated via a two-step procedure of CO2 reaction with OH- at cathode/electrolyte interface and subsequent CO2 generation at anode/electrolyte interface caused by a relatively low pH in the vicinity of the anode. Based on the proposed two-step carbon balance path, a systemic exploration of gases released in anolyte reveals the transformation of a HCO3- or OH- catholyte to a CO32- catholyte, which was further confirmed by pH measurement.