Surface Engineering of 2D Layered MnO2 with Co-Doping of Ni and Fe for Rechargeable Zinc-Air Battery

Surface engineering by doping has always been one of the well-applied effective strategies to improve the performance of transition metal oxides towards their application in energy conversion and storage devices. However, fewer studies have been conducted to understand the effect of co-doping of two or more transition metals in layered transition metal oxides. It is interesting to note how multiple metal ion doping could create a synergistic environment that modulates the electrocatalytic ability of the host lattice. In this work, we have sequentially introduced Fe, followed by Ni, into the delta-MnO2 lattice (NiFeMn) to occupy either interplanar spaces or to substitute the host lattice. Spectroscopic and high resolution microscopic characterizations suggested that the dopants have entered the in-planar voids, as well as in the inter-layer region. The electrochemical characterizations revealed the positive synergistic interaction of Fe and Ni in the MnO2 lattice improved overall bifunctional oxygen electrode activity parameters. In the case of a practical Zn-air battery, the electrode NiFeMn shows 2.4 times improved output power density, cyclic stability, and durability than FeMn, along with a roundtrip efficiency of 51 %, and specific capacity of 589.96 mAh g-1 corresponding to energy density of 470.67 mWh g-1. Our studies reveal that both Fe and Ni have played a major role as dopants and their synergistic interaction has significantly boosted the overall electrocatalytic properties of delta-MnO2.