For the widespread use of proton exchange membrane fuel cells (PEMFCs), cost reduction of the stack is indispensable. For this purpose, it is effective to reduce the use of Pt and other cell components by reducing the cell area with a high-power density of the cell while reducing the use of Pt catalyst per geometric area. However, automobile manufacturers have reported that reducing the amount of Pt used per geometric area increases the resistance of gas transport in the catalyst layer, resulting in greater performance loss during high power generation.(1-3) This gas transport resistance is found to be inversely proportional to the electrochemical Pt surface area per geometric area (ECSA), and is considered to stem from near the Pt surface.(4) Previously, our group proposed a mechanism in which a high-density layer of ionomer formed at the ionomer / Pt interface is the cause of the increase in gas transport resistance at a low Pt content (low Pt area) based on the analysis of oxygen transport resistance using a microelectrode and the molecular dynamics calculation.(5-9) In this presentation, we report the results of investigating the origin of gas transport resistance in the cathode catalyst layer (CL) using CLs with a low Pt loading using Vulcan® as the catalyst support. In the case of a catalyst layer using a solid carbon support such as Vulcan®, the resistance can be divided into two categories: resistance independent of pressure and Pt surface area (Knudsen diffusion resistance in secondary pores formed among carbon particles) and resistance independent of pressure and dependent on Pt surface area (resistance near Pt surface). We modified a method of Katayama et al.,(10) which uses CO to control the ECSA of an identical catalyst layer and measures the limiting current of oxygen reduction reaction, and examined a method of separating the Knudsen diffusion resistance and the resistance near Pt using the limiting current of hydrogen oxidation reaction.(11) Furthermore, this technique was applied to catalyst layers with different ionomer-to-carbon weight ratios with a constant ionomer coverage to separate the contribution of gas transport resistance in the ionomer from that at the ionomer / Pt interface. This electrochemical analysis revealed that the gas transport resistance at the interface of ionomer and Pt surface is dominant in the range of practical ionomer-to-carbon weight ratio. The small contribution of the secondary pores was supported by a microscopic model of the porous CL. In this analysis, the microscopic secondary porous model was reconstructed from 3-dimensional scanning electron microscopy (3D-SEM) images, and the molecular and Knudsen oxygen diffusion in the reconstructed model was simulated. The combined analyses indicate that the ionomer-Pt interfacial structure is a key for the design of the cathode CL.

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