Physical modeling of water fluxes in the catalyst layers of polymer electrolyte fuel cells

The ability to predict the electrochemical performance of the catalyst layer (CL) in polymer electrolyte fuel cells (PEFCs) hinges on a precise knowledge of the water balance. The key effective properties of this layer, like gas diffusivity and vaporization exchange rate constant, control water distribution and fluxes in the complete cell. Unfortunately, the knowledge of relevant properties of CLs is rare and not available with sufficient accuracy. A physical model of water fluxes in CLs is proposed to develop a methodology for the determination of the effective properties of CLs. For the purpose of this work, the CL is considered exclusively as a medium for vapor diffusion, liquid water permeation, and vaporization exchange. The presented model exploits an analogy of the water transport problem to the processes involved in charge transfer in a porous electrode, which is represented by the famous transmission line model (TLM). The expectation is that this analogy could lead to a diagnostic tool with similar capabilities as electrochemical impedance spectroscopy (EIS) in rationalizing the response of CLs to varying conditions and in extracting parameters of water transport and vaporization exchange. An analytical solution under steady state and isothermal conditions is presented that rationalizes the relation between controlled environmental conditions and the net water flux under partial saturation. The analysis of water flux data using this solution provides a method for the extraction of the net vaporization exchange rate, the activation energy of vaporization, vapor diffusivity, and the temperature exponent of the vapor diffusivity, which allows the transport mechanism of vapor diffusion in the CL to be identified. Transient analysis with a periodic perturbation is then explored. The overall impedance of water transport and the response function of a voltage change to a vapor change are analyzed for a specific scenario, where no effluence of liquid water from the CL is permitted. The methodology based on the transient analysis provides not only a way to extracting the effective properties of the CL, but also a way to estimate the liquid saturation in the CL.