Chi-Chang Hu

A cost-effective supercapacitor material of ultrahigh specific capacitances: spinel nickel cobaltite aerogels from an epoxide-driven sol-gel process.

Adv. Mater. 2010, 22, 347–351 2010 WILEY-VCH Verlag Gm The ever worsening energy depletion and global warming issues call for not only urgent development of clean alternative energies and emission control of global warming gases, but also more advanced energy storage and management devices. Supercapacitors, offering transient but extremely high powers, are probably the most important next generation energy storage device. To boost the specific capacitance of supercapacitors, the specific surface area of the electrode materials needs to be as high as possible to promote the electric double-layer capacitances and to accommodate a large amount of superficial electroactive species to participate in faradaic redox reactions. In addition, suitable pore sizes, 2–5 nm, of the porous electrode materials are critical to ease the mass transfer of electrolytes within the pores for fast redox reactions and double-layer charging/discharging. Aerogels are a class of mesoporous materials possessing highly specific surface areas and porosities, from which promising applications in a wide range of areas have been investigated. They are composed of 3D networks of nanoparticles with an average pore size of several nanometers, adjustably falling within the optimal pore sizes of 2–5 nm. Consequently, aerogels are a promising candidate for supercapacitor applications. As to the electrode material, electroactive materials possessing multiple oxidation states/structures that enable rich redox reactions for pseudocapacitance generation are desirable for supercapacitors. Transition metal oxides are such a class of materials that have drawn extensive and intensive research attention in recent years. Among them, RuO2 is themost prominent one with a specific capacitance as high as 1580F g , probably the highest ever reported. The commercialization of RuO2 based supercapacitors, however, is not promising because of the high cost and rareness of Ru. Spinel nickel cobaltite (NiCo2O4) is a low-cost, environmentally friendly transition metal oxide, which has been employed in electrocatalytic water splitting (oxygen evolution) and lithium ion batteries. Its application in supercapacitors, however, received much less attention. Nickel cobaltite has been reported to possess a much better electronic conductivity, at least two orders of magnitude higher, and higher electrochemical activity than those of nickel oxides and cobalt oxides. It is expected to offer richer redox reactions, including contributions from both nickel and cobalt ions, than the two corresponding single component oxides and is a potential cost-effective alternative for RuO2. Based on the above considerations, one would expect nickel cobaltite aerogels, with anticipated good electronic conductivity, low diffusion resistance to protons/cations, easy electrolyte penetration, and high electroactive areas to be a promising candidate for the construction of next-generation, ultrahighperformance supercapacitors. Traditionally, aerogels are prepared with sol–gel processes by taking corresponding alkoxides as the precursors. Alkoxides are generally expensive and sensitive to moisture and heat, requiring careful handling. Recently, to tackle these drawbacks, the epoxide synthetic route, enabling the use of low-cost and stable metal salts as the precursors, was successfully developed to prepare metal oxide aerogels. In this work, we reported the first successful preparation of nickel cobaltite aerogels with the epoxide-driven sol–gel process. The effects of the post-gel-drying calcination temperature on the critical properties of the product aerogels were investigated. At a starting Ni/Co ratio of 0.5 and a post-gel-drying calcination temperature of 200 8C, an optimal combination of composition, crystallinity, specific surface area, pore volume, and pore size was achieved to afford the nickel cobaltite aerogels that showed an extremely high-specific capacitance of 1400 F g 1 under a mass loading of 0.4 mg cm 2 at a sweep rate of 25mV s 1 within a potential window of 0.04 to 0.52V in a 1 M NaOH solution. The excellent reversibility and cycle stability of the product aerogels were also demonstrated. A stoichiometric mixture of nickel and cobalt chlorides was used as the precursor for the preparation of the nickel cobaltite aerogels. After the gel is dried in supercritical carbon dioxide, a post-gel-drying calcination is generally required to acquire preferred composition and/or better crystallinity of the products. The post-gel-drying calcination temperature is thus an important processing parameter to be studied. For referring convenience, we term the product aerogels as Ni–Co–O–T, with Tdenoting the calcination temperature. The T block is omitted for as-prepared samples. Also, for comparison purposes, NiO and Co3O4 aerogels were prepared, termed as Ni–O–T and Co–O–T, respectively. Figure 1a shows the X-ray diffraction (XRD) patterns of the as-prepared product aerogels and those samples calcined at 200 and 300 8C. Surprisingly, nickel cobaltite was formed even at the as-prepared condition. The diffraction peak located at the 2u value

Ideal capacitive behavior of hydrous manganese oxide prepared by anodic deposition

Amorphous hydrous manganese oxide (a-MnO2·nH2O) was anodically deposited onto a graphite substrate from a MnSO4·5H2O solution with pH of 6.4. The specific capacitance of this thick a-MnO2·nH2O deposit, measured from both cyclic voltammetry and chronopotentiometry, is about in a potential window of 1.0 V. The electrochemical reversibility of redox couples at/within the a-MnO2·nH2O was significantly influenced by the pH of the test electrolytes. This amorphous hydrous oxide exhibited ideal capacitive behavior (acceptable capacity, high reversibility and high pulse charge–discharge property) between 0 and 1.0 V in 0.1 M Na2SO4 indicating a promising electrode material for electrochemical supercapacitors.

How to Achieve Maximum Utilization of Hydrous Ruthenium Oxide for Supercapacitors

The utilization of hydrous ruthenium oxide (denoted as RuO x .nH 2 O) was promoted by annealing the oxide in air as well as by mixing it with conductive activated carbon (AC) due to the significant improvement in intra- and interparticle electronic conductivity, respectively. The maximum specific capacitance (C S,RuOx ) of RuO x .nH 2 O, 1340 F/g (measured at 25 aV/s), very close to the theoretic value, was obtained from a composite consisting of AC and RuO x .nH 2 O coated on graphite (denoted as AC-RuO x /G) with 10 wt % of sol-gel-derived RuO x .nH 2 O nanodots annealed in air at 200°C for 2 h. The UV absorption spectral features showed a shift in λ max to the red as the mean particle size of RuO x .nH 2 O nanodots was increased, attributable to the surface plasmon resonance phenomenon. The average particle size of highly uniform RuO x .nH 2 O nanodots, ranged from 2.05 to 3.01 nm, was estimated from the high-resolution transmission electron microscopy. The dependence of capacitive performance on the size and content of RuO x .nH 2 O nanodots, evidenced by cyclic voltammetry and electrical impedance spectroscopy results, revealed the important influences of interparticle electronic conductivities on the utilization of RuO x .nH 2 O. The RuO x .nH 2 O nanodots with and without annealing in air at 200°C for 2 h showed the amorphous structure from both the X-ray diffraction and electron diffraction analysis.

Design and tailoring of a hierarchical graphene-carbon nanotube architecture for supercapacitors

Stacking of individual graphene sheets (GS) is effectively inhibited by introducing one-dimensional carbon nanotubes (CNTs) to form a 3-D hierarchical structure which significantly enhances the electrochemical capacitive performances of GS-based composites. From SEM images, inserting proper quantity of CNTs as nanospacers can effectively impede the stacking of GS and enlarge the space between GS sheets, leading to obtain a highly porous nanostructure. The specific capacitance of GS-CNTs-9-1 (∼326.5 F g−1 at 20 mV s−1) is much higher than that of GS material (∼83 F g−1). Furthermore, the energy and power densities of GS-CNTs-9-1 are respectively as high as 21.74 Wh kg−1 and 78.29 kW kg−1, revealing that the hierarchical graphene-CNT architecture provides remarkable effects on enhancing the capacitive performance of GS-based composites. Therefore, the GS-CNT composites are promising carbon materials for supercapacitors.

Electric Double‐Layer Capacitors Based on Highly Graphitized Nanoporous Carbons Derived from ZIF‐67

Nanoporous carbons (NPCs) have large specific surface areas, good electrical and thermal conductivity, and both chemical and mechanical stability, which facilitate their use in energy storage device applications. In the present study, highly graphitized NPCs are synthesized by one-step direct carbonization of cobalt-containing zeolitic imidazolate framework-67 (ZIF-67). After chemical etching, the deposited Co content can be completely removed to prepare pure NPCs with high specific surface area, large pore volume, and intrinsic electrical conductivity (high content of sp(2) -bonded carbons). A detailed electrochemical study is performed using cyclic voltammetry and galvanostatic charge-discharge measurements. Our NPC is very promising for efficient electrodes for high-performance supercapacitor applications. A maximum specific capacitance of 238 F g(-1) is observed at a scan rate of 20 mV s(-1) . This value is very high compared to previous works on carbon-based electric double layer capacitors.

Nanostructures and Capacitive Characteristics of Hydrous Manganese Oxide Prepared by Electrochemical Deposition

Amorphous manganese oxide deposits with nanostructures (denoted as a-MnO x .nH 2 O) were electrochemically deposited onto graphite substrates from 0.16 M MnSO 4 .5H 2 O with pH 5.6 by means of the potentiostatic, galvanostatic, and potentiodynamic techniques. The maximum specific capacitance of a-MnO x .nH 2 O deposits plated in different modes, measured from cyclic voltammetry at 25 mV s -1 , is about 230 F g -1 in a potential window of 1.0 V. The high electrochemical reversibility, high-power characteristics, good stability, and improved frequency responses in 0.1 M Na 2 SO 4 for these nanostructured a-MnO x .nH 2 O deposits prepared by electrochemical methods demonstrate their promising potential in the application to electrochemical supercapacitors. The nanostructure of a-MnO x .nH 2 O, clearly observed by means of a scanning electron microscope, was found to depend strongly on the deposition mode. The similar capacitive performance of all deposits prepared in different modes was attributable to their nonstoichiometric nature with a very similar oxidation state, demonstrated by XPS spectra.

Cyclic Voltammetric Deposition of Hydrous Ruthenium Oxide for Electrochemical Capacitors

Hydrous ruthenium oxide-coated titanium electrodes (RuO{sub x}{center_dot}nH{sub 2}O/Ti) with high pseudocapacitance were prepared by cyclic voltammetry from an aqueous chloride solution in the {minus}200 to 1000 mV range. The growth rate of RuO{sub x}{center_dot}nH{sub 2}O, represented by i{sub p} (peak current) of the cyclic voltammograms, was constant up to cycle 120, but it decreased slightly between 120 and 240 cycles. Voltammetric responses studied by cyclic voltammetry as well as the charging and discharging behavior examined by chronopotentiometry in 0.5 M H{sub 2}SO{sub 4} demonstrated the suitability of RuO{sub x}{center_dot}H{sub 2}O for use in electrochemical capacitors. X-ray diffraction spectra exhibited an amorphous structure of this hydrous oxide film. The oxide consisted of mixed oxyruthenium species with various oxidation states as demonstrated by X-ray photoelectron spectroscopy, and the RuO{sub x}{center_dot}nH{sub 2}O surface showed a porous morphology.

Nanoarchitectured Graphene-Based Supercapacitors for Next-Generation Energy-Storage Applications

Tremendous development in the field of portable electronics and hybrid electric vehicles has led to urgent and increasing demand in the field of high-energy storage devices. In recent years, many research efforts have been made for the development of more efficient energy-storage devices such as supercapacitors, batteries, and fuel cells. In particular, supercapacitors have great potential to meet the demands of both high energy density and power density in many advanced technologies. For the last half decade, graphene has attracted intense research interest for electrical double-layer capacitor (EDLC) applications. The unique electronic, thermal, mechanical, and chemical characteristics of graphene, along with the intrinsic benefits of a carbon material, make it a promising candidate for supercapacitor applications. This Review focuses on recent research developments in graphene-based supercapacitors, including doped graphene, activated graphene, graphene/metal oxide composites, graphene/polymer composites, and graphene-based asymmetric supercapacitors. The challenges and prospects of graphene-based supercapacitors are also discussed.

Hydrogen and Oxygen Evolutions on Ru‐Ir Binary Oxides

The electrocatalytic activity of mixed Ru-Ir oxide electrodes fabricated by thermal decomposition are compared (using cyclic voltammetry and potentiodynamic technique), for their ability to evolve hydrogen and oxygen in both 1N H 2 SO 4 and 1N NaOH solutions. Cyclic voltammetry provides information about the redox transitions of surface oxyruthenium and oxiridium groups, and also generates an effective index, (voltammetric charge (q * )), which can be used to determine the electrocatalytic activity of the electrode

Improving the utilization of ruthenium oxide within thick carbon–ruthenium oxide composites by annealing and anodizing for electrochemical supercapacitors

The effects of annealing in air and anodizing on the capacitive behavior of carbon–ruthenium (denoted as C–Ru) composites fabricated by wet impregnation were investigated in 0.1 M H2SO4 by cyclic voltammetry (CV) and chronopotentiometry (CP). The utilization of Ru species within the thick composites (≈1000 μm) was greatly promoted by annealing in air at 240 °C for 8 h and anodizing in 0.1 M H2SO4 at 1.2 V for 1.5 h, due to the formation of Ru oxide and the transformation into a hydrous nature and the maximal specific capacity of Ru oxide (760 F g−1 based on RuO2) could be obtained. The crystalline information of the composites with annealing at different temperatures was obtained from X-ray diffraction (XRD) patterns. The morphology of C–Ru composites was examined by scanning electron microscope (SEM). The specific surface area and pore-size distribution of the composites with annealing and/or anodizing were analyzed by the BET method.