Prof. Dr. Volker Presser

In this study, carbide-derived carbon fibers from silicon oxycarbide precursor were synthesized by electrospinning of a commercially available silicone resin without adding a carrier polymer for the electrospinning process. The electrospun fibers were pyrolyzed yielding SiOC. Modifying the synthesis procedure, we were also able to obtain electrosprayed SiOC beads instead of fibers. After chlorine treatment, nanoporous carbon with a specific surface area of up to 2394 m2 g−1 was obtained (3089 m2 g−1 BET). Electrochemical characterization of the SiOC-CDC either as free-standing fiber mat electrodes or polymer-bound bead films was performed in 1 M tetraethylammonium tetrafluoroborate in acetonitrile (TEA-BF4 in ACN). The electrospun fibers presented a high gravimetric capacitance of 135 F g−1 at 10 mV s−1 and a very high power handling, maintaining 63% of the capacitance at 100 A g−1. Comparative data of SiOC-CDC beads and fibers show enhanced power handling for fiber mats only when the fiber network is intact, that is, a lowered performance was observed when using crushed mats that employ polymer binder.

Metallic lithium is the most widespread reference electrode in lithium ion battery research, but its high reactivity limits the usage primarily to conventional carbonate based electrolytes. Novel high power concepts, like hybrid supercapacitors, require lithium containing electrolytes with high ionic conductivity (e.g., acetonitrile), which are not always stable versus lithium. In the current work we face this issue by refining activated carbon as a quasi-reference electrode originally employed for conventional supercapacitors. Different commercially available carbon powders were examined as reference electrode materials and calibrated in lithium-salt containing acetonitrile versus Li+ intercalation/de-intercalation reaction of nanoparticulate Li4Ti5O12. The stability of the activated carbon reference electrode is highly affected by the salt employed and decreases in the following order: LiTFSI > LiClO4 > LiPF6 > LiBF4. Only a negligible impact of electrolyte solvent, pore size distribution and reference electrode binder was observed. Furthermore, activated carbon was functionalized (HNO3 treated) and de-functionalized (thermal annealing in vacuum or hydrogen) to investigate the impact of carbon functionalization on the reference electrode stability. Nitrogen and oxygen containing surface groups have been found to drastically improve long-term stability of activated carbon quasi-reference electrodes. Even after 15 days exposed to the electrolyte, the potential of HNO3 treated activated carbon is marginally altered by 10 mV.

A novel, two step synthesis is presented combining the formation of carbide-derived carbon (CDC) and redox-active vanadium pentoxide (V2O5) in a core-shell manner using solely vanadium carbide (VC) as the precursor. In a first step, the outer part of VC particles is transformed to nanoporous CDC owing to the in situ formation of chlorine gas from NiCl2 at 700 [degree]C. In a second step, the remaining VC core is calcined in synthetic air to obtain V2O5/CDC core-shell particles. Materials characterization by means of electron microscopy, Raman spectroscopy, and X-ray diffraction clearly demonstrates the partial transformation from VC to CDC, as well as the successive oxidation to V2O5/CDC core-shell particles. Electrochemical performance was tested in organic 1 M LiClO4 in acetonitrile using half- and asymmetric full-cell configuration. High specific capacities of 420 mA h g-1 (normalized to V2O5) and 310 mA h g-1 (normalized to V2O5/CDC) were achieved. The unique nanotextured core-shell architecture enables high power retention with ultrafast charging and discharging, achieving more than 100 mA h g-1 at 5 A g-1 (rate of 12C). Asymmetric cell design with CDC on the positive polarization side leads to a high specific energy of up to 80 W h kg-1 with a superior retention of more than 80% over 10 000 cycles and an overall energy efficiency of up to 80% at low rates.

Manganese oxide presents very promising electrochemical properties as an electrode material in supercapacitors, but there remain important open questions to guide further development of the complex manganese oxide/carbon/electrolyte system. Our work addresses specifically the influence of carbon ordering and the difference between outer and inner porosity of carbon particles for the application in aqueous 1 M Na2SO4 and 1 M LiClO4 in acetonitrile. Birnessite-type manganese oxide was hydrothermally hybridized on two kinds of carbon onions with only outer surface area and different electrical conductivity, and conventional activated carbon with a high inner porosity. Carbon onions with a high degree of carbon ordering, high conductivity, and high outer surface area were identified as the most promising material, yielding 179 F g-1. Pore blocking in activated carbon yields unfavorable electrochemical performances. The highest specific energy of 16.4 W h kg-1 was measured for a symmetric full-cell arrangement of manganese oxide coated high temperature carbon onions in the organic electrolyte. High stability during 10 000 cycles was achieved for asymmetric full-cells, which proved as a facile way to enhance the electrochemical performance stability.

Carbon onions are a relatively new member of the carbon nanomaterials family. They consist of multiple concentric fullerene-like carbon shells which are highly defective and disordered. Due to their small size of typically below 10 nm, the moderate surface area, and high conductivity they were used for supercapacitor applications. As electrode material, carbon onions provide fast charge/discharge rates resulting in high specific power but present comparable low specific energy. They improve the performance of activated carbon electrodes as conductive additive and show suitable properties as substrate for redox-active materials. This review provides a critical discussion of the electrochemical properties of different types of carbon onions as electrode material. It also compares general advantages and disadvantages of different carbon onion synthesis methods. The structure, physical and chemical properties of carbon onions, in particular nanodiamond-derived carbon onions, are described with emphasis on those parameters especially important for electrochemical energy storage systems, including among others structure, conductivity, and porosity. Although the primary focus of current research is on electrode materials for supercapacitors, the use of carbon onions as conductive additive for activated carbon and electro-active polymers, as well as substrate for redox-active species is also discussed.

Fluid catalytic cracking (FCC), which currently accounts for half of the worldwide petroleum refining efforts, relies on catalytic, aluminosilicate zeolite particles which slowly deactivate. As of yet, this FCC catalyst residue (FC3R) has no commercial outlet, resulting in abundant amounts of landfill-destined refuse. However, this overlooked waste has the right ingredients for the synthesis of some of today's emerging nanomaterials. High-carbon FC3R, sourced from a Uruguayan refinery, was identified as faujasite particles encased in graphitic carbon shells. We show that pulsed laser ablation of raw FC3R produces simultaneous deposition of single-wall carbon nanotubes and silica nanowires through vapour/solid-liquid-solid self-assembly in distinct zones of an oven-laser apparatus. This is an extreme revalorisation and provides a new untapped resource for research and applications in C- and Si-based nanomaterials and mesoscopic physics.

The 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.

Mixtures of polyvinylpyrrolidone/polyvinyl butyral (PVP/PVB) are attractive binders for the preparation of carbon electrodes for aqueous electrolyte supercapacitors. The use of PVP/PVB offers several key advantages: They are soluble in ethanol and can be used to spray coat or drain cast activated carbon (AC) electrodes directly on a current collector. Infrared spectroscopy and contact angle measurements show that the PVP-to-PVB ratio determines the degree of binder hydrophilicity. Within our study, the most favorable performance was obtained for AC electrodes with a composition of AC + 1.5 mass% PVP + 6.0 mass% PVB; such electrodes were mechanically stabile and water resistant with a PVP release of less than 5% of total PVP while PVB itself is water insoluble. Compared to when using PVDF, the specific surface area (SSA) of the assembled electrodes was 10% higher, indicating a reduced pore blocking tendency. A good electrochemical performance was observed in different aqueous electrolytes for composite electrodes with the optimized binder composition: 160 F g−1 at 1 A g−1 for 1 M H2SO4 and 6 M KOH and 120 F g−1 for 1 M NaCl. The capacitance was slightly reduced by 2.5% after cycling to 1.2 V with 1.28 A g−1 in 1 M NaCl for 10,000 times.

Electrospinning 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 m2g-1 (TiC/C) after carbothermal reduction.

Supercapacitors 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.