Understanding the Effects of Structural Water in Tungsten Oxide for High Power Electrochemical Capacitor Applications

Abstract

Typically, high surface area electrodes and low active material mass loadings are required to obtain both high power and energy in electrochemical devices. Structural water in bulk, hydrated oxides exists as a 2-D layer and could lead to enhanced interfacial charge transfer and solid-state ion transport for high-power applications. The use of bulk hydrated oxides and high mass loadings could also result in high areal capacitance, which is an important metric for energy storage applications. This research investigates the role of interlayer structural water in bulk, crystalline WO3·nH2O (0 ≤ n ≤ 2) for high power and high areal capacitance energy storage. WO3·2H2O was synthesized via precipitation of a tungstate salt in acid, and the structural water content was further controlled through thermal dehydration to yield anhydrous WO3. Morphology and structure of the obtained materials were characterized through electron microscopy, X-ray diffraction, and Raman spectroscopy. Electrochemical characterization was performed with slurry-cast electrodes and high mass loadings (> 4 mg cm-2) using three-electrode cyclic voltammetry at charge/discharge times ranging from minutes to seconds. These results show that hydrated WO3 exhibits excellent capacity retention, and more importantly, improved energy efficiency compared to the anhydrous material. We attribute the excellent high rate capability of hydrated WO3 to the presence of interlayer structural water. To further understand the role of structural water, three-dimensional Bode impedance analyses, electrochemical quartz crystal microbalance, and operando X-ray diffraction studies were investigated and will be presented. Overall, these studies aim to elucidate the role of structural water to present a new design strategy for high power and energy density storage based on bulk, hydrated transition metal oxides.