Inclusion of surface heterogeneity in bridging PEM fuel cell electrode contamination kinetics and transport via a competitive Langmuir-Freundlich isotherm

Abstract

This paper reports the development of mathematical model that bridges PEMFC transport phenomena with surface reaction kinetics in rough and heterogeneous electrodes, in which specifically the anode is modeled under carbon monoxide contamination. The mathematical bridging is done by converting surface concentration of reactants and contaminant into surface coverage of relevant adsorbates using the Langmuir-Freundlich isotherm to statistically include heterogeneity in binding site energetics. Thermodynamically optimized kinetic rate and equilibrium constants are calculated using coverage-dependent activation energies and provided as input to the HOR, ORR, and a kinetic-based limiting current model developed to solve for the distribution of activation overpotential. A novel model for a closed-form calculation of activation overpotential is proposed to allow numerical investigation via a galvanostatic approach. The kinetic reaction models are highly coupled with three-dimensional transport equations and solved iteratively under single-phase and steady-state conditions. Comparison is done with respect to two sets of available literature data in order to test the kinetic model validity under variation of CO concentrations and cell operating temperatures, in which good agreement is found. The results confirm that a Langmuir-Freundlich isotherm could be a more suitable isotherm compared to the extensively used Langmuir-specific isotherm for rough heterogeneous surfaces physically found in PEMFC catalysts. Results from the model show that rate of HOR is higher under the ribs of the cell since most of the active sites under channel are blocked with CO, with the effect exacerbated at lower temperatures. © 2014, Springer-Verlag Berlin Heidelberg.