Pt/Nanoporous Carbon Scaffold Catalyst Layer for Use in Symmetrical Proton Exchange Membrane Fuel Cells

Abstract

Carbon microstructures are an essential component of both the cathode and anode catalyst layer in PEMFCs, typically serving as the catalyst support [1], and it is known that the carbon porosity, surface area, and its hydrophobicity/hydrophilicity have a significant effect on electrocatalytic activity and transport issues. This is due to the impact of the carbon on catalyst dispersion, Nafion distribution, electronic conduction, and mass transport limitations [2-4]. Typically, carbon black is used as the catalyst support because of its high surface area, low cost, good electrical conductivity [4,5], but it is susceptible to corrosion and does not pack uniformly, thus resulting in uncontrolled pathways for reactants/products through the catalyst layer [6,7]. Recently, there has been an increasing interest in using mesoporous carbons as catalyst supports for fuel cell application, due to the better accessibility of the internal carbon surfaces, and the tethered Pt nanoparticles, to the reactants, and better mass transport of reactants and products during fuel cell operation [8,9].Our team has recently developed a novel nanoporous carbon scaffold (NCS), which is self-supported, scalable, and highly tunable (its monodisperse pore size can be controllably varied from 10 to 100 nm), giving specific surface areas of 200 to 600 m2 g-1. These films are fully percolating, have very low tortuosity and a 90% porosity, good electronic conductivity of 2-10 S cm-1 and are robust [10]. While prior work has involved the preparation and testing of an MEA that contained Pt/NCS on the cathode side of the separator, in this work, the focus was on the first-time testing of a symmetrical cell with Pt/NCS on both sides of a Nafion membrane. Here, the Pt nanoparticles (NPs) were deposited throughout the NCS material by wet impregnation of the chloroplatinic acid precursor (Fig. 1) and then infiltrated with Nafion by drop-casting. The thickness of the Pt/NCS films used in the current study was 25 ± 2 μm, the NCS pore size was 85 nm, and the Pt loading at the cathode was < 1 mg Pt cm-2. The Nafion®117 membrane was sandwiched between two NCS catalyst layers by hot pressing and the performance of these new symmetrical MEAs was evaluated in a fuel cell test station using different temperatures. This presentation will focus on the performance of the symmetrical MEA design in comparison to conventional ink-deposited systems, as well as the durability of these novel materials. Because the NCS has such a uniform structure, discussion will also be focused on the changes observed to the carbon itself as well as to the Pt NP size and distribution after testing under PEMFC conditions.