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.     

A Simple Electrochemical Route to Access Amorphous Mixed‐Metal Hydroxides for Supercapacitor Electrode Materials

Supercapacitors or electrochemical capacitors, as energy storage devices, require very stable positive electrode materials for useful applications. Although most positive electrodes are based on crystalline mixed‐metal hydroxides, their pseudocapacitors usually perform poorly or have a short cycle life. High activities can be achieved with amorphous phases. Methods to produce amorphous materials are also not typically amenable towards mixed‐metal compositions. It is demonstrated that electrochemistry in an ambient environment can be used to produce a series of amorphous mixed‐metal hydroxides with a homogeneous distribution of metals for use as positive electrode materials in a supercapacitor. The integrated performance of the amorphous ternary mixed‐metal hydroxide pseudocapacitor is superior to that of crystalline materials. The amorphous Ni‐Co‐Fe hydroxide supercapacitor is characterized by a long‐term cycling stability that retained 94% of its capacity after 20 000 cycles. This is much higher than the cycle life of crystalline devices. These results show the broad applicability of this methodology towards new electrode materials for high‐performance supercapacitors, especially amorphous mixed‐metal hydroxides, as advanced electrode materials.     

High Performance Flexible Supercapacitor Electrodes Composed of Ultralarge Graphene Sheets and Vanadium Dioxide

Little is known regarding the effect of the graphene lateral size on the electrochemical performance of hybrid graphene electrode. This work examines the electrochemical performance of a flexible hybrid supercapacitor electrode composed of ultralarge graphene oxide (UGO; mean lateral size of 47 ± 22 μm) and vanadium dioxide (VO₂) nanobelts, referring to a reference electrode composed of small scale graphene oxide (SGO; mean lateral size of 0.8 ± 0.5 μm) and VO₂.Thermal treatment converts UGO/VO₂ and SGO/VO₂ to URGO/VO₂ (denoted VURGO) and SRGO/VO₂ (denoted VSRGO) electrodes, respectively. The sheet resistance of the VURGO film (0.57 ± 0.03 kΩ sq.^–1) was two orders of magnitude lower than that of the VSRGO (55.74 ± 9.35 kΩ sq.^–1). The VURGO hybrid electrode showed a specific capacitance of 769 F g^?1, which was significantly better than the corresponding values for the VSRGO electrode (385 F/g). These results support the notion that the use of ultralarge graphene sheets (≈22 500 μm²) lowers the intersheet resistance due to the presence of fewer intersheet tunneling barriers. This article highlights the potential utility of URGO (as a conductive support) in hybrid electrode containing VO₂ nanobelts for high performance flexible hybrid supercapacitor.     

Electrospun Nanomaterials for Supercapacitor Electrodes: Designed Architectures and Electrochemical Performance

Electrospinning is the most facile and highly versatile approach to produce 1D polymeric, inorganic, and hybrid nanomaterials with a small diameter, controllable dimensions, and designed architectures. In particular, with large surface area, high porosity, low density, good directionality, and tunable composition, electrospun nanofibers and mats are regarded as ideal candidates for various kinds of electrochemical energy storage devices such as supercapacitors (SCs). In this review, the recent progress in electrospun electrode materials for SCs is presented, covering the architecture design and their electrochemical performance. After a brief introduction about SCs, the basic principles of the electrospinning technique are discussed. Following, attention is paid to the discussion of various electrospun nanofibers and mats including 1D carbons, metal oxides, metal sulfides, metal nitrides, conducting polymers and composite nanomaterials with various types of architectures as electrodes for SCs. The relationship between the composition, architecture, and the electrochemical performance is discussed in detail. Finally, some challenges and perspectives of future research of the electrospun nanofibers and mats for high performance SCs are highlighted. It is anticipated that this review would provide the researchers some inspiration for constructing new types of energy storage devices.     

Supercapacitor Electrodes Obtained by Directly Bonding 2D MoS2 on Reduced Graphene Oxide

Layered molybdenum disulfide (MoS₂) is deposited by microwave heating on a reduced graphene oxide (RGO). Three concentrations of MoS₂ are loaded on RGO, and the structure and morphology are characterized. The first layers of MoS₂ are detected as being directly bonded with the oxygen of the RGO by covalent chemical bonds (Mo‐O‐C). Electrochemical characterizations indicate that this electroactive material can be cycled reversibly between 0.25 and 0.8 V in 1 m HClO₄ solution for hybrids with low concentrations of MoS₂ layers (LCMoS₂/RGO) and between 0.25 and 0.65 V for medium (MCMoS₂/RGO) and high concentrations (HCMoS₂/RGO) of MoS₂ layers on graphene. The specific capacitance measured values at 10 mV s^?1 are 128, 265, and 148 Fg^?1 for the MoS₂/RGO with low, medium, and high concentrations of MoS₂, respectively, and the calculated energy density is 63 W h kg^?1 for the LCMoS₂/RGO hybrid. This supercapacitor electrode also exhibits superior cyclic stability with 92% of the specific capacitance retained after 1000 cycles.     

Layered Double Hydroxide Minerals as Possible Prebiotic Information Storage and Transfer Compounds

One of the fundamental difficulties when considering the origin of life on Earth is the identification of an emergent system that not only replicated, but also had the capacity to undergo discrete mutation in such a way that following generations might inherit and pass on the mutation. We speculate that the layered double hydroxide (LDH) minerals are plausible candidates for a proto-RNA molecule. We describe a hypothetical LDH-like system which, when intercalated with certain anions, forms crystals with a high degree of internal order giving rise to novel information storage structures in which replication fidelity is maintained, a concept we use to propose an explanation for interstratification in terephthalate LDHs. The external surfaces of these hypothetical crystals provide active sites whose structure and chemistry is dictated by the internal information content of the LDH. Depending on the LDH polytype, the opposing external surfaces of a crystal may give rise to reactive sites that are either complementary or mirror images of each other, and so may be chiral. We also examine similarities between these proposed “proto-RNA” structures and the DNA that encodes the hereditary information in life today, concluding with a hypothetical scenario wherein these proto-RNA molecules predated the putative RNA-world.     

Trinary Layered Double Hydroxides as High‐Performance Bifunctional Materials for Oxygen Electrocatalysis

Layered double hydroxides (LDHs) are a family of high‐profile layer materials with tunable metal species and interlayer spacing, and herein the LDHs are first investigated as bifunctional electrocatalysts. It is found that trinary LDH containing nickel, cobalt, and iron (NiCoFe‐LDH) shows a reasonable bifunctional performance, while exploiting a preoxidation treatment can significantly enhance both oxygen reduction reaction and oxygen evolution reaction activity. This phenomenon is attributed to the partial conversion of Co²⁺ to Co³⁺ state in the preoxidation step, which stimulates the charge transfer to the catalyst surface. The practical application of the optimized material is demonstrated with a small potential hysteresis (800 mV for a reversible current density of 20 mA cm^?2) as well as a high stability, exceeding the performances of noble metal catalysts (commercial Pt/C and Ir/C). The combination of the electrochemical metrics and the facile and cost‐effective synthesis endows the trinary LDH as a promising bifunctional catalyst for a variety of applications, such as next‐generation regenerative fuel cells or metal–air batteries.     

Single Ru Atoms Stabilized by Hybrid Amorphous/Crystalline FeCoNi Layered Double Hydroxide for Ultraefficient Oxygen Evolution

In view of the sluggish kinetics suppressing the oxygen evolution reaction (OER), developing efficient and robust OER catalysts is urgent and essential for developing efficient energy conversion technologies. Herein, hybrid amorphous/crystalline FeCoNi layered double hydroxide (LDH)-supported single Ru atoms (Ru SAs/AC-FeCoNi) are developed for enabling a highly efficient electrocatalytic OER. The amorphous outer layer in Ru SAs/AC-FeCoNi is composed of abundant defect sites and unsaturated coordination sites, which can serve as anchoring sites to stabilize single Ru atoms. The crystalline inner has a highly symmetric rigid structure, thereby strengthening the stability of support for a long-lasting OER. The synergistic effects endow this hybrid catalyst with extremely low overpotential (205 mV at 10 mA cm⁻²). Density functional theory calculation indicates that single Ru atoms stabilized by hybrid amorphous/crystalline FeCoNi LDH facilitate the formation of Ru–O* (rate-determining step), thus accelerating the OER process.     

Iron‐Based Supercapacitor Electrodes: Advances and Challenges

Supercapacitors (SCs) have great promise as the state‐of‐the‐art power source in portable electronics and hybrid vehicles. The performance of SCs is largely determined by the properties of the electrode material, and numerous efforts have been devoted to the explorations of novel electrode materials. Recently, iron‐based materials, including Fe₂O₃, Fe₃O₄, FeOOH, FeO_x, CoFe₂O₄, and MnFe₂O₄, have received considerable attention as very promising electrode materials for SCs due to their high theoretical specific capacitances, natural abundance, low cost, and non‐toxicity. However, most of these Fe‐based SC electrodes suffer from poor conductivity and/or electrochemical instability, which seriously impede their implementation as high‐performance electrodes for SCs. To settle these issues, substantial efforts have been made in improving their conductivity and cycling stability, and great processes have been achieved. Here, recent research advances in the rational design and synthesis of diverse Fe‐based nanostructured electrodes and their capacitive performance for SCs are presented. Besides, challenges and prospects of Fe‐based materials as advanced negative electrodes for SCs are also discussed.     

Quantifying the Role of Nanotubes in Nano:Nano Composite Supercapacitor Electrodes

Many promising supercapacitor electrode materials have high resistivity and require conductive additives to function effectively. However, the detailed role of the additive is not understood. Here, this question is resolved by applying a quantitative model for resistance‐limited supercapacitor electrodes to Co(OH)₂‐nanosheet/carbon nanotube composites. Without nanotubes, theory predicts and experiments show that while the low‐rate capacitance increases linearly with electrode thickness, the high rate capacitance decreases with thickness due to slow charging. Experiments supported by theory show that nanotube addition has two effects. First, the nanotube network effectively distributes charge, increasing the intrinsic electrode performance to the limit associated with its accessible surface area. Second, at high‐rate, the increased electrode conductivity shifts the rate‐limiting resistance from electrode to electrolyte, thus removing the thickness‐dependent capacitance falloff. Furthermore, the analysis quantifies the out‐of‐plane conductivity of the nanotube network, identifies the cross‐over from resistance‐limited to diffusion‐limited behavior, and allows full electrode modeling, facilitating rational design.     

Tuning Electronic Structure of NiFe Layered Double Hydroxides with Vanadium Doping toward High Efficient Electrocatalytic Water Oxidation

Binary NiFe layer double hydroxide (LDH) serves as a benchmark non‐noble metal electrocatalyst for the oxygen evolution reaction, however, it still needs a relatively high overpotential to achieve the threshold current density. Herein the catalyst's electronic structure is tuned by doping vanadium ions into the NiFe LDHs laminate forming ternary NiFeV LDHs to reduce the onset potential, achieving unprecedentedly efficient electrocatalysis for water oxidation. Only 1.42 V (vs reversible hydrogen electrode (RHE), ≈195 mV overpotential) is required to achieve catalytic current density of 20 mA cm^?2 with a small Tafel slope of 42 mV dec^?1 in 1 m KOH solution, which manifests the best of NiFe‐based catalysts reported till now. Electrochemical analysis and density functional theory +U simulation indicate that the high catalytic activity of NiFeV LDHs mainly attributes to the vanadium doping which can modify the electronic structure and narrow the bandgap thereby bring enhanced conductivity, facile electron transfer, and abundant active sites.     

A Layered Inorganic–Organic Open Framework Material as a 4 V Positive Electrode with High‐Rate Performance for K‐Ion Batteries

K‐ion batteries (KIBs) are promising for large‐scale energy storage owing to various advantages like the high abundance of potassium resources in the Earth's crust, high operational potentials, and high power due to fast diffusion of K⁺ ions. However, to realize the practical application of KIBs, electrode materials are needed with high operational voltage, good capacity, long cycle life, and low‐cost. This work reports a layered open framework material, K₂[(VOHPO₄)₂(C₂O₄)], composited with reduced graphene oxide (rGO) as a 4 V positive electrode material for KIBs. The material is prepared by a simple precipitation reaction at room temperature. The material demonstrates reversible K‐extraction/insertion with conventional carbonate ester KPF₆ solutions; however, with low specific capacity and low Coulombic efficiency. A high discharge capacity of >100 mAh g^?1 with good cycling stability and higher Coulombic efficiency is achieved in a highly concentrated electrolyte, 7 mol kg^?1 of potassium bis(fluorosulfonyl)amide (KFSA) in dimethoxyethane (DME) at 0.1 C rate. Due to the facile migration of K⁺ ions in the framework, the material exhibits excellent rate capability with a discharge capacity of 80 mAh g^?1 at 10 C rate, and a good capacity retention of 67% after 500 cycles at 2 C rate.     

A Strategy for Synthesis of Carbon Nitride Induced Chemically Doped 2D MXene for High‐Performance Supercapacitor Electrodes

A step‐by‐step strategy is reported for improving capacitance of supercapacitor electrodes by synthesizing nitrogen‐doped 2D Ti₂CT_x induced by polymeric carbon nitride (p‐C₃N₄), which simultaneously acts as a nitrogen source and intercalant. The NH₂CN (cyanamide) can form p‐C₃N₄ on the surface of Ti₂CT_x nanosheets by a condensation reaction at 500–700 °C. The p‐C₃N₄ and Ti₂CT_x complexes are then heat‐treated to obtain nitrogen‐doped Ti₂CT_x nanosheets. The triazine‐based p‐C₃N₄ decomposes above 700 °C; thus, the nitrogen species can be surely doped into the internal carbon layer and/or defect site of Ti₂CT_x nanosheets at 900 °C. The extended interlayer distance and c‐lattice parameters (c‐LPs of 28.66 Å) of Ti₂CT_x prove that the p‐C₃N₄ grown between layers delaminate the nanosheets of Ti₂CT_x during the doping process. Moreover, 15.48% nitrogen doping in Ti₂CT_x improves the electrochemical performance and energy storage ability. Due to the synergetic effect of delaminated structures and heteroatom compositions, N‐doped Ti₂CT_x shows excellent characteristics as an electrochemical capacitor electrode, such as perfectly rectangular cyclic voltammetry results (CVs, R² = 0.9999), high capacitance (327 F g^?1 at 1 A g^?1, increased by ≈140% over pristine‐Ti₂CT_x), and stable long cyclic performance (96.2% capacitance retention after 5000 cycles) at high current density (5 A g^?1).     

层状双金属氢氧化物及其在药物传递系统中的应用研究现状

层状双金属氢氧化物作为一种新型无机纳米载体材料,具有独特优势,近年来其在各类药物传递系统中的应用已成为研究热点。介绍层状双金属氢氧化物的制备与修饰,分类综述其在不同药物传递系统中的应用研究。     

Highly Stable Transparent Conductive Silver Grid/PEDOT:PSS Electrodes for Integrated Bifunctional Flexible Electrochromic Supercapacitors

Silver grids are attractive for replacing indium tin oxide as flexible transparent conductors. This work aims to improve the electrochemical stability of silver‐based transparent conductors. A silver grid/PEDOT:PSS hybrid film with high conductivity and excellent stability is successfully fabricated. Its functionality for flexible electrochromic applications is demonstrated by coating one layer of WO₃ nanoparticles on the silver grid/PEDOT:PSS hybrid film. This hybrid structure presents a large optical modulation of 81.9% at 633 nm, fast switching, and high coloration efficiency (124.5 cm² C^?1). More importantly, an excellent electrochemical cycling stability (sustaining 79.1% of their initial transmittance modulation after 1000 cycles) and remarkable mechanical flexibility (optical modulation decay of only 7.5% after 1200 compressive bending cycles) is achieved. A novel smart supercapacitor is presented that functions as a regular energy‐storage device and simultaneously monitors the level of stored energy by a rapid and reversible color variation even at high current charge/discharge conditions. The film sustains an optical modulation of 87.7% and a specific capacitance of 67.2% at 10 A g^?1 compared to their initial value at a current density of 1 A g^?1. The high‐performance silver grid/PEDOT:PSS hybrid transparent films exhibit promising features for various emerging flexible electronics and optoelectronic devices.     

Reduced Mesoporous Co3O4 Nanowires as Efficient Water Oxidation Electrocatalysts and Supercapacitor Electrodes

While electrochemical water splitting is one of the most promising methods to store light/electrical energy in chemical bonds, a key challenge remains in the realization of an efficient oxygen evolution reaction catalyst with large surface area, good electrical conductivity, high catalytic properties, and low fabrication cost. Here, a facile solution reduction method is demonstrated for mesoporous Co₃O₄ nanowires treated with NaBH₄. The high‐surface‐area mesopore feature leads to efficient surface reduction in solution at room temperature, which allows for retention of the nanowire morphology and 1D charge transport behavior, while at the same time substantially increasing the oxygen vacancies on the nanowire surface. Compared to pristine Co₃O₄ nanowires, the reduced Co₃O₄ nanowires exhibit a much larger current of 13.1 mA cm^‐2 at 1.65 V vs reversible hydrogen electrode (RHE) and a much lower onset potential of 1.52 V vs RHE. Electrochemical supercapacitors based on the reduced Co₃O₄ nanowires also show a much improved capacitance of 978 F g^‐1 and reduced charge transfer resistance. Density‐functional theory calculations reveal that the existence of oxygen vacancies leads to the formation of new gap states in which the electrons previously associated with the Co‐O bonds tend to be delocalized, resulting in the much higher electrical conductivity and electrocatalytic activity.