pH Matters When Reducing CO2 in an Electrochemical Flow Cell

The pH at the electrocatalyst surface plays a key role in defining the activity and selectivity of the CO2 reduction reaction (CO2RR). We report here operando Raman measurements of the catalyst surface in a customized CO2RR flow cell that enable the measure of pH. Using this flow cell, we were able to measure surface pH as a function of time, current density, and proximity to the catalyst surface during the electrolysis of bicarbonate solutions. We observed that increasing the current density from 0 to 200 mA cm–2 increased the surface pH from 8.5 to 10.3. We also show here that operation at elevated temperatures (70 °C) results in an increased surface pH and serves to suppress the competing and undesirable hydrogen evolution reaction.