Résumé
Due to their acidic environment, the present generation of proton-exchange membrane fuel cells (PEMFC) requires precious metal catalysts, in particular platinum, which is an obstacle to their large-scale deployment. In contrast, anion-exchange membrane fuel cells (AEMFCs) operate at high pH, facilitating the use of sustainable precious metal free catalysts. While high performance has been reached with platinum-group-metal (PGM) free cathodes in AEMFCs, the replacement of PGM-based anodes by PGM-free ones is a challenge. Nickel-based catalysts are the most promising PGM-free hydrogen oxidation reaction (HOR) catalysts in alkaline medium, but nickel suffers from early surface oxidation at potentials above 0.1 V vs. the reversible hydrogen electrode, which in turn blocks the HOR. In order to mitigate the surface oxidation of nickel at HOR potentials, two main approaches are studied in the literature: optimization of nickel intrinsic properties by alloying with other earth abundant elements, or core@shell nanostructuration of nickel by a protective carbon shell. We will report on recent results obtained with this second approach, and the preparation of core@shell Ni@NC catalysts, comprising a nitrogen-doped carbon shell, via the annealing of Ni-based MOFs. The latter were obtained either by autoclave or by a solvent-free mechanochemical synthesis. Our observations confirm the significant effect of pyrolysis atmosphere (NH3, H2, N2 and combinations) on the HOR activity. Other synthesis parameters will be shown to play a key role on the MOF structure and morphology as well as on the HOR activity of final materials. The materials were characterized by X-ray diffraction, electron microscopy, nitrogen adsorption, X-ray photoelectron spectroscopy (XPS), and electrochemical techniques, including AEMFC tests. The presentation will correlate the material’s structure to their activity and stability. In particular, it will be shown that Ni-based anodes require a specific break-in protocol to show any performance in AEMFC.