Electrochemical and magnetic properties of nanostructured CoMn2O4 and Co2MnO4

Abstract

In this study, we have focused on the synthesis of cobalt manganite nanostructures using a simplistic hydrothermal route. We have explored these spinels as alternative low-cost bifunctional electrocatalysts for oxygen reduction/evolution reactions (ORR/OER). Herein, we have developed energy-saving, facile and rapid synthetic methodologies for highly active spinel electrocatalysts. Two spinel phases, cubic Co₂MnO₄ and tetragonal CoMn₂O₄ have been successfully obtained by tuning the stoichiometric ratio of Co and Mn salts respectively. These CoMn₂O₄ and Co₂MnO₄ nanocubes have been used as bifunctional catalysts towards OER and ORR. Electrocatalytic experiments show that cubic Co₂MnO₄ nanocubes show five times higher activity towards ORR than tetragonal CoMn₂O₄ nanocubes while the tetragonal phase is a better electrocatalyst towards OER than the cubic Co₂MnO₄ phase. XPS studies revealed two types of oxygen (lattice O and surface adsorbed O species like OH⁻) and the efficiency of the catalyst could be related to the binding affinity of oxygen. This explains the better catalytic activity of cubic Co₂MnO₄ which has a large percentage of adsorbed oxygen species. The stability of the catalyst was confirmed by carrying out TEM studies on a sample after carrying out 25 cycles. Magnetization experiments reveal that both the tetragonal CoMn₂O₄ as well as cubic Co₂MnO₄ show hysteresis at 10 K and 100 K without reaching saturation, which confirms an existing ferrimagnetic order in the samples. Both the tetragonal and cubic phases show T_c ∼ 110 K and 150 K respectively.