Synthesis and electrochemical performance of manganese nitride as an oxygen reduction and oxygen evolution catalyst for zinc-air secondary batteries

Development of stable, high-performance and cost-effective bifunctional electrocatalysts that can replace baseline Pt- and Ir-based catalysts has been a central theme in metal-air batteries. Along this direction, transition metal-based oxides and nitrides have attracted attention due to their abundance, stability, and low cost. Here, Mn nitride, fabricated via annealing of Mn powder in N-2, is investigated for the first time as a candidate bifunctional electrode for rechargeable Zn-air batteries (ZABs). Three samples were prepared by nitridation of a Mn precursor, with particle size < 100 mu m, at 1100 A degrees C for 4-30 h. The morphology and microstructure of the fabricated samples were investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), secondary ion mass spectrometry (SIMS), and X-ray diffraction (XRD). Phase quantification for all samples was performed using Rietveld-based fitting. The samples treated for 4 and 10 h had the largest fraction (similar to 5%) of nitride phases (Mn4N and Mn6N2.58); the remaining material was primarily MnO. The nitride phases were not pure, but contained oxygen, resulting in the formation of pseudobinary phases. The oxygen reduction/evolution reaction (ORR/OER) performance of all samples was evaluated using rotating disk electrode (RDE) voltammetry in an alkaline electrolyte (0.1 M KOH). Among all the catalysts, the sample treated for 10 h exhibited the most positive ORR onset potential (-0.038 V vs. Hg/HgO) with good stability. The catalyst was incorporated into a practical ZAB and displayed a battery efficiency of 52.7% after 14 h of charge-discharge cycling (1 h/cycle) at a current density of 7.5 mA cm(-2).