Measurement of Water Transport Properties Through Membrane Electrode Assemblies

This paper presents an in situ method to characterize the effective water diffusivity De of cathode diffusion media in an operating fuel cell. The De values of self-made gas diffusion layers GDLs and a commercial GDL were experimentally determined to be about one order of magnitude lower than the theoretical value. The effects of several factors such as the inert gas component and cathode pressure as well as the presence of microporous layer MPL on De were studied, and the mechanism of water transport through cathode diffusion media was explored. It was found that the water transport is largely controlled by molecular and Knudsen diffusion through the catalyst layer. The MPL does not exhibit a dominant resistance to water transport. Interrelations of De with the net water transport coefficient through the membrane and membrane resistance were also examined, suggesting that good balance between suitable membrane hydration and cathode flooding avoidance can be achieved by optimizing cathode diffusion media. The capability to measure De reported in this work provides a diagnostic tool for screening cathode diffusion materials. Diffusion media or gas diffusion layers GDLs are a vital component for attainment of high performance in proton exchange membrane fuel cells PEMFC. One essential requirement for an ideal GDL is that it should have proper hydrophobicity so that excellent reactant supply can be attained while anode dehydration and cathode flooding are avoided. This is particularly critical for the cathode side because product water, existing as a liquid under many conditions, may accumulate and lead to flooding if the excessive water cannot be efficiently removed out of diffusion media. 1,2 A schematic of the GDL is shown in Fig. 1. It is typically a double-layer carbon-based porous substrate placed between the flow field and the catalyst layer CL. The first layer in contact with the flow field is a macroporous substrate of either carbon cloth or carbon paper. It is usually treated with polytetrafluoroethylene PTFE to increase hydrophobicity for favorable water management. It also serves as a current collector, a physical support of the CL, and an elastic component of a membrane electrode assembly MEA. The second layer adjacent to the CL is a thinner microporous layer MPL consisting of carbon or graphite particles mixed with a polymeric binder, usually PTFE. The MPL helps to improve the water management of a fuel cell. 1 A key function of the GDL is for water management. Much effort has been made to optimize its morphology, structure, and composition for this purpose, leading to enhanced fuel cell performance. 3-9 However, these approaches have been mostly guided by empiricism due to lack of effective measurement tools or characterization methods to evaluate the water retention/removal properties of GDLs. The net water transport coefficient through the membrane, , defined as the ratio of the net water flux from the anode to cathode to the protonic flux mol H2O/mol H + , is often used as a quantitative measure of water management in a PEMFC. When 0, the net water transported through the membrane from the anode to cathode combined with product water of oxygen reduction reaction ORR on the cathode is removed by cathode gas, thus termed cathode water removal. In contrast, when 0, product water will be removed via the anode fuel stream, thus termed anode water removal. 10 The minimum is equal to 0.5. In This case all product water of ORR is removed through the membrane into the anode. Numerous studies have examined the effects of operating conditions, e.g., stoichiometry and relative humidity of gas streams, current density, and pressure on the value.