Synthesis, characterization and electrochemical properties of iron doped phosphate tungsten heteropoly acid (Fe-PWA) and it’s bronze (Fe-PWB): Comparative study

Abstract

In this work, synthesized 12-tungstenphosphoric acid (H3PW12O40 × nH2O; PWA) was further ionically exchanged with Fe3+ ions, which led to the formation of the 12- tungstophosporic acid iron salt, (FePW12O40 × nH2O; Fe-PWA). Fe-PWA was then subjected to thermal analysis (TGA/DTA), determining the phase transition temperature of 576°C from Fe-PWA to its corresponding phosphate tungsten bronze doped with iron, Fe-PWB. Using the X-ray powder diffraction (XRPD), Fourier-transform infrared spectroscopy (FTIR), and Scanning electron microscopy with an energy dispersive X-ray spectroscopy (SEM-EDS) method, the obtained Fe-PWA and Fe-PWB were additionally characterized, and compared. Due to channels and cavities in their structures, Fe-PWA and Fe-PWB were next examined as electrode materials for aqueous rechargeable batteries. Electrochemical measurements were done in aqueous solutions of 6 M LiNO3 by cyclic voltammetry. Fe-PWA and Fe-PWB exhibit different redox processes, which are discussed thoroughly in this work. Electrochemical results are showing that within the Fe-PWA structure, more Li+ ions can be intercalated in the first discharge cycle, but consecutive cycling leads to a fast capacity fade. While the Fe- PWB redox process was stable during cycling, its specific capacity is limited by the material's poor electrical conductivity. Improvements in Fe-PWB conductivity must be addressed in future studies in order to boost material’s electrochemical performance.

Keywords

• heteropoly compound
• 12-tungstophosporic acid iron salt
• iron phosphate tungsten bronze
• keggin anion
• cyclic voltammetry