Integrated System of Solar Cells with Hierarchical NiCo2O4 Battery-Supercapacitor Hybrid Devices for Self-Driving Light-Emitting Diodes

Recent Advances in Flexible Wearable Supercapacitors: Properties, Fabrication, and Applications

A supercapacitor is a potential electrochemical energy storage device with high-power density (PD) for driving flexible, smart, electronic devices. In particular, flexible supercapacitors (FSCs) have reliable mechanical and electrochemical properties and have become an important part of wearable, smart, electronic devices. It is noteworthy that the flexible electrode, electrolyte, separator and current collector all play key roles in overall FSCs. In this review, the unique mechanical properties, structural designs and fabrication methods of each flexible component are systematically classified, summarized and discussed based on the recent progress of FSCs. Further, the practical applications of FSCs are delineated, and the opportunities and challenges of FSCs in wearable technologies are proposed. The development of high-performance FSCs will greatly promote electricity storage toward more practical and widely varying fields. However, with the development of portable equipment, simple FSCs cannot satisfy the needs of integrated and intelligent flexible wearable devices for long durations. It is anticipated that the combining an FSC and a flexible power source such as flexible solar cells is an effective strategy to solve this problem. This review also includes some discussions of flexible self-powered devices.

Effect of Interlayer Spaces and Interfacial Structures on High-Performance MXene/Ionic Liquid Supercapacitors: A Molecular Dynamics Simulation

The combination of high-capacitance MXenes and wide-electrochemical-window ionic liquids (ILs) has exhibited bright prospects in supercapacitors. Several strategies, such as surficial functionalization and interlayer spacing tuning, have been used to enhance the electrochemical performance of supercapacitors. However, the lack of theoretical guidance on these strategies, including the effects of the microenvironment in the interlayer of confined ILs, hindered the further exploration of such devices. Herein, we performed molecular dynamics simulations to comprehensively investigate the effects of the interlayer space and surface terminations of MXene electrodes on capacity. The results show that the electrical double layer (EDL) structure was found to form on the interface between the MXene electrode and ILs electrolyte by analyzing the ion number density and charge density in the nanometer confined spaces. Under the same potential, the -OH terminations significantly impact the ion orientation in the EDL, particularly near the electrode surface, where cations tend to align vertically, allowing the retention of more cations at the electrode surfaces. Interestingly, such an orientation distribution was decisively from the hydrogen bonds expressed by O-H···O between the -OH termination of MXene and -OH groups of ILs. The differential capacitances of the supercapacitors were calculated by the surficial electron density, and it showed that the capacitance is a nearly one-quarter increase in the 14 Å interlayer spacing compared with that of 10 Å under an applied potential of 2 V. At the same time, the Ti3C2(OH)2 electrode had a higher differential capacitance than the Ti3C2O2 electrode, which possibly originates from the stronger hydrogen bonds to contribute to the vertical aggregation of the cations. Our results highlighted the roles of the interlayer spacing distance and surface terminations of the MXene on the performance of the type of supercapacitor.

Single-Piece Membrane Supercapacitor with Exceptional Areal/Volumetric Capacitance via Double-Face Print of Electrode/Electrolyte Active Ink

Single-piece flexible supercapacitors (FSCs) have light and ultrathin superiorities, thereby having great potential in portable/wearable electronics. However, all the available single-piece FSCs are fabricated by in situ growth routes, which are incompatible with large-scale technology. This work designs a carboxymethyl cellulose/phytic acid/polyaniline ink, incorporating electrode with electrolyte active compositions. Based on the electrode/electrolyte active ink, a double-face print technique on mixed cellulose ester and nylon membranes to fabricate single-piece membrane-FSCs, where both sides of membranes can be utilized well, is proposed. Consequently, one FSC is measured to be only ≈0.785 cm² in area, ≈0.021 g in weight, and ≈200 µm in thickness, while it has exceptional areal and volumetric capacitances up to 757 mF cm^-2 and 37.8 F cm^-3 , respectively, based on the entire device. It also exhibits high flexibility with a capacitance retention of 98% after 2000 bend cycles from 0° to 180°. The state-of-the-art FSCs are expected to have exciting prospects in portable/wearable electronics, smart reading, and flexible displays. The preparation strategy renders the massive production of large-area and mini-size arrayed FSCs, and also the "do-it-yourself" or homemade preparation, which adds more interest and designability for general users.

Flexible Inorganic All-Solid-State Electrochromic Devices toward Visual Energy Storage and Two-Dimensional Color Tunability

Multicolor display has gradually become a sought-after trend for electrochromic devices due to its broadened application scope. Meanwhile, the advantages of inorganic electrochromic devices such as stable electrochemical performance and good energy storage ability also have great attraction in practical production applications. However, there are still huge challenges for inorganic electrochromic materials to achieve multicolor transformation due to their single-color hue change. Herein, we design an inorganic and multicolor electrochromic energy storage device (MEESD) exhibiting flexibility and all-solid-state merits. Prussian blue (PB) and MnO₂, as the asymmetrical electrodes of this MEESD, show good pseudocapacitance property, matching charge capacity, and obvious color change. As a typical electrochromic device, the MEESD shows a fast response of 0.5 s and good coloration efficiency of 144.2 cm²/C. As an energy storage device, the MEESD presents excellent rate capability and volumetric energy/power density (84.2 mWh cm^-3/23.3 W cm^-3). Its energy level can be visually monitored by color contrast and optical modulation. In the charging/discharging process, its color can obviously change to various degrees of yellow, green, and blue along with 40% wide optical modulation at 710 nm. Meanwhile, the stability of the MEESD in a common and humidity environment was analyzed in detail from electrochemical, optical, and energy storage aspects. This work provides feasible thoughts to design multifunctional electrochromic devices integrated with inorganic, flexible, all-solid-state, multicolor, and energy storage properties.

Electrochemical Self-Healing Nanocrystal Electrodes for Ultrastable Potassium-Ion Storage

The unique properties of self-healing materials hold great potential in battery systems, which can exhibit excellent deformability and return to its original shape after cycling. Herein, a Cu₃ BiS₃ anode material with self-healing mechanisms is proposed for use in ultrastable potassium-ion battery (PIB) and potassium-ion hybrid capacitor (PIHC). Different from the binder design, Cu₃ BiS₃ anode can exhibit the dual advantages of phase and morphological reversibility, further remaining original property after potassiation/depotassiation and exhibiting ultrastable cycling performance. The reversible electrochemical reconstruction during the continuous charge/discharge processes is beneficial to maintain the structure and function of the material. Furthermore, the conversion reactions during the charge and discharge process produce two advantages: i) suppressing the shuttle effect due to the formation of the heterostructure interface between Cu (111) and Bi (012); ii) Cu can avoid the agglomeration of Bi nanoparticles (NPs), further improving the electrochemical performance and long-cycle stability of the Cu₃ BiS₃ electrode. As a result, the Cu₃ BiS₃ electrode not only exhibits a long cycle life in half cells, but also 2000 cycles and 12000 cycles in PIB and PIHC full cells, respectively.

Liquid Metal-Templated Tin-Doped Tellurium Films for Flexible Asymmetric Pseudocapacitors

Liquid metals can be surface activated to generate a controlled galvanic potential by immersing them in aqueous solutions. This creates energized liquid-liquid interfaces that can promote interfacial chemical reactions. Here we utilize this interfacial phenomenon of liquid metals to deposit thin films of tin-doped tellurium onto rigid and flexible substrates. This is accomplished by exposing liquid metals to a precursor solution of Sn²⁺ and HTeO₂⁺ ions. The ability to paint liquid metals onto substrates enables us to fabricate supercapacitor electrodes of liquid metal films with an intimately connected surface layer of tin-doped tellurium. The tin-doped tellurium exhibits a pseudocapacitive behavior in 1.0 M Na₂SO₄ electrolyte and records a specific capacitance of 184.06 F·g^-1 (5.74 mF·cm^-2) at a scan rate of 10 mV·s^-1. Flexible supercapacitor electrodes are also fabricated by painting liquid metals onto polypropylene sheets and subsequently depositing tin-doped tellurium thin films. These flexible electrodes show outstanding mechanical stability even when experiencing a complete 180° bend as well as exhibit high power and energy densities of 160 W·cm^-3 and 31 mWh·cm^-3, respectively. Overall, this study demonstrates the attractive features of liquid metals in creating energy storage devices and exemplifies their use as media for synthesizing electrochemically active materials.

Progress on two-dimensional binary oxide materials

Two-dimensional van der Waals (2D vdW) materials have attracted much attention because of their unique electronic and optical properties. Since the successful isolation of graphene in 2004, many interesting 2D materials have emerged, including elemental olefins (silicene, germanene, etc.), transition metal chalcogenides, transition metal carbides (nitrides), hexagonal boron, etc. On the other hand, 2D binary oxide materials are an important group in the 2D family owing to their high structural diversity, low cost, high stability, and strong adjustability. This review systematically summarizes the research progress on 2D binary oxide materials. We discuss their composition and structure in terms of vdW and non-vdW categories in detail, followed by a discussion of their synthesis methods. In particular, we focus on strategies to tailor the properties of 2D oxides and their emerging applications in different fields. Finally, the challenges and future developments of 2D binary oxides are provided.

Designing flexible, smart and self-sustainable supercapacitors for portable/wearable electronics: from conductive polymers

The rapid development of portable/wearable electronics proposes new demands for energy storage devices, which are flexibility, smart functions and long-time outdoor operation. Supercapacitors (SCs) show great potential in portable/wearable applications, and the recently developed flexible, smart and self-sustainable supercapacitors greatly meet the above demands. In these supercapacitors, conductive polymers (CPs) are widely applied due to their high flexibility, conductivity, pseudo-capacitance, smart characteristics and moderate preparation conditions. Herein, we'd like to introduce the CP-based flexible, smart and self-sustainable supercapacitors for portable/wearable electronics. This review first summarizes the flexible SCs based on CPs and their composites with carbon materials and metal compounds. The smart supercapacitors, i.e., electrochromic, electrochemical actuated, stretchable, self-healing and stimuli-sensitive ones, are then presented. The self-sustainable SCs which integrate SC units with energy-harvesting units in one compact configuration are also introduced. The last section highlights some current challenges and future perspectives of this thriving field.

Construction of high-performance electrode materials of NiCo2O4 nanoparticles encapsulated in ultrathin N-doped carbon nanosheets for supercapacitors

Highly dispersed nitrogen doped carbon (N-C) is decomposed by 2-methylimidazole (C₄H₆N₂) and is used as a composite material with nickel cobaltite (NiCo₂O₄). The N-C and NiCo₂O₄ composites are obtained by a one-step hydrothermal method and subsequent calcination. In addition, N-C is used to control the morphology and structure of NiCo₂O₄ to obtain excellent capacitor materials. The N-C/NiCo₂O₄ electrode shows an excellent specific capacitance of 157.97 mA h g^-1 (1263.75 F g^-1) at 1 A g^-1. Herein, we successfully develop a N-C/NiCo₂O₄//AC asymmetric supercapacitor (ASC), which is prepared using N-C/NiCo₂O₄ as a cathode coupled with activated carbon (AC) as an anode at a voltage of 1.6 V. The prepared N-C/NiCo₂O₄//AC device shows an excellent volumetric energy density of 66.44 mW h kg^-1. The promising performance of N-C/NiCo₂O₄//AC illustrated its potential for portable supercapacitor applications.

Inkjet-Printed Ultrathin MoS2-Based Electrodes for Flexible In-Plane Microsupercapacitors

Flexible and wearable energy storage microdevice systems with high performance and safety are promising candidates for the electronics of on-chip integration. Herein, we demonstrate inkjet-printed ultrathin electrodes based on molybdenum disulfide (MoS₂) nanosheets for flexible and all-solid-state in-plane microsupercapacitors (MSCs) with high capacitance. The MoS₂ nanosheets were uniformly dispersed in the low-boiling point and nontoxic solvent isopropanol to form highly concentrated inks suitable for inkjet printing. The MSCs were assembled by printing the highly concentrated MoS₂ inks on a polyimide substrate with appropriate surface tension using a simple and low-cost desktop inkjet printer. Because of the two-dimensional structure of MoS₂ nanosheets, the as-assembled planar MSCs have high loadings of active materials per unit area, resulting in more flexibility and thinness than the capacitors with a traditional sandwich structure. These planar MSCs can not only possess any collapsible shape through the computer design but also exhibit excellent electrochemical performance (with a maximum energy density of 0.215 mW h cm^-3 and a high-power energy density of 0.079 W cm^-3), outstanding mechanical flexibility (almost no degradation of capacitance at different bending radii), good cycle stability (85.6% capacitance retention even after 10,000 charge-discharge cycles), and easy scale-up. Moreover, a blue light-emitting diode can be powered using five MSCs connected in series. The in-plane and low-cost MSCs with high energy densities have great application potential for integrated energy storage systems including wearable planar solar cells and other electronics.

High performance flexible hybrid supercapacitors based on nickel hydroxide deposited on copper oxide supported by copper foam for a sunlight-powered rechargeable energy storage system

Herein, an integrated system combining solar cells with a hybrid supercapacitor for operating a homemade windmill device was assembled, achieving energy conversion, storage and utilization. As a candidate for positive electrode of hybrid supercapacitor devices, battery-like Ni(OH)₂@CuO@Cu binder-free electrode was fabricated by a two-step process at ambient temperature. CuO@Cu was prepared by chemical oxidation method to act as the supporting electrode for electrochemical deposition of Ni(OH)₂. Various deposition times (30, 50, 90, 150 and 200 s) were investigated to optimize the energy storage characteristics of the resulting Ni(OH)₂@CuO@Cu electrode materials. Among all the samples, Ni(OH)₂@CuO@Cu-150 exhibited the largest areal capacity of 7063 mC cm^-2 at 20 mA cm^-2, and was therefore chosen as the positive electrode in a hybrid supercapacitor device. Using N-doped reduced graphene oxide on nickel foam (N-rGO/NF) as the negative electrode, a hybrid supercapacitor was assembled. It displayed good flexibility, cycling stability and high areal energy density of 130.4 μWh cm^-2 at a power density of 1.6 mW cm^-2. Two hybrid supercapacitor devices were connected in series to successfully lighten up a red LED for 12 min 39 s, while three devices assembled in series were able to successfully power a three-digit digital display for 1 min 28 s. Interestingly, the hybrid supercapacitor device, charged by solar cells, further operated a homemade windmill device for 59 s, achieving sunlight-powered integration system. All of the findings suggested the practical application potential of the hybrid supercapacitor based on Ni(OH)₂@CuO@Cu composite as energy storage device.

Wearable Textile Supercapacitors for Self-Powered Enzyme-Free Smartsensors

Wearable energy storage and flexible body biomolecule detection are two key factors for real-time monitoring of human health in a practical environment. It would be rather exciting if one wearable system could be used for carrying out both energy storage and biomolecule detection. Herein, carbon fiber-based NiCoO₂ nanosheets coated with nitrogen-doped carbon (CF@NiCoO₂@N-C) have been prepared via a simple electrochemical deposition method. Interestingly, being a dual-functional active material, CF@NiCoO₂@N-C exhibits excellent behaviors as a supercapacitor and prominent electrocatalytic properties, which can be applied for enzyme-free biosensor. It exhibits outstanding energy storage, high capacitive stability (94% capacitive retention after 10,000 cycles), and pre-eminent flexible ability (95% capacitive retention after 10,000 bending cycles), as well as high sensitivity for enzyme-free glucose detection (592  μA mM^-1). Moreover, the CF@NiCoO₂@N-C-based wearable supercapacitors would be used as self-powered energy systems for enzyme-free biosensors. Integrating with bluetooth, we have successfully developed a wearable self-powered enzyme-free smartsensor, remotely controlled using a smartphone for health monitoring in a practical environment. From this prospective study, it was found that the design of wearable self-powered smartsensors, demonstrating energy storage and enzyme-free biosensing in one system, provides a promising device for detecting body biomolecules, which has the potential to be implemented in the artificial intelligent fields.

Ultrathin 2D Metal-Organic Framework Nanosheets In situ Interpenetrated by Functional CNTs for Hybrid Energy Storage Device

The ultrathin nickel metal–organic framework (MOF) nanosheets in situ interpenetrated by functional carboxylated carbon nanotubes (C-CNTs) were successfully constructed. The incorporated C-CNTs effectively adjust the layer thickness of Ni-MOF nanosheets. The integrated hybrid MOF nanosheets delivered the boosted electrochemical performances and exhibited superior specific capacity of 680 C g−1 at 1 A g−1.

The controllable construction of two-dimensional (2D) metal–organic framework (MOF) nanosheets with favorable electrochemical performances is greatly challenging for energy storage. Here, we design an in situ induced growth strategy to construct the ultrathin carboxylated carbon nanotubes (C-CNTs) interpenetrated nickel MOF (Ni-MOF/C-CNTs) nanosheets. The deliberate thickness and specific surface area of novel 2D hybrid nanosheets can be effectively tuned via finely controlling C-CNTs involvement. Due to the unique microstructure, the integrated 2D hybrid nanosheets are endowed with plentiful electroactive sites to promote the electrochemical performances greatly. The prepared Ni-MOF/C-CNTs nanosheets exhibit superior specific capacity of 680 C g−1 at 1 A g−1 and good capacity retention. The assembled hybrid device demonstrated the maximum energy density of 44.4 Wh kg−1 at a power density of 440 W kg−1. Our novel strategy to construct ultrathin 2D MOF with unique properties can be extended to synthesize various MOF-based functional materials for diverse applications.

Core–shell ZnO@C:N hybrids derived from MOFs as long-cycling anodes for lithium ion batteries

ZnO@C:N hybrids are obtained that have a reversible capacity of 608 mA h g⁻¹ after 500 cycles.

FeSe2/carbon nanotube hybrid lithium-ion battery for harvesting energy from triboelectric nanogenerators

FeSe2-carbon nanotube (FeSe2-CNT) hybrid microspheres are investigated as anode materials for lithium ion batteries (LIBs), exhibiting a high specific capacity of 571.2 mA h g-1 at 0.5 A g-1 with excellent rate performance and cycling stability. The FeSe2-CNT hybrid LIBs could withstand the high-voltage pulse of triboelectric nanogenerators (TENGs) and be charged by TENGs directly for harvesting energy with high stability.