Electrochemical Supercapacitor Properties of RuO2-Based Crystalline Material

W. Sugimoto, T. Shibutani, Y. Murakami, and Y. Takasu

Department of Fine Materials Engineering, Faculty of Textile Science and Technology, Shinshu University,

3-15-1 Tokida, Ueda 386-8567, Japan Introduction Electrochemical supercapacitors have attracted increased interest due to their high power density and long cycle life compared to batteries, and high energy density compared to conventional capacitors.1) Supercapacitor systems based on metal oxide electrodes, in particular ruthenium oxide, offer the advantage of providing higher energy density compared to conventional carbon materials, and better electrochemical stability compared to polymer materials. Since the report of capacitance exceeding 720 F g-1 in an amorphous hydrated form of ruthenium oxide(a-RuO2·nH2O),2) many studies have centered on the increase in the capacitance of this material. However, the lack of abundance and cost of the precious metal are still major disadvantages for commercial production, which has led to increased efforts to improve the capacitance of ruthenium-oxide based material. A common method for increasing the electrochemical charge stored per RuO2 is the addition of a second metal oxide, such as TiO2 ,VOx ,MoO3 , etc.3,4) Recent studies have shown that crystalline material possessing rutile, perovskite, or pyrochlore structures are promising electrode materials for supercapacitors. We report here, the preparation of crystalline Ru1-xVxO2 particles with high surface area, which exhibit supercapacitve behavior comparable to that of a-RuO2·nH2O.5) Experimental The Ru-V binary oxide was prepared by a polymerizable-complex method using ethylene glycol and methanol as solvents, and RuCl3 and VO(OC3H7)3 as the metal source. Anhydrous citric acid was used as the complexing agent. The cation ratio in the products was determined by energy-dispersive X-ray (EDX) analysis. X-ray diffraction (XRD) was used to characterize the crystal structure. The surface area was determined using the Brunauer, Emmett, and Teller (BET) equation from N2 adsorption/desorption measurements. A beaker-type electrochemical cell was used for the electrochemical measurements. The cell was equipped with a working electrode, a platinum plate counter electrode, and a Ag/AgCl reference electrode. A Luggin capillary faced the working electrode at a distance of 2 mm. The working electrode was a Pt mesh painted with the active material mixed with 1 mass% PTFE as the binder. The amount of oxide painted on the Pt mesh was in the range of 5.0 to 10.0 mg. Cyclic voltammetry was carried out between 0.1 and 1.3 V vs. RHE at a scan rate of 2 mV s-1 in a 0.5 M H2SO4 solution at 25°C. The specific capacitance was calculated from the enclosed area of the anodic curves in the cyclic voltammogram (CV) of the 100th cycle. Results and Discussion The observed vanadium contents in the products were lower than the nominal ones due to the dissolution of excess V2O5. The XRD patterns of the products were successfully indexed based on a rutile-type structure. The diffraction peaks for the products deviated from that of pure RuO2, indicating some vanadium incorporation into

the rutile structure. The calculated lattice parameters and cation ratios, indicated the formation of single phase Ru1-xVxO2. A specific capacitance of 570 F g-1 based on the oxide mass was obtained for Ru0.35V0.65O2. This value amounts to 1,210 F g-1 based on the mass of RuO2 in the product. The capacitance decay for this sample was less than 8% after 50,000 cycles, which shows that vanadium in the active phase is not dissolved during the electrochemical measurements. The voltammetric charge per unit surface area of the products was higher for Ru0.35V0.65O2 than RuO2. This suggest that the incorporation of V4+ must have contributed to the enhancement in supercapacitive property. The high specific capacitance in amorphous hydrated ruthenium oxide (a-RuO2·nH2O) has lead many researchers to believe that the hydrated phase was essential in order to obtain high specific capacitance. The present result suggests that specific capacitance exceeding 1,000 F g-1 per mass of RuO2 can be achieved from anhydrous crystalline materials as well. References 1) B. E. Conway, Electrochemical Supercapacitors,

Kluwer Academic Publishers, Norwell, MA (1999). 2) J. P. Zheng and T. R. Jow, J. Electrochem. Soc., 142,

L6 (1995). 3) Y. Takasu, T. Nakamura, H. Ohkawauchi, and Y.

Murakami, J. Electrochem. Soc.,144, 2601 (1997). 4) Y. Takasu and Y. Murakami, Electrochim. Acta, 45,

4135 (2000). 5) W. Sugimoto, T. Shibutani, Y. Murakami, and Y.

Takasu, Electrochem. Solid-State Lett., 5, A170 (2002).

Figure. The variation in the capacitance per mass of RuO2 as a function of the amount of V4+ in Ru1-xVxO2.