Highly Stable Polybenzimidazole (PBI) Grafted graphene As Catalyst for Polymer Electrolyte Membrane Fuel Cells

Abstract

Graphene is a single atomic-tick graphite layer. The carbon atoms in the graphene plane are saturated due to the bonding structure: each has 3 covalent bonds with three neighboring carbon atoms in the graphene plane, the lone px e- form a long range conjugated system, a big π bond. This leads to some unique properties: excellent electric conductivity and chemical stability, which are critically needed for catalyst supports in polymer electrolyte membrane fuel cells (PEMFCs).[1] The catalyst support stability plays a significant role in determining the overall durability of PEMFC systems when PEMFCS are subjected to very high potential cycling , for example, from 1 V to 1.5 V. However, there are three major obstacles for graphene supported Pt based catalysts in PEMFCs: 1) there are limited bonding sites for catalyst landing on the graphene basal plane, which lead to migration/aggregations of Pt nanoparticles when subjected to harsh accelerated durability tests (ADTs); 2) it is very difficult for Nafion ionomer particles and graphene based catalyst mixing together to form a uniform catalyst ink, which results in the poor catalyst/ionomer interface. 3) For MEA preparation using ink spraying method, when ink is dried, the π-π interaction makes 2D graphene sheets easily restacking back to graphite structure, which could severely block the mass transport of gas and water, leading to poor MEA performance at high current. To overcome these barriers, we propose a novel approach to transfer the 2D graphene sheets into 3D composite with sufficient channels and pores for facile mass transport. This is realized by developing the highly stable hierarchical polybenzimidazole (PBI) -grafted nano-graphene supported Pt catalysts for PEMFCs and applying spacers during ink formulation. Nano- graphene, comparing with normal graphene, has smaller dimensions as the catalyst supports, which makes the pores/channels between graphene sheets much shorter, facilitating the mass transport. It is expected to see some extra voltage gains at the high current density. In order to construct appropriate pore structures in the catalyst layers, spacers are introduced. However, although the initial performance somehow increases with such modification, yet the support durability remains same and to some extents, decrease little bit. In this case, the hypothesis raised that during durability test, the spacers themselves are corroded and graphene sheets are restacked. To overcome this barrier, we apply more stable spacer that match the durability of graphene. Nano-silicon powder is very stable during voltage scan between 1.0V to 1.5V but is not conductible. To remedy this drawback, we coat highly graphitized carbon over its surface, which gives very promising durability and conductivity for overall MEA electrode. [1]. Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183