In this report, we have demonstrated the morphological effect on the electrochemical performances of hydrothermally synthesized NiMn₂O₄. Morphologies of the nanoparticles are varied using different surfactants and mineralizer during synthesis. Three different surfactants namely cetyl trimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS) and glucose and one mineralizer ammonium fluoride (NH₄F) are used all of which result in a spherically shaped particles with varying sizes. We have seen that addition of SDS results in an agglomeration of spherical nano-particles of NiMn₂O₄ with good interconnection between them. Brunauer–Emmett–Teller (BET) surface area analysis confirms this sample offers highest surface area with mesoporous structure amongst all these samples. A specific capacitance of 1937 F g⁻¹ is obtained for the same at 2 mV s⁻¹ scan rate in 1 M aqueous Na₂SO₄ electrolyte. A two-electrode asymmetric device has been fabricated using the sample as positive electrode and activated carbon as negative electrode. The device offers a specific capacitance of 270 F g⁻¹ with excellent cyclic stability and good power and energy density. The superior performance of the device has been validated by demonstrating the capability of the device to harvest energy from a solar cell and to deliver the same through a LED.