Molecular Insights into the Double-Layer Capacitance of Platinum Surfaces in Alkaline Media

Interpreting electrochemical capacitance measurements in terms of pure double-layer charging is particularly challenging on electrode surfaces that exhibit adsorption processes. Here, we combine electrochemical impedance spectroscopy (EIS) and ab initio molecular dynamics (AIMD) simulations to elucidate the molecular origins of the double-layer capacitance of Pt(111) and Pt(110) in alkaline media. Experimentally, we observe a systematic decrease in capacitance upon entering the hydrogen underpotential deposition (H_UPD) region. AIMD simulations reveal, however, that this decrease is not primarily caused by hydrogen adsorption itself. Instead, the capacitance response reflects a transition between two distinct regimes governed by the structure and polarization of interfacial species. Near the potential of zero charge (PZC), orientational polarization and charge-transfer contributions compensate each other, yielding a high capacitance. At more negative potentials, orientational saturation of the first water layer limits further screening while charge transfer continues to evolve, breaking this compensation and resulting in a low capacitance. This transition occurs for both bare and hydrogen-covered surfaces, demonstrating that the reduced capacitance observed in the H_UPD region at alkaline pH is not a unique consequence of hydrogen coverage. Instead, under the conditions studied here, the low capacitance arises predominantly from the large potential offset from the potential of zero charge.