Microporous Ruthenium Oxide Films for Energy Storage Applications

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Microporous Ruthenium Oxide Films for Energy Storage Applications J.P. Zheng and Q.L. Fang Department of Electrical and Computer Engineering Florida A&M University and Florida State University Tallahassee, FL 32310, USA ABSTRACT Highly porous ruthenium oxide films were prepared using ruthenium ethoxide solution at low temperatures. The specific capacitance of the ruthenium oxide film electrode made with ruthenium ethoxide solution is much higher than that made using the traditional ruthenium chloride solution. It was found that amorphous ruthenium oxide films with high porosity could be formed at temperatures of 100-200 oC. At temperatures above 250 oC, crystalline ruthenium oxide films were formed. Electrochemical capacitors were made with ruthenium oxide film electrodes and were tested under constant current charging and discharging. The maximum specific capacitance of 593 F/g was obtained from the electrode prepared at 200 oC. The interfacial capacitance increased linearly with increasing film thickness. A value of interfacial capacitance as high as 4 F/cm2 was obtained from the electrode prepared at 200 oC. INTRODUCTION Powder forms of amorphous and hydrous ruthenium oxides (RuO2•xH2O) have been formed by the sol-gel method and found to be promising materials for electrochemical (EC) capacitors with both high power density and high energy density [1,2]. The advantages of amorphous ruthenium oxides include high specific capacitance, high conductivity, and good electrochemical reversibility [3]. A maximum specific capacitance of 768 F/g has been obtained from an amorphous ruthenium oxide [3,4]. The high specific capacitance leads to superior energy density for capacitors made with amorphous ruthenium oxides. The maximum power density of the EC capacitor is determined mainly by the internal resistance of the capacitor, including its electrical and ionic resistances. From an analysis of the distribution of resistance inside a capacitor made of powder electrode materials and aqueous electrolytes, it was previously determined that over 50% of the total internal resistance of the capacitor is due to the electrical contact resistance [5]. This includes the powder electrode material contact, the electrode and the current collector contact, the current collector and the current collector contact from the neighboring cell, and the current collector contact and the end electrode contact. In contrast with the powder electrode, the film electrode has extremely low contact resistance [6]. In this paper, we report a new method to prepare amorphous ruthenium film electrodes on a metallic tantalum (Ta) substrate. These film electrodes are shown to have a higher specific capacitance than that of crystalline films. EXPERIMENTAL DETAILS Tantalum (Ta) foil was used as the substrate. The surface of the Ta foil was abraded with a brush made of palladium wire to remove the oxide layer and at the same time to make the surface rough. This was followed by vacuum heat treatment at 900 oC for ½ hour. The procedure GG9.17.1

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