ELECTROCHEMICALCAPACITORS：电化学电容器.doc

Electrochemical CapacitorsBackgroundDuring periods of high peak output, significant voltage drops are seen in conventional batteries making them incapable of satisfying the high power requirements of various consumer electronic devices. Electrochemical capacitors are attractive for load-leveling to alleviate this problem. Reducing the peak power demand on batteries will increase operating times for electronic devices such as lap-tops, cellular phones, camcorders, and other portable devices. On the same criteria capacitors also form a significant component of an electric vehicle based on any alternative fuel technology (batteries/natural gas/fuel cell). Current research focuses exclusively on the development of capacitors, which are based either on carbons or on metal oxides. However this approach does not satisfy the twin requirements of both high power and low cost. The later factor is critical in successful commercialization of an electric vehicle. Our ApproachWe have developed hybrid metal oxide-carbon supercapacitors with superior energy and power densities as compared to bare carbon using a novel thin film deposition process. The novel route to synthesizing these materials affords ease in controlling Ru metal loading and optimizing particle size. This leads to the development of RuOx-C materials with high rates of reaction and fast diffusion rates. This in turn would translate directly into capacitors with superior energy and power densities capable of satisfying the high power requirements of electronic devices and electric vehicles. In contrast to pseudo-capacitors, the capacitance of carbon-based materials is directly linked to their specific surface area. However, increasing this area beyond a certain point does not translate to a similar increase in the capacitance. The increased surface area arises due to the formation of micropores of size less than 10. These pores remain inaccessible to the electrolyte and hence do not form a double layer. Hence, there exists a maximum limit to the energy and power density realizable from any carbonaceous material. Alternatively, pseudocapacitors have significantly large energy densities as compared to carbon based double layer capacitors. This advantage of pseudocapacitors is offset by their high cost as compared to carbon. The advantages accrued from carbon based materials could be combined with those of the transition metal oxides leading to the development of hybrid electrochemical capacitors. Development of hybrid double layer-faradaic pseudo capacitors would result in utilizing both the faradaic capacitance of the metal oxide and the double layer capacitance of the carbons better. Few studies have focussed on developing supercapacitors utilizing both the double layer of carbon and the faradaic capacitance of metal oxides.The hybrid RuOx-C capacitors have been synthesized using an electroless deposition technique and also by colloidal method. In the electroless deposition technique, Ru is dispersed on the carbon matrix in the form of small particles (0.4mm). The effect of electrochemical oxidation and temperature treatment on the material performance has been studied extensively. Increasing the oxidation temperature reduces the proton transport rate and also increases the degree of crystalline of the deposits. This adversely affects the performance of the composite. Loading a small amount of Ru oxide (9 wt %) on carbon increases the capacitance from 98 F/g to 190 F/g. Novel RuO2nH2O/carbon composite electrode were also prepared by a colloidal method. XRD study showed that RuO2nH2O prepared from RuCl3nH2O and NaHCO3 was amorphous material when it was annealed below 150oC. FTRaman study confirmed that ruthenium bicarbonate converted ruthenium oxide after annealing even at 100oC. The specific capacitance of composite electrode was calculated from cyclic voltammerty. In the case of 40wt% Ru loaded on carbon, the specific capacitance increased over fifteen times higher compared to the bare carbon black. The specific capacitance RuO2nH2O synthesized by colloidal method was around 860F/g. Transmission electron microscopy indicated that the particle size of RuO2nH2O on carbon was 36 nm. Therefore, The observed high specific capacitance results from the increase of total active bulk volume that proton can diffuse by decreasing particle size. From rate capability test using constant power discharge, 40wt% Ru loaded single electrode showed 17Wh/kg of energy density at 4000W/kg of power density.The hybrid approach outlined here is focussed on combined the high surface area of carbons with the large specific capacitance of metal oxides. This would result in utilizing both the faradaic capacitance of the metal oxide and the double layer capacitance of the carbon better. Of the various metal oxides, Ru is the material of choice since it exhibits very fast reactions and complete reversibility two important criteria for a capacitor material. Traditionally, synthesizing carbon-Ru oxide composites has been achieved by either sputtering Ru on carbon or by sol-gel processes. These methods are expensive and process parameters are difficult to control leading to difficulties in replicating the final material composition and thickness. The alternate synthesis route suggested here is easily controllable and is very cheap. Further, as ruthenium is scarce and expensive, for commercial applications it is critical to utilize the Ru effectively. Since in this process Ru nano-par