Research progress in rare earths and their composites based electrode materials for supercapacitors

Supercapacitor is an imminent potential energy storage system,and acts as a booster to the batteries and fuel cells to provide necessary power density.In the last decade,carbon and carbonaceous materials,conducting polymers and transition metal oxide/hydroxide based electrode materials have been made to show a remarkable electrochemical performance.Rare-earth materials have attracted significant research attention as an electrode material for supercapacitor applications based on their physicochemical properties.In this review,rare earth metals,rare earth metal oxides/hydroxides,rare-earth metal chalcogenides,rare-earth metal/carbon composites and rare-earth metal/metal oxide composites based electrode materials are discussed for supercapacitors.We also discuss the energy chemistry of rare-earth metal-based materials.Besides the factors that affect the performance of the electrode materials,their evaluation methods and supercapacitor performances are discussed in details.Finally,the future outlook in rare-earth-based electrode materials is revealed towards its current developments for supercapacitor applications.

Recent advances in transition metal phosphide materials:Synthesis and applications in supercapacitors

Supercapacitors(SCs)are considered promising energy storge systems because of their outstanding power density,fast charge and discharge rate and long-term cycling stability.The exploitation of cheap and efficient electrode materials is the key to improve the performance of supercapacitors.As the battery-type materials,transition metal phosphides(TMPs)possess high theoretical specific capacity,good electrical conductivity and superior structural stability,which have been extensively studied to be electrode materials for supercapacitors.In this review,we summarize the up-to-date progress on TMPs materials from diversified synthetic methods,diverse nanostructures and several prominent TMPs and their composites in application of supercapacitors.In the end,we also propose the remaining challenges toward the rational discovery and synthesis of high-performance TMP electrodes materials for energy storage.

One-Step Hydrothermal Synthesis of a CoTe@rGO Electrode Material for Supercapacitors

CoTe@reduced graphene oxide(CoTe@rGO)electrode materials for supercapacitors were prepared by a one-step hydrothermal method in this paper.Compared with that of pure CoTe,the electrochemical performance of CoTe@rGO was significantly improved.The results showed that the optimal CoTe@rGO electrode material has a remarkably high specific capacitance of 810.6 F/g at a current density of 1 A/g.At 5 A/g,the synthesized material retained 77.2%of its initial capacitance even after 5000 charge/discharge cycles,thereby demonstrating good cycling stability.Moreover,even at a high current density of 20 A/g,the composite electrode retained 79.0%of its specific capacitance at 1 A/g,thus confirming its excellent rate performance.An asymmetric supercapacitor(ASC)with a wider potential window and higher energy density was assembled by using 3 M KOH as the electrolyte,the CoTe@rGO electrode as the positive electrode,and active carbon as the negative electrode.The operating voltage of the supercapacitor could be increased to 1.6 V,and its specific capacitance could reach 112.6 F/g at 1 A/g.The specific capacitance retention rate of the fabricated supercapacitor after 5000 charge/discharge cycles at 5 A/g was 87.1%,which confirms its excellent cycling stability.In addition,the ASC revealed a high energy density of 40.04 W·h/kg at a power density of 799.91 W/kg and a high power density of 4004.93 W/kg at an energy density of 33.43 W·h/kg.These results collectively show that CoTe@rGO materials have broad application prospects.

Metal-Organic Framework Materials for Electrochemical Supercapacitors

Exploring new materials with high stability and capacity is full of challenges in sustainable energy conversion and storage systems.Metal-organic frameworks(MOFs),as a new type of porous material,show the advantages of large specific surface area,high porosity,low density,and adjustable pore size,exhibiting a broad application prospect in the field of electrocatalytic reactions,batteries,particularly in the field of supercapacitors.This comprehensive review outlines the recent progress in synthetic methods and electrochemical performances of MOF materials,as well as their applications in supercapacitors.Additionally,the superiorities of MOFs-related materials are highlighted,while major challenges or opportunities for future research on them for electrochemical supercapacitors have been discussed and displayed,along with extensive experimental experiences.

Exploration and progress of high-energy supercapacitors and related electrode materials

As one of new electrical energy storage systems, supercapacitors possess higher energy density than conventional capacitors and larger power density than batteries, integrating substantial merits with high energy, large power delivery, long cycle life, obvious safety, and low cost. However, the unsatisfying energy density is the inhabiting issue for the wide commercial applications. As the energy density(E, W h kg?1) is directly proportional to specific capacitance(C, F g?1) and the square of operating voltage(V, V), in this review, we summarize the recent progress in two sections: the exploration of high-performance electrode materials to achieve high specific capacitance and the construction of high-voltage supercapacitor systems for high working voltage. The progressive explorations and developments in supercapacitors could guide the future research towards high-performance, low-cost, and safe energy storage devices.

Hierarchical graphite foil/CoNi2S4 flexible electrode with superior thermal conductivity for high-performance supercapacitors

Effective heat dissipation is a crucial issue in electrochemical energy storage devices. Thus, it is highly desirable to develop high-performance electrode materials with high thermal conductivity. Here, we report a facile one-step electrodeposition method to synthesize ternary cobalt nickel sulfide（CoNi2S4）flower-like nanosheets which are grown on graphite foil（GF） as binder-free electrode materials for supercapacitors. The as-fabricated GF/CoNi2S4 integrated electrode manifested an excellent thermal conductivity of 620.1 W·m-1·K-1 and a high specific capacitance of 881 F·g-2 at 5 mA cm-2, as well as good rate capability and cycling stability. Ultimately, the all-solid-state symmetric supercapacitor based on these advanced electrodes demonstrated superior heat dissipation performance during the galvanostatic charge-discharge processes. This novel strategy provides a new example of effective thermal management for potential applications in energy storage devices.

Enhancing Capacitance Performance of Ti3C2Tx MXene as Electrode Materials of Supercapacitor: From Controlled Preparation to Composite Structure Construction

Ti3C2Tx,a novel two-dimensional layer material,is widely used as electrode materials of supercapacitor due to its good metal conductivity,redox reaction active surface,and so on.However,there are many challenges to be addressed which impede Ti3C2Tx obtaining the ideal specific capacitance,such as restacking,re-crushing,and oxidation of titanium.Recently,many advances have been proposed to enhance capacitance performance of Ti3C2Tx.In this review,recent strategies for improving specific capacitance are summarized and compared,for example,film formation,surface modification,and composite method.Furthermore,in order to comprehend the mechanism of those efforts,this review analyzes the energy storage performance in different electrolytes and influencing factors.This review is expected to predict redouble research direction of Ti3C2Tx materials in supercapacitors.

Preface:Innovative electrode materials for supercapacitors

Portable electrical power sources play increasingly vital roles in our daily lives due to the widespread use of mobile electronic devices and electrical vehicles.Electrochemical capacitors,also referred as supercapacitors(SCs)or ultracapacitors,are an important type of energy storage system with superior advantages of rapid power delivery and recharging compared to other types of energy storage systems.In practice,SCs have played im-

Nanocarbon-based electrode materials applied for supercapacitors

As one of the promising energy storage and conversion systems,supercapacitors(SCs)are highly favored owing to their high power density and good service life.Among all the key components of supercapacitor devices,the design and investigation of electrode materials play an essential role in determining the whole electrochemical charge storage performance.Recently,nanocarbon-based materials(e.g.,graphene,carbon dots,graphene quantum dots,etc.)have been widely used as SC electrode materials because of their good physical structure and chemical properties,providing a new route to further improve the energy density and life span of SCs.Here,we review the latest progress of nanocarbon-based materials(including nanocarbon and nanocarbon-based composite materials)as electrode materials in SCs application.The recent progress of carbon dots,graphene,carbon nanotubes,and other nanocarbon materials electrodes is summarized,while the capacitance and energy density of the above nanocarbon electrodes still need to be improved.Then,the preparation and performance of nanocarbonbased composite electrodes comprising transition metal oxides,conductive polymer,and metal-organic framework derived porous carbon are reviewed.Finally,we outline major challenges and propose some ideas on building better nanocarbon-based SC electrodes.

Electrode materials derived from plastic wastes and other industrial wastes for supercapacitors

The present review not only devotes on the environmental consequences of plastic bag wastes and other industrial wastes observable in the landfills,in the oceans or elsewhere but also gives a new insight idea on conversion of them into worth material,carbon,for the best electrochemical supercapacitor.Transformation of plastic wastes into high-value materials is the incentive for plastic recycling,end-oflife handling case for plastic bag wastes in practice quite limited.The plastic recycling waste for reuse saves energy compared with manufacturing virgin materials.Herein,we identified several synthetic methods to convert plastic waste and other industrial wastes into carbon material for supercapacitor.Different kinds of carbon materials,including nanofiber,nanotube,graphene,mesoporous carbon,etc.,have been derived from plastic waste,and thus give a superior potential for transforming trash into a"gold capacitor".Finally,conclusions and future trends of high-voltage supercapacitors were made as well as the easy and mass production of high-performance electrode materials for supercapacitors.Our work offers a promising sustainable approach to handle plastic bags,waste,and other industrial wastes and provides a new avenue in supercapacitor applications and other areas.

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.

A bird nest-like manganese dioxide and its application as electrode in supercapacitors

A novel bird nest-like nanostructured MnO2(BNNS-MnO2) was prepared by a facile and cost-effective strategy. Their structures and morphologies were characterized by field emission scanning electron microscopy, transmission electron microscopy and powder X-ray diffraction. Capacitive behaviors were investigated by cyclic voltammetry and galvanostatic charge-discharge. The obtained nano-MnO2 possesses a well designed loose-assembled hierarchical nanoarchitecture with an appropriate crystallinity which gives rise to excellent performances as an electrode material for supercapacitors. A maximum specific capacitance of 917 F/g has been obtained at a current density of 5 mA/cm2 in 6 mol/L KOH aqueous solution, and a specific capacitance of 210 F/g has been maintained for 500 cycles. As the low cost of MnSO4 and KCr2O7 and the low reaction temperature, the present method avoids the requirements for complicated operations, time/energy-consuming and expensive reagents, and perhaps is ready for the industrialization of nano-MnO2 production.

Layered AI-substituted Cobalt Hydroxides/GO Composites for Electrode Materials of Supercapacitors

In this work, Al-substituted a-Co（OH）2/GO composites with supercapacitive properties were prepared by chemical co-precipitated method in which cobalt nitrate and aluminum nitrate were used as the raw material, and graphite oxide was employed as carrier. The as-prepared materials were characterized by X-ray diffraction （XRD）, field emission scanning electron microscopy （FESEM） and fourier transform infrared spectroscopy （FF-IR）. Cyclic voltammetry （CV） and galvanostatic charge/discharge measurements showed that the Al-substituted a-Co（OH）2/GO electrode material had excellent electrochemical capacitance. The specific capacitance of 1137 F·g-11 was achieved in 6 mol/L KOH solution at a current density of 1 A·g-1 within a potential range of 0-0.5 V. Moreover, only 12% losses of the initial specific capacitance were found after 500 cycles at a current density of 1 A·g-1.

Novel hierarchical yolk-shell α-Ni(OH)_(2)/Mn_(2)O_(3) microspheres as high specific capacitance electrode materials for supercapacitors

For high performance supercapacitors, novel hierarchical yolk-shell α-Ni(OH)_(2)/Mn_(2)O_(3) microspheres were controllably synthesized using a facile two-step method based on the solvothermal treatment. The unique α-Ni(OH)_(2) based yolk-shell microstructures decorated with numerous interconnected nanosheets and the heterocomposition features can synergistically enhance reactive site exposure and electron conduction within the microspheres, facilitate charge transfer between electrolyte and electrode materials, and release structural stress during OH− chemisorption/desorption. Moreover, the Mn2O3 sediments distributed over the α-Ni(OH)_(2) microspheres can serve as an effective protective layer for electrochemical reactions. Consequently, when tested in 1 mol·L^(−1) KOH aqueous electrolyte for supercapacitors, the yolk-shell α-Ni (OH)_(2)/Mn_(2)O_(3) microspheres exhibited a considerably high specific capacitance of 2228.6 F·g^(−1) at 1 A·g^(−1) and an impressive capacitance retention of 77.7% after 3000 cycles at 10 A·g^(−1). The proposed α-Ni(OH)_(2)/Mn_(2)O_(3) microspheres with hetero-composition and unique hierarchical yolk-shell microstructures are highly promising to be used as electrode materials in supercapacitors and other energy storage devices.

Highly densified carbon electrode materials towards practical supercapacitor devices

Supercapacitors are expected to bridge the gap between conventional electrostatic capacitors and batteries, but have not found significant application in primary energy devices, partly due to some unsolved problems in the elec- trode materials. A wide range of novel materials such as novel carbons have been investigated to increase the energy den- sity of the electrodes and the volumetric merits of the materi- als need to be specifically considered and evaluated, towards the practical application of these novel materials. In obser- vation of the intense research activity to improve the volu- metric performance of carbon electrodes, the density or mass loading is particularly important and shall be further opti- mized, both for commercially applied activated carbons and in novel carbon electrode materials such as graphene. In this review, we presented a brief overview of the recent progress in improving the volumetric performance of carbon-based su- percapacitor electrodes, particularly highlighting the devel- opment of densified electrodes by various technical strategies including the controlled assembly of carbon building blocks, developing carbon based hybrid composites and constructing micro- supercapacitors.

Application of covalent organic frameworks as electrode materials for supercapacitors

Supercapacitors have garnered significant attention due to their superior power density,excellent cycle stability,and fast charge/discharge rates.The properties of the electrode materials determine the charge storage performance of supercapacitors.Covalent organic frameworks(COFs)are a novel class of crystalline porous polymer materials with uniform nanoscale pores,large specific surface areas,and tunable redox-active groups within the framework,which can be tuned to suit specific applications.They have flexible molecular designs and synthetic strategies,demonstrating their strong application potential in the field of energy storage.Most COFs exhibit poor inherent conductivity and low utilization of active sites,resulting in incomplete electrochemical performance.Hybrid systems can be developed by incorporating redox-active groups,hydrogen bonding,or combining COFs with other materials to improve the energy storage performance of COFs materials.In this review article,the background of COFs is summarized,including a brief introduction,design strategy,and synthesis methods.It also discusses their applications in supercapacitors and is categorized into:pristine COFs,modified COFs,and COF matrix composites.Finally,the review addresses the challenges that remain for the practical application of these materials in supercapacitors.

Electrochemical Performance of Nickel Hydroxide/Activated Carbon Supercapacitors Using a Modified Polyvinyl Alcohol Based Alkaline Polymer Electrolyte

Polyvinyl alcohol （PVA）-sodium polyacrylate （PAAS）-KOH-H2O alkaline polymer electrolyte film with high ionic conductivity was prepared by a solution-casting method. Polymer Ni（OH）2/activated carbon （AC） hybrid supercapacitors with different electrode active material mass ratios （positive to negative） were fabricated using this alkaline polymer electrolyte, nickel hydroxide positive electrodes, and AC negative electrodes. Galvanostatic charge/ discharge and electrochemical impedance spectroscopy （EIS） methods were used to study the electrochemical performance of the capacitors, such as charge/discharge specific capacitance, rate charge/discharge ability, and charge/discharge cyclic stability. Experimental results showed that with the decreasing of active material mass ratio m（Ni（OH）2）/m（AC）, the charge/discharge specific capacitance increases, but the rate charge/discharge ability and the charge/discharge cyclic stability decrease.

Computational Insights into Charge Storage Mechanisms of Supercapacitors

Computational modeling methods,including molecular dynamics(MD)and Monte Carlo(MC)simulations,and density functional theory(DFT),are receiving booming interests for exploring charge storage mechanisms of electrochemical energy storage devices.These methods can effectively be used to obtain molecular scale local information or provide clear explanations for novel experimental findings that cannot be directly interpreted through experimental investigations.This short review is dedicated to emphasizing recent advances in computational simulation methods for exploring the charge storage mechanisms in typical nanoscale materials,such as nanoporous carbon materials,2 D MXene materials,and metal-organic framework electrodes.Beyond a better understanding of charge storage mechanisms and experimental observations,fast and accurate enough models would be helpful to provide theoretical guidance and experimental basis for the design of new high-performance electrochemical energy storage devices.

Recent advances in materials and device technologies for aqueous hybrid supercapacitors

Aqueous hybrid supercapacitors(AHSCs)offer potential safety and eco-friendliness compared with conventional electrochemical energy storage devices that use toxic and flammable organic electrolytes.They can serve as the bridge between aqueous batteries and aqueous supercapacitors by combining the advantages of high energy of the battery electrode and high power as well as long lifespan of the capacitive electrode.Over the past few decades,extensive research efforts have been devoted to developing advanced materials and fascinating device architectures for AHSCs.However,further development related to the compatibilities between the battery-type electrode and capacitive electrode remains stagnant mainly due to discrepancy encountered in terms of reaction kinetics and capacity.This review focuses on the recent progress made in the field of AHSCs via elucidating the main concepts on the design of battery and capacitive electrodes and emerging electrolytes.In particular,ingenious AHSCs that possess either better flexibility toward materials selection or better device functionality such as those with“dual-ion”energy storage mechanism and non-polarity feature are also discussed.Recent advances and unresolved issues in multivalent ion hybrid devices(in particular,zinc-ion AHSCs)are further outlined.Finally,future research directions and challenges for AHSCs are presented,which are anticipated to deliver higher energy and demonstrate greater multifunctionalities for more breakthrough technology applications.