Enhancing pseudocapacitive charge storage in polymer templated mesoporous materials

Beyond the Meso/Macroporous Boundary: Extending Capillary Condensation-Based Pore Size Characterization in Thin Films Through Tailored Adsorptives

The characterization of thin films containing nanopores with diameters exceeding 50 nm poses significant challenges, especially when deploying sorption-based techniques. Conventional volumetric physisorption or mercury intrusion methods have limited applicability in thin films due to constraints in sample preparation and nondestructive testing. In this context, ellipsometric porosimetry represents a viable alternative, leveraging its optical sensitivity to thin films. With existing setups relying on the capillary condensation of volatile compounds such as water, applicability is typically restricted to pore dimensions <50 nm. In this study, we introduce two high-molar-mass hydrocarbon adsorptives, namely ethylbenzene and n-nonane. These adsorptives exhibit substantial potential in improving the accuracy of physisorption measurements beyond mesoporosity (i.e., >50 nm). Specifically, with n-nonane, applicability is extended up to 80 nm pores. Our measurement guidelines propose a nondestructive, expeditious (<60 min), low-pressure (<0.03 bar) approach to investigate nanoporous thin films with potential adaptability to diverse structural architectures.

Surfactant effect on energy storage performance of hydrothermally synthesized Ni3V2O8

We report a study to improve the ternary oxide Ni3V2O8's electrochemical energy storage capabilities through correct surfactanization during hydrothermal synthesis. In this study, Ni3V2O8nanomaterials were synthesized in three different forms: one with a cationic surfactant (CTAB), one with an anionic surfactant (SLS), and one without any surfactant. FESEM study reveals that all the synthesized Ni3V2O8nanomaterials had a small stone-like morphology. The electrochemical study showed that anionic surfactant-assisted Ni3V2O8(NVSLS) had a maximum of 972 F g-1specific capacitance at 1 A g-1current density, whereas cationic surfactant-assisted Ni3V2O8(NVCTAB) had the lowest specific capacitance of 162 F g-1. The specific capacitance and the capacitance retention of the NVSLS(85% after 4000 cycles) based electrode was much better than that of the NVCTAB(76% after 4000 cycles) based electrode. The improved energy storage properties of the NVSLSelectrode are attributed to its high diffusion coefficient, high surface area, and enriched elemental nickel, as compared to the NVCTABelectrode. All these excellent electrochemical properties of NVSLSelectrode indicates their potential usage in asymmetric supercapacitor application.

Rough and Porous Micropebbles of CeCu2Si2 for Energy Storage Applications

Supercapacitors have attracted considerable attention due to their advantages, including being lightweight and having rapid charge-discharge, a good rate capability, and high cyclic stability. Electrodes are one of the most important factors influencing the performance of supercapacitors. Herein, a three-dimensional network of rough and porous micropebbles of CeCu2Si2 has been prepared using a one-step procedure and tested for the first time as a supercapacitor electrode. The synthesized material was extensively characterized in a three-electrode configuration using different electrochemical techniques, such as cyclic voltammetry (CV), galvanostatic charge and discharge (GCD) tests, and electrochemical impedance spectroscopy (EIS). CeCu2Si2 shows rather high mass-capacitance values: 278 F/g at 1 A/g and 295 F/g at 10 mV/s. Moreover, the material exhibits remarkable long-term stability: 98% of the initial capacitance was retained after 20,000 cycles at 10 A/g and the Coulombic efficiency remains equal to 100% at the end of the cycles.

Fibril-Type Textile Electrodes Enabling Extremely High Areal Capacity through Pseudocapacitive Electroplating onto Chalcogenide Nanoparticle-Encapsulated Fibrils

Effective incorporation of conductive and energy storage materials into 3D porous textiles plays a pivotal role in developing and designing high-performance energy storage devices. Here, a fibril-type textile pseudocapacitor electrode with outstanding capacity, good rate capability, and excellent mechanical stability through controlled interfacial interaction-induced electroplating is reported. First, tetraoctylammonium bromide-stabilized copper sulfide nanoparticles (TOABr-CuS NPs) are uniformly assembled onto cotton textiles. This approach converts insulating textiles to conductive textiles preserving their intrinsically porous structure with an extremely large surface area. For the preparation of textile current collector with bulk metal-like electrical conductivity, Ni is additionally electroplated onto the CuS NP-assembled textiles (i.e., Ni-EPT). Furthermore, a pseudocapacitive NiCo-layered double hydroxide (LDH) layer is subsequently electroplated onto Ni-EPT for the cathode. The formed NiCo-LDH electroplated textiles (i.e., NiCo-EPT) exhibit a high areal capacitance of 12.2 F cm^-2 (at 10 mA cm^-2 ), good rate performance, and excellent cycling stability. Particularly, the areal capacity of NiCo-EPT can be further increased through their subsequent stacking. The 3-stack NiCo-EPT delivers an unprecedentedly high areal capacitance of 28.8 F cm^-2 (at 30 mA cm^-2 ), which outperforms those of textile-based pseudocapacitor electrodes reported to date.

A Mechanically Flexible Necklace-Like Architecture for Achieving Fast Charging and High Capacity in Advanced Lithium-Ion Capacitors

Integration of fast charging, high capacity, and mechanical flexibility into one electrode is highly desired for portable energy-storage devices. However, a high charging rate is always accompanied by capacity decay and cycling instability. Here, a necklace-structured composite membrane consisting of micron-sized FeSe₂ cubes uniformly threaded by carbon nanofibers (CNF) is reported. This unique electrode configuration can not only accommodate the volumetric expansion of FeSe₂ during the lithiation/delithiation processes for structural robustness but also guarantee ultrafast kinetics for Li⁺ entry. At a high mass loading of 6.2 mg cm^-2 , the necklace-like FeSe₂ @CNF electrode exhibits exceptional rate capability (80.7% capacity retention from 0.1 to 10 A g^-1 ) and long-term cycling stability (no capacity decay after 1100 charge-discharge cycles at 2 A g^-1 ). The flexible lithium-ion capacitor (LIC) fabricated by coupling a pre-lithiated FeSe₂ @CNF anode with a porous carbon cathode delivers impressive volumetric energy//power densities (98.4 Wh L^-1 at 157.1 W L^-1 , and 58.9 Wh L^-1 at 15714.3 W L^-1 ). The top performance, long-term cycling stability, low self-discharge rate, and high mechanical flexibility make it among the best LICs ever reported.

Enhanced Electrodes for Supercapacitor Applications Prepared by Hydrothermal-Assisted Nano Sheet-Shaped MgCo2O4@ZnS

In this paper, we report on nanodisc-shaped MgCo2O4 wrapped with ZnS, achieved using the sol–gel-assisted hydrothermal method. This enhances the electrochemical performance, with the electrode delivering superior supercapacitive performance compared to MgCo2O4. Moreover, the nanodisc provides more active sites and allows smooth charge transfer during faradaic reactions. The nanodisc-shaped MgCo2O4 with ZnS delivers a capacitance of approximately 910 F/g at 1 A/g. The fabricated asymmetric capacitor is composed of MgCo2O4@ZnS and activated carbon (AC). The nanodisc-shaped MgCo2O4@ZnS provides more active sites and allows the smooth transport of electrons during long-term cycling. In addition, the electrode side reactions and electrolyte decomposition are significantly reduced due to the ZnS coating on the surface of the MgCo2O4, allowing this asymmetric capacitor to deliver an energy density of 43 Wh·kg−1 at 1454 W·kg−1. The performance of the asymmetric capacitor exhibits enhanced supercapacitive performance and opens a new way to investigate asymmetric supercapacitor devices.

High specific surface area triphenylamine-based covalent organic framework/polyaniline nanocomposites for supercapacitor application

Covalent organic frameworks (COFs) possess extraordinary porosity, structural diversity, and good electrochemical performance, and have broad application prospects in the field of energy storage. However, the low conductivity of COFs limits its further development. In this paper, the electrochemical performance of triphenylamine-based COFs (TPA-COFs) was improved by compounding with highly conductive polyaniline (PANI) using solvothermal synthesis process. The highly conductive polyaniline fibers can act as conductive path in the composite to accelerate the charge transfer rate of TPA-COFs. The π-π interaction between TPA-COFs and PANI effectively decreases the agglomeration degree of PANI. The good dispersion of composite results in that the specific surface area of TPA-COFs/PANI-20 is high as 1233.9 m2 g−1, which provides rich diffusion channels for electrolyte ions. Moreover, the strong π-π structure in the composites ensures the stability of the material skeleton. Thus, TPA-COFs/PANI composite exhibits excellent rate characteristics and cycling stability.

Insights into ZnO-based doped porous nanocrystal frameworks

Colloidal nanocrystals play a vital role in several applications. The doping of cations in the nanocrystal matrix enhances the optical, electrical, and magnetic properties. The number and well-defined distribution of the dopant are crucial to protect the nanocrystal from clustering. The XRD, XPS, and XAS instruments reveal the change in the lattice parameters, chemical states, and local coordination environment information. In addition of detecting the position and distribution of the dopant, the 4D-STEM detector mode gathers all types of real-space atomic-resolution images by collecting all diffraction datasets from each electron probe with high-speed and efficient detection. Dopant-host ligand type, reactions conditions, and reaction time optimization during synthesis are critical for the host and dopant reactivity balance. Pearson's hard/soft acids/bases theory would be a base for balancing the solubility of the dopant-host in the given solvents/surfactant. In addition, tuning the colloidal nanocrystals to secondary structures, which enhances the mass-/ions transport, can contribute a combination of properties that do not exist in the original constituents.

Rational design of covalent triazine frameworks based on pore size and heteroatomic toward high performance supercapacitors

A series of covalent triazine frameworks (CTFs) are prepared via ionothermal synthesis for supercapacitors. Due to the feature of adjustable pore structure and rich nitrogen, CTFs with regular structure can be used as a group of model compounds to further investigate the influence of pore size and heteroatom on supercapacitors. By comparing the performance of CTFs with different pore structures and nitrogen contents, the experimental results show that BPY-CTF with high specific surface area of 2278 m² g^-1, mesopores structure, and suitable nitrogen content displays a specific capacitance of 393.6 F g^-1 at 0.5 A g^-1. According to the results and analysis, the existence of mesopores largely enhance the contact area between the electrode material and electrolyte, and then boost the charge transfer. On the other hand, N-doping has a prominent effect on improving the Faradaic pseudo-capacitance and conductivity for CTF electrode materials. This work will inspire further research on the development of highly efficient electrode materials for energy storage devices.

Role of composition and texture on bifunctional catalytic performance of extruded Au–Cu alloys

Extruded Au–Cu alloys can be used as bifunctional catalysts for the electro-oxidation of CH3OH and HCOOH, and their catalytic activities can be improved based on alloying and appropriate texture.

Polyaniline nanowire arrays generated through oriented mesoporous silica films: effect of pore size and spectroelectrochemical response

Indium-tin oxide electrodes modified with vertically aligned silica nanochannel membranes have been produced by electrochemically assisted self-assembly of cationic surfactants (cetyl- or octadecyl-trimethylammonium bromide) and concomitant polycondensation of the silica precursors (tetraethoxysilane). They exhibited pore diameters in the 2-3 nm range depending on the surfactant used. After surfactant removal, the bottom of mesopores was derivatized with aminophenyl groups via electrografting (i.e., electrochemical reduction of in situ generated aminophenyl monodiazonium salt). These species covalently bonded to the ITO substrate were then exploited to grow polyaniline nanofilaments by electropolymerization of aniline through the nanochannels. Under potentiostatic conditions, the length of polyaniline wires is controllable by tuning the electropolymerization time. From cyclic voltammetry characterization performed either before or after dissolution of the silica template, it appeared that both the polyaniline/silica composite and the free polyaniline nanowire arrays were electroactive, yet with much larger peak currents in the latter case as a result of larger effective surface area offered to the electrolyte solution. At identical electropolymerization time, the amount of deposited polyaniline was larger when using the silica membrane with larger pore diameter. All polyaniline deposits exhibited electrochromic properties. However, the spectroelectrochemical data indicated more complete interconversion between the coloured oxidized form and colourless reduced polyaniline for the arrays of nanofilaments in comparison to bulky films. In addition, the template-free nanowire arrays (i.e., after silica dissolution) were characterized by faster electrochromic behaviour than the polyaniline/silica hybrid, confirming the potential interest of such polyaniline nano-brushes for practical applications.

Hierarchical flower-like architecture of nickel phosphide anchored with nitrogen-doped carbon quantum dots and cobalt oxide for advanced hybrid supercapacitors

The exploitation of hybrid supercapacitors with excellent electrochemical properties is of great significance for energy storage systems. Herein, a three-dimensional hierarchical flower-like architecture of nickel phosphide (Ni₂P) decorated with nitrogen-doped carbon quantum dots (N-CQDs) and cobalt oxide (Co₃O₄) is constructed by an effective two-step hydrothermal strategy followed by in situ phosphorization process. Introducing N-CQDs with superior electrochemical characteristics can not only induce the formation of N-CQDs deposited nickel hydroxide (Ni(OH)₂) flower-like architecture but also significantly enhance the electrochemical features of Ni(OH)₂ nanosheets. After combination with Co₃O₄ nanoparticles and phosphorization treatment, an advanced cathode of Ni₂P/Co₃O₄/N-CQDs with enriched surface phosphate ions is obtained, which possesses an ultra-high capacity of 1044 C g^-1 (2088 F g^-1) at 1 A g^-1 with a splendid rate capacity of 876 C g^-1 (1752 F g^-1) at 20 A g^-1. Moreover, a device assembled by Ni₂P/Co₃O₄/N-CQDs hierarchical flower-like architecture and p-phenylenediamine functionalized reduced graphene oxide (PPD/rGO) nanosheets depicts a commendable energy density of 53.5 Wh kg^-1 at 772.9 W kg^-1. This work provides a novel hierarchical multi-component electrode material with decent electrochemical capacities for hybrid supercapacitors, which has a broad prospect in energy storage devices.

Phyto-inspired Cu/Bi oxide-based nanocomposites: synthesis, characterization, and energy relevant investigation

A modified and sustainable approach is reported in this research for the synthesis of a spherical-shaped CuO–Bi₂O₃ electrode material for electrochemical studies. Aqueous extract derived from the plant Amaranthus viridis L. (Amaranthaceae) (AVL) was used as a reducing agent for morphological control of the synthesis of CuO–Bi₂O₃ nanocomposites. The modified nanomaterial revealed an average crystal size of 49 ± 2 nm, which matches very well with scanning electron microscopy (SEM) findings. Furthermore, the synthesized material was characterized using Fourier-transform infrared spectroscopy, field emission SEM and energy-dispersive spectroscopy. The optical band gap energy of 3.45 eV was calculated using a Tauc plot. Finally, the bioorganic framework-derived CuO–Bi₂O₃ electrode was tested for energy generating and storage applications and the results revealed a capacitance of 389 F g⁻¹ by cyclic voltammetry, with a maximum energy density of 12 W h kg⁻¹ and power density of 5 kW kg⁻¹. Hydrogen evolution reaction and oxygen evolution reaction studies showed good potential of CuO–Bi₂O₃ as an electrocatalyst for water splitting, with maximum efficiency of the electrode up to 16.5 hours.

Spherical-shaped CuO–Bi₂O₃ electrode material and its electrochemical studies.

Progress and Perspectives on Covalent-organic Frameworks (COFs) and Composites for Various Energy Applications

Covalent-organic frameworks (COFs), being a new member of the crystalline porous materials family, have emerged as important materials for energy storage/conversion/generation devices. They possess high surface areas, ordered micro/mesopores, designable structures and an ability to precisely control electro-active groups in their pores, which broaden their application window. Thanks to their low weight density, long range crystallinity, reticular nature and tunable synthesis approach towards two and three dimensional (2D and 3D) networks, they have been found suitable for a range of challenging electrochemical applications. Our review focuses on the progress made on the design, synthesis and structure of COFs and their composites for various energy applications, such as metal-ion batteries, supercapacitors, water-splitting and solar cells. Additionally, attempts have been made to correlate the structural and mechanistic characteristics of COFs with their applications.

Electrodeposition of the MnO2 on the Ag/Au Core-Shell Nanowire and Its Application to the Flexible Supercapacitor

Supercapacitors have received considerable attention as energy storage devices owing to their high power density, fast charge/discharge rate, and long cyclic life. Especially with an increasing demand for flexible and wearable devices, research on flexible supercapacitors has surged in recent years. The silver nanowire (Ag NW) network has been used as a flexible electrode owing to its excellent mechanical and electrical properties; however, its use as an electrode for flexible supercapacitors has been limited due to insufficient electrochemical stability. In this study, we proposed a method to resolve this issue. We employed a solution process that enabled the coating of the surface of Ag NW by a thin Au shell of ≈ 5 nm thickness, which significantly improved the electrochemical stability of the Ag NW network electrodes. Furthermore, we confirmed for the first time that MnO₂, which is one of the most widely used capacitive materials, can be directly electroplated on the AACS NW network electrode. Finally, we fabricated a high-performance and flexible solid-state supercapacitor using the suggested Ag/Au/MnO₂ core–shell NW network electrodes.

Enhancing energy storage capacity of iron oxide-based anodes by adjusting Fe (II/III) ratio in spinel crystalline

Supercapacitors, as promising energy storage candidates, are limited by their unsatisfactory anodes. Herein, we proposed a strategy to improve the electrochemical performance of iron oxide anodes by spinel-framework constraining. We have optimized the anode performance by adjusting the doping ratio of Fe (II/III) self-redox pairs. Structure and electronic state characterizations reveal that the Ni_xFe_3-xO₄was composed of Fe (II/III) and Ni (II/III) pairs in lattice, ensuring a flexible framework for the reversible reaction of Fe (II/III). Typically, when the ratio of Fe (II/III) is 0.91:1 (Fe (II/III)-0.91/1), the Ni_xFe_3-xO₄anode shows a remarkable electrochemical performance with a high specific capacitance of 1694 F g^-1at the current density of 2 A g^-1and capacitance retention of 81.58%, even at a large current density of 50 A g^-1. In addition, the obtained material presents an ultra-stable electrochemical performance, and there is no observable degradation after 5000 cycles. Moreover, an assembled asymmetric supercapacitor of Ni-Co-S@CC//Ni_xFe_3-xO₄@CC presents a maximum energy density of 136.82 Wh kg^-1at the power density of 850.02 W kg^-1. When the power density was close to 42 500 W kg^-1, the energy density was still maintained 63.75 Wh kg^-1. The study indicates that inherent performance of anode material can be improved by tuning the valence charge of active ions.

Ultrathin Carbon Deficient Molybdenum Carbide (α-MoC1-x) Enables High-Rate Mg-Ion-based Energy Storage

Dual-electron transfer with Mg²⁺-ion intercalation outperforms typical alkali metal-ion (Li⁺, Na⁺, K⁺) systems with superior charge storage efficiency while the neutral electrolytes can achieve a working voltage beyond the hydrolysis window of 1.23 V. Hence, aqueous Mg-ion electrolytes are promising for electrochemical energy storage devices to boost the energy density and solve the safety challenges synchronously. However, the Mg-based electrochemical energy storage (EES) devices are generally confined by poor rate performance due to the slow Mg²⁺ diffusion in the electrode materials. In this paper, we demonstrate that carbon-deficient carbide could function as a promising electrode material in Mg²⁺-ion-based EES. An electrode made of such carbide can operate over an extended window up to 2.4 V in 1 M magnesium acetate, showing superior performance of high capacitance (125.2 F/g), high energy density (25.1 Wh/kg), and high power density (3934.8 W/kg). Ab initio simulation reveals migration energy of Mg²⁺ being lower than that of Li⁺ diffusing from one carbon defect to another in the α-MoC_1-x lattice, supporting the experimental results that a symmetric supercapacitor made of α-MoC_1-x in an electrolyte based on Mg²⁺ outperforms electrolytes based on Li⁺.

Ordered mesoporous metal oxides for electrochemical applications: correlation between structure, electrical properties and device performance

Ordered mesoporous metal oxides with a high specific surface area, tailored porosity and engineered interfaces are promising materials for electrochemical applications. In particular, the method of evaporation-induced self-assembly allows the formation of nanocrystalline films of controlled thickness on polar substrates. In general, mesoporous materials have the advantage of benefiting from a unique combination of structural, chemical and physical properties. This Perspective article addresses the structural characteristics and the electrical (charge-transport) properties of mesoporous metal oxides and how these affect their application in energy storage, catalysis and gas sensing.

One-step carbonization of a nickel-containing nitrogen-doped porous carbon material for electrochemical supercapacitors

Improving the graphitization degree of materials by Ni metal catalysis, so as to enhance the electrical properties of the material.

Covalent organic frameworks (COFs) for electrochemical applications

Covalent organic frameworks are a class of extended crystalline organic materials that possess unique architectures with high surface areas and tuneable pore sizes. Since the first discovery of the topological frameworks in 2005, COFs have been applied as promising materials in diverse areas such as separation and purification, sensing or catalysis. Considering the need for renewable and clean energy production, many research efforts have recently focused on the application of porous materials for electrochemical energy storage and conversion. In this respect, considerable efforts have been devoted to the design and synthesis of COF-based materials for electrochemical applications, including electrodes and membranes for fuel cells, supercapacitors and batteries. This review article highlights the design principles and strategies for the synthesis of COFs with a special focus on their potential for electrochemical applications. Recently suggested hybrid COF materials or COFs with hierarchical porosity will be discussed, which can alleviate the most challenging drawback of COFs for these applications. Finally, the major challenges and future trends of COF materials in electrochemical applications are outlined.

Nanoparticle-Based Electrodes with High Charge Transfer Efficiency through Ligand Exchange Layer-by-Layer Assembly

Organic-ligand-based solution processes of metal and transition metal oxide (TMO) nanoparticles (NPs) have been widely studied for the preparation of electrode materials with desired electrical and electrochemical properties for various energy devices. However, the ligands adsorbed on NPs have a significant effect on the intrinsic properties of materials, thus influencing the performance of bulk electrodes assembled by NPs for energy devices. To resolve these critical drawbacks, numerous approaches have focused on developing unique surface chemistry that can exchange bulky ligands with small ligands or remove bulky ligands from NPs after NP deposition. In particular, recent studies have reported that the ligand-exchange-induced layer-by-layer (LE-LbL) assembly of NPs enables controlled assembly of NPs with the desired interparticle distance, and interfaces, dramatically improving the electrical/electrochemical performance of electrodes. This emerging approach also demonstrates that efficient surface ligand engineering can exploit the unique electrochemical properties of individual NPs and maximize the electrochemical performance of the resultant NP-assembled electrodes through improved charge transfer efficiency. This report focuses on how LE-LbL assembly can be effectively applied to NP-based energy storage/conversion electrodes. First, the basic principles of the LE-LbL approach are introduced and then recent progress on NP-based energy electrodes prepared via the LE-LbL approach is reviewed.

Metal-Free Carbon-Based Supercapacitors—A Comprehensive Review

Herein, metal-free heteroatom doped carbon-based materials are being reviewed for supercapacitor and energy applications. Most of these low-cost materials considered are also derived from renewable resources. Various forms of carbon that have been employed for supercapacitor applications are described in detail, and advantages as well as disadvantages of each form are presented. Different methodologies that are being used to develop these materials are also discussed. To increase the specific capacitance, carbon-based materials are often doped with different elements. The role of doping elements on the performance of supercapacitors has been critically reviewed. It has been demonstrated that a higher content of doping elements significantly improves the supercapacitor behavior of carbon compounds. In order to attain a high percentage of elemental doping, precursors with variable ratios as well as simple modifications in the syntheses scheme have been employed. Significance of carbon-based materials doped with one and more than one heteroatom have also been presented. In addition to doping elements, other factors which play a key role in enhancing the specific capacitance values such as surface area, morphology, pore size electrolyte, and presence of functional groups on the surface of carbon-based supercapacitor materials have also been summarized.

Synthesis of a Three-Dimensional Interconnected Oxygen-, Boron-, Nitrogen-, and Phosphorus Tetratomic-Doped Porous Carbon Network as Electrode Material for the Construction of a Superior Flexible Supercapacitor

To construct a high-performance next-generation carbon-based flexible supercapacitor, high porosity, large mass density, and high flexibility are three significant challenging goals. However, one side always affects another. Herein, high-density tetratomic-doped porous composite carbon derived from sustainable biomaterials is achieved via two-step processes of carbonization and acid-washing treatment. The assembled carbon-based electrodes are highly doped with various heteroatoms (B, O, N, and P) for 33.59 atom %, resulting in abundant porosity, high densities, high pseudocapacitive contribution for 84.5%, and superior volumetric capacitive performance. The fabricated flexible electrode exhibits high flexibility, high mass loading (316 mg cm^-3), and remarkable tensile strength (44.6 MPa). Generally, the volumetric performance is key and a significant parameter to appraise the electrochemical characteristics of flexible supercapacitors within a limited space. The aqueous symmetric supercapacitor demonstrates a high volumetric energy density and an excellent power density of 2.08 mWh cm^-3 and 498.4 mW cm^-3, respectively, along with 99.6% capacitance retention after 20 000 cycles, making it competitive to even some pseudocapacitors.

Fast-Charging Cathodes from Polymer-Templated Mesoporous LiVPO4F

Fast-charging cathodes with high operating voltages are critical to the development of high energy and power density lithium-ion batteries. One route to fast-charging battery materials is through the formation of nanoporous networks, but these methods are often limited by the high calcination temperatures required for synthesis. Here, we report the synthesis of carbon-coated nanoporous LiVPO₄F with excellent rate capabilities that can be stably cycled up to 4.6 V in standard LiPF₆ electrolytes. During charge and discharge at 30C, 110 mAh/g (70% of theoretical capacity) was obtained, and only 9% of capacity was lost after 2000 cycles at 20C. These materials also showed excellent stability, with little self-discharge, an open-circuit voltage of 4.2 V, and a discharge capacity of 139 mAh/g obtained after holding for 12 h. Rate capabilities were further demonstrated in a proof-of-concept full cell made with a nanostructured Nb₂O₅. These devices were able to deliver 200 mAh/g at 1C and 100 mAh/g at 30C. Finally, operando X-ray diffraction and electrochemical kinetics were further used to provide insight into the nature of fast charging in these materials.

A polythreaded MnII-MOF and its super-performances for dye adsorption and supercapacitors

One new polythreaded Mn^II-MOF was successfully prepared by employing a tridentate N-donor ligand with three long arms. Its excellent performances in dye adsorption and supercapacitor have been investigated in detail.

Great Enhancement of Carbon Energy Storage through Narrow Pores and Hydrogen-Containing Functional Groups for Aqueous Zn-Ion Hybrid Supercapacitor

The proton transfer mechanism on the carbon cathode surface has been considered as an effective way to boost the electrochemical performance of Zn-ion hybrid supercapacitors (SCs) with both ionic liquid and organic electrolytes. However, cheaper, potentially safer, and more environmental friendly supercapacitor can be achieved by using aqueous electrolyte. Herein, we introduce the proton transfer mechanism into a Zn-ion hybrid supercapacitor with the ZnSO₄ aqueous electrolyte and functionalized activated carbon cathode materials (FACs). We reveal both experimentally and theoretically an enhanced performance by controlling the micropores structure and hydrogen-containing functional groups (-OH and -NH functions) of the activated carbon materials. The Zn-ion SCs with FACs exhibit a high capacitance of 435 F g^-1 and good stability with 89% capacity retention over 10,000 cycles. Moreover, the proton transfer effect can be further enhanced by introducing extra hydrogen ions in the electrolyte with low pH value. The highest capacitance of 544 F g^-1 is obtained at pH = 3. The proton transfer process tends to take place preferentially on the hydroxyl-groups based on the density functional theory (DFT) calculation. The results would help to develop carbon materials for cheaper and safer Zn-ion hybrid SCs with higher energy.

Block copolymers for supercapacitors, dielectric capacitors and batteries

Block copolymer-based energy storage emerges as an active interdisciplinary research field. This topical review presents a survey of the recent advances in block copolymers for energy storage. In the first section, we introduce the background of electrochemical energy storage and block copolymer thermodynamics. In the second section, we discuss the current understandings of block copolymer chemistry, processing, pore size, and ionic conductivity. In the third section, we summarize the design principles and state-of-the-art applications of block copolymers in three energy storage devices, namely, supercapacitors, dielectric capacitors, and batteries. Lastly, we present our perspectives on future possible breakthroughs and associated challenges that are essential to propel the development of advanced block copolymers for energy storage. We expect the review to encourage innovative studies on integrating block copolymers into energy storage applications.

Full Gamut Wall Tunability from Persistent Micelle Templates via Ex Situ Hydrolysis

The predictive self-assembly of tunable nanostructures is of great utility for broad nanomaterial investigations and applications. The use of equilibrium-based approaches however prevents independent feature size control. Kinetic-controlled methods such as persistent micelle templates (PMTs) overcome this limitation and maintain constant pore size by imposing a large thermodynamic barrier to chain exchange. Thus, the wall thickness is independently adjusted via addition of material precursors to PMTs. Prior PMT demonstrations added water-reactive material precursors directly to aqueous micelle solutions. That approach depletes the thermodynamic barrier to chain exchange and thus limits the amount of material added under PMT-control. Here, an ex situ hydrolysis method is developed for TiO₂ that mitigates this depletion of water and nearly decouples materials chemistry from micelle control. This enables the widest reported PMT range (M:T = 1.6-4.0), spanning the gamut from sparse walls to nearly isolated pores with ≈2 Å precision adjustment. This high-resolution nanomaterial series exhibits monotonic trends where PMT confinement within increasing wall-thickness leads to larger crystallites and an increasing extent of lithiation, reaching Li_0.66 TiO₂ . The increasing extent of lithiation with increasing anatase crystallite dimensions is attributed to the size-dependent strain mismatch of anatase and bronze polymorph mixtures.

The NH4F-induced morphology control of hierarchical CoO@MnO2 core–shell arrays for high performance supercapacitor electrodes

High-performance hierarchical CoO@MnO₂ core–shell arrays with controllable morphology were fabricated by a facile two-step hydrothermal method.

Preparation of ZnFe2O4/α-Fe2O3 Nanocomposites From Sulfuric Acid Leaching Liquor of Jarosite Residue and Their Application in Lithium-Ion Batteries

Recycling Zn and Fe from jarosite residue to produce high value-added products is of great importance to the healthy and sustainable development of zinc industry. In this work, we reported the preparation of ZnFe₂O₄/α-Fe₂O₃ nanocomposites from the leaching liquor of jarosite residue by a facile chemical coprecipitation method followed by heat treatment at 800°C in air. The microstructure of the as-prepared ZnFe₂O₄/α-Fe₂O₃ nanocomposites were characterized by X-ray diffraction (XRD), Mössbauer spectroscopy, scanning transmission electron microscope (STEM), and X-ray photoelectron spectrum (XPS). The results demonstrated that the ZnFe₂O₄/α-Fe₂O₃ composites are composed of interconnected ZnFe₂O₄ and α-Fe₂O₃ nanocrystals with sizes in the range of 20–40 nm. When evaluated as anode material for Li-ion batteries, the ZnFe₂O₄/α-Fe₂O₃ nanocomposites exhibits high lithium storage activity, superior cyclic stability, and good high rate capability. Cyclic voltammetry analysis reveals that surface pseudocapacitive lithium storage has a significant contribution to the total stored charge of the ZnFe₂O₄/α-Fe₂O₃, which accounts for the enhanced lithium storage performance during cycling. The synthesis of ZnFe₂O₄/α-Fe₂O₃ nanocomposites from the leaching liquor of jarosite residue and its successful application in lithium-ion batteries open up new avenues in the fields of healthy and sustainable development of industries.

Design and Mechanisms of Asymmetric Supercapacitors

Ongoing technological advances in diverse fields including portable electronics, transportation, and green energy are often hindered by the insufficient capability of energy-storage devices. By taking advantage of two different electrode materials, asymmetric supercapacitors can extend their operating voltage window beyond the thermodynamic decomposition voltage of electrolytes while enabling a solution to the energy storage limitations of symmetric supercapacitors. This review provides comprehensive knowledge to this field. We first look at the essential energy-storage mechanisms and performance evaluation criteria for asymmetric supercapacitors to understand the wide-ranging research conducted in this area. Then we move to the recent progress made for the design and fabrication of electrode materials and the overall structure of asymmetric supercapacitors in different categories. We also highlight several key scientific challenges and present our perspectives on enhancing the electrochemical performance of future asymmetric supercapacitors.

Convergent Covalent Organic Framework Thin Sheets as Flexible Supercapacitor Electrodes

Flexible supercapacitors in modern electronic equipment require light-weight electrodes, which have a high surface area, precisely integrated redox moieties, and mechanically strong flexible free-standing nature. However, the incorporation of the aforementioned properties into a single electrode remains a great task. Herein, we could overcome these challenges by a facile and scalable synthesis of the convergent covalent organic framework (COF) free-standing flexible thin sheets through solid-state molecular baking strategy. Here, redox-active anthraquinone (Dq) and π-electron-rich anthracene (Da) are judiciously selected as two different linkers in a β-ketoenamine-linked two-dimensional (2D) COF. As a result of precisely integrated anthraquinone moieties, COF thin sheet exhibits redox activity. Meanwhile, π-electron-rich anthracene linker assists to improve the mechanical property of the free-standing thin sheet through the enhancement of noncovalent interaction between crystallites. This binder-free strategy offers the togetherness of crystallinity and flexibility in 2D COF thin sheets. Also, the synthesized porous crystalline convergent COF thin sheets are benefited with crack-free uniform surface and light-weight nature. Further, to demonstrate the practical utility of the material as an electrode in energy-storage systems, we fabricated a solid-state symmetrical flexible COF supercapacitor device using a GRAFOIL peeled carbon tape as the current collector.

Critical Role of the Crystallite Size in Nanostructured Li4Ti5O12 Anodes for Lithium-Ion Batteries

Lithium titanate Li₄Ti₅O₁₂ (LTO) is regarded as a promising alternative to carbon-based anodes in lithium-ion batteries. Despite its stable structural framework, LTO exhibits disadvantages, such as the sluggish lithium-ion diffusion and poor electronic conductivity. To modify the performance of LTO as an anode material, nanosizing constitutes a promising approach and the impact is studied here by a systematical experimental approach. Phase-pure polycrystalline LTO nanoparticles (NPs) with high crystallinity and crystallite sizes ranging from 4 to 12 nm are prepared by an optimized solvothermal protocol and characterized by several state-of-the-art technologies, including high-resolution transmission electron microscopy, X-ray diffraction (XRD), pair distribution function (PDF) analysis, Raman spectroscopy, and X-ray photoelectron spectroscopy. Through a wide array of electrochemical analyses, including charge/discharge profiles, cyclic voltammetry, and electrochemical impedance spectroscopy, a crystallite size of approx. 7 nm is identified as the optimum particle size. Such NPs exhibit as good reversible capacity as the ones with larger crystallite sizes but with a more pronounced interfacial charge storage. By decreasing the crystallite size to about 4 nm, the interfacial charge storage increases remarkably, however resulting in a loss of reversible capacity. An in-depth structural characterization using the PDF obtained from synchrotron XRD data indicates an enrichment in Ti for NPs with the small crystallite sizes, and this Ti-rich structure enables a higher Li storage. The electrochemical characterization confirms this result and furthermore points to a plausible reason as to why a higher Li storage in very small nanoparticles (4 nm) results in a loss in the reversible capacity.

Identifying the Origin and Contribution of Surface Storage in TiO2 (B) Nanotube Electrode by In Situ Dynamic Valence State Monitoring

Fundamental insight into the surface charging mechanism of TiO2 (B) nanomaterials is limited due to the complicated nature of lithiation behavior, as well as the limitations of available characterization tools that can directly probe surface charging process. Here, an in situ approach is reported to monitor the dynamic valence state of TiO2 (B) nanotube electrodes, which utilizes in situ X-ray absorption spectroscopy (XAS) to identify the origin and contribution of surface storage. A real-time correlation is elucidated between the rate-dependent electrode performance and dynamic Ti valence-state change. A continuous Ti valence state change is directly observed through the whole charging/discharging process regardless of charging rates, which proves that along with the well-known non-faradaic reaction, the surface charging process also originates from a faradaic reaction. The quantification of these two surface storage contributions at different charging rates is further realized through in situ dynamic valence state monitoring combined with traditional cyclic voltammetry measurement. The methodology reported here can also be applied to other electrode materials for the real-time probing of valence state change during electrochemical reactions, the quantification of the faradaic and non-faradaic reactions, and the eventual elucidation of electrochemical surface charging mechanisms.