Electrodeposition of Nanoscale Manganese Oxide for Electrochemical Energy Storage Devices

As society moves away from fossil fuels towards renewable energy sources, energy storage is becoming increasingly important. Manganese oxide is a good candidate material for multiple energy storage applications (e.g., rechargeable batteries and supercapacitors) because it is environmentally benign and has good catalytic and electrochemical properties. When used as a supercapacitor, manganese oxide stores charge through electric double layer and redox effects. To maximize these effects, the structure of manganese oxide deposits needs to be optimized; this means increasing the effective surface area of the deposit. In addition to the effective surface area, the crystal structure of the deposit also greatly affects both catalytic and electrochemical performance. Manganese oxide is polymorphic and it is important to understand the crystallography of the manganese oxide within a device, in order to optimize its properties.

In this work, nickel foam is coated with high surface area manganese oxide through a simple anodic electrodeposition procedure. Nickel foam is an excellent substrate for many energy storage devices because in addition to nickel’s good conductivity, the porosity of the foam increases surface area and also facilitates fluid flow through the substrate. The latter is important for applications where manganese oxide is being used as a catalyst.

The electrolyte used for deposition of manganese oxide contains manganese acetate, ammonium acetate and dimethyl sulfoxide. By varying deposition and post deposition annealing conditions, different microstructures are obtained. The microstructure, crystallography and chemical state of the manganese oxide deposits are characterized through scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). XPS analysis has been used to determine that manganese oxide is in the form of MnO₂. Preliminary XRD and TEM results indicate that the deposits obtained in this work are nanocrystalline with grains less than 20 nm in size.

Electrochemical tests such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) are used to determine performance (e.g., capacitance and cyclability) and to optimize the morphology and structure of the manganese oxide deposits. Preliminary CV measurements suggest that the manganese oxide deposits on nickel foam exhibit good areal and specific capacitance.