Thermal phenomena associated with water transport across a fuel cell membrane: Soret and Dufour effects

We present calculations of the coupling effects that take place when heat and water are transported across a membrane relevant to fuel cells, using the theory of non-equilibrium thermodynamics. Numerical results are given for the Nafion membrane bounded by surfaces of molecular thickness in contact with water vapor of varying relative humidity. Analytical expressions for thermal effects of water transport are given. We show how reversible heat transport (Dufour effects) can be understood in terms of coupling coefficients (heats of transfers). The sign of the enthalpy of adsorption of water in the membrane determines the sign of the coupling coefficient, the Dufour and Soret effect as well as thermal osmosis effects meaning that the effect can be large at interfaces. We show how data presented in the literature can be understood in terms of the presented theory. Using common estimates for transport properties in the membrane and its surface, we find that the more detailed equations predict a 1030% variation in the heat and mass fluxes as the membrane thickness drops below . Analysis of experiments on thermal osmosis suggests that more accurate measurements on the water content as a function of activity are required.