Recent advances in metal-based electrocatalysts: from fundamentals and structural regulations to applications in anion-exchange membrane fuel cells

Alkaline exchange membrane fuel cells (AEMFCs) have broad application prospects due to the use of low-cost, non-precious catalysts. Furthermore, a wide range of fuels, for example, carbon-neutral hydrogen (H2) and ammonia (NH3), can be directly used in H2-fueled AEMFCs and NH3-fueled AEM direct ammonia fuel cells (AEM-DAFCs). However, the development of the above-mentioned AEMFCs is hindered by the sluggish dynamics of the alkaline hydrogen oxidation reaction (HOR), ammonium oxidation reaction (AOR), and oxygen reduction reaction (ORR) and low efficiency catalysts for these electrode reactions. Thus, it is expected that the rational design and controlled synthesis of highly efficient, durable catalysts will enable AEMFCs to achieve comparable performance to or even a higher performance than that of proton exchange membrane fuel cells (PEMFCs), which usually require high-cost platinum group metals (PGMs). In particular, the proposed catalytic mechanism of these reactions in alkaline media is still under debate, especially of the HOR and AOR. Herein, we present an in-depth, comprehensive understanding of the alkaline HOR, AOR, and ORR based on metal catalysts, especially employing PGM-free catalysts, including the proposed mechanisms and the current development of catalysts and AEMFCs. Finally, we highlight the prevailing challenge of the mechanisms and catalysts for each reaction and outline the possible development directions for AEMFCs. We anticipate that this review will offer global scientific insights and a roadmap for the design of catalysts for alkaline electrode reactions to accelerate the further development of AEMFC technology. We present a comprehensive understanding of the alkaline hydrogen oxidation reaction (HOR), ammonium oxidation reaction (AOR), and oxygen reduction reaction (ORR) based on metal catalysts for H2 or NH3-fueled alkaline exchange membrane fuel cells.