Highly Stable Hierarchical Polybenzimidazole (PBI) Grafted Graphene/Nanographene Hybrids As Catalyst Supports for Polymer Electrolyte Membrane Fuel Cells

Abstract

Graphene synthesized from the exfoliation of graphite has a great potential in polymer electrolyte membrane fuel cell (PEMFC) applications.[1] The highly graphitic structure composed of the conjugated carbons endows graphene with extremely high electric conductivity and chemical/electrochemical stability. Improved stability is required for PEMFC catalyst support materials as the stability plays a critical role in determining the overall durability of PEMFC systems. While graphene is an attractive material, there are three major barriers when using graphene-supported Pt-based catalysts in PEMFCs: 1) the lack of bonding sites for catalyst landing on the basal plane of graphene causes the migration/aggregations of Pt nanoparticles when subjected to harsh accelerated stress tests (ASTs), 2) the highly hydrophobic surface of graphene is difficult to wet when mixed with the Nafion ionomer particles, leading to a poor catalyst/ionomer interface, 3) 2D graphene sheets tends to restack back to graphite structure through π-π interactions, which can severely block the O2 diffusion and retard the catalytic reactions, in particular when PEMFCs are operated at high current density (>1.5 A/cm2),and 4) the exfoliation of natural graphite using wet chemistry method usually yield graphene that contains some defects and oxygenated functional groups, which can de-stabilizes its conjugated electronic structure which negatively affects electronic conductivity and stability. Furthermore, when subjected to harsh potential cycling (e.g. 1.0-1.5 V; potentials which can be observed during start-up/shut-down or local fuel starvation), these defect sites are vulnerable to carbon corrosion. To overcome these challenges, we use a novel approach to make the 2D graphene sheets into 3D composite materials with many channels and pores to facilitate the facile mass transport by developing the highly stable hierarchical polybenzimidazole (PBI) -grafted graphene hybrids supported Pt catalysts for PEMFCs. PBI- functionalization is found to homogenize the chemical environment of graphene surface, resulting in a uniform dispersion of Pt nanoparticles that exposes more active sites for electro-catalytic reactions. In order to construct appropriate pore structures in the catalyst layers, spacers were introduced using graphitized carbon materials or metal oxides. The surface charge was imposed on the surfaces of these spacer-particles as opposed to PBI-graphene, so that the columbic attraction forces drive them to anchor onto graphene sheets. As a result, the secondary pore volume (characterized by mercury porosimetry) significantly increases, leading to the improved mass transport of reactants (H2 and O2) and water, and consequently an improvement in the PEMFCs performance at high current density (i.e. 2.0 A/cm2). Additionally, we prepared graphene nanoplatelet with smaller dimensions as the catalyst supports, which leads to extra voltage gains at the high current density. Covalent grafting polymers onto graphene sheets containing semi-flexible backbones on graphene defects can help to re-build the conjugated carbons and de-localize the electron, as well seal the dangling bond resulting from graphene synthesis. The PEMFC durability studies carried out by our research group has shown significant improvement on support stability when they were cycled between 1.0 V to 1.5 V for 5000 cycles compared to traditional carbon support materials,[RB1]during which the mass activity is only decreased by 47% for Pt/PBI-graphene and 25% for Pt/PBI-nanographene. The reported results are very close to DOE 2020 targets on catalyst support stability [2]. In addition, these materials show improved Pt stability. Strengthened interaction between functional groups and Pt particles, as characterized by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) greatly improves the catalyst stability. Our developed PBI-graphene supported Pt demonstrated a 27% decrease in mass activity and 15% loss of ECSA after 10,000 cycles between 0.6 V and 1.0 V. This exceeds the DOE 2020 targets for automotive PEM fuel cell electro-catalysts.[2] The unique advantages of the hierarchical PBI-functionalized graphene/nanographene hybrids demonstrates the potential for these materials to be the next generation of catalyst supports for PEMFCs. [1]. Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183 [2]. U.S. DRIVE FuelCell Tech Team Cell Component Accelerated Stress Test Protocols for PEM Fuel Cells (2013)