Nickel-based electrocatalysts for ammonia borane oxidation: enabling materials for carbon-free-fuel direct liquid alkaline fuel cell technology

This paper introduces a new class of carbon-supported nickel-based electrocatalysts (Ni₃Ag/C, Ni₃Pd/C and Ni₃Co/C) for the direct electrooxidation of ammonia borane (AB) in alkaline medium. The enabled anodic process opens the opportunity to build a novel concept of carbon-free-fuel energy conversion devices. Promising performances are reported for every studied catalyst. A trade-off is observed between the activity for the direct electrooxidation of AB and its decomposition (and related hydrogen release), as well as hydrogen oxidation reaction (HOR). The direct AB oxidation reaction (ABOR) onset is lower for the least noble materials (Ni₃Co/C), due to its smaller catalytic activity for AB decomposition and hydrogen evolution, combined with a non-negligible activity for AB oxidation. On the contrary, on Ni₃Pd/C, the noblest material, the onset of the ABOR is higher, because this material decomposes AB into hydrogen and evolves hydrogen, and then valorizes it. Moreover, Ni₃Co/C exhibits a better durability than Ni₃Ag/C, Ni₃Pd/C, and than what is reported for Pt/C and Pd/C nanocatalysts in the literature in the same experimental conditions: identical-location transmission electron microscopy (ILTEM) investigations demonstrated no significant morphological degradations after accelerated stress tests (ASTs) in alkaline medium. This study therefore demonstrates that a carbon-supported nanostructured noble-free catalyst (Ni₃Co/C) is both active and durable for the AB direct electrooxidation in alkaline medium. This result opens the way to direct liquid alkaline fuel cells fed with boron based fuels, a technology that could be both economically and industrially viable if noble-free catalysts are used.