Power limitations of supercapacitor and capacitance distribution operation associated with resistance in porous electrode devices

By analogy with the behavior of vacuum (air) capacitors, early perceptions of the properties of electrochemical supercapacitors; were that very high power-levels would be achievable because only capacitance (ca. 25 muF cm(-2)) originating from a double-layer of nanometre thickness was involved. However, for device applications, the required large capacitances (0.50 F g(-1)) are achievable only by use of high-area porous-C matrices (ca. 1 to 2 x 10(3) m(2) g(1)). Then the capacitance is only accessible through a complex series/parallel distribution of reduced current (RC) networks leading to a power-spectrum in which only a fraction of expected charge is accessible at high rates. The equivalent circuit then approximates to that of a transmission-line. Characterization of power limitation effects is demonstrated by results of experiments on porous-C electrodes by means of linear-sweep voltammetry, complementary dc charge/discharge curves and by real-time simulations of the behavior of a 5-element hardware RC circuit. The distribution of R and C elements in a porous electrode structure leads to a decline of energy-density with operating power-density as current drain is increased. It is shown how this can be represented by Ragone plots for capacitors, taking account of declining voltage with state of discharge. Practical comparisons are made with the behavior of three, non-aqueous capacitor modules. (C) 2002 Elsevier Science B.V. All rights reserved.