Nanosheets/nanoparticles-composed hierarchical manganese oxides enabled by molybdenum nitride rods for boosted oxygen reduction reaction electrocatalysis

Abstract

Catalyst microgeometry plays a more important role than activity in determining output performance of metal-air batteries. However, this is often overlooked. Herein, hierarchical manganese oxides anchored by one-dimensional molybdenum nitride rods (Mn/Mo2N) were constructed for efficient oxygen reduction reaction (ORR) electrocatalysis. The composition-optimized 10%Mn/Mo2N possessed a higher catalytic capability than commercial Pt/C catalyst in a homemade zinc-air battery (ZAB), even though the former exhibited a lower have-wave potential than the latter in rotating disk electrode (RDE) measurements. Moreover, the ZAB driven by 10%Mn/Mo2N possessed a larger discharging specific capacity than the one driven by Pt/C. This improved discharging performance was attributed to the high dimension of hierarchical features of Mn oxides anchored on Mo2N, which benefited the stabilization of active sites and accelerated mass transmission originating from the one-dimensional Mo2N rods. This inference was further confirmed by the high peak power densities of ZABs driven by high-dimensional 2%Mn/Mo2C-Mo2N, 5%Co/Mo2C-Mo2N, which exhibited relatively low catalytic activities in RDE measurements. For ORR electrocatalysts, the relationship between ORR activity and the corresponding ZAB power was built on the first attempt. This work highlighted the significance of hierarchical microgeometry, rather than the intrinsic activity, on boosting the electrocatalytic kinetics of a catalyst for efficiently practical electrochemical energy conversion.