Convective mass transfer to partially recessed and porous electrodes

The diffusional problem of a rotating porous electrode has been analysed based on the mass transfer equations for a partially blocked electrode. It is shown that the porous geometry leads to a dependence of the current on rotation rate identical to that corresponding to a coupled diffusion-activated electron transfer mechanism. The apparent rate constant, however, is related only to the geometry of the porous surface. It is shown that the characteristic diffusional length corresponds to the pore dimension modified by a term including the transition from linear to spherical diffusional geometry at the pore entrance. The theory is compared with experimental results for the reduction of hexacyanoferrate(III) and oxygen on platinum electrodes covered by an insulating film of nickel hydroxide. It is shown that the limiting currents from Koutecký–Levich plots give a much larger blocking factor than that derived from the analysis of a semilogarithmic current function. A comparison of these two methods for calculating the blocking factor provides information on the tortuosity coefficient of the film.