Free-standing Carbon Nanofiber Research Articles

Scalable NH4V4O10 clusters as functional network nodes on toughened porous carbon nanofibers for hybrid capacitive deionization

Layered vanadium-based compound (e.g., NH4V4O10) is acknowledged as an outstanding candidate for intercalation pseudocapacitance electrode of capacitive deionization (CDI) cells. However, the directed production of high-efficiency NH4V4O10-based self-supporting CDI electrodes and their structure–activity connection warrant more in-depth study. Herein, the delicate design and controllable construction of NH4V4O10 clusters on the freestanding porous carbon nanofiber (PCNF) scaffolds (the composite PCNFs/NH4V4O10 is defined as CNVO) using a new organic reducing agent of citric acid is proposed, where NH4V4O10 clusters as functional network nodes can improve the mechanical toughness, electrochemical capacitive performance, and desalination capability of the CNVO hybrid. The Na+ adsorption mechanism on CNVO is confirmed by ex-situ experimental characterizations and density functional theory (DFT) calculations. The hybrid CDI (HCDI) cell composed of the optimized CNVO cathode and PCNFs anode shows a desirable desalination capacity (45.07 mg g−1), a superior salt removal rate (16.4 mg g−1 min−1), an excellent energy utilization, and an acceptable cyclic regeneration ability in 500 mg L-1 NaCl solution. More importantly, the assembled HCDI system is promising for the desalination of reclaimed water from the printing and dyeing enterprise. This work highlights a scalable strategy that is experimentally and theoretically feasible to design and fabricate high-efficiency CDI electrodes.

Free-standing ultra-thin carbon nanofiber films with controllable thickness for lithium ion batteries

Thin and porous anode materials enable short ion migration distances and high-rate performance. However, the current freestanding anode materials are too thick (typically around 100 μm) for high-rate lithium ion batteries (LIBs). Here, we have developed a facile and scalable process to synthesize ultra-thin (<400 nm) freestanding films. In comparison to other chemical and physical deposition methods that yield thin films (<500 nm), our freestanding films do not require the substrate material, achieving a 100 % loading of active material. Our ultra-thin carbon nanofiber films exhibit an exceptional specific capacity of 599 mAh/g even after 10,000 cycles at a current density of 10 A/g. Compared to thick carbon nanofiber films, the utilization of ultra-thin carbon nanofiber films as anode materials exhibits a remarkable enhancement of more than tenfold in cycling performance

Hierarchical porous N-doped carbon fibers enable ultrafast electron and ion transport for Zn-ion storage at high mass loadings

Zinc-ion capacitors (ZICs) are a promising and safe energy storage system for portable electronics but usually limited by relatively low areal capacity and energy density. Although increasing the mass loading has been suggested, thick electrode layers and clogged pores unavoidably lead to sluggish electron transport and ion diffusion as well as “dead volume” of active materials. Herein, a millimeter-scale 3D thick-network electrode, composed of closely intertwined hierarchical porous N-doped and free-standing carbon nanofibers (HPNCFs), was fabricated via carbonization of nanoscale zeolitic imidazolate framework (ZIF-8) particles embedded in electrospun polyacrylonitrile (PAN) nanofibers. With fast electron/ion transport, large ion-accessible surface area and high utilization rate of active materials, the mass loading of the HPNCFs electrode can reach 48.67 mg cm−2 with a thickness of 4.46 mm. Moreover, the HPNCFs-based ZICs can yield a superior areal/gravimetric capacity of 4.13 F cm−2 /280 F g−1, superior rate capability up to 193.7 F g−1 even at 100 A g−1, high energy density of 99.6 Wh kg−1, and long-term stability for 40,000 cycles. The straightforward approach can provide an opportunity to prepare efficient thick electrodes that could be applied in electrocatalysis, energy storage/conversion, and other electrochemical energy-related techniques.

Efficient and Homogenous Precipitation of Sulfur Within a 3D Electrospun Heterocatalytic Rutile/Anatase TiO2-x Framework in Lithium–Sulfur Batteries

AbstractLithium–sulfur (Li–S) batteries can potentially outperform state-of-the-art lithium-ion batteries, but their further development is hindered by challenges, such as poor electrical conductivity of sulfur and lithium sulfide, shuttle phenomena of lithium polysulfides, and uneven distribution of solid reaction products. Herein, free-standing carbon nanofibers embedded with oxygen-deficient titanium dioxide nanoparticles (TiO2-x/CNFs) has been fabricated by a facile electrospinning method, which can support active electrode materials without the need for conductive carbon and binders. By carefully controlling the calcination temperature, a mixed phase of rutile and anatase was achieved in the TiO2-x nanoparticles. The hybridization of anatase/rutile TiO2-x and the oxygen vacancy in TiO2-x play a crucial role in enhancing the conversion kinetics of lithium polysulfides (LiPSs), mitigating the shuttle effect of LiPSs, and enhancing the overall efficiency of the Li–S battery system. Additionally, the free-standing TiO2-x/CNFs facilitate uniform deposition of reaction products during cycling, as confirmed by synchrotron X-ray imaging. As a result of these advantageous features, the TiO2-x/CNFs-based cathode demonstrates an initial specific discharge capacity of 787.4 mAh g−1 at 0.5 C in the Li–S coin cells, and a final specific discharge capacity of 584.0 mAh g−1 after 300 cycles. Furthermore, soft-packaged Li–S pouch cells were constructed using the TiO2-x/CNFs-based cathode, exhibiting excellent mechanical properties at different bending states. This study presents an innovative approach to developing free-standing sulfur host materials that are well suited for flexible Li–S batteries as well as for various other energy applications. Graphical Abstract

Improved performance of lithium–sulfur battery with a free-standing cobalt-doped vanadium nitride carbon nanofiber interlayer

Lithium–sulfur batteries (LSBs) are considered promising devices for next-generation secondary battery owing to their low cost and high energy density. However, the practical application of LSB is hindered by the sluggish redox kinetics of sulfur and the shuttling effect of polysulfide. To address the above problems, cobalt-doped vanadium nitride/three-dimensional carbon nanofiber (Co-VN/3DNC) composites were synthesized based on non-woven fabrics. The Co-VN/3DNC interlayer can serve as a physical barrier directly for the shuttle effect alleviating in LSBs. The introduction of the Co atom can enrich the d-electron number of VN, improving the catalysis of LSBs redox reaction. The growth of curved carbon nanotubes (CNTs) on the surface of the intermediate layer can expose active adsorption-catalytic sites, accelerating polysulfide redox kinetics for LSBs. The electrochemical performance of the LSB is greatly enhanced by the synergistic effect of Co, VN, and 3DNC. The LSB with Co-VN/3DNC interlayer demonstrate a capacity of 1207.7 mAh g−1 at a current density of 0.1C, and retain a specific capacity of 918.2 mAh g−1 even after 250 cycles. This work provides an effective strategy for the intermediate layers design to improve the electrochemical performance of LSBs.

Achieving a Quasi‐Solid‐State Conversion of Polysulfides via Building High Efficiency Heterostructure for Room Temperature Na–S Batteries

AbstractThe practical application of room temperature sodium–sulfur (RT Na–S) batteries are prevented by the sulfur insulation, the severe shuttling effect of high‐order sodium polysulfides (Na2Sn, 4 ≤ n ≤ 8), and the sluggish reaction kinetics. Therefore, designing an ideal host material to suppress the polysulfides shuttle process and accelerate the redox reactions of soluble NaPSs to Na2S2/Na2S is of paramount importance for RT Na–S batteries. Here, a quasi‐solid‐state transformation of NaPSs is realized by building high efficiency MoC‐W2C heterostructure in freestanding multichannel carbon nanofibers via electrospinning and calcination methods (MoC‐W2C‐MCNFs). The multichannel carbon nanofibers are interlinked micro‐mesoporous structures that can accommodate volume change of electrode materials and confine the entire redox process of NaPSs (restraining the polysulfides shuttle process). Meanwhile, the MoC‐W2C heterostructure with abundant heterointerfaces can facilitate electron/ion transport and accelerate conversion of NaPSs. Consequently, the S/MoC‐W2C‐MCNFs cathode delivers a high capacity of 640 mAh g−1 after 500 cycles at 0.2 A g−1 and an excellent reversible performance of 200 mAh g−1 after ultralong 3500 cycles at 4 A g−1. What's more, the heterostructure catalytic mechanism (a quasi‐solid‐state transformation) is proposed and confirmed in carbonate electrolyte by combining experimentally and theoretically.

ZnSe/SnSe Heterostructure Incorporated with Selenium/Nitrogen Co-Doped Carbon Nanofiber Skeleton for Sodium-Ion Batteries.

Effective strategies toward building exquisite nanostructures with enhanced structural integrity and improved reaction kinetics will carry forward the practical application of alloy-based materials as anodes in batteries. Herein, a free-standing 3D carbon nanofiber (CNF) skeleton incorporated with heterostructured binary metal selenides (ZnSe/SnSe) nanoboxes is developed for Na-ion storage anodes, which can facilitate Na+ ion migration, improve structure integrity, and enhance the electrochemical reaction kinetics. During the carbonization and selenization process, selenium/nitrogen (Se/N) is co-doped into the 3D CNF skeleton, which can improve the conductivity and wettability of the CNF matrices. More importantly, the ZnSe/SnSe heterostructures and the Se/N co-doping CNFs can have a synergistic interfacial coupling effect and built-in electric field in the heterogeneous interfaces of ZnSe/SnSe hetero-boundaries as well as the interfaces between the CNF matrix and the selenide heterostructures, which can enable fast ion/electron transport and accelerate surface/internal reaction kinetics for Na-ion storage. The ZnSe/SnSe@Se,N-CNFs exhibit superior Na-ion storage performance than the comparative ZnSe/SnSe, ZnSe and SnSe powders, which deliver an excellent rate performance (882.0, 773.6, 695.7, 634.2, and 559.0mAhg-1 at current rates of 0.1, 0.2, 0.5, 1, and 2Ag-1) and long-life cycling stability of 587.5mAhg-1 for 3500 cycles at 2Ag-1.

Designing Architected Nickel Hydroxide Cathodes for Rechargeable Alkaline Nickel–Zinc Batteries

Now that dendrite-suppressing Zn sponge anodes developed at the U.S. Naval Research Laboratory [1] offer a safer route to aqueous batteries that can power a wide range of end uses including electric vehicles, portable electronic devices, and backup energy storage, we need better cathodes. With high rate capability and deep theoretical utilization of the zinc (DOD ≥ 40%) now routine in alkaline electrolytes, storing and delivering more than one electron per metal-centered active material in the cathode is next. We focus on moving the Ni cathode past the 0.8–0.9 electrons per Ni characteristic of fabricating the cathode using β-Ni(OH)2 to tap the extra capacity inherent to α-Ni(OH)2 (theoretical: 1.67 electrons/Ni). Substituting Al3+ into α-Ni(OH)2 stabilizes the alpha phase from conversion to beta phase upon contact with and cycling in aqueous KOH and shifts positive the cell voltage for oxygen evolution (OER), the parasitic charging reaction that affects coulombic efficiency and limits cycle life [2]. We have now redesigned a classic powder composite Ni cathode (carbon black + Ni(OH)2 + binder) into an architected Ni electrode to improve charging efficacy. Using microwave-assisted deposition of Al-substituted α-Ni(OH)2 nanosheets onto a freestanding carbon nanofiber paper electrode wires the active material in 3D to the current-collecting carbon throughout the volume of the electrode. The architected Ni cathodes are tested in alkaline cells versus zinc sponge anodes and demonstrate that architected electron-wiring improves the capacity, rate, and cycle stability of Ni cathodes relative to conventional powder-composite electrodes. Varying the synthetic conditions achieves mass loadings that approach technologically relevant values. The enhanced electrochemical performance of architected Ni cathodes and architected Zn anodes provides a pathway to energy dense, safe, rechargeable Ni–Zn batteries.[1] Parker, J.F.; Chervin, C.N.; Pala, I.R.; Machler, M.; Burz, M.F.; Long, J.W.; Rolison, D.R. Rechargeable Nickel–3D Zinc Batteries: An Energy-Dense, Safer Alternative to Lithium-Ion. Science 2017, 356, 415–418 (10.1126/science.aak9991).[2] Kimmel, S.W.; Hopkins,B.J.; Chervin, C.N.; Skeele, N.L.; Ko, J.S.; DeBlock, R.H.; Long, J.W.; Parker, J.F.; Hudak, B.M.; Stroud, R.M.; Rolison, D.R.;Rhodes, C.P.Capacity and Phase Stability of Metal-Substituted α-Ni(OH)₂ Nanosheets in Aqueous Ni–Zn Batteries. Mater. Adv. 2021, 2, 3060–3074 (10.1039/D1MA00080B).

Hydrothermal growth of FeMoO4 nanosheets on electrospun carbon nanofibers as freestanding supercapacitor electrodes

In this study, an electrospinning method was used to produce highly conductive freestanding carbon nanofibers (CNFs). The freestanding CNFs are compatible with hierarchical growth techniques, such as hydrothermal processes, for the nanostructure engineering of supercapacitor electrodes with enhanced electrochemically active sites. Therefore, the flower-like FeMoO4@CNF nanosheets were investigated to improve the electrochemical performance using aqueous and neutral electrolytes (Na2SO4 and K2SO4). The optimized FeMoO4@CNF electrode exhibits areal capacitances of 252 and 220 mF·cm−2 at a high current density of 2.5 mA·cm−2 with Na2SO4 and K2SO4 electrolytes, respectively. The wide potential window (1.6 V) of the symmetric supercapacitor delivered maximum energy densities of 22.4 and 19.6 μWh·cm−2 at a power density of 2 mW·cm−2 for Na2SO4 and K2SO4 electrolytes, respectively.

Effects of different fabrication conditions on the performance of free-standing carbon nanofiber cathodes in lithium–oxygen batteries

Effects of different fabrication conditions on the performance of free-standing carbon nanofiber cathodes in lithium–oxygen batteries

Freestanding Carbon Nanofibers Derived from Biopolymer (Kraft Lignin) as Ultra-Microporous Electrodes for Supercapacitors

Freestanding Carbon Nanofibers Derived from Biopolymer (Kraft Lignin) as Ultra-Microporous Electrodes for Supercapacitors

Graphene sheets assembled into three-dimensional networks of carbon nanofibers: A nano-engineering approach for binder-free supercapacitor electrodes

The design and synthesis of flexible and free-standing carbon nanofibers membrane with good electrochemical properties have recently attracted much attention in energy storage applications. Herein, we fabricated graphene sheets-incorporated three-dimensional carbon nanofibers membrane (3D-GCNFs) as effective electrode materials for supercapacitors. The electrospun two-dimensional (2D) graphene oxide sheets-incorporated polyacrylonitrile nanofibers (GO/PAN NFs) were transformed into 3D membrane by a gas-forming technique using sodium borohydride. Subsequent thermal treatment converted the 2D-assembled GO/PAN NFs membrane into the low-density 3D-GCNFs membrane. The morphology and electrochemical performances of the as-prepared 3D membrane were systematically investigated using state-of-the-art techniques. It was found that the electrochemical performance of the as-obtained 3D-GCNFs membrane was greatly improved compared to the 2D counterparts. As a binder-free electrode, the materials exhibited a high areal capacitance of 318 mF/cm2 at a current density of 1 mA/cm2 and good cyclic stability of up to 98.5% after 5000 cycles. In addition, a symmetrical supercapacitor was constructed using the 3D-GCNFs membrane as both negative and positive electrodes. The device exhibited areal capacitance of 240 mF/cm2 at a current density of 1 mA/cm2, an energy density of 33.33 mWh/cm2, a power density of 1000 mW/cm2, and capacitance retention of 103% after 5000 cycles. The improved performance of the 3D-GCNFs membrane was attributed to its high electrical conductivity due to the absence of binders, high porosity, and conductive networks that form efficient channels for electrolyte penetration and effectively shorten the transmission distance. We believe that this study not only provides a reference for the introduction of 3D CNFs-based composite membranes, but also demonstrates their great potential for energy storage applications.

Cytomembrane-inspired engineering of oxygen transfer channel for modulating Ni/NiO heterostructure and boosting water oxidation

Development of low-cost and efficient non-precious metal electrocatalysts for oxygen evolution reaction (OER) is of predominant importance for promoting the large-scale application of water splitting. Herein, we use ZIF-8 as a sacrifice template, and successfully prepare the freestanding N-doped carbon nanofibers that decorated with Ni/NiO nanoparticles (denoted as xNi/NiO-NCNFs) via a scalable electrospinning-pyrolysis strategy. Inspired by the cytomembrane, we tune the dosage of ZIF-8 (x) to optimize the amount of micropores on the carbon layer that encapsulating the Ni species, which could act as oxygen transfer channels and enable suitable amount of oxygen to input to modulate the oxidation state of Ni. Moreover, the porous structure could facilitate the mass transfer of the reactants and contact with the active sites, as well as provide more channels for oxygen to release during the OER process. Benefiting from the synergistic effect of the Ni/NiO active sites, porous structure of carbon shell, and the doped N atoms, the 0.075Ni/NiO-NCNFs presents an ultralow overpotential of 210 mV to achieve a current density of 20 mA cm−2 in 0.1 M KOH solution and a low Tafel slope of 29.6 mV dec-1. This work provides a simple and cost-effective pathway to develop porous free-standing non-noble metal-based catalyst with high OER efficiency for potential sustainable electrochemical energy conversion.

Rational designed Cu-MOF@1D carbon nanofibers as free-standing and flexible electrode for robust electrochemical energy storage

The present study reported the performance of free-standing carbon nanofibers decorated with copper-metal-organic frameworks (Cu-MOFs). A green electrospinning technique was utilized to obtain nanofiber sheets that were carbonized and activated to produce activated carbon nanofiber sheets (ACNFs). These ACNFs consist of inherent oxygen functionalities of the lignin that can aid the growth of MOF structure on it in a single step, which motivated us to do this work. Cu-MOF particles were in-situ grown over the surface of ACNFs using a hydrothermal-assisted technique. The developed materials were characterized using FESEM, Raman Spectroscopy and BET analysis in order to identify morphological, porosity, and surface area transformations. Along with these, Cyclic Voltammetry (CV), Electrochemical Impedance Spectroscopy (EIS), and Galvanostatic Charge-Discharge (GCD) studies were performed to compare the electrochemical performance of the Cu-MOF, ACNF, and Cu-MOF@ACNF as electrode materials for supercapacitors. Among three electrodes, the Cu-MOF@ACNF sheet reveals a higher specific capacitance of 303.2 F/g compared to ACNF (203.3 F/g) and Cu-MOF (68.2 F/g) at the same current density of 1 A/g. Based on the higher performance of Cu-MOF@ACNF two types of symmetrical devices, i.e., aqueous (using electrolyte-impregnated filter paper) and solid-state devices (using a polymeric gel electrolyte), were fabricated. The all-solid-state supercapacitor device was found to feature high flexibility, stand-alone characteristics, and excellent performance as an electrode with a working potential window of 0–2.0 V. The device demonstrates a high energy density of 78.71 Wh/kg and a power density of 1050.0 W/kg, along with high coulombic efficiency (99.8 %) after 10,000 cycles. The achieved performance is superior to many published reports, revealing that the developed composite materials hold prospect for applications in green and high-performance flexible energy storage devices.

MOFs-derived niobium oxide nanoparticles/carbon nanofiber hybrid paper as flexible binder-free electrode for solid-state asymmetric supercapacitors

For the first time, a bottom-up approach was adopted to prepare free-standing orthorhombic Nb2O5-anchored carbon nanofibers (Nb2O5 @CNFs) hybrid paper via in-situ solvothermal synthesis of Nb-metal organic framework@CNFs hybrid and subsequent pyrolysis process. The nanocrystalline Nb2O5 decorated on the CNF surfaces improved the charge storage performance through synergistic effect between porous CNFs (EDLC) and Nb2O5 (pseudocapacitance). Moreover, the high surface area and core-shell structure of Nb2O5 @CNF hybrid reduce diffusion limitations and facilitate ionic transport. The Nb2O5 @CNF hybrid paper as a binder-free electrode delivered greater specific capacitance of 633.6 F/g (975.7 F/cm3) in 1 M H2SO4, compared to Li2SO4 electrolyte (524.3 F/g) at 0.5 A/g. However, the Nb2O5 @CNF electrode showed better cycling stability in neutral Li2SO4 electrolyte than in acidic H2SO4 electrolyte. The solid-state asymmetric supercapacitor (ASC) based on Nb2O5 @CNF paper and activated carbon (AC) electrodes delivered higher energy density of 62.1 Wh kg−1 at 292.7 W kg−1 in Li2SO4 electrolyte due to a wider working potential window (0–1.5 V), compared to H2SO4 electrolyte (55 Wh kg−1). The Li2SO4-based ASC cell is also capable of producing an energy density of up to 45.6 Wh kg−1, even at a high-power density of up to 5853.6 W kg−1. The as-assembled ASC demonstrated low self-discharge rate as well as excellent bending stability, indicating its excellent prospects in real-life applications.

Mo2 N-ZrO2 Heterostructure Engineering in Freestanding Carbon Nanofibers for Upgrading Cycling Stability and Energy Efficiency of Li-CO2 Batteries.

Li-CO2 batteries have attracted considerable attention for their advantages of CO2 fixation and high energy density. However, the sluggish dynamics of CO2 reduction/evolution reactions restrict the practical application of Li-CO2 batteries. Herein, a dual-functional Mo2 N-ZrO2 heterostructure engineering in conductive freestanding carbon nanofibers (Mo2 N-ZrO2 @NCNF) is reported. The integration of Mo2 N-ZrO2 heterostructure in porous carbons provides the opportunity to simultaneously accelerate electron transport, boost CO2 conversion, and stabilize intermediate discharge product Li2 C2 O4 . Benefiting from the synchronous advantages, the Mo2 N-ZrO2 @NCNF catalyst endows the Li-CO2 batteries with excellent cycle stability, good rate capability, and high energy efficiency even under high current densities. The designed cathodes exhibit an ultrahigh energy efficiency of 89.8% and a low charging voltage below 3.3V with a potential gap of 0.32V. Remarkably, stable operation over 400 cycles can be achieved even at high current densities of 50 µA cm-2 . This work provides valuable guidance for developing multifunctional heterostructured catalysts to upgrade longevity and energy efficiency of Li-CO2 batteries.

Near-perfect suppression of Li dendrite growth by novel porous hollow carbon fibers embedded with ZnO nanoparticles as stable and efficient anode for Li metal batteries

For the practical application of next-generation Li metal batteries (LMBs), a Li metal anode with high safety and efficiency is essential. However, LMBs still suffer from the problems caused by the growth of Li dendrites at the Li metal anode. In this study, we introduce a novel way that could dramatically suppress the growth of Li dendrites and improve the performance of LMBs. A free-standing porous hollow carbon nanofiber embedded with lithiophilic ZnO nanoparticles (P-HCNF@ZnO) is fabricated using a dual-nozzle electrospinning technique followed by carbonization. The nano-sized pores formed in the shell of hollow carbon fiber provide passages for Li ions to penetrate into the core space of the hollow fiber, and the lithiophilic ZnO particles play a decisive role in inducing and plating Li ions inside the core efficiently and uniformly. Therefore, Li ions are mostly electroplated/stripped on the internal surface of the porous hollow fibers and dendrites are rarely formed on their exterior surface even under fast lithiation conditions, while numerous Li dendrites are formed on the exterior surface of the non-porous hollow fiber electrode (HCNF@ZnO). As a result, the P-HCNF@ZnO electrode exhibits a very low over-potential of about 87 mV, a stable capacity retention of about 95% at a high current density of 1.0 mA cm−2, and a much higher performance than HCNF@ZnO in a symmetric cell test. Furthermore, in full cell tests, the LiFePO4 (LFP) battery made of lithiated P-HCNF@ZnO anode exhibits a high capacity of 175.3 mAh g−1 and a lower over-potential by 0.29 V than the reference sample. Analytical works and theoretical interpretations, elucidated that the Li metal anode made of the P-HCNF@ZnO significantly improves the performance of LMBs.

TiC nanoparticles supported on free-standing carbon nanofibers enabled high-performance Lithium–Sulfur batteries

Lithium–sulfur (Li–S) batteries hold great promise for substituting current energy-storage technologies owing to their exceptional advantage in energy density. The main challenge in developing practical Li–S batteries is the lack of efficient sulfur host which can simultaneously suppress the shuttle effect and improve the redox kinetics. Polar host materials manifest chemisorptive properties for localizing the mobile polysulfide intermediates, being promising in inhibiting the shuttle effect; however, their poor intrinsic conductivity hinders their role in enhancing the redox kinetics of subsequent conversion reactions. In this regard, a conductive polar host material is highly desirable for efficient and long-life Li–S batteries. Herein, we design and develop a free-standing sulfur host consisted of carbon nanofibers decorated with titanium carbides nanoparticles (TiC/CNFs) for high performance Li–S batteries. Benefiting from the intrinsic chemical polarity and high electric conductivity of TiC, the chemisorption and conversion kinetics of lithium polysulfides are simultaneously promoted, leading to the greatly enhanced battery performance. For example, the S@TiC/CNFs composite cathode retains a high capacity of 672 mA h g−1 after 1000 cycles and an excellent rate performance of 938 mA h g−1 at 5 C. Impressively, even at a super high sulfur loading of 7 mg cm−2, the TiC/CNFs can retain a capacity of 5 mA h (630 mA h g−1) after 100 cycles, highlighting the advantages of the polar conductive sulfur host design for efficient conversion of lithium polysulfides.

Free-Standing Carbon Nanofiber Composite Networks Derived from Bacterial Cellulose and Polypyrrole for Ultrastable Potassium-Ion Batteries.

Carbon materials derived from bacterial cellulose have been studied in lithium-ion batteries due to their low cost and flexible characteristics. However, they still face many intractable problems such as low specific capacity and poor electrical conductivity. Herein, bacterial cellulose is used as the carrier and skeleton to creatively realize the composite of polypyrrole on its nanofiber surface. After carbonization treatment, three-dimensional carbon network composites with a porous structure and short-range ordered carbon are obtained for potassium-ion batteries. The introduction of nitrogen doping from polypyrrole can increase the electrical conductivity of carbon composites and provide abundant active sites, improving the comprehensive performance of anode materials. The carbonized bacterial cellulose@polypyrrole (C-BC@PPy) anode exhibits a high capacity of 248 mA h g-1 after 100 cycles at 50 mA g-1 and a capacity retention of 176 mA h g-1 even over 2000 cycles at 500 mA g-1. Combined with density functional theory calculations, these results indicate that the capacity of C-BC@PPy is contributed by N-doped and defect carbon composite materials as well as pseudocapacitance. This study provides a guideline for the development of novel bacterial cellulose composites in the energy storage field.

Bimetallic CoMoO4 Nanosheets on Freestanding Nanofiber as Wearable Supercapacitors with Long-Term Stability

Currently, lightweight wearable energy storage devices are in great demand owing to their use in wearable electronics and energy-efficient electric vehicles. Freestanding carbon nanofibers replace the need for metal substrates while providing a rapid electrical network owing to their excellent electrical properties. Bimetallic oxides with multivalent oxidation states facilitate the rapid transfer of electrolytic ions owing to efficient Faradaic reactions, thereby enhancing the overall energy storage capability. In this study, CoOx@CNF was derived from ZIF-67 (zeolitic-imidazolate framework) @PAN-2MI fibers that were stabilized in air at 280°C and then annealed in argon at 900°C. Subsequently, Co was seeded on the annealed CoOx@CNF and subjected to a hydrothermal process in sodium molybdate dihydrate solution to grow CoMoO4 nanosheets, eventually forming bimetallic CoMoO4@CNF. The concentration of sodium molybdate solution was varied to determine the optimal growth conditions for CoMoO4 nanosheets. The energy density of the optimal bimetallic CoMoO4@CNF sample was 166.5 μWh cm-2 at a power density of 200 μW cm-2; this represented a nearly twofold increase compared to that of the single metallic CoOx@CNF. Powering humidity sensors using only one CoMoO4@CNF supercapacitor was demonstrated. The optimal sample remained stable during long-term galvanostatic charge and discharge cycles ( N cyc = 30,000 ) and retained 100% of its specific capacitance.