The development of energy storage and conversion technologies has intensified in recent years. Today, Polymer Electrolyte Membrane Fuel Cells (PEMFCs) has been recognized as one of the most important and environmentally friendly technologies. The catalytic layers of PEMFC, usually composed of Pt/carbon, gas voids and ionomer, are key elements to the cell performance. In particular, it has been recognized that the carbon support material play a critical role in terms of catalyst activity and stability. Nowadays, carbon blacks (especially Vulcan XC72) are the most commonly used for Pt catalysts. However, they suffer from thermochemical instability and from corrosion caused by electrochemical oxidation under fuel cell operating condition1). Much effort has been directed, therefore, to the synthesis of alternative carbon materials as catalyst supports for PEMFCs. One strategy to decrease carbon support corrosion is to use carbon with high extent of graphitization, which is due to decreased defect sites on the carbon structure, where carbon oxidation starts2). Graphene has attracted tremendous interest3). Indeed the combination of its high surface area, high conductivity and unique graphitized basal plane structure makes this 2D material a promising candidate for cathode catalyst support in PEMFCs. However those supports suffer from enough anchoring sites that leads to large catalyst particles during the synthesis and reduce consequently the active surface area.
The aim of this study was to prepare catalysts in which the Pt nanoparticles (NPs) (particle size 3~4 nm) were well dispersed on Few Graphene layers and compare their electro-catalytic performance and stability with commercial catalyst (Pt/C 50%). We will see that, in view of enhancing performance, 1D MWCNTs will be added to the 2D graphene material.
MATERIALS AND METHODS
Commercials Few Graphene Layers and MWCNTs were provided by Angstron Materials and Nanocyl societies respectively. Platinization of these carbon supports has been achieved by two different method: ethylene glycol and thermal treatment reduction of the precursor salt (H2PtCl6). The as-synthesized catalysts, denotes as Pt/G and Pt/MWCNTs, were characterized by Scanning Transmission Electron Microscopy (STEM) analyses, UV spectroscopy and electrochemical Rotating Disk Electrode (RDE) (0.5 M H2SO4 solution at a scan rate of 5 mV.s-1). To demonstrate the potential of this catalysts as fuel cell cathode under real operating conditions, Membrane Electrode Assembly (MEA) were prepared. Fuel cell tests were carried out by measuring polarization curves under automotive conditions (ie. 80°C H2/Air, stoichiometry 1.2/2).
RESULTS AND DISCUSSION
The CV curves for the Pt/G, Pt/MWCNTs and a mixture of Pt/(G-MWCNTs) with different ratios are shown in Fig. 1A.. Fig. 1B. shows the calculated current density (JO2) deduced from the polarization curves for ORR and the electrochemically active surface area (ECSA) of each catalyst compared with the commercial.
The results show that the best electrochemical performances are obtained on Pt/(G-MWCNTs) catalyst with ratio of (25:75). In this study, the addition of MWCNTs to the graphene matrix leads to a porous network structure which then facilitated the simultaneous access between Pt NPs and reactant reducing the mass transport limitation. Thus Pt/(G-MWCNTs) displayed good electroactivity when compared to commercial carbon blacks support. Polarization curves obtained with these catalysts are reported in Fig. 2..
These fuel-cell tests confirmed the previous ex-situ obtained results from the RDE analyses. A maximum value of 435 mW/cm² is achieved with the mix of graphene and MWCNTs, which is almost double the maximum power density obtain with pure graphene under the same conditions.
CONCLUSIONS
In summary, we have synthesized graphene and carbon nanotubes based catalyst for ORR reaction. Ex-situ electrochemical characterization have shown an enhance catalyst performance for Pt/(G-MWCNTs) compared with commercial Pt catalyst. Beyond these results, the future work would involve the optimization of the formulation of catalyst layer to further increase the catalytic performance under real working fuel cell environment.
REFERENCES
1. Wang, Y.J., Wilkinson, D.P. and Zhang, J. (2011). “Noncarbon Support Materials for Polymer Electrolyte Membrane Fuel Cell Electrocatalysts”, Chem. Rev., Vol. 111, p.7625-7651
2. Sharma, S. and Pollet, B.G. (2012). “Support materials for PEMFC and DMFC electrocatalysts-A review”, J. Pow. Sources, Vol. 208, p.96-119
3. Zhou, X., Qiao, J., Yang, L. and Zhang, J. (2014).”A Review of Graphene-Based Nanostructural Materials for Both Catalyst Supports and Metal-Free Catalysts in PEM Fuel Cell Oxygen Reduction Reactions”, Adv. Energy Mater., Vol. 4, p.1301523