Ti_(3)C_(2)T_(x) MXene/carbon composites for advanced supercapacitors:Synthesis,progress,and perspectives

MXenes are a family of two-dimensional(2D)layered transition metal carbides/nitrides that show promising potential for energy storage applications due to their high-specific surface areas,excellent electron conductivity,good hydrophilicity,and tunable terminations.Among various types of MXenes,Ti_(3)C_(2)T_(x) is the most widely studied for use in capacitive energy storage applications,especially in supercapacitors(SCs).However,the stacking and oxidation of MXene sheets inevitably lead to a significant loss of electrochemically active sites.To overcome such challenges,carbon materials are frequently incorporated into MXenes to enhance their electrochemical properties.This review introduces the common strategies used for synthesizing Ti_(3)C_(2)T_(x),followed by a comprehensive overview of recent developments in Ti_(3)C_(2)T_(x)/carbon composites as electrode materials for SCs.Ti_(3)C_(2)T_(x)/carbon composites are categorized based on the dimensions of carbons,including 0D carbon dots,1D carbon nanotubes and fibers,2D graphene,and 3D carbon materials(activated carbon,polymer-derived carbon,etc.).Finally,this review also provides a perspective on developing novel MXenes/carbon composites as electrodes for application in SCs.

Recent Research Progress of Paper-Based Supercapacitors Based on Cellulose

In recent years,paper-based functional materials have received extensive attention in the field of energy storage due to their advantages of rich and adjustable porous network structure and good flexibility.As an important energy storage device,paper-based supercapacitors have important application prospects in many fields and have also received extensive attention from researchers in recent years.At present,researchers have modified and regulated paper-based materials by different means such as structural design and material composition to enhance their electrochemical storage capacity.The development of paper-based supercapacitors provides an important direction for the development of green and sustainable energy.Therefore,it is of great significance to summarize the relevant work of paper-based supercapacitors for their rapid development and application.In this review,the recent research progress of paper-based supercapacitors based on cellulose was summarized in terms of various cellulose-based composites,preparation skills,and electrochemical performance.Finally,some opinions on the problems in the development of this field and the future development trend were proposed.It is hoped that this review can provide valuable references and ideas for the rapid development of paper-based energy storage devices.

Facile Fabrication of Cellulosic Paper-based Composites with Temperature-controlled Hydrophobicity and Excellent Mechanical Strength

In this paper,we presented a novel strategy to employ a plantderived carbohydrate polymer,i.e.,cellulose,to prepare a hydrophobic composite.Cellulose was used as a scaffold,and ethylene-propylene side by side(ES)fiber was thermally melted and then coated on the cellulose surface to achieve hydrophobicity.Experimental results revealed that the thermocoating ES fibers greatly increased the water contact angle of the cellulose scaffold from 25°to 153°while simultaneously enhanced the wet tensile strength of the composite approximately 6.7-fold(drying temperature of 170℃)compared with the pure cellulose paper.In particular,compared with other related research,the prepared cellulose-based composite possessed excellent hydrophobicity and superior mechanical strength,which introduces a new chemical engineering approach to prepare hydrophobic cellulose-based functional materials.

In Situ Directional Polymerization of Poly(1,3-dioxolane)Solid Electrolyte Induced by Cellulose Paper-Based Composite Separator for Lithium Metal Batteries

In traditional in situ polymerization preparation for solid-state electrolytes,initiators are directly added to the liquid precursor.In this article,a novel cellulose paper-based composite separator is fabricated,which employs alumina as the inorganic reinforcing material and is loaded with polymerization initiator aluminum trifluoromethanesulfonate.Based upon this,a separator-induced in situ directional polymerization technique is demonstrated,and the extra addition of initiators into liquid precursors is no longer required.The polymerization starts from the surface and interior of the separator and extends outward with the gradually dissolving of initiators into the precursor.Compared with its traditional counterpart,the separator-induced poly(1,3-dioxolane)electrolyte shows improved interfacial contact as well as appropriately mitigated polymerization rate,which are conducive to practical applications.Electrochemical measurement results show that the prepared poly(1,3-dioxolane)solid electrolyte possesses an oxidation potential up to 4.4 V and a high Li+transference number of 0.72.After 1000 cycles at 2 C rate(340 mA g^(−1)),the assembled Li||LiFePO_(4)solid battery possesses a 106.8 mAh g^(−1)discharge capacity retention and 83.5%capacity retention ratio,with high average Coulombic efficiency of 99.5%achieved.Our work may provide new ideas for the design and application of in situ polymerization technique for solid electrolytes and solid batteries.

Carbon nanomaterials and their composites for supercapacitors

As a type of energy storage device between traditional capacitors and batteries,the supercapacitor has the advantages of energy saving and environmental protection,high power density,fast charging and discharging speed,long cycle life,and so forth.One of the key factors affecting the performance of supercapacitor is the electrode material.Carbon materials,such as carbon nanotube,graphene,activated carbon,and carbon nanocage,are most widely concerned in the application of supercapacitors.The synergistic effect of composites can often obtain excellent results,which is one of the common strategies to increase the electrochemical performance of supercapacitors.To further improve the performance of binary composites,it is a relatively simple method to increase the components as the“bridge”between the two materials to form the ternary composites.The review mainly introduces the current research progress of supercapacitors with pure carbon nanomaterials and multistage carbon nanostructures(composites)as electrodes.The characteristics and application directions of different pure carbon nanomaterials are introduced in detail.Different ways of multilevel structure(material)composite have their own effects on the development of high-performance supercapacitors.We also highlight the recent advances related to these fields and provide our insight into high-energy supercapacitors.

High energy density performance of hydrothermally produced hydrous ruthenium oxide/multiwalled carbon nanotubes composite: Design of an asymmetric supercapacitor with excellent cycle life

Hydrous ruthenium oxide(h-Ru O) nanoparticles and its composite with multiwalled carbon nanotubes(h-Ru O/MWCNT) were synthesized by a simple hydrothermal method and proved to have potential application as hybrid supercapacitor material.The h-Ru Oand h-Ru O/MWCNT were characterized for their physico-chemical properties by PXRD,BET surface area,Raman,SEM-EDS and TEM techniques.The electrochemical performance of the materials were investigated,specific capacitance(Cs) of h-Ru Oand hRu O/MWCNT estimated by their cyclic voltammetric studies were found to be 604 and 1585 F/g respectively at a scan rate of 2 m V/s in the potential range 0–1.2 V.Further,this value was found to be nearly three times higher than that of pure h-Ru O.An asymmetric supercapacitor(AS) device was fabricated by employing h-Ru O/MWCNT as the positive electrode and activated carbon as the negative electrode.The device exhibited Cs of 61.8 F/g at a scan rate of 2 m V/s.Further,the device showed excellent long term stability for 20,000 cycles with 88% capacitance retention at a high current density of 25 A/g.

A dandelion-like carbon microsphere/MnO_2 nanosheets composite for supercapacitors

This article reported the electrochemical performance of a novel cabon microsphere/MnO2nanosheets(CMS/MnO2) composite prepared by a in situ self-limiting deposition method under hydrothermal condition. The results of scanning electron microscopy(SEM) and transmission electron microscopy(TEM) revealed that MnO2nanosheets homogeneously grew onto the surface of CMS to form a loose-packed and dandelion-like core/shell microstructure. The unique microstructure plays a basic role in electrochemical accessibility of electrolyte to MnO2active material and a fast diffusion rate within the redox phase. The results of cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectrometry indicated that the prepared CMS/MnO2composite presented high capacitance of 181 F g-1and long cycle life of 61% capacity retention after 2000 charge/discharge cycles in 1 mol/L Na2SO4solution, which show strong promise for high-rate electrochemical capacitive energy storage applications.

Ordered Mesoporous Carbon/SnO_2 Composites as the Electrode Material for Supercapacitors

A series of composites as electrode materials for supercapacitors were prepared via incipient wetness impregnation method utilizing ordered mesoporous carbon （OMC） and tin （IV） oxide （SnO2） with different ratio.The structure and electrochemical properties of the OMC/SnO2 composites were characterized by XRD,TEM and cyclic voltammetry （CV）.Pore characteristics were measured by nitrogen adsorption and desorption isotherms.The results show that the structure and electrochemical properties of the composites depend mainly on the loading amount of SnO2 in the ordered mesoporous carbon.The optimum amount of SnCl4 added is found to be 40 % （1.54 g ethanol-based SnCl4·5H2O added to 1 g OMC） of the saturated solution.The specific capacitance of the composite of optimum amount of SnCl4 （200 F g-1） is nearly three times of that of the pristine SnO2 （72 F g-1） at the scan rate of 5 mV s-1,and its specific capacitance is almost equal to that of the ordered mesoporous carbon （126 F g-1） at the scan rate of 200 mV s-1.Meanwhile,it has better specific volumetric energy density than OMC due to its higher density.Besides,in the potential range of 0-0.9 V the composite electrode material exhibits a stable cycle life after 500 cycles.

Graphene/polyaniline composite sponge of three-dimensional porous network structure as supercapacitor electrode

As a supercapacitor electrode, the graphene/polyaniline （PANI） composite sponge with a three-dimensional （3D） porous network structure is synthesized by a simple three-step method. The three steps include an in situ polymerization, freeze-drying and reduction by hydrazine vapor. The prepared sponge has a large specific surface area and porous network structure, so it is in favor of spreading the electrolyte ion and increasing the charge transfer efficiency of the system. The process of preparation is simple, easy to operate and low cost. The composite sponge shows better electrochemical performance than the pure individual graphene sponge while PANI cannot keep the shape of a sponge. Such a composite sponge exhibits specific capacitances of 487 F.g-1 at 2 mV/s compared to pristine PANI of 397 F.g-1.

Nickel Sulfide/Graphene/Carbon Nanotube Composites as Electrode Material for the Supercapacitor Application in the Sea Flashing Signal System

This work presents NiS/graphene/carbon nanotube （NiS/GNS/CNT） composites as electrode material for the supercapacitor application in sea flashing signal systems. NiS nanosheets were closely anchored on the conductive GNS-CNT networks. As a result, the NiS/GNS/CNT electrode showed a high specific capacitance of 2 377 F.g^-1 at 2 mV.s^-1 and good cycling stability compared with the pure NiS （1 599F.g^-1）. The enhanced electrochemical performances are attributed to the synergetic effect between the conductive carbon and the pseudo-capacitive NiS. The high performance supercapacitor may provide application in the sea flashing signal system.

Hollow Carbon Microspheres/MnO_2 Nanosheets Composites:Hydrothermal Synthesis and Electrochemical Behaviors

This article reported the electrochemical behaviors of a novel hollow carbon microspheres/manganese dioxide nanosheets(micro-HC/nano-MnO2) composite prepared by an in situ self-limiting deposition method under hydrothermal condition. The results of scanning electron microscopy reveal that MnO2 nanosheets homogeneously grow onto the surface of micro-HC to form a loose-packed microstructure. The quantity of MnO2 required in the electrode layer has thereby been reduced significantly, and higher specific capacitances have been achieved. The micro-HC/nano-MnO2 electrode presents a high capacitance of 239.0 F g-1 at a current density of 5 m A cm-2, which is a strong promise for high-rate electrochemical capacitive energy storage applications.

Preparation and electrochemical characterization of C/PANI composite electrode materials

Taking the nano-sized carbon black and aniline monomer as precursor and （NH4）2S2O6 as oxidant, the well coated C/polyaniline（C/PANI） composite materials were prepared by in situ polymerization of the aniline on the surface of well-dispersed nano-sized carbon black for supercapacitor. The micro-structure of the C/PANI composite electrode materials were analyzed by SEM. The electrochemical properties of C/ PANI and PANI composite electrode were characterized by means of the galvanostatic charge-discharge experiment, cyclic voltammetric measurement and impedance spectroscopy analysis. The results show that by adding the nano-sized carbon black in the process of chemical polymerization of the aniline, the polyaniline can be in situ polymerized and well-coated onto the carbon black particles, which may effectively improve the aggregation of particles and the electrolyte penetration. What’s more , the maximum of specific capacitance of C/PANI electrode 437.6F·g -1 can be attained. Compared with PANI electrode, C/PANI electrode shows more desired capacitance characteristics, smaller internal resistance and better cycle performance.

Preparation and Characterization of Co_(3)O_(4)/Graphene/Cellulose Nanofiber Composite Films

Nanocellulose has served as an eye-catching nanomaterial for constructing advanced functional devices with renewability,light weight,flexibility,and environmental friendliness.In this study,Co_(3)O_(4)/graphene/cellulose nanofiber(CNF)flexible composite films,in which the CNF acted as a spacer for the graphene,were prepared via a facile and scalable vacuum filtration method.The effects of the CNF on the microstructure,hydrophilicity,thermal stability,tensile strength,surface resistance,and electrochemical performance of the Co_(3)O_(4)/graphene/CNF composite films were systematically investigated.The results showed that the synergistic interaction of the CNF and graphene substantially improved the overall properties of the Co_(3)O_(4)/graphene/CNF composite films,particularly their hydrophilicity and tensile strength.Meanwhile,Co_(3)O_(4)/graphene/CNF composite films with a CNF content of 4%appeared to have the optimal electrochemical performance,with an area specific capacitance of 56 mF/cm^(2) and prominent capacitance retention of 95.6%at a current density of 1 A/g after 1000 cycles.This work demonstrated that the prepared Co_(3)O_(4)/graphene/CNF flexible composite films have great application potential in the field of flexible energy storage devices.

MOF(ZM)/Potassium Citrate-Derived Composite Porous Carbon and Its Electrochemical Properties

Metal-organic frameworks are compounds with a reticulated skeletal structure formed by chemically bonding inorganic and organic units that are widely used in many fields, such as photocatalysis, gas separation and energy storage, because of their unique structures. In this paper, we prepared a metal-organic framework [(μ2-2-methylimidazolyl)12-Zn(ii)6-H18O10]n(ZM) with well-developed pores and high specific surface area of MOFs by the solution method. And MOF-derived porous carbon was prepared by the direct charring method in an argon atmosphere using a mixture of ZM, ZM and potassium citrate as carbon precursors. Characterization analysis revealed that the maximum specific surface area of ZMPC-800-1:15 was 2014.97 m2⋅g−1, and the pore size structure was mainly mesoporous. At a current density of 1.0 A⋅g−1 the specific capacitance of ZMC-800 and ZMPC-800-1:15 was 121.3 F⋅g−1 and 226.6 F⋅g−1, respectively, with a substantial increase of 86.8%. The specific capacitance of ZMPC-800-1:15 decays to 168.8 F⋅g−1, with a decay rate of 25.5%, when the current density increases to 10.0 A⋅g−1. After 5000 constant current charge/ discharge cycles, the capacitance retention rate was still 96.41%. These results prove that the application of MOF-derived carbon materials in future supercapacitors is very promising.

A novel method for extracting and optimizing the complex permittivity of paper-based composites based on an artificial neural network model

Measuring the complex permittivity of ultrathin,flexible materials with a high loss tangent poses a substantial challenge with precision using conventional methods,and verifying the accuracy of test results remains difficult.In this study,we introduce a methodology based on a back-propagation artificial neural network(ANN)to extract the complex permittivity of paper-based composites(PBCs).PBCs are ultrathin and flexible materials exhibiting considerable complex permittivity and dielectric loss tangent.Given the absence of mature measurement methods for PBCs and a lack of sufficient data for ANN training,a mapping relationship is initially established between the complex permittivity of honeycomb-structured microwave-absorbing materials(HMAMs,composed of PBCs)and that of PBCs using simulated data.Leveraging the ANN model,the complex permittivity of PBCs can be extracted from that of HMAMs obtained using standard measurement.Subsequently,two published methods are cited to illustrate the accuracy and advancement of the results obtained using the proposed approach.Additionally,specific error analysis is conducted,attributing discrepancies to the conductivity of PBCs,the homogenization of HMAMs,and differences between the simulation model and actual objects.Finally,the proposed method is applied to optimize the cell length parameters of HMAMs for enhanced absorption performance.The conclusion discusses further improvements and areas for extended research.

Chinese ink-facilitated fabrication of paper-based composites as electrodes for supercapacitors

Commercial Chinese ink was employed to disperse pristine vapor-grown carbon nanofibers(VGCNFs)in aqueous suspensions via horizontal ball milling.The obtained suspension was used to fabricate conductive paper-based composites through filtration-deposition onto filter paper.It was found that the carbon black particles from the Chinese ink helped separate VGCNFs and acted as connection points between the VGCNFs,while the glue reinforced the conduction network.Thus,the VGCNF-ink/paper ternary composite showed sufficiently low sheet resistance.With merely 2.5 mg·cm^(−2)VGCNFs,the sheet resistance could be reduced to 4.5Ω·sq^(−1).As a proof of concept,these paper-based composites were directly used as electrodes of solid-state symmetric electronic double-layer capacitors(EDLCs)and the substrate for the electrodeposition of MnO_(2)to achieve higher electrochemical performances.The EDLCs fabricated with 2.5 mg·cm^(−2)VGCNFs showed a specific capacitance of 224 mF·cm^(−2)at a current density of 1 mA·cm^(−2),which was retained by 86.4%after 10,000 charge-discharge cycles.Moreover,thanks to the high electrical conductivity and the porous structure,the MnO_(2)decorated paper-based composites exhibited dramatically enhanced specific capacitance.It is believed that our finding offers an idea to directly utilize commercial Chinese ink for the fabrication of electrode materials.

Synthesis and characterization of porous cobalt oxide/copper oxide nanoplate as novel electrode material for supercapacitors

A promising Co3O4/Cu O composite electrode material was successfully synthesized through a facile hydrothermal and calcination process. Effects of the surfactants hexadecyltrimethyl ammonium bromide（CTAB） and polyvinylpyrrolidone（PVP） on the morphology and electrochemical performance of the composite were investigated. Powder X-ray diffraction（XRD）, scanning electron microscopy（SEM）, transmission electron microscopy（TEM） and nitrogen adsorption-desorption experiment were employed to characterize the microstructures and morphologies of the composite. Meanwhile, the electrochemical performances of the samples were studied using cyclic voltammetry（CV）, galvanostatic charge-discharge test and electrochemical impedance spectroscopy（EIS）. The results show that the porous Co3O4/Cu O-CTAB nanoplates own the best performance and exhibits a high specific capacitance of 398 F/g at 1 A/g with almost 100% capacitance retention over 2000 cycles, and it retains 90% of capacitance at 10 A/g.

High areal capacitance of Fe3O4-decorated carbon nanotubes for supercapacitor electrodes

A conceptually new approach has been developed for the fabrication of magnetite(Fe3O4)-decorated carbon nanotubes(M-CNTs)for negative electrodes of electrochemical supercapacitors.M-CNTs were prepared by an ultrasonic-assisted chemical synthesis method,which involved dispersion of functionalized CNTs in water,Fe3O4 formation on the CNTs surface,and particle extraction through liquid-liquid interface(PELLI).Palmitic acid was found to be an efficient new extractor for PELLI.The slurries produced after drying and redispersing M-CNTs and slurries obtained using PELLI were used for electrode fabrication.The electrodes prepared using PELLI showed superior performance due to reduced particle agglomeration.Testing results provided an insight into the influence of Fe3O4/CNTs mass ratio on the capacitance and capacitance retention at high charge-discharge rates.A capacitance of 5.82 F cm−2(145.4 F g−1)was achieved in Na2SO4 electrolyte using electrodes with high active mass of 40 mg cm−2 and ratio of active mass to current collector mass of 0.6.Good electrochemical performance was achieved at low impedance.The capacitance of the negative M-CNTs electrodes was comparable with capacitance of advanced positive MnO2-CNTs electrodes,which was beneficial for the fabrication of asymmetric devices.The asymmetric device has been fabricated,which showed promising performance in a voltage window of 1.6 V.

Flexible and disposable paper-based gas sensor using reduced graphene oxide/chitosan composite

Nitrogen dioxide(NO_(2))is a representative toxicant in air pollution that mostly arises from the exhaust gas released by automobiles.It is related to various respiratory diseases such as pneumonia and sudden infant death syndrome.Additionally,because the toxicity of nitrogen dioxide is high in overpopulated areas(i.e.,a capital or metropolis),the development of simple,practical,and facile sensors is highly needed.This work presents a flexible and disposable paper-based NO_(2)sensor based on a reduced graphene oxide/chitosan(r GO/CS)composite.The synthesized r GO/CS composite can be easily flexed and deformed into various shapes,which are attributed to chitosan molecules that function as a dispersion and reduction agent and support material.In addition,this composite can be attached to paper owing to its adhesive property;hence it can be utilized in versatile applications in a disposable manner.By analyzing the conductive change of the r GO/CS composite when it reacts with NO_(2),we can detect nitrogen dioxide in a concentration range of 0–100 ppm with a detection limit of 1 ppm.Moreover,we performed NO_(2)detection in the exhaust gas released by automobiles using the r GO/CS composite for practical application.The results indicated that the r GO/CS composite has the potential to be used in feasible gas sensing as a facile and disposable sensor under various conditions.

The synthesis of carbon microspheres film composed of nano-onions and its application as flexible supercapacitors

Flexible electrodes with superior mechanical and electrochemical properties are essential for flexible supercapacitors.A convenient and scalable colloidal film-assisted chemical vapor deposition(CF-CVD)method is developed for the one-step fabrication of the carbon microspheres films composed of carbon nano-onions(CMS-CNO films).The influence of growth conditions(such as growth temperature,time,and gas ratio)during CF-CVD process on the carbon structures and the growth mechanism of the CMS-CNO films have been investigated.By controlling the growth conditions,the controllable preparation of CMS-CNO films is realized.Such binder-free films can be used for the assembly of flexible supercapacitors,and unique architecture can achieve excellent performance.Benefitting from the composite of nano-micro zero dimensional structures,the performance of the film in supercapacitors is remarkably improved.At the current density of 5 mA cm^(-2),the area-specific capacity can be 903 mF cm^(-2).When the current density is increased to 500 mA cm^(-2),the area-specific capacity can be increased to 729 mF cm^(-2).This simple and low-cost preparation process and the superb electrochemical performance suggest great potential applications of CMS-CNO films in flexible supercapacitors.