Hydrogen oxidation reaction in alkaline media: From mechanism to recent electrocatalysts

The sluggish cathodic oxygen reduction reaction (ORR) of proton exchange membrane fuel cells (PEMFCs) heavily relies on the employment of a large quantity of unaffordable Pt-based electrocatalysts to accelerate the slow kinetics. As switching from acidic proton exchange membrane to alkaline hydroxide one, it is highly promising to completely replace platinum group metal (PGM)-based ORR electrocatalysts with PGM-free counterparts. However, anodic hydrogen oxidation reaction (HOR), with a fast kinetics in PEMFCs even at a low Pt loading of 50 mu g(Pt) cm(-2) or less, becomes two orders of magnitude slower in alkaline media and thus requires a high loading of PGM-based electrocatalysts to accelerate the reaction rate. Alkaline HOR has drawn a great number of recent attentions, yet a comprehensive review is missing. Herein, this review covers diverse possible alkaline HOR mechanisms, hardly comparable electrocatalysts evaluation methods, two seemingly contradictive activity descriptors, as well as design and synthesis of PGM-based and PGM-free electrocatalysts with controlled structural parameters. Wherever is appropriate, this review describes our own point of view on certain subjects. Finally, future research directions are suggested. This review will provide knowledge and insights on fundamental and practical issues for the development of advanced alkaline HOR electrocatalysts and mechanism.