Capacitance Measurements for Evaluating Electrochemical Double-Layer Models and Potentials of Zero Charge: A Reassessment.

Differential capacitances (DCAPs) derived from electrostatic Gouy-Chapman-type models for electrochemical double layers (DLs) typically show valley-, bell-, or camel-type profiles as a function of the potential, centered around the potential of zero charge. These DCAP profiles are routinely evaluated with measured potential dependencies of capacitances. Here, the influences of hydrogen evolution, oxygen reduction, and oxide formation on the potential dependence of the capacitance of a polished gold electrode are experimentally examined. These parasitic reactions are found to cause most of the potential-dependent capacitance features that are typically attributed to intrinsic DL properties. With these insights, the historical development of the literature regarding the development of the theoretical framework in relation to capacitance measurements is critically reevaluated. As a result, drawbacks of the 100-year-old Gouy-Chapman theory for the DL are identified. Moreover, DCAPs as differences of electrostatic states are discussed as unable to portray measured capacitances that result from capacitive-resistive and dynamic charge displacements in the DL. Hence, the links between theories and experiments are critically assessed, motivating the need for more advanced atomistic models to adequately portray the DL.