Phase and shape dependent electrochemical properties of BiPO4 by PVP assisted hydrothermal method for pseudocapacitors

Abstract

BiPO₄ materials with different shapes are obtained under hydrothermal conditions by simply varying the concentration of PVP (polyvinylpyrrolidone). The prepared materials are characterized using various techniques such as X-Ray Diffraction (XRD), Thermo Gravimetric/Differential Scanning Calorimetry (TG/DSC), Fourier Transform Infrared (FT-IR) spectra, Field Emission Scanning Electron Microscopy (FE-SEM) and Transmission Electron Microscopy (TEM). The XRD with Rietveld analysis infers the formation of a monazite-type monoclinic BiPO₄ structure. A transformation from monazite-type into trigonal BiPO₄ phase is observed upon increasing the PVP concentration. The FE-SEM images reveal the change of shape from irregular cubic structure to octahedral to spindle upon increasing the concentration of PVP from 0.25 g to 1.0 g and thereafter the shape begins to disintegrate (1.25 g). Similarly, the effect of reaction time on the shape of BiPO₄ is also studied. The electrochemical performance of pristine BiPO₄ electrode is studied in various aqueous electrolytes such as 1 M LiOH, 1 M NaOH, 1 M KOH, 0.5 M K₂SO₄ and 1 M KNO₃. Overall, the pristine BiPO₄ provides a maximum specific capacitance in 1 M KOH electrolyte and a specific capacitance of 89 F g⁻¹ is obtained at 5 mA cm⁻². At low PVP concentration, there is an increase in specific capacitance whereas it decreases at high concentrations (1.0 g and 1.25 g). The monazite-type monoclinic structure with rod shape particles exhibits a high specific capacitance (167 F g⁻¹ at 5 mA cm⁻²) and cycle life compared with other morphologies. The effective role of crystal phase and shape on the electrochemical performance of BiPO₄ is elucidated.