A life cycle assessment of hard carbon anodes for sodium-ion batteries.

Waste management is one of the biggest environmental challenges worldwide. Biomass-derived hard carbons, which can be applied to rechargeable batteries, can contribute to mitigating environmental changes by enabling the use of renewable energy. This study has carried out a comparative environmental assessment of sustainable hard carbons, produced from System A (hydrothermal carbonization (HTC) followed by pyrolysis) and System B (direct pyrolysis) with different carbon yields, as anodes in sodium-ion batteries (SIBs). We have also analysed different scenarios to save energy in our processes and compared the biomass-derived hard carbons with commercial graphite used in lithium-ion batteries. The life cycle assessment results show that the two systems display significant savings in terms of their global warming potential impact (A1: -30%; B1: -21%), followed by human toxicity potential, photochemical oxidants creation potential, acidification potential and eutrophication potential (both over -90%). Possessing the best electrochemical performance for SIBs among our prepared hard carbons, the HTC-based method is more stable in both environmental and electrochemical aspects than the direct pyrolysis method. Such results help a comprehensive understanding of sustainable hard carbons used in SIBs and show an environmental potential to the practical technologies. This article is part of the theme issue 'Bio-derived and bioinspired sustainable advanced materials for emerging technologies (part 2)'.

Biomass-derived carbon quantum dot sensitizers for solid-state nanostructured solar cells.

New hybrid materials consisting of ZnO nanorods sensitized with three different biomass-derived carbon quantum dots (CQDs) were synthesized, characterized, and used for the first time to build solid-state nanostructured solar cells. The performance of the devices was dependent on the functional groups found on the CQDs. The highest efficiency was obtained using a layer-by-layer coating of two different types of CQDs.

Hydrothermal carbon-based nanostructured hollow spheres as electrode materials for high-power lithium-sulfur batteries.

Carbon hollow spheres were produced using a sustainable approach, i.e. hydrothermal carbonization, using monosaccharides as carbon precursors and silica nanoparticles as hard-templates. Hydrothermal carbonization is an eco-efficient and cost-effective route to synthesize nanostructured carbonaceous materials from abundant biomass-derived molecules. After further thermal treatment under an inert atmosphere and removal of the silica-based core by chemical etching, porous hollow spheres depicting 5-8 nm thin shells were obtained. Subsequently, carbon-sulfur composites were synthesized via a melt diffusion method and used as nanostructured composites for cathodes in lithium-sulfur (Li-S) cells. The morphology of the hollow spheres was controlled and optimized to achieve improved electrochemical properties. Both high specific energies and high specific powers were obtained, due to the unique nanostructure of the hollow spheres. These results revealed that using optimized carbonaceous materials, it is possible to design sustainable Li-S cells showing promising electrochemical properties.

High-Density Lignin-Derived Carbon Nanofiber Supercapacitors with Enhanced Volumetric Energy Density.

Supercapacitors are increasingly used in short-distance electric transportation due to their long lifetime (≈15 years) and fast charging capability (>10 A g-1 ). To improve their market penetration, while minimizing onboard weight and maximizing space-efficiency, materials costs must be reduced (<10 $ kg-1 ) and the volumetric energy-density increased (>8 Wh L-1 ). Carbon nanofibers display good gravimetric capacitance, yet their marketability is hindered by their low density (0.05-0.1 g cm-3 ). Here, the authors increase the packing density of low-cost, free-standing carbon nanofiber mats (from 0.1 to 0.6 g cm-3 ) through uniaxial compression. X-ray computed tomography reveals that densification occurs by reducing the inter-fiber pore size (from 1-5 µm to 0.2-0.5 µm), which are not involved in double-layer capacitance. The improved packing density is directly proportional to the volumetric performances of the device, which reaches a volumetric capacitance of 130 F cm-3 and energy density of 6 Wh L-1 at 0.1 A g-1 using a loading of 3 mg cm-2 . The results outperform most commercial and lab-scale porous carbons synthesized from bioresources (50-100 F cm-3 , 1-3 Wh L-1 using 10 mg cm-2 ) and contribute to the scalable design of sustainable electrodes with minimal 'dead volume' for efficient supercapacitors.

The role of carbon dots - derived underlayer in hematite photoanodes.

Hematite is a promising candidate as photoanode for solar-driven water splitting, with a theoretically predicted maximum solar-to-hydrogen conversion efficiency of ∼16%. However, the interfacial charge transfer and recombination greatly limits its activity for photoelectrochemical water splitting. Carbon dots exhibit great potential in photoelectrochemical water splitting for solar to hydrogen conversion as photosensitisers and co-catalysts. Here we developed a novel carbon underlayer from low-cost and environmental-friendly carbon dots through a facile hydrothermal process, introduced between the fluorine-doped tin oxide conducting substrate and hematite photoanodes. This led to a remarkable enhancement in the photocurrent density. Owing to the triple functional role of carbon dots underlayer in improving the interfacial properties of FTO/hematite and providing carbon source for the overlayer as well as the change in the iron oxidation state, the bulk and interfacial charge transfer dynamics of hematite are significantly enhanced, and consequently led to a remarkable enhancement in the photocurrent density. The results revealed a substantial improvement in the charge transfer rate, yielding a charge transfer efficiency of up to 80% at 1.25 V vs. RHE. In addition, a significant enhancement in the lifetime of photogenerated electrons and an increased carrier density were observed for the hematite photoanodes modified with a carbon underlayer, confirming that the use of sustainable carbon nanomaterials is an effective strategy to boost the photoelectrochemical performance of semiconductors for energy conversion.

In Situ Synthesis of Fluorescent Carbon Dots/Polyelectrolyte Nanocomposite Microcapsules with Reduced Permeability and Ultrasound Sensitivity.

Designing and fabricating multifunctional nanocomposite microcapsules are considerable interests in both academic and industrial research aspects. This work first reports an innovative approach to in situ synthesize and assemble fluorescent carbon dots (CDs) into polyelectrolyte microcapsules, obtaining highly biocompatible nanocomposite microcapsules with excellent luminescence that facilitate imaging and identification in vitro, yet with the feasibility to load small molecules and ultrasound responsiveness to trigger their release. CDs are produced in situ in (PAH/PSS)4 microcapsule shells by carbonization of dextran molecules under relatively mild hydrothermal treatment. Compared with the collapsed and film-like (PAH/PSS)4 microcapsules, the novel composite microcapsules show a free-standing structure, smaller size, and thicker shell. CDs are proven to be fabricated and embedded in PAH/PSS multilayers, and the formed PAH/PSS/CD microcapsules are endowed with strong luminescence, as verified by the transmission electron microscopy, fluorescence spectra, and confocal laser scanning microscopy results. The in situ formation of CDs in capsule shells also empowers these capsules with ultrasound responsiveness and reduced permeability. The feasibility of encapsulation of small molecules (rhodamine B) and ultrasound-triggered release is also shown. Most importantly, due to the intrinsic biocompatible property and photostability of CDs, these fluorescent PAH/PSS/CD microcapsules show negligible cell toxicity and low photobleaching, which are impossible for capsules composited with conventional organic dyes and semiconductor quantum dots.

Biosourced nitrogen-doped microcellular carbon monoliths.

An original approach based on the hydrothermal carbonization of nitrogen-containing biomass derivatives within the continuous phase of a direct concentrated emulsion is reported for the synthesis of nitrogen-doped microcellular carbon monoliths. These biosourced foams show promising performances as intrinsic electrocatalysts in the oxygen reduction reaction. Preliminary catalytic properties of powdered versus monolithic samples are discussed and suggest interesting prospects for their introduction within electrochemical devices.

Emulsion-templated macroporous carbons synthesized by hydrothermal carbonization and their application for the enzymatic oxidation of glucose.

Carbon-based monoliths have been designed using a simple synthetic pathway based on using high internal phase emulsion (HIPE) as a soft template to confine the polymerization and hydrothermal carbonization of saccharide derivatives (furfural) and phenolic compounds (phloroglucinol). Monosaccharides can be isolated from the cellulosic fraction of lignocellulosic biomass and phloroglucinol can be extracted from the bark of fruit trees; however, this approach constitutes an interesting sustainable synthetic route. The macroscopic characteristics can be easily modulated; a high macroporosity and total pore volume of up to 98 % and 18 cm(3)g(-1) have been obtained, respectively. After further thermal treatment under inert atmosphere, the as-synthesized macroporous carbonized HIPEs (carbo-HIPEs) have shaping capabilities relating to interesting mechanical properties as well as a high electrical conductivity of up to 300 Sm(-1) . These conductive foams exhibit a hierarchical structure associated with the presence of both meso- and micropores that exhibit specific Brunauer-Emmett-Teller (BET) surface areas and DFT total pore volumes up to 730 m(2)g(-1) and 0.313 cm(3)g(-1) , respectively. Because of their attractive structural characteristics and intrinsic properties, these macroporous monoliths have been incorporated as a proof of principle within electrochemical devices as modified thin carbon disc electrodes. A promising two-fold improvement in the catalytic current is observed for the electrooxidation of glucose after the immobilization of a glucose oxidase-based biocatalytic mixture onto the carbo-HIPE electrodes compared to that observed if using commercial glassy carbon electrodes.