Hierarchical 3D All‐Carbon Composite Structure Modified with N‐Doped Graphene Quantum Dots for High‐Performance Flexible Supercapacitors

Flexible supercapacitors have shown enormous potential for portable electronic devices. Herein, hierarchical 3D all‐carbon electrode materials are prepared by assembling N‐doped graphene quantum dots (N‐GQDs) on carbonized MOF materials (cZIF‐8) interweaved with carbon nanotubes (CNTs) for flexible all‐solid‐state supercapacitors. In this ternary electrode, cZIF‐8 provides a large accessible surface area, CNTs act as the electrical conductive network, and N‐GQDs serve as highly pseudocapactive materials. Due to the synergistic effect and hierarchical assembly of these components, N‐GQD@cZIF‐8/CNT electrodes exhibit a high specific capacitance of 540 F g^?1 at 0.5 A g^?1 in a 1 m H₂SO₄ electrolyte and excellent cycle stability with 90.9% capacity retention over 8000 cycles. The assembled supercapacitor possesses an energy density of 18.75 Wh kg^?1 with a power density of 108.7 W kg^?1. Meanwhile, three supercapacitors connected in series can power light‐emitting diodes for 20 min. All‐solid‐state N‐GQD@cZIF‐8/CNT flexible supercapacitor exhibits an energy density of 14 Wh kg^?1 with a power density of 89.3 W kg^?1, while the capacitance retention after 5000 cycles reaches 82%. This work provides an effective way to construct novel electrode materials with high energy storage density as well as good cycling performance and power density for high‐performance energy storage devices via the rational design.     

“HOT” Alkaline Hydrolysis of Amorphous MOF Microspheres to Produce Ultrastable Bimetal Hydroxide Electrode with Boosted Cycling Stability

Nickel/cobalt hydroxide is a promising battery‐type electrode material for supercapacitors. However, its low cycle stability hinders further applications. Herein, Ni_0.7Co_0.3(OH)₂ core–shell microspheres exhibiting extreme‐prolonged cycling life are successfully synthesized, employing Ni‐Co‐metal–organic framework (MOF) as the precursor/template and a specific hydrolysis strategy. The Ni‐Co‐MOF and KOH aqueous solution are separated and heated to 120 °C before mixing, rather than mixing before heating. Through this hydrolysis strategy, no MOF residual exists in the product, contributing to close stacking of the hydroxide nanoflakes to generate Ni_0.7Co_0.3(OH)₂ microspheres with a robust core–shell structure. The electrode material exhibits high specific capacity (945 C g^?1 at 0.5 A g^?1) and unprecedented cycling performance (100% after 10 000 cycles). The fabricated asymmetric supercapacitor delivers an energy density of 40.14 Wh kg^?1 at a power density of 400.56 W kg^?1 and excellent cycling stability (100% after 20 000 cycles). As far as is known, it is the best cycling performance for pure Ni/Co(OH)₂.     

Vertically Aligned Graphene–Carbon Fiber Hybrid Electrodes with Superlong Cycling Stability for Flexible Supercapacitors

Graphene electrode–based supercapacitors are in high demand due to their superior electrochemical characteristics. A major bottleneck of using the supercapacitors for commercial applications lies in their inferior electrode cycle life. Herein, a simple and facile method to fabricate highly efficient supercapacitor electrodes using pristine graphene sheets vertically stacked and electrically connected to the carbon fibers which can result in vertically aligned graphene–carbon fiber nanostructure is developed. The vertically aligned graphene–carbon fiber electrode prepared by electrophoretic deposition possesses a mesoporous 3D architecture which enabled faster and efficient electrolyte‐ion diffusion with a gravimetric capacitance of 333.3 F g^?1 and an areal capacitance of 166 mF cm^?2. The electrodes displayed superlong electrochemical cycling stability of more than 100 000 cycles with 100% capacitance retention hence promising for long‐lasting supercapacitors. Apart from the electrochemical double layer charge storage, the oxygen‐containing surface moieties and α‐Ni(OH)₂ present on the graphene sheets enhance the charge storage by faradaic reactions. This enables the assembled device to provide an excellent gravimetric energy density of 76 W h kg^?1 with a 100% capacitance retention even after 1000 bending cycles. This study opens the door for developing high‐performing flexible graphene electrodes for wearable energy storage applications.     

Biomass‐Derived Nitrogen‐Doped Carbon Nanofiber Network: A Facile Template for Decoration of Ultrathin Nickel‐Cobalt Layered Double Hydroxide Nanosheets as High‐Performance Asymmetric Supercapacitor Electrode

The development of biomass‐based energy storage devices is an emerging trend to reduce the ever‐increasing consumption of non‐renewable resources. Here, nitrogen‐doped carbonized bacterial cellulose (CBC‐N) nanofibers are obtained by one‐step carbonization of polyaniline coated bacterial cellulose (BC) nanofibers, which not only display excellent capacitive performance as the supercapacitor electrode, but also act as 3D bio‐template for further deposition of ultrathin nickel‐cobalt layered double hydroxide (Ni‐Co LDH) nanosheets. The as‐obtained CBC‐N@LDH composite electrodes exhibit significantly enhanced specific capacitance (1949.5 F g^?1 at a discharge current density of 1 A g^?1, based on active materials), high capacitance retention of 54.7% even at a high discharge current density of 10 A g^?1 and excellent cycling stability of 74.4% retention after 5000 cycles. Furthermore, asymmetric supercapacitors (ASCs) are constructed using CBC‐N@LDH composites as positive electrode materials and CBC‐N nanofibers as negative electrode materials. By virtue of the intrinsic pseudocapacitive characteristics of CBC‐N@LDH composites and 3D nitrogen‐doped carbon nanofiber networks, the developed ASC exhibits high energy density of 36.3 Wh kg^?1 at the power density of 800.2 W kg^?1. Therefore, this work presents a novel protocol for the large‐scale production of biomass‐derived high‐performance electrode materials in practical supercapacitor applications.     

Ionic Liquid‐Controlled Growth of NiCo2S4 3D Hierarchical Hollow Nanoarrow Arrays on Ni Foam for Superior Performance Binder Free Hybrid Supercapacitors

A significant development in the design of a NiCo₂S₄ 3D hierarchical hollow nanoarrow arrays (HNA)‐based supercapacitor binder free electrode assembled by 1D hollow nanoneedles and 2D nanosheets on a Ni foam collector through controlling ionic liquid 1‐octyl‐3‐methylimidazolium chloride ([OMIm]Cl) concentration is reported. The unique NiCo₂S₄‐HNA electrode acquires high specific capacity (1297 C g^?1 at 1 A g^?1, 2.59 C cm^?2 at 2 mA cm^?2), excellent rate capability (maintaining 73.0% at 20 A g^?1), and long operational life (maintaining 92.4% after 10 000 cycles at 5 A g^?1), which are superior to those for 1D hollow nanoneedle arrays (HNN) and 2D porous nanoflake arrays (PNF). The outstanding electrochemical performance is attributed to the novel 3D structure with large specific surface, hollow cores, high porosity as well as stable architecture. In addition, a hybrid supercapacitor applying 3D NiCo₂S₄‐HNA as the positive electrode and active carbon as the negative electrode exhibits a high energy density of 42.5 Wh kg^?1 at a power density of 2684.2 W kg^?1 in an operating voltage of 1.6 V. Robust cycling stability is also expressed with 84.9% retention after repeating 10 000 cycles at 5 A g^?1, implying their great potential in superior‐performance supercapacitors.     

MoS2/NiS Yolk–Shell Microsphere‐Based Electrodes for Overall Water Splitting and Asymmetric Supercapacitor

Rational designing of the composition and structure of electrode material is of great significance for achieving highly efficient energy storage and conversion in electrochemical energy devices. Herein, MoS₂/NiS yolk–shell microspheres are successfully synthesized via a facile ionic liquid‐assisted one‐step hydrothermal method. With the favorable interface effect and hollow structure, the electrodes assembled with MoS₂/NiS hybrid microspheres present remarkably enhanced electrochemical performance for both overall water splitting and asymmetric supercapacitors. In particular, to deliver a current density of 10 mA cm^?2, the MoS₂/NiS‐based electrolysis cell for overall water splitting only needs an output voltage of 1.64 V in the alkaline medium, lower than that of Pt/C–IrO₂‐based electrolysis cells (1.70 V). As an electrode for supercapacitors, the MoS₂/NiS hybrid microspheres exhibit a specific capacitance of 1493 F g^?1 at current density of 0.2 A g^?1, and remain 1165 F g^?1 even at a large current density of 2 A g^?1, implying outstanding charge storage capacity and excellent rate performance. The MoS₂/NiS‐ and active carbon‐based asymmetric supercapacitor manifests a maximum energy density of 31 Wh kg^?1 at a power density of 155.7 W kg^?1, and remarkable cycling stability with a capacitance retention of approximately 100% after 10 000 cycles.     

Redox Tuning in Crystalline and Electronic Structure of Bimetal–Organic Frameworks Derived Cobalt/Nickel Boride/Sulfide for Boosted Faradaic Capacitance

The development of efficient electrode materials is a cutting‐edge approach for high‐performance energy storage devices. Herein, an effective chemical redox approach is reported for tuning the crystalline and electronic structures of bimetallic cobalt/nickel–organic frameworks (Co‐Ni MOFs) to boost faradaic redox reaction for high energy density. The as‐obtained cobalt/nickel boride/sulfide exhibits a high specific capacitance (1281 F g^?1 at 1 A g^?1), remarkable rate performance (802.9 F g^?1 at 20 A g^?1), and outstanding cycling stability (92.1% retention after 10 000 cycles). An energy storage device fabricated with a cobalt/nickel boride/sulfide electrode exhibits a high energy density of 50.0 Wh kg^?1 at a power density of 857.7 W kg^?1, and capacity retention of 87.7% (up to 5000 cycles at 12 A g^?1). Such an effective redox approach realizes the systematic electronic tuning that activates the fast faradaic reactions of the metal species in cobalt/nickel boride/sulfide which may shed substantial light on inspiring MOFs and their derivatives for energy storage devices.     

Fabrication of a 3D Hierarchical Sandwich Co9S8/α‐MnS@N–C@MoS2 Nanowire Architectures as Advanced Electrode Material for High Performance Hybrid Supercapacitors

Supercapacitors suffer from lack of energy density and impulse the energy density limit, so a new class of hybrid electrode materials with promising architectures is strongly desirable. Here, the rational design of a 3D hierarchical sandwich Co₉S₈/α‐MnS@N–C@MoS₂ nanowire architecture is achieved during the hydrothermal sulphurization reaction by the conversion of binary mesoporous metal oxide core to corresponding individual metal sulphides core along with the formation of outer metal sulphide shell at the same time. Benefiting from the 3D hierarchical sandwich architecture, Co₉S₈/α‐MnS@N–C@MoS₂ electrode exhibits enhanced electrochemical performance with high specific capacity/capacitance of 306 mA h g^?1/1938 F g^?1 at 1 A g^?1, and excellent cycling stability with a specific capacity retention of 86.9% after 10 000 cycles at 10 A g^?1. Moreover, the fabricated asymmetric supercapacitor device using Co₉S₈/α‐MnS@N–C@MoS₂ as the positive electrode and nitrogen doped graphene as the negative electrode demonstrates high energy density of 64.2 Wh kg^?1 at 729.2 W kg^?1, and a promising energy density of 23.5 Wh kg^?1 is still attained at a high power density of 11 300 W kg^?1. The hybrid electrode with 3D hierarchical sandwich architecture promotes enhanced energy density with excellent cyclic stability for energy storage.     

Self‐Assembled Nickel Pyrophosphate‐Decorated Amorphous Bimetal Hydroxides 2D‐on‐2D Nanostructure for High‐Energy Solid‐State Asymmetric Supercapacitor

To obtain a supercapacitor with a remarkable specific capacitance and rate performance, a cogent design and synthesis of the electrode material containing abundant active sites is necessary. In present work, a scalable strategy is developed for preparing 2D‐on‐2D nanostructures for high‐energy solid‐state asymmetric supercapacitors (ASCs). The self‐assembled vertically aligned microsheet‐structured 2D nickel pyrophosphate (Ni₂P₂O₇) is decorated with amorphous bimetallic nickel cobalt hydroxide (NiCo‐OH) to form a 2D‐on‐2D nanostructure arrays electrode. The resulting Ni₂P₂O₇/NiCo‐OH 2D‐on‐2D array electrode exhibits peak specific capacity of 281 mA hg^?1 (4.3 F cm^?2), excellent rate capacity, and cycling stability over 10 000 charge–discharge cycles in the positive potential range. The excellent electrochemical features can be attributed to the high electrical conductivity and 2D layered structure of Ni₂P₂O₇ along with the Faradic capacitance of the amorphous NiCo‐OH nanosheets. The constructed Ni₂P₂O₇/NiCo‐OH//activated carbon based solid‐state ASC cell operates in a high voltage window of 1.8 V with an energy density of 78 Wh kg^?1 (1.065 mWh cm^?3) and extraordinary cyclic stability over 10 000 charge–discharge cycles with excellent energy efficiency (75%–80%) over all current densities. The excellent electrochemical performance of the prepared electrode and solid‐state ASC device offers a favorable and scalable pathway for developing advanced electrodes.     

Efficient Supercapacitor Energy Storage Using Conjugated Microporous Polymer Networks Synthesized from Buchwald–Hartwig Coupling

Supercapacitors have received increasing interest as energy storage devices due to their rapid charge–discharge rates, high power densities, and high durability. In this work, novel conjugated microporous polymer (CMP) networks are presented for supercapacitor energy storage, namely 3D polyaminoanthraquinone (PAQ) networks synthesized via Buchwald–Hartwig coupling between 2,6‐diaminoanthraquinone and aryl bromides. PAQs exhibit surface areas up to 600 m² g^?1, good dispersibility in polar solvents, and can be processed to flexible electrodes. The PAQs exhibit a three‐electrode specific capacitance of 576 F g^?1 in 0.5 m H₂SO₄ at a current of 1 A g^?1 retaining 80–85% capacitances and nearly 100% Coulombic efficiencies (95–98%) upon 6000 cycles at a current density of 2 A g^?1. Asymmetric two‐electrode supercapacitors assembled by PAQs show a capacitance of 168 F g^?1 of total electrode materials, an energy density of 60 Wh kg^?1 at a power density of 1300 W kg^?1, and a wide working potential window (0–1.6 V). The asymmetric supercapacitors show Coulombic efficiencies up to 97% and can retain 95.5% of initial capacitance undergo 2000 cycles. This work thus presents novel promising CMP networks for charge energy storage.     

Rational Construction of Hollow Core‐Branch CoSe2 Nanoarrays for High‐Performance Asymmetric Supercapacitor and Efficient Oxygen Evolution

Metal selenides have great potential for electrochemical energy storage, but are relatively scarce investigated. Herein, a novel hollow core‐branch CoSe₂ nanoarray on carbon cloth is designed by a facile selenization reaction of predesigned CoO nanocones. And the electrochemical reaction mechanism of CoSe₂ in supercapacitor is studied in detail for the first time. Compared with CoO, the hollow core‐branch CoSe₂ has both larger specific surface area and higher electrical conductivity. When tested as a supercapacitor positive electrode, the CoSe₂ delivers a high specific capacitance of 759.5 F g^?1 at 1 mA cm^?2, which is much larger than that of CoO nanocones (319.5 F g^?1). In addition, the CoSe₂ electrode exhibits excellent cycling stability in that a capacitance retention of 94.5% can be maintained after 5000 charge–discharge cycles at 5 mA cm^?2. An asymmetric supercapacitor using the CoSe₂ as cathode and an N‐doped carbon nanowall as anode is further assembled, which show a high energy density of 32.2 Wh kg^?1 at a power density of 1914.7 W kg^?1, and maintains 24.9 Wh kg^?1 when power density increased to 7354.8 W kg^?1. Moreover, the CoSe₂ electrode also exhibits better oxygen evolution reaction activity than that of CoO.     

Phosphorus‐Mediated MoS2 Nanowires as a High‐Performance Electrode Material for Quasi‐Solid‐State Sodium‐Ion Intercalation Supercapacitors

Molybdenum disulfide (MoS₂) is a promising electrode material for electrochemical energy storage owing to its high theoretical specific capacity and fascinating 2D layered structure. However, its sluggish kinetics for ionic diffusion and charge transfer limits its practical applications. Here, a promising strategy is reported for enhancing the Na⁺‐ion charge storage kinetics of MoS₂ for supercapacitors. In this strategy, electrical conductivity is enhanced and the diffusion barrier of Na⁺ ion is lowered by a facile phosphorus‐doping treatment. Density functional theory results reveal that the lowest energy barrier of dilute Na‐vacancy diffusion on P‐doped MoS₂ (0.11 eV) is considerably lower than that on pure MoS₂ (0.19 eV), thereby signifying a prominent rate performance at high Na intercalation stages upon P‐doping. Moreover, the Na‐vacancy diffusion coefficient of the P‐doped MoS₂ at room temperatures can be enhanced substantially by approximately two orders of magnitude (10^?6–10^?4 cm² s^?1) compared with pure MoS₂. Finally, the quasi‐solid‐state asymmetrical supercapacitor assembled with P‐doped MoS₂ and MnO₂, as the positive and negative electrode materials, respectively, exhibits an ultrahigh energy density of 67.4 W h kg^?1 at 850 W kg^?1 and excellent cycling stability with 93.4% capacitance retention after 5000 cycles at 8 A g^?1.     

Interface‐Engineered Nickel Cobaltite Nanowires through NiO Atomic Layer Deposition and Nitrogen Plasma for High‐Energy,Long‐Cycle‐Life Foldable All‐Solid‐State Supercapacitors

The large‐scale application of supercapacitors (SCs) for portable electronics is restricted by low energy density and cycling stability. To alleviate the limitations, a unique interface engineering strategy is suggested through atomic layer deposition (ALD) and nitrogen plasma. First, commercial carbon cloth (CC) is treated with nitrogen plasma and later inorganic NiCo₂O₄ (NCO)/NiO core–shell nanowire arrays are deposited on nitrogen plasma–treated CC (NCC) to fabricate the ultrahigh stable SC. An ultrathin layer of NiO deposited on the NCO nanowire arrays via conformal ALD plays a vital role in stabilizing the NCO nanowires for thousands of electrochemical cycles. The optimized NCC/NCO/NiO core–shell electrode exhibits a high specific capacitance of 2439 F g^?1 with a remarkable cycling stability (94.2% over 20 000 cycles). Benefiting from these integrated merits, the foldable solid‐state SCs are fabricated with excellent NCC/NCO/NiO core–shell nanowire array electrodes. The fabricated SC device delivers a high energy density of 72.32 Wh kg^?1 at a specific capacitance of 578 F g^?1, with ultrasmall capacitance decline rate of 0.0003% per cycle over 10 000 charge–discharge cycles. Overall, this strategy offers a new avenue for developing a new‐generation high‐energy, ultrahigh stable supercapacitor for real‐life applications.     

TiO<Subscript>2</Subscript> crystalline structure and electrochemical performance in two-ply yarn CNT/TiO<Subscript>2</Subscript> asymmetric supercapacitors

Solid-state flexible energy storage devices play a crucial role in the development of wearable electronic textiles. In this study, we fabricated flexible asymmetric two-ply yarn supercapacitors from carbon nanotube yarns and surface-oxidized titanium filament. The crystalline structure of the TiO₂ surface layer can be adjusted to amorphous, anatase and rutile states by altering the annealing temperature. The titanium filament with a rutile TiO₂ surface layer produced at high annealing temperature showed far superior electrochemical performance over the filaments with amorphous and anatase TiO₂ surface layers. The as-prepared asymmetric two-ply yarn supercapacitors in aqueous gel electrolyte can achieve a durable operating voltage up to 1.4 V, with a maximum energy density of 11.7 Wh kg^?1 and a maximum power density of 2060 W kg^?1. The asymmetric two-ply yarn supercapacitors exhibited excellent flexibility and cycling stability over 1200 cycles at straight, twisted and bent states.     

Growth of cobalt–nickel layered double hydroxide on nitrogen-doped graphene by simple co-precipitation method for supercapacitor electrodes

Nitrogen-doped graphene/Co–Ni layered double hydroxide (RGN/Co–Ni LDH) is synthesized by a facile co-precipitation method. Transmission electron microscopy images indicated that the formation of Co–Ni(OH)₂ nanoflakes with the good dispersion anchored on the surfaces of the nitrogen-doped graphene sheets. The nitrogen-doped graphene composites delivered the enhanced electrochemical performances compared to the pure Co–Ni LDH due to the improved electronic conductivity and its hierarchical layer structures. The high specific capacitance of 2092 F g^?1 at current density of 5 mA cm^?2 and the rate retention of 86.5% at current density of 5–50 mA cm^?2 are achieved by RGN/Co–Ni LDH, higher than that of pure Co–Ni LDH (1479 F g^?1 and 76.5%). Moreover, the two-electrode asymmetric supercapacitor, with the RGN/Co–Ni LDH composites as the positive electrode and active carbon as the negative electrode material, exhibits energy density of 49.4 Wh kg^?1 and power density of 101.97 W kg^?1 at the current density of 5 mA cm^?2, indicating the composite has better capacitive behavior.     

High‐Performance 2.6 V Aqueous Asymmetric Supercapacitors based on In Situ Formed Na0.5MnO2 Nanosheet Assembled Nanowall Arrays

The voltage limit for aqueous asymmetric supercapacitors is usually 2 V, which impedes further improvement in energy density. Here, high Na content Birnessite Na_0.5MnO₂ nanosheet assembled nanowall arrays are in situ formed on carbon cloth via electrochemical oxidation. It is interesting to find that the electrode potential window for Na_0.5MnO₂ nanowall arrays can be extended to 0–1.3 V (vs Ag/AgCl) with significantly increased specific capacitance up to 366 F g^?1. The extended potential window for the Na_0.5MnO₂ electrode provides the opportunity to further increase the cell voltage of aqueous asymmetric supercapacitors beyond 2 V. To construct the asymmetric supercapacitor, carbon‐coated Fe₃O₄ nanorod arrays are synthesized as the anode and can stably work in a negative potential window of ?1.3 to 0 V (vs Ag/AgCl). For the first time, a 2.6 V aqueous asymmetric supercapacitor is demonstrated by using Na_0.5MnO₂ nanowall arrays as the cathode and carbon‐coated Fe₃O₄ nanorod arrays as the anode. In particular, the 2.6 V Na_0.5MnO₂//Fe₃O₄@C asymmetric supercapacitor exhibits a large energy density of up to 81 Wh kg^?1 as well as excellent rate capability and cycle performance, outperforming previously reported MnO₂‐based supercapacitors. This work provides new opportunities for developing high‐voltage aqueous asymmetric supercapacitors with further increased energy density.     

Direct Laser‐Patterned Micro‐Supercapacitors from Paintable MoS2 Films

Micrometer‐sized electrochemical capacitors have recently attracted attention due to their possible applications in micro‐electronic devices. Here, a new approach to large‐scale fabrication of high‐capacitance, two‐dimensional MoS₂ film‐based micro‐supercapacitors is demonstrated via simple and low‐cost spray painting of MoS₂ nanosheets on Si/SiO₂ chip and subsequent laser patterning. The obtained micro‐supercapacitors are well defined by ten interdigitated electrodes (five electrodes per polarity) with 4.5 mm length, 820 μm wide for each electrode, 200 μm spacing between two electrodes and the thickness of electrode is ～0.45 μm. The optimum MoS₂‐based micro‐supercapacitor exhibits excellent electrochemical performance for energy storage with aqueous electrolytes, with a high area capacitance of 8 mF cm^?2 (volumetric capacitance of 178 F cm^?3) and excellent cyclic performance, superior to reported graphene‐based micro‐supercapacitors. This strategy could provide a good opportunity to develop various micro‐/nanosized energy storage devices to satisfy the requirements of portable, flexible, and transparent micro‐electronic devices.     

Phosphorus‐Rich Colloidal Cobalt Diphosphide (CoP2) Nanocrystals for Electrochemical and Photoelectrochemical Hydrogen Evolution

Developing earth‐abundant and efficient electrocatalysts for photoelectrochemical water splitting is critical to realizing a high‐performance solar‐to‐hydrogen energy conversion process. Herein, phosphorus‐rich colloidal cobalt diphosphide nanocrystals (CoP₂ NCs) are synthesized via hot injection. The CoP₂ NCs show a Pt‐like hydrogen evolution reaction (HER) electrocatalytic activity in acidic solution with a small overpotential of 39 mV to achieve ?10 mA cm^?2 and a very low Tafel slope of 32 mV dec^?1. Density functional theory (DFT) calculations reveal that the high P content both physically separates Co atoms to prevent H from over binding to multiple Co atoms, while simultaneously stabilizing H adsorbed to single Co atoms. The catalytic performance of the CoP₂ NCs is further demonstrated in a metal–insulator–semiconductor photoelectrochemical device consisting of bottom p‐Si light absorber, atomic layer deposition Al–ZnO passivation layers, and the CoP₂ cocatalyst. The p‐Si/AZO/TiO₂/CoP₂ photocathode shows a photocurrent density of ?16.7 mA cm^?2 at 0 V versus reversible hydrogen electrode (RHE) and an output photovoltage of 0.54 V. The high performance and stability are attributed to the junction between p‐Si and AZO, the corrosion‐resistance of the pinhole‐free TiO₂ protective layer, and the fast HER kinetics of the CoP₂ NCs.     

Sulfur‐Deficient Bismuth Sulfide/Nitrogen‐Doped Carbon Nanofibers as Advanced Free‐Standing Electrode for Asymmetric Supercapacitors

The use of free‐standing carbon‐based hybrids plays a crucial role to help fulfil ever‐increasing energy storage demands, but is greatly hindered by the limited number of active sites for fast charge adsorption/desorption processes. Herein, an efficient strategy is demonstrated for making defect‐rich bismuth sulfides in combination with surface nitrogen‐doped carbon nanofibers (dr‐Bi₂S₃/S‐NCNF) as flexible free‐standing electrodes for asymmetric supercapacitors. The dr‐Bi₂S₃/S‐NCNF composite exhibits superior electrochemical performances with an enhanced specific capacitance of 466 F g^?1 at a discharge current density of 1 A g^?1. The high performance of dr‐Bi₂S₃/S‐NCNF electrodes originates from its hierarchical structure of nitrogen‐doped carbon nanofibers with well‐anchored defect‐rich bismuth sulfides nanostructures. As modeled by density functional theory calculation, the dr‐Bi₂S₃/S‐NCNF electrodes exhibit a reduced OH^? adsorption energy of ‐3.15 eV, compared with that (–3.06 eV) of defect‐free bismuth sulfides/surface nitrogen‐doped carbon nanofiber (df‐Bi₂S₃/S‐NCNF). An asymmetric supercapacitor is further fabricated by utilizing dr‐Bi₂S₃/S‐NCNF hybrid as the negative electrode and S‐NCNF as the positive electrode. This composite exhibits a high energy density of 22.2 Wh kg^?1 at a power density of 677.3 W kg^?1. This work demonstrates a feasible strategy to construct advanced metal sulfide‐based free‐standing electrodes by incorporating defect‐rich structures using surface engineering principles.     

Core–Shell Nitrogen‐Doped Carbon Hollow Spheres/Co3O4 Nanosheets as Advanced Electrode for High‐Performance Supercapacitor

Co₃O₄/nitrogen‐doped carbon hollow spheres (Co₃O₄/NHCSs) with hierarchical structures are synthesized by virtue of a hydrothermal method and subsequent calcination treatment. NHCSs, as a hard template, can aid the generation of Co₃O₄ nanosheets on its surface; while SiO₂ spheres, as a sacrificed‐template, can be dissolved in the process. The prepared Co₃O₄/NHCS composites are investigated as the electrode active material. This composite exhibits an enhanced performance than Co₃O₄ itself. A higher specific capacitance of 581 F g^?1 at 1 A g^?1 and a higher rate performance of 91.6% retention at 20 A g^?1 are achieved, better than Co₃O₄ nanorods (318 F g^?1 at 1 A g^?1 and 67.1% retention at 20 A g^?1). In addition, the composite is employed as a positive electrode to fabricate an asymmetric supercapacitor. The device can deliver a high energy density of 34.5 Wh kg^?1 at the power density of 753 W kg^?1 and display a desirable cycling stability. All of these attractive results make the unique hierarchical Co₃O₄/NHCS core–shell structure a promising electrode material for high‐performance supercapacitors.