羰基、羧基和羟基表面官能团对碳纳米管电容量的影响

分别采用混酸、空气、硝酸和高锰酸钾对碳纳米管进行氧化处理,以在其表面引入官能团,进而研究了表面官能团对碳纳米管电化学性能的影响.X-射线光电子谱分析表明:混酸氧化处理引入的官能团主要为羰基和羧基;空气氧化使碳纳米管表面链接较多的羟基,但羰基和羧基的含量最少;而硝酸处理和高锰酸钾处理引入了中等数量的羰基和羧基.经四种处理方法所得碳纳米管具有相近的比表面积和孔结构.通过比较它们的比电容发现:羰基和羧基贡献了最多的准电容,尤其羰基含量与碳纳米管的电容量呈正比关系;而羟基主要增强了双层电容,并未引入明显的准法拉第容量.由于羰基和羧基比羟基具有更低的电荷传递电阻,有利于快速的法拉第反应,从而引入准电容.     

High capacitance B/C/N composites for capacitor electrodes synthesized by a simple method

The B/C/N composites were synthesized by a very simple method, that is, carbonization at HTT = 800-1200 °C of the precursor prepared by drying a solution mixture of polyacrylamide and boric acid, followed by boiling in water to remove borate by-products. The amount of insoluble B species in the composite increased linearly from 4.8 to 18.6 mass% with raising HTT. The XRD and FT-IR revealed that turbostratic h-BN started to form at around 1000 °C as a by-product. By XPS, major B and N components in the composite were B-N bond, C-B-O type B, pyridinic N, pyrrolic N, and quaternary N. A fraction for B-N bond including h-BN in the total B or N components increased with raising HTT and it exceeded 50 at% between 900 and 1000 °C. It was suggested that in the composites formed at HTT > 1000 °C the amounts of h-BN increased, leading to reduction in other B and N components. The S_BET was almost unchanged up to 1000 °C, 410-420 m² g⁻¹. Large and broad redox peaks arisen from plural reactions appeared in the cyclic voltammogram (CV) measured in 1 mol dm⁻³ H₂SO₄ for the composites formed at HTT ≤ 1000 °C. These peaks disappeared in 1 mol dm⁻³ solutions of Na₂SO₄ and Li₂SO₄. By comparing CV with that for C/N composite formed from PAA by the MgO template method, the pseudo-capacitance owing to reactions of B-N and C-B-O components with protons was found to be added to commonly observed pseudo-capacitance for nitrogen-doped carbons. The capacitances for the composites formed at 850-950 °C exceeded 300 F g⁻¹ at 2 mV s⁻¹ in the acid electrolyte and the retention at 50 mV s⁻¹ was 78-80%. The shape of CV in the neutral electrolytes was trapezoid and the current density increased with lowering potential, suggesting adsorption and desorption of Na⁺ and Li⁺ ions. This was considered to be due to doped nitrogen, indicating the development of pseudo-capacitance. The capacitance per S_BET was 0.33-0.74 F m⁻² and 0.17-0.32 F m⁻², larger for lower HTT, in the acid and neutral electrolytes, respectively.     

Morphological reason for enhancement of electrochemical double layer capacitances of various acetylene blacks by electrochemical polarization

Enhancement of electrochemical capacitance and morphological variations of various acetylene blacks caused by electrochemical polarization are presented. Acetylene blacks of different mean particle diameters were modified by air-oxidation and heat treatment to diversify the morphologies of the acetylene blacks before electrochemical polarization. The various acetylene blacks were electrochemically oxidized at 1.6 V (vs. Ag/AgCl) for 10 s and the polarization step was repeated until the capacitance values did not change any longer. These polarization steps enhanced the capacitances of the acetylene blacks and the specific enhancement factors range from 2 to 5.5. Such an enhancement is strongly related to morphological modification as revealed by transmission electron microscopic observations. The electrochemical polarization resulted in formation of tiny graphene sheets on the wide graphitic carbon surfaces, which were most responsible for the observed capacitive enhancement. Although the pseudo-capacitance increased after polarization by forming oxygenated species on the surfaces, its contribution to the total capacitance was less than 10%. The mechanism of the formation of the tiny graphene sheets during the electrochemical oxidation is described schematically.     

Facile approach to prepare loose-packed cobalt hydroxide nano-flakes materials for electrochemical capacitors

Cobalt hydroxide nano-flakes are successfully synthesized by a facile chemical precipitation method. Electrochemical characterization is performed using cyclic voltammetry, chronopotentiometry and impedance spectroscopy, respectively. These cobalt hydroxide nano-flakes maintain high utilization at high rates of discharge. A maximum specific capacitance of 735 F g⁻¹ can be achieved in 2 M aqueous KOH with the potential range from −0.2 to 0.4 V (vs. SCE) in a half-cell setup configuration for the nano-flakes Co(OH)₂ electrode, suggesting its potential application in electrochemical capacitors. Furthermore, the effect of annealing temperatures on the electrochemical capacitance characteristics is also been systematically explored.     

One-dimensional growth and electrochemical properties of polyaniline deposited by a pulse potentiostatic method

One-dimensional growth of polyaniline (PANI) was conducted on carbon cloth through a pulse potentiostatic method. Hydrolysis of PANI was depressed, and the generated PANI film (PPM) displayed improved electroactivities. The specific capacitance of PPM was increased by 39% when compared to that of PANI film made by the conventional potentiostatic method (PM). The influences of the upper limit potential of the pulse potentiometry and the acidity of the polymerization solution on surface morphologies, electroactivities and conformation of the PANI films were studied by SEM, cyclic voltammetry, chronopotentiometry and UV-Vis spectrometry.     

Physical and electrochemical characterization of hydrous ruthenium oxide/ordered mesoporous carbon composites as supercapacitor

Ruthenium oxide/ordered mesoporous carbon composites materials were prepared by impregnating an ordered mesoporous carbon CMK-3 with RuCl₃ · xH₂O solution followed by annealing in nitrogen atmosphere from 80 to 400 °C. The content of ruthenium oxide in the composites ranged from 10.0 to 30.7 wt.%. X-ray diffraction (XRD), thermogravimetric analysis (TGA), nitrogen adsorption measurement and transmission electron microscopy (TEM) were used to characterize the composites. The results showed that the ruthenium oxide deposited on CMK-3 mesoporous carbon was hydrous and amorphous when annealed up to 400 °C. The specific capacitance of the composites was determined by cyclic voltammetry. Such composites had high specific capacitance, which was derived from the high specific surface area of CMK-3 mesoporous carbon and the pseudo-capacitance of amorphous RuO₂. In addition, the specific capacitance depended on the annealing temperature and the RuO₂ content. As the temperature increased, the specific capacitance decreased. In contrast, the specific capacitance increased with higher RuO₂ content and reached 633 F/g with a heavy content. However, as the RuO₂ content increased, its contribution to the pseudo-capacitance became poorer. The rate capability of the composite electrodes also decreased as a function of RuO₂ content, due to an increase in the equivalent series resistance (ESR) and the overall capacitance.     

Structure, morphology and electrochemical behaviour of manganese oxides prepared by controlled decomposition of permanganate

Hydrothermal decomposition of permanganate, conducted in a range of pH-controlled solutions (from strongly acidic to strongly basic), is used to prepare manganese dioxides that are well-suited for use as supercapacitor electrode materials. While permanganate is thermodynamically unstable, the kinetics of its decomposition in an aqueous environment are very slow, until the temperature is raised to 200 °C. Although the resultant materials are relatively crystalline and have low total pore volume, their prominent meso-porosity leads to good electrochemical performance. Best behaviour is obtained for material from permanganate decomposition in 0.01 M H₂SO₄ solution, for which composite electrodes (150 μm thick) yield 150 F g⁻¹ at 5 mV s⁻¹ in a 9 M KOH electrolyte.     

Pseudo-capacitance of composite electrode of ruthenium oxide with porous carbon in non-aqueous electrolyte containing imidazolium salt

Pseudo-capacitance of composite materials where ruthenium oxide particles are loaded on activated carbon has been evaluated in the electrolyte of 1-ethyl-3-methyl imidazolium tetrafluoroborate dissolved in acetonitrile. The composite materials prepared by conventional a sol-gel method have dispersed structure of ruthenium oxide particle of tens nanometer diameter on the surface of activated carbon. The extent of the pseudo-capacitance of the composite electrodes in the imidazolium salt electrolyte, estimated by the comparison of the capacitance per surface area of electrode in different non-aqueous electrolyte, is ca. 3-5 μF cm⁻² in addition to the double-layer capacitance of ca. 6 μF cm⁻², depending on the loading status of ruthenium oxide. The symmetric cell consisting of the composite electrode containing 18 wt% of ruthenium oxide and the imidazolium salt electrolyte provides cell capacitance based on the pseudo-capacitance by a constant-current test.     

Electrochemical properties of nanosized hydrous manganese dioxide synthesized by a self-reacting microemulsion method

Manganese dioxide has been synthesized by a new simple self-reacting microemulsion method. The synthesized MnO₂ has been found to be amorphous structure containing a moderate amount of water by X-ray diffraction, Fourier transform infrared spectroscopy and thermogravimetric analysis. Particles in a spherical shape with about 4 nm in diameter have been observed by transmission electron microscopy. Cyclic voltammetic tests have been performed between −0.5 and 0.5 V versus Hg/Hg₂SO₄ in 1 mol L⁻¹ Na₂SO₄ solution at sweep rates up to 50 mV s⁻¹. A specific capacitance value as high as 246.2 F g⁻¹ was obtained, which was much higher than 146.5 F g⁻¹ of MnO₂ prepared by chemical co-precipitation. After 600 cycles, only 6% decrease of specific capacitance was measured which indicated that such a material possesses good cycling property.     

Polypyrrole/carbon composite electrode for high-power electrochemical capacitors

Nano-thin polypyrrole (PPy) layers with thickness from ∼5 nm to several 10s nm were deposited on vapor grown carbon fibers (VGCF) by an in situ chemical polymerization. Using different concentrations of the pyrrole could control the thicknesses of deposited PPy layers. Surface morphology and thickness of the deposited PPy layers were confirmed by means of scanning electron microscopy and scanning transmission emission microscopy. Pseudo-capacitive behavior of the deposited PPy layers on VGCF investigated by means of cyclic voltammetry. Then, the PPy/VGCF composites were mixed with activated carbons (AC) at various mixing ratios. For the PPy/VGCF/AC composite electrodes, characteristics of specific capacitance and power capability were examined by half-cell tests. As results of this study, it was investigated that nano-thin PPy layer below ∼10 nm deposited on VGCF had high pseudo-capacitance and fast reversibility. Its specific capacitance per averaged weight of active material (PPy) was obtained as ∼588 F g⁻¹ at 30 mV s⁻¹ and maintained as ∼550 F g⁻¹ at 200 mV s⁻¹ of scan rate. Also, from the mixing 60 wt.% of the PPy/VGCF with 25 wt.% of AC, the PPy/VGCF/AC composite electrode exhibited higher power capability maintaining the specific capacitance per active materials of PPy and AC as ∼300 F g⁻¹ at 200 mV s⁻¹ in 6 M KOH.