The science and technology of ultracapacitors are reviewed for a number of electrode materials, including carbon, mixed metal oxides, and conducting polymers. More work has been done using microporous carbons than with the other materials and most of the commercially available devices use carbon electrodes and an organic electrolytes. The energy density of these devices is 3¯5 Wh/kg with a power density of 300¯500 W/kg for high efficiency (90¯95%) charge/discharges. Projections of future developments using carbon indicate that energy densities of 10 Wh/kg or higher are likely with power densities of 1¯2 kW/kg. A key problem in the fabrication of these advanced devices is the bonding of the thin electrodes to a current collector such the contact resistance is less than 0.1 cm2. Special attention is given in the paper to comparing the power density characteristics of ultracapacitors and batteries. The comparisons should be made at the same charge/discharge efficiency.

Ultracapacitors: Why, How, and Where is the Technology

In this Review, we aim to provide an updated summary of the research related to hollow micro- and nanostructures, covering both their synthesis and their applications. After a brief introduction to the definition and classification of the hollow micro-/nanostructures, we discuss various synthetic strategies that can be grouped into three major categories, including hard templating, soft templating, and self-templating synthesis. For both hard and soft templating strategies, we focus on how different types of templates are generated and then used for creating hollow structures. At the end of each section, the structural and morphological control over the product is discussed. For the self-templating strategy, we survey a number of unconventional synthetic methods, such as surface-protected etching, Ostwald ripening, the Kirkendall effect, and galvanic replacement. We then discuss the unique properties and niche applications of the hollow structures in diverse fields, including micro-/nanocontainers and rea...

Synthesis, Properties, and Applications of Hollow Micro-/Nanostructures

Flexible and wearable electronics are attracting wide attention due to their potential applications in wearable human health monitoring and care systems. Carbon materials have combined superiorities such as good electrical conductivity, intrinsic and structural flexibility, light weight, high chemical and thermal stability, ease of chemical functionalization, as well as potential mass production, enabling them to be promising candidate materials for flexible and wearable electronics. Consequently, great efforts are devoted to the controlled fabrication of carbon materials with rationally designed structures for applications in next-generation electronics. Herein, the latest advances in the rational design and controlled fabrication of carbon materials toward applications in flexible and wearable electronics are reviewed. Various carbon materials (carbon nanotubes, graphene, natural-biomaterial-derived carbon, etc.) with controlled micro/nanostructures and designed macroscopic morphologies for high-performance flexible electronics are introduced. The fabrication strategies, working mechanism, performance, and applications of carbon-based flexible devices are reviewed and discussed, including strain/pressure sensors, temperature/humidity sensors, electrochemical sensors, flexible conductive electrodes/wires, and flexible power devices. Furthermore, the integration of multiple devices toward multifunctional wearable systems is briefly reviewed. Finally, the existing challenges and future opportunities in this field are summarized.

Advanced Carbon for Flexible and Wearable Electronics.

Metal–organic frameworks (MOFs) have obtained increasing attention as a kind of novel electrode material for energy storage devices. Yet low capacity in most MOFs largely thwarts their application. In this study, an effective strategy was developed to improve the conductivity of MOFs by partially substituting Ni2+ in the Ni-MOF with Co2+ or Zn2+. The mixed-metal organic frameworks (M-MOFs) showed excellent electrochemical performance, which is attributed not only to the favorable paths for charge transport due to the presence of free pores, but also to the raised electrochemical double-layer capacitance (EDLC) at the enlarged specific surface area of the material. Meanwhile, the cycling stability of the assembled hybrid supercapacitors (M-MOFs//CNTs–COOH) is enhanced due to the alleviation of phase transformation during electrochemical cycling tests. More interestingly, the Co/Ni-MOF//CNTs–COOH also exhibited an excellent energy density (49.5 W h kg−1) and power density (1450 W kg−1) simultaneously. These values demonstrated the better performance of all the MOF materials in supercapacitors at present. In addition to broadening the application of MOFs, our study may open a new avenue for bridging the performance gap between batteries and supercapacitors.

Mixed-metallic MOF based electrode materials for high performance hybrid supercapacitors

Materials engineering plays a key role in the field of electrochemical energy storage, and considerable efforts have been made in recent years to fulfil the future requirements of electrochemical energy storage using novel functional electrode materials. Among the transition metal oxides, ternary metal oxides possess multiple oxidation states that enable multiple redox reactions. They have been reported to exhibit a higher supercapacitive performance than single component metal oxides, and seem to be a group of the most promising and low cost materials for pseudo-capacitors. This paper presents a state-of-the-art review on the research progress of developing ternary oxide nanostructures for supercapacitors, with the focus on the synthesis of one-dimensional (1-D), two-dimensional (2-D) and three-dimensional (3-D) nanostructures and their potential applications in energy storage devices. The remaining challenges toward the rational design of ternary oxide nanostructured electrodes for next-generation supercapacitors are also proposed.

Ternary oxide nanostructured materials for supercapacitors: a review

ZnCo2O4 nanowire cluster arrays (NWCAs) were directly grown on Ni foam via a facile hydrothermal method. The resulting products were analyzed by using X-ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The ZnCo2O4 NWCAs on Ni foam were directly used as integrated electrodes for supercapacitors and exhibited a high specific capacitance of 4.05 F cm−2 at 20 mA cm−2 (1620 F g−1 at 8 A g−1) in 3 M KOH aqueous solution, and an excellent cycling ability at various current densities up to 100 mA cm−2 (40 A g−1); 90% of the initial capacitance remained after 6000 cycles. Moreover, the asymmetric supercapacitor had a high energy density of 41.00 W h kg−1 at a power density of 384 W kg−1 and 16.63 W h kg−1 at a high power density of 2561 W kg−1. Such excellent rate properties and superior cycling life suggest that ZnCo2O4 NWCAs can not only be applied in high energy density fields, but also used in high power density applications.

Facile synthesis of ZnCo2O4 nanowire cluster arrays on Ni foam for high-performance asymmetric supercapacitors

In this report, ZnWO4 nanowall arrays (NWAs) were grown on Ni foam by a hydrothermal route and investigated for application in supercapacitors for the first time. The resulting NWAs were analyzed by using X-ray diffraction spectroscopy, scanning electron microscopy, and transmission electron microscopy. ZnWO4 NWAs exhibited high electrochemical property with a specific capacitance of 2.5 F cm−2 at a charge and discharge current density of 20 mA cm−2 (1250 F g−1 at current density of 10 A g−1). Furthermore, they showed an excellent cycling ability at different current densities up to 100 mA cm−2, and 92% (2.3 F cm−2) of the initial capacitance remained after 4000 cycles. The open network structure consisting of interconnected ZnWO4 NWAs directly grown on current collectors is advantageous for electron transport and electrolyte diffusion which can facilitate the electrochemical reaction. Our work not only demonstrated a facile hydrothermal method for ZnWO4 NWAs directly grown on Ni foam, but also revealed ZnWO4 to be a promising electrode for high-property supercapacitors.

Facile synthesis of ZnWO4 nanowall arrays on Ni foam for high performance supercapacitors

Highly sensitive and fast responding humidity sensors were fabricated based on Sb-doped ZnSnO3 fine nanoparticles, which were synthesized via a dual-hydrolysis-assisted liquid precipitation reaction and subsequent hydrothermal procedure. The nanoparticles were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and UV-visible spectrophotometry. The results demonstrated that ZnSnO3 nanocubes evolved into Sb-doped ZnSnO3 nanoparticles with doping by Sb. This evolution facilitated their application in humidity sensing. At room temperature, the resistance of the humidity sensors based on Sb-doped ZnSnO3 at 30% relative humidity (RH) was 130 times greater than that in air with 85% RH. The response and recovery times were 7.5 s and 33.6 s, respectively, when the sensors were switched between 30% and 85% RH. These results are much better than those reported so far. The present results may present new opportunities for the practical application of high-performance humidity sensors at room temperature.

Highly sensitive humidity sensors based on Sb-doped ZnSnO3 nanoparticles with very small sizes

In this study, we investigated the nanocomposites composed of graphene, SnOx and carbon fibers (CFs) for humidity sensing applications. The composites were obtained by an electrospinning method followed by addition of graphene. SnOx/CFs uniformly dispersed into graphene nanosheets. SnOx and graphene were proved to be promising sensing materials. The amorphous carbon fibers could supply more channels for transportation of protons or electrons. Therefore, the composites exhibited high humidity sensing performance. The resistance of the sensor increases two times with decreasing relative humidity from 55% to 30%. The sensitivity to humidity increased by almost 100% after adding graphene to SnOx/CF nanocomposites. The response and recovery times were 8 s and 6 s at 30% RH, respectively. The results demonstrated that rational design of nanocomposites with graphene could be a favorable strategy to improve humidity sensing properties.

Humidity sensors based on graphene/SnOx/CF nanocomposites

In this paper, ZnFe2O4 nanotubes were fabricated by a simple electrospinning method. The scanning electron microscopy images show that the obtained ZnFe2O4 nanotubes had a hollow structure and had a diameter of about 80–100 nm, and wall thickness of 15–20 nm. A humidity sensor based on as-prepared nanotubes was fabricated and exhibited good performance, high sensitivity, fast response and recovery and good stability. When RH switched between 75% and 35%, the resistance changed from 2.63 × 107 Ω to 1.65 × 109 Ω. The corresponding sensitivity was calculated to be 62.7. The response and recovery time was 11 s and 5.6 s respectively. The sensor also showed good stability and repeatability. These results indicated that the synthesized ZnFe2O4 nanotubes were promising candidates for high-performance humidity sensors.

Tao Fu

Papers

Facile synthesis of ZnCo2O4 nanowire cluster arrays on Ni foam for high-performance asymmetric supercapacitors

Facile synthesis of ZnWO4 nanowall arrays on Ni foam for high performance supercapacitors

Highly sensitive humidity sensors based on Sb-doped ZnSnO3 nanoparticles with very small sizes

Humidity sensors based on graphene/SnOx/CF nanocomposites

High-performance humidity sensors based on electrospinning ZnFe2O4 nanotubes