Low Parsons–Zobel slope of metal–solution interfaces: A theoretical comparison of three causes

Abstract Rational design of electrochemical energy conversion and storage devices hinges on a fundamental understanding of the electrical double layer (EDL) at the electrode–electrolyte interface. The classical Gouy–Chapman–Stern model of EDL predicts a unity Parsons–Zobel (PZ) slope, which is derived from the relation between inverse measured capacitance and inverse Gouy–Chapman capacitance. However, recent experiments on metals such as Au and Pt have revealed ultrahigh capacitances and ultralow PZ slopes, markedly deviating from the classical model. In this study, we employ density‐potential functional theoretic models to systematically compare three mechanisms: surface roughness, nonspecific ion attraction, and ion chemisorption with partial charge transfer. Our analysis reveals that only ion chemisorption fully accounts for the ultrahigh double layer capacitance and ultralow PZ slopes. The gleaned insights into the EDL on transition metals are informative for understanding local reaction environment in electrochemical energy conversion and storage devices.