Reaction and Mass Transport Simulation of Polymer Electrolyte Fuel Cell for the Analysis of the Key Factors Affecting the Output Performance in the Catalyst Layer

Abstract

Polymer electrolyte fuel cell (PEFC) has been developed as a power source for fuel cell vehicle (FCV). To have FCV used more commonly, it is required to reduce PEFC system cost[1]. Increasing output density is effective measures for cost reduction because it is necessary to reduce total cost of PEFC in addition to quantity of platinum catalyst. In recent years, high performance cathode catalyst layers (CLs) are proposed, which made contrivances to its catalyst structure[2-3]. However, the method for developing catalyst is trial and error and there is no clear design guideline for further improvement. The purpose of this study is to propose the optimum design of cathode catalyst (Pt/CB) by simulating the electrochemical reaction and PEFC transport phenomena. In this study, based on the our previous research[4], diffusion phenomenon in the catalyst is newly reflected on the 1D model to compare catalyst structures. Fig.1 shows the image of transport phenomena in the catalyst pore. As shown in the structure (A), if Pt is covered with ionomer, oxygen reduction reaction (ORR) rate per unit Pt surface area is reduced by sulfonic group in ionomer. Thus it is assumed that if Pt exists in the pore as shown in the structure (B), ORR rate increases compared with structure (A) because there is no sulfonic group in the pore. However, oxygen concentration on the surface of Pt decreases compared with structure (A) because oxygen diffusion resistance increases. In this research, reflecting this trade-off between ORR rate and diffusion resistance, we constructed the model to compare catalyst structures. Based on the porous electrode theory[4], ORR and mass transport at the fuel cell evaluation test were simulated. In this research, generated water was considered as vapor, carbon corrosion reaction and suppress elution of platinum were neglected. The temperature profile in the cell was presumed to be uniform (80 ℃). Fig.2 shows some catalyst structures for simulation. Spherical diameter of carbon black (CB) and Pt catalyst carrying rate of CB are same in all structures. Assuming that pores of the porous CB is filled with the generated water. In addition, diffusion coefficient of GDL and MPL are determined by the experimental correlation equation for porous structures of PEFC. Each structure was simulated under various exchange current density. And oxygen distribution and relative reaction ratio were investigated in the catalyst. Fig. 3 shows the current-voltage characteristic curves obtained from the simulation (Exchange current density in the pore is five times as large as covered with ionomer.) Str.2-6 has larger output density at low current density region. In this case, this result shows porous carbon black has larger output density than Str.1 for PEFC because practical voltage region is 0.6 V - 0.8 V. Fig.4 shows the radial relative reaction rate in the pore.Reaction rate decreasing for the center direction, we were able to reflect oxygen diffusion resistance and compare catalyst structures. Furthermore, we attempt to separate factors of performance degradation of fuel cell and evaluate each factors quantitatively to confirm the appropriateness of the model.