Production and Characterization of Electroactive Nickel Oxides Grown on Nickel Foam by Anodic Oxidation in KOH Melts for
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Production and Characterization of Electroactive Nickel Oxides Grown on Nickel Foam by Anodic Oxidation in KOH Melts for Supercapacitor Applications N. Tokmak 1 and M. Urgen 1 1 Istanbul Technical University, Department of Metallurgical and Materials Engineering, 34469, Maslak, Istanbul-Turkey ABSTRACT The role of experimental parameters on direct oxidation of nickel foams with anodic oxidation (anodization) in molten KOH and their contribution to capacitance properties are investigated. Temperature of the melt exhibited an important role on the nature of compounds formed by anodic oxidation. On the samples anodized at 280 and 300 °C stoichiometric NiO is formed. Samples anodized in the temperature range of 150-200 °C gave very high maximum currents in CV measurements indicating the formation of electroactive nickel compounds on them. The nature of these compounds is determined as alpha nickel oxy-hydroxide (α-Ni(OH)2) by micro Raman, XRD and FT-IR measurements. Other property that determines the capacity of these electrodes is the morphology of the electroactive layer, which is controlled mainly by the duration of the treatment. 30 minutes of anodic oxidation time is determined as the optimum value. Areal capacity of the samples anodically oxidized at 200 °C for 30 min using 0.8 V cell voltage are determined as 2.73 F.cm-2 and 1.58 F.cm-2 for 1 mA.cm-2 and 20 mA.cm-2 discharge current densities respectively. INTRODUCTION Nickel oxide/ hydroxide is one of the most promising electrode materials for supercapacitor application due to their electrochemical stability, environmental compatibility and low cost. The different strategies for forming electroactive nickel compounds can be summarized in two groups. First one is the electroactive Ni(OH)2 powder precipitation by chemical, hydrothermal methods followed by mixing with suitable polymer blends [1]. The second one is the production of them on the metallic substrate directly or in-directly. For in-direct production of Ni(OH)2 films, nickel substrates are treated in nickel ion containing solutions using chemical, electrochemical and microwave irradiation methods [2-7]. Synthesis in an autoclave [8-10], molten salt anodization [11], anodic oxidation in by potentiodynamic sweep anodization [12-14] and direct annealing [15] methods are the generally applied routes of production of electroactive Ni(OH)2 layer directly onto the nickel substrate without any usage of external nickel ion source. Most preferred method is direct oxidation of the metal since the direct electronic contact of electroactive compound with the substrate allows very easy transport of the electrons to the substrate. However, the electroactive nickel compounds are hydroxylated (alpha or beta Ni(OH)2 which is not easily produced by simple heating methods. Electrochemical anodic oxidation conducted in aqueous solutions does not also work satisfactorily for this purpose due to passivation of nickel in alkaline solutions and the requirement of high oxidation potentials. Thus, oxidation studies are generally c
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