On obtaining double-layer capacitance and potential of zero charge from voltammetry

The method of obtaining double-layer capacitance (Cdl) and the potential of zero charge (pzc) from voltammetry is examined numerically using a modified Poisson-Nernst-Planck theory considering ion steric effect and natural convection. A prerequisite of this method is that the pure double-layer charging region is well separated from interfacial reactions (such as hydrogen adsorption/desorption at Pt), if any. An additional tacit assumption is that the double layer reaches equilibrium at each potential during the potential scanning. The equilibrium assumption is violated to a greater extent when the potential scanning is faster because ion transport is more retarded, bringing into greater errors into the results obtained. A key quantity related to the equilibrium assumption is the time constant of double-layer charging which is given by, t0 = nλδ/D (n is a potential-dependent coefficient around 20, λ the Debye length, δ the thickness of diffusion layer, D the diffusion coefficient). Also discussed is how interfacial reactions hinder the double layer from achieving equilibrium. Considerations on optimizing the experimental setup for determining Cdl and pzc from voltammetry are presented.