Evaluation of Degradation Mechanisms of Polymer Electrolyte Fuel Cell Using DEM Simulation
Authors: Okamura Kaisei , Saito Yuki , So Magnus , Permatasari Agnesia , Yano Takeru , Inoue Gen 
Published: December 2024

Polymer electrolyte fuel cells (PEFCs), which have various advantages such as high power output, high conversion efficiency, and low operating temperature, are expected to be applied to heavy-duty vehicles with a high environmental impact, and further performance and durability improvements are required for practical use. Currently, the degradation of power generation performance due to long-term operation is an issue in the practical application of PEFCs and it is considered that one of the causes of this degradation is the corrosion of the carbon support. However, the dynamic process of catalyst layer structure change due to CB particle corrosion has not yet been clarified, and the relationship between power generation performance change and catalyst layer microstructure has not been linked. It is also considered difficult to do so experimentally with the current state of the art. In this study, we focused on analyzing the dynamic structural change behavior of the catalyst layer due to corrosion of the carbon support by using numerical model simulation. By using the catalyst layer created by the numerical model, we simulated the power generation reaction and to identify the factors that cause the degradation of power generation performance.

Discrete element method (DEM) was used to calculate the structure of catalyst layer by using carbon support, which is a branch-like aggregate of carbon black (CB) particles. The aggregate arrangement was determined based on the translational and rotational equations of motion for individual aggregates, elasticity between particles, and viscous damping1). Based on previous studies2) 3), the carbon corrosion reaction caused by the reaction between the products of the carbon surface reaction and the platinum oxidation reaction is considered, reflecting localized carbon corrosion within the catalyst layer. Also, we hypothesized that the porosity increases due to the shrinkage of CB particles in the early stage of corrosion, followed by a decrease in porosity and thickness due to the separation of the sintered parts of the CB particles that compose the aggregates. Furthermore, in this model, the fastening pressure is related to the timing of the structural collapse of the catalyst layer. Therefore, in this study, we performed calculations considering the neck breakdown of the aggregate due to the fastening pressure. The particles were assumed to be non-porous carbon (Vulcan), and calculations were performed under the same initial conditions as actual carbon black: primary particle size of 30 nm and aggregate diameter of 400 nm. Ionomers were assumed to be uniformly coated and not change in volume during degradations. The power generation reactions considering transport and diffusion were simulated for the catalyst layer structures fabricated by DEM, and the factors that cause differences in power generation performance were evaluated for each catalyst layer structure. In this study, performance degradation due to aggregation and desorption of platinum was neglected because we focused only on the effects of structural changes.

Dynamic degradation of the catalyst layer was calculated considering localized corrosion due to carbon oxidation and structural collapse due to fastening pressure.

The results showed a significant change in the catalyst layer structure despite the small amount of carbon corrosion. It was also suggested that the fastening pressure affects the amount of carbon corrosion when structural collapse occurs.

Acknowledgment

This study was supported by New Energy and Industrial Technology Development Organization (NEDO).

References

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[4]Gen Inoue et al., J. Pow. Sou., 327, 610-621, (2016)

[5] N. Macauley, et al., J. Electrochem. Soc., 165 F3148 (2018)