Kuo-Hsin Chang

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.

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.

The Synergistic Influences of OH − Concentration and Electrolyte Conductivity on the Redox Behavior of Ni ( OH ) 2 / NiOOH

The synergistic influences of the OH - concentration and electrolyte conductivity on the redox behavior of NiOOH/Ni(OH) 2 for nickel oxide-coated graphite electrodes are clearly demonstrated by voltammetric and impedance analyses. The increase in the OH - concentration and electrolyte conductivity effectively promote the utilization of active nickel species and the electrochemical reversibility of NiOOH/Ni(OH) 2 , indicating the simultaneous involvement of OH - and cations in the redox transition. The upper limit for utilizing Ni oxyhydroxides is mainly determined by the OH - concentration, which is facilely reached by increasing the electrolyte conductivity (adding Na 2 SO 4 ). The synergistic phenomena could be very important in the applications of Ni oxide-based batteries, supercapacitors, sensors, electrochromic devices, and organic synthesis.

Coalescence inhibition of hydrous RuO2 crystallites prepared by a hydrothermal method

Coalescence of particulates accompanied with crystal growth upon annealing at/above 200°C, found for hydrous RuO2 (RuO2∙nH2O) prepared by a sol-gel process, is effectively inhibited by the formation of RuO2∙nH2O nanocrystallites in a hydrothermal process. This thermal stability, attributable to the barrier originated from the lattice energy of crystallites, maintains high water content, nanocrystalline structure, and porous nature of RuO2∙nH2O annealed at elevated temperatures from 200to400°C. A hydrothermal derived RuO2-based supercapacitor with high specific capacitance (ca. 200Fg−1 measured at 100mAcm−2) and a cycle lifetime longer than 40000cycles, resulting from thermal stability, is demonstrated.

Pseudocapacitive Characteristics of Vanadium Oxide Deposits with a Three-Dimensional Porous Structure

A three-dimensional porous vanadium oxide was anodically deposited onto graphite substrates (denoted as VO x ·nH 2 O/G) at 0.7 V from an aqueous solution containing 25 mM VOSO 4 and 5 mM H 2 O 2 . Through annealing at temperatures up to 350°C, the thermal stability of VO x ·nH 2 O preserved its porous morphology and excellent capacitive performances in 3 M KCI (pH 2.4). X-ray photoelectron spectroscopic analysis revealed the mixed valence nature of oxy-/hydroxyl-vanadium species in which the amount of V 4+ species was not significantly affected by varying the annealing temperature. A maximal specific capacitance of ca. 150-160 F g -1 measured at 250 mV s -1 was obtained for this porous VO x ·nH 2 O annealed between 150 and 250°C. Only 9-17% loss in specific capacitance was found for these VO x ·nH 2 O when the scan rate of the cyclic voltammetry was increased from 25 to 250 mV s -1 , demonstrating a typical high power property, which was never found in the VO x ·nH 2 O-based supercapacitors.

Synthesis of activated carbon-surrounded and carbon-doped anatase TiO2 nanocomposites

A specially designed photocatalytic structure, i.e., carbon-doped anatase-TiO2 (A-TiO2−xCx) nanocrystallites surrounded with activated carbon (AC), is synthesized by a simple one-step carbonization of a self-assembled matrix consisting of titanium tetra-isopropoxide (TTIP) and triblock copolymer. The self-assembled copolymer homogeneously disperses TTIP and possesses a steric inhibition of A-TiO2 grain growth and aggregation during the carbonization step, reducing the average grain size of the A-TiO2 nanocrystallites. An additional merit of this simple process, the facile formation of visible-light-driven photocatalysts through carbon-doping, is attributable to the retention of few-nanometre-sized A-TiO2 clusters during the carbonization step. The porous AC of high specific surface area within the novel A-TiO2−xCx-AC nanocomposites shows excellent adsorption ability for concentrating organics at the vicinity of A-TiO2−xCx nanocrystallites. Unlike the A-TiO2 crystals, the synergy between C-doped A-TiO2 and AC definitely promotes the visible-light photo-induced degradation efficiency of organics.

Electron field emission from various morphologies of fluorinated amorphous carbon nanostructures

Unlike general fluorination, amorphous fluorinated carbon (a‐C:F) nanostructures have been synthesized directly and efficiently by an electron cyclotron resonance chemical vapor deposition (ECR–CVD) system using a mixture of C2H2, CF4, and Ar as precursors. The electron field-emission properties of the a‐C:F nanostructures were investigated. The a‐C:F nanoporous films with a low turnon field (1.8V∕μm) are apparently lower than other types of a‐C:F nanostructures. The a‐C:F nanostructures have a greater field-enhancement factor (2500–4000) than other nonaligned multiwall nanotubes. However, the a‐C:F nanostructures follow the Fowler–Nordheim characteristics only in the medium emission current region and they deviate from the characteristics in the low and high emission current regions.

Anodic composite deposition of RuO2/reduced graphene oxide/carbon nanotube for advanced supercapacitors

Anodic composite deposition is demonstrated to be a unique method for fabricating a ternary ruthenium dioxide/reduced graphene oxide/carbon nanotube (RuO2 xH2O/rGO/CNT, denoted as RGC) nanocomposite onto Ti as an advanced electrode material for supercapacitors. The rGO/CNT composite in RGCs acts as a conductive backbone to facilitate the electron transport between current collector and RuO2 xH2O nanoparticles (NPs), revealed by the high total specific capacitance (C(S,T) = 808 F g(-1)) of RGC without annealing. The contact resistance among RuO2 xH2O NPs is improved by low-temperature annealing at 150 °C (RGC-150), which renders slight sintering and enhances the specific capacitance of RuO2 xH2O to achieve 1200 F g(-1). The desirable nanocomposite microstructure of RGC-150 builds up the smooth pathways of both protons and electrons to access the active oxy-ruthenium species. This nanocomposite exhibits an extremely high C(S,T) of 973 F g(-1) at 25 mV s(-1) (much higher than 435 F g(-1) of an annealed RuO2 xH2O deposit) and good capacitance retention (60.5% with scan rate varying from 5 to 500 mV s(-1)), revealing an advanced electrode material for high-performance supercapacitors.

Microstructure tuning of mesoporous silica prepared by evaporation-induced self-assembly processes: interactions among solvent evaporation, micelle formation/packing and sol condensation

By varying the vapor pressure of solvents through changing the alkyl length of aliphatic alcohols or the aging temperature, the microstructures, including mesopore ordering length, porosity, and specific surface area, of silica templates are controllable in an evaporation-induced self-assembly (EISA) process. The microstructures of various silica powders are systematically characterized and compared by the small-angle X-ray scattering (SAXS), nitrogen adsorption/desorption isotherms, as well as scanning electron microscopic (SEM) and transmission electron microscopic (TEM) images. The microstructure ordering length of silica powders is found to decrease with reduction of the solvent evaporation rate tuned by the length of alkyl groups of alcohols, which is confirmed by the almost identical microstructure of silica powders prepared by using methanol and tetrahydrofuran (THF) which exhibit similar vapor pressures. The rate of micelle formation relative to the rate of silica precursor condensation, strongly depending on the solvent evaporation rate, determines the resultant organization of micelles and may cause morphological distortion and microdomain disorientation of silica templates. This concept is confirmed by the presence of completely and partially ordered microstructures of silica powders synthesized from the ethanol- and butanol-based precursor solutions, respectively, which are dried completely at a higher temperature in the aging process. The microstructure of mesoporous silica can thus be simply tuned by varying solvents and aging temperatures in the EISA process, which are promising for many applications.

Graphene: a novel template for controlling the microstructures of mesoporous silica

This study presents the use of graphene sheets as a novel template to control the microstructure of mesoporous silica. The impurity-doped silica is confirmed to be under the pseudo-softening state at temperatures ≥600 °C. The mechanical strength of graphene sheets, catalytically formed in the same temperature range by the presence of Co catalysts, forces the partial transformation of mesoporous impurity-doped silica from 2D hexagonal (p6mm) to the lamellar phase. A mechanism for tailoring the microstructure of mesoporous silica is proposed in this work. This study opens a new way for simply controlling the microstructure of mesoporous silica.