Influence of the Electrolyte pH on the Double Layer Capacitance of Polycrystalline Pt and Au Electrodes in Acidic Solutions

Abstract

A deeper understanding of electrified solid/liquid interfaces of polycrystalline materials is crucial for optimizing energy conversion and storage devices, such as fuel cells, electrolyzers, and supercapacitors. After more than a century of research, the double-layer capacitance (C_DL) has proven to be one of the few relatively easily experimentally accessible quantitative measures for characterizing such interfaces. However, despite their great importance, systematic C_DL measurements are still not frequently associated with other interfacial properties. This work investigates the effect of the electrolyte pH on the C_DL for polycrystalline platinum (Pt(pc)) and gold (Au(pc)) electrodes using cyclic voltammetry and impedance spectroscopy in acidic solutions with a pH ranging from 0 to 2 without adding any supporting electrolyte. Interestingly, under these conditions, the C_DL for the Pt(pc) electrode increases with increasing electrolyte pH, while the C_DL for the Au(pc) electrode shows the opposite trend. The increasing trend for Pt(pc) cannot be quantitatively described by the classical Stern model due to the stronger adsorption phenomenon on Pt surfaces. Moreover, positive linear trends with pH were found for the potentials of minimum C_DL values and the potentials of maximum entropy for both electrodes, which closely correlate with reaction activities. However, the transition potentials of the constant phase element exponent (an element commonly used to approximate the behavior of the double layer in experiments) are only observed for the Pt electrode due to the phase transitions within the hydrogen adsorption/desorption and double-layer regions. These findings pose an important step toward revealing the interplay between essential interfacial parameters, which is crucial for a complete understanding of the electrical double layer.