In practice, carbon electrodes are highly heterogeneous with surface defects and organic functional groups in abundance when used in aqueous electrolyte media. This seems to translate into an electron transfer process occurring at a surface consisting of microscopically small and distinctly different regions that must be sufficiently close so that complete overlap of diffusion layers occurs on the measurement timescale, presumably allowing the entire surface to be successfully modelled by approximating the mass transfer by planar diffusion.