Electrochemical CO 2 Reduction in Acidic Electrolytes: Spectroscopic Evidence for Local pH Gradients

Inspired by recent advances in electrochemical CO reduction (CO R) under acidic conditions, herein we leverage in situ spectroscopy to inform the optimization of CO R at low pH. Using attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) and fluorescent confocal laser scanning microscopy, we investigate the role that alkali cations (M ) play on electrochemical CO R. This study hence provides important information related to the local electrode surface pH under bulk acidic conditions for CO R, both in the presence and absence of an organic film layer, at variable [M ]. We show that in an acidic electrolyte, an appropriate current density can enable CO R in the absence of metal cations. In situ local pH measurements suggest the local [H ] must be sufficiently depleted to promote H O reduction as the competing reaction with CO R. Incrementally incorporating [K ] leads to increases in the local pH that promotes CO R but only at proton consumption rates sufficient to drive the pH up dramatically. Stark tuning measurements and analysis of surface water structure reveal no change in the electric field with [M ] and a desorption of interfacial water, indicating that improved CO R performance is driven by suppression of H mass transport and modification of the interfacial solvation structure. In situ pH measurements confirm increasing local pH, and therefore decreased local [CO ], with [M ], motivating alternate means of modulating proton transport. We show that an organic film formed via in situ electrodeposition of an organic additive provides a means to achieve selective CO R (FE ∼ 65%) over hydrogen evolution reaction in the presence of strong acid (pH 1) and low cation concentrations (≤0.1 M) at both low and high current densities.