Elaboration par voie solvothermale de matériaux graphéniques : vers des électrodes de PEMFC sans platine

Keywords

PEMFC, Graphene, Doping, Oxygen electro-reduction

Subject details

Our society has nowadays to face a growing energy demand throughout the world, linked to scarcer fossil energy. Together with the global warming, this has contributed to development of new energy forms, e.g. wind solar or geothermal energies. Storage of energy in the form of dihydrogen has aroused a huge interest in the scientific community as well as in energy and transport industrial branches. Fuel cells provided with proton exchanging membranes (PEMFC) allow dihydrogen to be converted into electrical energy without harmful emissions at a temperature level not too far from the ambient one. The large-scale development of this type of generator is currently hindered by its fabrication cost and its still insufficient reliability. This is in particular the case for the cathode (positive electrode) where oxygen reduction occurs. Most FC electrodes still consist of platinum nanoparticles dispersed on the surface of carbon-based support. This solution is not optimal because of the sluggishness of oxygen reduction reaction (ORR). Moreover, in spite of its significant reduction for two decades, the amount of platinum required for sufficient power density is still important. Besides, platinum catalysts are prone to ageing by corrosion and aggregation which reduces its efficiency. Finally, platinum is expensive, its extraction and refinery generate approx. 20 kg CO2 per g. Widely studied in the literature, nitrogen-doped carbonaceous materials have a catalytic activity for the oxygen reduction reaction (ORR), which is the most limiting reaction in PEMFCs. An elaboration method based on a solvothermal reaction between sodium and an alcohol containing nitrogen atoms, followed by a pyrolysis treatment, has been developed in the Carbonaceous Materials team. It allows to obtain doped graphenic materials, presenting a strong three-dimensional character, a very developed porosity and a catalytic activity for the ORR in a PEMFC. Electrical power densities higher than 3 mW.cm-2 have been reached. Several avenues of improvement are being considered: • As suggested in former studies, these graphenic structures can be doped with other elements of the p block (S, P, B, Se). DFT investigations of the properties of the C-O bond revealed that double functionalization N/S, N/B or N/P on the C support allows improvement in the catalytic activity. Even though most modelling works are performed at zero potential, without taking in account interaction with the electrode interface, this type of doping appears really promising. • Incorporation of transition metals e.g. Co or Fe could allow improvement of their performances. Metallic macrocycles such as porphyrin or phtalocyanin complexes centered on Fe or Co atoms and deposited on their carbon support, are considered as promising catalysts after their pyrolysis over 500°C. Active sites for ORR would be M-NxC groups (with M=Fe or Co) formed in the pyrolysis stage. The chemical, textural and structural properties of the produced samples will be analyzed by different complementary techniques, in particular TGA, XRD, TEM, Raman and XPS spectroscopies, gas physisorption. The new materials prepared will be evaluated in a first step in half-stack allowing to study the reaction. This method gives access to the main electrochemical characteristics of the materials produced and allows a fast and efficient screening of functionalized graphene materials. The most promising materials will be produced in larger quantities, deposited on a membrane with an anodic layer to form an assembly: these assemblies will be tested in a complete stack.