Experimental and theoretical analyses of ion exchange membranes for fuel cell applications
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Description
Recently, the need to develop high-efficiency power generators has been a major concern of several industrial concerns. A promising candidate is the fuel cell, which is highly efficient, generates relatively benign waste products, and maintains quiet operation Two of the principal factors that limit long-term fuel cell performance are water management and corrosion of cell components by the electrolyte. In solid-polymer-electrolyte (SPE) fuel-cell systems, the electrolyte is confined to the pores of an ion-exchange membrane and corrosion effects are minimized. The membrane maintains a low ionic resistance when fully saturated, and the energy output can be quite high Besides corrosion and water management, one must consider the higher cost of the electrodes and membrane materials. The commercially available membrane of choice for high-energy-density cells is the perfluorosulfonic acid (PSA) Nafion (Registered trademark of E. I. du Pont de Nemours and Company) 117 membrane. Since the PSA membrane is an expensive component of SPE fuel cells, it is important to identify promising, less-costly membranes and determine their transport characteristics Three ion-exchange membranes provide by RAI Research Corporation are analysed in this dissertation to determine membrane conductivity, coion and water partition and diffusion coefficients, and membrane porosity, using standard radiotracer and electrochemical techniques. The equilibrium absorption and transport parameters are combined with a macrohomogeneous transport model to evaluate counterion diffusivity, membrane electrokinetic permeability, and membrane pore diameter. A critical evaluation is made as to the applicability of the RAI membranes to hydrogen/oxygen fuel cells Experimental techniques to determine equilibrium absorption and transport rates in ion-exchange membranes have been developed in parallel with the formation of new mathematical models. The mathematical model presented in this dissertation represents a major extension of existing electrokinetic membrane transport models and includes such effects as ion hydration and variable solvent dielectric constant. In addition, the relevant equations are solved in a two-dimensional coordinate system in order to accommodate a nonuniform pore diameter. Computed fluxes for alkali metal sulfate salts are compared with experimental data for a Nafion membrane to evaluate the accuracy of the model