Oxygen Transport Resistance in PEMFC Catalyst Layer Based on Ultra-Micro Electrode (UME) Theory

Abstract

Hydrogen fuel cell is a promising zero-emission automotive powertrain for larger vehicles with long-distance requirements. However, its cost must be reduced to achieve real commercial use. Minimizing use of precious metal catalysts and increasing the fuel cell power density have been the most viable means to reduce the stack cost. Focus of advanced research is shifting to improve the fuel cell power density (> 1 W/cm2) with low Pt-loaded catalyst layers (≤ 0.125 mgPt/cm2 total). One of the emerging issues with the low loaded fuel cell electrode is the dramatic increase of oxygen transport resistance with Pt loading reduction. This has been observed for both diluted (with addition of carbon particles to maintain the layer thickness) and undiluted (thinner) electrodes. The mechanism for the transport resistance increase in low Pt loaded electrode is not well understood and the quantitative modeling of the resistance is scarce. In this work we propose a simple explanation to the oxygen transport in the catalyst layer based on the assumption that each active Pt particle functions as a semi-spherical nano-electrode, which is covered by a layer of proton conducting matter (ionomer, water film, or a combination of two). Without any other assumption, the oxygen transport resistance in the catalyst layer can be analytically determined, for the case of limiting current density, as (see image file for the equation) This work identifies several critical factors that influence the oxygen transport resistance, including the CL thickness, the Pt particle density, and the diffusivities of oxygen in proton conducting phase and in CL voids. Figure 1 shows the influence of the CL thickness and the Pt particle density, the two most influencing factors, on the oxygen transport resistance, as an example. From this work it is possible to reduce the oxygen transport resistance by optimizing the layer structure. (see image file for the plot) Figure 1. The influence of the CL thickness (left) and the Pt particle density (right) on the oxygen transport resistance in the catalyst layer. Figure 1