Advancements in Asymmetric Supercapacitors: From Historical Milestones to Challenges and Future Directions

Numerous challenges, like the uninterrupted supply of electricity, stable and reliable power, and energy storage during non-operational hours, arise across various industries due to the absence of advanced energy storage technologies. With the continual technological advancements in portable electronics, green energy, and transportation, there are inherent limitations in their innovative production. Thus, ongoing research is focused on pursuing sustainable energy storage technologies. An emerging solution lies in the development of asymmetric supercapacitors (ASCs), which offer the potential to extend their operational voltage limit beyond the thermodynamic breakdown voltage range of electrolytes. This is achieved by employing two distinct electrode materials, presenting an effective solution to the energy storage limitations faced by ASCs. The current review concentrates on the progression of working materials to develop authentic pseudocapacitive energy storage systems (ESS). Also, evaluates their ability to exceed energy storage constraints. It provides insights into fundamental energy storage mechanisms, performance evaluation methodologies, and recent advancements in electrode material strategies. The review approaches developing high-performance electrode materials and achieving efficient ASC types. It delves into critical aspects for enhancing the energy density of ASCs, presenting debates and prospects, thereby offering a comprehensive understanding and design principles for next-generation ASCs in diverse applications.

Unraveling the energy storage mechanism in graphene-based nonaqueous electrochemical capacitors by gap-enhanced Raman spectroscopy

Graphene has been extensively utilized as an electrode material for nonaqueous electrochemical capacitors. However, a comprehensive understanding of the charging mechanism and ion arrangement at the graphene/electrolyte interface remain elusive. Herein, a gap-enhanced Raman spectroscopic strategy is designed to characterize the dynamic interfacial process of graphene with an adjustable number of layers, which is based on synergistic enhancement of localized surface plasmons from shell-isolated nanoparticles and a metal substrate. By employing such a strategy combined with complementary characterization techniques, we study the potential-dependent configuration of adsorbed ions and capacitance curves for graphene based on the number of layers. As the number of layers increases, the properties of graphene transform from a metalloid nature to graphite-like behavior. The charging mechanism shifts from co-ion desorption in single-layer graphene to ion exchange domination in few-layer graphene. The increase in area specific capacitance from 64 to 145 µF cm-2 is attributed to the influence on ion packing, thereby impacting the electrochemical performance. Furthermore, the potential-dependent coordination structure of lithium bis(fluorosulfonyl) imide in tetraglyme ([Li(G4)][FSI]) at graphene/electrolyte interface is revealed. This work adds to the understanding of graphene interfaces with distinct properties, offering insights for optimization of electrochemical capacitors.

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.

Solution-Processable and Printable Two-Dimensional Transition Metal Dichalcogenide Inks

Two-dimensional (2D) transition metal dichalcogenides (TMDs) with layered crystal structures have been attracting enormous research interest for their atomic thickness, mechanical flexibility, and excellent electronic/optoelectronic properties for applications in diverse technological areas. Solution-processable 2D TMD inks are promising for large-scale production of functional thin films at an affordable cost, using high-throughput solution-based processing techniques such as printing and roll-to-roll fabrications. This paper provides a comprehensive review of the chemical synthesis of solution-processable and printable 2D TMD ink materials and the subsequent assembly into thin films for diverse applications. We start with the chemical principles and protocols of various synthesis methods for 2D TMD nanosheet crystals in the solution phase. The solution-based techniques for depositing ink materials into solid-state thin films are discussed. Then, we review the applications of these solution-processable thin films in diverse technological areas including electronics, optoelectronics, and others. To conclude, a summary of the key scientific/technical challenges and future research opportunities of solution-processable TMD inks is provided.

Hybrid and Asymmetric Supercapacitors: Achieving Balanced Stored Charge across Electrode Materials

An electrochemical capacitor configuration extends its operational potential window by leveraging diverse charge storage mechanisms on the positive and negative electrodes. Beyond harnessing capacitive, pseudocapacitive, or Faradaic energy storage mechanisms and enhancing electrochemical performance at high rates, achieving a balance of stored charge across electrodes poses a significant challenge over a wide range of charge-discharge currents or sweep rates. Consequently, fabricating hybrid and asymmetric supercapacitors demands precise electrochemical evaluations of electrode materials and the development of a reliable methodology. This work provides an overview of fundamental aspects related to charge-storage mechanisms and electrochemical methods, aiming to discern the contribution of each process. Subsequently, the electrochemical properties, including the working potential windows, rate capability profiles, and stabilities, of various families of electrode materials are explored. It is then demonstrated, how charge balancing between electrodes falters across a broad range of charge-discharge currents or sweep rates. Finally, a methodology for achieving charge balance in hybrid and asymmetric supercapacitors is proposed, outlining multiple conditions dependent on loaded mass and charge-discharge current. Two step-by-step tutorials and model examples for applying this methodology are also provided. The proposed methodology is anticipated to stimulate continued dialogue among researchers, fostering advancements in achieving stable and high-performance supercapacitor devices.

NiCo-compounds inside and outside N-doped carbon nanotubes to construct a double-enhanced hierarchical structure for high energy density supercapacitors

Attaining a high energy density that aligns with practical application requirements is a crucial indicator in the advancement of supercapacitors. In this paper, a hybrid hierarchical electrode structure of N-doped carbon nanotube (NCNT) spheres encapsulated with NiCo-Se nanoparticles (NPs) and coated with nickel-cobalt layered double hydroxide (NiCo-LDH) multilayer nanosheets was successfully synthesized on a nickel foam (NF) substrate. The self-supporting strategy enables nickel-cobalt Prussian blue analogues (Ni-Co PBAs) to be directly attached to the NF surface, which results in fluffy NCNTs with a high length-diameter ratio and considerable yield and greatly enhances the conductivity of the electrode material. The synergistic interaction between the dual transition metal compounds inside and outside the NCNTs enables the hybrid electrode material to achieve an impressive specific capacity of 1899 F g-1 (211.0 mA h g-1) at 1 A g-1. The asymmetric supercapacitor (ASC) exhibits an excellent energy density of 57.6 W h kg-1 at a power density of 798 W kg-1. This study not only provides an attractive strategy for obtaining CNTs with excellent properties from Ni-Co PBA and synthesizing hybrid electrodes with efficient synergistic effects, but also achieves a high energy density that aligns with the practical application demands of supercapacitors.

Recycling and Reusing of Graphite from Retired Lithium-ion Batteries: A Review

The proliferation of rechargeable lithium-ion batteries (LIBs) over the past decade has led to a significant increase in the number of electric vehicles (EVs) powered by these batteries reaching the end of their lifespan. With retired EVs becoming more prevalent, recycling and reusing their components, particularly graphite, has become imperative as the world transitions toward electric mobility. Graphite constitutes ≈20% of LIBs by weight, making it a valuable resource to be conserved. This review presents an in-depth analysis of the current global graphite mining landscape and explores potential opportunities for the "second life" of graphitefrom depleted LIBs. Various recycling and reactivation technologies in both industry and academia are discussed, along with potential applications for recycled graphite forming a vital aspect of the waste management hierarchy. Furthermore, this review addresses the future challenges faced by the recycling industry in dealing with expired LIBs, encompassing environmental, economic, legal, and regulatory considerations. In conclusion, this review provides a comprehensive overview of the developments in recycling and reusing graphite from retired LIBs, offering valuable insights for forthcoming large-scale recycling efforts.

One-step simultaneous liquid phase exfoliation-induced chirality in graphene and their chirality-mediated microRNA delivery

Graphene (G) has established itself as an exciting prospect for a broad range of applications owing to its remarkable properties. Recent innovations in chiral nanosystems have led to sensors, drug delivery, catalysis, etc. owing to the stereospecific interactions between various nanosystems and enantiomers. As the molecular structure of G itself is achiral introducing chirality in G by simple attachment of a functional group (a chiral ligand) on the G nanosheet may result in more diverse applications. Herein, we demonstrate direct liquid phase exfoliation and chiral induction in G nanosheets abbreviated as l-graphene and d-graphene in the presence of chiral l-tyrosine and d-tyrosine and by applying high-temperature sonication. The obtained exfoliated nanosheets demonstrated stable chirality confirmed by circular dichroism. Fourier transform infrared (FTIR) spectra, Raman spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and differential scanning calorimetry (DSC) showed functional, structural, morphological, surface, and thermal characteristics of l-graphene and d-graphene. The hemo-compatibility of these chiral graphenes was evaluated for the very first time utilizing human red blood cells. Lastly, for the very first time, an attempt was made to explore enantiomeric binding between chiral l-graphene and d-graphene with microRNA (miR-205) and their possibility towards chirality-mediated gene delivery in prostate cancerous cells.

Model Approach to Thermal Conductivity in Hybrid Graphene-Polymer Nanocomposites

The thermal conductivity of epoxy nanocomposites filled with self-assembled hybrid nanoparticles composed of multilayered graphene nanoplatelets and anatase nanoparticles was described using an analytical model based on the effective medium approximation with a reasonable amount of input data. The proposed effective thickness approach allowed for the simplification of the thermal conductivity simulations in hybrid graphene@anatase TiO2 nanosheets by including the phenomenological thermal boundary resistance. The sensitivity of the modeled thermal conductivity to the geometrical and material parameters of filling particles and the host polymer matrix, filler's mass concentration, self-assembling degree, and Kapitza thermal boundary resistances at emerging interfaces was numerically evaluated. A fair agreement of the calculated and measured room-temperature thermal conductivity was obtained.

A Review of Yarn-Based One-Dimensional Supercapacitors

Energy storage in a one-dimensional format is increasingly vital for the functionality of wearable technologies and is garnering attention from various sectors, such as smart apparel, the Internet of Things, e-vehicles, and robotics. Yarn-based supercapacitors are a particularly compelling solution for wearable energy reserves owing to their high power densities and adaptability to the human form. Furthermore, these supercapacitors can be seamlessly integrated into textile fabrics for practical utility across various types of clothing. The present review highlights the most recent innovations and research directions related to yarn-based supercapacitors. Initially, we explore different types of electrodes and active materials, ranging from carbon-based nanomaterials to metal oxides and conductive polymers, that are being used to optimize electrochemical capacitance. Subsequently, we survey different methodologies for loading these active materials onto yarn electrodes and summarize innovations in stretchable yarn designs, such as coiling and buckling. Finally, we outline a few pressing research challenges and future research directions in this field.

Evaluating the appropriateness of γ-graphyne derivatives as electrode materials for supercapacitors

DFT calculations were used to study the quantum capacitance of pure, B/Al/Si/N/P-doped, and defective γ-graphyne. Due to the direct relationship between capacitance and electronic states around the Fermi level, structures' electronic properties were evaluated by DOS plots. The results of integrated specific quantum capacitance in the range of water stability potential show an improvement of capacity in each p and n-type doping. The calculated cohesive energies of doped structures reflect the stability enhancement. Also, the stability/capacitance of single and double vacancies in two distinct positions (sp and sp2) were examined. The results illustrate stability retention and quantum capacitance improvement of these defective structures. Among the doped structures, the maximum quantum capacitance is 2251.10 F/gr belonging to the aluminum doped structure (in the sp position). For the defective structures, the maximum quantum capacitance is 4221.69 F/gr belonging to removing two sp carbon atoms. These quantum capacitances significantly improved compared to the pristine structure (1216.87 F/gr) and many other structures. These stunning results can contribute to the design of appropriate structures as electrode materials for high-efficiency supercapacitors.

Electrospray Deposition of PEDOT:PSS on Carbon Yarn Electrodes for Solid-State Flexible Supercapacitors

The increasing demand for flexible electronic devices has risen due to the high interest in electronic textiles (e-textiles). Consequently, the urge to power e-textiles has sparked enormous interest in flexible energy storage devices. One-dimensional (1D) configuration supercapacitors are the most promising technology for textile applications, but often their production involves complex synthesis techniques and expensive materials. This work unveils the use of the novel electrospray deposition (ESD) technique for the deposition of poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS). This deposition methodology on conductive carbon yarns creates flexible electrodes with a high surface area. The deposition conditions of PEDOT:PSS were optimized, and their influence on the electrochemical performance of a 1D symmetric supercapacitor with a cellulose-based gel as an electrolyte and a separator was evaluated. The tests herein reported show that these capacitors exhibited a high specific capacitance of 72 mF g^-1, an excellent cyclability of more than 85% capacitance retention after 1500 cycles, and an outstanding capability of bending.

N-Doped Graphene-like Film/Silicon Structures as Micro-Capacitor Electrodes

Currently, the miniaturization of portable and autonomous devices is challenging for modern electronics. Graphene-based materials have recently emerged as one of the ideal candidates for supercapacitor electrodes, while Si is a common platform for direct component-on-chip integration. We have proposed the direct liquid-based CVD of N-doped graphene-like films (N-GLFs) on Si as a promising way to achieve solid-state on-chip micro-capacitor performance. Synthesis temperatures in the range from 800 °C to 1000 °C are investigated. Capacitances and electrochemical stability of the films are evaluated using cyclic voltammetry, as well as galvanostatic measurements and electrochemical impedance spectroscopy in 0.5 M Na₂SO₄. We have shown that N-doping is an efficient way to improve the N-GLF capacitance. 900 °C is the optimal temperature for the N-GLF synthesis with the best electrochemical properties. The capacitance rises with increasing film thickness which also has an optimum (about 50 nm). The transfer-free acetonitrile-based CVD on Si yields a perfect material for microcapacitor electrodes. Our best value of the area-normalized capacitance (960 mF/cm²) exceeds the world's achievements among thin graphene-based films. The main advantages of the proposed approach are the direct on-chip performance of the energy storage component and high cyclic stability.

Study on the Application of Nitrogen-Doped Holey Graphene in Supercapacitors with Organic Electrolyte

We present a facile low-cost method to produce nitrogen-doped holey graphene (N-HGE) and its application to supercapacitors. A composite of N-HGE and activated carbon (AC) was used as the electrode active material in organic-electrolyte supercapacitors, and the performances were evaluated. Melamine was mixed into graphite oxide (GO) as the N source, and an ultra-rapid heating method was used to create numerous holes during the reduction process of GO. X-ray photoelectron spectra confirmed the successful doping with 2.9-4.5 at.% of nitrogen on all samples. Scanning electron micrographs and Raman spectra revealed that a higher heating rate resulted in more holes and defects on the reduced graphene sheets. An extra annealing step at 1000 °C for 1 h was carried out to further eliminate residual oxygen functional groups, which are undesirable in the organic electrolyte system. Compared to the low-heating-rate counterpart (N-GE-15), N-HGE boosted the specific capacity of the supercapacitor by 42 and 22% at current densities of 0.5 and 20 A/g, respectively. The effects of annealing time (0.5, 1, and 2 h) at 1000 °C were also studied. Longer annealing time resulted in higher capacitance values at all current densities due to the minimized oxygen content. Volumetric specific capacitances of 49 and 24 F/cm³ were achieved at current densities of 0.5 and 20 A/g, respectively. For the high-power-density operation at 31,000 W/kg (or 10,000 W/L), an energy density as high as 11 Wh/kg (or 3.5 Wh/L) was achieved. The results indicated that N-HGE not only improved the conductivity of the composite supercapacitors but also accelerated ion transport by way of shortened diffusion paths through the numerous holes all over the graphene sheets.

An Overview of the Emerging Technologies and Composite Materials for Supercapacitors in Energy Storage Applications

Energy storage is one of the challenges currently confronting the energy sector. However, the invention of supercapacitors has transformed the sector. This modern technology's high energy capacity, reliable supply with minimal lag time, and extended lifetime of supercapacitors have piqued the interest of scientists, and several investigations have been conducted to improve their development. However, there is room for improvement. Consequently, this review presents an up-to-date investigation of different supercapacitor technologies' components, operating techniques, potential applications, technical difficulties, benefits, and drawbacks. In addition, it thoroughly highlights the active materials used to produce supercapacitors. The significance of incorporating every component (electrode and electrolyte), their synthesis approach, and their electrochemical characteristics are outlined. The research further examines supercapacitors' potential in the next era of energy technology. Finally, concerns and new research prospects in hybrid supercapacitor-based energy applications that are envisaged to result in the development of ground-breaking devices, are highlighted.

Recent advances in supramolecular self-assembly derived materials for high-performance supercapacitors

The key preponderance of supramolecular self-assembly strategy is its ability to precisely assemble various functional units at the molecular level through non-covalent bonds to form multifunctional materials. Supramolecular materials have the merits of diverse functional groups, flexible structure, and unique self-healing properties, which make them of great value in the field of energy storage. This paper reviews the latest research progress of the supramolecular self-assembly strategy for the advanced electrode materials and electrolytes for supercapacitors, including supramolecular self-assembly for the preparation of high-performance carbon materials, metal-based materials and conductive polymer materials, and its beneficial effects on the performance of supercapacitors. The preparation of high performance supramolecular polymer electrolytes and their application in flexible wearable devices and high energy density supercapacitors are also discussed in detail. In addition, at the end of this paper, the challenges of the supramolecular self-assembly strategy are summarized and the development of supramolecular-derived materials for supercapacitors is prospected.

The Emerging Layered Hydroxide Plates with Record Thickness for Enhanced High-Mass-Loading Energy Storage

The past decade has witnessed the development of layered-hydroxide-based self-supporting electrodes, but the low active mass ratio impedes its all-around energy-storage applications. Herein, the intrinsic limit of layered hydroxides is broken by engineering F-substituted β-Ni(OH)₂ (Ni-F-OH) plates with a sub-micrometer thickness (over 700 nm), producing a superhigh mass loading of 29.8 mg cm^-2 on the carbon substrate. Theoretical calculation and X-ray absorption spectroscopy analysis demonstrate that Ni-F-OH shares the β-Ni(OH)₂ -like structure with slightly tuned lattice parameters. More interestingly, the synergy modulation of NH₄ ⁺ and F^- is found to serve as the key enabler to tailor these sub-micrometer-thickness 2D plates thanks to the modification effects on the (001) plane surface energy and local OH^- concentration. Guided by this mechanism, the superstructures of bimetallic hydroxides and their derivatives are further developed, revealing they are a versatile family with great promise. The tailored ultrathick phosphide superstructure achieves a superhigh specific capacity of 7144 mC cm^-2 and a superior rate capability (79% at 50 mA cm^-2 ). This work highlights a multiscale understanding of how exceptional structure modulation happens in low-dimensional layered materials. The as-built unique methodology and mechanisms will boost the development of advanced materials to better meet future energy demands.

Facile synthesis of crumpled nitrogen-doped porous carbon nanosheets with ultrahigh surface area for high-performance supercapacitors

Porous carbon nanosheets are currently considered excellent electrode materials for high-performance supercapacitors. However, their ease of agglomeration and stacking nature reduce the available surface area and limit the electrolyte ion diffusion and transport, thereby leading to low capacitance and poor rate capability. To solve these problems, we report an adenosine blowing and KOH activation combination strategy to prepare crumpled nitrogen-doped porous carbon nanosheets (CNPCNS), which exhibit much higher specific capacitance and rate capability compared to flat microporous carbon nanosheets. The method is simple and capable of one-step scalable production of CNPCNS with ultrathin crumpled nanosheets, ultrahigh specific surface area (SSA), microporous and mesoporous structure and high heteroatom content. The optimized CNPCNS-800 with a thickness of 1.59 nm has an ultrahigh SSA of 2756 m² g^-1, high mesoporosity of 62.9% and high heteroatom content (2.6 at% for N, 5.4 at% for O). Consequently, CNPCNS-800 presents an excellent capacitance, high rate capability and long cycling stability both in 6 M KOH and EMIMBF₄. More importantly, the energy density of the CNPCNS-800-based supercapacitor in EMIMBF₄ can reach up to 94.9 W h kg^-1 at 875 W kg^-1 and is still 61.2 W h kg^-1 at 35 kW kg^-1.

Nanocellulose-based electrodes and separator toward sustainable and flexible all-solid-state supercapacitor

Nanocellulose, as the most abundant natural nanomaterial with sustainability, biodegradability, and excellent mechanical properties, has been widely applied in modern electronic systems, particularly, in the flexible electrochemical energy storage devices. Herein, a reduced graphene oxide (RGO)/cellulose nanocrystal/cellulose nanofiber (RCC) composite membrane was prepared by using a one-pot method. Compared to the pure RGO membranes, the RCC composite membranes exhibited better mechanical properties and hydrophilicity. Furthermore, due to the synergistic effect of nanocellulose and RGO sheets, the RCC composite membrane exhibited a specific capacitance as high as 171.3 F·cm^-3. Consequently, a nanocellulose-based symmetric flexible all-solid-state supercapacitor (FASC) was constructed, in which two RCC composite membranes served as electrodes and a porous cellulose nanofiber membrane acted as separator. This fabricated FASC demonstrated a high volumetric specific capacitance of 164.3 F·cm^-3 and a satisfactory energy density of 3.7 mW·h·cm^-3, which exceeded that of many other FASCs ever reported. This work may open a new avenue in design of next-generation nanocellulose based, sustainable and flexible energy storage device.

Recent advances in epitaxial heterostructures for electrochemical applications

Construction of epitaxial heterostructures is crucial for boosting the electrochemical properties of various materials, however a review dedicated to this attractive topic is still lacking. In this Minireview, a timely summary on the achievements of epitaxial heterostructure design for electrochemical applications is provided. We first introduce the synthesis strategies to provide fundamental understanding on how to create epitaxial interfaces between different components. Secondly, the superiorities of epitaxial heterostructures in electrocatalysis, supercapacitors and batteries are highlighted with the underlying structure-property relationship elucidated. Finally, a discussion on the challenges and future prospects of this field is presented.

Effect of Surface Treatment on Performance and Internal Stacking Mode of Electrohydrodynamic Printed Graphene and Its Microsupercapacitor

Microelectronic devices are developing rapidly in portability, wearability, and implantability. This puts forward an urgent requirement for the delicate deposition process of materials. Electrohydrodynamic printing has attracted academic and industrial attention in preparing ultrahigh-density microelectronic devices as a new noncontact, direct graphic, and low-loss thin film deposition process. In this work, a printed graphene with narrow line width is realized by combining the electrohydrodynamic printing and surface treatment. The line width of printed graphene on the hydrophobic treatment surface reduced from 80 to 28 μm. The resistivity decreased from 0.949 to 0.263 Ω·mm. Unexpectedly, hydrophobic treatment can effectively induce random stacking of electrohydrodynamic printed graphene, which avoids parallel stacking and agglomeration of graphene sheets. The performance of printed graphene is thus effectively improved. After optimization, a graphene planar supercapacitor with a printed line width of 28 μm is successfully obtained. Its capacitance can reach 5.39 mF/cm² at 50 mV/s, which is twice higher than that of the untreated devices. The device maintains 84.7% capacitance after 5000 cycles. This work provides a reference for preparing microelectronic devices by ultrahigh precision printing and a new direction for optimizing two-dimensional material properties through stacking adjustment.

Collagen and Silk Fibroin as Promising Candidates for Constructing Catalysts

A catalyst determines the mechanism of an organic chemical reaction, thus enabling the commercially viable formation of desired material products. Biopolymers offer new opportunities for the construction of catalysts by virtue of their biocompatibility, environmental benignity, and sustainability, as well as their low cost. Biopolymers are especially useful as carriers and precursors in catalysis application. The employment of biocompatible and biosustainable collagen and silk fibroin materials will revolutionize state-of-the-art electronic devices and systems that currently rely on conventional technologies. In this review, we first consider the ordered hierarchical structure, origin, and processing methods of collagen and silk fibroin. Then, the unique advantages and applicability of collagen and silk fibroin for constructing catalysts are summarized. Moreover, a summary of the state-of-the-art design, fabrication, and application of collagen- and silk fibroin-based catalysts, as well as the application of collagen- and silk-based catalysts, is presented by focusing on their roles as carriers and precursors, respectively. Finally, challenges and prospects are assessed for the construction and development of collagen and silk fibroin-based catalysts.

Electrochemical Double-Layer Capacitor based on Carbon@ Covalent Organic Framework Aerogels

High energy demand results in comprehensive research of novel materials for energy sources and storage applications. Covalent organic frameworks (COFs) possess appropriate features such as long-range order, permanent porosity, tunable pore size, and ion diffusion pathways to be competitive electrode materials. Herein, we present a deep electrochemical study of two COF-aerogels shaped into flexible COF-electrodes (ECOFs) by a simple compression method to fabricate an electrochemical double-layer capacitor (EDLC). This energy storage system has considerable interest owing to its high-power density and long cycle life compared with batteries. Our result confirmed the outstanding behavior of ECOFs as EDLC devices with a capacity retention of almost 100 % after 10 000 charge/discharge cycles and, to our knowledge, the highest areal capacitance (9.55 mF cm^-2 ) in aqueous electrolytes at higher scan rates (1000 mV s^-1 ) for COFs. More importantly, the hierarchical porosity observed in the ECOFs increases ion transport, which permits a fast interface polarization (low τ₀ values). The complete sheds light on using ECOFs as novel electrode material to fabricate EDLC devices.

Strain engineering of graphene on rigid substrates

Graphene with a large tensile strain is a promising candidate for the new "straintronics'' applications. The current approaches of strain engineering on graphene are mainly realized by flexible or hollow substrates. In this work, a novel method for strained graphene on a rigid substrate assisted by PDMS stretching and interface adjustments is proposed. The Raman spectra show that the maximum strain of graphene on the SiO₂/Si substrate is ∼1.5%, and multiple characterizations demonstrate its high cleanness, flatness, integrity, and reliable electrical performance. The successful strain engineering is attributed to the protection of a layer of formvar resin and the interfacial capillary force of the buffering liquid. We believe this technique can advance strain-related fundamental studies and applications of two-dimensional materials.

Conductive Hydrogels Based on Industrial Lignin: Opportunities and Challenges

The development of green materials, especially the preparation of high-performance conductive hydrogels from biodegradable biomass materials, is of great importance and has received worldwide attention. As an aromatic polymer found in many natural biomass resources, lignin has the advantage of being renewable, biodegradable, non-toxic, widely available, and inexpensive. The unique physicochemical properties of lignin, such as the presence of hydroxyl, carboxyl, and sulfonate groups, make it promising for use in composite conductive hydrogels. In this review, the source, structure, and reaction characteristics of industrial lignin are provided. Description of the preparation method (physical and chemical strategies) of lignin-based conductive hydrogel is elaborated along with their several important properties, such as electrical conductivity, mechanical properties, and porous structure. Furthermore, we provide insights into the latest research advances in industrial lignin conductive hydrogels, including biosensors, strain sensors, flexible energy storage devices, and other emerging applications. Finally, the prospects and challenges for the development of lignin-conductive hydrogels are presented.

Reduced Graphene Oxide Derived from Low-Grade Coal for High-Performance Flexible Supercapacitors with Ultrahigh Cyclability

Preparation of reduced graphene oxide (RGO) from abundant and inexpensive low-grade coal is regarded as one of the most promising methods for utilizing this resource in a high-value and environmentally sustainable manner. As the main precursor for the fabrication of RGO, graphene oxide (GO) can be extracted from low-grade coal such as lignite, but its size is just in the range of tens to hundreds of nanometers, which limits its practical application. Herein, we demonstrate that large-size RGO sheets can be prepared in large quantities by the pretreatment of lignite using the high temperature-high pressure (HTHP) method. The RGO electrode after the reduction reaction by 50 mM NaBH₄ at 105 °C features porosity and high conductivity, which can facilitate high electrochemical reaction efficiency. Thus, we also demonstrate the use of lignite-derived RGO for supercapacitor electrode materials with high performance. The lignite-derived RGO supercapacitor can deliver outstanding volumetric capacitance (30.6 F cm^-3), high energy density (4.2 mW h cm^-3), excellent flexibility (79.5% retention of the initial capacitance at 180° bending), and a long lifespan (112.3% retention of the initial capacitance after 20,000 cycles). It is believed that the proposed large-size RGO based on reasonable optimization of inferior lignite will offer a new prospect for next-generation energy storage applications.

Compacted N-Doped 3D Bicontinuous Nanoporous Graphene/Carbon Nanotubes@Ni-Doped MnO2 Electrode for Ultrahigh Volumetric Performance All-Solid-State Supercapacitors at Wide Temperature Range

Developing wide temperature range flexible solid-state supercapacitors with high volumetric energy density is highly desirable to meet the demands of the rapidly developing field of miniature consumer electronic devices and promote their widespread adoption. Herein, high-quality dense N-doped 3D porous graphene/carbon nanotube (N-3DG/CNTs) hybrid films are prepared and used as the substrate for the growth of Ni-doped MnO₂ (Ni-MnO₂ ). The integrated and interconnected architecture endows N-3DG/CNTs@Ni-MnO₂ composite electrodes' high conductivity and fast ion/electron transport pathway. Subsequently, 2.4 V solid-state supercapacitors are fabricated based on compacted N-3DG/CNTs@Ni-MnO₂ positive electrodes, which exhibit an ultrahigh volumetric energy density of 78.88 mWh cm^-3 based on the entire device including electrodes, solid-state electrolyte, and packing films, excellent cycling stability up to 10 000 cycles, and a wide operating temperature range from -20 to 70 °C. This work demonstrates a design of flexible solid-state supercapacitors with exceptional volumetric performance capable of operation under extreme conditions.

Two-Dimensional Hybrid Nanosheet-Based Supercapacitors: From Building Block Architecture, Fiber Assembly, and Fabric Construction to Wearable Applications

Fiber-based supercapacitors (F-SCs) have inspired widespread interest in the fields of wearable technology, energy, and carbon neutralization due to their highly deformable flexibility, fast charging/discharging capability, long-term stability, and energy conservation ability. In this review, we summarize the latest developments for fabricating fibrous electrodes of F-SCs where advanced micro two-dimensional (2D) building blocks (e.g., MXene and graphene) are chemically assembled and constructed into ordered mesofibers and multifunctional macrofabrics. Diverse fundamental principles of 2D hybrid nanosheets with respect to surface controls, pseudocapacitive modifications, and microstructural manipulations, promoting rapid electron transfer and charge conduction, are introduced. Additionally, various spinning methods for assembling and fabricating sophisticated fibers with advanced nano/microstructures, including hierarchical skeletons, anisotropic backbones, surface/entire porous frameworks, and vertical-aligned networks, for boosting ionic kinetic transport/storage are presented. Likewise, the structure-activity relationships between the porous structure and electrochemical performance are clarified. Moreover, multifunctional fabrics in terms of high flexibilities/strengths, superior electrical conductivities, and stabilized operations, which realize large energy density, deformable capability, and robust stability under harsh conditions, are emphasized. In particular, the potential power-supply applications, including flexible electronic devices, self-powered functions, and energy-sensor systems, are highlighted. Finally, a short conclusion and outlook, along with the current challenges and future opportunities of next-generation F-SCs, are proposed.

Tetra germanium nonaselenide enwrapped with reduced graphene oxide and functionalized carbon nanotubes (Ge4Se9/RGO/FCNTs) hybrids for improved energy storage performances

The development of multifunctional layered semiconductor materials and their carbonaceous hybrids as acceptable positive electrode materials for supercapacitor application is of key interest. Ternary germanium selenide (Ge₄Se₉) with reduced graphene oxide (RGO) and functionalized carbon nanotube (FCNT) hybrids were successfully synthesized by following a one-step hydrothermal approach, and their electrochemical energy storage performance toward supercapacitor (SC) applications was investigated. It was observed that the specific capacitance of Ge₄Se₉/RGO/FCNTs was 440 F g^-1 at 1 A g^-1 in an acidic (1 M H₂SO₄) medium. Further, the material showed 83% retention of its own initial value of capacitance with 98% coulombic efficiency after 5000 galvanostatic charge-discharge cycles. Considering the two-dimensional (2D) layered structures of MXenes with their greater stability, exceptional hydrophilicity, and pseudocapacitive behavior in aqueous electrolytes makes them an alternative for the fabrication of asymmetric SC devices. The above findings about MXenes suggest the design of an asymmetric device using MXene as the negative electrode material and as-prepared Ge₄Se₉/RGO/FCNTs as the positive electrode material in a similar electrolyte media. The fabricated Ge₄Se₉/RGO/FCNTs//MXenes displayed a higher specific capacitance of 102 F g^-1 at 1 A g^-1, with an acceptable energy density (E.D.) of 32 W h kg^-1 and a power density (P.D.) of 1071 W kg^-1. Furthermore, over long-term repeated 5000 GCD cycles the fabricated device retained 92% of its initial capacitance and good reversibility (96% coulombic efficiency), making the Ge₄Se₉/RGO/FCNT//MXenes assembly a preferable electrode material for enhancing asymmetric SC performance.

Sub-Second Joule-Heated RuO2-Decorated Nitrogen- and Sulfur-Doped Graphene Fibers for Flexible Fiber-type Supercapacitors

Graphene-based fiber-shaped supercapacitors (FSSCs) have received considerable attention as potential wearable energy storage devices owing to their simple operating mechanism, flexibility, and long-term stability. To date, energy storage capacities of supercapacitors have been significantly improved via strategies such as heteroatom doping and the incorporation of pseudocapacitive metal oxides. Herein, we develop a novel and scalable direct-hybridization method that combines heteroatom doping and metal oxide hybridization for the fabrication of high-performance FSSCs. Using porous and highly conductive nitrogen and sulfur co-doped graphene fibers (NS-GFs) as self-heating units, we successfully convert ruthenium hydroxide anchored to the surface into ruthenium oxide nanoparticles after programmed sub-second electrothermal annealing without structural damage of the fibers. The resulting fibers show an increased gravimetric capacitance of 68.88 F g^-1 compared to that of the pristine NS-GF (8.32 F g^-1), excellent cyclic stability maintaining 96.67% of the initial capacitance after 20 000 continuous charging/discharging cycles, and good mechanical flexibility. The findings of this work advocate a successful Joule heating strategy for preparing high-performance graphene-based metal oxide hybrid FSSCs for use in energy storage applications.

Small Size, Big Impact: Recent Progress in Bottom-Up Synthesized Nanographenes for Optoelectronic and Energy Applications

Bottom-up synthesized graphene nanostructures, including 0D graphene quantum dots and 1D graphene nanoribbons, have recently emerged as promising candidates for efficient, green optoelectronic, and energy storage applications. The versatility in their molecular structures offers a large and novel library of nanographenes with excellent and adjustable optical, electronic, and catalytic properties. In this minireview, recent progress on the fundamental understanding of the properties of different graphene nanostructures, and their state-of-the-art applications in optoelectronics and energy storage are summarized. The properties of pristine nanographenes, including high emissivity and intriguing blinking effect in graphene quantum dots, superior charge transport properties in graphene nanoribbons, and edge-specific electrochemistry in various graphene nanostructures, are highlighted. Furthermore, it is shown that emerging nanographene-2D material-based van der Waals heterostructures provide an exciting opportunity for efficient green optoelectronics with tunable characteristics. Finally, challenges and opportunities of the field are highlighted by offering guidelines for future combined efforts in the synthesis, assembly, spectroscopic, and electrical studies as well as (nano)fabrication to boost the progress toward advanced device applications.

Role of Cr Doping on the Structure, Electronic Structure, and Electrochemical Properties of BiFeO3 Nanoparticles

BiFe1−xCrxO3, (0 ≤ x ≤ 10) nanoparticles were prepared through the sol−gel technique. The synthesized nanoparticles were characterized using various techniques, viz., X-ray diffraction, high-resolution field emission scanning electron microscopy (HRFESEM), energy dispersive spectroscopy (EDS), UV−Vis absorption spectroscopy, photoluminescence (PL), dc magnetization, near-edge X-ray absorption spectroscopy (NEXAFS) and cyclic voltammetry (CV) measurements, to investigate the structural, morphological, optical, magnetic and electrochemical properties. The structural analysis showed the formation of BiFeO3 with rhombohedral (R3c) as the primary phase and Bi25FeO39 as the secondary phase. The secondary phase percentage was found to reduce with increasing Cr content, along with reductions in crystallite sizes, lattice parameters and enhancement in strain. Nearly spherical shape morphology was observed via HRFESEM with Bi, Fe, Cr and O as the major contributing elements. The bandgap reduced from 1.91 to 1.74 eV with the increase in Cr concentration, and PL spectra revealed emissions in violet, blue and green regions. The investigation of magnetic field (H)-dependent magnetization (M) indicated a significant effect of Cr substitution on the magnetic properties of the nanoparticles. The ferromagnetic character of the samples was found to increase with the increase in the Cr concentration and the increase in the saturation magnetization. The Fe (+3/+4) was dissolved in mixed-valence states, as found through NEXAFS analysis. Electrochemical studies showed that 5%-Cr-doped BFO electrode demonstrated outstanding performance for supercapacitors through a specific capacitance of 421 F g−1 measured with a scan rate of 10 mV s−1. It also demonstrated remarkable cyclic stability through capacitance retention of >78% for 2000 cycles.

Highly Porous Holey Carbon for High Areal Energy Density Solid-State Supercapacitor Application

Biomass materials are perceived as sustainable, carbon-rich precursors for the fabrication of carbon materials. In this study, we demonstrated the capacitance performance of biomass-derived carbon, produced by using golden shower tree seeds (GTs) as carbon precursors and potassium ferrate (K₂FeO₄) as the activation agent. The as-prepared porous carbon (GTPC) possessed an ultrahigh specific surface area (1915 m² g^-1) and abundant pores. They also exhibited superior electrochemical performance, owing to their well-constructed porous structure, high surface area, and optimized porous structure. Optimized activated carbon (GTPC-1) was used to assemble a symmetric solid-state supercapacitor device with poly(vinyl alcohol) (PVA)/H₂SO₄ as a solid-state gel electrolyte. The device exhibited a maximum areal energy density of 42.93 µWh cm^-2 at a power density of 520 µW cm^-2.

Hierarchical nanoarchitectonics of ordered mesoporous carbon from lignin for high-performance supercapacitors

The synthesis of ordered mesoporous carbons (OMCs) with hierarchical pore structure is significant for supercapacitor applications as electrode material. In this study, the ordered mesoporous carbons with hierarchical pore structure (HOMC) are synthesized via solvent evaporation induced self-assembly (EISA) method using lignin from walnut shell as carbon precursor and Co²⁺ ion as crosslinking agent, followed by removal of metal by diluted acid and chemical activation with KHCO₃. The prepared HOMC material has a large specific surface area of 2033 m² g^-1 and high pore volume of 1.59 cm³ g^-1, and it shows good electrochemical performance as the electrode of supercapacitor with high specific supercapacitances of 286 and 206 F g^-1 in 6 M KOH aqueous solutions at 0.2 and 20 A g^-1, respectively. The assembled HOMC-based symmetric supercapacitors provides a specific energy density of 13.5 Wh kg^-1 at a high power density of 44.3 kW kg^-1 and keep good cycling stability after 5000 cycle tests. The superior electrochemical performance is ascribed to the long range ordered parallel mesoporous channels, hierarchical porous structure, high specific surface area and appropriate microporous/mesoporous ratio. The materials prepared in this study have the potential to be used in the fields of adsorption, energy storage and capacitance deionization.

Diamond Supercapacitors: Towards Durable, Safe, and Biocompatible Aqueous-Based Energy Storage

Durable and safe energy storage is required for the next generation of miniature bioelectronic devices, in which aqueous electrolytes are preferred due to the advantages in safety, low cost, and high conductivity. While rechargeable aqueous batteries are among the primary choices with relatively low power requirements, their lifetime is generally limited to a few thousand charging/discharging cycles as the electrode material can degrade due to electrochemical reactions. Electrical double layer capacitors (EDLCs) possess increased cycling stability and power density, although with as-yet lower energy density, due to quick electrical adsorption and desorption of ions without involving chemical reactions. However, in aqueous solution, chemical reactions which cause electrode degradation and produce hazardous species can occur when the voltage is increased beyond its operation window to improve the energy density. Diamond is a durable and biocompatible electrode material for supercapacitors, while at the same time provides a larger voltage window in biological environments. For applications requiring higher energy density, diamond-based pseudocapacitors (PCs) have also been developed, which combine EDLCs with fast electrochemical reactions. Here we inspect the properties of diamond-related materials and discuss their advantages and disadvantages when used as EDLC and PC materials. We argue that further optimization of the diamond surface chemistry and morphology, guided by computational modelling of the interface, can lead to supercapacitors with enhanced performance. We envisage that such diamond-based supercapacitors could be used in a wide range of applications and in particular those requiring high performance in biomedical applications.

Facile preparation of flexible binder-free graphene electrodes for high-performance supercapacitors

A facile two-step strategy to prepare flexible graphene electrodes has been developed for supercapacitors using thermal reduction of graphene oxide (GO) and thermally reduced graphene oxide (TRGO) composite films. The tunable porous structure of the GO/TRGO film provided channels to release the high pressure generated by CO₂ gas. The graphene electrode obtained from reduced-GO/TRGO (1 : 1 in mass ratio) film showed great flexibility and high film density (0.52 g cm⁻³). Using the EMI-BF₄ electrolyte with a working voltage of 3.7 V, the as-fabricated free-standing reduced-GO/TRGO (1 : 1) film achieved a great gravimetric capacitance of 180 F g⁻¹ (delivering a gravimetric energy density of 85.6 W h kg⁻¹), a volumetric capacitance of 94 F cm⁻³ (delivering a volumetric energy density of 44.7 W h L⁻¹), and a 92% retention after 10 000 charge/discharge cycles. In addition, the solid state flexible supercapacitor with the free-standing reduced-GO/TRGO (1 : 1) film as the electrodes and the EMI-BF₄/poly (vinylidene fluoride hexafluopropylene) (PVDF-HFP) gel as the electrolyte also demonstrated a high gravimetric capacitance of 146 F g⁻¹ with excellent mechanical flexibility, bending stability, and electrochemical stability. The strategy developed in this study provides great potentials for the synthesis of flexible graphene electrodes for supercapacitors.

A supercapacitor electrode is developed with a free-standing graphene film by a facile two-step strategy. The graphene electrode achieved a gravimetric capacitance of 180 F g⁻¹ and a volumetric capacitance of 94 F cm⁻³.

Porous organic polymers for high-performance supercapacitors

With the aim of addressing the global warming issue and fossil energy shortage, eco-friendly and sustainable renewable energy technologies are urgently needed. In comparison to energy conversion, studies on energy storage fall behind and remain largely to be explored. By storing energy from electrochemical processes at the electrode surface, supercapacitors (SCs) bridge the performance gap between electrostatic double-layer capacitors and batteries. Organic electrode materials have drawn extensive attention because of their special power density, good round trip efficiency and excellent cycle stability. Porous organic polymers (POPs) have drawn extensive attention as attractive electrode materials in SCs. In this review, we present and discuss recent advancements and design principles of POPs as efficient electrode materials for SCs from the perspectives of synthetic strategies and the structure-performance relationships of POPs. Finally, we put forward the outlook and prospects of POPs for SCs.

Banana Peel and Conductive Polymers-Based Flexible Supercapacitors for Energy Harvesting and Storage

Flexible supercapacitors are highly demanding due to their wearability, washability, lightweight property and rollability. In this paper, a comprehensive review on flexible supercapacitors based on conductive polymers such as polypyrrole (PPy), polyaniline (PANI) and poly(3,4-ethylenedioxtthiophne)-polystyrene sulfonate (PEDOT:PSS). Methods of enhancing the conductivity of PEDOT:PSS polymer using various composites and chemical solutions have been reviewed in detail. Furthermore, supercapacitors based on carbonized banana peels and methods of activation have been discussed in point. This review covers the up-to-date progress achieved in conductive polymer-based materials for supercapacitor electrodes. The effect of various composites with PEDOT:PSS have been discussed. The review result indicated that flexible, stretchable, lightweight, washable, and disposable wearable electronics based on banana peel and conductive polymers are highly demanding.

Foldable batteries: from materials to devices

Wearable electronics is a growing field that has important applications in advanced human-integrated systems with high performance and mechanical deformability, especially foldable characteristics. Although foldable electronics such as rollable TVs (LG signature OLED R) or foldable smartphones (Samsung Galaxy Z fold/flip series) have been successfully established in the market, these devices are still powered by rigid and stiff batteries. Therefore, to realize fully wearable devices, it is necessary to develop state-of-the-art foldable batteries with high performance and safety in dynamic deformation states. In this review, we cover the recent progress in developing materials and system designs for foldable batteries. The Materials section is divided into three sections aimed at helping researchers choose suitable materials for their systems. Several foldable battery systems are discussed and the combination of innovative materials and system design that yields successful devices is considered. Furthermore, the basic analysis process of electrochemical and mechanical properties is provided as a guide for researchers interested in the evaluation of foldable battery systems. The current challenges facing the practical application of foldable batteries are briefly discussed. This review will help researchers to understand various aspects (from material preparation to battery configuration) of foldable batteries and provide a brief guideline for evaluating the performance of these batteries.

Optimization of the electrodeposition process of a polypyrrole/multi-walled carbon nanotube fiber electrode for a flexible supercapacitor

In the large-scale production of flexible supercapacitors, given the poor interface stability and the low mass loading of functional films on the fiber electrode, cyclic voltammetry (CV) and constant current (CC) electrodeposition methods were adopted to prepare polypyrrole/multi-walled carbon nanotubes (PPy/MWCNTs) on the surface of polyacrylonitrile (PAN) carbon cloth to explore the optimization process. The surface morphology and structural properties of the flexible electrode were characterized, and the electrical and electrochemical properties were studied. The research indicated that the PPy/MWCNTs were uniformly distributed on the fiber surface in the form of a linear structure and were amorphous and rich in carbon, nitrogen, and oxygen functional groups. A higher deposition current density helped improve the degree of coating of the MWCNTs with PPy and the number of oxygen-containing functional groups. The electrical and electrochemical properties of the flexible electrode prepared using the CC method were excellent; the electrochemical properties of the samples in the bent state were not significantly different from those in the straightened state. Using CC and CV methods, the conductivities of the samples were 32.4 S cm⁻¹ and 24.1 S cm⁻¹, the area-specific capacitance values were C 96.24 mF cm⁻² and 46.18 mF cm⁻² at a scan rate of 100 mV s⁻¹, the equivalent series resistance R_s values were 2.74 Ω and 4.67 Ω, the specific capacitance retention rates were 94.4% and 88.3% after 1000 cycles, and the capacitance retention rates were 89.7% and 80.6% after 5000 cycles, respectively. The differences in the performances of the flexible electrodes using the same preparation solvent and different preparation processes were due to the higher deposition current density of the CC method compared with that of CV. The former enhanced the polymerization degree of the PPy/MWCNT flexible electrode and improved the electrochemical performance. The presented research results are significant for the optimization of large-scale production processes.

The difference between the electrical and electrochemical properties of PPy/MWCNTs flexible electrodes was found to be due to the constant current method having a higher deposition current density, leading to a higher degree of polymerization.

Food seasoning-derived gel polymer electrolyte and pulse-plasma exfoliated graphene nanosheet electrodes for symmetrical solid-state supercapacitors

The symmetrical solid-state supercapacitors using graphene nanosheet electrodes and table salt-derived green gel polymer electrolyte which provide a stable energy storage device, and good electrochemical capability are introduced.

Sandwich-like NiCo-LDH/rGO with Rich Mesopores and High Charge Transfer Capability for Flexible Supercapacitors

Layered double metal hydroxides (LDHs) have been widely used in the energy storage field due to adjustable composition and interlayer spacing. However, easy to agglomerate, poor electrical conductivity, and large...