New High Performance Proton Conducting Membranes for Hotter, Drier Operation of Polymer Electrolyte Fuel Cells

摘要

Proton exchange membrane (PEM) fuel cells are still the most desirable component of future zero emission, high efficiency automobiles. However, their unit cost, ease of operation, and reliability must be reduced which includes eliminating the humidifier from the fuel cell system. Currently the commercial proton exchange membrane (PEM) is fabricated from a perfluorosulfonic acid (PFSA) polymer such as Nafion~R. Unfortunately PFSA ionomers must be fully hydrated to achieve their maximum proton conductivities and practical levels of proton conductivity can only be achieved in vehicles operating at an inlet RH of 80% which still necessitates the use of a humidifier and undesirable complex water management and recovery systems. To achieve the goal of a PEM that can operate at temperatures from freezing to 120oC using dry inlet gases it will be necessary to develop new PEMs that are based on new chemistries. We have created this new class of PEM by polymerizing heteropoly acid (HPA) monomers with appropriate co-monomers to produce films with unique proton conducting properties. While HPA have been previously polymerized they have not until now been fabricated into proton conducting films. We use this methodology to fabricate hybrid HPA in which olefinic functionalities are introduced to fabricate hybrid HPA monomers. By changing the chemistry of the other components of the system and the loading of the HPA we are building a mechanistic picture of proton conduction in these systems. DC conductivity and EIS are used to measure the proton conductivity and NMR is used to measure diffusion coefficients and the films water content. NMR is also used to probe the molecular motions in the film. We have recently concluded a series of experiments of in- situ SAXS measurements at the advanced photon source under varying conditions of temperature and humidity which are giving us additional insights in to the morphology of these unique polymers. This chemistry is now expanded to three new polymer backbones which promote enhanced phase separation and new morphologies. We will discuss these new systems in terms of structure activity relationships. The analogous system using Zr to promote proton conduction in a poly phosphonic acid polymer will also be compared and discussed.