(62d) Surface pH Measurement on Aqueous CO2 Reduction Electrocatalysts Using a Rotating Ring Disk Electrode

Understanding pH changes at a CO₂ electroreduction catalyst surface is vital to optimization of activity and selectivity. The pH influences concentration of reactant CO₂, reaction mechanism, and the electrochemical potential to which experiments are referenced. Determination of near surface pH is complicated by the buffering ability of reactant CO₂, itself, and by the fact that CO₂ reduction and hydrogen evolution both consume protons, thus changing the pH. Previous attempts to model and measure surface pH gradients under reaction conditions have suffered from neglected or poorly defined convection of CO₂ to the electrode surface. Here we utilize a rotating ring disk electrode (RRDE) to control reaction and convection conditions at the CO₂ electroreduction disk, while directly measuring pH in real-time with a potentiometric pH-sensing iridium oxide ring. The well-defined transport within the boundary layer of the RRDE enables accurate determination of electrocatalyst surface pH by solving the convection-diffusion-reaction equations. The effects of buffer strength and of cation size on the pH gradient are evaluated. Interchangeable disks reveal pH differences between planar metals and high-surface area electrocatalysts.