Pseudocapacitive contributions to charge storage in highly ordered mesoporous group V transition metal oxides with iso-oriented layered nanocrystalline domains

A Facile Approach To Improve Electrochemical Capacitance of Carbons by in Situ Electrochemical Oxidation

A facile approach of in situ electrochemical oxidation has been utilized to modify carbons, including activated carbon, mesoporous few-layer carbon, graphite, carbon fiber, and carbon nanotube, which induces oxygen-containing functional groups on its surface and simultaneously enhances its wettability, contributing to the improvement of capacitance. By this approach, the capacitance of commercialized activated carbon is increased by 86% in an acidic electrolyte, reaching 320 F g^-1, of which more than 96% was maintained after 10 000 cyclic tests. The huge improvement stems from electrochemical redox reactions enabled by oxygen-associated groups, which do not adversely affect the porous structure and electrical conductivity. Such improvement will put carbon-based electrochemical capacitors into more practical application areas.

Anion-Based Pseudocapacitance of the Perovskite Library La1- xSr xBO3-δ (B = Fe, Mn, Co)

We have synthesized a library of perovskite oxides with the composition La_{1- x}Sr _xBO_3-δ ( x = 0-1; B = Fe, Mn, Co) to systematically study anion-based pseudocapacitance. The electrochemical capacitance of these materials was evaluated by cyclic voltammetry and galvanostatic charging/discharging in 1 M KOH. We find that greater oxygen vacancy content (δ) upon systematic incorporation of Sr²⁺ linearly increases the surface-normalized capacity with a slope controlled by the B-site element. La_0.2Sr_0.8MnO_2.7 exhibited the highest specific capacitance of 492 F g^-1 at 5 mV s^-1 relative to the Fe and Co oxides. In addition, the first all-perovskite asymmetric pseudocapacitor has been successfully constructed and characterized in neutral and alkaline aqueous electrolytes. We demonstrate that the asymmetric pseudocapacitor cell voltage can be increased by widening the difference between the B-site transition metal redox potentials in each electrode resulting in a maximum voltage window of 2.0 V in 1 M KOH. Among the three pairs of asymmetric pseudocapacitors constructed from SrCoO_2.7, La_0.2Sr_0.8MnO_2.7, and brownmillerite (BM)-Sr₂Fe₂O₅, the BM-Sr₂Fe₂O₅//SrCoO_2.7 combination performed the best with a high energy density of 31 Wh kg^-1 at 450 W kg^-1 and power density of 10 000 W kg^-1 at 28 Wh kg^-1.

Evidence for Fast Lithium-Ion Diffusion and Charge-Transfer Reactions in Amorphous TiO x Nanotubes: Insights for High-Rate Electrochemical Energy Storage

The charge-storage kinetics of amorphous TiO _x nanotube electrodes formed by anodizing three-dimensional porous Ti scaffolds are reported. The resultant electrodes demonstrated not only superior storage capacities and rate capability to anatase TiO _x nanotube electrodes but also improved areal capacities (324 μAh cm^-2 at 50 μA cm^-2 and 182 μAh cm^-2 at 5 mA cm^-2) and cycling stability (over 2000 cycles) over previously reported TiO _x nanotube electrodes using planar current collectors. Amorphous TiO _x exhibits very different electrochemical storage behavior from its anatase counterpart as the majority of its storage capacity can be attributed to capacitive-like processes with more than 74 and 95% relative contributions being attained at 0.05 and 1 mV s^-1, respectively. The kinetic analysis revealed that the insertion/extraction process of Li⁺ in amorphous TiO _x is significantly faster than in anatase structure and controlled by both solid-state diffusion and interfacial charge-transfer kinetics. It is concluded that the large capacitive contribution in amorphous TiO _x originates from its highly defective and loosely packed structure and lack of long-range ordering, which facilitate not only a significantly faster Li⁺ diffusion process (diffusion coefficients of 2 × 10^-14 to 3 × 10^-13 cm² s^-1) but also more facile interfacial charge-transfer kinetics than anatase TiO _x.

T-Nb2O5 nanoparticle enabled pseudocapacitance with fast Li-ion intercalation

Orthorhombic Nb₂O₅ (T-Nb₂O₅) nanocrystallites are successfully fabricated through an evaporation induced self-assembly (EISA) method guided by a commercialised triblock copolymer - Pluronic F127. We demonstrate a morphology transition of T-Nb₂O₅ from continuous porous nanofilms to monodisperse nanoparticles by changing the content of Pluronic F127. The electrochemical results show that the optimized monodisperse Nb-2 with a particle size of 20 nm achieves premier Li-ion intercalation kinetics and higher rate capability than mesoporous T-Nb₂O₅ nanofilms. Nb-2 presents an initial intercalation capacity of 528 and 451 C g^-1 at current densities of 0.5 and 5 A g^-1 and exhibited a stable capacity of 499 C g^-1 after 300 charge/discharge cycles and 380 C g^-1 after 1000 cycles, respectively. We would expect this copolymer guided monodispersion of T-Nb₂O₅ nanoparticles with high Li⁺ intercalation performance to open up a new window for novel EES technologies.

Advanced Energy Storage Devices: Basic Principles, Analytical Methods, and Rational Materials Design

Tremendous efforts have been dedicated into the development of high-performance energy storage devices with nanoscale design and hybrid approaches. The boundary between the electrochemical capacitors and batteries becomes less distinctive. The same material may display capacitive or battery-like behavior depending on the electrode design and the charge storage guest ions. Therefore, the underlying mechanisms and the electrochemical processes occurring upon charge storage may be confusing for researchers who are new to the field as well as some of the chemists and material scientists already in the field. This review provides fundamentals of the similarities and differences between electrochemical capacitors and batteries from kinetic and material point of view. Basic techniques and analysis methods to distinguish the capacitive and battery-like behavior are discussed. Furthermore, guidelines for material selection, the state-of-the-art materials, and the electrode design rules to advanced electrode are proposed.

Nonaqueous Hybrid Lithium-Ion and Sodium-Ion Capacitors

Hybrid metal-ion capacitors (MICs) (M stands for Li or Na) are designed to deliver high energy density, rapid energy delivery, and long lifespan. The devices are composed of a battery anode and a supercapacitor cathode, and thus become a tradeoff between batteries and supercapacitors. In the past two decades, tremendous efforts have been put into the search for suitable electrode materials to overcome the kinetic imbalance between the battery-type anode and the capacitor-type cathode. Recently, some transition-metal compounds have been found to show pseudocapacitive characteristics in a nonaqueous electrolyte, which makes them interesting high-rate candidates for hybrid MIC anodes. Here, the material design strategies in Li-ion and Na-ion capacitors are summarized, with a focus on pseudocapacitive oxide anodes (Nb₂ O₅ , MoO₃ , etc.), which provide a new opportunity to obtain a higher power density of the hybrid devices. The application of Mxene as an anode material of MICs is also discussed. A perspective to the future research of MICs toward practical applications is proposed to close.

Hydrothermal Preparation, Crystal Chemistry, and Redox Properties of Iron Muscovite Clay

The development of functional materials based on Earth-abundant, environmentally benign compositions is critical for ensuring their commercial viability and sustainable production. Here we present an investigation into the crystal chemistry and electrochemical properties of the muscovite clay KFe_2.75Si_3.25O₁₀(OH)₂. We first report a low-temperature hydrothermal reaction that allows for a significant degree of control over sample crystallinity, particle morphology, and cation distribution through the lattice. A complex sequence of stacking faults is identified and characterized using a combination of Mössbauer spectroscopy and total scattering neutron experiments. We then show the existence of a reversible electrochemical process using galvanostatic cycling with complementary cyclic voltammetry suggesting that the redox activity occurs primarily on the surface of the particles. We conclude by determining that the ability to (de)intercalate Li ions from the material is hindered by the strong negative charge on the transition metal silicate layers, which prevents the displacement of the interlayer K ions. This work calls attention to a hugely Earth-abundant family of minerals that possesses useful electrochemical properties that warrant further exploration.

Tuning Porosity and Surface Area in Mesoporous Silicon for Application in Li-Ion Battery Electrodes

This work aims to improve the poor cycle lifetime of silicon-based anodes for Li-ion batteries by tuning microstructural parameters such as pore size, pore volume, and specific surface area in chemically synthesized mesoporous silicon. Here we have specifically produced two different mesoporous silicon samples from the magnesiothermic reduction of ordered mesoporous silica in either argon or forming gas. In situ X-ray diffraction studies indicate that samples made in Ar proceed through a Mg₂Si intermediate, and this results in samples with larger pores (diameter ≈ 90 nm), modest total porosity (34%), and modest specific surface area (50 m² g^-1). Reduction in forming gas, by contrast, results in direct conversion of silica to silicon, and this produces samples with smaller pores (diameter ≈ 40 nm), higher porosity (41%), and a larger specific surface area (70 m² g^-1). The material with smaller pores outperforms the one with larger pores, delivering a capacity of 1121 mAh g^-1 at 10 A g^-1 and retains 1292 mAh g^-1 at 5 A g^-1 after 500 cycles. For comparison, the sample with larger pores delivers a capacity of 731 mAh g^-1 at 10 A g^-1 and retains 845 mAh g^-1 at 5 A g^-1 after 500 cycles. The dependence of capacity retention and charge storage kinetics on the nanoscale architecture clearly suggests that these microstructural parameters significantly impact the performance of mesoporous alloy type anodes. Our work is therefore expected to contribute to the design and synthesis of optimal mesoporous architectures for advanced Li-ion battery anodes.

Oxygen vacancies enhance pseudocapacitive charge storage properties of MoO3-x

The short charging times and high power capabilities associated with capacitive energy storage make this approach an attractive alternative to batteries. One limitation of electrochemical capacitors is their low energy density and for this reason, there is widespread interest in pseudocapacitive materials that use Faradaic reactions to store charge. One candidate pseudocapacitive material is orthorhombic MoO₃ (α-MoO₃), a layered compound with a high theoretical capacity for lithium (279 mA h g^-1 or 1,005 C g^-1). Here, we report on the properties of reduced α-MoO_3-x(R-MoO_3-x) and compare it with fully oxidized α-MoO₃ (F-MoO₃). The introduction of oxygen vacancies leads to a larger interlayer spacing that promotes faster charge storage kinetics and enables the α-MoO₃ structure to be retained during the insertion and removal of Li ions. The higher specific capacity of the R-MoO_3-x is attributed to the reversible formation of a significant amount of Mo⁴⁺ following lithiation. This study underscores the potential importance of incorporating oxygen vacancies into transition metal oxides as a strategy for increasing the charge storage kinetics of redox-active materials.

Long-Lasting Nb2O5-Based Nanocomposite Materials for Li-Ion Storage

Advanced nanostructured hybrid materials can help us overcome the electrochemical performance limitations of current energy storage devices. In this study, three-dimensional porous carbon nanowebs (3D-CNWs) with numerous included orthorhombic Nb₂O₅ (T-Nb₂O₅) nanoparticles were fabricated using a microbe-derived nanostructure. The 3D-CNW/T-Nb₂O₅ nanocomposites showed an exceptionally stable long-term cycling performance over 70 000 cycles, a high reversible capacity of ∼125 mA h g^-1, and fast Li-ion storage kinetics in a coin-type two-electrode system using Li metal. In addition, energy storage devices based on the above nanocomposites achieved a high specific energy of ∼80 W h kg^-1 together with a high specific power of ∼5300 W kg^-1 and outstanding cycling performance with ∼80% capacitance retention after 35 000 cycles.

Solid state microwave synthesis of highly crystalline ordered mesoporous hausmannite Mn3O4films

Microwave calcination of ordered micelle templated manganese carbonate films leads to highly crystalline, ordered mesoporous manganese oxide, while similar temperatures in a furnace lead to disordered, amorphous manganese oxide.

Aqueous tantalum polyoxometalate reactivity with peroxide

Peroxide ligation promotes linking of Ta-polyoxometalates, the solution speciation elucidated by small and wide angle and total X-ray scattering.

Pseudocapacitive materials for electrochemical capacitors: from rational synthesis to capacitance optimization

Among various energy-storage devices, electrochemical capacitors (ECs) are prominent power provision but show relatively low energy density. One way to increase the energy density of ECs is to move from carbon-based electric double-layer capacitors to pseudocapacitors, which manifest much higher capacitance. However, compared with carbon materials, the pseudocapacitive electrodes suffer from high resistance for electron and/or ion transfer, significantly restricting their capacity, rate capability and cyclability. Rational design of electrode materials offers opportunities to optimize their electrochemical performance, leading to devices with high energy density while maintaining high power density. This paper reviews the different approaches of electrodes striving to advance the energy and power density of ECs.

Modulation of Crystal Surface and Lattice by Doping: Achieving Ultrafast Metal-Ion Insertion in Anatase TiO2

We report that an ultrafast kinetics of reversible metal-ion insertion can be realized in anatase titanium dioxide (TiO₂). Niobium ions (Nb⁵⁺) were carefully chosen to dope and drive anatase TiO₂ into very thin nanosheets standing perpendicularly onto transparent conductive electrode (TCE) and simultaneously construct TiO₂ with an ion-conducting surface together with expanded ion diffusion channels, which enabled ultrafast metal ions to diffuse across the electrolyte/solid interface and into the bulk of TiO₂. To demonstrate the superior metal-ion insertion rate, the electrochromic features induced by ion intercalation were examined, which exhibited the best color switching speed of 4.82 s for coloration and 0.91 s for bleaching among all reported nanosized TiO₂ devices. When performed as the anode for the secondary battery, the modified TiO₂ was capable to deliver a highly reversible capacity of 61.2 mAh/g at an ultrahigh specific current rate of 60 C (10.2 A/g). This fast metal-ion insertion behavior was systematically investigated by the well-controlled electrochemical approaches, which quantitatively revealed both the enhanced surface kinetics and bulk ion diffusion rate. Our study could provide a facile methodology to modulate the ion diffusion kinetics for metal oxides.

Multidimensional materials and device architectures for future hybrid energy storage

Electrical energy storage plays a vital role in daily life due to our dependence on numerous portable electronic devices. Moreover, with the continued miniaturization of electronics, integration of wireless devices into our homes and clothes and the widely anticipated ‘Internet of Things', there are intensive efforts to develop miniature yet powerful electrical energy storage devices. This review addresses the cutting edge of electrical energy storage technology, outlining approaches to overcome current limitations and providing future research directions towards the next generation of electrical energy storage devices whose characteristics represent a true hybridization of batteries and electrochemical capacitors.

With the continued miniaturization of electronics, there are increasing efforts to engineer small, powerful energy storage devices. Here the authors review the cutting edge of this rapidly developing field, highlighting the most promising materials and architectures for our future energy storage requirements.

Partially Single-Crystalline Mesoporous Nb2 O5 Nanosheets in between Graphene for Ultrafast Sodium Storage

Partially single-crystalline mesoporous Nb2 O5 nanosheets with orthorhombic structure in between graphene are scalably fabricated via a simple nanocasting method. The well-designed architecture provides numerous open and short channels for fast diffusion of sodium ion and good electronic conductivity, resulting in an enhanced electrochemical performance and a favorable high-rate behavior for sodium storage.

Electrochemical Energy Storage Applications of CVD Grown Niobium Oxide Thin Films

We report here on the controlled synthesis, characterization, and electrochemical properties of different polymorphs of niobium pentoxide grown by CVD of new single-source precursors. Nb2O5 films deposited at different temperatures showed systematic phase evolution from low-temperature tetragonal (TT-Nb2O5, T-Nb2O5) to high temperature monoclinic modifications (H-Nb2O5). Optimization of the precursor flux and substrate temperature enabled phase-selective growth of Nb2O5 nanorods and films on conductive mesoporous biomorphic carbon matrices (BioC). Nb2O5 thin films deposited on monolithic BioC scaffolds produced composite materials integrating the high surface area and conductivity of the carbonaceous matrix with the intrinsically high capacitance of nanostructured niobium oxide. Heterojunctions in Nb2O5/BioC composites were found to be beneficial in electrochemical capacitance. Electrochemical characterization of Nb2O5/BioC composites showed that small amounts of Nb2O5 (as low as 5%) in conjunction with BioCarbon resulted in a 7-fold increase in the electrode capacitance, from 15 to 104 F g(-1), while imparting good cycling stability, making these materials ideally suited for electrochemical energy storage applications.

Mesoporous LixMn2O4 Thin Film Cathodes for Lithium-Ion Pseudocapacitors

Charge storage devices with high energy density and enhanced rate capabilities are highly sought after in today's mobile world. Although several high-rate pseudocapacitive anode materials have been reported, cathode materials operating in a high potential range versus lithium metal are much less common. Here, we present a nanostructured version of the well-known cathode material, LiMn2O4. The reduction in lithium-ion diffusion lengths and improvement in rate capabilities is realized through a combination of nanocrystallinity and the formation of a 3-D porous framework. Materials were fabricated from nanoporous Mn3O4 films made by block copolymer templating of preformed nanocrystals. The nanoporous Mn3O4 was then converted via solid-state reaction with LiOH to nanoporous LixMn2O4 (1 < x < 2). The resulting films had a wall thickness of ∼15 nm, which is small enough to be impacted by inactive surface sites. As a consequence, capacity was reduced by about half compared to bulk LiMn2O4, but both charge and discharge kinetics as well as cycling stability were improved significantly. Kinetic analysis of the redox reactions was used to verify the pseudocapacitive mechanisms of charge storage and establish the feasibility of using nanoporous LixMn2O4 as a cathode in lithium-ion devices based on pseudocapacitive charge storage.

High-Rate Intercalation without Nanostructuring in Metastable Nb2O5 Bronze Phases

Nanostructuring and nanosizing have been widely employed to increase the rate capability in a variety of energy storage materials. While nanoprocessing is required for many materials, we show here that both the capacity and rate performance of low-temperature bronze-phase TT- and T-polymorphs of Nb2O5 are inherent properties of the bulk crystal structure. Their unique "room-and-pillar" NbO6/NbO7 framework structure provides a stable host for lithium intercalation; bond valence sum mapping exposes the degenerate diffusion pathways in the sites (rooms) surrounding the oxygen pillars of this complex structure. Electrochemical analysis of thick films of micrometer-sized, insulating niobia particles indicates that the capacity of the T-phase, measured over a fixed potential window, is limited only by the Ohmic drop up to at least 60C (12.1 A·g(-1)), while the higher temperature (Wadsley-Roth, crystallographic shear structure) H-phase shows high intercalation capacity (>200 mA·h·g(-1)) but only at moderate rates. High-resolution (6/7)Li solid-state nuclear magnetic resonance (NMR) spectroscopy of T-Nb2O5 revealed two distinct spin reservoirs, a small initial rigid population and a majority-component mobile distribution of lithium. Variable-temperature NMR showed lithium dynamics for the majority lithium characterized by very low activation energies of 58(2)-98(1) meV. The fast rate, high density, good gravimetric capacity, excellent capacity retention, and safety features of bulk, insulating Nb2O5 synthesized in a single step at relatively low temperatures suggest that this material not only is structurally and electronically exceptional but merits consideration for a range of further applications. In addition, the realization of high rate performance without nanostructuring in a complex insulating oxide expands the field for battery material exploration beyond conventional strategies and structural motifs.

A mini review of designed mesoporous materials for energy-storage applications: from electric double-layer capacitors to hybrid supercapacitors

In recent years, porous materials have attracted significant attention in various research fields because of their structural merits. In particular, well-designed mesoporous structures with two- or three-dimensionally interconnected pores have been recognized as electrode materials of particular interest for achieving high-performance electrochemical capacitors (ECs). In this mini review, recent progress in the design of mesoporous electrode materials for ECs, from electric double-layer capacitors (EDLCs) and pseudocapacitors (PCs) to hybrid supercapacitors (HSCs), and research challenges for the development of new mesoporous electrode materials has been discussed.

Electrochemical capacitors: mechanism, materials, systems, characterization and applications

This article reviews the latest progress in electrochemical capacitors (i.e.supercapacitors), including materials, charge storage mechanisms, systems, characterization and applications.

Superior high-rate capability of Na3(VO0.5)2(PO4)2F2 nanoparticles embedded in porous graphene through the pseudocapacitive effect

Na₃(VO_0.5)₂(PO₄)₂F₂ nanoparticles embedded in porous graphene as the cathode material for sodium-ion batteries can show superior high-rate capability through the pseudocapacitive effect.

Preparation and performance of novel enhanced electrochemical capacitors based on graphene constructed self-assembled Co3O4 microspheres

Transition metal oxide nanostructures is one of the current investigation focuses for supercapacitors.

General fabrication of mesoporous Nb2O5 nanobelts for lithium ion battery anodes

Mesoporous Nb₂O₅ NBs with good crystallinity and large specific area have been synthesized by sequential annealing of solid Nb₂O₅ NBs in NH₃ and air,exhibiting an enhanced capacity and rate capability than that of solid Nb₂O₅ NBs.

Superelastic Few-Layer Carbon Foam Made from Natural Cotton for All-Solid-State Electrochemical Capacitors

Flexible/stretchable devices for energy storage are essential for future wearable and flexible electronics. Electrochemical capacitors (ECs) are an important technology for supplement batteries in the energy storage and harvesting field, but they are limited by relatively low energy density. Herein, we report a superelastic foam consisting of few-layer carbon nanowalls made from natural cotton as a good scaffold to growth conductive polymer polyaniline for stretchable, lightweight, and flexible all-solid-state ECs. As-prepared superelastic bulk tubular carbon foam (surface area ∼950 m(2)/g) can withstand >90% repeated compression cycling and support >45,000 times its own weight but no damage. The flexible device has a high specific capacitance of 510 F g(-1), a specific energy of 25.5 Wh kg(-1) and a power density of 28.5 kW kg(-1) in weight of the total electrode materials and withstands 5,000 charging/discharging cycles.

Hierarchical Nanostructured WO3 with Biomimetic Proton Channels and Mixed Ionic-Electronic Conductivity for Electrochemical Energy Storage

Protein channels in biologic systems can effectively transport ions such as proton (H(+)), sodium (Na(+)), and calcium (Ca(+)) ions. However, none of such channels is able to conduct electrons. Inspired by the biologic proton channels, we report a novel hierarchical nanostructured hydrous hexagonal WO3 (h-WO3) which can conduct both protons and electrons. This mixed protonic-electronic conductor (MPEC) can be synthesized by a facile single-step hydrothermal reaction at low temperature, which results in a three-dimensional nanostructure self-assembled from h-WO3 nanorods. Such a unique h-WO3 contains biomimetic proton channels where single-file water chains embedded within the electron-conducting matrix, which is critical for fast electrokinetics. The mixed conductivities, high redox capacitance, and structural robustness afford the h-WO3 with unprecedented electrochemical performance, including high capacitance, fast charge/discharge capability, and very long cycling life (>50,000 cycles without capacitance decay), thus providing a new platform for a broad range of applications.

Fabrication of Nb2O5 nanosheets for high-rate lithium ion storage applications

Nb₂O₅ nanosheets are successfully synthesized through a facile hydrothermal reaction and followed heating treatment in air. The structural characterization reveals that the thickness of these sheets is around 50 nm and the length of sheets is 500 ~ 800 nm. Such a unique two dimensional structure enables the nanosheet electrode with superior performance during the charge-discharge process, such as high specific capacity (~184 mAh·g⁻¹) and rate capability. Even at a current density of 1 A·g⁻¹, the nanosheet electrode still exhibits a specific capacity of ~90 mAh·g⁻¹. These results suggest the Nb₂O₅ nanosheet is a promising candidate for high-rate lithium ion storage applications.

Nanoscale spinel LiFeTiO4 for intercalation pseudocapacitive Li+ storage

Nanosized spinel materials were found to allow a large-amount of Li⁺ intercalation storage without compromising kinetics.

Hierarchically superstructured prussian blue analogues: spontaneous assembly synthesis and applications as pseudocapacitive materials

Hierarchically superstructured Prussian blue analogues (hexacyanoferrate, M=Ni(II) , Co(II) and Cu(II) ) are synthesized through a spontaneous assembly technique. In sharp contrast to macroporous-only Prussian blue analogues, the hierarchically superstructured porous Prussian blue materials are demonstrated to possess a high capacitance, which is similar to those of the conventional hybrid graphene/MnO2 nanostructured textiles. Because sodium or potassium ions are involved in energy storage processes, more environmentally neutral electrolytes can be utilized, making the superstructured porous Prussian blue analogues a great contender for applications as high-performance pseudocapacitors.

Nb2O5/graphene nanocomposites for electrochemical energy storage

Here we report the synthesis of Nb₂O₅/graphene nanocomposites, through a simple hydrothermal method, with Nb₂O₅ nanoparticles anchored on reduced graphene oxide sheets.

Fast lithium-ion storage of Nb2O5 nanocrystals in situ grown on carbon nanotubes for high-performance asymmetric supercapacitors

Nanocomposites of Nb₂O₅ NCs in situ grown on CNTs are successfully developed with excellent rate capability, leading to the successful fabrication of asymmetric supercapacitors with high energy and power density and long-term cycling stability.

Mesoscopically structured nanocrystalline metal oxide thin films

This review describes the main successful strategies that are used to grow mesostructured nanocrystalline metal oxide and SiO₂ films via deposition of sol-gel derived solutions. In addition to the typical physicochemical forces to be considered during crystallization, mesoporous thin films are also affected by the substrate-film relationship and the mesostructure. The substrate can influence the crystallization temperature and the obtained crystallographic orientation due to the interfacial energies and the lattice mismatch. The mesostructure can influence the crystallite orientation, and affects nucleation and growth behavior due to the wall thickness and pore curvature. Three main methods are presented and discussed: templated mesoporosity followed by thermally induced crystallization, mesostructuration of already crystallized metal oxide nanobuilding units and substrate-directed crystallization with an emphasis on very recent results concerning epitaxially grown piezoelectric structured α-quartz films via crystallization of amorphous structured SiO₂ thin films.

Advanced hybrid supercapacitor based on a mesoporous niobium pentoxide/carbon as high-performance anode

Recently, hybrid supercapacitors (HSCs), which combine the use of battery and supercapacitor, have been extensively studied in order to satisfy increasing demands for large energy density and high power capability in energy-storage devices. For this purpose, the requirement for anode materials that provide enhanced charge storage sites (high capacity) and accommodate fast charge transport (high rate capability) has increased. Herein, therefore, a preparation of nanocomposite as anode material is presented and an advanced HSC using it is thoroughly analyzed. The HSC comprises a mesoporous Nb2O5/carbon (m-Nb2O5-C) nanocomposite anode synthesized by a simple one-pot method using a block copolymer assisted self-assembly and commercial activated carbon (MSP-20) cathode under organic electrolyte. The m-Nb2O5-C anode provides high specific capacity with outstanding rate performance and cyclability, mainly stemming from its enhanced pseudocapacitive behavior through introduction of a carbon-coated mesostructure within a voltage range from 3.0 to 1.1 V (vs Li/Li(+)). The HSC using the m-Nb2O5-C anode and MSP-20 cathode exhibits excellent energy and power densities (74 W h kg(-1) and 18,510 W kg(-1)), with advanced cycle life (capacity retention: ∼90% at 1000 mA g(-1) after 1000 cycles) within potential range from 1.0 to 3.5 V. In particular, we note that the highest power density (18,510 W kg(-1)) of HSC is achieved at 15 W h kg(-1), which is the highest level among similar HSC systems previously reported. With further study, the HSCs developed in this work could be a next-generation energy-storage device, bridging the performance gap between conventional batteries and supercapacitors.

Self-Assembled α-Fe2O3 mesocrystals/graphene nanohybrid for enhanced electrochemical capacitors

Self-assembled α-Fe2O3 mesocrystals/graphene nanohybrids have been successfully synthesized and have a unique mesocrystal porous structure, a large specific surface area, and high conductivity. Mesocrystal structures have recently attracted unparalleled attention owing to their promising application in energy storage as electrochemical capacitors. However, mesocrystal/graphene nanohybrids and their growth mechanism have not been clearly investigated. Here we show a facile fabrication of short rod-like α-Fe2O3 mesocrystals/graphene nanohybrids by self-assembly of FeOOH nanorods as the primary building blocks on graphene under hydrothermal conditions, accompanied and promoted by concomitant phase transition from FeOOH to α-Fe2O3. A systematic study of the formation mechanism is also presented. The galvanostatic charge/discharge curve shows a superior specific capacitance of the as-prepared α-Fe2O3 mesocrystals/graphene nanohybrid (based on total mass of active materials), which is 306.9 F g(-1) at 3 A g(-1) in the aqueous electrolyte under voltage ranges of up to 1 V. The nanohybrid with unique sufficient porous structure and high electrical conductivity allows for effective ion and charge transport in the whole electrode. Even at a high discharge current density of 10 A g(-1), the enhanced ion and charge transport still yields a higher capacitance (98.2 F g(-1)), exhibiting enhanced rate capability. The α-Fe2O3 mesocrystal/graphene nanohybrid electrode also demonstrates excellent cyclic performance, which is superior to previously reported graphene-based hematite electrode, suggesting it is highly stable as an electrochemical capacitor.

Polymeric micelle assembly for preparation of large-sized mesoporous metal oxides with various compositions

Here we report the synthesis of mesoporous metal oxide materials with various compositions by assembly of spherical polymeric micelles consisting of triblock copolymer poly(styrene-b-2-vinyl pyridine-b-ethylene oxide) (PS-b-PVP-b-PEO) with three chemically distinct units. The PVP block interacts strongly with the inorganic precursors for the target compositions. The hydrophobic PS block is kinetically frozen in the precursor solutions, enabling the spherical micelles to remain in a stable form. The frozen PS cores serve as templates for preparing robust mesoporous materials. The PEO corona helps the micelles to stay well dispersed in the precursor solutions, which plays a key role in the orderly arrangement of the micelles during solvent evaporation. This approach is based on assembly of the stable micelles using a simple, highly reproducible method and is widely applicable toward numerous compositions that are difficult for the formation of mesoporous structures.