Electrochemical CO₂ Reduction in Acidic Electrolytes: Spectroscopic Evidence for Local pH Gradients

Abstract

High Resolution Image Download MS PowerPoint Slide Inspired by recent advances in electrochemical CO 2 reduction (CO 2 R) under acidic conditions, herein we leverage in situ spectroscopy to inform the optimization of CO 2 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 2 R. This study hence provides important information related to the local electrode surface pH under bulk acidic conditions for CO 2 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 2 R in the absence of metal cations. In situ local pH measurements suggest the local [H + ] must be sufficiently depleted to promote H 2 O reduction as the competing reaction with CO 2 R. Incrementally incorporating [K + ] leads to increases in the local pH that promotes CO 2 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 2 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 2 ], 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 2 R (FE CO 2 R ∼ 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.

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