Advanced Graphene-Based Binder-Free Electrodes for High-Performance Energy Storage

Ultrastable monolithic electrodes with single-atom platinum-oxygen sites for efficient hydrogen evolution in acidic conditions

PtNC single-atom catalysts (SACs) single-atom catalysts (SACs) are promising for acidic hydrogen evolution reaction (HER) but suffer from instability at high current densities, limiting their large-scale application. Herein, PtO bonds are constructed to securely anchor atomically dispersed Pt for single-atom (SA) catalysis, utilizing etched vertical graphene (EVG) nanosheets as monolithic supports (Pt-SAs/EVG). Compared to PtNC, the resultant PtO4 coordination demonstrates improved stability while maintaining significant catalytic activity. When applying this catalyst in the acidic HER, a high turnover frequency (34.6 s-1) is achieved at 70 mV, accompanied by exceptional durability exceeding 100 h at -100 mA cm-2. Theoretical analyses indicate that the PtO bonds confer stability to the Pt atoms, facilitating the efficient adsorption of protons and the subsequent desorption of hydrogen. The prepared Pt-SAs/EVG can also be directly employed as the cathode to afford stable operation at 0.5 A cm-2 in a proton exchange membrane electrolyzer cell. This study offers novel insights into enhancing the performance of SACs for industrial applications in electrocatalysis.

Oxidative MnO2 Template Assisted Electrochemical Fabrication of Graphene/Polypyrrole Supercapacitor Electrodes

Improving the morphological structure of active materials is a reliable strategy for the fabrication of high-performance supercapacitor electrodes. In this study, we introduce a feasible approach to constructing the graphene/polypyrrole (PPy) composite film implanted onto the current collector through a two-step electrochemical deposition method utilizing MnO2 as an intermediary template. The reduced graphene oxide (rGO) hydrogel film is first hydrothermally grown on a carbon cloth (CC) substrate to obtain a porous rGO@CC electrode on which MnO2 is electrodeposited. Then the as-prepared rGO/MnO2@CC electrode is subjected to the electrochemical polymerization of pyrrole, with MnO2 acting as an oxidizing template to facilitate the oxidative polymerization of pyrrole, ultimately yielding an rGO/PPy composite film on CC. The PPy synthesized via this methodology exhibits a distinctive interconnected structure, resulting in superior electrochemical performance compared with the electrode with PPy directly electrodeposited on rGO@CC. The optimized electrode achieves an impressive specific capacitance of 583.6 F g-1 at 1 A g-1 and retains 83% of its capacitance at 20 A g-1, with a capacitance loss of only 9.5% after 5000 charge-discharge cycles. The corresponding all-solid-state supercapacitor could provide a high energy density of 22.5 Wh kg-1 and a power density of 4.6 kW kg-1, with a capacitance retention of 82.7% after 5000 charge-discharge cycles. Furthermore, the device also demonstrates good flexibility performance upon bending at 90 and 180°. This work presents an innovative method for the preparation of carbon material/conducting polymer electrodes with specific structural characteristics and superior performance.

Production of Large Specific Capacitance by Electrodes with Low Active Mass and Synergistic Mechanisms

Decoration of vanadium nitride nanoparticles on carbon nanotubes creates electrodes with three different energy storage mechanisms that operate synergistically to give a high specific capacitance with a low active mass. Calculation and measurements further indicate the power and energy density to be as high as 105-106 W/kg and 102 Wh/kg, respectively. Particle attachment also greatly improves the capacitive coefficient, including ionic transmittance, charge transfer, porosity, and conductivity. Corrosion tests based on the Tafel method reveal the corrosion potential and current of electrodes as low as -0.721 V and 7.53 × 10-4 A, respectively.

Utilization of 2D materials in aqueous zinc ion batteries for safe energy storage devices

Aqueous rechargeable battery has been an intense topic of research recently due to the significant safety issues of conventional Li-ion batteries (LIBs). Amongst the various candidates of aqueous batteries, aqueous zinc ion batteries (AZIBs) hold great promise as a next generation safe energy storage device due to its low cost, abundance in nature, low toxicity, environmental friendliness, low redox potential, and high theoretical capacity. Yet, the promise has not been realized due to their limitations, such as lower capacity compared to traditional LIB, dendrite growth, detrimental degradation of electrode materials structure as ions intercalate/de-intercalate, and gas evolution/corrosion at the electrodes, which remains a significant challenge. To address the challenges, various 2D materials with different physiochemical characteristics have been utilized. This review explores fundamental physiochemical characteristics of widely used 2D materials in AZIBs, including graphene, MoS2, MXenes, 2D metal organic framework, 2D covalent organic framework, and 2D transition metal oxides, and how their characteristics have been utilized or modified to address the challenges in AZIBs. The review also provides insights and perspectives on how 2D materials can help to realize the full potential of AZIBs for next-generation safe and reliable energy storage devices.

Fabrication of superhydrophilic porous carbon materials through a porogen-free method: Surface and structure modification promoting the two-electron oxygen reduction activity

HYPOTHESIS

Electrochemical manufacture of H₂O₂ through the two-electron oxygen reduction reaction (2e^- ORR), providing prospects of the distributed production of H₂O₂ in remote regions, is considered a promising alternative to the energy-intensive anthraquinone oxidation process.

EXPERIMENTS

In this study, one glucose-derived oxygen-enriched porous carbon material (labeled as HGC₅₀₀) is developed through a porogen-free strategy integrating structural and active site modification.

FINDINGS

The superhydrophilic surface and porous structure together promote the mass transfer of reactants and accessibility of active sites in the aqueous reaction, while the abundant CO species (e.g., aldehyde groups) are taken for the main active site to facilitate the 2e^- ORR catalytic process. Benefiting from the above merits, the obtained HGC₅₀₀ possesses superior performance with a selectivity of 92 % and mass activity of 43.6 A g_cat^-1 at 0.65 V (vs. RHE). Besides, the HGC₅₀₀ can operate steadily for 12 h with the accumulation of H₂O₂ reaching up to 4090±71 ppm and a Faradic efficiency of 95 %. The H₂O₂ generated from the electrocatalytic process in 3 h can degrade a variety of organic pollutants (10 ppm) in 4-20 min, displaying the potential in practical applications.

On the Road to the Frontiers of Lithium-Ion Batteries: A Review and Outlook of Graphene Anodes

Graphene has long been recognized as a potential anode for next-generation lithium-ion batteries (LIBs). The past decade has witnessed the rapid advancement of graphene anodes, and considerable breakthroughs are achieved so far. In this review, the aim is to provide a research roadmap of graphene anodes toward practical LIBs. The Li storage mechanism of graphene is started with and then the approaches to improve its electrochemical performance are comprehensively summarized. First, morphologically engineered graphene anodes with porous, spheric, ribboned, defective and holey structures display improved capacity and rate performance owing to their highly accessible surface area, interconnected diffusion channels, and sufficient active sites. Surface-modified graphene anodes with less aggregation, fast electrons/ions transportation, and optimal solid electrolyte interphase are discussed, demonstrating the close connection between the surface structure and electrochemical activity of graphene. Second, graphene derivatives anodes prepared by heteroatom doping and covalent functionalization are outlined, which show great advantages in boosting the Li storage performances because of the additionally introduced defect/active sites for further Li accommodation. Furthermore, binder-free and free-standing graphene electrodes are presented, exhibiting great prospects for high-energy-density and flexible LIBs. Finally, the remaining challenges and future opportunities of practically available graphene anodes for advanced LIBs are highlighted.

Integrated Gas Diffusion Electrode with High Conductivity Obtained by Skin Electroplating for High Specific Power Density Fuel Cell

Smaller volume/weight and higher output power/energy density are always the goals of electrochemistry energy devices. Here, a simple strategy is proposed to prepare an integrated gas diffusion electrode (GDE) with high conductivity through skin electroplating. The skin electroplating is the combination of magnetron sputtering and spatial confinement electroplating. The electroplated metal obtained by skin electroplating is uniformly, continuously, and tightly attached to the surface of carbon paper like a layer of skin. Uniform and continuous electroplating metal layer endows the integrated electrode excellent conductivity with the square resistance as low as 27 mΩ sq^-1 . In application, the self-breathing fuel cell with 1 cm² active area can harvest ultrahigh volume specific power density (20.9 kW L^-1 ). Additionally, the weight of the fuel cell stack (23 W) with the integrated electrode is only 20 g, which is only 7% of the commercial stack with the same power. The mass specific power density reaches 1150 W kg^-1 , which is 15 times of the commercial stack. Outstandingly, the stack can charge 4 mobile phones at the same time. More importantly, the skin electroplating provides an effective strategy to improve the specific power density of other energy devices including Zn-air batteries, Li-air batteries, and so on.

Identifying the Active Sites of Heteroatom Graphene as a Conductive Membrane for the Electrochemical Filtration of Organic Contaminants

The dopants of sulfur, nitrogen, or both, serving as the active sites, into the graphitic framework of graphene is an efficient strategy to improve the electrochemical performance of electrochemical membrane filtration. However, the covalent bonds between the doped atoms and the substrate that form different functional groups have a significant role in the specific activity for pollutant degradation. Herein, we found that the singly doped heteroatom graphene (NG and SG) achieved superior removal efficiency of pollutants as compared with that of the double doped heteroatom graphene (SNG). Mechanism studies showed that the doped N of NG presented as graphitic N and substantially increased electron transfer, whereas the doped S of SG posed as -C-SOx-C- provided more adsorption sites to improve electrochemical performance. However, in the case of SNG, the co-doped S and N cannot form the efficient graphitic N and -C-SOx-C- for electrochemical degradation, resulting in a low degradation efficiency. Through the fundamental insights into the bonding of the doped heteroatom on graphene, this work furnishes further directives for the design of desirable heteroatom graphene for membrane filtration.

Recent Advances of Carbon Materials in Anodes for Aqueous Zinc Ion Batteries

Carbon-based materials have been successfully applied in the zinc ion batteries to improve the energy storage capability and durability of zinc anodes. In this review, four types of carbon materials (conventional carbons, fiber-like carbons, carbon nanotubes, graphene and other 2D carbon materials) are introduced based on the electrode preparation, physicochemical property and battery performance. Several modification strategies are also illustrated, such as heteroatom doping, hierarchical design and metal/carbon composites. Besides the discussion of existing issues of zinc anodes, the structure-performance relationships are analyzed in depth. Finally, conclusive remarks of this review are summarized and prospects of the future improvement are proposed.

3D Continuously Porous Graphene for Energy Applications

Constructing bulk graphene materials with well-reserved 2D properties is essential for device and engineering applications of atomically thick graphene. In this article, the recent progress in the fabrications and applications of sterically continuous porous graphene with designable microstructures, chemistries, and properties for energy storage and conversion are reviewed. Both template-based and template-free methods have been developed to synthesize the 3D continuously porous graphene, which typically has the microstructure reminiscent of pseudo-periodic minimal surfaces. The 3D graphene can well preserve the properties of 2D graphene of being highly conductive, surface abundant, and mechanically robust, together with unique 2D electronic behaviors. Additionally, the bicontinuous porosity and large curvature offer new functionalities, such as rapid mass transport, ample open space, mechanical flexibility, and tunable electric/thermal conductivity. Particularly, the 3D curvature provides a new degree of freedom for tailoring the catalysis and transport properties of graphene. The 3D graphene with those extraordinary properties has shown great promises for a wide range of applications, especially for energy conversion and storage. This article overviews the recent advances made in addressing the challenges of developing 3D continuously porous graphene, the benefits and opportunities of the new materials for energy-related applications, and the remaining challenges that warrant future study.

Recent Advances in Self-Powered Piezoelectric and Triboelectric Sensors: From Material and Structure Design to Frontier Applications of Artificial Intelligence

The development of artificial intelligence and the Internet of things has motivated extensive research on self-powered flexible sensors. The conventional sensor must be powered by a battery device, while innovative self-powered sensors can provide power for the sensing device. Self-powered flexible sensors can have higher mobility, wider distribution, and even wireless operation, while solving the problem of the limited life of the battery so that it can be continuously operated and widely utilized. In recent years, the studies on piezoelectric nanogenerators (PENGs) and triboelectric nanogenerators (TENGs) have mainly concentrated on self-powered flexible sensors. Self-powered flexible sensors based on PENGs and TENGs have been reported as sensing devices in many application fields, such as human health monitoring, environmental monitoring, wearable devices, electronic skin, human–machine interfaces, robots, and intelligent transportation and cities. This review summarizes the development process of the sensor in terms of material design and structural optimization, as well as introduces its frontier applications in related fields. We also look forward to the development prospects and future of self-powered flexible sensors.

Mo-doped Co3O4 ultrathin nanosheet arrays anchored on nickel foam as a bi-functional electrode for supercapacitor and overall water splitting

Simple preparation, favorable price and environmental protection have been a long-term challenge in the field of electrochemistry. Herein, we studied and prepared a bifunctional Mo-doped Co₃O₄ ultrathin nanosheets, which has been validated as an effective binder-free electrode material for electrocatalytic water splitting and supercapacitors. The material has a large specific surface area, high electrical conductivity and exposure to more active sites, breaking down the limited performance and range of use of transition metal oxides. Benefiting from intriguing ultrathin property and conductivity, OER and HER process of 0.4Mo-Co₃O₄ have a small Tafel slope of 83.7 and 98 mV dec^-1, respectively. The current density at 10 mA cm^-2 show a low overpotential of 315 and 79 mV and significant stability. The water electrolytic device requires a potential of 1.64 V to reach 10 mA cm^-2, and the potential change is negligible after 12 h of continuous electrolysis. In addition, the manifest improved electrochemical performance of 0.3Mo-Co₃O₄ as supercapacitor electrode material shows high areal capacitance 2815 mF cm^-2 at 1 mA cm^-2, excellent rate performance (85% at 10 mA cm^-2) and retains 90% of the initial capacitance by cycling 5000 at a current density of 10 mA cm^-2. Moreover, 0.3Mo-Co₃O₄||0.3Mo-Co₃O₄ symmetrical supercapacitor has a maximum volumetric energy density of 1.25 mW h cm^-3 at a power density of 7.1 mW cm^-3 and superior cycle life. The influence of doping on electrochemical performance was studied by changing the content of doped metal ions, which is of great significance for the exploration of supercapacitor and electrocatalytic hydrolysis of bifunctional electrode materials.

Highly Sensitive Ultrastable Electrochemical Sensor Enabled by Proton-Coupled Electron Transfer

Electrochemical sensors are critical to artificial intelligence by virtue of capability of mimicking human skin to report sensing signals. But their practical applications are restricted by low sensitivity and limited cycling stability, which result from piezoionic mechanism with insufficient sensing response. Here, we report a highly sensitive ultrastable sensor based on proton-coupled electron transfer, which is different from piezoionic mechanism. The sensor gives a high sensing signal output of 117 mV, which is 16 times higher than that of counterpart device (7 mV). It delivers excellent working stability with performance retention as high as 99.13% over 10 000 bending cycles in air, exceeding that of the best-known sensors reported previously. The flexible sensor displays high sensitivity in detecting real-time signals of human activities with large and subtle deformations, including wrist bending, moving speed, pulse wave and voice vibration. Smart functions, such as braille language and handwriting recognitions, are demonstrated for artificial intelligence.

Self-Templated Formation of Fluffy Graphene-Wrapped Ni5P4 Hollow Spheres for Li-Ion Battery Anodes with High Cycling Stability

Transition-metal phosphides have gained great importance in the field of energy conversion and storage such as electrochemical water splitting, fuel cells, and Li-ion batteries. In this study, a rationally designed novel fluffy graphene (FG)-wrapped monophasic Ni₅P₄ (Ni₅P₄@FG) is in-situ-synthesized using a chemical vapor deposition method as a Li-ion battery anode material. The porous and hollow structure of Ni₅P₄ core is greatly helpful for lithium-ion diffusion, and at the same time, the cilia-like graphene nanosheet shell provides an electron-conducting layer and stabilizes the solid electrolyte interface formed on the Ni₅P₄ surface. The Ni₅P₄@FG sample shows a high reversible capacity of 739 mAh g^-1 after 300 cycles at a specific current density of 500 mA g^-1. The high capacity, superior cycling stability, and improved rate capability of Ni₅P₄@FG are ascribed to its unique hierarchical structure. Moreover, the present efficient fabrication methodology of Ni₅P₄@FG has potential to be developed as a general method for the synthesis of other transition-metal phosphides.

Theories and models of supercapacitors with recent advancements: impact and interpretations

Supercapacitors provide remarkable eco-friendly advancement in energy conversion and storage with a huge potential to control the future economy of the entire world. Currently, industries focus on the design and engineering aspects of supercapacitors with high performance (high energy), flexibility (by the use of composite polymer based electrolytes), high voltage (ionic liquid) and low cost. The paper reviews the modelling techniques like Empirical modelling, Dissipation transmission line models, Continuum models, Atomistic models, Quantum models, Simplified analytical models etc. proposed for the theoretical study of Supercapacitors and discusses their limitations in studying all the aspects of Supercapacitors. It also reviews the various software packages available for Supercapacitor (SC) modelling and discusses their advantages and disadvantages. The paper also reviews the Experimental advancements in the field of electric double layer capacitors (EDLCs), pseudo capacitors and hybrid/asymmetric supercapacitors and discusses the commercial progress of supercapacitors as well.

Engineering flexible carbon nanofiber concatenated MOF-derived hollow octahedral CoFe2O4 as an anode material for enhanced lithium storage

We constructed a necklace-like structure consisting of hollow CoFe₂O₄ octahedral nanoparticles encapsulated in N-doped carbon nanofibers (CoFe₂O₄@CNFs), which deliver an excellent electrochemical performance as free-standing anodes for LIBs.

Microstructure and electrochemical performance of 3D hierarchical porous graphene/polyaniline composites

3D hierarchical porous graphene/polyaniline composites show outstanding performance in energy storage field due to their unique microstructure and properties.

Hierarchical Co-doped SnS2@Ni(OH)2 double-shell crystalline structure on carbon cloth with gradient pore distribution for superior capacitance

Hierarchical Co-doped SnS₂@Ni(OH)₂ double-shell nanosheet arrays are coated on carbon cloth, the vertically aligned arrays with gradient pore distribution can facilitate the charge/ion transfer rate, thus improve the energy storage performance.

Silicon-Nanographite Aerogel-Based Anodes for High Performance Lithium Ion Batteries

To increase the energy storage density of lithium-ion batteries, silicon anodes have been explored due to their high capacity. One of the main challenges for silicon anodes are large volume variations during the lithiation processes. Recently, several high-performance schemes have been demonstrated with increased life cycles utilizing nanomaterials such as nanoparticles, nanowires, and thin films. However, a method that allows the large-scale production of silicon anodes remains to be demonstrated. Herein, we address this question by suggesting new scalable nanomaterial-based anodes. Si nanoparticles were grown on nanographite flakes by aerogel fabrication route from Si powder and nanographite mixture using polyvinyl alcohol (PVA). This silicon-nanographite aerogel electrode has stable specific capacity even at high current rates and exhibit good cyclic stability. The specific capacity is 455 mAh g⁻¹ for 200^th cycles with a coulombic efficiency of 97% at a current density 100 mA g⁻¹.

Which is the most effective pristine graphene electrode for energy storage devices: aerogel or xerogel?

The morphological design of graphene materials is definitely important since their electrochemical properties as an electrode in energy storage devices are mainly dominated by their charge accessibility and active area. In this work, we present a systematic investigation on the prospects of a pristine graphene aerogel and a pristine graphene xerogel as electrode materials for both supercapacitors and lithium-ion batteries. We confirm that the graphene aerogel has a significantly higher surface area, needed for effective charge storage, than the xerogel, which offers a clear advantage for supercapacitors. In terms of battery performance, the quality of the pristine graphene raw materials is a more critical factor than their shape owing to the lithium intercalation mechanism. As a result, the graphene aerogel supercapacitors exhibited a specific capacitance of about 700 F g-1 at 10 mV s-1 in 1 M LiPF6 electrolyte, which is 3.6 times higher than the values for the xerogel devices. On the other hand, the electrochemical battery performances of the graphene aerogel and xerogel show no significant difference considering their high specific capacity of about 380 mA h g-1 at 1C. Further, the surface control kinetics of the graphene aerogel are much more dominant in the supercapacitor and battery applications than those of the xerogel. This study provides more practical approaches in order to design electrodes using lightweight, high-performance, and low-cost materials for the effective use of energy storage systems.

Graphene Aerogel Broken to Fragments for a Piezoresistive Pressure Sensor with a Higher Sensitivity

The porous and elastic reduced graphene aerogel (rGA) is a promising active material for piezoresistive pressure sensors (PRSs) to realize an electronic skin. Due to the specific working mechanism and the limitation of the rGA's monolithic conductive network, the PRSs based on rGA suffer from a limited change of resistance with mechanical deformation, so they show poor sensitivity and cannot detect low pressures. Here we aim to improve the sensitivity of the PRS and make it suitable for a low-pressure system (0.5-8 kPa) through an effective method. The monolithic rGA is broken into small pieces by cutting (named as CGA). The sensitivity of the PRS based on CGA can be improved by 10 times that of the uncut rGA (named as UCGA). The resistance variation ratio of CGA (0.96) is 1.45 times of the resistance variation ratio of the UCGA (0.66). By using a package of elastic polypropylene thin films (PP), the cycle stability performance of CGA remains stable after 4200 cycles. The CGA can detect the pulse of a human being with sensitivity higher than the UCGA and the ordinary sensors. This method is simple, effective, and universal to improve the sensitivity of PRS based on porous and elastic materials.

Facile and Scalable Fabrication of Nitrogen-Doped Porous Carbon Nanosheets for Capacitive Energy Storage with Ultrahigh Energy Density

Porous carbon materials are the most commonly used electrode materials for supercapacitors because of their abundant structures, excellent conductivities, and chemical stability. However, the manufacture of carbon materials possessing sizable pores and remarkable wettability with the electrolyte remains challenging. Herein, we developed a facile and industrially scalable method for the production of nitrogen-doped porous carbon nanosheets (PNDC-4) with excellent pore size distribution, large specific surface area (>1200 m² g^-1), high conductivity (>700 S m^-1), and superb wettability either in aqueous or organic electrolyte. Particularly, PNDC-4 shows a high capacitance of 387 F g^-1 (1 A g^-1) in a three-electrode system with 3 M KOH and 80 F g^-1 (1 A g^-1) in a symmetric two-electrode system with EMIMBF₄. The device exhibits an ultrahigh energy density of 81 W h kg^-1 at a power density of 1.3 kW kg^-1 and can still maintain at 60.8 W h kg^-1 when the power density is increased to 266.6 kW kg^-1. Moreover, the devices show superb stability that 94% of its initial capacitance is still maintained after 100 000 cycles at 20 A g^-1.

Morphology-controlled synthesis of CoMoO4 nanoarchitectures anchored on carbon cloth for high-efficiency oxygen oxidation reaction

Novel CoMoO₄ nanoarrays with different morphologies are anchored on a carbon cloth via a simple hydrothermal method by adjusting the Co/Mo atom ratio. The in situ growth and tight immobilization of the CoMoO₄ nanocomposite on the carbon cloth can facilitate the electrolyte infiltration and electrons transfer rate at the contact interface. Therefore, the free-standing electrode of CoMoO₄/carbon cloth with interconnected nanosheets shows superior electrocatalytic activity, and the overpotential of 286 mV is obtained at 15 mA cm⁻² in alkaline solution. Moreover, the catalyst also exhibits a small Tafel slope of 67 mV dec⁻¹ as well as good stability. The relationship between the active material morphology, contact interface and the electrocatalytic performance is also discussed. As the carbon cloth is commercially available, this simple but effective structural controlling method demonstrates a new large-scale practical electrode fabrication technique for high performance OER electrodes and large-scale water splitting.

Novel CoMoO₄ nanoarrays with different morphologies are anchored on a carbon cloth via a simple hydrothermal method by adjusting the Co/Mo atom ratio.

High-yield scalable graphene nanosheet production from compressed graphite using electrochemical exfoliation

Electrochemical exfoliation is a promising bulk method for producing graphene from graphite; in this method, an applied voltage drives ionic species to intercalate into graphite where they form gaseous species that expand and exfoliate individual graphene sheets. However, a number of obstacles have prevented this approach from becoming a feasible production route; the disintegration of the graphite electrode as the method progresses is the chief difficulty. Here we show that if graphite powders are contained and compressed within a permeable and expandable containment system, the graphite powders can be continuously intercalated, expanded, and exfoliated to produce graphene. Our data indicate both high yield (65%) and extraordinarily large lateral size (>30 μm) in the as-produced graphene. We also show that this process is scalable and that graphene yield efficiency depends solely on reactor geometry, graphite compression, and electrolyte transport.

Flexible Micro-Supercapacitors Based on Naturally Derived Juglone

Recently, great efforts have been devoted to designing and fabricating flexible, lightweight, wearable, and miniaturized supercapacitors. At the same time, the exploration of green, renewable, and biocompatible energy-storage materials has been attracting intensive attention. By taking fabrication and configuration design into consideration, the naturally derivable juglone molecule was exploited as an active charge-storage material, and integrated into flexible and micro-supercapacitor devices. The polypyrrole/juglone-composite-based supercapacitors exhibit significant energy-storage capabilities with high specific capacitance and long cyclability, which are comparable to that of conventional electrode materials. This study presents a new way for developing flexible, lightweight, portable, and/or wearable electronic devices with biocompatible and environmentally friendly attributes.

A high-performance supercapacitor electrode based on tremella-like NiC2O4@NiO core/shell hierarchical nanostructures on nickel foam

Tremella-like nickel oxalate@nickel oxide (NiC₂O₄@NiO) core/shell hierarchical nanostructures have been successfully synthesized on nickel foam, using Ni foam as a current collector, a Ni source and a three-dimensional (3D) substrate, through a facile hydrothermal method followed by an electrochemical activation process. The prepared samples can be directly used as binder-free electrodes for supercapacitors. The tremella-like morphology, together with the NiC₂O₄ nanoblocks on 3D Ni foam, significantly increases the amount of active sites for redox reactions and the conductivity of the electrode material, shortens the diffusion pathway for ions, facilitates the effective penetration of the electrolyte, and lowers the intrinsic equivalent series resistance, demonstrating good potential for energy storage application. This material has a high specific capacitance of 2287.09 F g^-1 at 1 A g^-1, a good cycling stability (remaining 95% after 10 000 cycles) and a good rate capability (83.2% retention upon increasing the current density by 10 times).

Hollow nanostructures of metal oxides as next generation electrode materials for supercapacitors

Hollow nanostructures of copper oxides help to stabilize appreciably higher electrochemical characteristics than their solid counter parts of various morphologies. The specific capacitance values, calculated using cyclic voltammetry (CV) and charge-discharge (CD) studies, are found to be much higher than the values reported in literature for copper oxide particles showing  intriguing morphologies or even composites with trendy systems like CNTs, rGO, graphene, etc. The proposed cost-effective synthesis route makes these materials industrially viable for application in alternative energy storage devices. The improved electrochemical response can be attributed to effective access to the higher number of redox sites that become available on the surface, as well as in the cavity of the hollow particles. The ion transport channels also facilitate efficient de-intercalation, which results in the enhancement of cyclability and Coulombic efficiency. The charge storage mechanism in copper oxide structures is also proposed in the paper.

Controllable synthesis of MnO2 nanostructures anchored on graphite foam with different morphologies for a high-performance asymmetric supercapacitor

MnO₂ nanostructures with different morphologies (nanowires, nanowire bundles, flower-like nanosheet bundles) were synthesized via a simple and surfactant-free hydrothermal method.

Graphene hybridization for energy storage applications

Graphene hybridization principles and strategies for various energy storage applications are reviewed from the view point of material structure design, bulk electrode construction, and material/electrode collaborative engineering.

Three-Dimensional Binder-Free Nanoarchitectures for Advanced Pseudocapacitors

The ever-increasing energy demands for electrification of transportation and powering of portable electronics are driving the pursuit of energy-storage technologies beyond the current horizon. Pseudocapacitors have emerged as one of the favored contenders to fill in this technology gap, owing to their potential to deliver both high power and energy densities. The high specific capacitance of pseudocapacitive materials is rooted in the various available oxidation states for fast surface or near-surface redox charge transfer. However, the practical implementation of pseudocapacitors is plagued by the insulating nature of most pseudocapacitive materials. The wealth of the research dedicated to addressing these critical issues has grown exponentially in the past decade. Here, we briefly survey the current progress in the development of pseudocapacitive electrodes with a focus on the discussion of the recent most exciting advances in the design of three-dimensional binder-free nanoarchitectures, including porous metal/graphene-based electrodes, as well as metal-atom/ion-doping-enhanced systems, for advanced supercapacitors with comparable energy density to batteries, and high power density.

One-Step Device Fabrication of Phosphorene and Graphene Interdigital Micro-Supercapacitors with High Energy Density

Rational engineering and simplified fabrication of high-energy micro-supercapacitors (MSCs) using graphene and other 2D nanosheets are of great value for flexible and integrated electronics. Here we develop one-step mask-assisted simplified fabrication of high-energy MSCs (PG-MSCs) based on the interdigital hybrid electrode (PG) patterns of stacking high-quality phosphorene nanosheets and electrochemically exfoliated graphene in ionic liquid electrolyte. The hybrid PG films with interdigital patterns were directly manufactured by layer-by-layer deposition of phosphorene and graphene nanosheets with the assistance of a customized interdigital mask, and directly transferred onto a flexible substrate. The resultant patterned PG films present outstanding uniformity, flexibility, conductivity (319 S cm^-1), and structural integration, which can directly serve as binder- and additive-free flexible electrodes for MSCs. Remarkably, PG-MSCs deliver remarkable energy density of 11.6 mWh cm^-3, outperforming most nanocarbon-based MSCs. Moreover, our PG-MSCs show outstanding flexibility and stable performance with slight capacitance fluctuation even under highly folded states. In addition, our simplified mask-assisted strategy for PG-MSCs is highly flexible for simplified production of parallelly and serially interconnected modular power sources, without need of conventional metal-based interconnects and contacts, for designable integrated circuits with high output current and voltage.

Interconnected 3 D Network of Graphene-Oxide Nanosheets Decorated with Carbon Dots for High-Performance Supercapacitors

Interconnected 3 D nanosheet networks of reduced graphene oxide decorated with carbon dots (rGO/CDs) are successfully fabricated through a simple one-pot hydrothermal process. The as-prepared rGO/CDs present appropriate 3 D interconnectivity and abundant stable oxygen-containing functional groups, to which we can attribute the excellent electrochemical performance such as high specific capacitance, good rate capability, and great cycling stability. Employed as binder-free electrodes for supercapacitors, the resulting rGO/CDs exhibit excellent long-term cycling stability (ca. 92 % capacitance retention after 20 000 charge/discharge cycles at current density of 10 A g^-1 ) as well as a maximum specific capacitance of about 308 F g^-1 at current density of 0.5 A g^-1 , which is much higher than that of rGO (200 F g^-1 ) and CDs (2.2 F g^-1 ). This work provides a promising strategy to fabricate graphene-based nanomaterials with greatly boosted electrochemical performances by decoration of with CDs.