THEORETICAL APPROACH TO FARADAIC ADMITTANCE OF HYDROGEN ABSORPTION REACTION ON METAL MEMBRANE-ELECTRODE

Faradaic admittance of hydrogen absorption reaction (har) on metal membrane electrode has been derived on the basis of ''two models'' of the har under the permeable boundary condition by using an extension of Armstrong's kinetic approach. In the case of the har through adsorbed phase (model A), two limiting cases of the Faradaic admittance are considered depending upon the magnitude of rate of hydrogen transfer from bulk to metal surface. One involves the diffusion-controlled har for fast rate of hydrogen transfer and the other is the interface-controlled har for slow rate of hydrogen transfer from bulk to metal surface. Depressing and/or tailing of the second semicircle, often found in the complex-plane impedance spectra for hydrogen evolution reaction (her), is fairly explained by the diffusion-controlled har. In the case of the direct har without passing through the adsorbed phase (model B), the Faradaic admittance is differentiated by the absence of adsorption capacitance from that based upon the model A.