Synthesis of Carbon coated Nano-Na<inf>4</inf>Ni<inf>3</inf>(PO<inf>4</inf>)<inf>2</inf>P<inf>2</inf>O<inf>7</inf> as a Novel Cathode Material for Hybrid Supercapacitors

A new cathode material Na4Ni3(PO4)2P2O7 has been successfully synthesized by facile solution combustion synthesis (SCS) technique using various fuels such as glycine, urea and hexamine. The effects of fuels on crystal structure, morphology and electrochemical performance have been investigated. TGA results show three steps of combustion synthesis process and carbon content present in the Na4Ni3(PO4)2P2O7 prepared by using hexamine. The orthorhombic non-centrosymmetric space group Pn21a of Na4Ni3(PO4)2P2O7 is identified from powder XRD. SEM and TEM image of carbon coated Na4Ni3(PO4)2P2O7 shows non-uniform spherical nanoparticles with porous structure. The electrochemical performances of the materials are investigated in 2 M NaOH aqueous electrolyte. The pseudocapacitive nature of Na4Ni3(PO4)2P2O7 is recognized by diffusion controlled reversible redox reaction of Ni(II)/Ni(III) in alkaline aqueous electrolyte. Carbon coated Na4Ni3(PO4)2P2O7 exhibits enhanced pseudocapacitance with high capacitance of 1094 F g-1 and good cycling stability relative to pristine Na4Ni3(PO4)2P2O7 (584 F g-1) and maricite NaNiPO4 (368 F g-1) due to reversible redox reaction of Ni(II)/Ni(III), assisted by porous nature and high electrical conductivity of the sample.