On the behavior of the porous rotating disk electrode

The behavior of the current from a porous rotating disk electrode (PRDE) as a function of the rotation rate, electrode geometry, and porosity at large overpotentials is studied experimentally. The current shows a much richer behavior than the flat RDE, including a sigmoidal dependence on the rotation rate. At low rotation rates, the measured current is qualitatively similar to that of an RDE, with the current increasing with the square of the rotation rate. However, at a critical rotation rate, which we term the lower critical rotation rate and which depends on the radius, the thickness, and the porosity of the porous disk, the current increases much more rapidly than predicted by the Levich theory. At a larger, upper critical rotation rate, the current increases more slowly and then approaches a plateau. The plateau value greatly exceeds that predicted by Levich and is about 20 times greater than that for a flat electrode. The results are explained in terms of the angular velocity dependence of the ratio of the effective electrochemical reaction time to the residence time of the fluid in the porous disk. A theory is presented that collapses the data onto a universal curve. (c) 2006 The Electrochemical Society. [DOI: 10.1149/1.2403082] All rights reserved.