Boron Element Nanowires Electrode for Supercapacitors

The pursuit of new categories of active materials as electrodes of supercapacitors remains a great challenge. Herein, for the first time, elemental boron as a superior electrode material of supercapacitors is reported, which exhibits significantly high capacitances and excellent rate performance in all alkaline, neutral, and acidic electrolytes. Notably, boron nanowire‐carbon fiber cloth (BNWs‐CFC) electrodes achieve a capacitance up to 42.8 mF cm^?2 at a scan rate of 5 mV s^?1 and 60.2 mF cm^?2 at a current density of 0.2 mA cm^?2 in the acidic electrolyte. Moreover, in all these three kinds of electrolytes, BNWs‐CFC electrodes demonstrate a decent cycling stability with >80% capacitance retention after 8000 charging/discharging cycles. The Dominating energy storage mechanism of BNWs in the different electrolytes is analyzed by looking into the kinetics of the electrochemical process. Subsequently, the BNWs‐CFC electrode is used to fabricate a flexible solid‐state supercapacitor, which reveals a specific capacitance up to 22.73 mF cm^?2 and good mechanical performance after 1000 bending cycles. This study opens a new avenue to explore elemental boron‐based new nanomaterials for the application of energy storage with superior electrochemical performance.     

Large‐Scale Conductive Yarns Based on Twistable Korean Traditional Paper (Hanji) for Supercapacitor Applications: Toward High‐Performance Paper Supercapacitors

A simple and scalable method to fabricate a yarn‐type supercapacitor with a large specific capacitance without the aid of traditional pseudocapacitive electrode materials such as conducting polymers and metal oxides is reported. The yarn‐type supercapacitors are made from twisting reduced graphene oxide (rGO) or/and single‐walled carbon nanotubes (SWNTs)‐coated Korean traditional paper (KTP). The yarn‐type paper supercapacitor displays surprisingly enhanced electrochemical capacitance values, showing synergistic effect between rGO and SWNTs (500 times larger than performance of yarn‐type rGO‐coated paper supercapacitors). Coating rGO or/and SWNTs on KTP gives good morphology to the composite film, in which porosity increases and mean pore diameter decreases. The yarn‐type rGO/SWNT paper supercapacitor shows good mechanical strength, high flexibility, excellent electrochemical performance, and long‐life operation. The yarn‐type supercapacitor has an excellent electrochemical performance with a specific capacitance of 366 F g^?1 at scan rate of 25 mV s^?1 and high stability without any degradation in electrical performance up to 10 000 charge–discharge cycles. The average capacitance of rGO/SWNT@KTP yarn‐type supercapacitors is seven times higher than that of sheet‐type supercapacitors at scan rate of 500 mV s^?1. The lighting of a red light‐emitting diode (LED) is demonstrated by the yarn‐type paper supercapacitor without connecting supercapacitors in series.     

Mixed Metal Sulfides for Electrochemical Energy Storage and Conversion

Mixed metal sulfides (MMSs) have attracted increased attention as promising electrode materials for electrochemical energy storage and conversion systems including lithium‐ion batteries (LIBs), sodium‐ion batteries (SIBs), hybrid supercapacitors (HSCs), metal–air batteries (MABs), and water splitting. Compared with monometal sulfides, MMSs exhibit greatly enhanced electrochemical performance, which is largely originated from their higher electronic conductivity and richer redox reactions. In this review, recent progresses in the rational design and synthesis of diverse MMS‐based micro/nanostructures with controlled morphologies, sizes, and compositions for LIBs, SIBs, HSCs, MABs, and water splitting are summarized. In particular, nanostructuring, synthesis of nanocomposites with carbonaceous materials and fabrication of 3D MMS‐based electrodes are demonstrated to be three effective approaches for improving the electrochemical performance of MMS‐based electrode materials. Furthermore, some potential challenges as well as prospects are discussed to further advance the development of MMS‐based electrode materials for next‐generation electrochemical energy storage and conversion systems.     

Metal–Organic Framework Derived Honeycomb Co9S8@C Composites for High‐Performance Supercapacitors

Unique nanostructures always lead to extraordinary electrochemical energy storage performance. Here, the authors report a new strategy for using Metal‐organic frameworks (MOFs) derived cobalt sulfide in a carbon matrix with a 3D honeycombed porous structure, resulting in a high‐performance supercapacitor with unrivalled capacity of ≈1887 F g^‐1 at the current density of 1 A g^‐1. The honeycomb‐like structure of Co₉S₈@C composite is loosely adsorbed, with plentiful surface area and high conductivity, leading to improved Faradaic processes across the interface and enhanced redox reactions at active Co₉S₈ sites. Therefore, the heterostructure‐fabricated hybrid supercapacitor, using activated carbon as the counter electrode, demonstrates a high energy density of 58 Wh kg^‐1 at the power density of 1000 W kg^‐1. Even under an ultrahigh power density of 17 200 W kg^‐1, its energy density maintains ≈38 Wh kg^‐1. The hybrid supercapacitor also exhibits suitable cycling stability, with ≈90% capacity retention after 10 000 continuous cycles at the current density of 5 A g^‐1. This work presents a practical method for using MOFs as sacrificial templates to synthesize metal‐sulfides for highly efficient electrochemical energy storage.     

Hierarchical Fabric Decorated with Carbon Nanowire/Metal Oxide Nanocomposites for 1.6 V Wearable Aqueous Supercapacitors

Aqueous asymmetric supercapacitors (ASCs) may offer comparable or higher energy density than electric double‐layer capacitors (EDLCs) based on organic electrolytes. As such, ASCs may be more suitable for integration into smart textiles, where the use of flammable organic solvents is not acceptable. However, reported ASC devices typically suffer from poor rate capability and low areal loadings. This study demonstrates the development of nitrogen‐doped carbon (N‐C) nanowire/metal oxide (Fe₂O₃ and MnO₂) nanocomposite electrodes directly produced on the internal surface of a conductive fabric for use as high‐rate electrodes for solid‐state ASCs. The N‐C nanowires provide fast and efficient pathways for electrons, while short diffusion paths within nanosized metal oxides enable fast ion transport, leading to greatly enhanced performance at high rates. The porous structure of the fabric enables high areal capacitance loading in each electrode (≈150 mF cm^?2). Both electrodes show high specific capacitance of ≈180 F g^?1 (Fe₂O₃) and ≈250 F g^?1 (MnO₂) and excellent rate capability. Solid‐state ASCs assembled by using an aqueous gel electrolyte operate at 1.6 V and deliver over 60 mF cm^?2 during ≈50 s charging/discharging time and over 30 mF cm^?2 for ≈5 s discharge.     

CoNi2S4‐Graphene‐2D‐MoSe2 as an Advanced Electrode Material for Supercapacitors

3D CoNi₂S₄‐graphene‐2D‐MoSe₂ (CoNi₂S₄‐G‐MoSe₂) nanocomposite is designed and prepared using a facile ultrasonication and hydrothermal method for supercapacitor (SC) applications. Because of the novel nanocomposite structures and resultant maximized synergistic effect among ultrathin MoSe₂ nanosheets, highly conductive graphene and CoNi₂S₄ nanoparticles, the electrode exhibits rapid electron and ion transport rate and large electroactive surface area, resulting in its amazing electrochemical properties. The CoNi₂S₄‐G‐MoSe₂ electrode demonstrates a maximum specific capacitance of 1141 F g^?1, with capacitance retention of ≈108% after 2000 cycles at a high charge–discharge current density of 20 A g^?1. As to its symmetric device, 109 F g^?1 at a scan rate of 5 mV s^?1 is exhibited. This pioneering work should be helpful in enhancing the capacitive performance of SC materials by designing nanostructures with efficient synergetic effects.     

High‐Performance Fiber‐Shaped All‐Solid‐State Asymmetric Supercapacitors Based on Ultrathin MnO2 Nanosheet/Carbon Fiber Cathodes for Wearable Electronics

Flexible fiber‐shaped supercapacitors have shown great potential in portable and wearable electronics. However, small specific capacitance and low operating voltage limit the practical application of fiber‐shaped supercapacitors in high energy density devices. Herein, direct growth of ultrathin MnO₂ nanosheet arrays on conductive carbon fibers with robust adhesion is exhibited, which exhibit a high specific capacitance of 634.5 F g^?1 at a current density of 2.5 A g^?1 and possess superior cycle stability. When MnO₂ nanosheet arrays on carbon fibers and graphene on carbon fibers are used as a positive electrode and a negative electrode, respectively, in an all‐solid‐state asymmetric supercapacitor (ASC), the ASC displays a high specific capacitance of 87.1 F g^?1 and an exceptional energy density of 27.2 Wh kg^?1. In addition, its capacitance retention reaches 95.2% over 3000 cycles, representing the excellent cyclic ability. The flexibility and mechanical stability of these ASCs are highlighted by the negligible degradation of their electrochemical performance even under severely bending states. Impressively, as‐prepared fiber‐shaped ASCs could successfully power a photodetector based on CdS nanowires without applying any external bias voltage. The excellent performance of all‐solid‐state ASCs opens up new opportunity for development of wearable and self‐powered nanodevices in near future.     

Sulfidation of NiMn‐Layered Double Hydroxides/Graphene Oxide Composites toward Supercapacitor Electrodes with Enhanced Performance

Supercapacitors can deliver high‐power density and long cycle stability, but the limited energy density due to poor electronic and ionic conductivity of the supercapacitor electrode has been a bottleneck in many applications. A strategy to prepare microflower‐like NiMn‐layered double hydroxides (LDH) with sulfidation is delineated to reduce the charge transfer resistance of supercapacitor electrode and realize faster reversible redox reactions with notably enhanced specific capacitance. The incorporation of graphite oxide (GO) in NiMn LDH during sulfidation leads to simultaneous reduction of GO with enhanced conductivity, lessened defects, and doping of S into the graphitic structure. Cycling stability of the sulfidized composite electrode is enhanced due to the alleviation of phase transformation during electrochemical cycling test. As a result, this sulfidation product of LDH/GO (or LDHGOS) can reach a high‐specific capacitance of 2246.63 F g^?1 at a current density of 1 A g^?1, and a capacitance of 1670.83 F g^?1 is retained at a high‐current density of 10 A g^?1, exhibiting an outstanding capacitance and rate performance. The cycling retention of the LDHGOS electrode is also extended to ≈ 67% after 1500 cycles compared to only ≈44% of the pristine NiMn LDH.