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- ItemCarbons and electrolytes for advanced supercapacitors(Hoboken, NJ : Wiley, 2014) Presser, VolkerElectrical energy storage (EES) is one of the most critical areas of technological research around the world. Storing and efficiently using electricity generated by intermittent sources and the transition of our transportation fleet to electric drive depend fundamentally on the development of EES systems with high energy and power densities. Supercapacitors are promising devices for highly efficient energy storage and power management, yet they still suffer from moderate energy densities compared to batteries. To establish a detailed understanding of the science and technology of carbon/carbon supercapacitors, this review discusses the basic principles of the electrical double-layer (EDL), especially regarding the correlation between ion size/ion solvation and the pore size of porous carbon electrodes. We summarize the key aspects of various carbon materials synthesized for use in supercapacitors. With the objective of improving the energy density, the last two sections are dedicated to strategies to increase the capacitance by either introducing pseudocapacitive materials or by using novel electrolytes that allow to increasing the cell voltage. In particular, advances in ionic liquids, but also in the field of organic electrolytes, are discussed and electrode mass balancing is expanded because of its importance to create higher performance asymmetric electrochemical capacitors.
- ItemCapacitive deionization using biomass-based microporous salt-templated heteroatom-doped carbons(Hoboken, NJ : Wiley, 2015) Porada, Slawomir; Schipper, Florian; Aslan, Mesut; Antonietti, Markus; Presser, Volker; Fellinger, Tim-PatrickMicroporous 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.
- ItemGraphitization as a universal tool to tailor the potential-dependent capacitance of carbon supercapacitors(Hoboken, NJ : Wiley, 2014) Weingarth, Daniel; Zeiger, Marco; Jäckel, Nicolas; Aslan, Mesut; Feng, Guang; Presser, VolkerMost 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.
- ItemAcid‐Base Interactions of Pyrazine, Ethyl Acetate, Di‐alcohols, and Lysine with the cyclic Alumosiloxane (Ph2SiO)8[Al(O)OH]4 in View of Mimicking Al2O3(H2O) Surface Reactions(Hoboken, NJ : Wiley, 2020) Veith, Michael; Kolano, David; Huch, VolkerThe etherate of (Ph2SiO)8[Al(O)OH]4 can be transformed into the pyrazine adduct (Ph2SiO)8[Al(O)OH]4·3N(C2H2)2N (1), the ethyl acetate adduct (Ph2SiO)8[Al(O)OH]4·3H3C‐C(O)OC2H5 (2), the 1,6‐hexane diol adduct (Ph2SiO)8[Al(O)OH]4·2HO–CH2(CH2)4CH2–OH (3) and the 1,4‐cyclohexane diol adduct (Ph2SiO)8[Al(O)OH]4·4HO–CH(CH2CH2)2CH–OH (4). In all compounds the OH groups of the starting material bind to the bases through O–H···N (1) or O–H···O hydrogen bonds (2, 3, 4) as found from single‐crystal X‐ray diffraction analyses. Whereas in 1 only three of the central OH groups bind to the pyrazines, in 2 two of them bind to the same carbonyl oxygen atom of the ethyl acetate resulting in an unprecedented O–H···O···H–O double hydrogen bridge. The hexane diol adduct 3 in the crystal forms a one‐dimensional coordination polymer with an intramolecularly to two OH groups grafted hexane diol loop, while the second hexane diol is connecting intermolecularly. In the cyclohexane diol adduct 4 all OH groups of the central Al4(OH)4 ring bind to different diols, leaving one alcohol group per diol uncoordinated. These “free” OH groups form an (O‐H···)4 assembly creating a three‐dimensional overall structure. When reacting with (Ph2SiO)8[Al(O)OH]4 lysine loses water, turns into the cyclic 3‐amino‐2‐azepanone, and transforms through chelation of one of the aluminum atoms the starting material into a new polycycle. The isolated compound has the composition (Ph2SiO)12[Al(O)OH]4[Al2O3]2·4 C6H12N2O·6(CH2)4O (5).