Energy Materials

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.

The structural characterisation of nanoporous carbons is a challenging task as they generally lack long-range order and can exhibit diverse local structures. Such characterisation represents an important step towards understanding and improving the properties and functionality of porous carbons, yet few experimental techniques have been developed for this purpose. Here we demonstrate the application of nuclear magnetic resonance (NMR) spectroscopy and pair distribution function (PDF) analysis as new tools to probe the local structures of porous carbons, alongside more conventional Raman spectroscopy. Together, the PDFs and the Raman spectra allow the local chemical bonding to be probed, with the bonding becoming more ordered for carbide-derived carbons (CDCs) synthesised at higher temperatures. The ring currents induced in the NMR experiment (and thus the observed NMR chemical shifts for adsorbed species) are strongly dependent on the size of the aromatic carbon domains. We exploit this property and use computer simulations to show that the carbon domain size increases with the temperature used in the carbon synthesis. The techniques developed here are applicable to a wide range of porous carbons, and offer new insights into the structures of CDCs (conventional and vacuum-annealed) and coconut shell-derived activated carbons.

Thermal energy is abundantly available, and especially low-grade heat is often wasted in industrial processes as a by-product. Tapping into this vast energy reservoir with cost-attractive technologies may become a key element for the transition to an energy-sustainable economy and society. We propose a novel heat-to-current converter which is based on the temperature dependence of the cell voltage of charged supercapacitors. Using a commercially available supercapacitor, we observed a thermal cell-voltage rise of around 0.6 mV K-1 over a temperature window of 0 [degree]C to 65 [degree]C. Within our theoretical model, this can be used to operate a Stirling-like charge-voltage cycle whose efficiency is competitive to the most-efficient thermoelectric (Seebeck) engines. Our proposed heat-to-current converter is built from cheap materials, contains no moving parts, and could operate with a plethora of electrolytes which can be chosen for optimal performance at specific working temperatures. Therefore, this heat-to-current converter is interesting for small-scale, domestic, and industrial applications.

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 ([small tau] = 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.

A new approach to produce carbide-derived carbon nanospheres of 20-200 nm in diameter based on a novel soft-templating technique is presented. Platinum catalysis 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, resulting particles offer ideal spherical shape, very high specific surface area (up to 2347 m2/g), and large micro/mesopore volume (up to 1.67 cm3/g). The internal pore structure of the nanospheres is controllable by the composition of the oil phase within the miniemulsions. The materials are highly suitable for electrochemical double-layer capacitors with high specific capacitances in aqueous 1 M Na2SO4 solution (110 F/g) and organic 1 M tetraethylammonium tetrafluoroborate in acetonitrile (130 F/g).