How zinc substitution affects phosphate tungsten heteropoly acid (PWA) and its bronze (PWB): Structural and electrochemical comparisons with Fe and Co analogues

In this study, a novel zinc-doped phosphor-tungsten salt (Zn-PWA) was synthesized via the Zn2+ ion exchange of protons in H3PW12O40 × nH2O, followed by controlled thermal transformation into its corresponding zinc phosphate-tungsten bronze (Zn-PWB) by sintering process. Thermo-gravimetric and differential thermal analysis (TGA/DTA) revealed that the phase transition from Zn-PWA into Zn-PWB occurs at 580°C, signaling a critical structural reorganization from a Keggin-type heteropoly salt lattice into an open-framework bronze structure. Structural and morphological characterization via X-ray powder diffraction (XRPD), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS) confirmed the preservation of the primary polyanionic structural units in Zn-PWA and the successful formation of the bronze phase after thermal treatment. The electrochemical properties by cyclic voltammetry (CV) of both materials were evaluated separately in aqueous LiNO3 solution. Comparative XRPD and electrochemical studies demonstrated that the thermally derived Zn-PWB exhibits highest crystallinity and significantly enhanced redox stability compared to its heteropoly salt precursor, as well as the highest capacity obtained compared to Fe-PWB and Co-PWB analogues.