(Invited) Computational and Experimental Characterization of PEFC Electrode Structures
Authors: Yamakawa Shunsuke , Kajiya Shuji , Yatsu Ryosuke , Hasegawa Naoki , Suzuki Takahisa , Shinozaki Kazuma , Jinnouchi Ryosuke 
Published: December 2024

In polymer electrolyte fuel cells (PEFCs) that demand high-efficiency and high-power-density for vehicle applications, elucidating the structure of the cathode catalyst layer is crucial to maximize the catalytic activity, proton conductivity, electric conductivity, and mass transport. Complex hierarchical primary particle, aggregate, and agglomerate structures of the cathode catalyst layer, which contains ionomer and platinum (Pt) catalysts supported on carbon particles, hinder the identification of bottlenecks in power generation. Particularly in recent years, emerging high-performance mesoporous carbon has introduced further complexity [1,2]. This advanced material is known to improve the catalytic activity by reducing the poisoning of Pt active sites. However, the gas transport resistance of mesoporous carbon remains unknown. Pt particles may be located both inside and outside the mesopores of the primary particles. The resistance to gas transport for Pt particles inside mesopores, which are less likely to be covered by ionomers that significantly hinder gas transport [3,4], is expected to be low. However, if Pt is located too deeply within the mesopores, or if the pores are submerged, the gas transport resistance could become substantial.
Here, we elucidate impacts of ionomers and water on gas transport resistance in a catalyst layer composed of mesoporous and solid carbons by combined experiments and simulations. A pressure-independent gas transport resistance (Rother) can be determined from the limiting current of the hydrogen oxidation reaction (HOR) under various relative humidity (RH) conditions [3]. Separate ex-situ N2 and water adsorption isotherms of the catalyst layer powders allow for determining the volume ratio of ionomers within the mesopores and the RH-dependent water content in the catalyst layer, respectively. Moreover, computational simulations based on information from 3-D virtual models of the catalyst layers, reconstructed from 3-D transmission electron microscopy images and the isotherms, can reveal further details about the interactions between pore structure, Pt placement, ionomer distribution, and humidity. Our experiments have shown that at low RH, the mesoporous carbon exhibits a lower Rother than the solid carbon. However, as RH increases, Rother sharply increases, and the difference in Rother between mesoporous and solid carbons decreases. The experiments also displayed a hysteresis of RH-dependent Rother similar to that in the water adsorption isotherm. All these results suggest significant impacts of liquid water condensed in the mesopores. The simulations indicated that the lower Rother of mesoporous carbon at low RH can be reproduced by assuming that gas transport resistance at the ionomer/Pt interface is reduced in the mesopores, where Pt particles are not fully covered by ionomers owing to limited ionomer penetrations. The simulations also showed that Rother significantly increases, as observed in the experiments, when the mesopores are filled with liquid water. These findings suggest that optimizing the mesopore structure and distributions of Pt particles, ionomers, and water is crucial for enhancing power density.
References:

[1] V. Yarlagadda et al., ACS Energy Letters 3 (2018) 618–621.
[2] T. Takeshita et al., J. Electroanal. Chem. 871 (2020) 114250.
[3] K. Shinozaki et al., J. Power Sources 565 (2023) 232909.
[4] R. Jinnouchi et al., Nat. Commun. 12 (2021) 4956.