Synthesis and electrochemical performances of maricite-NaMPO4 (M = Ni, Co, Mn) electrodes for hybrid supercapacitors†
Sodium metal phosphates, NaMPO4 (M = Mn, Co and Ni) were successfully synthesized by a solution combustion synthesis (SCS) method using glycine-nitrate as a precursor. An XRD Rietveld refinement method revealed the crystal structure and lattice parameters of NaMPO4 (M = Mn, Co and Ni). For the first time, the crystal structure parameters of the orthorhombic NaNiPO4 maricite-type phase were evaluated. Similarly, it was identified that the NaCoPO4 and NaMnPO4 have high temperature hexagonal and maricite phases, respectively. The calculated BET specific surface areas (SBET) of NaMnPO4, NaCoPO4 and NaNiPO4 were 17.7, 22.6 and 18.7 m2 g−1, respectively. The NaMPO4 (M = Mn, Co and Ni) electrode exhibits good specific capacitance in 1 M NaOH electrolyte, when compared with 1 M Na2SO4, 1 M NaNO3 and 1 M NaCl. This difference in specific capacitance was analysed based on the influence of electrolyte anions (Cl−, SO42−, OH− and NO3−) and pH conditions of the electrolyte solution. Overall, maricite-NaNiPO4 nanoparticles provided a high specific capacitance of 368 F g−1 compared to NaMnPO4 (163 F g−1) and NaCoPO4 (249 F g−1) in 1 M NaOH electrolyte. Subsequently, a hybrid supercapacitor (AC‖NaNiPO4) was fabricated and it delivered a good specific capacitance and cyclic stability compared to the commercially available device.