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

The double layer on transition metals features chemical metal-water interactions and chemisorption of partially charged adsorbates, which have not been adequately described in previous double layer models developed for mercury-like metals. A new model with an improved description of both is developed herein. The first water layer is treated statistically, considering chemical interactions with the metal and a continuous spectrum of water orientational states. Chemisorption processes are described using Langmuir adsorption isotherms with distributed equilibrium potentials. In addition, the surface dipole moment induced by partially charged chemisorbates, markedly changing double layer behaviors, is considered. The model is then used to study 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 provides an alternative interpretation for recent capacitance data of Pt(111)-aqueous solution interfaces calculated from cyclic voltammetry (CV). The present interpretation is appealing for it does not introduce any unknown metal-ion interactions as previously, and for it resolves the discrepancy that the CV-based capacitance is several fold higher than that obtained using electrochemical impedance spectroscopy. The implications and limitations of the model are discussed.