Improved electrochemical performance and charge storage mechanism of NiMnCoO₄ by XPS study

Ni and Mn-based oxides are gaining more attention in supercapacitor applications due to their high abundance and high specific capacitance. However, the charge storage mechanism is not yet clearly understood. In this study, we have synthesized NiMn₂O₄ and NiMnCoO₄ by hydrothermal method followed by annealing at a temperature of 400 °C for 4 h and tried to understand their charge storage mechanism by XPS study. The capacitance property of synthesized materials NiMn₂O₄ (NMO) and NiMnCoO₄ (NMCO) is evaluated for supercapacitor application. The NiMn₂O₄ exhibits a specific capacitance of 260 F g⁻¹ when cycled at 0.6 A g⁻¹. At the same specific current, NiMnCoO₄ exhibits 298.5 F g⁻¹. When both the samples were cycled at 2 A g⁻¹, the capacitance retentions of NMO and NMCO are found to be 80% and 100%, respectively after 5000 cycles. The XPS study clearly indicates the involvement of Mn²⁺/Mn³⁺ and Mn³⁺/Mn⁴⁺ redox reaction in the charge storage of NiMn₂O₄ while that of Co²⁺/Co³⁺ redox couple in the charge storage process of NiMnCoO₄. This result clearly indicates the higher specific capacitance and better cycling stability of NiMnCoO₄ over NiMn₂O₄. ^{Although, there are a lot} of reports on the capacitance property of NMO, there is rare report on NMCO material. So, we have studied their electrochemical properties and charge storage mechanism by using XPS for supercapacitor application.