Dissolved CO2 Modulates the Electrochemical Capacitance on Gold Electrodes

The presence of CO2 at an electrified interface between an aqueous electrolyte and metal electrode is the prerequisite for many electrochemical CO2 capture technologies. To understand the behavior of dissolved CO2 at an aqueous electrified interface, we characterized the electrochemical interface of planar gold electrodes with cyclic voltammetry, electrochemical impedance spectroscopy (EIS), electrochemical surface plasmon resonance (EC-SPR), and attenuated total reflectance surface-enhanced infrared spectroscopy (ATR-SEIRAS). Under all investigated conditions, we  observed a decrease in the electrochemical capacitance upon saturation of the electrolyte with CO2, as compared to an electrolyte saturated with Ar. EIS and EC-SPR showed that this capacitance reduction was also potential dependent: it reached a minimum near the point of zero charge, and became more significant as the applied potential moved further away from the point of zero charge. Hybrid quantum-classical simulations of the gold/aqueous electrolyte interface indicate that bicarbonate decreases the capacitance and modifies the composition of the electric double layer. In addition to the binding of bicarbonate under positive bias, we propose that molecular CO2 can be induced by applied potential to concentrate in the diffuse layer of the electric double layer, leading to a reduction in the electrochemical capacitance under both negative and positive bias. This work advances the understanding of non-Faradaic effects of dissolved CO2 at aqueous electrified interfaces of relevance for electrochemical CO2 capture.