Combustion-Synthesized KNiPO4: A Non-toxic, Robust, Intercalating Battery-Type Pseudocapacitive Electrode for Hybrid Supercapacitors as a Large-Scale Energy Storage Solution

Abstract

Polyanionic-type electrode material made of (PO4)n− polyhedral covalently bonded with Ni–O linkage is envisaged as a novel electrode material for intercalative battery-type hybrid supercapacitors. Highly porous, flake-type KNiPO4 showed robust electrochemical performances as a result of its open framework structure and active participation of the Ni2+/3+ redox couple that results in superior pseudocapacitive intercalating charge storage in the aqueous KOH electrolyte. The KNiPO4 electrode shows the specific charge storage equivalent to 168.5 mAh/g (capacitance of 935 F/g) at 1 A/g current rate in the potential window of 0.65 V in the aqueous 2 M KOH electrolyte. KNiPO4 electrodes exhibit excellent long-term cycle stability at 10 A/g for 5000 cycles with 87% of the initial capacity retention of the electrode and coulombic efficiency (η = td/tc) equivalent to 95.1% after 5000 cycles. Further, in full cell hybrid supercapacitor (HSC) mode in which porous KNiPO4 acted as the positive electrode and activated carbon (AC) functioned as the negative electrode, in the voltage window of 1.6 V, the highest energy density equivalent to 200 Wh/kg and power density equivalent to ∼819 W/kg were obtained at 1 A/g current rate. At a higher current rate (10 A/g), the hybrid supercapacitor attains a very high power density equivalent to 7981 W/kg with a retention of energy density close to 75 Wh/kg with superior cyclic stability. Coulombic efficiency of the full cell [asymmetric supercapacitor (ASC) mode] has lost only 3.4% with excellent capacity retention (92.3%) of its initial value after 2200 cycles. The robust performance and long cycle life of the electrode in full cells confirm the applicability of the material to power implantable biomedical devices. Further, high power performance coupled with superior cyclic stability coupled with strong electrochemical energy storage properties of the KNiPO4 electrode makes it suitable for bulk, grid-level charge storage applications.