Selective synthesis of monodisperse bimetallic nickel–cobalt phosphates with different nanoarchitectures for battery-like supercapacitors

Abstract

This work reports the fabrication of monodisperse nickel–cobalt phosphate particles with varying structures via the solvothermal reaction of nickel–cobalt glycerate spheres with triethyl phosphate (TEP) in different solvents followed by subsequent calcination in air at 600 °C. The choice of solvent affects the morphology of the resulting nickel–cobalt phosphate precursor obtained from the solvothermal reaction. It is found that alcohol-based solvents (pure ethanol and ethanol/butanol mixture) favour the formation of monodisperse plate-like particles, while the use of a water/ethanol mixture promotes the generation of rod-like particles. The optimized amorphous nickel–cobalt phosphate sample prepared using pure ethanol followed by calcination in air at 600 °C (E-NiCo-TEP-600) exhibits a battery-like behaviour with a high specific capacity of 620 C g⁻¹ (specific capacitance of 1550 F g⁻¹) in 6.0 M KOH at a current density of 2 A g⁻¹. Furthermore, the asymmetric supercapacitor (ASC) fabricated using E-NiCo-TEP-600 as the cathode and commercial activated carbon (AC) as the anode (E-NiCo-TEP-600//AC ASC) displays a maximum energy density of 45 W h kg⁻¹ at a power density of 750 W kg⁻¹. The stability test reveals the good long-term stability of this ASC with a high capacitance retention of ∼100% after 5000 cycles at a high current density of 10 A g⁻¹. The porous nature along with the high structural disorder and abundant mesopores in the amorphous nickel-cobalt phosphate plate-like particles (E-NiCo-TEP-600) can promote faster diffusion of electrolyte ions and better electrolyte ion penetration, leading to a higher electrochemical performance. These results indicate the promising potential of porous amorphous bimetallic phosphates for supercapacitors.