(277a) New Membrane Morphologies for PEM Fuel Cells

Polymeric membranes play a crucial role during the generation of electricity in hydrogen/air and direct methanol proton-exchange membrane (PEM) fuel cells. The membrane in such devices performs three functions: (1) it physically separates the positive and negative electrodes (so there is no electrical short circuit), (2) it prevents mixing of the fuel and oxidant, and (3) it provides pathways for proton transport between the electrodes. For any fuel cell, the membrane must have good mechanical properties in the wet and dry states and be chemically stable under fuel cell operating conditions. DuPont's Nafion® (a perfluorosulfonic acid polymer) has many attractive properties and has been widely studied in PEM fuel cells, but it does not meet all performance criteria. In a hydrogen/air fuel cell, for example, Nafion loses water and the conductivity drops at temperatures greater than 80^oC, unless the water activity in the feed gases is near unity. Nafion has also been used in direct methanol fuel cells, but high methanol crossover (permeation) leads to low power output due to cathode depolarization. Historically, the general strategy for developing new membranes for electrochemical fuel cell applications has focused on: (1) synthesizing new polymers with a high ion-exchange capacity, (2) using block copolymers which self assemble into nano-phase domains, (3) combining the desirable properties of two different polymers in a single membrane by blending the polymers prior to membrane casting, and (4) impregnating a functional polymer into a microporous inert support, where the support provides mechanical properties that the functional polymer does not possess. There have been some successes with these approaches, but they all have limitations. Two new strategies will be described in this talk: (1) uniaxial polymer film stretching to alter the polymer crystallinity and/or nanomorphology (pre-stretched recast perfluorosulfonic acid polymer films) and (2) composite membranes fabricated by forced assembly (proton conducting ionomeric nano-fiber composite films). Membrane fabrication methods will be described, physical property data relevant to PEM fuel cell applications (e.g., membrane conductivity, solvent swelling, and mechanical properties) will be presented, and the results of fuel cell performance and durability tests will be discussed.