High-performance Flexible Energy Storage Research Articles

Hierarchical Ni@Ni(OH)2 core-shell hybrid arrays on cotton cloth fabricated by a top-down approach for high-performance flexible asymmetric supercapacitors

A simple and cost-efficient top-down strategy is used to prepare a high conductive Ni microspheres @ Ni(OH)2 nanoflakes core-shell on cotton (Ni@N NF-CT) flexible electrode via a palladium-free catalytic electroless deposition process followed by electrochemical oxidation with further optimized cyclic voltammetry cycle times. Taking the advantages of the interfacial Ni microspheres and Ni(OH)2 nanoflakes which accelerates the electronic transport through the increased electro-active surface and the shortened ion diffusion distance, the as-prepared electrode exhibits the outstanding electrochemical performances such as ultrahigh areal specific capacitance (5.17 F cm−2/0.53 mAh cm−2 at a current density of 5 mA cm−2), the high rate capability (78% capacitance retention at a current density of 25 mA cm−2) and the well cycling stability (89.5% capacitance retention after 1500 charge/discharge cycles). A flexible asymmetric supercapacitor (FASC) is successfully fabricated using Ni @ Ni(OH)2 core-shell on cotton as the positive electrode and active carbon cloth (ACC) as the negative electrode, which manifests a high energy density of 0.597 mWh cm−2 (1.91 F cm−2/1.44 mAh cm−2) at a power density of 3.68 mW cm−2 (5 mA cm−2) in 2.0 M KOH aqueous electrolyte. The device is also super flexible as demonstrated by powering the red LEDs under various bending conditions. This facile fabrication process demonstrates the low-cost production of Ni @ Ni(OH)2 core-shell on cotton as a promising material for application in the high-performance flexible electrochemical energy storage devices.

In situ growth of Cu(OH)2@FeOOH nanotube arrays on catalytically deposited Cu current collector patterns for high-performance flexible in-plane micro-sized energy storage devices

Rationally designed interdigitated electrodes based on Cu(OH)2@FeOOH nanotube arrays are facilely converted in situ from catalytically deposited Cu current collector patterns for high-performance flexible micro-supercapacitor energy storage devices.

Interlaced NiMn-LDH nanosheet decorated NiCo2O4 nanowire arrays on carbon cloth as advanced electrodes for high-performance flexible solid-state hybrid supercapacitors.

The development of flexible energy storage devices for portable and wearable electronics has aroused increasing interest. In this work, three-dimensional hierarchical NiCo2O4@NiMn-LDH nanowire/nanosheet arrays have been successfully fabricated on carbon cloth through a facile hydrothermal and calcination synthetic method. Benefiting from the sophisticated hybrid nanoarchitectures with desirable structure and components, the optimized NiCo2O4@NiMn-LDH hybrid electrode is found to deliver a remarkable specific capacity of 278 mA h g-1 at 2 mA cm-2 and a good rate capability of 89.1% retention at 20 mA cm-2. Detailed analysis of the reaction kinetics for the hybrid electrode clearly indicates the dominant diffusion-controlled contribution to the total capacity. In addition, a flexible solid-state hybrid supercapacitor is assembled by taking NiCo2O4@NiMn-LDH and activated carbon as the cathode and anode, respectively, which manifests a maximum energy density of 47 W h kg-1 at a power density of 357 W kg-1 as well as an excellent long-term cycling stability (95.6% retention after 5000 cycles over 8 mA cm-2). Our work demonstrates the great potential of this core/shell hybrid nanostructure as an advanced battery-type electrode for high-performance flexible energy storage devices.

High-performance flexible all-solid-state supercapacitor constructed by free-standing cellulose/reduced graphene oxide/silver nanoparticles composite film

A 400-μm-thick free-standing and flexible cellulose/RGO/AgNPs composite film was fabricated by chemical reduction of cellulose/GO/AgNPs film with hot hydrazine vapor, which was obtained via the reaction of GO with silver–ammonia complex in the presence of cellulose fibers, followed by dewatering with assistance of filtration. The 400-μm-thick cellulose/RGO/AgNPs film was systematically characterized and found to be conductive enough with sheet resistance of merely 0.17 Ω sq−1 for direct use as electrode. When measured in a three-electrode setup, such flexible electrode exhibited outstanding supercapacitive performances with the highest areal specific capacitance of 1242.7 mF cm−2 (corresponding to 31.1 F cm−3), excellent rate capability and remarkable cycling stability with the capacitance retention of 99.6% after extensive charge/discharge at a high current density of 60 mA cm−2 for more than 10,000 cycles. Furthermore, a symmetric flexible all-solid-state supercapacitor cell of cellulose/RGO/AgNPs//cellulose/RGO/AgNPs was assembled. It delivered the maximum areal specific capacitance of 683.8 mF cm−2 (corresponding to 8.6 F cm−3), possessed desirable rate capability, showed no substantial variation of supercapacitive behavior when being bent, and offered the highest areal energy density of 95.0 μWh cm−2 (corresponding to 1.19 mWh cm−3), which outperformed that of many existing graphene-based symmetric flexible supercapacitors and lots of asymmetric flexible supercapacitors. The impressive electrochemical properties and facile preparation process of cellulose/RGO/AgNPs film make it a promising candidate for constructing next-generation high-performance flexible energy storage devices.

Synthesis of cobalt phosphate nanoflakes for high-performance flexible symmetric supercapacitors

Herein, leaf-like Co3(PO4)2 was synthesized via a simple and low-cost approach without adding any additional surfactant. Such leaf-like Co3(PO4)2 showed the high crystallinity as proved by spectra analysis. The electrochemical performance of as-synthesized Co3(PO4)2 was firstly characterized in a three-electrode system, which shows the specific capacitance of 410 F g− 1 at the current density of 1.0 A g− 1. Furthermore, flexible symmetric supercapacitor was fabricated with PVA-KOH gel electrolyte, exhibiting the specific capacitance of 165 F g− 1 at a current density of 0.5 A g− 1 with a high energy density of 52.8 Wh kg− 1 at a power density of 756 W kg− 1. This superior performance is attributed to the unique morphology and fast surface redox reaction. These excellent results suggested that Co3(PO4)2 nanoflakes are promising materials for potential application in high-performance flexible energy storage devices.

All-in-One Compact Architecture toward Wearable All-Solid-State, High-Volumetric-Energy-Density Supercapacitors.

High-performance flexible energy storage devices are an important prerequisite to the utilization of various advanced wearable electronics, such as healthcare sensors and smart textiles. In this work, we design a wearable all-solid-state, all-in-one asymmetric supercapacitor by integrating current collectors, a separator, and negative and positive electrodes into a thin, flexible, and porous polyamide nanofiber film. The positive and negative electrodes are, respectively, electrodeposited onto each side of the carbon nanotube-modified porous polyamide nanofiber film to form the integrated and compact asymmetric cell. The all-in-one thin-film asymmetric supercapacitor is binder-, additive-, and metal current collector-free, which can effectively decrease the cost, simplify the assembly procedures, and increase the energy density. The assembled flexible all-in-one asymmetric supercapacitor with a compact structure shows high gravimetric and volumetric specific capacitances of 70 F g-1 and 3.1 F cm-3 under a current density of 0.5 A g-1 in a neutral polyvinyl alcohol/LiCl gel electrolyte, respectively. Additionally, the all-in-one asymmetric cell displays a favorable volumetric energy density of 1.1 W h L-3, which is among the highest compared with other reported flexible solid-state supercapacitors. Notably, multiple cell units can be integrated in one piece of polyamide nanofiber film and connected in series to satisfy the need of high output voltage.

Novel polyaniline/manganese hexacyanoferrate nanoparticles on carbon fiber as binder-free electrode for flexible supercapacitors

Abstract To accomplish high-performance flexible energy storage devices, a new class of flexible electrodes with prosperous constructions bearing exceptional electrical conductivity, exclusive porosity, and outstanding mechanical stability is extremely needed. Here, we have successfully fabricated a highly flexible electrode with nanocomposite of polyaniline/manganese hexacyanoferrate PANI/MnHCF nanocomposite (PANI/MnHCF) on highly conductive carbon fiber (FCF). The nanocomposite exhibits an ultra-high specific capacitance of 730 F g−1 at a current density of 1 A g−1 with exceptional rate capability (350 F g−1 capacitance retention at a current density of 20 A g−1), and outstanding cycling performance (capacitance retention of ∼85% after 1000 cycles). The PANI/MnHCF/FCF electrode shows better electrochemical performance due to their exclusive properties such as excellent electrical conductivity, unique porous network and outstanding mechanical flexibility. Furthermore, the higher surface area of PANI/MnHCF nanocomposite facilitates the transport of electrons and electrolyte ions during rapid charge/discharge process. This present work offers a simple, scalable and cost-effective method for fabrication of other PANI/MnHCF nanocomposite based electrode, which is applicable for providing energy in practical flexible portable devices.

A Freestanding and Long-Life Sodium-Selenium Cathode by Encapsulation of Selenium into Microporous Multichannel Carbon Nanofibers.

Selenium cathode has attracted more and more attention because of its comparable volumetric capacity but much higher electrical conductivity than sulfur cathode. Compared to Li-Se batteries, Na-Se batteries show many advantages, including the low cost of sodium resources and high volumetric capacity. However, Na-Se batteries still suffer from the shuttle effect of polyselenides and high volumetric expansion, resulting in the poor electrochemical performance. Herein, Se is impregnated into microporous multichannel carbon nanofibers (Se@MCNFs) thin film with high flexibility as a binder-free cathode material for Na-Se batteries. The fibrous unique structure of the Se@MCNFs is beneficial to alleviate the volume change of Se during cycling, improve the utilization of active material, and suppress the dissolution of polyselenides into electrolyte. The freestanding Se@MCNF thin-film electrode exhibits high discharge capacity (596 mA h g-1 at the 100th cycle at 0.1 A g-1 ) and excellent rate capability (379 mA h g-1 at 2 A g-1 ) for Na-Se batteries. In addition, it also shows long cycle life with a negligible capacity decay of 0.067% per cycle over 300 cycles at 0.5 A g-1 . This work demonstrates the possibility to develop high performance Na-Se batteries and flexible energy storage devices.

Synthesis of three-dimensional porous reduced graphene oxide hydrogel/carbon dots for high-performance supercapacitor

A composite composed of reduced graphene oxide hydrogel/carbon dots (rGH/CDs) is prepared hydrothermally. The composite has a three-dimensional (3D) interconnected network structure and exhibits good electrical conductivity and mechanical robustness, making it ideal electrode materials in supercapacitors. The carbon dots (CDs) in the reduced graphene oxide hydrogel promotes electron transport and reduces the internal resistance and charge transfer resistance in addition to providing a large surface area. The flexible solid-state supercapacitor comprising the 130μm thick rGH/CDs electrode delivers excellent performance including high gravimetric specific capacitance of 264Fg−1 (up to 301Fg−1 for a 40μm thick electrode), areal specific capacitance of 394mFcm−2 (up to 432Fcm−2 for a 200μm thick electrode), excellent cycling stability (9.1% deterioration after 5000cycles), larger energy density (35.3Whkg−1), as well as high power density (516Wkg−1). This study demonstrates the tremendous potential of rGH/CDs in high-performance flexible energy storage devices.

Flexible and robust reduced graphene oxide/carbon nanoparticles/polyaniline (RGO/CNs/PANI) composite films: Excellent candidates as free-standing electrodes for high-performance supercapacitors

Reduced graphene oxide/carbon nanoparticles (RGO/CNs) composite films with good flexibility and high mechanical strength were prepared via a facile hydrothermal process of the β-cyclodextrin modified graphene oxide (β-CD/GO) hydrogel in presence of ascorbic acid (vitamin C), in which the GO was reduced into reduced graphene oxide (RGO) and β-CD converted into carbon nanoparticles (CNs). The resulted CNs could enlarge the interlayer distance of graphene sheets not only to prevent their agglomeration, but also to improve the transfer speed of the electrolyte ions throughout the film electrodes during the charge/discharge process. The flexible and robust RGO/CNs composite films were then used as highly conductive supports for the electrodeposition of polyaniline (PANI) to further improve the electrochemical performance. As flexible and robust paper-like electrodes, the reduced graphene oxide/carbon nanoparticles/polyaniline (RGO/CNs/PANI) composite films exhibited high electrochemical activity, such as high specific capacitance of 787.3 F/g at the current density of 1 A/g and 564.0 F/g at the current density of 10 A/g, as well as excellent cyclic stability. The results demonstrated the significant potential application of the proposed RGO/CNs/PANI composite films as binder-free and free-standing electrodes for high-performance flexible energy storage devices.

Atomic layer deposition of TiO2 on nitrogen-doped carbon nanofibers supported Ru nanoparticles for flexible Li-O2 battery: A combined DFT and experimental study

Flexible Li-O2 batteries have attracted worldwide research interests and been considered to be potential alternatives for the next-generation flexible devices. Nitrogen-doped carbon nanofibers (N-CNFs) prepared by electrospinning are used as flexible substrate and an amorphous TiO2 layer is coated by atomic layer deposition (ALD) and then decorated with Ru nanoparticles. The Ru/N-CNFs@TiO2 composite is directly used as a free-standing electrode for Li-O2 batteries and the electrode delivers a high specific capacity, improved round-trip efficiency and good cycling ability. The superior electrochemical performance can be attributed to the amorphous TiO2 protecting layer and superior catalytic activity of Ru nanoparticles. Based on density functional theory (DFT) calculations from first principles, the carbon electrode after coating with TiO2 is more stable during discharge/charge process. The analysis of Li2O2 on three different interfaces (Li2O2/N-CNFs, Li2O2/TiO2, and Li2O2/Ru) indicates that the electron transport capacity was higher on Ru and TiO2 compared with N-CNFs, therefore, Li2O2 could be formed and decomposed more easily on the Ru/N-CNFs@TiO2 cathode. This work paves a way to develop the free-standing cathode materials for the future development of high-performance flexible energy storage systems.

Eco‐Friendly Red Seaweed‐Derived Electrolytes for Electrochemical Devices

Green electrolytes composed of kappa‐carrageenan (κ‐Cg), 1‐butyl‐3‐methyl‐1H‐imidazolium chloride ([Bmim]Cl) ionic liquid, and glycerol (Gly) are prepared in aqueous solution using a simple, clean, fast and low‐cost procedure. A flexible membrane incorporating 50% wt [Bmim]Cl and 50% wt Gly with respect to κ‐Cg exhibits the highest ionic conductivity values (8.47 × 10−4/2.45 × 10−3 S cm−1 at 20/66 °C, under anhydrous conditions, and 5.49 × 10−2/0.186 S cm−1 at 30/60 °C, at a relative humidity of 98%). Tests of room temperature air/hydrogen fuel cells incorporating κ‐Cg, κ‐Cg/Gly, and κ‐Cg/Gly/[Bmim]Cl membranes demonstrate that these predominantly protonic conductors electrolytes are particularly well suited for the design and fabrication of eco‐friendly electrochemical devices whose operation does not require the flow of gases and does not lead to water formation. These new materials have excellent application prospects in high performance (flexible) energy storage devices (supercapacitors and batteries) and electrochromic devices.

Rational Design of Hierarchical Carbon/Mesoporous Silicon Composite Sponges as High-Performance Flexible Energy Storage Electrodes.

Nanostructuring silicon (Si) and combining Si with carbon shells have been studied in recent Li-ion battery electrodes, yet it remains a grand challenge to overcome the low electrical conductivity and associated volume change of Si. Here, by first coating a mesoporous SiO2 (meso-SiO2) onto carbon nanotube (CNT) networks and then converting it into a meso-Si layer covered by carbon, we obtained a freestanding, highly porous composite sponge electrode consisting of three-dimensionally interconnected sandwiched carbon-Si-CNT skeletons. In this hierarchical structure, the macropores among the sponge connect to mesopores in the meso-Si layer so that Li+ diffusion is facilitated, whereas the underlying CNT networks serve as conductive paths for electrons transport. Meanwhile, the outer carbon coating on meso-Si could buffer the volume expansion and prevent material shedding. As a result, our sandwiched carbon-Si-CNT electrodes exhibit large specific capacity, high rate capability and long cycle life. The combination of carbon-wrapped meso-Si and CNT sponges might be a potential strategy for developing efficient electrodes in various energy storage systems.

A flexible carbon electrode based on traditional cotton woven fabrics with excellent capacitance

A flexible and stretchable three-dimensional carbon electrode (SCF) based on the natural woven fabric was developed by coupling a sublimation generating pores method and facile pyrolyzing procedure. The SCF electrode possessed good mechanical flexibility, which was attributed to the increase in the specific surface area and the amount of the pores. Furthermore, the SCF electrode exhibited excellent capacitance property (157.89 F g−1 at 2 m s−1), resulting from the synergistic effect between the double-layer capacitance (large specific surface area, high porosity and excellent conductivity) and the pseudocapacitance of O and N heteroatoms. The SCF will be employed as a promising candidate for flexible energy storage devices owing to the excellent wearable and electrochemical performance. The one-step strategy can provide a novel route for low-cost production of high-performance flexible energy storage materials from the natural fabrics.

High-Performance Flexible Supercapacitors obtained via Recycled Jute: Bio-Waste to Energy Storage Approach

In search of affordable, flexible, lightweight, efficient and stable supercapacitors, metal oxides have been shown to provide high charge storage capacity but with poor cyclic stability due to structural damage occurring during the redox process. Here, we develop an efficient flexible supercapacitor obtained by carbonizing abundantly available and recyclable jute. The active material was synthesized from jute by a facile hydrothermal method and its electrochemical performance was further enhanced by chemical activation. Specific capacitance of 408 F/g at 1 mV/s using CV and 185 F/g at 500 mA/g using charge-discharge measurements with excellent flexibility (~100% retention in charge storage capacity on bending) were observed. The cyclic stability test confirmed no loss in the charge storage capacity of the electrode even after 5,000 charge-discharge measurements. In addition, a supercapacitor device fabricated using this carbonized jute showed promising specific capacitance of about 51 F/g, and improvement of over 60% in the charge storage capacity on increasing temperature from 5 to 75 °C. Based on these results, we propose that recycled jute should be considered for fabrication of high-performance flexible energy storage devices at extremely low cost.

3D nitrogen-doped graphene foam with encapsulated germanium/nitrogen-doped graphene yolk-shell nanoarchitecture for high-performance flexible Li-ion battery

Flexible electrochemical energy storage devices have attracted extensive attention as promising power sources for the ever-growing field of flexible and wearable electronic products. However, the rational design of a novel electrode structure with a good flexibility, high capacity, fast charge–discharge rate and long cycling lifetimes remains a long-standing challenge for developing next-generation flexible energy-storage materials. Herein, we develop a facile and general approach to three-dimensional (3D) interconnected porous nitrogen-doped graphene foam with encapsulated Ge quantum dot/nitrogen-doped graphene yolk-shell nano architecture for high specific reversible capacity (1,220 mAh g−1), long cycling capability (over 96% reversible capacity retention from the second to 1,000 cycles) and ultra-high rate performance (over 800 mAh g−1 at 40 C). This work paves a way to develop the 3D interconnected graphene-based high-capacity electrode material systems, particularly those that suffer from huge volume expansion, for the future development of high-performance flexible energy storage systems.

High-performance flexible all-solid-state supercapacitors based on densely-packed graphene/polypyrrole nanoparticle papers

Graphene-based all-solid-state supercapacitors (ASSSCs) have received increasing attention. It’s a great challenge to fabricate high-performance flexible solid-state supercapacitors with high areal and volumetric energy storage capability, superior electron and ion conductivity, robust mechanical flexibility, as well as long term stability. Herein, we report a facile method to fabricate flexible ASSSCs based on densely-packed reduced graphene oxide (rGO)/polypyrrole nanoparticle (PPy NP) hybrid papers with a sandwich framework, which consists of well-separated and continuously-aligned rGO sheets. The incorporation of PPy NPs not only provides pseudocapacitance but also facilitates the infiltration of gel electrolyte. The assembled ASSSCs possess maximum areal and volumetric specific capacitances of 477mF/cm2 and 94.9F/cm3 at 0.5mA/cm2. They also exhibit little capacitance deviation under different bending states, excellent cycling stability, small leakage current and low self-discharge characteristics. Additionally, the maximum areal and volumetric energy densities of 132.5μWh/cm2 and 26.4mWh/cm3 are achieved, which indicate that this hybrid paper is a promising candidate for high-performance flexible energy storage devices.

Steamed water engineering mechanically robust graphene films for high-performance electrochemical capacitive energy storage

Developing versatile methods to fabricate flexible graphene film electrodes with favorable mechanical strength and desirably tailored areal and volumetric capacitances are very challenging for high-performance capacitive energy storages. Here, we present a simple yet versatile method to regulate the structures of scalable free-standing reduced graphene oxide (rGO) films for high-performance flexible supercapacitors. Steamed water with a high pressure and a moderately high temperature in closed vessels was used to prepare reduced graphene oxide with regulated structures, and the resultant rGO films exhibited favorable mechanical robustness (with modulus and tensile strength higher than 0.28GPa and 5.9MPa respectively) as well as excellently controllable areal and volumetric capacitances (with a highest gravimetric specific capacitance, a highest areal specific capacitance, and a highest volumetric capacitance up to 340F/g, 915mF/cm2, and 326F/cm3, respectively), revealing the versatile behavior of this regulation technique for high-performance flexible energy storage. In addition, a typical assembled all-solid-state supercapacitor based on as-fabricated graphene films shows large gravimetric and areal specific capacitances, high energy density and power density, as well as excellent capacitance stability, highlighting its great potential for high-performance flexible energy storage devices.

High-performance flexible energy storage and harvesting system for wearable electronics.

This paper reports on the design and operation of a flexible power source integrating a lithium ion battery and amorphous silicon solar module, optimized to supply power to a wearable health monitoring device. The battery consists of printed anode and cathode layers based on graphite and lithium cobalt oxide, respectively, on thin flexible current collectors. It displays energy density of 6.98 mWh/cm2 and demonstrates capacity retention of 90% at 3C discharge rate and ~99% under 100 charge/discharge cycles and 600 cycles of mechanical flexing. A solar module with appropriate voltage and dimensions is used to charge the battery under both full sun and indoor illumination conditions, and the addition of the solar module is shown to extend the battery lifetime between charging cycles while powering a load. Furthermore, we show that by selecting the appropriate load duty cycle, the average load current can be matched to the solar module current and the battery can be maintained at a constant state of charge. Finally, the battery is used to power a pulse oximeter, demonstrating its effectiveness as a power source for wearable medical devices.

Highly Flexible Graphene/Mn3O4 Nanocomposite Membrane as Advanced Anodes for Li-Ion Batteries.

Advanced electrode design is crucial in the rapid development of flexible energy storage devices for emerging flexible electronics. Herein, we report a rational synthesis of graphene/Mn3O4 nanocomposite membranes with excellent mechanical flexibility and Li-ion storage properties. The strong interaction between the large-area graphene nanosheets and long Mn3O4 nanowires not only enables the membrane to endure various mechanical deformations but also produces a strong synergistic effect of enhanced reaction kinetics by providing enlarged electrode/electrolyte contact area and reduced electron/ion transport resistance. The mechanically robust membrane is explored as a freestanding anode for Li-ion batteries, which delivers a high specific capacity of ∼800 mAh g(-1) based on the total electrode mass, along with superior high-rate capability and excellent cycling stability. A flexible full Li-ion battery is fabricated with excellent electrochemical properties and high flexibility, demonstrating its great potential for high-performance flexible energy storage devices.