Analysis of the Effect of Pt Loading on Mass and Specific Activity in PEM Fuel Cells

Abstract

Combined experimental and simulation techniques were applied to evaluate the validity of the commonly applied Tafel-approximation for the calculation of the mass and specific activities. A one dimensional agglomerate-based macro-homogeneous model was developed and solved analytically and numerically to understand the effects of exchange current density, porosity, agglomerate size, Nafion-film thickness and Pt loading on the mass activity. Porosity higher than 60% or agglomerates bigger than 100 nm at higher loadings may cause the loss of the validity of the Tafel-approximation and consequently the mass and specific activities decrease. Since these effect do not fully interpret the measured data, a new structural parameter was introduced. The real pore structure was estimated by using fractal theory. The volumetric catalyst density was corrected by the fractal dimension measured by Hg-porosimetry, which finally gave a good agremment with the experimental values.