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Porous Crystalline Organosilica Materials as High Surface Area Support in Proton Exchange Membranes for Fuel Cells

Brett A. Holmberg, Deryn Chu, and Sol Gilman. Sensors and Electron Devices Directorate, U.S. Army Research Laboratory, 2800 Powder Mill Road, AMSRD-ARL-SE-DC, Adelphi, MD 20783-1197

Proton exchange membrane (PEM) cost and durability have been identified as key hurdles to implementation of proton exchange membrane fuel cell (PEMFC) technology on a large scale. Peroxide radicals generated during PEMFC operation significantly decrease PEM lifetime. Thermal and humidity cycling during fuel cell operation and the resultant swelling and shrinking of existing PEM materials also lead to premature failure. PEMFC failure is realized by gas crossover through PEM pinholes or catalyst layer delamination from the PEM, and both effects worsen with increasing operation temperature and decreasing humidity. Moderate temperature (110 to 140C) fuel cell operation under low relative humidity conditions (~25%) has been proposed to take advantage of reduced carbon monoxide catalyst poisoning effects and improved cell power output. These conditions would allow the fuel cell to operate on reformed hydrogen without bulky balance of plant hydrogen purifiers or humidifiers. Clearly, a robust, non-swelling PEM material is needed for operation at both conventional and moderate temperatures. This work explores the potential of nanoscale-engineered crystalline organosilica materials as cost effective, highly durable PEM materials.

Crystalline organosilica materials can be non-swelling and extremely stable in acidic hydrothermal environments, in addition to being inexpensive and simple to produce. Crystalline organosilica materials with large internal surface areas and tunable pore diameters can be synthesized with tethered acid functional groups, making them strong solid acids and good candidates as PEM materials. Thus far, proton conductivity as high as ~0.01 S/cm has been reported for organosilica based materials (compared to Nafion, 0.1 S/cm, water saturated, 20C). No optimization attempts for organosilica PEM fuel cell applications have been reported despite the potential of the material.

Crystalline organosilica mesoporous materials (OMMs) are directly synthesized with mercaptopropyl side chains and framework ethylene, benzene, and biphenyl groups. The template is extracted and the mercapto groups are oxidized to sulfonic acid groups to generate significant proton conductivity. Preliminary OMMs with benzene framework groups and propylsulfonic acid side chains have exhibited proton conductivity as high as 9.010-3 S/cm. The optimization of proton conductivity of the organosilica materials is ongoing. The development of OMMs as membranes for fuel cell performance testing is being performed using both organosilica crosslinking and evaporation induced self assembly techniques. With optimization, the developed OMM PEM are expected to exhibit proton conductivity comparable to existing PEM materials while delivering lower costs and improved durability. These improvements will possibly lead to longer lasting fuel cells that are capable of running on liquid fuel reformate instead of stored hydrogen.