| The chemical kinetics and catalytic mechanisms at the interface of the catalyst, electrolyte and electrode, known as the triple-phase, or interphase region of a polymer electrolyte membrane fuel cell (PEMFC), are continually being developed analytically through modeling. Better understanding of this region is considered by the fuel cell community, a critical component in the development of practical, low cost fuel cells.;Although the chemical kinetic parameters such as the rate constants for the electrocatalyst processes in the interphase region are difficult to measure, methods are explored in this paper for estimating the rate constant pre-exponential using statistical thermodynamics and partition functions. Furthermore, partition functions are shown to be viable to qualitatively identify the rate determining step (RDS).;A mechanistic model was developed for the oxidation reduction reaction (ORR) at the cathodic interphase region of a single cell polymer electrolyte membrane fuel cell, (PEMFC). Rate constants for these mechanisms will be compared using preexponential values calculated from both published data and partition functions. The forward and reverse rate expressions will be estimated for transition state catalytic surface reactions occurring in the triple-phase region of a PEMFC oxygen cathode. Several ideal mechanisms were proposed for the catalytic oxygen reduction and the hydrogen oxidation reactions, identifying the elementary reactions and intermediates, for formation of an H2O monolayer with and without hydroxide. Expressions were derived for the adsorption isotherms and reaction rates in terms of reaction kinetics, statistical thermodynamics, heterogeneous catalysis, and electrochemical charge transfer. I have developed expressions for the adsorption isotherms that I attempt to solve using MathCad, an equation solver software to find the surface species populations theta i of O2, O, H, OH and H2O under steady state conditions. These isotherms can provide insight into fuel cell parameters, such as the amount of reactants adsorbed on the electrodes, activity in bulk solution, and the electrical state of the systems, at a given temperature and reactant concentrations. The model was formulated for ideal interphase conditions and does not address mass transport, surface diffusion, or thermal management. These were excluded from the model to explore only the heterogeneous catalytic processes, but are discussed briefly on how to incorporate these for future development and expansion of this model. Even without these additional features the model was very stiff and did not converge to solution.;As a result of this work are promising methods to identify adsorption isotherms for heterogeneous catalysis. Further work will need to be done on the model to improve its solver and modify the input parameters. |
