(722d) Unifying the Hydraulic, Electro-Osmotic, and Diffusive Perspectives on Water Transport in Ionomer Membranes

Charles W. Monroe

Department of Chemical Engineering, University of Michigan

Modern studies of water transport in ionomer membranes for PEM fuel cells usually take into account the traditional diffusive mode of transport, in which water-activity gradients drive water flux, and the pressure-diffusive transport mode, in which gradients in the local pressure induce chemical-potential gradients that drive flux [1]. Some models also treat the ionomer gel as a porous medium, through which water flows via hydraulic permeation [2,3]. These two perspectives are seemingly contradictory, in that one set of phenomena is diffusive, and governed by material (and charge) balances, while the other is controlled by the momentum balance. In fact, a general transport model can be constructed in which the diffusional and hydraulic transport modes are taken to occur in parallel, and are coupled.

A general transport model will be presented that is applicable to swollen ionomers under isothermal, non-isobaric conditions in general. The case of a Nafion membrane (for which most transport properties are readily available) will be treated in particular detail. The basis of the multicomponent, momentum-coupled mass- and charge-transport model in irreversible thermodynamics will be addressed.

In addition to water diffusivity, ionic conductivity, and the electro-osmotic coefficient, the model also incorporates the membrane compressibility, alongside other thermodynamic factors. The theoretical construct will be used to illustrate a number of effects on transport that arise when water-swelled ionomers are constrained spatially, or subjected to external compressive forces. Theoretical analysis will be used to predict several effects observed in experiments, such as stresses exerted by constrained membranes subjected to water flux, and constant-current diffusion potentials that appear to change with the applied mechanical load. The model will be useful to describe practical PEM fuel-cell membranes constrained within reactor assemblies.

References

[1] D.M. Bernardi and M. W. Verbrugge, J. Electrochem. Soc. 142(1992) 2477.

[2] C. Fabiani, G. Scibona, and B. Scuppa, J. Membrane Sci. 16(1983) 51.

[3] Q. Duan, H. Wang, and J. Benziger, J. Membrane Sci. 392-393 (2012) 88.