Molecular picture of electric double layers with weakly adsorbed water

Water adsorption energy Eads is a key physical quantity in sustainable chemical technologies such as (photo)electrocatalytic water splitting, water desalination, water harvesting. In many of these applications, electrode surface is operated outside the point (potential) of zero charge, which attracts counter-ions to form the electric double layer (EDL) and controls the surface properties. Nevertheless, the relationship between water adsorption energy Eads and EDLs is not fully understood, in particular for electrode surfaces with weakly adsorbed water. Here, by applying density functional theory-based finite-field molecular dynamics simulations, we have studied the effect of water adsorption energy Eads on surface acidity and the Helmholtz capacitance of BiVO4 as an example of metal oxide electrodes with weakly chemisorbed water. This allows us to establish the effect of Eads on the coordination number, the H-bond network, the orientation of chemisorbed water by comparing a oxide series composed of BiVO4, TiO2 and SnO2. In particular, it is found that a positive correlation exists between the degree of asymmetry ∆CH in the Helmholtz capacitance and the strength of Eads. This correlation is verified and extended further to graphene-like systems with physisorbed water, where EDLs is controlled by electronic charge rather than proton charge as in the oxide series. Therefore, this work reveals a general relationship between water adsorption energy Eads and EDLs, which is relevant to both electrochemical reactivity and electrowetting of aqueous interfaces.