The key challenges and future opportunities of electrochemical capacitors

Electrochemical capacitors(ECs)with unique merits of fast charge/discharge rate and long cyclability are one of the representative electrochemical energy storage systems,possessing wide applications in power electronics and automotive transportation,etc.[1,2].Furthermore.

Alternatingly stacked thin film electrodes-based compact aqueous hybrid electrochemical capacitors for hundred-volts AC line filtering

Filtering capacitor with compact configuration and a wide range of operating voltage has been attracting increasing attention for the smooth conversion of the electric signal in modern circuits.Lossless integration of capacitor units can be regarded as one of the efficient ways to achieve a wider voltage range,which has not yet been fully conquered due to the lack of rational designs of the electrode structure and integration technology.This study presents an alternatingly stacked assemble technology to conveniently fabricate compact aqueous hybrid integrated filtering capacitors on a large scale,in which a unit consists of rGO/MXene composite film as a negative electrode and PEDOT:PSS based film as a positive electrode.Benefiting from the synergistic effect of rGO and MXene components,and morphological characteristics of PEDOT:PSS,the capacitor unit exhibits outstanding AC line filtering with a large areal specific energy density of 1,015 μF V^(2)cm^(-2)(0.28 μW h cm^(-2)) at 120 Hz.After rational integration,the assembled capacitors present compact/lightweight configuration and lossless frequency response,as reflected by almost constant resistor-capacitor time constant of 0.2 ms and dissipation factor of 15% at120 Hz,identical to those of the single capacitor unit.Apart from standing alone steadily on a flower,a small volume(only 8.1 cm^(3)) of the integrated capacitor with 70 units connected in series achieves hundred-volts alternating current line filtering,which is superior to most reported filtering capacitors with sandwich configuration.This study provides insight into the fabrication and application of compact/ultralight filtering capacitors with lossless frequency response,and a wide range of operating voltage.

Intedrcalation in Two-Dimensional Transition Metal Carbides and Nitrides(MXenes) toward Electrochemical Capacitor and Beyond

Since its discovery in 2011,the emerging family of two-dimensional(2D)transition metal carbides,nitrides,and carbonitrides(denoted as MXenes)has shown tremendous promise in the field of energy storages,especially electrochemical capacitors(ECs).The intercalation of foreign species,including but not limited to proton,cations,organic ions,and solvents,is demonstrated as one of the dominant facts during the energy storage process of MXenes,through which interlayer spacing of MXenes can be reversibly tuned,followed with reversible redox reactions on the surface of MXenes.Such a mechanism provides MXenes extremely high capacitance up to 1500 F cm-3,in aqueous acidic electrolyte.In addition,because of the versatile terminations on their surface,the intercalation of ions into MXenes can simultaneously induce the reversible transformation of their electronic structure to trigger some other phenomenon,for example,electrochromic effect,which has great potential on electrochromic smart window—an extended application of ECs.Accordingly,regulating and facilitating the intercalation in MXenes is of great significance for MXene-based ECs.In this review,we summarize the recent progress on the intercalation in MXenes towards ECs,discussing on the intercalated species,working mechanisms,and methods to promote the intercalation.Furthermore,we prospect the future research directions of intercalation of MXenes in ECs,such as the combination of simulation and experiment on finding the best intercalation species,precisely controlling the interlayer spacing and beyond,to boost the electrochemical performance of MXene toward practical applications and multi-functional devices.

A review of neutral pH polymer electrolytes for electrochemical capacitors:Transitioning from liquid to solid devices

The development of neutral pH polymer electrolytes has enabled high-performance solid-state,thin,and flexible electrochemical capacitors(ECs)to provide power for future consumer electronics and Internet-of-Thing devices.Notwithstanding their promising prospect,there is still some lack of understandings or disconnections from fundamental science to practical applications of these electrolytes.In this review,we provide an overview of stateof-the-art studies on ECs with neutral pH electrolytes in both liquid and solid configurations.Starting from the fundamental studies on the voltage window and ion conduction of salt species in liquid solution to polymer electrolytes,key considerations in developing neutral pH polymer electrolytes are discussed.The performance of the polymer electrolytes along with their enabled solid symmetric and asymmetric EC devices,as well as some enhanced functionalities are presented.The future directions for research on neutral pH polymer electrolytes are proposed,expected to provide reference for further enriching the fundamental knowledge and improving the device performances.

AC Line Filter Electrochemical Capacitors:Materials,Morphology,and Configuration

Recently,more and more supercapacitors(SCs)have been developed as AC line filter capacitors,which are generally named AC line filter electrochemical capacitors(FECs).Compared to traditional bulky aluminum electrolytic capacitors(AECs),FECs have higher capacity and lower space occupancy,which makes them a strong competitor.However,different from the common SCs for energy storage,it is necessary to consider the frequency response of the SCs for AC line filtering,where the contradiction between frequency response and specific capacitance is a challenge.The researchers have proposed different solutions from the perspective of materials,morphology,and configuration for this challenge.Based on the above background,in this review,we briefly introduce the principle and parameters of AC line filter electrochemical capacitors.We systematically summarize the state-of-the-art progresses of FECs and discuss their possible application and development in the future.The development of FECs can greatly promote the planarization,integration,and miniaturization of filter capacitors,and provide a new solution for the utilization of green and unstable energy.

Renewable biomass-derived carbons for electrochemical capacitor applications

Biomass is rich,renewable,sustainable,and green resources,thereby excellent raw material for the fabrication of carbon materials.The diversity in structure and morphology of biomass are relevant in obtaining carbon materials with dif-ferent structures and performances.The inherent ordered porous structure of biomass also benefits the activation process to yield porous carbons with ultra-high specific surface area and pore volume.Besides,obtained biomass-derived carbons(BDCs)are hard carbon with porous morphology,stable structure,supe-rior hardness/strength,and good cycling performances when used in electro-chemical capacitors(ECs).The inherent N,S,P,and O elements in biomass yield naturally self-doped N,S,P,and O BDCs with unique intrinsic structures.In this paper,the synthesis approaches and applications of BDCs in ECs are reviewed.It shows that BDCs electrochemical performances are highly determined by their pore structures,specific surface areas,heteroatoms doping,graphitization degree,defects,and morphologies.The electrochemical performances of BDCs can further be improved by compositing with other materials,such as graphene,carbon nanofibers/nanotubes,transition metal oxides or hydroxides,and con-ducting polymers.The future challenges and outlooks of BDCs are also provided.

Self-assembly of nanoparticles at solid-liquid interface for electrochemical capacitors

Self-assembly of nanoparticles at solid-liquid interface could be promising to realize the assembled functions for various applications,such as rechargeable batteries,supercapacitors,and electrocatalysis.This review summarizes the self-assembly of the nanoparticles at solid-liquid interface according to the different driving forces of assembly,including hydrophilic-hydrophobic interactions,solvophobic and electrostatic interaction.To be specific,the self-assembly can be divided into the following two types:surfactant-assisted self-assembly and direct self-assembly of Janus particles(inorganic and amphiphilic copolymer-inorganic Janus nanoparticles).Using the emulsion stabilized by nanoparticles as the template,the self-assembly constructed by the interaction of the nanostructure unit(including metal,metal oxide,and semiconductor,etc.)not only possesses the characteristic of nanostructure unit,but also exhibits the excellent assembly performance in electrochemistry aspect.The application of these assemblies in the area of electrochemical capacitors is presented.Finally,the current research progress and perspectives toward the self-assembly of nanoparticles at stabilized solid-liquid interface are proposed.

Preparation and Application of Nano-composite Poly(vinyl alcohol) Gel Electrolyte in Electrochemical Capacitor

A nano-composite polymer gel electrolyte was prepared using titanium oxide nanowire,poly(vinyl alcohol)(PVA),lithium salt and organic solvent N-methyl-2-pyrrolidone(NMP).The obtained electrolyte has the potential for application in electrochemical capacitor,the PVA in it is in an amorphous state.The ionic conductivities of electrolytes increased after addition of the nanowire,and the electrolyte with 3%(ω) of nanowire exhibited the highest ionic conductivity of 3.2 mS/cm at 20℃,as measured by electrochemical impedance spectroscopy.The temperature dependence of the conductivity was found to be in agreement with the Arrhenius equation.Functioning as separator and electrolyte,this nano-composite PVA gel electrolyte was used to assemble the electrochemical capacitor with active carbon film as electrodes.The compositing of nanowire may extend the life of electrochemical capacitors as they keep more than 90% of their capacitance after 5000 cycles of charging and discharging.

LaSr_2Mn_2O_7 Ruddlesden-Popper manganites for oxygen reduction and electrochemical capacitors

LaSr2 Mn2 O7,a Ruddlesden-Popper type bilayered perovskite,was investigated as oxygen catalyst for oxygen reduction reaction(ORR) and active electrode material for electrochemical capacitors in KOH solutions.XPS results show that Mn takes a mixed Mn2+,Mn3+and Mn4+valence state with an average valence of 3.4.As catalyst towards ORR,LaSr2 Mn2 O7 and LaSr2 Mn2 O7/acetylene black composite favor four electron pathways for ORR.LaSr2 Mn2 O7/C composite shows a comparable onset potential for ORR to that of Pt/C electrode.The improved ORR activity of LaSr2 Mn2 O7/acetylene black composite is attributed to the synergy between catalytic LaSr2 Mn2 O7 and conductive acetylene black.As electrode material for electrochemical capacitors,LaSr2 Mn2 O7 exhibits a maximum specific capacitance of 167.2 F/g with 62%faradic contribution at a scan rate of 1 mV/s in cyclic voltammetry test.

Fabrication and Electrochemical Properties of Carbon Nanotube-based Composite Electrodes for Electrochemical Capacitor Applications

1 Results Electrochemical capacitors (ECs) are expected to be used in hybrid electric vehicles in combination with batteries or fuel cells because of their higher power density than batteries. ECs using electrical double layer capacitance of carbon based materials and pseudocapacitance of transition metal oxides are called electrochemical double layer capacitors (EDLC) and supercapacitors (or pseudocapacitor), respectively. Transition metal oxides are considered the best candidates for high energy dens...

Review on supercapacitors:Technologies and performance evaluation

The development of electrochemical capacitors(i.e.supercapacitors)have attracted a lot of attention in recent years because of the increasing demand for efficient,high-power energy storage.Electrochemical capacitors(ECs)are particularly attractive for transportation and renewable energy generation applications,taking advantage of their superior power capability and outstanding cycle life.Over the past decade,various advanced electrode materials and cell design are being studied to improve the energy density of ECs.Hybrid Li-ion capacitors and pseudo-capacitors that utilize fast surface redox reactions of metal oxide and doped polymers are the prime candidates being considered.This paper is concerned with the metrics being used to describe the performance of ECs and how the metrics are evaluated by testing devices and how the data from the testing are best interpreted.Emphasize is on relating testing of advanced ECs using materials more complex than activated carbons to testing electric double-layer capacitors(EDLCs)using carbon in both electrodes.A second focus of the paper is projecting the potential of the advanced materials and ionic liquid electrolytes for the development of complete EC cells having an energy density more than a factor of ten greater the energy density of the EDLC devices currently on the market.This potential was evaluated by calculating the performance(energy and power)of a series of ECs that utilize the advanced materials that have been studied by electrochemists over the past 10-15 years.The capacitance and resistance of the advanced ECs were calculated utilizing specific capacitance(F/g or F/cm^(3))and porosity data for the electrode materials and ionic conductivity of the electrolytes.It was concluded that hybrid ECs can be developed with energy densities of at least 50 Wh/kg,70 Wh/L with efficient power greater than 3 k W/kg.Continued research on micro-porous carbons with specific capacitance of 200 F/g and greater is needed.to achieve these EC performance goals.

Conductive metal-organic frameworks: Recent advances in electrochemical energy-related applications and perspectives

Metal-organic frameworks(MOFs),typically constructed with metallic nodes and organic linkers,have influenced the development of modular solid materials.Their adjustable molecular structure provides a remarkable variety of MOF-based solid-state structures towards diverse applications.However,the low conductivity of traditional MOFs extremely hinders their applications in electronic and electrochemical devices.The emerging conductive MOFs,generally possessing twodimensional layered structures,are endowed with both the structural merits of common MOFs and exceptional electronic/ionic conductivities.Besides,the selection and optimization of ligands and metal centers,as well as synthetic methods enormously affects the intrinsic conductivity of conductive MOFs.The distinctive crystal structures and superb conductivity promise their appealing applications in electrochemical energy-related fields.In the review,we mainly summarize representative crystal features,conducting mechanisms and recent advances in rational design and synthesis of conductive MOFs,along with their versatile applications as electrodes for electrochemical capacitors and rechargeable batteries,and as catalysts towards electrocatalysis.Finally,the involved challenges and future trends/prospects of the conductive MOFs for electrochemical energyrelated applications are further proposed.

Influence of gas-diffusion-layer current collector on electrochemical performance of Ni(OH)_(2) nanostructures

We report the electrochemical performance of Ni(OH)_(2) on a gas diffusion layer(GDL).The Ni(OH)_(2) working electrode was successfully prepared via a simple method,and its electrochemical performance in 1 M NaOH electrolyte was investigated.The electrochemical results showed that the Ni(OH)_(2)/GDL provided the maximum specific capacitance value(418.11 F·g^(−1))at 1 A·g^(−1).Furthermore,the Ni(OH)_(2) electrode delivered a high specific energy of 17.25 Wh·kg^(−1) at a specific power of 272.5 W·kg^(−1) and retained about 81%of the capacitance after 1000 cycles of galvanostatic charge–discharge(GCD)measurements.The results of scanning electron microscopy(SEM)coupled with energy-dispersive X-ray spectroscopy(EDS)revealed the occurrence of sodium deposition after long-time cycling,which caused the reduction in the specific capacitance.This study results suggest that the light-weight GDL,which can help overcome the problem of the oxide layer on metal–foam substrates,is a promising current collector to be used with Ni-based electroactive materials for energy storage applications.

SiO2 stabilizes electrochemically active nitrogen in few-layer carbon electrodes of extraordinary capacitance

Pyrrolic and pyridinic N dopants can dramatically increase the electrochemical activities of carbon and conducting polymers.Although N-doped conducting polymers suffer from rapid degradation,their carbon counterpart of extraordinary capacitance has remarkable rate performance and cycling endurance thanks to carbon’s excellent electrical conductivity.But high nitrogen content and high electrical conductivity are difficult to achieve in a high-surface-area carbon,because the high chemical vapor deposition(CVD)temperature required for obtaining high conductivity also destabilizes under-coordinated pyrrolic and pyridinic nitrogen and tends to lower the surface area.Here we resolve this dilemma by using SiO2 as an effective N-fixation additive,which stabilizes the N-rich nano few-layer sp2-carbon construct in1000℃CVD.This enables a scalable sol-gel/CVD fabrication process for few-layer carbon electrodes of extraordinary capacitance(690 F g^-1).The electrodes have excellent rate performance and can maintain90%of their initial capacitance after 30,000 cycles,thus potentially suitable for practical applications.

Recent advances and future perspectives for aqueous zinc-ion capacitors

Ion-hybrid capacitors are expected to combine the high specific energy of battery-type materials and the superior specific power of capacitor-type materials and are considered as a promising energy storage technique.In particular,aqueous zinc-ion capacitors(ZIC),possessing the merits of high safety,cost-efficiency and eco-friendliness,have been widely explored with various electrode materials and electrolytes to obtain excellent electrochemical performance.In this review,we first summarize the research progress on enhancing the specific capacitance of capacitor-type materials and review the research on improving the cycling capability of battery-type materials under high current densities.Then,we look back on the effects of electrolyte engineering on the electrochemical performance of ZIC.Finally,we propose research challenges and development directions for ZIC.This review provides guidance for the design and construction of high-performance ZIC.

Review of the role of ionic liquids in two-dimensional materials

Ionic liquids(ILs)are expected to be used as readily available“designer”solvents,characterized by a number of tunable properties that can be obtained by modulating anion and cation combinations and ion chain lengths.Among them,its high ionicity is outstanding in the preparation and property modulation of two-dimensional(2D)materials.In this review,we mainly focus on the ILs-assisted exfoliation of 2D materials towards large-scale as well as functionalization.Meanwhile,electric-field controlled ILs-gating of 2D material systems have shown novel electronic,magnetic,optical and superconducting properties,attracting a broad range of scientific research activities.Moreover,ILs have also been extensively applied in various field practically.We summarize the recent developments of ILs modified 2D material systems from the electrochemical,solar cells and photocatalysis aspects,discuss their advantages and possibilities as“designer solvent”.It is believed that the design of ILs accompanying with diverse 2D materials will not only solve several scientific problems but also enrich materials design and engineer of 2D materials.

2020 roadmap on pore materials for energy and environmental applications

Porous materials have attracted great attention in energy and environment applications,such as metal organic frameworks(MOFs),metal aerogels,carbon aerogels,porous metal oxides.These materials could be also hybridized with other materials into functional composites with superior properties.The high specific area of porous materials offer them the advantage as hosts to conduct catalytic and electrochemical reactions.On one hand,catalytic reactions include photocatalytic,p ho toe lectrocatalytic and electrocatalytic reactions over some gases.On the other hand,they can be used as electrodes in various batteries,such as alkaline metal ion batteries and electrochemical capacitors.So far,both catalysis and batteries are extremely attractive topics.There are also many obstacles to overcome in the exploration of these porous materials.The research related to porous materials for energy and environment applications is at extremely active stage,and this has motivated us to contribute with a roadmap on ’porous materials for energy and environment applications’.

2020 roadmap on two-dimensional materials for energy storage and conversion

Energy storage and conversion have attained significant intere st owing to its important applications that reduce CO2 emission through employing green energy.Some promising technologies are included metalair batteries,metal-sulfur batteries,metal-ion batteries,electrochemical capacitors,etc.Here,metal elements are involved with lithium,sodium,and magnesium.For these devices,electrode materials are of importance to obtain high performance.Two-dimensional(2 D) materials are a large kind of layered structured materials with promising future as energy storage materials,which include graphene,black phosporu s,MXenes,covalent organic frameworks(COFs),2 D oxides,2 D chalcogenides,and others.Great progress has been achieved to go ahead for 2 D materials in energy storage and conversion.More researchers will join in this research field.Under the background,it has motivated us to contribute with a roadmap on ’two-dimensional materials for energy storage and conversion.

Transformation of Supercapacitive Charge Storage Behaviour in a Multi elemental Spinel CuMn_(2)O_(4) Nanofibers with Alkaline and Neutral Electrolytes

Electrode material has been cited as one of the most important determining factors in classifying an energy storage system’s charge storage mechanism,i.e.,as battery-type or supercapacitive-type.In this paper,we show that along with the electrode material,the electrolyte also plays a role in determining the charge storage behaviour of the system.For the purpose of our research,we chose multi-elemental spinal type CuMn_(2)O_(4) metal oxide nanofibers to prove the hypothesis.The material is synthesized as nanofibers of diameter~120 to 150 nm in large scales by a pilot scale electrospinning set up.It was then tested in three different electrolytes(1 M KOH,1 M Na_(2)SO_(4) and 1 M Li_(2)SO_(4)),two of which are neutral and the third is alkaline(KOH).The cyclic voltammograms and the galvanostatic charge-discharge of the electrode material in a three-electrode sys-tem measurement showed that it exhibit different charge storage mechanism in different electrolyte solutions.For the neutral electrolytes,a capacitive behaviour was observed whereas a battery-type behaviour was seen for the alkaline electrolyte.This leads us to conclude that the charge storage mechanism,along with the active material,also depends on the electrolyte used.