Alkaline hydrogen oxidation reaction on Ni-based electrocatalysts: From mechanistic study to material development

Alkaline exchange membrane fuel cells (AEMFCs), as an important clean energy conversion technology, exhibit the possibility of employing complete platinum-group-metal-free catalysts. In the anode, Nickel is considered almost the only active PGM-free element for hydrogen oxidation reaction (HOR) but is trapped by deficient activity and stability. Developing novel Ni-based catalysts with high activity and robust stability is crucial through various elaborate approaches with the guidance of theoretical understandings. However, the catalytic mechanisms in the alkaline electrolyte are still in debate. Against the backdrop, we aim to provide a comprehensive understanding of alkaline HOR on Ni-based catalysts, including the proposed mechanistic viewpoints, the development of the Ni-based materials with promising strategies to optimize both the activity and stability via the optimized coordination environment and the current advances of the AEMFC devices with Ni-anodes. Prevailing possible mechanisms with controversial activity descriptors like the H/OH binding energies are reviewed firstly, from the studies based on PGM model electrodes to those on Ni-based ones. Subsequently, the recent progress in Ni-based catalysts with enhanced activity is discussed based on various strategies by changing the chemical and coordination environment of the active sites. The influence factors and the overcoming methods for the poor stability, and the performance of the AEMFC devices constructed by Ni-based anode catalysts are briefly summarized. We finally highlight the challenges of the mechanisms insights and catalysts design and outline possible research directions for Ni-based materials toward alkaline HOR. Hopefully, this review will provide a roadmap for scientific insights and materials design for the alkaline HOR on Ni-based catalysts that propel the further development of the AEMFC techniques.