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.

This 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.

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.

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.