(220d) Interfacial Transport Resistances and Coupled Transport

Authors: 
Benziger, J., Princeton University
Kevrekidis, I. G., Princeton University
Thermodynamic fluxes in membranes are proportional to thermodynamic forces (gradients of chemical potentials, temperature, etc.) via a matrix of phenomenological coefficients. Onsager’s relations imply that the matrix is symmetric, which is a crucial result as the number of unknown coefficients is reduced. We show that for certain classes of non-equilibrium thermodynamic models, in addition to Onsager’s relations the transport coefficients have to share the same functional dependence on the local thermodynamic state of the system. We prove this statement for thermodynamic models of coupled water and proton transport in ionomer membranes. Thence classical irreversible thermodynamic models and experimental data should be checked for consistency. Additionally, these newly identified constraints further reduce the number of experiments needed to describe the phenomenological coefficients.

These functional constraints have been confirmed by detailed studies of water and proton fluxes across Nafion based membrane electrode assemblies. For small electric potential differences (ΔΦ) the water and proton fluxes are correlated; both increase linearly with applied potential. The electro-osmotic drag coefficient is constant, functionally independent of the applied electrical potential, water activity and temperature. For larger applied electric potential differences the water and proton fluxes are no linearly dependent on applied potential; the electro-osmotic drag coefficient increases with increasing applied electrical potential and increasing water activity. The transition between intramembrane transport limited flux and interfacial transport limited flux corresponds to the limits of applicability of the standard Onsager reciprocal relationships for coupled transport.