Influence of calcination temperature on structural, morphological, and electrochemical properties of Zn2P2O7 nanostructure

Abstract Ammonium zinc phosphate hydrates (NH4ZnPO4: NZPO) precursor powder was synthesized by a facile hydrothermal method at 200°C for 8 h. Effect of calcination temperature on structure, morphology and electrochemical properties of zinc pyrophosphate nanostructure (Zn2P2O7: ZPO Nstr) was studied by calcination NZPO precursor powder at 500 (ZPO5), 600 (ZPO6), 700 (ZPO7) and 800 (ZPO8) °C under argon flow for 1 h. Calcination temperature significantly affect structure of samples, since the X-ray diffraction (XRD) analysis confirmed a monoclinic phase with space group P21/n (Z = 4) in NZPO precursor powder, an orthorhombic phase of γ-Zn2P2O7 with space group Pbcm in ZPO5 Nstr, and a monoclinic phase of space group I2/c (Z = 12) in ZPO6, ZPO7 and ZPO8 Nstr. Moreover, the crystallite size of calcined ZPO Nstr increased with increasing calcination temperature. The micrographs of all ZPO Nstr revealed by scanning electron microscope (SEM) indicated the dependence of morphology on calcination temperature i.e. a dense agglomeration of perfect rectangular bars of smooth surface with irregular size intercalated by small incomplete grown crystals with small cracked fractures spread on their surfaces was observed in ZPO5 Nstr. For ZPO6 and ZPO7 Nstr, similar morphology was observed with a less dense agglomeration. Selected area electron diffraction (SAED) patterns of all samples obtained by transmission electron microscope (TEM) confirmed their crystal structures. The analyzed data of X-ray absorption near edge spectroscopy (XANES) indicated the presence of Zn2+ cations in all ZPO Nstr samples. Electrochemical properties of all active ZPO Nstr working electrodes studied by the cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectrum (EIS) using a three–electrode system in a 3 M KOH electrolyte displayed the pseudo-capacitor performance correlated with Faradaic redox reaction. In addition, the specific capacitance (Cs) was found to decrease at higher calcination temperature. The highest Cs of 102.9 F g−1 at 1 A g−1 with 81.58% retention of cycling stability after 1000 cycle test was achieved in ZPO5 Nstr electrode.