Synthesis, characterization and electrochemical properties of cobalt-doped phosphate tungsten heteropoly acid and its bronze

Abstract

Heteropoly acids and their compounds are a fascinating class of multifunctional materials for use in various fields: medicine, magnetism, catalysis and nonlinear optics, as well as for electrochemistry and battery materials. This study used tungsten-phosphate heteropoly acid to synthesize and characterize its Co doped salt (Co-PWA) and tungsten-phosphate bronze (Co-PWB). Thermal analysis was used to determine Co-PWA salt phase transition into Co-PWB bronze occurring at 588 °C. Both samples were further characterized using Fourier transform infrared spectroscopy, X-ray powder diffraction and scanning electron microscopy containing energy dispersive X-ray spectroscopy, and by use of electrochemical examina-tions. Cyclic voltammetry (as a rapid analytical method) showed that both materials yielded low capacities in an aqueous solution of LiNO<inf>3</inf>. However, a “slow” analytical method, chronopotentiometry, in which more places of a crystal lattice are occupied with ions (as compared to cyclic voltammetry), yielded solid and stable discharge capacity, making Co-PWB attractive as a potential electrode material for aqueous Li-ion batteries. The results obtained fill the gap in the scientific literature dealing with similar materials.