Proton exchange membrane fuel cell (PEMFC) is one of promising next-generation power sources for both passenger owned vehicles (POVs) and heavy duty vehicles (HDVs). For its wider distributions, challenging performance targets for POVs and HDVs were set, for example, by US DRIVE Fuel Cell Tech Team, Million Mile Fuel Cell Truck and the New Energy and Industrial Technology Development Organization in Japan [1]. Both targets indicate that the efficiency and power density need to be significantly enhanced to realize better fuel economy and compact implementations in mobility systems. In this study, multi-physics simulations of a single cell [2] were carried out to examine a set of realistic properties of components that can realize the target performances. After discussing the significance of reaction and mass transport resistances in the cathode catalyst layer (CL), we will show detailed breakdowns of relevant overpotentials in the state-of-the-art cathode CLs determined by a combined experimental and theoretical analysis. The theoretical analysis is based on a mathematical single cell model [3] which assumes that the porous CL is composed of homogeneous spherical carbon particles involving primary pores, where oxygen-accessible Pt nanoparticles are deposited. The proton and oxygen mass transports inside and outside the primary particles are analytically and numerically solved using effective mass transport resistances determined to reproduce experimentally measured limiting current densities and impedance spectra. The analysis indicated that the oxygen diffusion in the secondary pores outside the primary particles does not significantly contribute to the overpotential while the oxygen transport is significantly limited in the primary pores and at the ionomer-catalyst interface. Significant reduction of the catalytic activity is also caused at the ionomer-catalyst interface. The small contribution of the secondary pores was supported by a microscopic model of the porous CL, too. 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. Further detailed analysis using microelectrode experiments and molecular dynamics simulations [4-6] indicate that both the activity and the interfacial permeation can be significantly enhanced by using a highly oxygen permeable ionomer while the advanced interface still partially suffer from the interfacial resistance.

References

[1] https://www.nedo.go.jp/content/100944011.pdf.

[2] N. Nonoyama and Y. Ikogi ECS Trans. 16 13 (2008).

[3] M. Shibata, K. Shinozaki, S. Kajiya, N. Hasegawa, S. Yamakawa, T. Suzuki and R. Jinnouchi to be submitted.

[4] R. Jinnouchi, N. Kitano and Y. Morimoto Electrochim. Acta 188 767 (2016).

[5] K. Kudo, R. Jinnouchi and Y. Morimoto Electrochimica Acta 209 682 (2016).

[6] R. Jinnouchi, K. Kudo, K. Kodama, N. Kitano, T. Suzuki, S. Minami, K. Shinozaki, N. Hasegawa, A. Shinohara Nat. Commun. 12 4956 (2021).