Open Access

Mechanical properties of plants and underlying structure-property relationships are important for agricultural purposes as well as for biomimetic concepts. In this study, the effect of mechanical stimulation on morphology and bending properties of the stalk was investigated for Sorghum bicolor (Poaceae), a widely used drought-tolerant biomass grass. An experimental set-up allowing for defined growth and mechanical perturbation (flexing) during a defined growth period was designed. Mechanical properties of individual internodes of the stalk were determined by three-point bending tests. We found that the three investigated lines showed differences in their general bending strength in the non-stimulated condition. However, similar high range of bending strength values was measured for all plant lines after they underwent the mechanical stimulation procedure. The anatomy of internode cross-sections was examined to evaluate structure-property relationships. An increased thickness of the outer sclerenchymatous tissue was observed for internodes with higher bending strength values. Dried internodes fail under lower strains but showed higher bending strength. These findings show that mechanosensitivity in sorghum is dependent on genetic as well as environmental factors. The experimental system presented here offers new straight-forward possibilities for S. bicolor line selection for applications requiring mechanical strength of the stalk.

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.

In this study, we explore carbon onions (diameter below 10 nm), for the first time, as a substrate material for lithium sulfur cathodes. We introduce several scalable synthesis routes to fabricate carbon onion-sulfur hybrids by adopting in situ and melt diffusion strategies with sulfur fractions up to 68 mass%. The conducting skeleton of agglomerated carbon onions proved to be responsible for keeping active sulfur always in close vicinity to the conducting matrix. Therefore, the hybrids are found to be efficient cathodes for Li-S batteries, yielding 97-98% Coulombic efficiency over 150 cycles with a slow fading of the specific capacity (ca. 660 mA h g-1 after 150 cycles) in long term cycle test and rate capability experiments.

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.

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 (sp 2 vs. sp 3 ) 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.

We present a study on the pseudocapacitive properties of birnessite-type MnO 2 grafted on highly graphitized onion-like carbon (OLC/MnO 2 ). In a three-electrode setup, we evaluated two different substrates, namely a platinum disc and nickel foam. The OLC/MnO 2 nanohybrid exhibited a large specific capacitance (C sp ) 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 C sp 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/MnO 2 nanohybrid, compared to literature reports, promises new opportunities for the development of aqueous-based pseudocapacitors.

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.

Performance 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.

Energy storage technologies are at the focal point of current research activities, propelled by the increasing demand on mobile storage for portable electronics and the rapidly growing implementation of renewable energy sources. To fulfill the requirements of an advanced energy storage device - high power and high energy - a hybridization of redox-active materials and electrical double-layer materials can be used. In this PhD thesis, several strategies and material architectures will be employed: Wet-chemical carbon coating procedures with transition metal oxides, carbon functionalization with redox-active surface groups, and the synthesis of core-shell particles. A special focus was on the optimization of the carbon substrate properties, such as porosity, surface area, and structure. It was shown that carbon nanomaterials, like carbon onions, with exclusively outer surface area and high conductivity are ideal substrates for battery and pseudocapacitor hybrids. This is exemplified for pseudocapacitive manganese oxide, a quinone-based surface functionalization, and lithium-sulfur batteries. Coating nanoporous carbons led to substantial pore blocking and thus a reduction of the specific surface area and the resulting capacity. To take advantage from the high double-layer capacitance, nanoporous carbons can be effectively used in core-shell particles, with the battery material in the core and the nanoporous carbon in the shell, which avoids the pore blocking issues.