Pore-scale study of effects of relative humidity on reactive transport processes in catalyst layers in PEMFC

High surface area carbon (HSC) particles can be adopted to increase the specific surface area of catalyst layer (CL) in proton exchange membrane fuel cells. Relative humidity (RH) has a significant effect on the Pt activity inside HSC particles, and the underlying mechanisms need to be further investigated. In this study, a pore-scale model considering the effects of RH on the reactive transport processes inside the CLs is developed. Two kinds of liquid water distributions affected by the RH, including capillary condensation in pores and ultra-thin liquid film on Pt surface, are considered. The liquid water distribution, Pt activity and local oxygen transport resistance (Rlocal) under different RH are studied in detail. It is found that as RH increases from 0.3 to 1.0, the condensed water in micropores of HSC particles increases, resulting in an increase in reactive surface area by about 43 %. Combined effect of the RH, Pt loading, I/C ratio and different kinds of carbon particles is investigated. It is found that due to the lack of sufficient reaction sites, compared with that under a high Pt loading, R-local under a low Pt loading is more sensitive to the RH. Besides, since the Pt activity inside HSC particles depends on the condensed water, the R-local of HSC particles is more sensitive to the RH than solid carbon particles. Finally, the R-local at low ionomer content is more sensitive to RH due to low ionomer coverage on Pt particles. The present study provides a pore-scale model for investigating the coupled effects of RH and CL porous structures on local transport pro-cesses, and can facilitate the optimization of CL nanoscale structures for better cell performance.

Automated summary

A pore-scale model is developed to investigate the effects of relative humidity (RH) on the proton exchange membrane fuel cell (PEMFC) catalyst layer (CL). The study shows that an increase in RH can enhance the reactive surface area of the CL by considering the liquid water distribution of capillary condensation and ultra-thin liquid film. The results also indicate that the effects of RH on the CL are more pronounced under low Pt loading and with high surface area carbon particles. Furthermore, the local transport processes in the CL can be optimized for better cell performance by considering the coupled effects of RH and CL porous structures.