Modelling of Electrocatalytic Double Layers with Refined Treatment of Metal-Water Interactions and Chemisorption

The double layer at metal-aqueous interfaces in electrocatalytic systems has two features that are inadequately described by classical double layer models, namely, metal-water chemical interactions and chemisorption of partially charged adsorbates. A modified model is developed herein to encompass these two features. Specifically, the first-layer water molecules are treated statistically, considering chemical interactions and a continuous spectrum of water orientational states. In the latter aspect, this model improves over the often-used two-state water models. In addition, the chemisorption processes are assumed to have distributed equilibrium potentials, and the partially charged chemisorbates form dipole moments. With these modifications, the present double layer provides insights into how the potential of zero charge and the double-layer capacitance are influenced by the first-layer water molecules and partially charged chemisorbates. The model is then employed to fit recent experimental capacitance data of Pt(111)-aqueous interfaces that are calculated from cyclic voltammetry (CV). Quantitative agreement between experiments and the model is obtained, based on the assumption that the CV-based capacitance data are not the pure double-layer capacitance but include chemisorption capacitances. This assumption is supported by the observation that the CV-based capacitance values are several fold higher than the values obtained using electrochemical impedance spectroscopy, a technique that can separate pure double-layer charging and chemisorption processes. The model and the explanation of experimental data are critically discussed.