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

The method of obtaining double-layer capacitance (C-dl) 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, t(0) = n lambda delta/D (n is a potential-dependent coefficient around 20, lambda. the Debye length, delta 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 C-dl and pm from voltammetry are presented.