Role of Cu Substitution on the Pseudocapacitive Performance of Nano-platelet Shaped Spinels, NixCuzCoyO4 {x=1-z, y=2-z, z=0.2}

Abstract

AB2O4 type 3d transition metal nano-platelets NixCuzCoyO4 {x=1-z, y=2-z, z=0.2}, exhibiting pseudocapacitance behavior in aqueous electrolytes has been examined as a positive electrode in an asymmetric hybrid cell against a double layer coke carbon. Synthesis of these single phase spinels has been accomplished via a modified solvothermal route which yields individual nano-platelets (∼50nm size) as confirmed by STEM, high resolution TEM (HRTEM) and SAED images. It is evident from XPS analysis that the copper substitution occupies both tetrahedral and octahedral coordination in the host spinel. Indexed XRD patterns reveal Cu substitution affects the cell volume as compared with pristine spinel. It has been found that substitution of up to 20 mol% Ni or Co by Cu leads to a significant increase of specific capacitance (Fg−1) compared to pristine counterpart (NiCo2O4). More pronounced effects on the capacitance have been realized specifically with Cu substitution at Ni site (octahedral, called A-site herein after) namely Ni0.8Cu0.2Co2O4 rather than Co site (tetrahedral, called B-site) of NiCo1.8Cu0.2O4 spinels. The former composition delivers an average capacitance of up to 458Fg−1 whereas only a 244Fg−1 was deduced from the latter. A hybrid asymmetric aqueous cell comprised of Ni0.8Cu0.2Co2O4(+)//activated carbon (AC) (-) electrode couple renders ∼72Fg−1 of cell capacitance with a good cycle stability (over 1000 cycles) within a potential window of 1.5V. It has been deduced that the hybrid cell delivers a power density of 12kWkg−1 and energy density of 7Whkg−1 at room temperature.