Transition from Battery to Pseudocapacitor Behavior via Structural Water in Tungsten Oxide

The kinetics of energy storage in transition metal oxides are usually limited by solid-state diffusion, and the strategy most often utilized to improve their rate capability is to reduce ion diffusion distances by utilizing nanostructured materials. Here, another strategy for improving the kinetics of layered transition metal oxides by the presence of structural water is proposed. To investigate this strategy, the electrochemical energy storage behavior of a model hydrated layered oxide, WO 3 ·2H 2 O, is compared with that of anhydrous WO 3 in an acidic electrolyte. It is found that the presence of structural water leads to a transition from battery-like behavior in the anhydrous WO 3 to ideally pseudocapacitive behavior in WO 3 ·2H 2 O. As a result, WO 3 ·2H 2 O exhibits significantly improved capacity retention and energy efficiency for proton storage over WO 3 at sweep rates as fast as 200 mV s –1, corresponding to charge/discharge times of just a few seconds. Importantly, the energy storage of WO 3 ·2H 2 O at such rates is nearly 100% efficient, unlike in the case of anhydrous WO 3 . Pseudocapacitance in WO 3 ·2H 2 O allows for high-mass loading electrodes (>3 mg cm –2 ) and high areal capacitances (>0.25 F cm –2 at 200 mV s –1 ) with simple slurry-cast electrodes. These results demonstrate a new approach for developing pseudocapacitance in layered transition metal oxides for high-power energy storage, as well as the importance of energy efficiency as a metric of performance of pseudocapacitive materials.