Nitrogen doping regulates microcrystal structure of interconnected Mn oxide particles/rods for oxygen reduction electrocatalysis

Abstract

Manganese (Mn) oxide possessing considerable catalytic activity toward oxygen reduction reaction (ORR) exhibits promising potential in driving energy conversion technologies. However, small specific surface area, poor electron conductivity, and more importantly uncontrollable microgeometry of crystal greatly restraint their activity. Herein, a crystal regulation strategy induced by nitrogen doping is developed to construct interconnected Mn oxide nano-particles/rods (N-MnOx-Y) with different contents of [MnO6] octahedron for ORR. In N-MnOx-Y, "Y" refers to the mass ratio of melamine to α-MnO2 nanorods during calcination. Nitrogen doping induced increased content of crystalline [MnO6] octahedron is the primary factor boosting the intrinsic activity of N-MnOx-Y. The overall activity of N-MnOx-Y is also determined by electrochemically active area (ECSA) which is closely related to nitrogen doping. The crystal optimal N-MnOx-30 with high content of crystalline [MnO6] octahedron and large ECSA exhibits the highest half-wave potential for ORR and the largest kinetic current density (jk) among the N-MnOx-Y catalysts. When used as a cathodic catalyst, a homemade zinc-air battery driven by N-MnOx-30 delivers a peak power density higher than the one driven by Pt/C, along with better rate performance and a larger discharging specific capacity. This impressive electrocatalytic performance of N-MnOx-30 superior to Pt/C originates from the synergistic effect between crystalline [MnO6] octahedron, active area exposure, oxygen vacancy, Mn-N sites, and the three-dimensional hierarchical structure, which benefiting electronic/mass transmission and structural stability. This work offers a feasible strategy for constructing hierarchical and crystal tunable metal oxides electrocatalysts for efficient energy conversion.