Implementing MOF-derived Nickel Catalysts as Anion Exchange Membrane Fuel Cell Anode

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 passivation at potentials above ca 0.1 V vs. the reversible hydrogen electrode (RHE), 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 break-in protocol to show any performance in AEMFC.