ELECTROLYTE INCORPORATION INTO COMPOSITE ELECTRODES FOR PROTON-EXCHANGE MEMBRANE FUEL CELLS AND LITHIUM-ION BATTERIES

This dissertation describes research on the preparation and characterization of composite electrodes for use in proton-exchange membrane (PEM) fuel cells and lithium ion batteries. The general focus of the research was on high-surface-area carbon supports for platinum catalysts in fuel cells, and integration of electrolytes, particularly fluoropolymer electrolytes, into composite electrodes both batteries and fuel cells. Results are described for work in the following three specific topical areas. 1. Carbon nanofibers for use as platinum (Pt) catalyst supports in fuel cells were prepared by carbonization of electrospun acrylic fibers. The resulting carbon nanofibers were found to contain mainly micropores. Following grinding to a powder form, the carbon nanofibers were used as supports for Pt nanoparticles. The pulverized carbon nanofibers were found to be not suitable as supports for Pt catalysts because the microporosity of the individual carbon nanofibers cannot provide continuous porous channels in the electrode. As a result, the Pt utilization was found to be low. 2. Mesoporous carbon composites containing nanoscale embedded zirconia particles (ZCS) were prepared and found to be highly porous and electrically conductive. Surface modification of the composites with organic compounds having phenylphosphonic acid groups (e.g., phenylphosphonic acid, m-sulfophenylphosphonic acid, or sulfonated fluoropolymer ionomer having terminal phosphonic acid groups) was accomplished by simple exposure of the carbon composite to organophosphonate solutions. Nanoscale ZrO2 surfaces present in the composite skeleton acted as reactive sites for anchoring of phosphonates through formation of robust Zr–O–P bonding. Proton-exchange sites were introduced onto the nanocomposite surface by grafting m-sulfophenylphosphonic acid or a sulfonated fluoropolymer ionomer. Modification with the ionomer provided an increase in proton-exchange capacity relative to that found following modification with m-sulfophenylphosphonic acid because of the higher SO3H content of the ionomer. Even higher proton-exchange capacity was achieved using ionomer solution to which inert salt (Na2SO4) had been added to cause ionomer chains to become less extended in solution, thereby allowing more ionomer to fit into mesopores in the carbon composite support. Platinum nanoparticles were deposited onto the carbon composite supports with the platinum crystalline structure and size retained. Ionomer attachment to the Pt/ZCS composites was accomplished and the resulting materials were found to be effective catalysts for the oxygen reduction reaction. 3. Carbon-coated LiFePO4 and acetylene carbon black were blended with a short-side-chain perfluorosulfonate ionomer in lithium form to prepare composite cathodes. The cathodes were tested in a full-cell configuration against Li4Ti5O12 anodes using LiPF6-EC/DEC electrolytes. Comparison was made with cathodes prepared using poly-vinylidene difluoride (PVDF) as a nonionic fluoropolymer binder. At a discharge rate of C/5, both cathode types exhibited similar voltage profiles and charge-discharge capacities. However, under higher rate discharge conditions (e.g., \u3e 1C, up to 5C) cathodes prepared using ionomer binder showed better discharge rate capability than cathodes having the nonionic binder. This phenomenon was more pronounced when the salt concentration in the electrolyte was low. These findings suggest that the use of ionic binders can help compensate for electrolyte depletion from the electrode porous space, as lithium ions are intercalated into lithium-deficient LiFePO4 particles during rapid discharging