Preparation of High Performance Polymer Electrolyte Membrane Fuel Cells (PEMFC): Graphene As a Carbon Support

Abstract

Numerous publications have discussed the importance and the need for a high performance and a long-term stability for Polymer Electrolyte Membrane Fuel Cells (PEMFC), which has a triple phase boundary, consisting of platinum nanoparticules, a carbon support and an ionomer such as Nafion® (manufactured by DuPont). The constitutive material is also called Membrane Electrode Assemblies (MEA), and the performance of MEA depends mostly on a balance between electronic and protonic conductivity. The major barrier for large scale developments of PEMFC is the high cost of Pt which is the best known catalyst for the Oxygen Reduction Reaction (ORR). The Layer-by-layer (LbL) technique has been widely used for the design of nanostructured films. Recently, Michel et al. have already proven the possibility to design fast prepared electrodes for fuel cell made by the alternated spraying of conducting polymers exhibiting promising results in terms of fuel cell performance. Graphene, as a carbon support, discovered in 2004, is an interesting material for electrochemical systems due to its exceptional physical properties and its remarkable surface area. In this work, Polydopamine (PDA) layers with high surface coverage were formed on graphene by spontaneous oxidative polymerization of dopamine. This simple incorporation accelerates the charge transfer due to efficient proton-coupled electron transfer of PDA which could be a candidate to increase the performance of PEMFC. In the present work, fuel cell membranes were prepared according to the spray assisted LbL deposition method. The spray of the [Nafion / (Pt/graphene-PDA)] nanocomposite membrane were sprayed on both sides of the membrane using isopropanol as the solvent. The cells comprising such membranes were analysed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermo gravimetric analysis (TGA), and cyclic voltammetry (CV). All materials were used in this study as received from Sigma-Aldrich. Functionalization of Graphene with Pt : 100 mg of graphene (oxidized 10 min under sonication in concentrated nitric acid) in 20 ml ethylene glycol (EG) were stirred under sonication for 10 min, 100 mg Chloroplatinic acid hexahydrate (H2PtCl6.6H2O) in 30 ml EG were put into the solution with agitation, and then the solution was heated to 140°C for 1.5 h under reflux. The solution was cooled down to room temperature under stirring for 24 h; Pt/Graphene were collected by filtration and then washed with deionized water. Synthesis of polydopamine modified Graphene : 100 mg of Pt/Graphene and 0.1 mg ml-1 of dopamine hydrochloride were added to 10 mM Tris–HCl (pH 8.5) (100 mL), respectively, and the solution was stirred for 24 h at room temperature. The Pt/Graphene-PDA was rinsed with deionized water. MEA preparation:Membrane Electrode Assembly (MEA) was prepared by the alternated spraying of conducting materials onto Nafion® 117 membranes previously cleaned and protonated. Pt/Graphene-PDA were dispersed in 100ml isopropanol under sonication for 15 min, and Nafion® perfluorinated solution was added to the suspension to obtain a stable dispersed solution. The suspension was sprayed onto the two sides of membrane. Two different catalyst supports were prepared. Firstly, graphene functionalized with Pt nanoparticles (Pt/Graphene) and secondly graphene functionalized with Pt nanoparticles wrapped with PDA (Pt/Graphene-PDA). For each catalyst supports, quantity of sprayed solution was varied on the MEA, to study the influence of the thickness and the concentration. The MEAs were sandwiching by two pieces gas diffusion layer, then fixing between two bipolar plates with flow field. The electrodes were fed with 150 mL.min-1 H2 and 75 mL.min-1 of high-purity O2 with water in 100% humidifier at atmospheric pressure, the electrodes temperature were set to 80°C. Polarization curves were collected in galvanostatic mode by using a FuelCon Evaluator C50 test bench (FuelCon, AG, Germany). Maximum power densities of 350 mW.cm-² and 270 mW.cm-2 were obtained for (Pt/Graphène-PDA) and (Pt/Graphène) respectively. We observed that the maximum of these polarization curves increased with the number of successive tests, until a limit value.