Performance and Durability of Carbon-Free Ionomer-Free PEFC Cathode Using Pt Nanosheet Catalyst

Abstract

Polymer electrolyte fuel cells (PEFCs) have been attracting attention as highly efficient power sources and have been commercialized for use in residential power plants and automobiles. However, further widespread adoption requires the development of low-platinum catalysts. Additionally, for installation in heavy duty vehicles such as large trucks, high durability at high temperatures is necessary. Pt nanosheets (Pt(ns)), consisting of several atomic layers of Pt with a flat structure, possess high mass activity and durability due to the high specific surface area and low surface energy [1]. Conventional PEFC electrodes consisting of carbon-supported platinum (Pt/C) catalyst and ionomer suffer from degradation due to dissolution/redeposition, Ostwald ripening, and agglomeration of Pt, corrosion of the carbon support, and Pt poisoning by ionomer. Therefore, using unsupported Pt(ns) for an electrode that consists of neither carbon nor ionomer is expected to eliminate the degradation factors, resulting in a highly active and durable electrode. The purpose of this study is to develop highly active and durable electrodes by using Pt(ns) and eliminating degradation factors by making them carbon-free and ionomer-free.Polycrystalline Pt(ns) was synthesized by interlayer oxidation of graphene, as described elsewhere [2]. Catalyst ink was prepared, and cathode catalyst layers were created by dropping or spraying it onto carbon paper. A membrane-electrode assembly (MEA) was made by hot-pressing the cathode catalyst layer together with an anode made of Pt/C onto an electrolyte membrane. Power generation was conducted at 80˚C by supplying H2 to the anode and O2 or air to the cathode, each humidified under ambient pressure. Durability was evaluated by repeatedly applying cyclic potentials between 1.0 V and 1.5 V, supplying N2 to the anode and cathode, and examining changes in the cyclic voltammogram.The I-V curves of MEAs with ionomer-free carbon-free Pt(ns) cathodes fabricated using drop-coating and spray-coating methods and measured using air and oxygen as the cathode supply gases are shown in Fig. 1. As demonstrated in the figure, the activation overpotential at low current density was high, suggesting a low Pt utilization. The shape of the I-V curve of MEA using the spray method at high current density indicated that the concentration overpotential was suppressed under oxygen conditions but was high under air conditions. On the other hand, the concentration overpotential of the MEA using the drop method was high even under oxygen conditions. This may be attributed to the small spaces between the restacked nanosheets, which inhibited diffusion.A start-stop simulated potential cycle test was conducted for the MEA using ionomer-free carbon-free Pt(ns) cathode. The CV curves measured after a certain number of cycles are shown in Fig. 2. As shown in the figure, the change in the shape of the CV was small, and the ECSA remaining rate of the electrode was above 80% even after 8000 cycles, whereas that of the Pt/C cathode was less than 50% after 4000 cycles. This suggests that carbon-free ionomer-free Pt(ns) electrode has the potential as highly durable low Pt electrodes.AcknowledgementPart of this paper is based on results obtained from a project, JPNP20003, subsidized by the New Energy and Industrial Technology Development Organization (NEDO).