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- 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.
- ItemAn electrochemical in situ study of freezing and thawing of ionic liquids in carbon nanopores(Cambridge : Royal Society of Chemistry, 2014) Weingarth, Daniel; Drumm, Robert; Foelske-Schmitz, Annette; Kotz, Rüdiger; Presser, VolkerRoom temperature ionic liquids (RTILs) are an emerging class of electrolytes enabling high cell voltages and, in return, high energy density of advanced supercapacitors. Yet, the low temperature behavior, including freezing and thawing, is little understood when ions are confined in the narrow space of nanopores. This study shows that RTILs may show a tremendously different thermal behavior when comparing bulk with nanoconfined properties as a result of the increased surface energy of carbon pore walls. In particular, a continuous increase in viscosity is accompanied by slowed-down charge-discharge kinetics as seen with in situ electrochemical characterization. Freezing reversibly collapses the energy storage ability and thawing fully restores the initial energy density of the material. For the first time, a different thermal behavior in positively and negatively polarized electrodes is demonstrated. This leads to different freezing and melting points in the two electrodes. Compared to bulk, RTILs in the confinement of electrically charged nanopores show a high affinity for supercooling; that is, the electrode may freeze during heating.
- ItemPerformance evaluation of conductive additives for activated carbon supercapacitors in organic electrolyte(Amsterdam : Elsevier, 2016) Jäckel, Nicolas; Weingarth, Daniel; Schreiber, Anna; Krüner, Benjamin; Zeiger, Marco; Tolosa Rodriguez, Aura Monserrat; Aslan, Mesut; Presser, VolkerIn 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.
- ItemEnhanced electrochemical energy storage by nanoscopic decoration of endohedral and exohedral carbon with vanadium oxide via atomic layer deposition(Washington D.C. : American Chemical Society, 2016) Fleischmann, Simon; Jäckel, Nicolas; Zeiger, Marco; Krüner, Benjamin; Grobelsek, Ingrid; Formanek, Petr; Choudhury, Soumyadip; Weingarth, Daniel; Presser, VolkerAtomic layer deposition (ALD) is a facile process to decorate carbon surfaces with redox-active nanolayers. This is a particularly attractive route to obtain hybrid electrode materials for high performance electrochemical energy storage applications. Using activated carbon and carbon onions as representatives of substrate materials with large internal or external surface area, respectively, we have studied the enhanced energy storage capacity of vanadium oxide coatings. While the internal porosity of activated carbon readily becomes blocked by obstructing nanopores, carbon onions enable the continued deposition of vanadia within their large interparticle voids. Electrochemical benchmarking in lithium perchlorate in acetonitrile (1 M LiClO4) showed a maximum capacity of 122 mAh/g when using vanadia coated activated carbon and 129 mAh/g for vanadia coated carbon onions. There is an optimum amount of vanadia between 50 and 65 wt % for both substrates that results in an ideal balance between redox-activity and electrical conductivity of the hybrid electrode. Assembling asymmetric (charge balanced) full-cells, a maximum specific energy of 38 Wh/kg and 29 Wh/kg was found for carbon onions and activated carbon, respectively. The stability of both systems is promising, with a capacity retention of ∼85–91% after 7000 cycles for full-cell measurements.
- ItemEnhanced capacitance of nitrogen-doped hierarchically porous carbide-derived carbon in matched ionic liquids(Cambridge : Royal Society of Chemistry, 2015) Ewert, Julia K.; Weingarth, Daniel; Denner, Christine; Friedrich, Martin; Zeiger, Marco; Schreiber, Anna; Jäckel, Nicolas; Presser, Volker; Kempe, RhettSupercapacitors combine efficient electrical energy storage and performance stability based on fast electrosorption of electrolyte ions at charged interfaces. They are a central element of existing and emerging energy concepts. A better understanding of capacitance enhancement options is essential to exploit the full potential of supercapacitors. Here, we report a novel hierarchically structured N-doped carbon material and a significant capacitance enhancement for a specific ionic liquid. Our studies indicate that matching of the electrode material and the ionic liquid specifically leads to a constant normalized resistance of the electrode material (voltage window up to ±1 V vs. carbon) and a significant enhancement of the specific capacitance. Such effects are not seen for standard organic electrolytes, non-matched ionic liquids, or non-N-doped carbons. A higher N-doping of the electrode material improves the symmetric full cell capacitance of the match and considerably increases its long-term stability at +3 V cell voltage. This novel observance of enhanced specific capacitance for N-doped carbons with matched ionic liquid may enable a new platform for developing supercapacitors with enhanced energy storage capacity.
- ItemTracking the structural arrangement of ions in carbon supercapacitor nanopores using in situ small-angle X-ray scattering(Cambridge : Royal Society of Chemistry, 2015) Prehal, C.; Weingarth, Daniel; Perre, Emilie; Lechner, R.T.; Amenitsch, H.; Paris, O.; Presser, VolkerThe charge storage mechanism and ion arrangement inside electrically charged carbon nanopores is a very active research field with tremendous importance for advanced electrochemical technologies, such as supercapacitors or capacitive deionization. Going far beyond the state of art, we present for the first time a comprehensive study of tracking ion electrosorption in aqueous electrolytes during charging and discharging of porous carbon electrodes using in situ X-ray scattering. We provide novel and quantitative insights into the local concentration of anions and cations and demonstrate that the global number of ions within the pores does not vary during charging and discharging. In addition, we have unique access to the spatial arrangement of ions inside carbon nanopores by using a simple, yet powerful two-phase model. Applying this model to our data, we show that double-layer formation is accomplished by a unique combination of preferred counter-ion adsorption directly at the pore wall which drains ions from their local surrounding inside carbon nanopores. Effectively, this leads to a situation which globally appears as ion swapping.
- ItemCarbon flow electrodes for continuous operation of capacitive deionization and capacitive mixing energy generation(Cambridge : Royal Society of Chemistry, 2014) Porada, S.; Weingarth, Daniel; Hamelers, H.V.M.; Bryjak, M.; Presser, Volker; Biesheuvel, P.M.Capacitive technologies, such as capacitive deionization and energy harvesting based on mixing energy (“capmix” and “CO2 energy”), are characterized by intermittent operation: phases of ion electrosorption from the water are followed by system regeneration. From a system application point of view, continuous operation has many advantages, to optimize performance, to simplify system operation, and ultimately to lower costs. In our study, we investigate as a step towards second generation capacitive technologies the potential of continuous operation of capacitive deionization and energy harvesting devices, enabled by carbon flow electrodes using a suspension based on conventional activated carbon powders. We show how the water residence time and mass loading of carbon in the suspension influence system performance. The efficiency and kinetics of the continuous salt removal process can be improved by optimizing device operation, without using less common or highly elaborate novel materials. We demonstrate, for the first time, continuous energy generation via capacitive mixing technology using differences in water salinity, and differences in gas phase CO2 concentration. Using a novel design of cylindrical ion exchange membranes serving as flow channels, we continuously extract energy from available concentration differences that otherwise would remain unused. These results may contribute to establishing a sustainable energy strategy when implementing energy extraction for sources such as CO2-emissions from power plants based on fossil fuels.