Development of Nanoporous Electodes Architecture for Polymer Electrolyte Membrane Fuel Cells using Structure-Directing Agents
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Polymer electrolyte membrane fuel cells (PEMFCs), which directly convert the chemical energy of fuels to electrical energy, have attracted much attention for various applications in transportation as well as stationary and portable power generation due to their high efficiencies and low emissions. Though various technical breakthroughs have been achieved in the past, many challenges such as cost reduction and performance improvement, and durability enhancement must be overcome before commercialization of PEMFCs is possible. Therefore, one of the main objectives in PEMFC technology is the development of high-performing electrocatalyst support materials by a low-cost route. Various forms of ordered mesoporous carbons (OMCs) are good candidates for support materials due to their high electrical conductivity, large surface area, interconnected porous structure, chemical stability, and their wide availability. In this study, we introduce a 'one-pot' synthetic strategy using the unique properties of amphiphilic block copolymers as structure directing agents to synthesize mesoporous carbon-based nanocomposites with functionalities such as nanoparticle incorporation and controlled composition and pore size. We prepared ordered 2-D hexagonal, large-pore mesoporous carbon/silica composites with highly dispersed intermetallic PtPb nanocatalysts for use as anode catalysts in direct formic acid fuel cells. Uniform and large pores, dispersed with small intermetallic PtPb nanocrystals, enable pore backfilling with ionomers and formation of the desired triple-phase boundary in single cells. The materials show more than 10 times higher mass activities and significantly lower onset potentials for formic acid oxidation as compared with commercial Pt/C, as well as high stability due to better resistance toward CO poisoning. In single cells, the maximum power density was higher than that of commercial Pt/C, and the stability was highly improved compared with commercial Pd/C. Although the resulting materials showed high activity and stability for formic acid oxidation, the size of the intermetallic nanoparticles was much larger (ca. 12 nm) than that of commercial Pt/C (2 nm) and Pd/C (7 nm). Because the electrocatalytic activity is increases linearly with a decrease in particle size, we also investigated a modified “one-pot” method that enables small-sized nanoparticles to be encapsulated within OMC-based nanocomposites without nanoparticle aggregation or coalescence. During the synthetic process, metal-support interactions, dependent on the differing chemical properties of support materials, can affect the size and distribution of metal nanoparticles in the final materials. The resulting aggregation-free, small-sized intermetallic PtPb and Pt3Co nanocomposites were successfully employed as anodes for formic acid (FA) oxidation and as cathodes for the oxidation reduction reaction (ORR), respectively, and showed superior catalytic performance over commercial Pt/C.