High-Energy Flexible Supercapacitor-Synergistic Effects of Polyhydroquinone and RuO2· xH2O with Microsized, Few-Layered, Self-Supportive Exfoliated-Graphite Sheets

A metal-organic framework-derived α-MnS/MWCNT composite as a promising pseudocapacitive material for a flexible quasi-solid-state asymmetric supercapacitor device

The low conductivity of traditionally used pseudocapacitive materials like transition metal oxides has forced researchers to look for alternative materials. Transition metal sulfides are being investigated as viable alternative materials and have shown promising results. In this work, an α-MnS/MWCNT composite is selected as the active material for supercapacitor application. α-MnS has better conductivity than many transition metal oxides but has an extremely low specific surface area (10.5 m2 g-1), which reduces its specific capacitance. Metal-organic framework (MOF)-derived materials are known to possess higher specific surface area and favorable pore size distribution. Herein, α-MnS/MWCNT composites are synthesized via two routes: the conventional solvothermal technique and the MOF route, and their performance is compared. It is proved that the α-MnS/MWCNT composite synthesized through the MOF route shows a favorable porous structure and better performance than the composite synthesized through the conventional route. It shows a specific surface area of 47.6 m2 g-1 and a specific capacitance of 546.3 F g-1 at 1 A g-1 with a mass loading of 1.5 mg cm-2 in 3 M KOH under a 3-electrode configuration. A flexible quasi-solid-state asymmetric supercapacitor device is fabricated with MOF-derived α-MnS/MWCNT as the positive electrode material, and the device achieved a potential window of 1.4 V, a specific capacitance of 82.5 F g-1 at 1 A g-1 and a capacitance retention of 90.1% after 5000 cycles at 10 A g-1. The results clearly indicate that transition metal sulfides like MOF-derived α-MnS can be a viable alternative to traditional materials like transition metal oxides. The assembled device has the potential to power flexible, wearable electronics.

Low-temperature carbonized MXene/protein-based eggshell membrane composite as free-standing electrode for flexible supercapacitors

The demerits of the carbonized eggshell membrane (EM), such as high cost, high brittleness, immutable shape and size, greatly limit its application in demanding supercapacitors as free-standing electrode. Herein, the reconstituted EM (REM) with good flexibility and excellent size-customizability is developed, which is due to their fibrous structure and abundant surface polar groups. Ti₃C₂ nanosheet (a typical MXene) with ultra-high electrical conductivity and good electrochemical activity is then coated on REM surface, and undergoes a low-temperature carbonization (350 °C) to prepare CREM/T. Multi-functions of Ti₃C₂ are exhibited: (1) constructing a conductive network on REM surface by randomly stacking to yield a high electrical conductivity of 78.1 S cm^-1, (2) being as a protective mold to remain the inherent flexibility and porosity of REM during carbonization, (3) creating nanopores by inducing self-activation, and (4) yielding a large capacitance of 1729 mF cm^-2 at 0.5 mA cm^-2 and a high rate capability of 82 % after increasing the current density by 50 folds. Furthermore, an all-EM-based supercapacitor is fabricated with REM as the separator and CREM/T as the electrode. It delivers a high energy density of 16.1 μW h cm^-2 at 1301 μW cm^-2, and shows stable capacitive behaviors during bending.

COF-Based Electrodes with Vertically Supported Tentacle Array for Ultrahigh Stability Flexible Energy Storage

As an emerging porous crystal polymer, covalent organic frameworks (COFs) possess unique characteristics, such as high porosity, excellent stability, diverse topologies, designable open channels, and functional tunability. However, limited by the solid powder form, most COFs display low active site utilization and weak binding force with the current collector. In this pioneering research, we integrate redox-active COFs onto carbon fiber surfaces (AC-COFs) via strong covalent bridging. The 2,6-diaminoanthraquinone (DAAQ) pillars embedded on the carbon fiber surface are the key to precisely controlling the growth direction of COFs. The obtained tentacle-like array vertically supported on the surface of the carbon fiber can effectively induce charge transfer and prevent COFs from aggregating/collapsing. The strong covalent coupling and increase of accessible active sites contributed to the high specific capacitance of AC-COFs electrode (1034 mF cm^-2). In addition, the COF-based flexible electrode retains an initial capacitance of 98% after 20000 charge-discharge cycles. The flexible all-solid-state symmetric supercapacitor is assembled by PVA/H₂SO₄ gel electrolyte with an areal capacitance of 715 mF cm^-2. Besides, a red LED can be easily powered by three-bending AC-COFs//AC-COFs devices. The innovative synthesis strategy opens up new opportunities to develop high-performance flexible energy storage devices based on COFs.

Smart Electronic Textile-Based Wearable Supercapacitors

Electronic textiles (e-textiles) have drawn significant attention from the scientific and engineering community as lightweight and comfortable next-generation wearable devices due to their ability to interface with the human body, and continuously monitor, collect, and communicate various physiological parameters. However, one of the major challenges for the commercialization and further growth of e-textiles is the lack of compatible power supply units. Thin and flexible supercapacitors (SCs), among various energy storage systems, are gaining consideration due to their salient features including excellent lifetime, lightweight, and high-power density. Textile-based SCs are thus an exciting energy storage solution to power smart gadgets integrated into clothing. Here, materials, fabrications, and characterization strategies for textile-based SCs are reviewed. The recent progress of textile-based SCs is then summarized in terms of their electrochemical performances, followed by the discussion on key parameters for their wearable electronics applications, including washability, flexibility, and scalability. Finally, the perspectives on their research and technological prospects to facilitate an essential step towards moving from laboratory-based flexible and wearable SCs to industrial-scale mass production are presented.

In Situ Grown Ultrafine RuO2 Nanoparticles on GeP5 Nanosheets as the Electrode Material for Flexible Planar Micro-Supercapacitors with High Specific Capacitance and Cyclability

GeP₅, as the most representative phosphorus-based material in two-dimensional layered phosphorous compounds, has shown a fairly bright application prospect in the field of energy storage because of its ultrahigh electrical conductivity. However, high-yield exfoliation methods and effective structure construction strategies for GeP₅ nanosheets are still missing, which completely restricts the further application of GeP₅-based nanocomposites. Here, we not only improved the yield of GeP₅ nanosheets by a liquid nitrogen-assisted liquid-phase exfoliation technique but also constructed the GeP₅@RuO₂ nanocomposites with the 0D/2D heterostructure by in situ introduction of ultrafine RuO₂ nanoparticles on highly conductive GeP₅ nanosheets using a simple hydrothermal synthesis method, and then applying it to micro-supercapacitors (MSCs) as electrode materials through a mask-assisted vacuum filtration technique. It is precisely because of the synergy of the electrical double-layer material, GeP₅ nanosheets and the pseudocapacitance material RuO₂ nanoparticles that endows the GeP₅@RuO₂ electrode with outstanding electrochemical performance in micro-supercapacitors with a large specific capacitance of 129.5 mF cm^-2/107.9 F cm^-3, high energy density of 17.98 μWh cm^-2, remarkable long-term cycling stability with 98.4% capacitance retention after 10 000 cycles, the exceptional mechanical stability, outstanding environmental stability, and excellent integration features. This work opens up a new avenue to construct GeP₅-based nanocomposites as a most promising novel electrode material for practical application in flexible portable/wearable micro-nanoelectronic devices.

Hydrogen Bonding-Reinforced Hydrogel Electrolyte for Flexible, Robust, and All-in-One Supercapacitor with Excellent Low-Temperature Tolerance

Flexible supercapacitors are promising energy storage devices for emerging wearable electronics. However, due to the poor mechanical strength, complicated device manufacturing process, and unsatisfactory low-temperature tolerance, their overall performance for practical applications is hindered. Herein, we report a hydrogen bonding-reinforced, dual-crosslinked poly(vinyl alcohol), acrylic acid, and H₂SO₄ (PVA-AA-S) hydrogel electrolyte for all-in-one flexible supercapacitors. The PVA-AA-S hydrogel demonstrates excellent compressive/tensile properties and high ionic conductivity. It tolerates compressive stress of 0.53 MPa and is stretchable up to 500%. The hydrogel-based all-in-one supercapacitor shows promising electrochemical performance under various harsh conditions. The device energy density and power density reach up to 14.2 μWh cm^-2 and 0.94 mW cm^-2, respectively. Furthermore, it retains nearly 80% capacitance after being stored at -35 °C for 23 days. The excellent performance of the hydrogel electrolyte originates from its abundant strong hydrogen bonding between polymer chains and water molecules.