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Thermal conductivity and temperature profiles in carbon electrodes for supercapacitors
The thermal conductivity of supercapacitor film electrodes composed of activated carbon (AC), AC with 15 mass% multi-walled carbon nanotubes (MWCNTs), AC with 15 mass% onion-like carbon (OLC), and only OLC, all mixed with polymer binder (polytetrafluoroethylene), has been measured. This was done for dry electrodes and after the electrodes have been saturated with an organic electrolyte (1 M tetraethylammonium-tetrafluoroborate in acetonitrile, TEA-BF4). The thermal conductivity data was implemented in a simple model of generation and transport of heat in a cylindrical cell supercapacitor systems. Dry electrodes showed a thermal conductivity in the range of 0.09-0.19 W K-1 m-1 and the electrodes soaked with an organic electrolyte yielded values for the thermal conductivity between 0.42 and 0.47 W K-1 m-1. It was seen that the values related strongly to the porosity of the carbon electrode materials. Modeling of the internal temperature profiles of a supercapacitor under conditions corresponding to extreme cycling demonstrated that only a moderate temperature gradient of several degrees Celsius can be expected and which depends on the ohmic resistance of the cell as well as the wetting of the electrode materials.
Graphitization as a universal tool to tailor the potential-dependent capacitance of carbon supercapacitors
Most efforts to improve the energy density of supercapacitors are currently dedicated to optimized porosity or hybrid devices employing pseudocapacitive elements. Little attention has been given to the effects of the low charge carrier density of carbon on the total material capacitance. To study the effect of graphitization on the differential capacitance, carbon onion (also known as onion-like carbon) supercapacitors are chosen. The increase in density of states (DOS) related to the low density of charge carriers in carbon materials is an important effect that leads to a substantial increase in capacitance as the electrode potential is increased. Using carbon onions as a model, it is shown that this phenomenon cannot be related only to geometric aspects but must be the result of varying graphitization. This provides a new tool to significantly improve carbon supercapacitor performance, in addition to having significant consequences for the modeling community where carbons usually are approximated to be ideal metallic conductors. Data on the structure, composition, and phase content of carbon onions are presented and the correlation between electrochemical performance and electrical resistance and graphitization is shown. Highly graphitic carbons show a stronger degree of electrochemical doping, making them very attractive for enhancing the capacitance.
Performance evaluation of conductive additives for activated carbon supercapacitors in organic electrolyte
In this study, we investigate two different activated carbons and four conductive additive materials, all produced in industrial scale from commercial suppliers. The two activated carbons differed in porosity: one with a narrow microporous pore size distribution, the other showed a broader micro-mesoporous pore structure. Electrochemical benchmarking was done in one molar tetraethylammonium tetrafluoroborate in acetonitrile. Comprehensive structural, chemical, and electrical characterization was carried out by varied techniques. This way, we correlate the electrochemical performance with composite electrode properties, such as surface area, pore volume, electrical conductivity, and mass loading for different admixtures of conductive additives to activated carbon. The electrochemical rate handling (from 0.1 A g−1 to 10 A g−1) and long-time stability testing via voltage floating (100 h at 2.7 V cell voltage) show the influence of functional surface groups on carbon materials and the role of percolation of additive particles.
Capacitive deionization using biomass-based microporous salt-templated heteroatom-doped carbons
Microporous carbons are an interesting material for electrochemical applications. In this study, we evaluate several such carbons without/with N or S doping with regard to capacitive deionization. For this purpose, we extent the salt-templating synthesis towards biomass precursors and S-doped microporous carbons. The sample with the largest specific surface area (2830 m2 g−1) showed 1.0 wt % N and exhibited a high salt-sorption capacity of 15.0 mg g−1 at 1.2 V in 5 mM aqueous NaCl. While being a promising material from an equilibrium performance point of view, our study also gives first insights to practical limitations of heteroatom-doped carbon materials. We show that high heteroatom content may be associated with a low charge efficiency. The latter is a key parameter for capacitive deionization and is defined as the ratio between the amounts of removed salt molecules and electrical charge.