In situ growth of nickel ammonium phosphate ribbons on nickel foam for supercapacitor applications

We report the in-situ growth of the nickel ammonium phosphate (NAP) ribbons on nickel (Ni) foam in a single step process using hydrothermal approach. The morphology, structure and elemental analysis of the NAP-ribbons grown on Ni foam were analyzed by scanning electron microscope (SEM), energy-dispersive x-ray analysis (EDX) and x-ray diffraction. The NAP-coated Ni foam sample was sonicated in ethanol and the obtained suspension was observed through AFM analysis to analyze the lateral dimensions of the NAP-ribbons. The obtained NAP ribbons powder was also characterized by x-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Further, the NAP ribbons based electrodes were electrochemically characterized by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge discharge analysis (GCD). NAP ribbons grown on Ni foam as electrodes achieved a specific capacitance (C_sp) of 1196 Fg⁻¹ at 1 mVs⁻¹ and 1188 Fg⁻¹ at 1 Ag⁻¹. The charge storage mechanism was identified and quantified by evaluating the CV data in 1–10 mVs⁻¹ scan rate range. It was established that at low scan rate the charge was dominantly stored by diffusion of electrolyte ions within the gaps of the NAP ribbons resulting in high energy density of 105.5 WhKg⁻¹ at 1 Ag⁻¹. NAP electrodes also demonstrated good cyclic stability by showing 92 % charge retention after 5000 cycles. A symmetric 2-electrode device comprised of NAP ribbons electrodes gave C_sp of 309 Fg⁻¹ at 1 mVs⁻¹ with energy density of 42.06 Wh/Kg and power density of 0.5 KW/Kg at 1 A/g current density, which is an exceptional for a symmetric supercapacitor device. The study confirms that NAP ribbons have a promising potential to be employed in the next-generation energy storage devices.