The improvement of mechanical properties of polymer-based nanocomposites is usually obtained through a strong polymer–silica interaction. Most often, precipitated silica nanoparticles are used as filler. In this work, we study the synergetic effect occurring between dual silica-based fillers in a styrene-butadiene rubber (SBR)/polybutadiene (PBD) rubber matrix. Precipitated Highly Dispersed Silica (HDS) nanoparticles (10 nm) have been associated with spherical Stöber silica nanoparticles (250 nm) and anisotropic nano-Sepiolite. By imaging filler at nano scale through Scanning Transmission Electron Microscopy, we have shown that anisotropic fillers align only in presence of a critical amount of HDS. The dynamic mechanical analysis of rubber compounds confirms that this alignment leads to a stiffer nanocomposite when compared to Sepiolite alone. On the contrary, spherical 250 nm nanoparticles inhibit percolation network and reduce the nanocomposite stiffness.
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.
For application in mobile power supply systems, polymer electrolyte membrane fuel cells (PEMFC) are promising candidates due to their high efficiency and low emission. In principle, the fuel cell technology is long ready for its commercial market launch, hindered by its high cost, mainly caused by the price of the electrode catalyst which consisted mostly of noble metal platinum unsupported or supported on carbon support. An other major constraint is the limited long term stability of the catalyst support itself subjected to the severe conditions of the fuel cell environment. Indeed, standard carbon supports tend to corrode in environments of high water content, acidic pH, elevated temperatures (50-90°C), high potential (0.6-1.2 V), and high oxygen content. Moreover, the presence of platinum also accelerates the carbon corrosion. Hence, nowadays it is more and more important to develop new solutions to improve the PEMFC tolerance to corrosion. In the present work, our team worked on improving the PEMFC performance parameters, which include enhanced power density, increased catalyst utilization and reduce cost with the incorporation of alternative materials like Multi-Walled Carbon Nanotubes (MWCNTs) in combination with polydopamine (PDA) into electrodes architecture. The role of MWCNTs was to confer a high electronic conductivity and help to form a porous network. On the other side the role of PDA was to promote the proton conductivity similarly to ionomers such as Nafion. The fuel cell polarization test shows a maximum power density of 780 mW.cm−2 and a Pt utilization of 6051 mW.mg(Pt)−1. The Pt utilization reached in this work is almost three times higher than for Pt/MWCNTs electrodes containing the same Pt loading. In this study, fuel cell membrane were prepared applying sprayed layer-by-layer assembly deposition method from polydopamine (PDA) and multi-walled carbon nanotubes (MWCNTs). Pt nanoparticles were attached to the MWCNTs in a reaction of selective heterogeneous nucleation. MWCNTs were coated with mussel-inspired PDA. After functionalization, Nafion perfluorinated resin solution was add to the suspension to obtain a stable dispersed solution for LBL assembly. The MEAs were sandwiching by two pieces gas diffusion layer and fed with H2/O2. The originality of this work was to prove that PDA acts as a fantastic protective layer against carbon corrosion. To this aim, we performed cyclic voltammetry measurments (CV) on both MWCNTs and PDA-MWCNTs systems in order to simulate harsh conditions occuring in a PEMFC electrodes in acidic environment. Transmission Electron Microscopy (TEM) obersations were also realized showing a partial covering of MWCNTs by PDA, estimated at 20% using a X-ray Photoelectron Spectroscopy (XPS) theoritical model. Nethertheless, a such covering leads to significative resistance to corrosion in the range of potential and number of cycles studied. This study showed a simple preparation technique for high performance and long lasting advanced electrode structures which succeeded in incorporating PDA into a porous MWCNTs network. As far as we know this is the first time that Pt/MWCNTs-PDA is used in a real PEMFC environment. PDA-MWCNTs supports exhibit a better oxidation resistance than MWCNTs. This work opens the way to the manufacturing of efficient, stable and easily prepared PEMFC electrodes.
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