The Origin of the Constant Phase Element Behavior of Pt(111) Near the Potential of Zero Charge

Despite the Pt(111)/HClO₄ interface being a model system in electrochemistry, its electric double-layer (EDL) behavior deviates significantly from classical models. In the double-layer region (0.40-0.60 V_RHE), electrochemical impedance spectroscopy reveals non-ideal capacitance behavior, which is best described by a constant phase element rather than an ideal capacitor. The extent of non-ideal capacitive behavior can be quantified using the CPE exponent (α), in which we observe two distinct regimes: a decrease at potentials away from the potential of zero charge (PZC), and a pronounced minimum at the PZC itself. To interpret these findings, we combine experimental data with two-dimensional numerical simulations solving the coupled Poisson-Nernst-Planck equations. Our findings show that non-ideal capacitive behavior at potentials away from the PZC arises from finite mass transport within the EDL and an inhomogeneous current/potential distribution across the disc electrode, which results from the cell and electrode geometry. The anomalous α minimum at potentials closer to the PZC is the result of a second potential-dependent variable, which we propose to be electrowetting that alters the geometry of the wetted edge of the disk electrode in the hanging meniscus configuration and therefore the corresponding local electric field. These results highlight the critical roles of cell geometry, edge effects, and electrolyte concentration in modulating frequency dispersion. This work provides new physical insights into capacitance dispersion at well-defined interfaces and lays the foundation for more accurate models of electrochemical systems involving confined geometries and interfacial heterogeneity.