Structured Electrodes Induce Local pH as a Primary Determinant of CO₂ Reduction Selectivity

Highly structured electrodes permit high operating currents to be achieved as well as enhanced selectivity, especially for the carbon dioxide reduction (CDR) reaction. An understanding of the origins of CDR selectivity is complicated by the intricate internal structure of high surface area electrodes, which may modify the microenvironment in which catalysis occurs. We report here enhanced faradaic efficiency of the reduction of CO₂ to CO (FE(CO)) on photolithographically fabricated Au microwire (MW) electrodes as compared with their planar counterpart. We demonstrate that this selectivity arises from the local pH gradient at the electrode-solution interface, as a result of the electrode microstructure. By employing confocal microscopy with a ratiometric pH sensor, we can construct in situ maps of the local pH at the electrode-solution interface. When normalized for local pH, the same dependence of FE(CO) is observed for the fabricated MW and planar electrodes, and the selectivity is a direct result of the HCO₃^-/CO₃^2- equilibrium.