Fibrous mesoporous Ni1 - xO nanosponge with excellent pseudocapacitive properties was synthesized by controlled calcination of [Ni(H2O)(4)(4-nba)(2)]center dot 2H(2)O; a 4-nitrobenzoate (4-nba) coordinated compound. Control experiments with compounds such as [Ni(H2O)(6)](4-nba)(2)center dot 2H(2)O; a non-coordinated 4-nba compound and the [Ni(H2O)(6)]Cl-2, without any 4-nba resulted in the formation of only agglomerated NiO nanoparticles. This proved the significance of 4-nba ligation with Ni(2+)in [Ni(H2O)(4)(4-nba)(2)]center dot 2H(2)O to produce highly porous, sponge-like nanostructures, without the need of hydrothermal or solvothermal treatment. The detailed structural characterization confirmed the formation of highly crystalline, mesoporous Ni1 - xO nanosponge. Further, the results obtained from electrochemical investigation by cyclic voltammetry and galvanostatic charge-discharge studies demonstrated that the fibrous, nanosponge-like Ni1 - xO exhibited good electrochemical properties with a high specific capacitance of 1236.84 Fg(-1)at a current density of 1 Ag(-1)and excellent capacitance retention of similar to 95% even after 10000 charge-discharge cycles at the current density of 6 Ag-1. The improved pseudocapacitive performance and extremely high cycling stability were attributed to extensive mesoporous network, effective distribution of electroactive sites and an improved electrode-electrolyte interaction due to fibrous and spongy nature of the material. The synthesis process is simple and scalable for industrial applications. [GRAPHICS] .