Steady‐State Polarization at Porous, Flow‐Through Electrodes with Small Pore Diameter: I . Reversible Kinetics

Abstract

This paper considers the effect of mass transfer and ohmic limitations on the current‐polarization relations for porous flow‐through electrodes with fine pores, for reversible one‐electron transfer reactions. The model includes mass transfer by both axial forced convection and axial diffusion. A new analytical solution is presented which reduces to a previous solution when axial diffusion is negligible. The effects of axial diffusion and ohmic potential drop in the pore electrolyte on the current and concentration distributions inside the electrode are investigated. Experimental confirmation is presented for some of the predictions and a quantitative comparison between theory and experiment is presented. It is concluded that axial diffusion effects are negligible under many experimental conditions, e.g., normal diffusion coefficient, flow rate, electrode length, etc. The results demonstrate the validity of the one‐dimensional model presented here under these conditions.

Steady‐State Polarization at Porous, Flow‐Through Electrodes with Small Pore Diameter: II . Irreversible Kinetics for One‐Electron and Consecutive Two‐Electron Transfer Reactions

“…It is expected, therefore, that as A increases the effect of axial diffusion becomes detectable at smaller values of 0. This is consistent with the product A0 being the controlling parameter for axial diffusion, which was concluded from the analytical solutions in part I (22).…”

“…Formally, at x = --0, R(--0) --1, since no reaction occurs before x = 0. As discussed before (22), however, when the reaction approaches reversible kinetics (large to), the concentration change at x : 0 approaches a 5 function and the concentration gradient from the +0 direction can be nonzero. In practice, some reaction will occur on the outer face and in the two-dimensional pore mouth so that a gradual change of concentration gradient would occur over x = • Equation [5] is put into a form suitable for solution with these boundary conditions by using the substitu-…”

“…The relation between current and concentration is given by (22) i(x) ----1 --R(x) + OdR(x)/dx [8] And since dR(x)/dx -~ 0 at x --1, Eq. [7] and [8]…”

“…Determination of the diffusion coefficient and kinetic parameters of this couple on basal and edge planes of nonporous pyrolytic graphite were reported elsewhere (21). The porous electrodes were prepared from SP1 graphite used previously (22). The graphite particles are flakes of 20-30 #m average diameter and 0.3-0.4 #m thickness.…”

“…The theoretical predictions are tested experimentally by using the SnZ+/Sn4+ redox couple which is highly irreversible, and has reasonably wellunderstood kinetics (19)(20)(21). The corresponding model for reversible kinetics has been presented and tested in part I (22).…”