Electrodeposition of nano-spherical α-MnO۲ thin film for energy storagesupercapacitor electrode

In recent years, research on the electrochemical energy storage devices such as batteries andelectrochemical capacitors (ECs) has attracted much attention because of their wide applications in various mobile tools including cell phones and laptops and also transportation utilities such aselectric or hybrid electric vehicles. Among the various energy storage devices, supercapacitors are extensively studied due to their high power density and good rate-capability that allow them to be used as an auxiliary devices combined with secondary batteries. Among various electroactive materials for pseudocapacitive electrodes, manganese dioxide (MnO۲) is an attractive candidate for substituting ruthenium oxide due to its high theoretical specific capacitance (۱۳۷۰ F g-۱), natural abundance, low cost and non-toxicity. However, poor electrical conductivity of MnO۲ (۱۰-۵-۱۰-۶ Scm-۱) leads to its low electrochemical utilization upon charge storage and hence its low practicalcapacitance and poor power density. In general, reducing particle size and increasing specific surface area of active material can facilitate fast and easy charged particles (electrolyte ions andelectrons) insertion/deinsertion into the electrode material, leading to enhance practical discharge capacity and high-rate performance. Furthermore, the use of polymeric binders to paste theelectrode materials and conductive agents to current collectors in conventional supercapacitor electrodes is detrimental because of limitation of ion and electron transport within the electrode.This drawback can be overcome by direct deposition of active material in the absence of binder on the current collector surface. Electrochemical deposition or electrodeposition is one of variousapproach for fabricating binder free energy storage electrodes.In the present study, nanostructured spherical MnO۲ electrodes were galvanostatically deposited onto graphite substrates and then subjected to heat treatment at ۲۰۰ °C for ۲ h in air. The electrolyte was an aqueous solution of ۰.۱ M manganese acetate and ۰.۰۵ M sodium acetate. MnO۲ wasdeposited onto graphite substrate at room temperature by applying a constant current density of ۲۵mA cm-۲ for ۲۰ s. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM) and nitrogen adsorption-desorptionmeasurement were used to study the structure and morphology of the prepared films. The pseudocapacitive properties of the as-prepared electrode were investigated using cyclic voltammetry and chronopotentiometry charge-discharge measurements. All the tests were carriedout at room temperature in a three-electrode electrochemical cell containing ۰.۵ M Na۲SO۴ aqueous solution using an Ag/AgCl electrode as a reference electrode and a platinum plate as a counter electrode. According to the electrochemical measurements, the capacitance of MnO۲ electrodes arises mainly from pseudocapacitance, which is attributed to the reversible redox reaction betweenMn۴+ and Mn۳+. This transition process is based on the insertion/deinsertion of protons (H۳O+) or alkali metal cations into the MnO۲ structure. The corresponding reduction/oxidation reaction is given as follows:𝑀(....)𝑂𝑂𝑀where M+ represents H۳O+ and alkali cations (Li+, Na+ and K+). According to the results obtained from charge-discharge tests, the MnO۲ electrode exhibited high specific capacitances of ۰.۵۲, ۰.۴۲, ۰.۳۶ and ۰.۲۹ F cm-۲ at discharge current densities of ۱, ۵, ۱۰ and ۲۰ mA cm-۲, respectively. Consequently, the as-prepared binder-free MnO۲ electrode can be a very promising electrode for high-performance supercapacitors.

کلیدواژه ها:

Manganese oxide ، Electrodeposition ، Supercapacitor ، Electrochemical performances