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- 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.
- ItemNiobium carbide nanofibers as a versatile precursor for high power supercapacitor and high energy battery electrodes(London [u.a.] : RSC, 2016) Tolosa, Aura; Krüner, Benjamin; Fleischmann, Simon; Jäckel, Nicolas; Zeiger, Marco; Aslan, Mesut; Grobelsek, Ingrid; Presser, VolkerThis study presents electrospun niobium carbide/carbon (NbC/C) hybrid nanofibers, with an average diameter of 69 ± 30 nm, as a facile precursor to derive either highly nanoporous niobium carbide-derived carbon (NbC–CDC) fibers for supercapacitor applications or niobium pentoxide/carbon (Nb2O5/C) hybrid fibers for battery-like energy storage. In all cases, the electrodes consist of binder-free and free-standing nanofiber mats that can be used without further conductive additives. Chlorine gas treatment conformally transforms NbC nanofiber mats into NbC–CDC fibers with a specific surface area of 1508 m2 g−1. These nanofibers show a maximum specific energy of 19.5 W h kg−1 at low power and 7.6 W h kg−1 at a high specific power of 30 kW kg−1 in an organic electrolyte. CO2 treatment transforms NbC into T-Nb2O5/C hybrid nanofiber mats that provide a maximum capacity of 156 mA h g−1. The presence of graphitic carbon in the hybrid nanofibers enabled high power handling, maintaining 50% of the initial energy storage capacity at a high rate of 10 A g−1 (64 C-rate). When benchmarked for an asymmetric full-cell, a maximum specific energy of 86 W h kg−1 was obtained. The high specific power for both systems, NbC–CDC and T-Nb2O5/C, resulted from the excellent charge propagation in the continuous nanofiber network and the high graphitization of the carbon structure.
- ItemMXene as a novel intercalation-type pseudocapacitive cathode and anode for capacitive deionization(London [u.a.] : RSC, 2016) Srimuk, Pattarachai; Kaasik, Friedrich; Krüner, Benjamin; Tolosa, Aura; Fleischmann, Simon; Jäckel, Nicolas; Tekeli, Mehmet C.; Aslan, Mesut; Suss, Matthew E.; Presser, VolkerIn this proof-of-concept study, we introduce and demonstrate MXene as a novel type of intercalation electrode for desalination via capacitive deionization (CDI). Traditional CDI cells employ nanoporous carbon electrodes with significant pore volume to achieve a large desalination capacity via ion electrosorption. By contrast, MXene stores charge by ion intercalation between the sheets of its two-dimensional nanolamellar structure. By this virtue, it behaves as an ideal pseudocapacitor, that is, showing capacitive electric response while intercalating both anions and cations. We synthesized Ti3C2-MXene by the conventional process of etching ternary titanium aluminum carbide i.e., the MAX phase (Ti3AlC2) with hydrofluoric acid. The MXene material was cast directly onto the porous separator of the CDI cell without added binder, and exhibited very stable performance over 30 CDI cycles with an average salt adsorption capacity of 13 ± 2 mg g−1.
- 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.
- 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.
- 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.
- ItemElectrospinning of ultrafine metal oxide/carbon and metal carbide/carbon nanocomposite fibers(London : RSC Publishing, 2015) Atchison, Jennifer S.; Zeiger, Marco; Tolosa, Aura; Funke, Lena M.; Jäckel, Nicolas; Presser, VolkerElectrospinning has emerged as a facile technology for the synthesis of ultrafine fibers and even nanofibers of various materials. While carbon nanofibers have been extensively investigated, there have also been studies reported on metal oxide and metal carbide fibers. Yet, comparative studies, especially following the same general synthesis approach, are lacking. In our comprehensive study, we use a sol gel process by which a carrier polymer (cellulose acetate or polyvinylpyrrolidone) is mixed with titanium butoxide, zirconium(IV) acetylacetonate, or niobium n-butoxide to yield nanotextured titania/carbon, zirconia/carbon, or niobia/carbon nonwoven textiles. Carbothermal reduction between 1300 °C and 1700 °C effectively transforms the metal oxide/carbon fibers to metal carbide/carbon nanocomposite while preserving the fiber integrity. As a beneficial effect, the fiber diameter decreases compared to the as-spun state and we obtained ultrafine fibers: 294 ± 108 nm for ZrC/C, 122 ± 28 nm for TiC/C, and 65 ± 36 nm for NbC/C. The highly disordered and porous nature of the carbon matrix engulfing the metal carbide nanocrystals enables a high specific surface area of up to 450 m2 g−1 (TiC/C) after carbothermal reduction.
- ItemHigh performance stability of titania decorated carbon for desalination with capacitive deionization in oxygenated water(London : RSC Publishing, 2016) Srimuk, Pattarachai; Ries, Lucie; Zeiger, Marco; Fleischmann, Simon; Jäckel, Nicolas; Tolosa, Aura; Krüner, Benjamin; Aslan, Mesut; Presser, VolkerPerformance stability in capacitive deionization (CDI) is particularly challenging in systems with a high amount of dissolved oxygen due to rapid oxidation of the carbon anode and peroxide formation. For example, carbon electrodes show a fast performance decay, leading to just 15% of the initial performance after 50 CDI cycles in oxygenated saline solution (5 mM NaCl). We present a novel strategy to overcome this severe limitation by employing nanocarbon particles hybridized with sol–gel-derived titania. In our proof-of-concept study, we demonstrate very stable performance in low molar saline electrolyte (5 mM NaCl) with saturated oxygen for the carbon/metal oxide hybrid (90% of the initial salt adsorption capacity after 100 cycles). The electrochemical analysis using a rotating disk electrode (RDE) confirms the oxygen reduction reaction (ORR) catalytic effect of FW200/TiO2, preventing local peroxide formation by locally modifying the oxygen reduction reaction.