Operando Observation of (Bi)carbonate Precipitation during Electrochemical CO2 Reduction in Strongly Acidic Electrolytes

Precipitation of (bi)carbonate salts during the electrochemical CO2 reduction (CO2RR) has been identified as a major cause of degradation and one of the main challenges to be overcome before implementing this technology on the industrial scale. Recently, the use of acidic electrolytes has been suggested as a promising strategy to avoid this undesirable precipitation of carbonates and maximize the conversion of CO2 into valuable products. However, direct experimental evidence supporting this claim, as well as an understanding of degradation mechanisms in acidic electrolytes, is currently missing. In this study, we use operando synchrotron wide-angle X-ray scattering to provide unambiguous experimental proof that substantial (bi)carbonate precipitation takes place in Ag and Cu gas diffusion electrodes (GDEs) during CO2RR even in strongly acidic electrolytes (pH 1). The (bi)carbonate signal intensity increases over time, correlating with a loss of selectivity for carbon products and an increased level of H-2 production. Electrochemical measurements coupled with products analysis also show that a minimum current is required to activate the CO2RR and suppress hydrogen evolution and that this current is higher for more acidic electrolytes. These findings provide experimental evidence for high local pH close to the catalyst even in acidic electrolytes. Our results confirm that the CO2RR performed in an acidic electrolyte can yield high selectivity to carbon products, provided sufficiently large current densities are applied to provoke local proton depletion in the vicinity of the catalyst, but that detrimental (bi)carbonate precipitation and selectivity losses occur even under these conditions.