High-performance Flexible Energy Storage Devices Research Articles

Ambient-air in situ fabrication of high-surface-area, superhydrophilic, and microporous few-layer activated graphene films by ultrafast ultraviolet laser for enhanced energy storage

Microporous graphene-based (MPG) films have attracted tremendous attention in micro-energy storage devices due to their unique characteristics of high specific surface area (SSA), high flexibility and high conductivity. However, current fabrication technologies generally provide overall activation in the form of powder in the bulky tube furnace, and multi-stepped thin film forming process, hurdling their practical utilizations that require efficient, cost-effective and controllable methods for large-area MPG films. Herein, a one-step and scalable laser induction & activation technique is reported for in situ fabrication of decimeter-level few-layer MPG films by utilizing ultrafast ultraviolet laser as a mobile heat source acting on microstructured polyimide film coated with KOH in air, which enable directly regulating the activation area on a flexible substrate. The resulting laser-induced and activated graphene (LIAG) films have interconnected hierarchical porous structures containing ultra- (0.5–0.8 nm) and super-micropores (~1.2 nm), high SSA (>1300 m2 g−1), small amount of doped potassium (~0.50 wt%), and super-hydrophilicity (maximum droplet spreading velocity reaches 424.7 mm s−1). With the well-balanced properties of SSA, heteroatom doping, bulk density, and crystalline size, the LIAG micro-supercapacitors with interdigital electrodes of 35 µm width gap achieve a maximum areal capacitance of 128.4 mF cm−2, which outperforms state-of-the-art laser-processed carbon-based micro-supercapacitors. Additionally, it achieves almost the highest comprehensive evaluation considering processing precision, efficiency, cost and environmental friendliness. The facile, efficient, binder-free fabricability and roll-to-roll process compatibility would provide a rapid route for high-performance flexible energy storage devices.

Scalable CNTs/NiCoSe2 Hybrid Films for Flexible All-Solid-State Asymmetric Supercapacitors.

The rapidly developing wearable flexible electronics makes the development of high-performance flexible energy storage devices, such as all-solid-state supercapacitors (SCs), particularly important. Herein, we report the fabrication of CNTs/NiCoSe2 hybrid films on carbon cloth (CC) through a facile co-electrodeposition method based on flexible electrodes for all-solid-state SCs. The NiCoSe2 sheets grown on CNTs uniformly with a diameter of 50-100 nm act as the active materials. The CNTs in the hybrid films act as the scaffold to offer more deposition sites for NiCoSe2 and provide a conductive network to facilitate the transfer of electrons. Moreover, the one-step electrodeposition process avoids the usage of any organic binders. Benefiting from the high intrinsic reactivity and unique 3D architecture, the obtained CNTs/NiCoSe2 electrode delivers high specific capacity (218.1 mA h g-1) and satisfactory durability (over 5000 cycles). Remarkably, the CNTs/NiCoSe2//AC flexible all-solid-state (FASS) ASC provides remarkable energy density (112.2 W h kg-1) within 0-1.7 V and maintains 98.1% of its initial capacity after 10,000 cycles. In addition, this flexible ASC device could be fabricated at a large scale (5 × 6 cm2), and the LED arrays (>3.7 V) can be easily lighted up by three ASCs in series, showing its potential practical application.

Laser printing-based high-resolution metal patterns with customizable design and scalable fabrication of high-performance flexible planar micro energy storage devices

The ever-increasing demand for light, thin, flexible, and small-sized smart electronics has developed a market for planar micro energy storage devices with high performance, flexibility, and robust integration, that is not mature yet. Here, a high-resolution patterned platinum (Pt) layer that can be designed/shaped as required is prepared by printing sacrificial patterns (SPs) using a commercial laser printer. MnO2-based micro-supercapacitors (MSCs) are successfully prepared on commercial Polyethylene terephthalate (PET) via electrodeposition. Because of the large surface area and large number of active sites of the nano-branch-shaped MnO2, the MnO2/Pt MSCs exhibit excellent volumetric specific capacitance (615.14F cm−3) as well as an ultrahigh volumetric power density (11.677 Wh cm−3) and energy density (54.679 mWh cm−3). In addition, the MnO2/Pt MSCs possess excellent flexibility and integration, with capacitance retention when bend to 180° and can be designed to connect in series or parallel for delivering the desired output voltage and capacitance. Furthermore, planar Zn-MnO2 batteries are successfully prepared through step-by-step electrodeposition. Simultaneously, a flexible energy conversion storage system is constructed by connecting solar cells and electrical appliances with the fabricated MSCs, demonstrating that this straightforward strategy is extremely promising for future application in microcircuits and flexible wearable smart textiles.

Hierarchically nanobranch structured freestanding metallic mesh electrode for high-performance transparent flexible supercapacitor

The intense interest in wearable electronics as a futuristic technology advancing the daily life of human being is now motivating the synergetic development of high-performance transparent flexible energy storage devices, presenting challenges for supercapacitor electrodes in terms of their electrical, optical and mechanical properties. Here, we proposed a unique supercapacitor electrode with hierarchical Ni@MnO2 nanobranch structures supported on a transparent and flexible freestanding Ni-mesh via a facile electrochemical deposition method. The hierarchically nanostructured electrodes provide a large surface area for fast ion transportation, thus strongly enhancing the supercapacitor device performance (19.65 mF/cm2) which is nearly an order of magnitude compared to the device with planar electrodes (2.1 mF/cm2), while still maintaining the high transparency (77%) and super-flexibility. Besides, the device delivers a long cycling life of 98.6% retention after 10,000 cycles, and stable working performance when bended to a series of bending angles and even after repeated folding, presenting outstanding mechanical flexibility for conformal integration with curved surfaces. This hierarchically nanostructured electrode offers a facile way for capacitance enhancement in developing wearable energy-storage devices with both high optical transmittance and mechanical reliability.

Interconnected δ-MnO2 nanosheets anchored on activated carbon cloth as flexible electrode for high-performance aqueous asymmetric supercapacitors

Abstract For aqueous supercapacitors, the working voltage is normally limited below 2 V because of the electrochemical stability of water, which limits improvement of the energy density and thus severely hinders their practical application. Herein, a flexible electrode of interconnected δ-MnO2 nanosheets anchored on activated carbon cloth with a potential window (vs Ag/AgCl) extending to 1.2 V is developed using simple one-step water bath only at 40 °C. A high specific capacitance up to 360.5 F g−1 at 1 A g−1 combining with good rate performance and electrochemical stability is delivered. Folding test indicates that the charge storage performance of the electrode has neglected degradation during the 2000 times of folding. Furthermore, a 2.4 V aqueous asymmetric supercapacitor is assembled based on the device configuration of activated carbon//δ-MnO2 and a high energy density of 49.8 Wh kg−1 at the power density of 1198.4 W kg−1 and good electrochemical stability, i.e., 90.6% capacitance retention after 5000 cycles can be achieved. Thanks to the good energy storage performance, high flexibility and simple preparation of the material, it is believed that this work provides a valuable exploration to develop high-performance flexible aqueous asymmetric energy storage devices.

Low-cost and low-density carbonized facial tissue supported uniform NiCo2S4 nanotubes for high capacity flexible solid-state supercapacitors

Recently, rapid development of intelligent wearable electronic equipment has triggered great needs for flexible and lightweight portable energy storage systems. Here, we demonstrate a low-cost ($ 0.5 m-2) and low density carbonized facial tissue paper (FCP) for high-conductive NiCo2S4 nanotubes assisted novel ultra-flexible solid-state supercapacitors (SSCs). The hydrothermal synthesized FCP-900@NiCo2S4 electrodes in this paper only have a density of 5 mg cm−2 and show large specific capacitance of 3115 mF cm−2 at 2 mA cm−2. Based on the high performance of FCP-900@NiCo2S4 electrodes, the as assembled SSCs can deliver an energy density of 1.2 mWh cm−3, high power density of 58.16 mW cm−3. It also shows great bending-state capacitive performance with high capacitance retention after 5000 cycles, providing a new opportunity for developing high-performance flexible wearable energy storage devices.

NiCo2O4 nanosheets sheathed SiC@CNTs core-shell nanowires for high-performance flexible hybrid supercapacitors

Electrode materials with hierarchical self-supporting core–shell structures, with the metric of structural advantages and synergetic effect for different components, have been widely applied in supercapacitor. Besides, interface designing would improve the bonding of different components and further enhance the stability of electrochemical performance. In this work, by the introduction of CNTs layer to construct the conductive and rugged interface on SiC nanowires (NWs), the formed core–shell SiCNWs@CNTs network were served as conductive skeleton for supporting NiCo2O4 nanosheets (NSs). Benefiting from the unique hierarchical structure with designed interface, the formed SiCNWs@CNTs@NiCo2O4NSs electrode exhibits exceptional electrochemical performance with high specific capacitance of 2302F g−1 (319.7 mAh g−1) at 1 A g−1, excellent rate capability (86.3% capacitance retention at 20 A g−1) and outstanding cycling stability (95% capacitance retention after 5000 cycles). Furthermore, the hybrid supercapacitor assembled SiCNWs@CNTs@NiCo2O4NSs and activated carbon (AC), exhibits a high energy density of 64.2 Wh kg−1 at a power density of 0.79 kW kg−1, long cycle life and good flexibility. More impressively, this work provides a facile method for rationally constructing electrode materials with hierarchical structures for high-performance flexible energy storage devices.

Flexible Solid-State Supercapacitor Using 2D White Graphene

Future inventions and technologies like miniaturize electronics with various functionalities, wearable devices, and bio-implantable sensors have demand for reliable and efficient flexible energy storage devices that can stand mechanical deformation. The flexibility of energy storage devices depends upon configuration, and assembly design to integrate all components into a single flexible unit. In this work, we exhibit a flexible all solid-state supercapacitor which consists of two flexible electrodes, fabricated by modified activated carbon with 2D nanostructure of hexagonal boron nitride (h-BN) ionomers. The electrodes are separated by a nano-engineered electrolyte membrane developed in our previous work. The surface-functionalized h-BN (FhBN) ionomers and nanocomposite electrolyte membranes showed superionic conduction with through- and in-plane ion conductivity of 0.1 S.cm−1 and 0.41 S.cm−1, respectively, which are 14 and 7 times higher than the commercial Nafion ionomer and membrane. The performance of the flexible supercapacitor was validated by cyclic voltammetry, constant current charge/discharge tests, and electrochemical impedance spectroscopy (EIS). These tests are performed in symmetric assembly of supercapacitor and displayed outstanding areal-specific capacitance which was ~2 times higher than that of a supercapacitor made of commercial Nafion electrolyte. Furthermore, we observed that the FhBN-based flexible supercapacitor can conserve its capacitance during the bending test under the mechanical stress, demonstrating its outstanding mechanical flexibility while maintaining its electrochemical performance. With the exceptional mechanical and electrical robustness, the FhBN-based flexible supercapacitors promises an exciting candidate for high-performance flexible energy storage devices in wearable electronics.

In-situ fabrication of carbon-metal fabrics as freestanding electrodes for high-performance flexible energy storage devices

Hierarchical 1D carbon structures are attractive due to their mechanical, chemical and electrochemical properties however the synthesis of these materials can be costly and complicated. Here, through the combination of inexpensive acetylacetonate salts of Ni, Co and Fe with a solution of polyacrylonitrile (PAN), self-assembling carbon-metal fabrics (CMFs) containing unique 1D hierarchical structures can be created via easy and low-cost heat treatment without the need for costly catalyst deposition nor a dangerous hydrocarbon atmosphere. Microscopic and spectroscopic measurements show that the CMFs form through the decomposition and exsolution of metal nanoparticle domains which then catalyze the formation of carbon nanotubes through the decomposition by-products of the PAN. These weakly bound nanoparticles form structures similar to trichomes found in plants, with a combination of base-growth, tip-growth and peapod-like structures, where the metal domain exhibits a core(graphitic)-shell(disorder) carbon coating where the thickness is in-line with the metal-carbon binding energy. These CMFs were used as a cathode in a flexible zinc-air battery which exhibited superior performance to pure electrospun carbon fibers, with their metallic nanoparticle domains acting as bifunctional catalysts. This work therefore unlocks a potentially new category of composite metal-carbon fiber based structures for energy storage applications and beyond.

Enhanced Electrochemical Performance of Cooper Sulfides Coated Electrochemical Modified Carbon Cloth and Its Application in Flexible Supercapacitors

All High-performance flexible electrochemical energy storage devices such as supercapacitors (SCs) and lithium ion batteries (LIBs) are highly desirable as a result of the ever-increasing demands for stretchable electronics. In this work, an effective two-step strategy contain surface and structural modulation and hydrothermal method is developed to grow cooper sulfides onto a highly active surface with pores and numerous functional oxygenic groups. Arising from rough surface and increased active sites, the as-prepared activated loosen cooper sulfides coated electrochemical modified carbon cloths (CuS@ECC) electrode not only achieves a large capacitance (410 F cm−2 at 1 mA cm−2) and fast kinetics but also shows excellent cycling durability (maintain 95% initial capacitance after 3000 cycles). This dual modification strategy may throw light on the rational design of new generation advanced composited electrodes for high-performance flexible supercapacitors.

Oxygen Groups Immobilized on Micropores for Enhancing the Pseudocapacitance

Understanding the effect of heteroatoms on the capacitance of doped carbon provides an effective strategy to boost its electrochemical performance. Here we show that the pseudocapacitive behavior is closely related to the chemical sites of bonded oxygen, which can be enhanced by immobilizing oxygen functional groups on micropores. Additionally, the doping styles have a significant influence on the capacitance retention ability. High capacitance of 3457 mF cm–2 achieved for the as-prepared oxygen-doped carbon cloth also promises its application to flexible solid-state supercapacitors (SSCs). These findings can open up new opportunities for future design and application of advanced heteroatom-doped materials for high-performance flexible energy storage devices.

Free-standing MXene film modified by amorphous FeOOH quantum dots for high-performance asymmetric supercapacitor

The rapid development of the portable and wearable electronics creates a high demand for better performance energy storage devices with high areal capacity as well as excellent flexibility. Herein, a free-standing Ti3C2/FeOOH quantum dots (QDs) hybrid film has been designed and synthesized by a simple electrostatic self-assembly, where amorphous FeOOH QDs not only act as pillars to prevent restacking of the Ti3C2 nanosheets but also provide considerable capacitance as active materials. The Ti3C2/FeOOH QDs hybrid film exhibits 2.3 times higher areal capacitance (485 mF cm−2) than the pristine Ti3C2 film and good cycle stability (94.8% retention over 5000 cycles) in neutral electrolyte. Moreover, an asymmetric device has been prepared by combining the hybrid film as negative electrode and MnO2 on carbon cloth as positive electrode, which operates at an output potential difference of 1.6 V. The device delivers the maximum energy density of 40 μW h cm−2 and the maximum power density of 8.2 mW cm−2 with 82% capacitance retention after 3000 cycles. The facile preparation process and excellent electrochemical properties of the free-standing Ti3C2/FeOOH QDs hybrid film make it a promising candidate for constructing high-performance flexible energy storage devices.

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.

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.