Metal-Organic Framework Derived Spindle-like Carbon Incorporated α-Fe2O3 Grown on Carbon Nanotube Fiber as Anodes for High-Performance Wearable Asymmetric Supercapacitors

3D Hydrangea Macrophylla-like Nickel-Vanadium Metal-Organic Frameworks Formed by Self-Assembly of Ultrathin 2D Nanosheets for Overall Water Splitting

The development of highly efficient and low-cost bifunctional noble metal-free electrocatalysts for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is an effective strategy for improving efficiency. Herein, novel three-dimensional (3D) bimetallic metal-organic frameworks containing Ni and V with adjustable stoichiometry were synthesized on nickel foam successfully. Notably, Ni2V-MOFs@NF only require rather low overpotentials of 244 and 89 mV for the OER and HER, respectively, and expedites overall water splitting with 1.55 V at 10 mA cm^-2 with robust durability during the 80 h test. The high efficiency of the novel obtained electrocatalysts should be attributed to the particular morphological design of the two-dimensional (2D) ultrathin nanosheets self-assembling into a 3D nanoflower and the electronic structure regulation resulting from the synergetic interaction between nickel and vanadium. Subsequent theoretical calculations reveal the following conclusions: (I) the exceptional electronic conductivity of Ni2V-MOFs shows enhanced optimization as a result of electronic structure reconstruction, (II) the energy barrier reduction of the rate-limiting step is responsible for the enhanced dynamics of Ni2V-MOFs for the OER, and (III) the facilitation of the adsorption of H⁺ and H₂O plays a key role in progressing the HER catalytic activity of Ni2V-MOFs.

A General Electrodeposition Strategy for Fabricating Ultrathin Nickel Cobalt Phosphate Nanosheets with Ultrahigh Capacity and Rate Performance

Transition-metal phosphates/phosphides possess promising theoretical electrochemical characteristics and exhibit great potential in advanced supercapacitors. Unfortunately, limited by the processing techniques and overall structure, their specific capacity and rate performance are still unsatisfactory. Herein, we report the fabrication of transition-metal phosphate electrodes with an ultrathin sheetlike array structure by one-step electrodeposition at room temperature. As a proof-of-concept, a transition-metal phosphate member of NiCo(HPO₄)₂·3H₂O with an ultrathin nanosheet structure (thickness ∼2.3 nm) was synthesized and investigated. The as-prepared NiCo(HPO₄)₂·3H₂O electrode showcases an ultrahigh specific capacity of 1768.5 C g^-1 at 2 A g^-1 (the highest value for transition-metal phosphates/phosphides reported to date), superb rate performance of 1144.8 C g^-1 at 100 A g^-1, and excellent electrochemical stability. Moreover, the transition-metal phosphate nanosheet array can be uniformly deposited on various conductive substrates, demonstrating the generality of our strategy. Therefore, this simple electrodeposition strategy provides an opportunity to fabricate ultrathin transition-metal phosphate nanosheet materials that can be used for energy storage/conversion, electrocatalysis, and other electrochemical energy-related devices.

Enhanced Cycling Performance of Rechargeable Zinc-Air Flow Batteries Using Potassium Persulfate as Electrolyte Additive

Zinc-air batteries (ZABs) offer high specific energy and low-cost production. However, rechargeable ZABs suffer from a limited cycle life. This paper reports that potassium persulfate (KPS) additive in an alkaline electrolyte can effectively enhance the performance and electrochemical characteristics of rechargeable zinc-air flow batteries (ZAFBs). Introducing redox additives into electrolytes is an effective approach to promote battery performance. With the addition of 450 ppm KPS, remarkable improvement in anodic currents corresponding to zinc (Zn) dissolution and limited passivation of the Zn surface is observed, thus indicating its strong effect on the redox reaction of Zn. Besides, the addition of 450 ppm KPS reduces the corrosion rate of Zn, enhances surface reactions and decreases the solution resistance. However, excess KPS (900 and 1350 ppm) has a negative effect on rechargeable ZAFBs, which leads to a shorter cycle life and poor cyclability. The rechargeable ZAFB, using 450 ppm KPS, exhibits a highly stable charge/discharge voltage for 800 cycles. Overall, KPS demonstrates great promise for the enhancement of the charge/discharge performance of rechargeable ZABs.

Surfactant-Mediated Morphological Evolution of MnCo Prussian Blue Structures

In the preparation of nanomaterials, the kinetics and thermodynamics in the reaction can significantly affect the structures and phases of nanocrystals. Therefore, people are keen to adopt various synthetic strategies to accurately assemble the target nanocrystals, and reveal the underlying mechanism of the formation of specific structures. In this work, the total reaction time is adjusted to let the prepared MnCo Prussian blue analogous (MnCoPBA) crystals show four evolving morphological changes at different stages with the assistance of sodium dodecyl sulfate. Furthermore, it is clearly observed that the epitaxial growth along the (100) plane on the shell of MnCoPBA nanocrystals is favored, and the thermodynamics and kinetics in the morphology change process are analyzed in detail. Through the simple pyrolysis, MnCoPBA crystals can be successfully converted into the corresponding carbon composites, of which Mn₂ Co₂ C nanoparticles are evenly distributed in highly graphitized carbon matrix. Among them, PBA-III-700 performs good oxygen reduction reaction performance in alkaline solution with the half-wave potential of 0.801 V and diffusion-limited current density of 5.36 mA cm^-2 , and its zinc-air battery exhibits the peak power density of 103.4 mW cm^-2 competitive with commercial Pt/C.

Novel electrode geometry for high performance CF/Fe2O3 based planar solid state micro-electrochemical capacitors

A novel geometry of sharp-edged electrodes for planar micro-electrochemical capacitors is utilized for an enhanced performance compared to the conventionally used interdigitated electrodes. The sharp-edged electrode geometry achieves a 68% enhancement in the electric field at the sharp-edge of the electrodes as compared to interdigitated electrodes. Moreover, carbon foam with high specific surface area loaded with iron oxide nanoparticles allows a large mass loading for the pseudocapacitance in addition to electric double layer capacitance (EDLC). Thus, an enhancement of 235% was obtained in both the areal specific capacitance and energy density when the performance was compared with the interdigitated electrode based supercapacitors. Moreover, an excellent cycling stability (∼99.5%) over 10 000 charge-discharge cycles was also achieved. The high-performance architecture of sharp-edged electrodes paves a way for smart electrochemical capacitors using an efficient planar structure in combination with high-loading materials for large pseudocapacitance as well as EDLC.

Porous Carbon-Based Supercapacitors Directly Derived from Metal-Organic Frameworks

Numerously different porous carbons have been prepared and used in a wide range of practical applications. Porous carbons are also ideal electrode materials for efficient energy storage devices due to their large surface areas, capacious pore spaces, and superior chemical stability compared to other porous materials. Not only the electrical double-layer capacitance (EDLC)-based charge storage but also the pseudocapacitance driven by various dopants in the carbon matrix plays a significant role in enhancing the electrochemical supercapacitive performance of porous carbons. Since the electrochemical capacitive activities are primarily based on EDLC and further enhanced by pseudocapacitance, high-surface carbons are desirable for these applications. The porosity of carbons plays a crucial role in enhancing the performance as well. We have recently witnessed that metal-organic frameworks (MOFs) could be very effective self-sacrificing templates, or precursors, for new high-surface carbons for supercapacitors, or ultracapacitors. Many MOFs can be self-sacrificing precursors for carbonaceous porous materials in a simple yet effective direct carbonization to produce porous carbons. The constituent metal ions can be either completely removed during the carbonization or transformed into valuable redox-active centers for additional faradaic reactions to enhance the electrochemical performance of carbon electrodes. Some heteroatoms of the bridging ligands and solvate molecules can be easily incorporated into carbon matrices to generate heteroatom-doped carbons with pseudocapacitive behavior and good surface wettability. We categorized these MOF-derived porous carbons into three main types: (i) pure and heteroatom-doped carbons, (ii) metallic nanoparticle-containing carbons, and (iii) carbon-based composites with other carbon-based materials or redox-active metal species. Based on these cases summarized in this review, new MOF-derived porous carbons with much enhanced capacitive performance and stability will be envisioned.

Flexible supercapacitor electrodes using metal-organic frameworks

Advancements in the field of flexible and wearable devices require flexible energy storage devices to cater their power demands. Metal-ion batteries (such as lithium-ion batteries, sodium-ion batteries, etc.) and electrochemical capacitors (also called supercapacitors or ultracapacitors) have achieved great interest in the recent past due to their superior energy storage characteristics like high power density and long cycle life. A major bottleneck of using metal-ion batteries in wearable devices is their lack of flexibility. Low power density, toxicity and flammability due to organic electrolytes inhibit them from safe on-body device applications. On the other hand, supercapacitors can be made with aqueous electrolytes, making them a safer alternative for wearable applications. Metal-organic frameworks (MOFs) are novel candidates as electrode materials due to their salient features such as large surface area, three-dimensional porous architecture, permeability to foreign entities, structural tailorability, etc. Though pristine MOFs suffer from poor intrinsic conductivity, this can be rectified by preparing composites with other electronically conducting materials. MOF-based electrodes are highly promising for flexible and wearable supercapacitors since they exhibit good energy and power densities. This review focuses on the new developments in the field of MOF-based composite electrodes for developing flexible supercapacitors.

A High-Energy-Density Hybrid Supercapacitor with P-Ni(OH)2 @Co(OH)2 Core-Shell Heterostructure and Fe2 O3 Nanoneedle Arrays as Advanced Integrated Electrodes

Transition metal hydro/oxides (TMH/Os) are treated as the most promising alternative supercapacitor electrodes thanks to their high theoretical capacitance due to the various oxidation states and abundant cheap resources of TMH/Os. However, the poor conductivity and logy reaction kinetics of TMH/Os severely restrict their practical application. Herein, hierarchical core-shell P-Ni(OH)₂ @Co(OH)₂ micro/nanostructures are in situ grown on conductive Ni foam (P-Ni(OH)₂ @Co(OH)₂ /NF) through a facile stepwise hydrothermal process. The unique heterostructure composed of P-Ni(OH)₂ rods and Co(OH)₂ nanoflakes boost the charge transportation and provide abundant active sites when used as the intergrated cathode for supercapacitors. It delivers an ultrahigh areal specific capacitance of 4.4 C cm^-2 at 1 mA cm^-2 and the capacitance can maintain 91% after 10 000 cycles, showing an ultralong cycle life. Additionally, a hybrid supercapacitor composed with P-Ni(OH)₂ @Co(OH)₂ /NF cathode and Fe₂ O₃ /CC anode shows a wider voltage window of 1.6 V, a remarkable energy density of 0.21 mWh cm^-2 at the power density of 0.8 mW cm^-2 , and outstanding cycling stability with about 81% capacitance retention after 5000 cycles. This innovative study not only supplies a newfashioned electronic apparatus with high-energy density and cycling stability but offers a fresh reference and enlightenment for synthesizing advanced integrated electrodes for high-performance hybrid supercapacitors.

Polypyrrole-coated Fe2O3 nanotubes constructed from nanoneedles as high-performance anodes for aqueous asymmetric supercapacitors

Asymmetric supercapacitors (ASCs) show promising potential for electrochemical energy storage applications. However, the energy density of ASCs is limited by the poor electrochemical performance of anodes. To achieve high-performance ASCs, herein, Fe₂O₃ nanotubes constructed from Fe₂O₃ nanoneedles were fabricated by employing MnO₂ nanotubes as a self-sacrificing template, and then a layer of polypyrrole (PPy) was coated through an in situ chemical oxidative polymerization method to enhance their performance. The electrochemical tests indicate that the resultant PPy-coated Fe₂O₃ nanotubes (Fe₂O₃@PPy) exhibit a high areal capacitance of 530 mF cm^-2 at 1 mA cm^-2 and good cycling stability, which are superior to those of the Fe₂O₃ nanotubes. The superior performance of the Fe₂O₃@PPy nanotubes can be attributed to the synergistic effect between the PPy shell and Fe₂O₃ core, in which the conducting PPy shell not only works as a superhighway for charge transport, but also stabilizes the Fe₂O₃ nanotubes during charge-discharge processes. When the Fe₂O₃@PPy nanotubes were assembled with MnO₂ nanotubes, the as-assembled ASCs possess a high cell voltage of 2.0 V and deliver a high energy density of up to 51.2 Wh kg^-1 at a power density of 285.4 W kg^-1 with an excellent cycling stability (83.5% capacitance retention over 5000 cycles).

Three-Dimensional SiC/Holey-Graphene/Holey-MnO2 Architectures for Flexible Energy Storage with Superior Power and Energy Densities

Although nanostructured materials have recently enabled a dramatic improvement of the current energy-storage units in portable electronics with enhanced functionality, it is still challenging to provide a cost-efficient solution to attain the ultrahigh energy and power densities of supercapacitors (SCs) since nearly arbitrary electrodes are limited to the thinner porous structure with de facto rather low mass loading (∼1 mg cm^-2) because of the huge limitations of pronounced impaired ion transport in subnanometer pores in thicker compact electrodes. In this contribution, we report the fabrication of a macro/mesoporous hybrid hierarchical nanocomposite SiC/holey-graphene/holey-MnO₂ (SiC/HG/h-MnO₂) with tailored porosity by knitting together the quasi-aligned single-crystalline doped 3C-SiC nanowire array and in situ surface-reduced holey graphene framework into a three-dimensional quasi-ordered structure, which enables the mass growth of ultrathin h-MnO₂ nanosheets at approximately practical levels of mass loading. The produced synergistically favorable interconnected porous architecture allows for the highly efficient electron transfer and rapid ion transport up to interior surfaces of the network. Remarkably, the all-solid-state flexible asymmetric supercapacitors (ASCs) made with SiC/HG/h-MnO₂ and SiC/graphitic carbon (GC) nanoarrays are mechanically robust and show a high areal capacity (0.32 mWh cm^-2) and a high rate capability (280 mW cm^-2) at ultrahigh mass loading (6.5 mg cm^-2), much higher than most of previous superior SCs in aqueous or gelled electrolytes and thus offer an entirely new prototype of textile-based ASCs, which represents a critical step toward practical applications for various portable electronics.

Boosting the energy density of supercapacitors by encapsulating a multi-shelled zinc-cobalt-selenide hollow nanosphere cathode and a yolk-double shell cobalt-iron-selenide hollow nanosphere anode in a graphene network

The practical exploration of electrode materials with complex hollow structures is of considerable significance in energy storage applications. Mixed-metal selenides (MMSs) with favorable architectures emerge as new electrode materials for supercapacitor (SC) applications owing to their excellent conductivity. Herein, a facile and effective metal-organic framework (MOF)-derived strategy is introduced to encapsulate multi-shelled zinc-cobalt-selenide hollow nanosphere positive and yolk-double shell cobalt-iron-selenide hollow nanosphere negative electrode materials with controlled shell numbers in a graphene network (denoted as G/MSZCS-HS and G/YDSCFS-HS, respectively) for SC applications. Due to the considerable electrical conductivity and unique structures of both electrodes, the G/MSZCS-HS positive and G/YDSCFS-HS negative electrodes exhibit remarkable capacities (∼376.75 mA h g^-1 and 293.1 mA h g^-1, respectively, at 2 A g^-1), superior rate performances (83.4% and 74%, respectively), and an excellent cyclability (96.8% and 92.9%, respectively). Furthermore, an asymmetric device (G/MSZCS-HS//G/YDSCFS-HS) has been fabricated with the ability to deliver an exceptional energy density (126.3 W h kg^-1 at 902.15 W kg^-1), high robustness of 91.7%, and a reasonable capacity of 140.3 mA h g^-1.

Mesoporous Carbon Microfibers for Electroactive Materials Derived from Lignocellulose Nanofibrils

The growing adoption of biobased materials for electronic, energy conversion, and storage devices has relied on high-grade or refined cellulosic compositions. Herein, lignocellulose nanofibrils (LCNF), obtained from simple mechanical fibrillation of wood, are proposed as a source of continuous carbon microfibers obtained by wet spinning followed by single-step carbonization at 900 °C. The high lignin content of LCNF (∼28% based on dry mass), similar to that of the original wood, allowed the synthesis of carbon microfibers with a high carbon yield (29%) and electrical conductivity (66 S cm^-1). The incorporation of anionic cellulose nanofibrils (TOCNF) enhanced the spinnability and the porous morphology of the carbon microfibers, making them suitable platforms for electrochemical double layer capacitance (EDLC). The increased loading of LCNF in the spinning dope resulted in carbon microfibers of enhanced carbon yield and conductivity. Meanwhile, TOCNF influenced the pore evolution and specific surface area after carbonization, which significantly improved the electrochemical double layer capacitance. When the carbon microfibers were directly applied as fiber-shaped supercapacitors (25 F cm^-3), they displayed a remarkably long-term electrochemical stability (>93% of the initial capacitance after 10 000 cycles). Solid-state symmetric fiber supercapacitors were assembled using a PVA/H₂SO₄ gel electrolyte and resulted in an energy and power density of 0.25 mW h cm^-3 and 65.1 mW cm^-3, respectively. Overall, the results indicate a green and facile route to convert wood into carbon microfibers suitable for integration in wearables and energy storage devices and for potential applications in the field of bioelectronics.

High-Performance and Ultraflexible Aqueous Rechargeable Lithium-Ion Batteries Developed by Constructing All Binder-free Electrode Materials

Aqueous rechargeable lithium-ion batteries (ARLIBs) as alternative energy storage devices have attracted tremendous attention because of their low cost and high safety. However, it is still a significant challenge to develop flexible high-performance ARLIBs for powering wearable devices because of the lack of all binder-free electrode materials. In this study, we develop one-step hydro-/solvothermal methods to design binder-free electrodes of LiCoO₂ polygonal-sheeted arrays and rugby ball-shaped NaTi₂(PO₄)₃ on carbon nanotube fibers as the cathode (LCO@CNTF) and the anode (NTP@CNTF). Both the electrodes are prepared at low temperatures without an extra calcination process, which is a great improvement for the growth process. The electrodes deliver remarkable capacity and extraordinary rate performance in a saturated Li₂SO₄ solution. Meanwhile, because of the synergy of LCO@CNTF and NTP@CNTF, an impressive capacity of 45.24 mA h cm^-3 and an admirable energy density of 67.86 mW h cm^-3 are achieved for the assembled quasi-solid-state fiber-shaped flexible ARLIB (FARLIB), which outperform most reported fiber-shaped aqueous rechargeable batteries. More encouragingly, our FARLIB possesses good flexibility, with a 94.74% capacity retention after bending 3000 times. Thus, this work represents a significant step toward developing FARLIBs and provides a new prospect in the design of wearable energy storage devices.

Carbon Transition-metal Oxide Electrodes: Understanding the Role of Surface Engineering for High Energy Density Supercapacitors

Supercapacitors store electrical energy by ion adsorption at the interface of the electrode-electrolyte (electric double layer capacitance, EDLC) or through faradaic process involving direct transfer of electrons via oxidation/reduction reactions at one electrode to the other (pseudocapacitance). The present minireview describes the recent developments and progress of carbon-transition metal oxides (C-TMO) hybrid materials that show great promise as an efficient electrode towards supercapacitors among various material types. The review describes the synthetic methods and electrode preparation techniques along with the changes in the physical and chemical properties of each component in the hybrid materials. The critical factors in deriving both EDLC and pseudocapacitance storage mechanisms are also identified in the hope of pointing to the successful hybrid design principles. For example, a robust carbon-metal oxide interaction was identified as most important in facilitating the charge transfer process and activating high energy storage mechanism, and thus methodologies to establish a strong carbon-metal oxide contact are discussed. Finally, this article concludes with suggestions for the future development of the fabrication of high-performance C-TMO hybrid supercapacitor electrodes.

Superstructured α-Fe2O3 nanorods as novel binder-free anodes for high-performing fiber-shaped Ni/Fe battery

Fiber-shaped energy storage devices areindispensableparts of wearable and portable electronics. Aqueous rechargeable Ni/Fe battery is a very appropriate energy storage device due to their good safety without organic electrolytes, high ionic conductivity, and low cost. Unfortunately, the low energy density, poor power density and cycling performance hinder its further practical applications. In this study, in order to obtain high performance negative iron-based material, we first synthesized α-iron oxide (α-Fe₂O₃) nanorods (NRs) with superstructures on the surface of highly conductive carbon nanotube fibers (CNTFs), then electrically conductive polypyrrole (PPy) was coated to enhance the electron, ion diffusion and cycle stability. Theas-prepared α-Fe₂O₃@PPy NRs/CNTF electrode shows a high specific capacity of 0.62 Ah cm^-3 at the current density of 1 A cm^-3. Furthermore, the Ni/Fe battery that was assembled by the above negative electrode shows a maximum volumetric energy density of 15.47 mWh cm^-3 with 228.2 mW cm^-3 at a current density of 1 A cm^-3. The cycling durability and mechanical flexibility of the Ni/Fe battery were tested, which show good prospect for practical application. In summary, these merits make it possible for our Ni/Fe battery to have practical applications in next generation flexible energy storage devices.

Metal-Organic Frameworks with Enhanced Photodynamic Therapy: Synthesis, Erythrocyte Membrane Camouflage, and Aptamer-Targeted Aggregation

Here, ferric oxide-loaded metal-organic framework (FeTCPP/Fe₂O₃ MOF) nanorice was designed and constructed by the liquid diffusion method. The introduction of iron metal nodes and the loading of Fe₂O₃ can effectively catalyze the Fenton reaction to produce hydroxyl radicals (^•OH) and overcome the hypoxic environment of tumor tissue by generating oxygen. The monodispersity and porosity of the porphyrin photosensitizers in the MOF structure exposed more active sites, which promoted energy exchange between porphyrin molecules and oxygen molecules for photodynamic therapy (PDT) treatment. Therefore, the generated hydroxyl radicals and singlet oxygen (¹O₂) can synergistically act on tumor cells to achieve the purpose of improving tumor therapy. Then the erythrocyte membrane was camouflaged to enhance blood circulation and tissue residence time in the body, and finally, the targeted molecule AS1411 aptamer was modified to achieve the high enrichment of MOF photosensitizers on a tumor domain. As a result, the MOF nanorice camouflaged by the erythrocyte membrane can effectively reduce side effects and improve the therapeutic effect of PDT and chemo-dynamic therapy (CDT). The study not only improved the efficacy of PDT and CDT in essence from the MOF nanorice but also used the camouflage method to further concentrate FeTCPP/Fe₂O₃ on the tumor sites, achieving the goal of multiple gains. These results will provide theoretical and practical directions for the development of tumor-targeted MOF nanomaterials.

Moderate oxygen-deficient Fe(III) oxide nanoplates for high performance symmetric supercapacitors

As a promising anode material for supercapacitors, Fe₂O₃ has been widely studied but still face the problem of low conductivity. Inducing oxygen vacancy (Vo) into Fe₂O₃ is a widely used approach to tune the conductivity to enhance its capacitive performance, but there is little research on the influence of Vo content. Herein, we report the effect of Vo in Fe₂O₃ nanoplates with various content. We tuned the Vo content by annealing at 200-500 °C. XPS and EPR were taken to characterize the Vo content, ranging from 11% to 26%. Electrochemical results show that FO-300 with 17% Vo has the highest capacity of 301 mAh g^-1, and the capacity of the highest Vo content's (26% Vo) is only 107 mAh g^-1. The symmetric supercapacitor based on FO-300 shows a considerably high energy density of 58.5 Wh kg^-1 at a power density of 9.32 kW kg^-1 and remains 84.6% after 12,000 cycles.

Carbon intermediate boosted Fe-ZIF derived α-Fe2O3 as a high-performance negative electrode for supercapacitors

Earth-abundant Fe₂O₃ is a promising material for the negative electrode of supercapacitors by virtue of its wide potential windows. However, the unsatisfactory electrical conductivity and poor ionic diffusion rate within Fe₂O₃ results in degraded electrochemical performance. In this work, to address these issues, we demonstrate an easy method to synthesize Fe-based zeolitic imidazolate framework (Fe-ZIF) derived α-Fe₂O₃@C with remarkable supercapacitive properties. The as-obtained α-Fe₂O₃@C electrode, with the particular benefit of dispersed distribution of carbon, enabling fast electrochemical response, presents a prospective specific capacitance of 161 Fg^-1 at a current density of 1 Ag^-1. Furthermore, by using the α-Fe₂O₃@C architecture as the negative electrode, we fabricated a supercapacitor with Na_0.5MnO₂ as the positive electrode. Our supercapacitor shows a high energy density of 25 Whkg^-1, while the corresponding power density is 2400 Wkg^-1 at a current density of 2 Ag^-1.

All-Metal Phosphide Electrodes for High-Performance Quasi-Solid-State Fiber-Shaped Aqueous Rechargeable Ni-Fe Batteries

Aqueous secondary Ni-Fe batteries with superior energy density, cost-effectiveness, and outstanding safety contribute significantly toward the development of portable and wearable energy storage devices with high performance. However, the common electrode materials are nickel/iron or their oxides which have suffered from poor conductivity and cycle performance. As an ideal candidate to address these issues, metal phosphides may offer outstanding theoretical specific capacity, low conversion potential, and impressive redox. In this study, one novel type of high-performance flexible Ni-Fe battery with binder-free electrodes on conductive fiber substrates is successfully designed and fabricated. Carbon nanotube fibers with the direct grown hierarchical NiCoP nanosheet arrays and FeP nanowire arrays are fabricated first using hydrothermal synthesis and then the pursuant gas phosphating process. With the assistance of the PVA-KOH gel electrolyte, our fiber-shaped aqueous rechargeable battery (FARB) presents negligible capacity loss after bending 3000 times. Meanwhile, the assembled FARB has a significant capacity of 0.294 mA h/cm² under the current density of 2 mA/cm² and a high energy density of 235.6 μW h/cm².

Two-step synthesis of millimeter-scale flexible tubular supercapacitors

Flexible supercapacitors have been demonstrated to be ideal energy storage devices owing to their lightweight and flexible nature and their high power density. However, conventional film-shaped devices struggle to meet the requirements of application in complicated situations, including medical instruments and wearable electronics. Here we report a hollow-structured flexible tubular supercapacitor prepared from a scalable method with the same diameter as electric wires. This new supercapacitor design allows for a large specific capacitance of 102 F g^-1 at a current density of 1 A g^-1 with excellent air-working stability over 10,000 cycles. It also shows a high energy density of 14.2 Wh kg^-1 with good rate capability even at a current density of 10 A g^-1, which is superior to commercial devices (3-10 Wh kg^-1). Moreover, the device delivers a stable energy storage capacity when encountering different flexible conditions, such as elongated, tangled and bent states, showing wide potentials in flexible and even wearable applications. Especially, it retains stable specific capacitance even after 500 bending cycles with a bending angle of 180°. The two-step fabrication method of these flexible tubular supercapacitors may allow for possible mass production, as they could be easily integrated with other functional components, and used in realistic scenarios that conventional film devices struggle to realize.

Stitching of Zn3(OH)2V2O7·2H2O 2D Nanosheets by 1D Carbon Nanotubes Boosts Ultrahigh Rate for Wearable Quasi-Solid-State Zinc-Ion Batteries

Several layer-structured vanadates of two-dimensional (2D) nanosheet morphologies have been investigated recently for flexible quasi-solid-state aqueous zinc-ion batteries (ZIBs), where one of the challenging issues is the poor electronic conductivity and mechanical stability especially in the cross-2D nanosheet direction, leading to insufficient rate capability and mechanical stability and shortened cycle life. Herein, we have devised a strategy of using one-dimensional (1D) carbon nanotubes (CNTs) to stitch zinc pyrovanadate (Zn₃(OH)₂V₂O₇·2H₂O, CNT-stitched ZVO) 2D nanosheets that are directly grown on oxidized CNT fiber (CNT-stitched ZVO NSs@OCNT fiber). With the CNT-stitched 2D nanosheet structure, the open frameworks of ZVO provide required spacing for reversible Zn²⁺ (de)intercalation, and the stitching CNTs offer the desperately needed electronic conductivity and mechanical robustness across the ZVO 2D nanosheets. As a result, the fiber-shaped quasi-solid-state ZIB, assembled using the CNT-stitched ZVO NSs@OCNT as the cathode and Zn NSs@CNT fiber (electrodeposited zinc nanosheets on CNT fiber) as the anode, demonstrates an ultrahigh rate capability (69.7% retention after a 100-fold increase in current density), an impressively stack volumetric energy density of 71.6 mWh cm^-3, together with a long-term stability (88.6% retention after 2000 cycles). The present work proves the proof-of-concept of developing 2D nanosheets purposely stitched together by 1D conducting nanotubes/nanowires as a class of advanced cathodes for quasi-solid-state ZIBs in future portable electronics.

Cu-MOF derived Cu-C nanocomposites towards high performance electrochemical supercapacitors

For the development of asymmetric supercapacitors with higher energy density, the study of new electrode materials with high capacitance is a priority. Herein, the electrochemical behavior of nano copper in alkaline electrolyte is first discovered. It is found that there are two obvious reversible redox symmetric peaks in the range of -0.8-0.2 V in the alkaline electrolyte, corresponding to the conversion of copper into cuprous ions, and then converting cuprous ions into copper ions, indicating that the nanocomposite electrode has the characteristics of a pseudocapacitive reaction. It has a specific capacitance of up to 318 F g-1 at a current density of 1 A g-1, which remains at nearly 100% after 10 000 cycles at the same current density. When assembled with a Ni(OH)2-based electrode into an asymmetric supercapacitor, the device shows excellent capacitive behavior and good reaction reversibility. At 0.4 A g-1, the supercapacitor delivers a reversible capacity of 8.33 F g-1 with an energy density of 13.5 mW h g-1. This study first discovers the electrochemical behavior of nano copper, which can provide a new research idea for further expanding the negative electrodes of supercapacitors with higher energy density.

Rationally designed Ni2P/Ni/C as a positive electrode for high-performance hybrid supercapacitors

Ni₂P/Ni/C is fabricated a simple simultaneous carbonization and phosphidation process. It displays exceptional rate performance with excellent cycling ability, mainly resulting from accelerated charge transfer ability and stable porous structure.

Ni–Mo modified metal–organic frameworks for high-performance supercapacitance and enzymeless H2O2 detection

The growth process for A(B)-Ni_xMo_y-MOFs@AAC hybrids.

Conversion Synthesis of Self-Standing Potassium Zinc Hexacyanoferrate Arrays as Cathodes for High-Voltage Flexible Aqueous Rechargeable Sodium-Ion Batteries

Prussian blue analogs exhibit great promise for applications in aqueous rechargeable sodium-ion batteries (ARSIBs) due to their unique open framework and well-defined discharge voltage plateau. However, traditional coprecipitation methods cannot prepare self-standing electrodes to meet the needs of wearable energy storage devices. In this work, a water bath method is reported to grow microcube-like K₂ Zn₃ (Fe(CN)₆ )₂ ·9H₂ O on carbon cloth (CC) using Zn nanosheet arrays as the zinc source and reducing agent, directly serving as a self-standing cathode. Benefiting from fast ion diffusion and high conductivity, the cathode delivers a high areal capacity of 0.76 mAh cm^-2 at 0.5 mA cm^-2 and excellent capacity retention of 57.9% as the current density increases to 20 mA cm^-2 . By coupling with NaTi₂ (PO₄ )₃ grown on CC as an anode, a quasi-solid-state flexible ARSIB with a high output voltage plateau of 1.6 V is successfully assembled, exhibiting a superior areal capacity of 0.56 mAh cm^-2 and energy density of 0.92 mWh cm^-2 . In particular, the device shows admirable mechanical flexibility, maintaining 90.3% of initial capacity after 3000 bending cycles. This work is anticipated to open a new avenue for the rational design of self-standing electrodes used in high-voltage flexible ARSIBs.

Hierarchical Nanosheets/Walls Structured Carbon-Coated Porous Vanadium Nitride Anodes Enable Wide-Voltage-Window Aqueous Asymmetric Supercapacitors with High Energy Density

The energy density of aqueous asymmetric supercapacitors (ASCs) is usually limited by low potential windows and capacitances of both anode and cathode. Herein, a facile strategy to fabricate hierarchical carbon-coated porous vanadium nitride nanosheet arrays on vertically aligned carbon walls (CC/CW/p-VN@C) as anode for aqueous ASCs is reported. The potential window of CC/CW/p-VN@C electrode can be stably extended to -1.3 to 0 V (vs Ag/AgCl) with greatly improved specific capacitance (604.8 F g-1 at 1 A g-1), excellent rate capability (368 F g-1 at 60 A g-1), and remarkable electrochemical stability. To construct ASCs, a Birnessite Na0.5MnO2 nanosheet arrays (CC/CW/Na0.5MnO2) cathode is similarly built. Benefiting from the matchable potential windows and high specific capacitances of the rationally designed anode and cathode, aqueous CC/CW/p-VN@C||CC/CW/Na0.5MnO2 ASCs with a wide voltage window of 2.6 V are fabricated. Moreover, the ASCs showcase an ultrahigh energy density up to 96.7 W h kg-1 at a high power density of 1294 W kg-1, and excellent cycling stability (92.5% retention after 10 000 cycles), outperforming most of previously reported ASCs and even comparable to that of organic electrolyte supercapacitors (SCs). This efficient strategy for fabricating 2.6 V aqueous ASCs suggests a promising research system for high energy density SCs.

A Flexible Microsupercapacitor with Integral Photocatalytic Fuel Cell for Self-Charging

With the rapid advancement in different kinds of portable electronics, self-powered systems with small volume and high-performance characteristics have attracted great attention in recent years. It would be rather exciting if one integrated system can not only convert recyclable energy or waste to electricity but also store energy at the same time. Here, flexible all-in-one energy chips composed of urea-based photocatalytic fuel cells (PFCs) and asymmetric microsupercapacitors (AMSCs) are designed on the same plane for powering small portable electronics. The planar PFC consisting of TiO₂ photoanode and Ag counter electrode, utilizing urea as fuel, can produce a stable energy output (highest power density of 3.04 μW cm^-2 in 1 M urea solution under a UV intensity of 30 mW cm^-2) while purify this wasted water simultaneously. Besides, the AMSC comprised of NiCoP@NiOOH positive electrode and zeolite imidazolide framework derived carbon (ZIF-C) negative electrode achieves a high areal capacitance of 54.7 mF cm^-2 at 0.5 mA cm^-2 and an excellent energy density of 13.9 μWh cm^-2 at the power density of 270.5 μW cm^-2. Its stability can be confirmed by 86% capacitance retention after 8000 electrochemical cycles and almost no decay after 500 bending cycles. Four PFCs and two AMSCs can be easily constructed into an energy chip and power small electronics. This eco-friendly and self-sustainable system has great potential in future portable electronics.

Oxygen vacancy-engineered Fe2O3 nanoarrays as free-standing electrodes for flexible asymmetric supercapacitors

The charge storage performance of Fe₂O₃ nanoarrays (NAs) as negative electrodes are limited by their poor conductivity and rate capability. Herein, we have reported the delicate interfacial engineering on carbon cloth (CC) fibers and oxygen vacancy (V_O) generation on Fe₂O₃ nanorod arrays to boost the capacitive performance. Polydopamine-derived nitrogen-doped carbon layers were fabricated on CC fibers to govern the growth of FeOOH NAs. Rich V_Os were generated in Fe₂O₃ NAs to construct a unique heterostructure with a crystalline core and amorphous shell via successive N₂ thermal treatment and chemical reduction. Optimized by 2 h chemical reduction, the V_O-rich Fe₂O₃ NA electrode, featuring a charged voltage of -1.10 V, exhibited a high areal specific capacitance of 2.63 F cm^-2 at 0.5 mA cm^-2 and 0.12 F cm^-2 even at 60 mA cm^-2. Impressively, 86.7% specific capacitance was retained after 10 000 cycles at 10 mA cm^-2. The flexible asymmetric supercapacitor by assembling free-standing CN-Fe₂O₃-2 h (negative electrode) and MnO₂ (positive electrode) showed an energy density of 1.33 mW h cm^-3 at 15.4 mW cm^-3. To the best of our knowledge, these results are the record performance for Fe₂O₃-based electrodes. The two-step interfacial engineering reported in this study may open a new door in the design of high energy-density electrodes for advanced energy storage.

A dual metal organic framework based on copper-iron clusters integrated sulphur doped graphene as a porous material for supercapacitor with remarkable performance characteristics

Herein, a novel bimetallic metal organic framework (MOF) using copper and iron as the metal centers with 1,3,5-tricarboxylic acid as a ligand (CuFeBTC) and its composite with sulphur doped graphene (S-GNS) have been investigated for supercapacitive performance. The synthesis of materials has been carried out using a facile wet chemical route. The physicochemical characterization of the materials employing various structural and surface techniques has been performed which confirms the successful formation of nanocomposite. The capacitive behavior of CuFeBTC, S-GNS and CuFeBTC/S-GNS has been systematically examined using 1 M Na₂SO₄ as an electrolyte in a three and two electrode assembly. The electrochemical studies reveal that CuFeBTC/S-GNS electrode demonstrates highest specific capacitance of 1164.3 F g^-1 at 0.5 A g^-1 with suffice rate capability as compared to CuFeBTC and S-GNS electrodes. Moreover, a symmetric supercapacitor is configured using the CuFeBTC/S-GNS nanocomposite electrodes which deliver remarkable energy and power output of 96.57 Wh kg^-1 and 1595.12 W kg^-1 at an operating voltage of 1.8 V. The as-fabricated symmetric supercapacitor displays competent energy storage retention of 50.2 Wh kg^-1 even at current density of 20.0 A g^-1 with high power density 26973.13 W kg^-1. These deliverables epitomize the latest performance record of bimetallic MOFs based supercapacitors, suggesting that CuFeBTC/S-GNS is a promising active material for high performance electrochemical energy storage applications.

Flexible and High-Voltage Coaxial-Fiber Aqueous Rechargeable Zinc-Ion Battery

Extensive efforts have been devoted to construct a fiber-shaped energy-storage device to fulfill the increasing demand for power consumption of textile-based wearable electronics. Despite the myriad of available material selections and device architectures, it is still fundamentally challenging to develop eco-friendly fiber-shaped aqueous rechargeable batteries (FARBs) on a single-fiber architecture with high energy density and long-term stability. Here, we demonstrate flexible and high-voltage coaxial-fiber aqueous rechargeable zinc-ion batteries (CARZIBs). By utilizing a novel spherical zinc hexacyanoferrate with prominent electrochemical performance as cathode material, the assembled CARZIB offers a large capacity of 100.2 mAh cm^-3 and a high energy density of 195.39 mWh cm^-3, outperforming the state-of-the-art FARBs. Moreover, the resulting CARZIB delivers outstanding flexibility with the capacity retention of 93.2% after bending 3000 times. Last, high operating voltage and output current are achieved by the serial and parallel connection of CARZIBs woven into the flexible textile to power high-energy-consuming devices. Thus, this work provides proof-of-concept design for next-generation wearable energy-storage devices.

Scalable Wire-Type Asymmetric Pseudocapacitor Achieving High Volumetric Energy/Power Densities and Ultralong Cycling Stability of 100 000 Times

Wire-shaped asymmetric pseudocapacitors with both pseudocapacitive cathode and anode are promising in facilitating device assembly and provide highly efficient power sources for wearable electronics. However, it is a great challenge to simultaneously obtain high energy and power as well as ultralong cycling life for practical demands of such devices. Herein, a device design with new cathode/anode coupling is proposed to achieve excellent comprehensive performance in a wire-type quasi-solid-state asymmetric pseudocapacitor (WQAP). The hierarchical α-MnO2 nanorod@δ-MnO2 nanosheet array cathode and MoO2@C nanofilm anode are directly grown on flexible tiny Ti wires by well-established hydrothermal and electrodeposition techniques, which ensures rapid charge/mass transport kinetics and the sufficient utilization of pseudocapacitance. The nanoarray/film electrode also facilitates integration with gel electrolyte of polyvinyl alcohol-LiCl, guaranteeing the durability. The resulting WQAP with 2.0 V voltage delivers high volumetric energy and power densities (9.53 mWh cm-3 and 22720 mW cm-3, respectively) as well as outstanding cycling stability over 100 000 times, surpassing all the previously reported WQAPs. In addition, the device can be facilely connected in parallel or in series with minimal internal resistance, and be fabricated at the 1 m scale with excellent flexibility. This work opens the way to develop high-performance integrated wire supercapacitors.

Fish-Scale-Like Intercalated Metal Oxide-Based Micromotors as Efficient Water Remediation Agents

With compelling virtues of a large specific surface area, abundant active sites, and fast interfacial transport, nanomaterials have been demonstrated to be indispensable tools for water remediation applications. Accordingly, micro/nanomotors made by nanomaterials would also benefit from these properties. Though tuning the surface architecture on demand becomes a hot topic in the field of nanomaterials, there are still limited reports on the design of active surface architectures in chemically driven tubular micro/nanomachines. Here, a unique architecture composed of a fish-scale-like intercalated (FSI) surface structure and an active layer with 5 nm nanoparticles is constructed, which composes of Fe₂O₃ and ramsdellite MnO₂, Mn₂O₃, in the tubular micromotor using a versatile electrodeposition protocol. Tailoring the electrodeposition parameters enables us to modulate the active MnO₂ surface structure on demand, giving rise to a pronounced propulsion performance and catalytic activity. Upon exposure to the azo-dye waste solution, the degradation efficacy greatly raises by around 22.5% with FSI micromotor treatment when compared to the normal compact motors, owing to the synergistic effect between the Fe-related Fenton reaction and a large catalytic area offered by the hierarchically rough inner surface. Such unique micromachines with a large active surface area have great potential for environmental and biomedical applications.

Dual-Purpose 3D Pillared Metal-Organic Framework with Excellent Properties for Catalysis of Oxidative Desulfurization and Energy Storage in Asymmetric Supercapacitor

This study proposes an approach for improving catalysis of oxidative desulfurization (ODS) of diesel fuel under mild reaction conditions and enhancing supercapacitor (SC) properties for storage of a high amount of charge. Our approach takes advantage of a novel dual-purpose cobalt(II)-based metal-organic framework (MOF), [Co(2-ATA)₂(4-bpdb)₄] _n (2-ATA: 2-aminoterephthalic acid and 4-bpdb: N, N-bis-pyridin-4-ylmethylene-hydrazine as the pillar spacer), which is called NH₂-TMU-53. Due to the stability of the used compound, we decided to evaluate the capability of this compound as a novel electrode material for storing energy in supercapacitors, and also to investigate its catalytic capabilities. It is demonstrated that the addition of H₂O₂ as an oxidant enhances the efficiency of sulfur removal, which indicates that NH₂-TMU-53 can efficiently catalyze the ODS reaction. According to the kinetics results, the catalyzed process follows pseudo-first-order kinetics and exhibits 15.57 kJ mol^-1 activation energy. Moreover, with respect to the radical scavenging evaluations, the process is governed by direct catalytic oxidation rather than indirect oxidative attack of radicals. Furthermore, NH₂-TMU-53 was applied as an electrode material for energy storage in SCs. This material is used in the three-electrode system and shows a specific capacitance of 325 F g^-1 at 5 A g^-1 current density. The asymmetric supercapacitor of NH₂-TMU-53//activated carbon evaluates the further electrochemical activity in real applications, delivers the high power density (2.31 kW kg^-1), high energy density (50.30 Wh kg^-1), and long cycle life after 6000 cycles (90.7%). Also, the asymmetric supercapacitor practical application was demonstrated by a glowing red light-emitting diode and driving a mini-rotating motor. These results demonstrate that the fabricated device presents a good capacity for energy storage without pyrolyzing the MOF structures. These findings can guide the development of high-performance SCs toward a new direction to improve their practical applications and motivate application of MOFs without pyrolysis or calcination.