The limited amount of fossil fuels on earth drives the development of alternative energy to support the daily energy consumption of mankind. Scientists have devoted to develop several kinds of renewable energies and made significant progress. Though those renewable energies generate power when working, it is also important to have devices able to store the excess power. Therefore, capacitors become essential. Capacitors based on electrochemical energy storage such as lithium ion batteries [1], alkaline batteries [2,3], and supercapacitors [4 - 6] have been progressively studied in the past several decades. The central part of electrochemical property is the electrode material. Transition metal oxides have been widely investigated in various pseudocapacitive materials [7]. Tungsten oxide, an n-type semiconductor possessing excellent electron transport properties and chemical stabilities, has gained large attention and its characteristics of low cost, facile synthesis, good conductivity, and high theoretical capacity [8] makes itself a promising material as an electrode. Qiu group prepared flowerlike WO₃ negative electrode using electrochemical deposition [9]; Samu et al. [10] reported nanoporous tungsten trioxide for supercapacitor electrode; Yao et al. [11] demonstrated flexible WO3 nanofibers electrode through a facile hydrothermal approach. In this study, we prepared WO₃ film using the ultrasonic spray method [12] and demonstrated its potential as an electrode in capacitors. Details for the performance of WO3 film in energy storage devices will be discussed.

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