Physical modeling of the electrochemical impedance spectra for the O2 reduction reaction in HTPEM fuel cells’ cathodic electrochemical interface

Abstract

In this study we propose a microkinetic transition state pathway for the oxygen reduction reaction, (ORR), for the high temperature polymer electrolyte (HTPEM) fuel cell cathodic electrode, focusing on the importance of the cathodic impedance spectrum features, in the kinetic low current density, (lcd), regime of operation when ORR activation power losses are dominant. The ORR impedance spectrum involves a linear high frequency part feature and two arcs depending on the double layer capacitance, (Cdl), value. The high frequency linear part of the spectrum stems from the finite ionic, (H+), resistance in the catalyst layer. Our simulations revealed two arcs: all charge transfer reaction steps appear under the same high frequency arc directly related to the Cdl of the electrochemical interface, (EI), while the low frequency, (LF), arc originates from the relaxation of the adsorbed surface reaction intermediates on the catalyst surface, which is caused by the depletion of OHad on the surface. The kinetic model provides two Tafel slopes in the low and high current density (hcd) regime equal to 60 (mV/dec) and 180 (mV/dec) respectively.