Effect of a Finite-Contact-Angle Meniscus on Kinetics in Porous Electrode Systems

Abstract

The behavior of the meniscus in a static gas-electrode–gas-electrolyte system has been studied to clarify the mechanism(s) involved in the electrochemical dissolution of gaseous reactants in porous electrodes. Concurrent optical and electrochemical measurements in a specially designed cell, constructed from Teflon and fused quartz, demonstrated conclusively that although metastable thin films could be produced on the electrode by immersion followed by slow draining under high potentials (i.e., > 0.8 V with respect to the normal hydrogen electrode), these films broke down irreversibly at lower potentials, and were never observed to form spontaneously. The most stable configuration was a meniscus with a finite contact angle which was between 1° and 3°. On the basis of the observed geometry of the meniscus and a finite contact angle, a detailed second-order nonlinear nonhomogeneous differential equation of the form d2ηdx2 + F(x) dηdx + A[exp(kη) − exp(k2η)]G(x)[1 + B G(x) exp(k1η)] = 0, where F(x), G(x), A, B, k1, and k2 include only the pertinent experimental conditions (meniscus shape, exchange current, diffusion coefficient, etc.), was derived involving both concentration and activation polarization and the IR drop in the meniscus. This equation was solved numerically in complete form on a digital computer. The computed relations of current to potential were within 10% of the experimentally obtained ones for both H2 and O2 in 1N H2SO4 on Pt electrodes, without the inclusion of “calibrating” factors. The results of the computations show that the meniscus is effectively divided into three regions. In the first, near the tip, the reaction kinetics on the electrode surface is the primary limiting factor. The second region has mixed diffusion and activation limitation up to the zone where the meniscus is so thick that negligible additional currents can be drawn. This zone starts the third (inactive) region which extends down into the bulk of the solution. The experimental conditions (potential, gas pressures, etc.) determine the location of the boundaries of these regions and thus the over-all behavior of the meniscus. At moderate polarizations, over 90% of the current is produced within a very small (10−4–10−5 cm) distance from the meniscus tip.