Unveiling the anode reaction environment in a CO electrolyzer to provide a guideline for anode development

The use of 3d transition metals, such as Ni-based electrodes for the oxygen evolution reaction (OER) is inhibited in membrane electrode assembly (MEA) CO 2 electrolyzers owing to the local acidic media. We unveiled the reaction environment of the anode during the CO 2 reduction reaction (CO 2 RR) with ex situ characterization and in situ / operando pH measurements to challenge the employment of Ni-based electrodes for OER in MEA. Despite the neutral anolyte, the reaction environment became acidic because of the transported carbonate ions and drastic pH fluctuations (acidic to weakly alkaline) created by the nonuniform ion and water distribution. This pH fluctuation causes a rapid phase transition of Ni between hydrated hydroxide and dissolved ions, causing severe degradation and dissolution. Based on these insights, we propose a guideline for selecting OER electrodes for MEA CO 2 electrolyzers: good acidic stability with no phase transition under high anodic potential between acidic and weakly alkaline media is required. In situ pH measurements are carried out to elucidate the CO 2 RR anode environment. It is revealed that local pH near the anode goes through severe fluctuation, and a guideline for the development of anode material for CO 2 RR is suggested.