Abstract
The mass transport in the porous media in PEMFC involves multiscale and two-phase flow. The complicated mass transport not only influences the cell performance but also interacts with the degradation in various forms. In past decades, many efforts have been carried out to understand the mass transport in distinct porous components. However, the performance losses resulting from mass transport limitations necessitates further clarification. Besides, the porous components of PEMFC undergoes complicated harsh condition in realistic operation, and experiences diverse degradation scenarios, in the form of not only material degradation but also porous microstructure damages. The interactions between mass transport and performance degradation exacerbates each other and both leads to severe performance decay. Therefore, in this work, a series of experimental studies have been implemented to understand the mass transport in multiscale porous components in PEMFC. The relationships between component degradation and mass transport losses, as well as the cell performance are studied.
First the performance degradation of a single PEMFC is studied using an accelerated stress testing (AST). The mechanisms and contributions of different degradations caused by carbon corrosion are clarified. The results indicate that the kinetic degradation follows a quicker decay at the initial stage and a steady decay at a later stage. Besides kinetic degradation, the increase of mass transport loss is more involved.
The deterioration of water transport leads to severe mass transport limitation. To account for the mass transport limitation from distinct porous components and their variations due to the damage of microstructure, the performance is characterized under various operating conditions. The oxygen transport resistance in porous media are also measured and identified. The primary increase in mass transport loss occurs in catalyst layer (CL) due to its worse water transport and increased oxygen transport resistance caused by microstructure damages. The introduction of micro porous layer (MPL) significantly improve the cell water balance and thus cell performance. Moreover, it also alleviates the degradation in CL and thus less degradation, though MPL itself also suffers from carbon corrosion and results in higher oxygen transport resistance. The gas diffusion layer (GDL) does not shows apparent increase in resistance to oxygen diffusion. Yet the characterization under various operating conditions indicates that the aged GDL tends to retain more liquid water due to potential loss of hydrophobicity, which could cause significant increases in mass transport loss under certain conditions, for instance, high cathode relative humidity (RH). However, the water transport could be improved by the heat generation and also the optimization of operating parameters.
As the water transport greatly affects the mass transport in porous components and thus the cell performance, three methods are proposed following the principles of water transport in porous medium. By regulating the PTFE loading, hydrophobicity gradient from high at CL side to low at GDL side show better porosity distribution and thus better performance. The MPL penetration into GDL substrate plays an important role in porosity profile within GDL. Shadow MPL penetration results in optimal porosity and thus improved performance. Moreover, the cracks/pores on MPL surface are found to be important to the water drainage pathways. By arranging MPL with different cracks/pores at anode and cathode, the water transport balance could be optimized, and thus good cell performance could be achieved.