Carbonaceous Nanowire Supports for Polymer Electrolyte Membrane Fuel Cells

Polymer Electrolyte Fuel Cells, (PEMFCs) typically utilize Pt or Pt alloy electrocatalysts supported on high surface area carbons. The carbon support plays a fundamental role in the performance and durability of the fuel cell. Optimization of multi-component transport, i.e. gases, water, protons and electrons is essential for increasing power density and minimizing the Pt loading. Very high surface area carbon supports provide the best dispersion of the electrocatalyst, however their oxidative stability is diminished with respect to more graphitized, and lower surface area carbons. Conventional carbon supports possess typically spherical or flake-platelet morphologies, which are not necessarily the optimal geometries for supporting the ion conducting network of thin film polymer incorporated into the electrocatalyst layer. We have developed a group of unconventional carbonaceous support materials based on the pyrolysis of high surface area polypyrrole nanowires. The formation of the polymer from the monomeric precursor must be templated with appropriate molecules in order to achieve the desired morphology. We investigated the ability of various carbohydrates and dyes to form ionically-linked two-part soft templates for the formation of polypyrrole nanowire networks. In contrast to previous studies[1-3], the monomer was oxidized to pyrrole by a chemical rather than an electrochemical process. High yields of nanostructured products were obtained using relatively small amounts of low-cost carbohydrate and dye template materials, substituting for expensive heparin, previously used as the templating agent.  Generally, an increase in ionic groups on the carbohydrates favored the formation of the desired fibers over spheroid globules.  While the degree of ionic functionality of the carbohydrate was a factor, the ionic strength of the dye was not as important as the substitutional elements on the dye backbone.  Varying the concentration and the two-part ratio of the templates influenced the length and diameter of the nanofiber segments within the nanowire network.  Pyrolysis of the nanowires yielded fibers containing significant residual nitrogen, 8-11%, as well as folded graphitic domains. The resulting highly graphitic carbonaceous material (20-30nm diameter ~ 200nm long, 90 m²/g surface area), possessed a high density of nucleation sites enabled the formation of smaller 2.4nm Pt particles compared to commercial catalysts despite significantly higher support surface loadings of about 2mg/m².  Fuel cell testing at loading of less than 0.1mg/cm² yielded superior performance at 80°C and 60% relative humidity as compared similar loadings of conventional catalysts. Accelerated stress testing of the catalyst showed similar durability to highly graphitized supported catalysts.

 

1. W.S. Shi, D. Ge, J. Wang, Z. Jiang, L. Ren, Q. Zhang, Macromol. Rapid Commun., 27, 926–930 (2006).
2. W.S. Shi, P. Liang, D. Ge, J. Wang, Q. Zhang, Chem. Commun., 2414–2416 (2007).J-M. Sansiñena, M. S. Wilson, F. H. Garzón , ECS Transactions, 50 (2) 1693-1699 (2012)
3. J-M. Sansiñena, M. S. Wilson, F. H. Garzón , ECS Transactions, 50 (2) 1693-1699 (2012)