Electrochemical CO<sub>2</sub> Reduction in Acidic\nElectrolytes: Spectroscopic Evidence for Local pH Gradients

Inspired by recent advances in electrochemical CO<sub>2</sub> reduction\n(CO<sub>2</sub>R) under acidic conditions, herein we leverage in situ\nspectroscopy to inform the optimization of CO<sub>2</sub>R at low\npH. Using attenuated total reflection surface-enhanced infrared absorption\nspectroscopy (ATR-SEIRAS) and fluorescent confocal laser scanning\nmicroscopy, we investigate the role that alkali cations (M<sup>+</sup>) play on electrochemical CO<sub>2</sub>R. This study hence provides\nimportant information related to the local electrode surface pH under\nbulk acidic conditions for CO<sub>2</sub>R, both in the presence and\nabsence of an organic film layer, at variable [M<sup>+</sup>]. We\nshow that in an acidic electrolyte, an appropriate current density\ncan enable CO<sub>2</sub>R in the absence of metal cations. In situ\nlocal pH measurements suggest the local [H<sup>+</sup>] must be sufficiently\ndepleted to promote H<sub>2</sub>O reduction as the competing reaction\nwith CO<sub>2</sub>R. Incrementally incorporating [K<sup>+</sup>]\nleads to increases in the local pH that promotes CO<sub>2</sub>R but\nonly at proton consumption rates sufficient to drive the pH up dramatically.\nStark tuning measurements and analysis of surface water structure\nreveal no change in the electric field with [M<sup>+</sup>] and a\ndesorption of interfacial water, indicating that improved CO<sub>2</sub>R performance is driven by suppression of H<sup>+</sup> mass transport\nand modification of the interfacial solvation structure. In situ pH\nmeasurements confirm increasing local pH, and therefore decreased\nlocal [CO<sub>2</sub>], with [M<sup>+</sup>], motivating alternate\nmeans of modulating proton transport. We show that an organic film\nformed via in situ electrodeposition of an organic additive provides\na means to achieve selective CO<sub>2</sub>R (FE<sub>CO<sub>2</sub>R</sub> ∼ 65%) over hydrogen evolution reaction in the presence\nof strong acid (pH 1) and low cation concentrations (≤0.1 M)\nat both low and high current densities.