Electrochemical capacitance of porous NiO–CeO₂ binary oxide synthesized via sol–gel technique for supercapacitor

Abstract

Nanocrystalline NiO–CeO₂ binary oxide as a novel electrode material for ultracapacitor was synthesized via glycol assisted citrate sol–gel method. Unique cubic phases with aggregated crystalline microstructure of NiO–CeO₂ mixed oxides were examined by X-ray diffraction and transmission electron microscope. The observed electrochemical measurements further reveal the strong pseudocapacitance features of the mixed oxides at different current density. Binary oxide annealed at 500 °C shows an optimum specific capacitance (C _s ) of 305 Fg⁻¹ at the constant current density of 1 Ag⁻¹. The achieved C _s value undoubtedly certifies that in combination with NiO the structural stability and redox property of CeO₂ have been enhanced. Especially, increasing the calcination temperature binary oxide has shown well reversible redox features which confirm the high chemical and thermal stability of CeO₂ and it could be involve in the charge storage process effectively by their strong Ce³⁺/Ce⁴⁺ redox couples. Capacity retention and cyclic stability of the electrode was quite good, only ∼5 % capacity fading was observed after 1,000 cycles. Moreover, binary oxide calcined at 700 °C exhibits a specific capacitance of 167 Fg⁻¹ at the constant current density of 1 Ag⁻¹ which states that presence of CeO₂ with NiO have controlled the grain growth and maintains their porous microstructure even at 700 °C. This facilitates to the redox process at both NiO and CeO₂ active surfaces at elevated temperature significantly.