Electrochemical supercapacitors : scientific fundamentals and technological applications / B. E. Conway

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Systems for electrochemical energy production originated with Volta´s discov¬ery in 1800 of "voltaic electricity" and were developed in various forms during the nineteenth century. Toward the end of that period, reversibly chargeable bat-teries for electrical energy storage and utilization became a major development in applied electrochemistry and during the present century have been improved to a high state of the art. They also represent a large fraction of the economic activity in industrial electrochemistry. In relatively recent years, but originating with Becker´s patent in 1957, a new type of electrochemically reversible energy storage system has been devel¬oped that uses the capacitance associated with charging and discharging of the double layer at electrode interfaces or, complementarily, the pseudocapacitance associated with electrosorption processes or surface redox reactions. In the first case, large interfacial capacities of many tens of farads per gram of active elec¬trode material can be achieved at high-area carbón powders, fibers, or felts, while, in the second case, large pseudocapacitances can be developed at certain high-area oxides or conducting polymers where extents of Faradaic charge (Q) transfer are functionally related to the potential of the electrode (V), giving rise to a derivative corresponding to a capacitance dQldV. These large specific-valué capacitors, especially of the double-layer type, are perceived as electrical energy storage systems that can offer high power-den-sity in discharge and recharge, and cycle lives on the order of 105 to 106, many times those of conventional batteries. A variety of uses of such electrochemical or so-called "supercapacitors" are now recognized and a new direction of power-source development, complementary to that of batteries, is well established