Latest advances in supercapacitors: from new electrode materials to novel device designs

CoS2/carbon network flexible film with Co-N bond/π-π interaction enables superior mechanical properties and high-rate sodium ion storage

Flexible electrodes based on conversion-type materials have potential applications in low-cost and high-performance flexible sodium-ion batteries (FSIBs), owing to their high theoretical capacity and appropriate sodiation potential. However, they suffer from flexible electrodes with poor mechanical properties and sluggish reaction kinetics. In this study, freestanding CoS2 nanoparticles coupled with graphene oxides and carbon nanotubes (CoS2/GO/CNTs) flexible films with robust and interconnected architectures were successfully synthesized. CoS2/GO/CNTs flexible film displays high electronic conductivity and superior mechanical properties (average tensile strength of 21.27 MPa and average toughness of 393.18 KJ m-3) owing to the defect bridge for electron transfer and the formation of the π-π interactions between CNTs and GO. In addition, the close contact between the CoS2 nanoparticles and carbon networks enabled by the Co-N chemical bond prevents the self-aggregation of the CoS2 nanoparticles. As a result, the CoS2/GO/CNTs flexible film delivered superior rate capability (213.5 mAh g-1 at 6 A g-1, better than most reported flexible anode) and long-term cycling stability. Moreover, the conversion reaction that occurred in the CoS2/GO/CNTs flexible film exhibited pseudocapacitive behavior. This study provides meaningful insights into the development of flexible electrodes with superior mechanical properties and electrochemical performance for energy storage.

Cellulose nanofiber-based hybrid hydrogel electrode with superhydrophilicity enabling flexible high energy density supercapacitor and multifunctional sensors

Flexible hybrid hydrogels (GO/AC/CNFn) with a 3D porous network structure and superhydrophilic property are synthesized by cross-linking and self-assembling graphene oxide (GO) and activated carbon (AC) with cellulose nanofiber (CNF) during microwave hydrothermal process. In this ternary composite hydrogel, CNF molecular chains bridge GO sheets to build the 3D skeleton and anchor AC particles within GO nanosheets, forming ordered architecture of GO/AC/CNFn hydrogel that simultaneously possesses high flexibility and excellent mechanical integrity. When using this hydrogel as additive-free electrode, the presence of AC provides developed porous structure and density to promote high volumetric capacitance, while the heteroatom nitrogen groups tune the surface property of the composite with increased electrical conductivity. Benefited from the optimized structure, GO/AC/CNF1 electrode delivers an ultra-high mass specific capacitance of 627 F/g and volume specific capacitance of 618 F/cm3 at 0.5 A/g in three-electrode system in 1 M H2SO4 electrolyte, which is kinetically demonstrated to be essentially originated from the capacitive contributions. The energy density reaches 32.2 Wh/kg at a power density of 150 W/kg for the fabricated flexible solid-state symmetric supercapacitor. Moreover, the obtained flexible device could sensitively response at varied physiological signals, shedding fresh lights on their potential applications in signal sensors and portable electronics.

GCMe: Efficient Implementation of the Gaussian Core Model with Smeared Electrostatic Interactions for Molecular Dynamics Simulations of Soft Matter Systems

In recent years, molecular dynamics (MD) simulations have emerged as an essential tool for understanding the structure, dynamics, and phase behavior of charged soft matter systems. To explore phenomena across greater length and time scales in MD simulations, molecules are often coarse-grained for better computational performance. However, commonly used force fields represent particles as hard-core interaction centers with point charges, which often overemphasizes the packing effect and short-range electrostatics, especially in systems with bulky deformable organic molecules and systems with strong coarse-graining. This underscores the need for an efficient soft-core model to physically capture the effective interactions between coarse-grained particles. To this end, we implement a soft-core model uniting the Gaussian core model with smeared electrostatic interactions that is phenomenologically equivalent to recent theoretical models. We first parametrize it generically using water as the model solvent. Then, we benchmark its performance in the OpenMM toolkit for different boundary conditions to highlight a computational speedup of up to 34 × compared to commonly used force fields and existing implementations. Finally, we demonstrate its utility by investigating how boundary polarizability affects the adsorption behavior of a polyelectrolyte solution on perfectly conducting and nonmetal boundaries.

Efficient Fabrication of Disordered Graphene with Improved Ion Accessibility, Ion Conductivity, and Density for High-Performance Compact Capacitive Energy Storage

High-performance compact capacitive energy storage is vital for many modern application fields, including grid power buffers, electric vehicles, and portable electronics. However, achieving exceptional volumetric performance in supercapacitors is still challenging and requires effective fabrication of electrode films with high ion-accessible surface area and fast ion diffusion capability while simultaneously maintaining high density. Herein, a facile, efficient, and scalable method is developed for the fabrication of dense, porous, and disordered graphene through spark-induced disorderly opening of graphene stacks combined with mechanical compression. The obtained disordered graphene achieves a high density of 1.18 g cm-3, sixfold enhanced ion conductivity compared to common laminar graphene, and an ultrahigh volumetric capacitance of 297 F cm-3 in ionic liquid electrolyte. The fabricated stack cells deliver a volumetric energy density of 94.2 Wh L-1 and a power density of 13.7 kW L-1, representing a critical breakthrough in capacitive energy storage. Moreover, the proposed disordered graphene electrodes are assembled into ionogel-based all-solid-state pouch cells with high mechanical stability and multiple optional outputs, demonstrating great potential for flexible energy storage in practical applications.

MXene materials in electrochemical energy storage systems

MXenes, due to their unique geometric structure, rich elemental composition, and intrinsic physicochemical properties, have multi-functional applications. In the field of electrochemical energy storage, MXenes can be used as active components, conductive agents, supports, and catalysts in ion-intercalated batteries, metal-sulfur batteries, and supercapacitors. The electrochemical performance of MXene materials is closely related to their distinctive physical and chemical properties, which depend on their geometry, surface functional groups, and elemental composition. How to regulate MXene materials to optimize electrochemical functions is a key scientific challenge. Herein, we correlated the function of MXene materials with their interlayer structure, surface functional groups, and specific catalytic sites, analyzed the electrochemical function of MXene materials, and showed how to design the electrochemical function of MXene materials based on ion/electron transport. Additionally, this feature article provides an outlook on the opportunities and challenges for MXenes, offering theoretical and technical guidance on using MXene materials in energy storage systems.

A Highly Robust and Conducting Ultramicroporous 3D Fe(II)-Based Metal-Organic Framework for Efficient Energy Storage

Exploitation of metal-organic framework (MOF) materials as active electrodes for energy storage or conversion is reasonably challenging owing to their poor robustness against various acidic/basic conditions and conventionally low electric conductivity. Keeping this in perspective, herein, a 3D ultramicroporous triazolate Fe-MOF (abbreviated as Fe-MET) is judiciously employed using cheap and commercially available starting materials. Fe-MET possesses ultra-stability against various chemical environments (pH-1 to pH-14 with varied organic solvents) and is highly electrically conductive (σ = 0.19 S m-1) in one fell swoop. By taking advantage of the properties mentioned above, Fe-MET electrodes give prominence to electrochemical capacitor (EC) performance by delivering an astounding gravimetric (304 F g-1) and areal (181 mF cm-2) capacitance at 0.5 A g-1 current density with exceptionally high cycling stability. Implementation of Fe-MET as an exclusive (by not using any conductive additives) EC electrode in solid-state energy storage devices outperforms most of the reported MOF-based EC materials and even surpasses certain porous carbon and graphene materials, showcasing superior capabilities and great promise compared to various other alternatives as energy storage materials.

Hydrothermal Synthesis of β-NiS Nanoparticles and Their Applications in High-Performance Hybrid Supercapacitors

In this work, β-NiS nanoparticles (NPs) were efficiently prepared by a straightforward hydrothermal process. The difference in morphology between these NiS NPs was produced by adding different amounts of thiourea, and the corresponding products were denoted as NiS-15 and NiS-5. Through electrochemical tests, the specific capacity (Cs) of NiS-15 was determined to be 638.34 C g-1 at 1 A g-1, compared to 558.17 C g-1 for NiS-5. To explore the practical application potential of such β-NiS NPs in supercapacitors, a hybrid supercapacitor (HSC) device was assembled with activated carbon (AC) as an anode. Benefitting from the high capacity of the NiS cathode and the large voltage window of the device, the NiS-15//AC HSC showed a high energy density (Ed) of 43.57 W h kg-1 at 936.92 W kg-1, and the NiS-5//AC HSC provided an inferior Ed of 37.89 W h kg-1 at 954.79 W kg-1. Both HSCs showed excellent cycling performance over 6000 cycles at 10 A g-1. The experimental findings suggest that both NiS-15 and NiS-5 in this study can serve as potential cathodes for high-performance supercapacitors. This current synthesis method is simple and can be extended to the preparation of other transition metal sulfide (TMS)-based electrode materials with exceptional electrochemical properties.

Powering the Future by Iron Sulfide Type Material (FexSy) Based Electrochemical Materials for Water Splitting and Energy Storage Applications: A Review

Water electrolysis is among the recent alternatives for generating clean fuels (hydrogen). It is an efficient way to produce pure hydrogen at a rapid pace with no unwanted by-products. Effective and cheap water-splitting electrocatalysts with enhanced activity, specificity, and stability are currently widely studied. In this regard, noble metal-free transition metal-based catalysts are of high interest. Iron sulfide (FeS) is one of the essential electrocatalysts for water splitting because of its unique structural and electrochemical features. This article discusses the significance of FeS and its nanocomposites as efficient electrocatalysts for oxygen evolution reaction (OER), hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), and overall water splitting. FeS and its nanocomposites have been studied also for energy storage in the form of electrode materials in supercapacitors and lithium- (LIBs) and sodium-ion batteries (SIBs). The structural and electrochemical characteristics of FeS and its nanocomposites, as well as the synthesis processes, are discussed in this work. This discussion correlates these features with the requirements for electrocatalysts in overall water splitting and its associated reactions. As a result, this study provides a road map for researchers seeking economically viable, environmentally friendly, and efficient electrochemical materials in the fields of green energy production and storage.

Effect of a solvothermal method using DMF on the dispersibility of rGO, application of rGO as a CDI electrode material, and recovery of sp2-hybridized carbon

Graphene is prized for its large surface area and superior electrical properties. Efforts to maximize the electrical conductivity of graphene commonly result in the recovery of sp2-hybridized carbon in the form of reduced graphene oxide (rGO). However, rGO shows poor dispersibility and aggregation when mixed with other materials without hydrophilic functional groups, This could lead to electrode delamination, agglomeration, and reduced efficiency. This study focuses on the impact of solvothermal reduction on the dispersibility and capacitance of rGO compared with chemical reduction. The results show that the dispersibility of rGO-D obtained through solvothermal reduction using N,N-dimethylformamide improved compared to that obtained through chemical reduction (rGO-H). Furthermore, when utilized as a material for CDI, an improvement in deionization efficiency was observed in the AC@rGO-D-based CDI system compared to AC@rGO-H and AC. However, the specific surface area, a key factor affecting CDI efficiency, was higher in rGO-H (249.572 m2 g-1) than in rGO-D (150.661 m2 g-1). While AC@rGO-H is expected to exhibit higher deionization efficiency due to its greater specific surface area, the opposite was observed. This highlights the effect of the improved dispersibility of rGO-D and underscores its potential as a valuable material for CDI applications.

Chitosan Oligosaccharide Laser Lithograph: A Facile Route to Porous Graphene Electrodes for Flexible On-Chip Microsupercapacitors

In this study, a convenient chitosan oligosaccharide laser lithograph (COSLL) technology was developed to fabricate laser-induced graphene (LIG) electrodes and flexible on-chip microsupercapacitors (MSCs). With a simple one-step CO2 laser, the pyrolysis of a chitosan oligosaccharide (COS) and in situ welding of the generated LIGs to engineering plastic substrates are achieved simultaneously. The resulting LIG products display a hierarchical porous architecture, excellent electrical conductivity (6.3 Ω sq-1), and superhydrophilic properties, making them ideal electrode materials for MSCs. The pyrolysis-welding coupled mechanism is deeply discussed through cross-sectional analyses and finite element simulations. The MSCs prepared by COSLL exhibit considerable areal capacitance of over 4 mF cm-2, which is comparable to that of the polyimide-LIG-based counterpart. COSLL is also compatible with complementary metal-oxide-semiconductor (CMOS) and micro-electro-mechanical system (MEMS) processes, enabling the fabrication of LIG/Au MSCs with comparable areal capacitance and lower internal resistance. Furthermore, the as-prepared MSCs demonstrate excellent mechanical robustness, long-cycle capability, and ease of series-parallel integration, benefiting their practical application in various scenarios. With the use of eco-friendly biomass carbon source and convenient process flowchart, the COSLL emerges as an attractive method for the fabrication of flexible LIG on-chip MSCs and various other advanced LIG devices.

Hierarchical core-shelled CoMo layered double hydroxide@CuCo2S4 nanowire arrays/nickel foam for advanced hybrid supercapacitors

The development of core-shelled heterostructures with the unique morphology can improve the electrochemical properties of hybrid supercapacitors (HSC). Here, CuCo2S4 nanowire arrays (NWAs) are vertically grown on nickel foam (NF) utilizing hydrothermal synthesis. Then, CoMo-LDH nanosheets are uniformly deposited on the CuCo2S4 NWAs by electrodeposition to obtain the CoMo-LDH@CuCo2S4 NWAs/NF electrode. Due to the superior conductivity of CuCo2S4 (core) and good redox activity of CoMo-LDH (shell), the electrode shows excellent electrochemical properties. The electrode's specific capacity is 1271.4 C g-1 at 1 A g-1, and after 10, 000 cycles, its capacity retention ratio is 92.2 % at 10 A g-1. At a power density of 983.9 W kg-1, the CoMo-LDH@CuCo2S4 NWAs/NF//AC/NF device has an energy density of 52.2 Wh kg-1. This indicates that CoMo-LDH@CuCo2S4/NF has a great potential for supercapacitors.

Cation-induced Ti3C2Tx MXene@melamine sponge aerogels with large layer spacing and high strength for high-performance supercapacitors

The self-assembled aerogels are considered as an efficient strategy to address the aggregation and restacking of Ti3C2Tx MXene nanosheets for high-performance supercapacitors. However, the low mechanical strength of the MXene aerogel results in the structural collapse of the self-standing supercapacitor electrode materials. Herein, a low-cost melamine sponge (MS) absorbed different cations (H+, K+, Mg2+, Fe2+, Co2+, Ni2+ and Al3+), serves as a carrier and crosslinker for loading MXene hydrogel induced by the absorbed cations on the skeleton surface and the pores of MS, resulting in the high loading mass MXene aerogels with high mechanical strength. The experimental results show that the Mg-Ti3C2Tx@MS aerogel exhibits the maximum area capacitance of 702.22 mF cm-2 at 3 mA cm-2, and the area capacitance is still 603.12 mF cm-2 even at 100 mA cm-2, indicating the high rate capability with a capacitance retention of 85.89 %. It is worth noting that the constructed asymmetric supercapacitor with activated carbon achieves high energy densities of 104.53 μWh cm-2 and 93.87 μWh cm-2 at 800 μW cm-2 and 7999 μW cm-2, respectively. Furthermore, the asymmetric supercapacitor shows the high cycling stability with 90.2 % capacity retention after 10,000 cycles. This work provides a feasible strategy to prepare Ti3C2Tx MXene aerogels with large layer spacing and high strength for high-performance supercapacitors.

Enhancing Interfacial Capacitance by Boron Doping in Vertically Porous Carbon Toward High-Performance AC Filtering Electrochemical Capacitors

Electrochemical capacitors (ECs) show great perspective in alternate current (AC) filtering once they simultaneously reach ultra-fast response and high capacitance density. Nevertheless, the structure-design criteria of the two key properties are often mutually incompatible in electrode construction. Herein, it is proposed that combining vertically oriented porous carbon with enhanced interfacial capacitance (Ci) can efficiently solve this issue. Theoretically, the density function theory calculation shows that the Ci of a carbon electrode can be enhanced by boron doping due to the corresponding compact induced charge layer. Experimentally, the vertical-oriented boron-doped graphene nanowalls (BGNWs) electrodes, whose Ci is enhanced from 4.20 to 10.16 µF cm-2 upon boron doping, are prepared on a large scale (480 cm2) using a hot-filament chemical vapor deposition technique (HFCVD). Owing to the high Ci and vertically oriented porous structure, BGNWs-based EC has a high capacitance density of 996 µF cm-2 with a phase angle of - 79.4° at 120 Hz in aqueous electrolyte and a high energy density of 1953 µFV2 cm-2 in organic electrolyte. As a result, the EC is capable of smoothing 120 Hz ripples for 60 Hz AC filtering. These results provide enlightening insights on designing high-performance ECs for high-frequency applications.

Reduced graphene oxide-wrapped ZnS-SnS2 heterojunction bimetallic hollow cubic boxes as high-magnification and long lifespan supercapacitor anode materials

Metal sulfides have attracted extensive attention due to their excellent electrochemical performance. However, issues such as poor conductivity and severe volume expansion during charge and discharge processes affect the applications of sulfides as electrode materials. Here, a combination of coprecipitation and high-temperature sulfidation methods are employed to synthesize a ZnS-SnS2 composite with a hollow cubic structure, which is further composited with reduced graphene oxide (RGO) to form ZnS-SnS2 hollow cubic boxes encapsulated in a conductive framework of reduced graphene oxide (RGO) (denoted as ZnS-SnS2@RGO) for electrode materials. The hollow structure effectively alleviates the pulverization of ZnS-SnS2@RGO caused by volume expansion during charge and discharge processes. The heterogeneous structure formed by ZnS and SnS2 effectively reduces the electron transfer resistance of the material. The use of RGO wrapping enhances the conductivity of the ZnS-SnS2 hollow cubic boxes, and RGO's dispersion effect on the ZnS-SnS2 cubes improves particle agglomeration, further mitigating volume expansion of the material. These results indicate the outstanding electrochemical performance of heterostructural ZnS-SnS2 hollow cubic electrodes encapsulated with reduced graphene oxide as a conductive framework. The fabrication process provides a novel approach for addressing volume expansion and poor conductivity issues in other pseudocapacitive materials.

MXene Key Composites: A New Arena for Gas Sensors

With the development of science and technology, the scale of industrial production continues to grow, and the types and quantities of gas raw materials used in industrial production and produced during the production process are also constantly increasing. These gases include flammable and explosive gases, and even contain toxic gases. Therefore, it is very important and necessary for gas sensors to detect and monitor these gases quickly and accurately. In recent years, a new two-dimensional material called MXene has attracted widespread attention in various applications. Their abundant surface functional groups and sites, excellent current conductivity, tunable surface chemistry, and outstanding stability make them promising for gas sensor applications. Since the birth of MXene materials, researchers have utilized the efficient and convenient solution etching preparation, high flexibility, and easily functionalize MXene with other materials to prepare composites for gas sensing. This has opened a new chapter in high-performance gas sensing materials and provided a new approach for advanced sensor research. However, previous reviews on MXene-based composite materials in gas sensing only focused on the performance of gas sensing, without systematically explaining the gas sensing mechanisms generated by different gases, as well as summarizing and predicting the advantages and disadvantages of MXene-based composite materials. This article reviews the latest progress in the application of MXene-based composite materials in gas sensing. Firstly, a brief summary was given of the commonly used methods for preparing gas sensing device structures, followed by an introduction to the key attributes of MXene related to gas sensing performance. This article focuses on the performance of MXene-based composite materials used for gas sensing, such as MXene/graphene, MXene/Metal oxide, MXene/Transition metal sulfides (TMDs), MXene/Metal-organic framework (MOF), MXene/Polymer. It summarizes the advantages and disadvantages of MXene composite materials with different composites and discusses the possible gas sensing mechanisms of MXene-based composite materials for different gases. Finally, future directions and inroads of MXenes-based composites in gas sensing are presented and discussed.

Robotic wireless capsule endoscopy: recent advances and upcoming technologies

Wireless capsule endoscopy (WCE) offers a non-invasive evaluation of the digestive system, eliminating the need for sedation and the risks associated with conventional endoscopic procedures. Its significance lies in diagnosing gastrointestinal tissue irregularities, especially in the small intestine. However, existing commercial WCE devices face limitations, such as the absence of autonomous lesion detection and treatment capabilities. Recent advancements in micro-electromechanical fabrication and computational methods have led to extensive research in sophisticated technology integration into commercial capsule endoscopes, intending to supersede wired endoscopes. This Review discusses the future requirements for intelligent capsule robots, providing a comparative evaluation of various methods' merits and disadvantages, and highlighting recent developments in six technologies relevant to WCE. These include near-field wireless power transmission, magnetic field active drive, ultra-wideband/intrabody communication, hybrid localization, AI-based autonomous lesion detection, and magnetic-controlled diagnosis and treatment. Moreover, we explore the feasibility for future "capsule surgeons".

Multi-Interface Engineering of MXenes for Self-Powered Wearable Devices

Self-powered wearable devices with integrated energy supply module and sensitive sensors have significantly blossomed for continuous monitoring of human activity and the surrounding environment in healthcare sectors. The emerging of MXene-based materials has brought research upsurge in the fields of energy and electronics, owing to their excellent electrochemical performance, large surface area, superior mechanical performance, and tunable interfacial properties, where their performance can be further boosted via multi-interface engineering. Herein, a comprehensive review of recent progress in MXenes for self-powered wearable devices is discussed from the aspects of multi-interface engineering. The fundamental properties of MXenes including electronic, mechanical, optical, and thermal characteristics are discussed in detail. Different from previous review works on MXenes, multi-interface engineering of MXenes from termination regulation to surface modification and their impact on the performance of materials and energy storage/conversion devices are summarized. Based on the interfacial manipulation strategies, potential applications of MXene-based self-powered wearable devices are outlined. Finally, proposals and perspectives are provided on the current challenges and future directions in MXene-based self-powered wearable devices.

Fabrication of Polypyrrole Hollow Nanospheres by Hard-Template Method for Supercapacitor Electrode Material

Conducting polymers like polypyrrole, polyaniline, and polythiophene with nanostructures offers several advantages, such as high conductivity, a conjugated structure, and a large surface area, making them highly desirable for energy storage applications. However, the direct synthesis of conducting polymers with nanostructures poses a challenge. In this study, we employed a hard template method to fabricate polystyrene@polypyrrole (PS@PPy) core-shell nanoparticles. It is important to note that PS itself is a nonconductive material that hinders electron and ion transport, compromising the desired electrochemical properties. To overcome this limitation, the PS cores were removed using organic solvents to create hollow PPy nanospheres. We investigated six different organic solvents (cyclohexane, toluene, tetrahydrofuran, chloroform, acetone, and N,N-dimethylformamide (DMF)) for etching the PS cores. The resulting hollow PPy nanospheres showed various nanostructures, including intact, hollow, buckling, and collapsed structures, depending on the thickness of the PPy shell and the organic solvent used. PPy nanospheres synthesized with DMF demonstrated superior electrochemical properties compared to those prepared with other solvents, attributed to their highly effective PS removal efficiency, increased specific surface area, and improved charge transport efficiency. The specific capacitances of PPy nanospheres treated with DMF were as high as 350 F/g at 1 A/g. And the corresponding symmetric supercapacitor demonstrated a maximum energy density of 40 Wh/kg at a power density of 490 W/kg. These findings provide new insights into the synthesis method and energy storage mechanisms of PPy nanoparticles.

Superior Electrochemical Water Splitting and Energy-Storage Performances of In Situ Fabricated Charge-Separated Metal Organophosphonate Single Crystals

The design and exploration of advanced materials as a durable multifunctional electrocatalyst toward sustainable energy generation and storage development is the most perdurable challenge in the domain of renewable energy research. Herein, a facile in situ solvothermal approach has been adopted to prepare a methylviologen-regulated crystalline metal phosphonate compound, [C12H14N2][Ni(C11H11N2)(H2hedp)2]2•6H2O (NIT1), (H4hedp = 1-hydroxyethane 1,1-diphosphonic acid) and well characterized by several techniques. The as-prepared NIT1 displays excellent bifunctional electrocatalytic activity with dynamic stability toward oxygen evolution reaction (η10 = 288 mV) and hydrogen evolution reaction (η10 = 228 mV) in alkaline (1.0 M KOH) and acidic mediums (0.5 M H2SO4), respectively. Such a low overpotential and Tafel slope (68 mV/dec for OER; 56 mV/dec for HER) along with long-term durability up to 20 h of NIT1 make it superior to benchmark the electrocatalyst and various nonprecious metal-based catalysts under similar experimental condition. Further, the electrochemical supercapacitor measurements (in three-electrode system) reveal that the NIT1 electrode possesses much higher specific capacity of 187.6 C g-1 at a current density of 2 A g-1 (272 C g-1 at 5 mV s-1) with capacitance retention of 75.2% over 10,000 cycles at 14 A g-1 (Coulombic efficiency > 99%) in 6 M KOH electrolyte medium. Finally for a practical application, an asymmetric supercapacitor device (coin cell) is assembled by NIT1 material. The as-fabricated device delivers the maximum energy density of 39.4 Wh kg-1 at a power density of 450 W kg-1 and achieves a wide voltage window of 1.80 V. Notably, the device endures a remarkable cycle performance with cyclic retention of 92% (Coulombic efficiency > 99%) even after 14,000 charge/discharge cycles at 10 A g-1. Nevertheless, the extraordinary electrochemical activities toward OER and HER as well as the high-performance device fabrication for LED illumination of such a noble metal-free lower-dimensional charge-transfer compound are truly path breaking and would be promising for the development of advanced multifunctional materials.

The application of plasma technology for the preparation of supercapacitor electrode materials

With the rapidly growing demand for clean energy and energy interconnection, there is an urgent need for rapid and high-capacity energy storage technologies to realize large-scale energy storage, transfer energy, and establish the energy internet. Supercapacitors, which have advantages such as high specific capacitance, fast charging and discharging rates, and long cycle lifetimes, are being widely used in electric vehicles, information technology, aerospace, and other fields. The performance of supercapacitors is crucially dependent on electrode materials. These can be categorized into electric double-layer capacitors and pseudocapacitors, primarily made from carbon and transition metal oxides, respectively. However, effectively monitoring the physicochemical properties of electrode materials during preparation and processing is challenging, which limits the improvement of supercapacitors' performance. Plasma materials preparation technology can effectively affect the materials preparation processing by energetic electrons, ions, free radicals, and multiple effects in plasma, which are easily manipulated by operation parameters. Therefore, plasma material preparation technology is considered a promising method to precisely monitor the physicochemical and electrochemical properties of energy storage materials and has been widely studied. This paper provides an overview of plasma materials preparation mechanisms, and details of the plasma technology application in the preparation of transition metal hybrids, carbon, and composite electrode materials, as well as a comparison with traditional methods. In conclusion, the advantages, challenges, and research directions of plasma materials preparation technology in the field of electrode materials preparation are summarized.

Supramolecular polymers with dual energy storage mechanism for high-performance supercapacitors

In this paper, we prepared the supramolecular polymers (MWCNT-APP-s) with a dual energy storage mechanism as the electrode materials by the coordination of four transition metal ions with the small molecule chelator (APP) and functionalized carbon nanotubes, respectively. Among four MWCNT-APP-s, MWCNT-APP-Fe has the characteristics of moderate micropore/mesopore, significant hydrophobicity, redox property and functional groups. Interestingly, the redox reaction of Fe3+/Fe2+ and -CN-/-CN- transformation give MWCNT-APP-Fe an energy storage basis of pseudocapacitance, while MWCNTs and the micro/mesopore structure in MWCNT-APP-Fe provide a double-layer energy storage platform. As expected, on base of the dual energy storage mechanism, the symmetric supercapacitor assembled with MWCNT-APP-Fe has a higher specific capacity (Cs, 421 F g-1 at 1 mV s-1) as well as a long-lasting stability of 94.8% capacity retention with 99% Coulombic efficiency after 10,000 cycles at 20 mV s-1. More notably, the relevant aqueous Zn2+ hybrid supercapacitor provides a high capacity (Cm) of 191 mAh g-1 at 0.5 A g-1 and a long duration of over 2000 cycles at 50 A g-1, with a capacity retention of 92.4%. In summary, MWCNT-APP-Fe with a dual energy storage mechanism enables a potential application as an electrode material for high-performance supercapacitor.

Temperature-Dependent Electrochemical Performance of Ta-Substituted SrCoO3 Perovskite for Supercapacitors

Developing new electrode materials with good temperature-dependent electrochemical performance has become a great issue for the deployment of hybrid supercapacitors with wide temperature tolerance. In this work, a series of Ta-substituted SrCo1-x Tax O3-δ (x=0.05, 0.10, 0.15, 0.20) perovskites have been studied as positive electrodes for hybrid supercapacitors in terms of their structures, elemental valence states and electrochemical performances. Incorporating Ta into SrCoO3-δ perovskite not only stabilizes the crystallite structure but also notably improves electrochemical activities. The SrCo0.95 Ta0.05 O3-δ @CC delivers the highest specific capacity (Qsp ) of 227.91 C g-1 at 1 A g-1 , which is attributed to the highest oxygen vacancy content and the fastest oxygen diffusion kinetics. The hybrid supercapacitor SrCo0.95 Ta0.05 O3-δ @CC//AC@CC exhibits a high energy density of 22.82 Wh kg-1 @775.09 W kg-1 and a stable long-term cycle life (5000 cycles) with 90.7 % capacity retention. As temperature increases from 25 to 85 °C, the capacitance properties are improved at elevated temperatures for both electrode and device due to the increased electrolyte conductivity. The outstanding electrochemical results present that SrCo1-x Tax O3-δ perovskite holds good prospects for hybrid supercapacitors with wide temperature tolerance.

Flexible Linker-Based Triazine-Functionalized 2D Covalent Organic Frameworks for Supercapacitor and Gas Sorption Applications

Covalent organic frameworks (COFs) having a large surface area, porosity, and substantial amounts of heteroatom content are recognized as the ideal class of materials for energy storage and gas sorption applications. In this work, we have synthesized four different porous COF materials by the polycondensation of a heteroatom-rich flexible triazine-based trialdehyde linker, namely 2,4,6-tris(4-formylphenoxy)-1,3,5-triazine (TPT-CHO), with four different triamine linkers. Triamine linkers were chosen based on differences in size, symmetry, planarity, and heteroatom content, leading to the synthesis of four different COF materials named IITR-COF-1, IITR-COF-2, IITR-COF-3, and IITR-COF-4. IITR-COF-1, synthesized within 24 h from the most planar and largest amine monomer, exhibited the largest Brunauer-Emmett-Teller (BET) surface area of 2830 m2 g-1, superior crystallinity, and remarkable reproducibility compared to the other COFs. All of the synthesized COFs were explored for energy and gas storage applications. It is shown that the surface area and redox-active triazene rings in the materials have a profound effect on energy and gas storage enhancement. In a three-electrode setup, IITR-COF-1 achieved an electrochemical stability potential window (ESPW) of 2.0 V, demonstrating a high specific capacitance of 182.6 F g-1 with energy and power densities of 101.5 Wh kg-1 and 298.3 W kg-1, respectively, at a current density of 0.3 A g-1 in 0.5 M K2SO4 (aq) with long-term durability. The symmetric supercapacitor of IITR-COF-1//IITR-COF-1 exhibited a notable specific capacitance of 30.5 F g-1 and an energy density of 17.0 Wh kg-1 at a current density of 0.12 A g-1. At the same time, it demonstrated 111.3% retention of its initial specific capacitance after 10k charge-discharge cycles. Moreover, it exhibited exceptional CO2 capture capacity of 25.90 and 10.10 wt % at 273 and 298 K, respectively, with 2.1 wt % of H2 storage capacity at 77 K and 1 bar.

CoNiO2/Co3O4 Nanosheets on Boron Doped Diamond for Supercapacitor Electrodes

Developing novel supercapacitor electrodes with high energy density and good cycle stability has aroused great interest. Herein, the vertically aligned CoNiO2/Co3O4 nanosheet arrays anchored on boron doped diamond (BDD) films are designed and fabricated by a simple one-step electrodeposition method. The CoNiO2/Co3O4/BDD electrode possesses a large specific capacitance (214 mF cm-2) and a long-term capacitance retention (85.9% after 10,000 cycles), which is attributed to the unique two-dimensional nanosheet architecture, high conductivity of CoNiO2/Co3O4 and the wide potential window of diamond. Nanosheet materials with an ultrathin thickness can decrease the diffusion length of ions, increase the contact area with electrolyte, as well as improve active material utilization, which leads to an enhanced electrochemical performance. Additionally, CoNiO2/Co3O4/BDD is fabricated as the positive electrode with activated carbon as the negative electrode, this assembled asymmetric supercapacitor exhibits an energy density of 7.5 W h kg-1 at a power density of 330.5 W kg-1 and capacity retention rate of 97.4% after 10,000 cycles in 6 M KOH. This work would provide insights into the design of advanced electrode materials for high-performance supercapacitors.

Porous materials as effective chemiresistive gas sensors

Chemiresistive gas sensors (CGSs) have revolutionized the field of gas sensing by providing a low-power, low-cost, and highly sensitive means of detecting harmful gases. This technology works by measuring changes in the conductivity of materials when they interact with a testing gas. While semiconducting metal oxides and two-dimensional (2D) materials have been used for CGSs, they suffer from poor selectivity to specific analytes in the presence of interfering gases and require high operating temperatures, resulting in high signal-to-noise ratios. However, nanoporous materials have emerged as a promising alternative for CGSs due to their high specific surface area, unsaturated metal actives, and density of three-dimensional inter-connected conductive and pendant functional groups. Porous materials have demonstrated excellent response and recovery times, remarkable selectivity, and the ability to detect gases at extremely low concentrations. Herein, our central emphasis is on all aspects of CGSs, with a primary focus on the use of porous materials. Further, we discuss the basic sensing mechanisms and parameters, different types of popular sensing materials, and the critical explanations of various mechanisms involved throughout the sensing process. We have provided examples of remarkable performance demonstrated by sensors using these materials. In addition to this, we compare the performance of porous materials with traditional metal-oxide semiconductors (MOSs) and 2D materials. Finally, we discussed future aspects, shortcomings, and scope for improvement in sensing performance, including the use of metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), and porous organic polymers (POPs), as well as their hybrid counterparts. Overall, CGSs using porous materials have the potential to address a wide range of applications, including monitoring water quality, detecting harmful chemicals, improving surveillance, preventing natural disasters, and improving healthcare.

Soluble starch-derived porous carbon microspheres with interconnected and hierarchical structure by a low dosage KOH activation for ultrahigh rate supercapacitors

The developed porous structure and high density are essential to enhance the bulk performance of carbon-based supercapacitors. Nevertheless, it remains a significant challenge to optimize the balance between the porous structure and the density of carbon materials to realize superior gravimetric and areal electrochemical performance. The soluble starch-derived interconnected hierarchical porous carbon microspheres were prepared through a simple hydrothermal treatment succeeded by chemical activation with a low dosage of KOH. Due to the formation of interconnected spherical morphology, hierarchical porous structure, reasonable mesopore volume (0.33 cm3 g-1) and specific surface area (1162 m2 g-1), the prepared carbon microsphere has an ultrahigh capacitance of 394 F g-1 @ 1 A g-1 and a high capacitance retention of 62.7 % @ 80 A g-1. The assembled two-electrode device displays good cycle stability after 20,000 cycles and an ultra-high energy density of 11.6 Wh kg-1 @ 250 W kg-1. Moreover, the sample still exhibits a specific capacitance of 165 F g-1 @ 1 A g-1 at a high mass loading of 10 mg cm-2, resulting in a high areal capacitance of 1.65 F cm-2. The strategy proposed in this study, via a low-dose KOH activation process, provides the way for the synthesis of high-performance porous carbon materials.

Ultrathin Dielectric Triggered Charge Injection Dynamics for High-Performance Metal Organic Framework/MXene Supercapacitors

A MOF-MXene-BN three-component heterostructure exhibits impressive pseudocapacitive behavior with fast charge injection facilitated by an ultrathin dielectric h-BN. To address the MOF's low electronic conductivity, a 2D NiCo-MOF is grown on MXene nanosheets, enhancing conductivity and providing abundant redox-active sites. BN (boron nitride) serves a dual purpose, preventing restacking and facilitating charge injection toward NiCo-MOF. Synergistic contributions of 2D materials and a heterostructure with favorable charge injection dynamics among MOF, MXene, and BN contribute to enhanced electrochemical performance. Charge transfer mechanisms are elucidated using distribution of relaxation time technique to analyze complex EIS data and to differentiate electrode kinetics based on their respective relaxation time constants. An asymmetric supercapacitor, MOF-MXene-BN//activated carbon, achieves a specific capacity of 798 C/g, an energy density of 81 Wh/kg at 365 W/kg, and 81% capacitance retention over 5,000 cycles.

Application of Copper-Sulfur Compound Electrode Materials in Supercapacitors

Supercapacitors (SCs) are a novel type of energy storage device that exhibit features such as a short charging time, a long service life, excellent temperature characteristics, energy saving, and environmental protection. The capacitance of SCs depends on the electrode materials. Currently, carbon-based materials, transition metal oxides/hydroxides, and conductive polymers are widely used as electrode materials. However, the low specific capacitance of carbon-based materials, high cost of transition metal oxides/hydroxides, and poor cycling performance of conductive polymers as electrodes limit their applications. Copper-sulfur compounds used as electrode materials exhibit excellent electrical conductivity, a wide voltage range, high specific capacitance, diverse structures, and abundant copper reserves, and have been widely studied in catalysis, sensors, supercapacitors, solar cells, and other fields. This review summarizes the application of copper-sulfur compounds in SCs, details the research directions and development strategies of copper-sulfur compounds in SCs, and analyses and summarizes the research hotspots and outlook, so as to provide a reference and guidance for the use of copper-sulfur compounds.

Progress of charge carrier dynamics and regulation strategies in 2D CxNy-based heterojunctions

Two-dimensional carbon nitrides (CxNy) have gained significant attention in various fields including hydrogen energy development, environmental remediation, optoelectronic devices, and energy storage owing to their extensive surface area, abundant raw materials, high chemical stability, and distinctive physical and chemical characteristics. One effective approach to address the challenges of limited visible light utilization and elevated carrier recombination rates is to establish heterojunctions for CxNy-based single materials (e.g. C2N3, g-C3N4, C3N4, C4N3, C2N, and C3N). The carrier generation, migration, and recombination of heterojunctions with different band alignments have been analyzed starting from the application of CxNy with metal oxides, transition metal sulfides (selenides), conductive carbon, and Cx'Ny' heterojunctions. Additionally, we have explored diverse strategies to enhance heterojunction performance from the perspective of carrier dynamics. In conclusion, we present some overarching observations and insights into the challenges and opportunities associated with the development of advanced CxNy-based heterojunctions.

Fabrication of Mesoporous Materials Based on Supramolecular Self-Assembly of Guanosine Monophosphonate-Nickel Chloride (GMP-Ni) for High-Performance Hybrid Supercapacitors

Supramolecular self-assembly of nickel chloride and guanosine mono-phosphonate (GMP) and nickel (Ni)-based GMP-Ni and their calcinated mesoporous electrode materials GMP-Ni-500 and GMP-Ni-700 at 500 and 700 °C, respectively, have been fabricated. GMP-Ni, GMP-Ni-500, and GMP-Ni-700 are examined for their supercapacitor performance in a three-electrode configuration. The electrochemical tests demonstrate the mesoporous battery-type nature of GMP-Ni-500 which exhibited a specific capacity (Cs) of about 289 C g-1 at 0.5 A g-1 current density. In addition, a cost-effective and simple asymmetric supercapacitor device has been fabricated with battery-type GMP-Ni-500 as a cathode material and capacitive-type activated carbon (AC) as an anodic material. In an operating voltage window of 0 to 1.5 V, hybrid supercapacitors (HSCs) based on GMP-Ni-500//AC exhibited a remarkable performance with a specific capacity (Cs) of 144 C g-1 at 0.5 A g-1. For the HSC device, the maximum of 66% capacity retention has been observed after 5000 charging/discharging cycles at 5 A g-1. Furthermore, the HSC device demonstrates a high energy density of 24 W h kg-1 at a power density of 297 W kg-1. The molecular transformation was established by employing theoretical calculations. These results suggest that our HSC has outstanding potential in technology development for next-generation commercial applications.

MXene-Based Micro-Supercapacitors: Ink Rheology, Microelectrode Design and Integrated System

MXenes have shown great potential for micro-supercapacitors (MSCs) due to the high metallic conductivity, tunable interlayer spacing and intercalation pseudocapacitance. In particular, the negative surface charge and high hydrophilicity of MXenes make them suitable for various solution processing strategies. Nevertheless, a comprehensive review of solution processing of MXene MSCs has not been conducted. In this review, we present a comprehensive summary of the state-of-the-art of MXene MSCs in terms of ink rheology, microelectrode design and integrated system. The ink formulation and rheological behavior of MXenes for different solution processing strategies, which are essential for high quality printed/coated films, are presented. The effects of MXene and its compounds, 3D electrode structure, and asymmetric design on the electrochemical properties of MXene MSCs are discussed in detail. Equally important, we summarize the integrated system and intelligent applications of MXene MSCs and present the current challenges and prospects for the development of high-performance MXene MSCs.

Investigation of Hybrid Electrodes of Polyaniline and Reduced Graphene Oxide with Bio-Waste-Derived Activated Carbon for Supercapacitor Applications

Graphene-based materials have been widely studied in the field of supercapacitors. However, their electrochemical properties and applications are still restricted by the susceptibility of graphene-based materials to curling and agglomeration during production. This study introduces a facile method for synthesizing reduced graphene oxide (rGO) nanosheets and activated carbon based on olive stones (OS) with polyaniline (PAni) surface decoration for the development of supercapacitors. Several advanced techniques were used to examine the structural properties of the samples. The obtained PAni@OS-rGO (1:1) electrode exhibits a high electrochemical capacity of 582.6 F·g-1 at a current density of 0.1 A·g-1, and an energy density of 26.82 Wh·kg-1; thus, it demonstrates potential for efficacious energy storage. In addition, this electrode material exhibits remarkable cycling stability, retaining over 90.07% capacitance loss after 3000 cycles, indicating a promising long cycle life. Overall, this research highlights the potential of biomass-derived OS in the presence of PAni and rGO to advance the development of high-performance supercapacitors.

In situ selective selenization of ZIF-derived CoSe2 nanoparticles on NiMn-layered double hydroxide@CuBr2 heterostructures for high performance supercapacitors

As an emerging energy storage device, the practical application of supercapacitors (SCs) is currently constrained by their low energy density. Enhancing the capacitance of supercapacitors by leveraging the synergistic effect of multiple components in composite electrodes with well-designed structures can effectively increase their energy density. Here, a wire-sheet-particle hierarchical heterostructured CoSe2@NiMn-layered double hydroxide (NiMn-LDH) @Cu1.8Se/Copper foam (CF) electrode is synthesized via phase pseudomorphic transformation process achieved by selective selenization for Cu and Co elements. Benefiting from the stable support structure of CuBr2, the large specific surface area of NiMn-LDH, and the excellent conductivity of CoSe2, the prepared binder-free electrode shows excellent electrochemical properties. The CoSe2@NiMn-LDH@Cu1.8Se hybrid electrode exhibits a superior specific areal capacitance of 7064 mF cm-2 at 2 mA cm-2 and a stable cyclic performance with 80.11 % capacitance retention after 10,000 cycles. Furthermore, the assembled CoSe2@NiMn-LDH@Cu1.8Se/CF//AC (activated carbon) asymmetric supercapacitor (ASC) achieves an energy density of 36.6 Wh kg-1 when the power density is 760.6 W Kg-1 and retains 87.35 % of the initial capacitance after 5000 cycles. Overall, this pioneering research provided new insight for preparing supercapacitor electrode materials by selective selenization and ration design of the structures.

Chitosan modified graphene oxide with MnO2 deposition for high energy density flexible supercapacitors

To obtain a flexible composite electrode material with excellent electrochemical performance, chitosan (CS)/graphene oxide (GO) composite pretreated from microwave hydrothermal is adopted as the carbon substrate, and MnO2 active material is uniformly deposited on their surface through anodic electrodeposition. In this composite system, CS penetrates into graphene sheets as small molecule units, forming NH-C=O groups with GO via dehydration condensation, which effectively inhibits the stacking of GO and improves the specific surface area, conductivity, as well as the wettability of the carbon support. MnO2 bonding with heteroatom N from CS enables high active material loadings and forms stable three-dimensional network structure, facilitating the enhanced electrochemical performance. Results indicate that increasing depositing MnO2 amount leads to more defective structures of the composite, which promotes their electrochemical performance when used as electrode material. The area specific capacitance of the optimal composite reaches 3553.74 mF/cm2 at 5 mA/cm2 in 1 M Na2SO4 electrolyte. Kinetic analysis shows the energy storage process is capacitance-dominated, with the redox reactions of MnO2 being the main contributor. The prepared asymmetric solid supercapacitor delivers an energy density high up to 0.585 mWh/cm2 at power density of 3000 mW/cm2, and their excellent flexibility makes them promising candidates as flexible sensor.

Dual-mode fluorescence and electrochemiluminescence sensors based on Ru-MOF nanosheets for sensitive detection of apoE genes

A fluorescence-electrochemiluminescence (FL-ECL) dual-mode sensor for apoE gene detection has been developed, leveraging the unique properties of ruthenium metal organic framework nanosheets (RuMOFNSs). The system utilizes the quenching effect of the Ru(bpy)32+ ECL signal by ferrocene, leading to the synthesis of a multi-electron electrical signal marker, bisferrocene. By immobilizing the P-DNA on RuMOFNSs, bisferrocene quenches both FL and ECL signals. The addition of T-DNA and the consequent formation of double-stranded DNA enable the ExoIII enzyme to excise the bisferrocene fragment, restoring the signals. The sensor demonstrates wide detection linear ranges (1 fM to 1 nM for FL and 0.01 fM to 10 pM for ECL) and remarkable sensitivity (0.048 fM for FL and 0.016 fM for ECL). The dual-mode design offers enhanced reliability through a self-correction feature, reducing false positives. Compared to single-mode sensors, the dual-mode sensor shows significant advantages. Real-world testing confirms the sensor's capacity for robust detection in actual samples, underscoring its promising application in early disease diagnosis. This innovative approach opens up avenues for multi-signal response sensors, offering significant potential for diagnostic technologies.

The Realization of Uniform Growth of Conductive MOFs on LDHs and Their High Performance in Supercapacitors

Metal-organic frameworks (MOFs) have been widely investigated as functional materials with excellent properties. However, most MOFs are of poor electrical conductivity, which hinders their further application in electrochemical fields. Fortunately, the emergence of intrinsically conductive MOFs (c-MOFs) alleviates this problem. Layered double hydroxides (LDHs) possess Faraday redox reactivity, which is favorable to capacitors. In this paper, we combined c-MOFs with LDHs and prepared a series of NiCo-LDH@M-HHTP(-EtOH) (M=Ni or Co; HHTP=2,3,6,7,10,11-hexahydroxytriphenylene) multilayer nanoarrays, and the effects of solvent on the morphology and energy storage properties of the materials were investigated. When NiCo-LDH@Co-HHTP-EtOH is applied as an electrode material in supercapacitors, it exhibits a capacitance of 830 F g-1 at 1 A g-1 . Furthermore, it exhibits high energy density and excellent rate performance when assembled in aqueous asymmetric supercapacitors.

Facile synthesis of morphology-controlled hybrid structure of ZnCo2O4 nanosheets and nanowires for high-performance asymmetric supercapacitors

Controllable synthesis of electrode materials with desirable morphology and size is of significant importance and challenging for high-performance supercapacitors. Herein, we propose an efficient hydrothermal approach to controllable synthesis of hierarchical porous three-dimensional (3D) ZnCo2O4 composite films directly on Ni foam substrates. The composite films consisted of two-dimensional (2D) nanosheets array anchored with one-dimensional (1D) nanowires. The morphologies of ZnCo2O4 arrays can be easily controlled by adjusting the concentration of NH4F. The effect of NH4F in the formation of these 3D hierarchical porous ZnCo2O4 nanosheets@nanowires films is systematically investigated based on the NH4F-independent experiments. This unique 3D hierarchical structure can help enlarge the electroactive surface area, accelerate the ion and electron transfer, and accommodate structural strain. The as-prepared hierarchical porous ZnCo2O4 nanosheets@nanowires films exhibited inspiring electrochemical performance with high specific capacitance of 1289.6 and 743.2 F g-1 at the current density of 1 and 30 A g-1, respectively, and a remarkable long cycle stability with 86.8% capacity retention after 10 000 cycles at the current density of 1 A g-1. Furthermore, the assembled asymmetric supercapacitor using the as-prepared ZnCo2O4 nanosheets@nanowires films as the positive electrode and active carbon as negative electrode delivered a high energy density of 39.7 W h kg-1 at a power density of 400 W kg-1. Our results show that these unique hierarchical porous 3D ZnCo2O4 nanosheets@nanowires films are promising candidates as high-performance electrodes for energy storage applications.

Effect of synthesis methods on the activity of NiO/Co3O4 as an electrode material for supercapacitor: in the light of X-ray diffraction study

The formation of heterostructures by combining individual components (NiO and Co3O4) is a preferred approach to enhance electrochemical performance as it leads to improved charge transfer and surface reaction kinetics. In the present work, a NiO/Co3O4 composite was prepared by two methods. First, neat NiO and Co3O4 were prepared by adopting the hydrothermal method followed by the formation of the composite (i) by a hydrothermal route (NC-Hydro) and (ii) by a calcination route (NC-Cal). NC-Hydro composite shows a specific capacity of 176 C g-1 at 1 A g-1 of current density in the three-electrode system in a 2 M KOH solution as an electrolyte with 90% cyclic retention after 5000 cycles at 4 A g-1. NC-Cal shows a specific capacity of 111 C g-1 at 1 A g-1 with 75% cyclic retention. The coulombic efficiency of NC-Hydro was 86.3% while for NC-Cal it was 42.3%. The reason behind the superior electrochemical performance of NC-Hydro in comparison to NC-Cal may be the large interlayer spacing and lattice parameters of the former, which provide large space for redox reactions. The unit cell volume of the composites was more than that of the constituents. This study reveals that the composites prepared by the hydrothermal method have superior electrochemical properties in comparison to composites prepared by the calcination method.

"Visualization" Gas-Gas Sensors Based on High Performance Novel MXenes Materials

The detection of toxic, harmful, explosive, and volatile gases cannot be separated from gas sensors, and gas sensors are also used to monitor the greenhouse effect and air pollution. However, existing gas sensors remain with many drawbacks, such as lower sensitivity, lower selectivity, and unstable room temperature detection. Thus, there is an imperative need to find more suitable sensing materials. The emergence of a new 2D layered material MXenes has brought dawn to solve this problem. The multiple advantages of MXenes, namely high specific surface area, enriched terminal functionality groups, hydrophilicity, and good electrical conductivity, make them among the most prolific gas-sensing materials. Therefore, this review paper describes the current main synthesis methods of MXenes materials, and focuses on summarizing and organizing the latest research results of MXenes in gas sensing applications. It also introduces the possible gas sensing mechanisms of MXenes materials on NH3 , NO2 , CH3 , and volatile organic compounds (VOCs). In conclusion, it provides insight into the problems and upcoming challenges of MXenes materials for gas sensing.

Molecular Understanding of Charging Dynamics in Supercapacitors with Porous Electrodes and Ionic Liquids

Porous electrodes and ionic liquids could significantly enhance the energy storage of supercapacitors. However, they may reduce the charging dynamics and power density due to the nanoconfinement of porous electrodes and the high viscosity of ionic liquids. A comprehensive understanding of the charging mechanism in porous supercapacitors with ionic liquids provides a crucial theoretical foundation for their design optimization. Here, we review the progress of molecular simulations of the charging dynamics in supercapacitors consisting of porous electrodes and ionic liquids. We highlight and delve into the breakthroughs in the ion transport and charging mechanism for electrodes with subnanometer pores and realistic porous structures. We also discuss future directions for the charging dynamics of supercapacitors.

Recent advances in the utilization of covalent organic frameworks (COFs) as electrode materials for supercapacitors

Due to their excellent stability, ease of modification, high specific surface area, and tunable redox potentials, covalent organic frameworks (COFs) as potential electrodes in supercapacitors (SCs) have raised much research interest because these materials can enable the achievement of high electric double-layer supercapacitance and high pseudocapacitance. Here, the design strategies and SC applications of COF-based electrode materials are summarized. The detailed principles are introduced first, followed by discussions on strategies with diverse examples. The updated advances in design and applications are also discussed. Finally, in the outlook section, we provide some guidelines on the rational design of COF-based electrode materials for high-performance SCs, which we hope will inspire novel concepts for COF-based supercapacitors.

Ultralow-resistance electrochemical capacitor for integrable line filtering

Electrochemical capacitors are expected to replace conventional electrolytic capacitors in line filtering for integrated circuits and portable electronics1-8. However, practical implementation of electrochemical capacitors into line-filtering circuits has not yet been achieved owing to the difficulty in synergistic accomplishment of fast responses, high specific capacitance, miniaturization and circuit-compatible integration1,4,5,9-12. Here we propose an electric-field enhancement strategy to promote frequency characteristics and capacitance simultaneously. By downscaling the channel width with femtosecond-laser scribing, a miniaturized narrow-channel in-plane electrochemical capacitor shows drastically reduced ionic resistances within both the electrode material and the electrolyte, leading to an ultralow series resistance of 39 mΩ cm2 at 120 Hz. As a consequence, an ultrahigh areal capacitance of up to 5.2 mF cm-2 is achieved with a phase angle of -80° at 120 Hz, twice as large as one of the highest reported previously4,13,14, and little degradation is observed over 1,000,000 cycles. Scalable integration of this electrochemical capacitor into microcircuitry shows a high integration density of 80 cells cm-2 and on-demand customization of capacitance and voltage. In light of excellent filtering performances and circuit compatibility, this work presents an important step of line-filtering electrochemical capacitors towards practical applications in integrated circuits and flexible electronics.

Recent advances in biopolymers-based carbon materials for supercapacitors

Supercapacitors as potential candidates for novel green energy storage devices demonstrate a promising future in promoting sustainable energy supply, but their development is impeded by limited energy density, which can be addressed by developing high-capacitance electrode materials with efforts. Carbon materials derived from biopolymers have received much attention for their abundant reserves and environmentally sustainable nature, rendering them ideal for supercapacitor electrodes. However, the limited capacitance has hindered their widespread application, resulting in the proposal of various strategies to enhance the capacity properties of carbon electrodes. This paper critically reviewed the recent research progress of biopolymers-based carbon electrodes. The advances in biopolymers-based carbon electrodes for supercapacitors are presented, followed by the strategies to improve the capacitance of carbon electrodes which include pore engineering, doping engineering and composite engineering. Furthermore, this review is summarized and the challenges of biopolymer-derived carbon electrodes are discussed. The purpose of this review is to promote the widespread application of biopolymers in the domain of supercapacitors.

Microfluidic-Assembled Covalent Organic Frameworks@Ti3 C2 Tx MXene Vertical Fibers for High-Performance Electrochemical Supercapacitors

The delicate design of innovative and sophisticated fibers with vertical porous skeleton and eminent electrochemical activity to generate directional ionic pathways and good faradic charge accessibility is pivotal but challenging for realizing high-performance fiber-shaped supercapacitors (FSCs). Here, hierarchically ordered hybrid fiber combined vertical-aligned and conductive Ti3 C2 Tx MXene (VA-Ti3 C2 Tx ) with interstratified electroactive covalent organic frameworks LZU1 (COF-LZU1) by one-step microfluidic synthesis is developed. Due to the incorporation of vertical channels, abundant redox active sites and large accessible surface area throughout the electrode, the VA-Ti3 C2 Tx @COF-LZU1 fibers express exceptional gravimetric capacitance of 787 F g-1 in a three-electrode system. Additionally, the solid-state asymmetric FSCs deliver a prominent energy density of 27 Wh kg-1 , capacitance of 398 F g-1 and cycling life of 20 000 cycles. The key to high energy storage ability originates from the decreased ions adsorption energy and ameliorative charge density distribution in vertically aligned and active hybrid fiber, accelerating ions transportation/accommodation and interfacial electrons transfer. Benefiting from excellent electrochemical performance, the FSCs offer sufficient energy supply to power watches, flags, and digital display tubes as well as be integrated with sensors to detect pulse signals, which opens a promising route for architecting advanced fiber toward the carbon neutrality market beyond energy-storage technology.

Metal-Organic Frameworks and Their Derivatives-Based Nanostructure with Different Dimensionalities for Supercapacitors

With the urgent demand for the achievement of carbon neutrality, novel nanomaterials, and environmentally friendly nanotechnologies are constantly being explored and continue to drive the sustainable development of energy storage and conversion installations. Among various candidate materials, metal-organic frameworks (MOFs) and their derivatives with unique nanostructures have attracted increasing attention and intensive investigation for the construction of next generation electrode materials, benefitting from their unique intrinsic characteristics such as large specific surface area, high porosity, and chemical tunability as well as the interconnected channels. Nevertheless, the poor electrochemical conductivity severely limits their application prospects, hence a variety of nanocomposites with multifarious structures have been designed and proposed from different dimensionalities. In this review, recent advances based on MOFs and their derivatives in different dimensionalities ranging from 1D nanopowders to 2D nanofilms and 3D aerogels, as well as 4D self-supporting electrodes for supercapacitors are summarized and highlighted. Furthermore, the key challenges and perspectives of MOFs and their derivatives-based materials for the practical and sustainable electrochemical energy conversion and storage applications are also briefly discussed, which may be served as a guideline for the design of next-generation electrode materials from different dimensionalities.

Enhancing supercapacitor performance using nanosheet-covered nanotube structures Co3S4/Ni0.96S@CNTs with carbon nanotubes as conductive substrates

Transition metal sulfides have broad application prospects as supercapacitor electrode materials. However, their poor structural stability and conductivity hinder improvements in their electrochemical performance. Therefore, the introduction of highly conductive carbon nanotubes (CNTs) as sulfide growth substrates is considered to improve the microstructure and electrochemical performance of electrode materials. In this study, a highly conductive CNT solution was sprayed onto a nickel foam current collector, and Co3S4/Ni0.96S was successfully constructed on a CNT conductive substrate using a combination of hydrothermal and electrochemical deposition methods, forming a unique nanosheet-covered nanotube structure Co3S4/Ni0.96S@CNTs. The addition of an appropriate concentration of CNTs can not only serve as a substrate for the growth of Co3S4/Ni0.96S, but also effectively maintain the overall nanosheet structure. Thus, the Co3S4/Ni0.96S@CNTs (2-CSNS@CNTs) have a stable structure and a wide range of electrochemical reaction sites, ensuring excellent conductivity and cycling stability. The electrode material 2-CSNS@CNTs exhibited a specific capacity of 1427.05 C g-1 at 1 A g-1. Additionally, the asymmetric supercapacitor 2-CSNS@CNTs exhibited a high energy density of 53.76 W h kg-1 at 800 W kg-1 and a capacity retention rate of 68.5% at 10 A g-1 after 1000 cycles.

Constructing Interconnected Microporous Structures in Carbon by Homogeneous Activation as a Sustainable Electrode Material for High-Performance Supercapacitors

Microporous carbon attracts attention as an electrode material for supercapacitors. However, a large number of deep and distorted mesoporous and macroporous structures are usually created by non-uniform etching, resulting in underutilized internal space. Homogeneous activation has been considered by researchers as a necessary condition for the formation of interconnected microporous structures in carbon materials. Herein, a simple strategy of hydrothermal introduction of defects followed by homogeneous activation for the preparation of microporous carbon was developed for the synthesis of electrode materials for high-performance supercapacitors. The optimized sample with defect-enriched microporous structure and large specific surface area has a specific capacity of 315 F g-1 (1 A g-1) in KOH solution, and the assembled symmetric supercapacitor achieves a high energy density of 7.3 Wh kg-1 at a power density of 250 W kg-1. This work is interesting because it not only demonstrates that rational design of electrode materials is important to boost the performance of supercapacitors, but also provides inspiration for the design of efficient supercapacitors in the future.

Enhanced photocatalytic degradation of crystal violet dye and high-performance electrochemical supercapacitor applications of hydrothermally synthesised magnetic bifunctional nanocomposite (Fe3O4/ZnO)

The present investigation employed a facile hydrothermal approach for the fabrication of Fe3O4/ZnO dual-functional magnetic nanocomposite. Supercapacitor and visible-light-driven photocatalytic applications of the material were explored. X-ray diffraction, Fourier transform infrared spectra, ultraviolet-visible diffuse reflectance spectra (UV-vis/DRS), field emission scanning electron microscopy (FE-SEM), energy dispersive x-ray spectroscopy, and vibrating sample magnetometer were used to analyse the nanocomposite's structural, morphological, optical, and magnetic properties. The FE-SEM analysis demonstrated that the surface morphology of Fe3O4, ZnO, and the Fe3O4/ZnO nanocomposite consisted of nanoparticles, nanoflakes, and nanoparticles adhered to the nanoflakes, respectively. The maximum specific capacitance of the electrode based on the Fe3O4/ZnO nanocomposite was measured to be 736.36 Fg-1at a scan rate of 5 mVs-1. The electrode also demonstrated remarkable cycling stability, retaining 86.5% of its capacitance even after 3000 cycles. The Fe3O4/ZnO nanocomposite was found to have an optical bandgap of 2.7 eV, an average particle size of 22.5 nm, and a saturation magnetization of 68.7 emu g-1. The photocatalysis experiment was conducted using the optimised settings, which included a pH of 7.0, a dye concentration of 30 mg l-1, a catalyst dose of 1 g l-1, and a contact time of 120 min. The Fe3O4/ZnO nanocomposite exhibited a notable degradation efficiency towards crystal violet dye upon exposure to visible light, achieving a degradation efficiency of 96.9%. This performance surpassed that of pure ZnO, which attained a degradation efficiency of 70.2%. The nanocomposite exhibited a rate constant of 2.80 × 10-2min-1, which was found to be notably higher than that of pure ZnO (0.8 × 10-2min-1), as determined through modelling (pseudo-first order linear fit). The radical scavenger experiments indicated that the superoxide radicals and hydroxyl radicals are the primary reactive species. The Fe3O4/ZnO photocatalyst can be effectively isolated using a bar magnet. Remarkably, the photocatalytic efficiency of the material remained almost entirely intact even after undergoing four cycles of recycling. In addition, this research opens up exciting new possibilities for use in fields like energy storage and pollution control.

Strategies for Advanced Supercapacitors Based on 2D Transition Metal Dichalcogenides: From Material Design to Device Setup

Recently, the development of new materials and devices has become the main research focus in the field of energy. Supercapacitors (SCs) have attracted significant attention due to their high power density, fast charge/discharge rate, and excellent cycling stability. With a lamellar structure, 2D transition metal dichalcogenides (2D TMDs) emerge as electrode materials for SCs. Although many 2D TMDs with excellent energy storage capability have been reported, further optimization of electrode materials and devices is still needed for competitive electrochemical performance. Previous reviews have focused on the performance of 2D TMDs as electrode materials in SCs, especially on their modification. Herein, the effects of element doping, morphology, structure and phase, composite, hybrid configuration, and electrolyte are emphatically discussed on the overall performance of 2D TMDs-based SCs from the perspective of device optimization. Finally, the opportunities and challenges of 2D TMDs-based SCs in the field are highlighted, and personal perspectives on methods and ideas for high-performance energy storage devices are provided.

Multifunctional metal selenide-based materials synthesized via a one-pot solvothermal approach for electrochemical energy storage and conversion applications

Highly-efficient electroactive materials with distinctive electrochemical features, along with suitable strategies to prepare hetero-nanoarchitectures incorporating two or more transition metal selenides, are currently required to increase charge storage ability. Herein, a one-pot solvothermal approach is used to develop iron-nickel selenide spring-lawn-like architectures (FeNiSe SLAs) on nickel (Ni) foam. The porous Ni foam scaffold not only enables the uniform growth of FeNiSe SLAs but also serves as an Ni source. The effect of reaction time on their morphological and electrochemical properties is investigated. The FeNiSe-15 h electrode shows high areal capacity (493.2 μA h cm-2) and superior cycling constancy. The as-assembled aqueous hybrid cell (AHC) demonstrates high areal capacity and a decent rate capability of 59.4% (50 mA cm-2). The AHC exhibits good energy and power densities, along with excellent cycling stability. Furthermore, to confirm its practicability, the AHC is employed to drive portable electronic appliances by charging it with wind energy. The electrocatalytic activity of FeNiSe-based materials to complete the oxygen evolution reaction (OER) is explored. Among them, the FeNiSe-15 h catalyst shows good OER performance at a current density of 50 mA cm-2. This general synthesis approach may initiate a strategy of advanced metal selenide-based materials for multifunctional applications.