PERMEABILITY, STRUCTURAL CHANGES, FLOW REGIME TRANSITIONS IN FUEL CELL CATALYST LAYERS AND A MODEL FOR PREDICTING TRAPPED SATURATION FOR TWO-PHASE FLOW IN POROUS MEDIA

Abstract

Proper water management in Proton Exchange Membrane (PEM) fuel cell is important to achieve high performance. Understanding the percolation of the produced water at the cathode catalyst layer (CL) is critical for any robust water management technique. In this study, an ex-situ experimental setup is used to study percolation in the CL at different injection rates and relative humidity (RH) conditions. The results show that increasing the flow rate force the liquid to flow through the bulk of the pores due to the dominant viscous effect. On the other hand, at low injection rates, the capillary becomes dominant and liquid flow along the roughness of pores surfaces. At low flow rates, a big wetted area is captured, compared to high flow rates tests, but the liquid saturation is lower. Another set of testing were done at a fixed flow rate and a varied RH conditions, where permeability is calculated based on the steady percolation pressure. The Permeability of the CL for both gas and liquid decreases as RH increases, and that more likely related to ionomer swelling. A correlation between the permeability and water content (λ) is derived. A sharp decrease in the permeability is observed at low water content (λ < 3), beyond that there are no significant changes. Moreover, low and high RH conditions show significant effects on the structure of CL and flow regime. Static contact angle measurements at a range of RH also indicate possible morphological changes in the CL. In addition, fractional flow theory (FFT) model is adapted to study immiscible displacement of two-phase flow in a porous medium. The resulting model accurately predicts trapped saturation that occurs during imbibition and drainage of incompressible fluids for any capillary number. It also accurately predicts the fluid-fluid front displacement and critical capillary number at which trapped saturation begins to decrease. The unique aspect of this model is incorporation of a scaling factor, suggested by Médici and Allen (2016), that captures the propensity for gas or liquid holdup for any porous media and fluid pair.