Synthesis of Zinc Doped Phosphate Tungsten Bronzes and its Redox Activity in Aqueous Solution of LiNO3

Abstract

Phosphate tungsten bronzes (WPB) have been intensively investigated for many applications due to their interesting chemical, optical, electrical, and mechanical properties. Their most known usages are as a pigment in traditional ceramics, a chemical sensor in different electric and electrochromic devices, and an electrocatalyst in fuel cells. Phosphate tungsten bronzes have a specific layered structure formed by the collapse of the Keggin anion at a temperature around 602 °C. Its three-dimensional network structure consists of interconnected PO4 tetrahedra and WO6 octahedra. In such a structure, pentagonal and hexagonal openings (cavities, channels) are formed, allowing diffusion of small ions like Li+. Due to WPB's metallic properties, the high oxidation state of tungsten, and the existence of different channels for the accommodation of Li+ ions, make this material suitable as electrode material in Li-ion batteries. The redox activity of these materials has already been tested, and the obtained results have encouraged further research of these compounds as potential electrode materials. According to the literature, WPB materials are defined by electrochemical instability and decrease of columbic capacity upon intercalation/deintercalation processes. In order to try to stabilize the structure of WPB upon intercalation/deintercalation processes, WPB materials were doped with divalent cations. In this work, synthesized 12-tungstenphosphoric acid (H3PW12O40 ∙ nH2O, WPA) was further ionically exchanged with Zn2+ ions, which gave 12-tungsten phosphoric acids of the transition metal (ZnPW12O40 ∙ nH2O, ZnWPA). ZnWPA was then subjected to thermal analysis, which determined the phase transition temperature (when the Keggin anion collapses). The temperature of collapsing the Keggin anion is about 600 °C, and at this temperature, ZnWPA was heated for 10 minutes to obtain phosphate tungsten bronzes doped with zinc (ZnPW8O26, ZnWPB). Obtained ZnWPB was further characterized by XRD and FTIR, which confirmed the formation of the desired structure. The initial electrochemical measurements were done in the aqueous solution of LiNO3 (6 M) by cyclic voltammetry. Initial cycling at a scan rate of 20 mV s-1 shows good electrochemical stability of ZnWPB. Obtained results open new directions toward further research of ZnWPB as potential electrode material for rechargeable batteries.