Cation-Mediated Pseudocapacitance Dominates the Interfacial Charging of α-Fe₂O₃(0001) in an Alkaline Electrolyte

The electric double-layer at the electrode-electrolyte interface is crucial for electrocatalytic reactions in electrochemical applications, such as water splitting. On metal oxide surfaces in aqueous electrolytes, such as α-Fe₂O₃(0001), proton exchange between interfacial water and surface groups (e.g., Fe-O-(H)) varies with pH and potential. This process induces pseudocapacitive charging alongside standard double-layer charging. Using impedance spectroscopy, the effect of cation concentration and pH on the adsorption pseudocapacitance originating from deprotonation of Fe-O-(H) was studied. Results show that both the double-layer capacitance and adsorption pseudocapacitance remain largely unaffected by the electrolyte concentration and pH within the 'double-layer' window. However, the charge transfer resistance (<i>R</i> _ct) was found to be inversely proportional to the NaOH concentration but remained constant between pH 12 and 14 at a fixed Na⁺ concentration. The concentration-independent double-layer capacitance suggests a Helmholtz or compact-type layer, with negligible diffuse layer contributions to the capacitance. Consequently, no diffuse layer effects are expected on the reaction kinetics, whether pseudocapacitive or Faradaic. Interestingly, the correlation between cation concentration and <i>R</i> _ct implies that cations mediate the proton-coupled electron transfer (PCET) acid-base reactions. This results in a cation-coupled PCET (CCPCET) mechanism that determines the current in the 'double-layer' window. Thus, the observed current in the 'double-layer' window of α-Fe₂O₃(0001) is predominantly cation-mediated and pseudocapacitive rather than attributable to traditional double-layer charging.