MODELLING AND SIMULATION OF DIFFERENT TYPES OF CYLINDRICAL AGGLOMERATES IN CATHODE CATALYST LAYER OF PEM FUEL CELLS

Abstract

A Mathematical model for PEM fuel cell cathode catalyst layer has been developed to study the reaction kinetics, transportation of reactants and products through the agglomerates by considering two types of cylindrical agglomerates in cathode catalyst layer. The first type of cylindrical agglomerates consisted of mixture of carbon/Pt particles and a perfluorosulfonated ionomer (PFSI) and the second type was the mixture of carbon/Pt particles and water-filled pores. The performance of the PFSI filled agglomerate is explained on the basis of Thiele-modulus and the characteristics of the water filled type are studied with the help of Nernst—Planck and Poisson equations. Simultaneous Nernst-Plank, Poission and oxygen diffusion equations is discritised by using central difference method and the simulation results obtained using Visual C++ showed that, when proton concentration on the agglomerate surface was increased, the proton penetration depth and the effectiveness factor was decreased for water filled agglomerates. It has been found that in order to achieve more effectiveness factor, proton concentrations on the agglomerate surface (CH,1) should be less than 0.6x10-4 μm. The effect of agglomerate radius (Ra) on the catalyst utilization also has been studied, the results showed that with the reduction of Ra proton penetration depth and the effectiveness was increased, whereas the same value of Ra and cylindical agglomerate (L), the maximum effectiveness factor was obtained for radius of less than of 0.05μm. On the other. hand, maximum effectiveness factor was obtained at the Ra and L value of 0.2 and 0.05 pm, respectively. In addition to that, the effect ,of cathode transfer coefficient (ac) on the electrochemical reaction rate study, showed that the maximum reaction rate was obtained on the agglomerate surface when a. < 1, and inside the agglomerate when a. >1. Moreover, the results dictated that water filled agglomerates showed larger effectiveness factors as compared to PFSI-filled agglomerates at the cathode transfer coefficient (at) close to 1, whereas at ac = 0.75 the effectiveness factor for PFSI was close to 1. Finally, we compared the cylindrical and spherical agglomerate models to determine the best suited model for optimum catalyst utilization, the results showed that the spherical agglomerates has high effectiveness factor as compared to cylindrical agglomerates.