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

Novel Prussian White@MnO2-Based Inorganic Electrochromic Energy Storage Devices with Integrated Flexibility, Multicolor, and Long Life

Flexible electrochromic devices have attracted considerable attention in recent years due to their great potential in smart multifunction electrochromic energy storage devices and wearable intelligent electronics. Herein, we present an inorganic flexible Li-based electrochromic energy storage device (EESD) by combining a Prussian white@MnO₂-composited electrode (PWM) and sputtering-made WO₃ electrode. The synergistic effect of Prussian white and MnO₂ plays a positive role both in energy storage and electrochromic property of the EESD. Its energy level can be quantified by the transmittance spectrum and chrominance difference, and its charging-discharging process can be monitored in real time by optical modulation at special wavelength. Specifically, the EESD can endure a 10,000 times cyclic voltammetry cycle without obvious degradation at wide voltage windows (-2 to 2.5 V) and realize a high coloration efficiency (77.6 cm²/C) with 35% optical modulation at 510 nm. In terms of energy storage performance, the EESD demonstrates excellent volumetric energy/power density (1.25 W cm^-3/13.2 mWh cm^-3) and remarkable stability with close to 98.3% capacitance retention and 99.4% coulombic efficiency after more than 4000 cycles. Its charging and discharging degree can be visualized in different spectral regions. There are 40% transmittance change for charging in the blue light region (450-480 nm) and 45% transmittance change for discharging in the red light region (620-750 nm). Based on its multicolor property, a quantitative indicator of charge state is achieved by the linear dependence of real-time chrominance change as stored or released charge. The ∼11 mC/cm² stored charge capacity can cause an ∼11 increase in chrominance difference ΔE value, while ∼7 mC/cm² discharge capacity can cause a ΔE value increase of ∼4. This work provides an efficient strategy to develop portable multicolor-integrated EESDs toward high performance and long stability.

Cobalt vanadium chalcogenide microspheres decorated with dendrite-like fiber nanostructures for flexible wire-typed energy conversion and storage microdevices

The increasing energy demand for next-generation portable and miniaturized electronics has drawn tremendous attention to develop microscale energy storage and conversion devices with light weight and flexible characteristics. Herein, we report the preparation of flower-like cobalt vanadium selenide/nickel copper selenide (CoVSe/NiCuSe) microspheres with three-dimensional hierarchical structure of micropore growth on copper wire for a flexible fiber microsupercapacitor (microSC) and overall water splitting. The CoV-LDH microspheres are anchored on the dendrite-like NiCu nanostructured Cu wire using a hydrothermal method (CoV-LDH/NiCu@CW). The sulfidation and selenization of CoV-LDH/NiCu was carried out through the ion-exchange reaction of OH^- with sulfide and selenide ions to obtain CoVS/NiCuS@CW and CoVSe/NiCuSe@CW electrodes, respectively. Benefitting from the unique structure, the flower-like CoVSe/NiCuSe@CW microspheres exhibit better electrochemical performance compared with other as-prepared fiber-shaped electrodes. As an electrode active material for microSC, CoVSe/NiCuSe microspheres exhibit a specific capacitance of 35.40 F cm^-3 at 4 mA cm^-2, and maintain 281.25 F cm^-3 even at a high current density of 83 mA cm^-2, indicating outstanding charge storage capacitance and excellent rate capability. Moreover, the assembled flexible solid-state asymmetric microSCs based on flower-like CoVSe/NiCuSe microspheres-coated Cu wire as the positive electrode and polypyrrole/reduced graphene oxide-coated carbon fiber as the negative electrode manifests a maximum energy density of 20.17 mW h cm^-3 at a power density of 624.32 mW cm^-3 and remarkable cycling stability (96.7% after 5000 cycles) with good mechanical stability. As an electrocatalyst for oxygen and hydrogen evolution reactions in alkaline medium, the CoVSe/NiCuSe electrode delivers an overpotential of 297 mV and 165 mV at 100 mA cm^-2. Furthermore, the CoVSe/NiCuSe-based electrolysis cell for overall water splitting presents a low cell voltage (1.7 V at 50 mA cm^-2) as well as high durability.

Co-MnO2 Nanorods for High-Performance Sodium/Potassium-Ion Batteries and Highly Conductive Gel-Type Supercapacitors

Manganese dioxide (MnO2 ) is considered as a strong candidate in the field of new-generation electronic equipment. Herein, Co-MnO2 has excellent electrochemical properties in tests as the cathode electrode of sodium-ion batteries and potassium-ion batteries. The rate performance remains at 50.2 mAh g-1 at 200 mA g-1 for sodium-ion batteries. X-ray diffraction (XRD) is utilized to evaluate the crystal structure transition from Co0.2 -MnO2 to NaMnO2 with discharge to 1 V, proving that Co-doping does indeed facilitate the acceleration of ion transport and support layer spacing to stabilize the structure of MnO2 . Subsequently, highly conductive (0.0848 S cm-1 ) gel-type supercapacitors are prepared by combining Co0.2 -MnO2 , potassium hydroxide (KOH), and poly(vinyl alcohol) (PVA) together. Co0.2 -MnO2 provides capacitive behavior and strengthens the hydrogen bonds between molecules. KOH acts as an ion crosslinker to enhance hydrogen bond and as electrolyte to transport ions. 5 wt% Co0.2 -MnO2 @KOH/PVA has superb mechanical endurance, appreciable electrical conductivity, and ideal capacitive behavior. The quasi-solid-state supercapacitor demonstrates stabilized longevity (86.5% at 0.2 mA cm-3 after 500 cycles), which can greatly promote the integration of flexible energy storage fabric devices.

Metal–organic framework induced hybrid NiCo2S4/PPy structures with unique interface features for high performance flexible energy storage devices

In this work, we report hybrid-structure NiCo2S4/PPy nanosheets through hydrothermal synthesis and electrodeposition methods. Meanwhile, the NiCo2S4/PPy-110 samples exhibit a high specific capacitance of 983 C g−1 at 1 A g−1 and excellent stability.

Advanced aqueous sodium-ion capacitors based on Ni0.25Mn0.75O nanoparticles encapsulated in electrospinning carbon nanofibers

Manganese oxides are promising cathode material candidates with appropriate positive potential windows for low-cost and safe aqueous sodium-ion capacitors (ASICs).

Realizing high performance flexible supercapacitors by electrode modification

It is well known that the structure of an electrode material seriously affects its electrochemical performance. In this study, we prepare hybrid structured NiCo₂S₄@PPy nanoarchitectures by a hydrothermal method and subsequent electrodeposition process. The specific capacitance of the obtained sample is 1733.23 C g⁻¹ at 1 A g⁻¹. The assembled asymmetric device presents an energy density of 59.59 W h kg⁻¹ at 1404.04 W kg⁻¹. The excellent electrochemical performance can be attributed to the synergistic effect between the high theoretical specific capacitance of the NiCo₂S₄ sheets and the superior cycling stability of the PPy film. The device also shows an outstanding mechanical flexibility at different bending angles.

We prepare hybrid structured NiCo₂S₄@PPy nanoarchitectures by a hydrothermal method and subsequent electrodeposition process. The assembled asymmetric device presents an energy density of 59.59 W h kg⁻¹ at 1404.04 W kg⁻¹.

Interfacial Control of NiCoP@NiCoP Core-Shell Nanoflake Arrays as Advanced Cathodes for Ultrahigh-Energy-Density Fiber-Shaped Asymmetric Supercapacitors

Efficient improvement of the energy density and overall electrochemical performance of fiber-shaped asymmetric supercapacitors (FASCs) for practical applications in portable and wearable electronics requires highly electrochemically active materials and a rational design. Herein, two-step phosphorization (TSP) processes are performed to directly grow 3D well-aligned NiCoP@NiCoP (NCP@NCP TSP) nanoflake arrays (NFAs) on carbon nanotube fibers (CNTFs). Profiting from the metallic characteristics and excellent electrochemical performance of NiCoP and the hierarchical design of the core-shell heterostructure, the NCP@NCP TSP NFAs/CNTF hybrid electrode exhibits significantly improved electrochemical performance. The as-fabricated NCP@NCP TSP NFAs/CNTF electrode possesses an ultrahigh areal capacitance of 10 035 mF cm^-2 at a current density of 1 mA cm^-2 , with excellent rate capability and cycling stability. Furthermore, an FASC device with a maximum operating voltage of 1.6 V is assembled by adopting NCP@NCP TSP NFAs/CNTF as a positive electrode, hierarchical TiN@VN core-shell heterostructure nanowire arrays (NWAs)/CNTF as negative electrode, and KOH-PVA as a gel electrolyte. The FASC device exhibits a high areal capacitance of 430.4 mF cm^-2 and an ultrahigh energy density of 51.02 mWh cm^-3 . Thus, the rationally designed NiCoP@NiCoP electrode is a promising candidate for incorporation into next-generation wearable and portable energy-storage devices.

Surface and Interface Engineering of Nanoarrays toward Advanced Electrodes and Electrochemical Energy Storage Devices

The overall performance of electrochemical energy storage devices (EESDs) is intrinsically correlated with surfaces and interfaces. As a promising electrode architecture, 3D nanoarrays (3D-NAs) possess relatively ordered, continuous, and fully exposed active surfaces of individual nanostructures, facilitating mass and electron transport within the electrode and charge transfer across interfaces and providing an ideal platform for engineering. Herein, a critical overview of the surface and interface engineering of 3D-NAs, from electrode and interface designs to device integration, is presented. The general merits of 3D-NAs and surface/interface engineering principles of 3D-NA hybrid electrodes are highlighted. The focus is on the use of 3D-NAs as a superior platform to regulate the interface nature and unveiling new mechanism/materials without the interference of binders. The engineering and utilization of the surface of 3D-NAs to develop flexible/solid-state EESDs with 3D integrated electrode/electrolyte interfaces, or 3D triphase interfaces involving other active species, which are characteristic of (quasi-)solid-state electrolyte infiltration into the entire device, are also considered. Finally, the challenges and future directions of surface/interface engineering of 3D-NAs are outlined. In particular, potential strategies to obtain electrode charge balance, optimize the multiphase solid-state interface, and attain 3D solid electrolyte infiltration are proposed.

High areal capacitance of manganese oxide electrodes with cerium as rare earth modification

Manganese oxides have attracted wide attention as promising electrode materials for high-energy density supercapacitors. However, the electrochemical performance of the manganese oxide materials deteriorates considerably with the increase in mass loading due to their moderate electronic and ionic conductivities. This phenomenon leads to low areal capacitance, which limits the practical application of these materials. Herein, we perform a potentiostatic electrodeposition of manganese oxides with Ce as rare earth (RE) modification on a nickel (Ni) foam substrate to achieve high areal capacitance. Under optimum conditions, manganese oxide nanosheets are axially grown on Ni foam to form a hierarchically porous network nanostructure, which ensures facile ionic and electric transport. The Ce-modified manganese oxide with the Mn:Ce molar ratio of 1:0.1 yields an outstanding areal capacitance of 3.67 F cm^-2 at 2 mA cm^-2 and a good rate capability compared with the capacitance of 2.59 F cm^-2 at 2 mA cm^-2 of pure manganese oxide without the addition of Ce. This result verifies the importance of Ce modification to manganese oxides. Our results suggest the important role played by the RE element Ce in enhancing the electrochemical performance of high areal capacitance manganese oxide electrodes, which is essential to bringing them one step toward further practical applications.

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.

Transition metal sulfide-laminated copper wire for flexible hybrid supercapacitor

The CV profiles of transition metal sulphide electrodes on a Cu-wire (a), and (b) capacitive (light to dark: blue and red) and diffusion-controlled (dark: blue and red) contributions of the hybrid Cu@CoS/NiCo₂S₄ and Cu@ZnCo₂S₄ electrodes.

Microfluidic-Architected Nanoarrays/Porous Core-Shell Fibers toward Robust Micro-Energy-Storage

Methods enabling the controllable fabrication of orderly structural and active nanomaterials, along with high-speed ionic pathways for charge migration and storage are highly fundamental in fiber-shaped micro-supercapacitors (MSCs). However, due to fiber-electrodes with compact internal microstructure and less porosity, MSCs usually display a low energy density. Here, an innovative microfluidic strategy is proposed to design ordered porous and anisotropic core-shell fibers based on nickel oxide arrays/graphene nanomaterials. Owing to the homogeneous microchannels reaction, the graphene core maintains a uniformly anisotropic porous structure, and the nickel oxide shell keeps steadily vertically aligned nanosheets. The MSC presents an ultrahigh energy density (120.3 µWh cm-2) and large specific capacitance (605.9 mF cm-2). This higher performance originates from the microfluidic-architected core-shell fiber with abundant ionic channels (plentiful micro-/mesopores), large specific-surface-area (425.6 m2 g-1), higher electrical conductivity (176.6 S cm-1), and sufficient redox activity, facilitating ions with quicker diffusion and greater accumulation. Considering those outstanding properties, a wearable self-powered system, converting and storing solar energy into electric energy, is designed to light up displays. This microfluidic strategy offers an effective way to design new structural materials, which will advance the development of next-generation wearable/smart industries.