Enhanced electrochemical performance: Synergetic effect of time-dependent synthesis and redox additive concentration on ammonium nickel phosphate hydrate

Abstract

Transition metal phosphates (TMP) have emerged as compelling candidates for supercapacitors because of their exceptional structural, chemical, and electrochemical properties. For the enhancement of energy density (Ed) of TMP, the use of redox-additive (RA) electrolytes is a cutting-edge trend because of the tuning between the electrode and electrolyte, which is much simpler than the fabrication of complex electrodes. Ammonium nickel phosphate hydrate powders (ANPh's) have been prepared via the hydrothermal method with varying reaction times (3, 6, and 9 h) at the constant reaction temperature of 180°C. The formation of ANPh's has been confirmed by structural and compositional analysis. The morphological analysis revealed the microplate-like architecture of ANPh's. The specific capacitance (Cs) of the optimized ANPh_6 electrode (EANPh_6) in KOH electrolyte observed 496 F g−1 with an Ed of 11.03 Wh kg−1 and a power density (Pd) of 404 W kg−1 at 10 mA cm−2 with 78.79 % retention after the 5000th galvanostatic charge-discharge (GCD) cycle at 100 mA cm−2. However, to overcome the problem of poor Ed and capacitance retention in aqueous electrolytes, the electrochemical properties of EANPh_6 are studied in RA electrolytes with varying concentrations (0.010 M to 0.020M) results in enhancement in electrochemical performance by improving redox reactions at the EANPh_6 and KOH electrolyte interface. The EANPh_6 with 0.015 M potassium ferricyanide K4Fe(CN)6 boosted 3 times greater Cs 1516 F g−1 and Ed 33.70 Wh kg−1 with Pd 487 W kg−1 at 10 mA cm−2 with enhanced cyclic stability of 87.54 % over the 5000 GCD cycles at 100 mA cm−2. The symmetric aqueous state device, EANPh_6// EANPh_6, achieved Cs, Ed, and Pd of 48.51 F g−1, 1.07 Wh kg−1, and 292.68 W kg−1 at 30 mA cm−2 current density with remarkable capacitance retention of 98 % over 5000 GCD cycles at 80 mA cm−2. Finally, it is anticipated that identifying the optimized electrode with an appropriate concentration of RA will be one of the eminent techniques to boost the electrochemical properties.