Marco Zeiger

Review: carbon onions for electrochemical energy storage

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 large external surface area, and high conductivity they are used for supercapacitor applications. As electrode materials, carbon onions provide fast charge/discharge rates resulting in high specific power but present comparatively low specific energy. They improve the performance of activated carbon electrodes as conductive additives and show suitable properties as substrates for redox-active materials. This review provides a critical discussion of the electrochemical properties of different types of carbon onions as electrode materials. It also compares the general advantages and disadvantages of different carbon onion synthesis methods. The 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 the structure, conductivity, and porosity. Although the primary focus of current research is on electrode materials for supercapacitors, the use of carbon onions as conductive additives and for redox-active species is also discussed.

Enhanced capacitance of nitrogen-doped hierarchically porous carbide-derived carbon in matched ionic liquids

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.

A high-rate aqueous symmetric pseudocapacitor based on highly graphitized onion-like carbon/birnessite-type manganese oxide nanohybrids

We present a study on the pseudocapacitive properties of birnessite-type MnO2 grafted on highly graphitized onion-like carbon (OLC/MnO2). In a three-electrode setup, we evaluated two different substrates, namely a platinum disc and nickel foam. The OLC/MnO2 nanohybrid exhibited a large specific capacitance (Csp) of 295 and 323 F g−1 (at 1 A g−1) for the Pt disc and Ni foam, respectively. In addition, the Ni foam substrate exhibited much higher rate capability (power density) than the Pt disc. A symmetrical two-electrode device, fabricated with the Ni foam, showed a large Csp of 254 F g−1, a specific energy density of 5.6 W h kg−1, and a high power density of 74.8 kW kg−1. These values have been the highest for onion-based electrodes so far. The device showed excellent capacity retention when subjected to voltage-holding (floating) experiments for 50 h. In addition, the device showed a very short time constant (τ = 40 ms). This high rate handling ability of the OLC/MnO2 nanohybrid, compared to literature reports, promises new opportunities for the development of aqueous-based pseudocapacitors.

Improved capacitive deionization performance of mixed hydrophobic/hydrophilic activated carbon electrodes.

Capacitive deionization (CDI) is a promising salt removal technology with high energy efficiency when applied to low molar concentration aqueous electrolytes. As an interfacial process, ion electrosorption during CDI operation is sensitive to the pore structure and the total pore volume of carbon electrodes limits the maximum salt adsorption capacity (SAC). Thus, activation of carbons as a widely used method to enhance the porosity of a material should also be highly attractive for improving SAC values. In our study, we use easy-to-scale and facile-to-apply CO2-activation at temperatures between 950 °C and 1020 °C to increase the porosity of commercially available activated carbon. While the pore volume and surface area can be significantly increased up to 1.51 cm(3) g(-1) and 2113 m(2) g(-1), this comes at the expense of making the carbon more hydrophobic. We present a novel strategy to capitalize on the improved pore structure by admixing as received (more hydrophilic) carbon with CO2-treated (more hydrophobic) carbon for CDI electrodes without using membranes. This translates into an enhanced charge storage ability in high and low molar concentrations (1 M and 5 mM NaCl) and significantly improved CDI performance (at 5 mM NaCl). In particular, we obtain stable CDI performance at 0.86 charge efficiency with 13.1 mg g(-1) SAC for an optimized 2:1 mixture (by mass).

Dispersion analysis of carbon nanotubes, carbon onions, and nanodiamonds for their application as reinforcement phase in nickel metal matrix composites

Dispersions of multi-wall carbon nanotubes, onion-like carbon, and nanodiamonds in ethylene glycol are produced using a homogenizer and an ultrasonic bath, altering the treatment time. The dispersed particles are then used as reinforcement phase for nickel matrix composites. These nanoparticles are chosen to represent different carbon hybridization states (sp2 vs. sp3) or a different particle geometry (0D vs. 1D). This allows for a systematic investigation of the effect of named differences on the dispersibility in the solvent and in the composite, as well as the mechanical reinforcement effect. A comprehensive suite of complementary analytical methods are employed, including transmission electron microscopy, Raman spectroscopy, dynamic light scattering, sedimentation analysis, zeta-potential measurements, scanning electron microscopy, electron back scatter diffraction, and Vickers microhardness measurements. It can be concluded that the maximum achievable dispersion grade in the solvent is similar, not altering the structural integrity of the particles. However, nanodiamonds show the best dispersion stability, followed by onion-like carbon, and finally multi-walled carbon nanotubes. The distribution and agglomerate sizes of the particles within the composites are in good agreement with the dispersion analysis, which is finally correlated with a maximum grain refinement by a factor of 3 and a maximum mechanical reinforcement effect for nanodiamonds.

Electrospinning of ultrafine metal oxide/carbon and metal carbide/carbon nanocomposite fibers†

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 m2 g−1 (TiC/C) after carbothermal reduction.

Faradaic deionization of brackish and sea water via pseudocapacitive cation and anion intercalation into few-layered molybdenum disulfide

This work establishes molybdenum disulfide/carbon nanotube electrodes for the desalination of high molar saline water. Capitalizing on the two-dimensional layered structure of MoS2, both cations and anions can be effectively removed from a feed water stream by faradaic ion intercalation. The approach is based on the setup of capacitive deionization (CDI), where an effluent water stream is desalinated via the formation of an electrical double-layer at two oppositely polarized carbon electrodes. Yet, CDI can only be effectively applied to low concentrated solutions due to the intrinsic limitation of the electrosorption mechanism. By replacing the conventional porous carbon with MoS2/CNT binder-free electrodes, deionization of sodium and chloride ions was achieved by ion intercalation instead of ion electrosorption. This enabled stable desalination performance over 25 cycles in various molar concentrations, with salt adsorption capacities of 10, 13, 18, and 25 mg g−1 in 5, 25, 100, and 500 mM NaCl aqueous solutions, respectively. This novel approach of faradaic deionization (FDI) paves the way towards a more energy-efficient desalination of brackish water and even sea water.

Tuning pseudocapacitive and battery-like lithium intercalation in vanadium dioxide/carbon onion hybrids for asymmetric supercapacitor anodes

The study presents the synthesis of vanadium oxide/carbon onion hybrid materials. Flower-like vanadium oxide nanostructures nucleate on carbon onion nanoparticles under hydrothermal conditions, forming a highly intertwined network. By varying the amount of added carbon onions during the synthesis, the number of possible nucleation sites can be adjusted, resulting in the preferential growth of vanadium dioxide in either P21/c or C2/m space group. When employed as a lithium intercalation electrode, P21/c VO2 exhibits capacitor-like (pseudocapacitive) lithium intercalation, whereas C2/m VO2 shows battery-like intercalation peaks with a maximum capacity of 127 mA h g−1. By selecting an optimum ratio and thereby combining both intercalation mechanisms, enhanced kinetics with discharge capacities of 45 mA h g−1 and 29 mA h g−1 at high rates of 50 A g−1 and 100 A g−1 (equal to 394C and 788C) are obtained. This behavior can be translated to a device level by using the material as anodes in asymmetric supercapacitors with activated carbon cathodes, yielding a maximum specific energy of 45 W h kg−1 and a high power of 58 kW kg−1, while longevity over 5000 charge/discharge cycles is demonstrated.

Vanadium pentoxide/carbide-derived carbon core–shell hybrid particles for high performance electrochemical energy storage

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 °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.

Emulsion soft templating of carbide-derived carbon nanospheres with controllable porosity for capacitive electrochemical energy storage

A new approach to produce highly porous carbide-derived carbon nanospheres of 20–200 nm diameter based on a novel soft-templating technique is presented. A platinum catalyst is used for the cross-linking of liquid (allylhydrido)polycarbosilane polymer chains with para-divinylbenzene within oil-in-water miniemulsions. Quantitative implementation of the pre-ceramic polymer can be achieved allowing precise control over the resulting materials. After pyrolysis and high-temperature chlorine treatment, the resulting particles offer a spherical shape, very high specific surface area (up to 2347 m2 g−1), and large micro/mesopore volume (up to 1.67 cm3 g−1). The internal pore structure of the nanospheres is controllable by the composition of the oil phase within the miniemulsions. The materials are highly suitable to be used as supercapacitor electrodes with high specific capacitances in aqueous 1 M Na2SO4 solution (110 F g−1) and organic 1 M tetraethylammonium tetrafluoroborate in acetonitrile (130 F g−1).