Optimization of cathode catalyst layer for direct methanol fuel cells: Part II: Computational modeling and design

The cathode catalyst layer in direct methanol fuel cells (DMFCs) features a large thickness and mass transport loss due to higher Pt loading, and therefore must be carefully designed to increase the performance. In this work, the effects of Nafion loading, porosity distribution, and macro-pores on electrochemical characteristics of a DMFC cathode CL have been studied with a macro-homogeneous model, to theoretically interpret the related experimental results. Transport properties in the cathode catalyst layers are correlated to both the composition and microstructure. The optimized ionomer weight fraction (22%) is found to be much smaller than that in H2 polymer electrolyte fuel cells, as a result of an optimum balance of proton transport and oxygen diffusion. Different porosity distributions in the cathode CLs are investigated and a stepwise distribution is found to give the best performance and oxygen concentration profile. Influence of pore defects in the CLs is discussed and the location of macro-pores is found to play a dual role in affecting both oxygen transport and proton conduction, hence the performance. The reaction zone is extended toward the membrane side and the proton conduction is facilitated when the macro-pores are near the gas diffusion layer.