Strontium Ruthenate Perovskites with High Specific Capacitance for use in Electrochemical Capacitors
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Under the materials that exhibit pseudocapacitive behavior in an electrochemical capacitor the family of metallic conducting oxides has gained very much attention. The amorphous form of hydrous ruthenium oxide is considered to be the one of the materials with the largest specific capacitances. Values up to 768 F/g are reported [1]. Devices for use in electric vehicles as peak power sources using this kind of material are under development [2]. A major drawback of this material is its high cost. It was our intention to screen the metallic conducting perovskites for pseudocapacitive behavoir. Perovskites are widely used in electrochemical energy storage and conversion, but mainly at higher temperatures either as electrodes or as proton conducting electrolytes in fuel cells. [3, 4]. Under the metallic conducting perovskites are ruthenates [5] and we started with strontium ruthenate SrRuO 3 [6, 7]. Modifications in the chemical composition by doping on either the A or B site of the perovskite ABO 3 were investigated on their influence of the capacitance [7, 8]. Because the results are fairly topical, some important facts are elucidated in this paper again. Moreover here a proposal on the charge storage mechanism is given. It is the aim of this paper to illustrate our approach to increase the capacitive properties of SrRuO 3 by doping and by introducing a new preparation technique which multiplied the capacitance value by a factor of 25.
637 Mat. Res. Soc. Symp. Proc. Vol. 496 ©1998 Materials Research Society
EXPERIMENTAL The sample perovskites were prepared by a coprecipitation process: A stoichiometric aqueous solution of strontium nitrate and ruthenium chloride RuC13 was slowly added to 3 M KOH under vigorous stirring. After 30 minutes the solution was filtered and the precipitate washed several times with distilled water until chloride was removed completely. The precursor was dried and calcined at 500 to 800 'C for several hours in air. Raney type samples were prepared by a pyrolysis process. A solution of RuC13 and Sr(N0 3)2 with the stoichiometric ratio of Ru:Sr = 1:5/2 was given dropwise in an alumina crucible kept at 500 'C in an upright standing tube furace. The powder was kept at the desired temperature for 10 to 20 minutes, was then removed quickly from the crucible and ground by a mortar and pestle. Excess soluble phases were removed by washing the sample several times with water. Phase composition was determined using a Siemens D 5000 X-ray powder diffractometer, equipped with a graphite secondary monochrornator, Bragg-Brentano focusing and Cu K.
radiation. BET surface area by nitrogen adsorption was determined with a Carlo-ErbaInstruments Sorptomatic model 1990. Electrode preparation was done by painting a slurry of the sample with Triton-X-100 on one side of a nickel foil, previously etched in concentrated HCI to clean and to roughen the surface. The foil was 15 mm x 15 rmn and spot welded to a nickel wire as a current lead before, To adhere the coating and to burn out the binder the electrodes were
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