Low‐Temperature Growth of Hard Carbon with Graphite Crystal for Sodium‐Ion Storage with High Initial Coulombic Efficiency: A General Method

Practical application of hard carbon materials in sodium‐ion batteries (SIBs) is largely limited by their low initial coulombic efficiency (ICE), which may be improved by increasing the graphitization degree. However, biomass‐derived hard carbon is usually nongraphitizable and extremely difficult to graphitize by direct heating even at 3000 °C. Herein, a general strategy is reported for fabricating hard carbon materials with graphite crystals at 1300 °C promoted by external graphite that serves as a crystal template for the growth of graphite crystals. The graphite crystals enable the contacted pseudographitic domains with a high‐level ordered structure, large domain size, and low defects, leading to an enhanced ICE. The obtained hard carbon materials with graphite crystals, using the carbonized eggshell membranes, and sucrose‐derived microsphere as precursors, achieve very high ICE of 89% and 91% with reversible capacity of 310 and 301 mA h g^?1, respectively. Therefore, using external graphite to promote high‐level ordering pseudographitic domains at low temperature is quite useful to improve ICE for SIB applications.     

Capillary Encapsulation of Metallic Potassium in Aligned Carbon Nanotubes for Use as Stable Potassium Metal Anodes

Metallic potassium (K) is a desirable anode for potassium secondary batteries due to its low electrode potential in nonaqueous electrolytes and high theoretical capacity. Nevertheless, instability caused by dendritic growth, large volume changes, and parasitic side reactions hamper its practical application. Here, an anode containing metallic K is fabricated by infiltrating an aligned carbon nanotube membrane (ACM) with molten K because of its good wettability to molten K due to the strong capillary forces. The K metal is spatially distributed on the 3D ACM framework, which offers sufficient electrode/electrolyte contact for charge transfer. The robust ACM host provides a large number of K nucleation sites and physically confines the K deposited there, thus mitigating dimensional changes during cycling. The pathways for electrons and ions in the anode are associated to form a mixed conducting network, which is beneficial for the electrochemical redox. Consequently, the anode shows stable plating/stripping profiles with low polarization in symmetric cells using conventional carbonate‐based electrolytes. In addition, dendrite growth is suppressed, and the anode demonstrates excellent suitability when paired with a Prussian blue cathode in a full cell. This design strategy is expected to provide a way to address the problems with using metallic K anodes.     

Variations in carbon source–sink relationships in subalpine fir across elevational gradients

• Cold‐adapted trees display acclimation in both carbon source and carbon sink capacity to low‐temperature stress at their upper elevational range limits. Hence a balanced carbon source–sink capacity might be required for their persistence and survival at the elevational tree limits.
• The present study examined the spatial dynamics of carbon source–sink relationship in subalpine fir (Abies fargesii) trees along elevational gradients in the northern slope of the temperate region and in the southern slope of the subtropics in terms of climate in the Qinling Mountain range, north‐central China.
• The results showed that non‐structural carbohydrate (NSC) concentrations in both the source and sink tissues increased with the increase in elevation. The ratio of carbon source–sink displayed a consistent decreasing trend with the increase in elevation and during growing season, showing that it was lowest at a ratio of 2.93 in the northern slope and at a ratio of 2.61 in the southern slope at the upper distribution elevations in the late growing season. Such variations of carbon source–sink ratio might be attributable to the balance between carbon source and sink activities, which changed seasonally across the elevational distribution range.
• We concluded that a ratio of carbon source–sink of at least 2.6 might be essential for subalpine fir trees to persist at their upper range limits. Therefore, a sufficient source–sink ratio and a balanced source–sink relationship might be required for subalpine fir trees to survive and develop at their upper elevational distribution limits.

     

Allometry of fine roots in forest ecosystems

Theoretical predictions regarding fine root production are needed in many ecosystem models but are lacking. Here, we expand the classic pipe model to fine roots and predict isometric scaling relationships between leaf and fine root biomass and among all major biomass production components of individual trees. We also predict that fine root production scales more slowly against increases in leaf production across global forest ecosystems at the stand level. Using meta‐analysis, we show fine root biomass scales isometrically against leaf biomass both at the individual tree and stand level. However, despite isometric scaling between stem and coarse root production, fine root production scales against leaf production with a slope of about 0.8 at the stand level, which probably results from more rapid increase of turnover rate in leaves than in fine roots. These analyses help to improve our understandings of allometric theory and controls of belowground C processes.     

Dual emission carbon dots from carotenoids: Converting a single emission to dual emission

Dual emission carbon dots have a high potential for use as fluorescence‐based sensors with higher selectivity and sensitivity. This study demonstrated the possibility of conversion of a biological molecular system with a single emission peak to a double emission carbon dots system. This report is the first to describe the synthesis of dual emission carbon dots by tuning the electronic environment of a conjugated system. Here we prepared carbon dots from a natural extract, from which carotenoids were used as a new source for carbon dots. Formation of the carbon dots was confirmed by images obtained under a transmission electron microscope as well as from a dynamic light scattering study. The prepared carbon dots system was characterized and its optical property was monitored. The study showed that, after irradiation with microwaves, the fluorescence intensity of the whole system changed, without any change in the original peak position of the carotenoid but with the appearance of an additional peak. A Fourier transform infrared study confirmed breaking of the conjugated system. When using ethylene glycol as a surface passivating agent added to these carotenoid carbon dots, the dual emission spectra became more distinct.     

Boosting Oxygen Evolution Kinetics by Mn–N–C Motifs with Tunable Spin State for Highly Efficient Solar‐Driven Water Splitting

Solar‐driven water splitting is in urgent need for sustainable energy research, for which accelerating oxygen evolution kinetics along with charge migration is the key issue. Herein, Mn³⁺ within π‐conjugated carbon nitride (C₃N₄) in form of Mn–N–C motifs is coordinated. The spin state (e_g orbital filling) of Mn centers is regulated by controlling the bond strength of Mn–N. It is demonstrated that Mn serves as intrinsic oxygen evolution reaction (OER) site and the kinetics is dependent on its spin state with an optimized e_g occupancy of ≈0.95. Specifically, the governing role of e_g occupancy originates from the varied binding strength between Mn and OER intermediates. Benefiting from the rapid spin state‐mediated OER kinetics, as well as extended optical absorption (to 600 nm) and accelerated charge separation by intercalated metal‐to‐ligand state, Mn–C₃N₄ stoichiometrically splits pure water with H₂ production rate up to 695.1 µmol g^?1 h^?1 under simulated sunlight irradiation (AM1.5), and achieves an apparent quantum efficiency of 4.0% at 420 nm, superior to most solid‐state based photocatalysts to date. This work for the first time correlates photocatalytic redox kinetics with the spin state of active sites, and suggests a nexus between photocatalysis and spin theory.     

Carbon Quantum Dots–Modified Interfacial Interactions and Ion Conductivity for Enhanced High Current Density Performance in Lithium–Sulfur Batteries

Significant progress has achieved for developing lithium–sulfur (Li–S) batteries with high specific capacities and excellent cyclic stability. However, some critical issues emerge when attempts are made to raise the areal sulfur loading and increase the operation current density to meet the standards for various industrial applications. In this work, polyethylenimine‐functionalized carbon dots (PEI‐CDots) are designed and prepared for enhancing performance of the Li–S batteries with high sulfur loadings and operation under high current density situations. Strong chemical binding effects towards polysulfides and fast ion transport property are achieved in the PEI‐CDots‐modified cathodes. At a high current density of 8 mA cm^?2, the PEI‐CDots‐modified Li–S battery delivers a reversible areal capacity of 3.3 mAh cm^?2 with only 0.07% capacity decay per cycle over 400 cycles at 6.6 mg sulfur loading. Detailed analysis, involving electrochemical impedance spectroscopy, cyclic voltammetry, and density functional theory calculations, is done for the elucidation of the underlying enhancement mechanism by the PEI‐CDots. The strongly localized sulfur species and the promoted Li⁺ ion conductivity at the cathode–electrolyte interface are revealed to enable high‐performance Li–S batteries with high sulfur loading and large operational current.     

3D Printing of Tunable Energy Storage Devices with Both High Areal and Volumetric Energy Densities

Developing advanced supercapacitors with both high areal and volumetric energy densities remains challenging. In this work, self‐supported, compact carbon composite electrodes are designed with tunable thickness using 3D printing technology for high‐energy‐density supercapacitors. The 3D carbon composite electrodes are composed of the closely stacked and aligned active carbon/carbon nanotube/reduced graphene oxide (AC/CNT/rGO) composite filaments. The AC microparticles are uniformly embedded in the wrinkled CNT/rGO conductive networks without using polymer binders, which contributes to the formation of abundant open and hierarchical pores. The 3D‐printed ultrathick AC/CNT/rGO composite electrode (ten layers) features high areal and volumetric mass loadings of 56.9 mg cm^?2 and 256.3 mg cm^?3, respectively. The symmetric cell assembled with the 3D‐printed thin GO separator and ultrathick AC/CNT/rGO electrodes can possess both high areal and volumetric capacitances of 4.56 F cm^?2 and 10.28 F cm^?3, respectively. Correspondingly, the assembled ultrathick and compact symmetric cell achieves high areal and volumetric energy densities of 0.63 mWh cm^?2 and 1.43 mWh cm^?3, respectively. The all‐component extrusion‐based 3D printing offers a promising strategy for the fabrication of multiscale and multidimensional structures of various high‐energy‐density electrochemical energy storage devices.