Development of Carbon-Based Integrated Electrode (GDL + Catalyst Layer) Using the Electrospinning Deposition Method

Abstract

Introduction The Polymer Electrolyte Fuel Cell (PEFC) is a promising technology for automobile application. With the current focus on developing hydrogen powered trucks and buses, operation with high-current density is strongly required. One of the approaches to produce high-current density is reducing the thickness of the gas diffusion layer (GDL). However, doing so also sacrifices the structural integrity and durability of the GDL. This study takes an approach of creating an integrated electrode (IE) as shown in Figure 1, by fabricating the GDL and catalyst layer (CL) as one body through the electrospinning deposition (ESD) method, to allow for a thinner yet durable structure. The CL is made of mesoporous carbon (MC), since MC is of high interest after its use in the second-generation TOYOTA MIRAI [1]. More specifically, this study utilized MC fiber (MCF) sheets previously developed in our research group [2]. Experimental In this research, both GDL and CL were made using the ESD method, allowing for the fabrication of fiber-based sheets of desired thicknesses. The precursor solution the GDL was made of polyacrylonitrile (PAN) dissolved in dimethylformamide. For the MCF precursor solution, Pluronic-F127, phloroglucinol, formaldehyde and triethyl orthoacetate were dissolved in the solvents of distilled water, ethanol and hydrochloric acid, and then mixed with a viscous solution of polyvinyl alcohol (PVA) dissolved in water. The GDL was fabricated first, followed by ESD of the MCF layer. The resulting integrated layer was first heat treated in air to induce polymerization of the MCF precursor, and then put through pyrolysis under nitrogen flow for carbonization. Subsequently, the GDL side was coated with PTFE and heat-treated to provide hydrophobicity. While the MCF layer was decorated with Pt nanoparticles using Pt-acetylacetonate precursor, followed by a heat treatment, and finally impregnated with Nafion ionomer to complete the IE. For MEA fabrication, firstly a contact layer (0.03 mg Pt/cm2) was spray-printed onto a Nafion 212 membrane using 46.5% Pt/KB (TEC10E50E), in order to reduce the contact resistance between the IE and Nafion 212 membrane. A 1 cm2 IE was pressed on the contact layer forming the cathode. The anode was made of 46.5% Pt/KB (TEC10E50E), spray-printed to get 0.3 mg Pt/cm2. Results and discussion The IE was successfully fabricated without separation of the two layers, GDL and MCF. The thickness of the GDL was effectively reduced to 50-100 μm as expected. From SEM observation, fiber diameter was found to be 0.1-0.5 μm fibers. Fibers were found to be non-porous based on the nitrogen sorption measurement. On the other hand, the MCF layer was expected to have mesopores, and a mesopore distribution around 4-6 nm was confirmed through nitrogen sorption measurements. The resulting fiber diameter was between 2-4 μm. Details regarding electrochemical evaluation of the MEAs, after further introduction of PTFE, Pt nanoparticles and Nafion ionomer to the IE, will be further discussed.