Water Transport in MEAs

The molar flux of water generated in the ORR at the cathode at a typical current density of 1 A cm is 0.05 mol m s . At the same current density, the water flux due to electro-osmosis is 0.05-0.1 mol m s . These processes, whose rates are linked to the current density, induce nonuniform distributions of water within the MEA, which can have a negative impact on performance. Extreme situations of PEM dehydration on the anode side or excessive accumulation of liquid water in electrode layers on the cathode side could cause fuel cell failure. 

A highly water-permeable PEM would facilitate water removal via liquid transport toward the anode, alleviating the problem of cathode flooding and anode dehydration. Erom a system perspective, it is deemed beneficial to make use of internal humidification of CLs and PEM by water that is produced at the cathode. This mode of internal water management obviates the need for external humidifiers. It demands, however, precise control of water permeation rates through the PEM and of vaporization rates in partially saturated porous electrodes. Therefore, it is cmcial to know how relevant parameters of water transport (diffusion, hydraulic permeation, electro-osmotic drag, vaporization, and condensation) depend on PEM morphology and thermodynamic conditions. 

During the past decade, a number of experimental approaches have been developed to study water fluxes in PEM, under ex situ and in situ conditions. The objective is to measure water fluxes for materials with varying structure and composition, as a function of applied gradients, in relative humidity and liquid pressure (Adachi et al., 2009 Duan et al., 2012 Romero and Merida, 2009 Satterfield and Benziger, 2008). 

Ideally, water flux data should be complemented by independent data on PEM structure and water sorption. An analysis of the data demands water flux models that provide a means to deconvolute contributions of different flux mechanisms and extract the relevant parameters. The problem of isolation and parameter extraction for 

---