Improved low‐temperature performance of novel asymmetric supercapacitor with capacitor/battery‐capacitor construction

A concept for designing capacitor/battery‐capacitor asymmetric supercapacitor is proposed to improve low‐temperature capacitance, which consists of a capacitor‐type electrode (C) and a capacitor/battery‐type composite electrode (NiO/C). This construction overcomes the capacitor‐battery asymmetric supercapacitor's shortcoming of losing capacitance characteristics. By adjusting the NiO/C mass ratio to 1/2, the new NiO/C–C asymmetric supercapacitor maintains excellent capacitance feature (rectangular CV curves and symmetrical charge/discharge profiles) as well as enlarging the work potential to 1.5 V, showing improved low‐temperature capacitance in comparison with C‐C and NiO‐C constructions. It is believed to come from the decreased total inner resistance and charge‐transfer resistance due to the substitution of NiO electrode with NiO/C composite electrode in the asymmetric supercapacitor. Copyright © 2016 John Wiley & Sons, Ltd.     

A novel electrode material for electric double-layer capacitors

In this study, a novel electrode material, modified activated carbon aerogel, has been developed for electric double-layer capacitors (EDLCs). This novel material was produced by the activation of carbon aerogel under CO₂ flow, followed by surface modification with a surfactant, sodium oleate. It has been characterized by BET measurement and BJH method for its surface area and pore-size distribution, and by constant-current charge–discharge technique, cyclic voltammetry and electrochemical impedance spectrum (EIS) for its specific capacitance, equivalent series resistance and power capability. It was found that, after the surface modification, the wettability of the organic electrolyte based on non-polar organic solvent (i.e., propylene carbonate) to the activated carbon aerogel was improved greatly, which resulted in a lower internal resistance and a higher usable surface area. As results, a higher specific capacitance, energy density and power capability were achieved for the capacitor using the modified activated carbon aerogel electrodes than those without the surface modification. The effects from the surface modification became more significant at higher charge–discharge rates, at which the wettability of the electrolyte to the electrode material usually becomes more important and critical.     

Study on the surface modification of metal hydride electrodes with cobalt

A novel method has been applied to the surface modification of the metal hydride (MH) electrode of the MH/Ni batteries. Both sides of the electrode were plated with a thin cobalt film about 0.15 μm using vacuum evaporate plating technology and the effect of the electrode on the performance of the MH/Ni batteries was examined. It was found that the surface modification could enhance the electrode conductivity and decrease the battery ohmic resistance. After surface modification, the discharge capacity at 5C (8.5 A) was increased by 115 mAh and discharge voltage was increased by 0.04 V, the resistance of the batteries was also decreased by 18%. The batteries with modified electrode exhibited satisfactory durability. The remaining capacity of the modified batteries was 93% of the initial capacity even after 500 cycles. The inner pressure of the batteries during overcharging was lowered and the charging efficiency of the batteries was improved.     

Poly(o-toluidine) for carbon fabric electrode modification to enhance the electrochemical capacitance and conductivity

Capacitance and conductivity enhancements of activated carbon fabric employed as electrodes of electrochemical capacitors (ECs) were achieved by electrochemical deposition of conducting poly(o-toluidine) (POT). Potentiodynamic polymerization of o-toluidine onto the carbon in H₂SO₄ was employed for this carbon modification. The capacitance of the activated carbon was enhanced by superimposing the psuedocapacitance of poly(o-toluidine) onto the double-layer capacitance of the carbon. Deposition of polyaniline was also conducted for the purpose of comparison. With the presence of the electron-donating methyl group in the phenyl ring, poly(o-toluidine) is more effective than polyaniline in enhancing the capacitance of the carbon fabric. The specific capacitance of the electrodes increased with the amount of poly(o-toluidine) deposited and more than twice of that of the bare carbon can be achieved. However, the capacitance per unit weight of the deposited polymer decreased with the extent of deposition, probably due to an increase of the ion migration resistance that increasingly obstructs some polymer from the access of ions. The resistance of the carbon electrode was found to decrease upon polymer deposition, and this was found to be more significant with poly(o-toluidine) than with polyaniline. The low resistance resulting from poly(o-toluidine) deposition enabled the assembly of capacitors of relatively high power densities, more than three times of that of a capacitor with the bare carbon.     

Capacitance of porous carbon electrode in mixed salt non-aqueous electrolytes

Capacitances of a porous carbon electrode in non-aqueous electrolytes containing tetraethylamonium tetrafluoroborate (TEABF₄) and a lithium salt with various compositions have been investigated for the potential use in electric double layer capacitor. In the electrolyte prepared by dissolving TEABF₄ and LiBF₄ into the mixed solvent of ethylene carbonate (EC) with diethyl carbonate (DMC), an activated carbon fiber (ACF) electrode exhibits a larger capacitance than in TEABF₄ single salt electrolyte on cyclic voltammograms. The symmetrical capacitor cell containing the LiBF₄-TEABF₄ mixed salt electrolyte also exhibits larger capacitance on a constant-current test compared with that containing the TEABF₄ single salt electrolyte, while the capacitance degradation is observable for this cell at a significant extent, while the test under controlled potential of the ACF electrode to −0.2 to 1.0 V vs. Ag provides somewhat stable capacitance over 30 cycles.     

Enhanced electrochemical performance of nickel hydroxide electrode with monolayer hollow spheres composed of nanoflakes

Nickel hydroxide electrodes with hollow spheres were fabricated using a PS (polystyrene) sphere template and electrochemical deposition. The nickel hydroxide grew perpendicular to the electrode substrate during anodic deposition and around the PS spheres during cathodic deposition. After the removal of the PS template, hollow spheres or open hollow spheres were formed via cathodic deposition. The nickel hydroxide electrode with hollow spheres and nanoflakes showed greatly enhanced electrochemical performance in alkaline solution compared with the bare nickel hydroxide electrode. The opening of the hollow spheres facilitated easy electrolyte transport to the reaction sites and led to a further increase in the specific capacitance of the nickel hydroxide electrode. The specific capacitance of the electrode with the open hollow spheres reached 800 F g⁻¹, which was much higher than that of the bare electrode (224 F g⁻¹) and the hollow-sphere electrode (342 F g⁻¹) at a discharge current density of 10 A g⁻¹.     

Significantly enhanced charge conduction in electric double layer capacitors using carbon nanotube-grafted activated carbon electrodes

Carbon nanotube (CNT)-grafting by chemical vapor deposition was conducted to reduce the resistance of activated carbon fiber serving as an electrode for electric double layer capacitors. Sputtering deposition of Ni catalyst particles led to a uniform growth of CNTs on the carbon fiber surface through the tip-growth mechanism. Because sputtering deposition ensures little pore blockage (in comparison with wet-impregnation), the surface area decrease of the carbon fiber due to Ni loading was minimized. By using H₂SO₄ aqueous solution as the electrolyte, a capacitor cell assembled with the CNT-grafted fiber showed higher electron and electrolyte-ion conductivities relative to a cell assembled with the bare fiber. By increasing the discharging current density from 1 to 150 mA cm⁻², the bare fiber exhibited a capacitance loss of 17% while the CNT-grafted fiber showed a mitigated capacitance loss of only 7%. This developed CNT-grafting technique renders activated carbon fiber a promising electrode material for a variety of electrochemical applications.     

LaNi4.5Al0.5 alloy doped with Au used as anodic materials in a borohydride fuel cell

Fuel cells using borohydride as the fuel have received much attention because of their high thermodynamic cell voltage. Using rare-earth hydrogen storage alloys as the anodic catalyst materials instead of noble metals showed high catalytic activity both in the electrochemical oxidation and the hydrolysis of borohydride. In this work, we doped Au to modify the surface structure of LaNi_4.5Al_0.5 alloy by a self-reduction reaction method. The surface of the alloy particles was evenly covered with Au after treatment. The largest discharge current density increased from about 150 mA cm⁻² (discharge to −0.6 V versus Hg/HgO electrode) with the parent alloy to 250 mA cm⁻² with the Au-doped alloy. This finding suggested that the electrochemical catalytic activity of the alloy was enhanced after modification with Au. Fuel utilization also increased after modification with Au.     

Facile synthesis of three‐dimensional graphene networks by magnetron sputtering for supercapacitor electrode

Three‐dimensional (3D) graphene network deposition on Ni foam without any conductive agents and polymer binders was successfully synthesized by radio frequency magnetron sputtering at low temperature. The direct and close contact between graphene and Ni foam is beneficial to the enhanced conductivity of the electrode, as well as the improvement of ion diffusion/transport into the electrode. As a result, the 3D graphene network deposition on Ni foam electrode delivered a high specific capacitance of 122.0 F g⁻¹ at 1.0 A g⁻¹ and excellent cycle stability with capacitance retention of 99.0% after 1000 charge–discharge cycles. The work shows a new way to facile synthesis of 3D graphene network for various applications in the future. Copyright © 2016 John Wiley & Sons, Ltd.     

Properties of containing Sn nanoparticles activated carbon fiber for a negative electrode in lithium batteries

Activated carbon fiber (ACF) containing Sn nanoparticles were prepared by impregnation and were investigated as a negative electrode material in lithium batteries. The tin particle size was controlled by selecting an ACF with an adequate surface structure. This Sn/ACF composite cycled versus Li metal showed a first discharge capacity as high as 200 mAh g⁻¹ compared to the pristine ACF which showed only 87 mAh g⁻¹. Excellent cyclability with these composites was obtained with ACF BET SSA as large as 2000 m² g⁻¹ and 30 wt.% Sn.     

Influence of high temperature treatment of porous carbon on the electrochemical performance in supercapacitor

Mesoporous carbon (MoC), prepared by the template method from phenol resin, commercial mesoporous carbon fiber (ACF) and microporous activated carbon (MiC) were heat-treated under 1200 °C in nitrogen. The samples before and after high temperature heat treatment (HTT) were used as electrodes in 30% KOH electrolyte for supercapacitor. The structure and electrochemical properties of these samples were characterized by X-ray diffraction (XRD), galvanotactic charge–discharge, cyclic voltammetry (CV) and electrochemical impedance spectra, respectively. Results showed that HTT caused a remarkable increase of mesoporous ratio in texture accompanied with a significant reduction of surface area. After HTT, the layer–layer space among graphite crystallite decreased and the degree of graphitization was improved. The capacitance values of mesoporous carbons increased and a more stable tendency for specific capacitance was obtained compared with the contrary performance of MiC-1200. The cyclic voltammetry of the samples at different sweep rates was close to the rectangular shape, which represented the lower ESR and higher power density. It was also found that high temperature treatment could improve the electrical conductivity and decrease the impedance of the electrode remarkably. The improved specific capacitance and the better conductivity of mesoporous carbons could be ascribed to the expanding and reorganization of crystallite structure as well as increase of mesopore ratio.     

Fabrication and electrochemical performances of porous carbon nanotubes/polyaniline-modified carbon nanotube fiber

通过低电压电泳沉积的方法在碳纳米管纤维（CNF）表面沉积多孔碳纳米管（CNTs）,然后在其表面电化学沉积一层聚苯胺（PANI）,得到CNTs@PANI三维多孔网络结构修饰的核-鞘型纤维电极材料。通过扫描电镜、透射电镜和拉曼光谱表征电极材料表面形貌和微观结构,并利用电化学工作站测试电化学性能,研究结果表明,沉积的多孔CNTs结构可以为PANI提供更多的氧化还原反应活性位点,而PANI也具有固定CNTs的作用,在电流密度为1 mA/cm²时,CNTs和PANI修饰的电极面积比电容达77.28 mF/cm²。以聚二甲基硅氧烷薄膜为基底、PVA-H₃PO₄为电解质制备的对称型固态柔性超级电容器在电流密度为0.25 mA/cm²时,面积比电容为61.25 mF/cm²,恒流充放电4000次后,电容值仍维持在80%,并且串联两个电容器可以点亮电压为1.8 V的LED灯泡。     

Electrochemical and capacitive properties of thin-layer carbon black electrodes

Electrochemical properties and porous-structure-dependent capacitive ability of commercial carbon blacks, Black Pearls 2000^® (BP) and Vulcan^® XC 72R (XC), were investigated in H₂SO₄ solution by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The capacitance in-depth profile is correlated to microscopic appearance of carbon blacks in the form of a thin layer applied over Au substrate from water suspensions of BP and XC. The capacitance calculated from voltammetric charge was found to depend on the sweep rate, due to porosity of investigated materials. Impedance (EIS) characteristics upon frequency-dependent charge/discharge process indicate transmission line electric behavior of BP and XC. Capacitance and resistance values obtained by simulations of EIS data, enabled estimation of capacitance and resistance profile throughout carbon black porous electrodes. Capacitance of BP carbon layer increases going from the outer surface towards the bulk of the layer. External capacitance originates from capacitive characteristics of the macroscopic surface consisting of relatively large agglomerates, while internal capacitance originates from “inner” surface of micro-porous agglomerates. Contrary to BP, opposite distribution of the total capacitance to external and internal part was found for XC, caused by its loose structure and considerably lower real surface area in comparison to BP. The XC morphology makes additionally the pseudocapacitive contribution of surface functionalities more pronounced, which indirectly shifts also the “internal” double-layer capacitive response to higher frequencies through the effect of increased wettability of the layer. Thus, the capacitance of XC surface directly exposed to the electrolyte is larger than that of the inner one, which makes it a “fully-utilized” capacitor, while increased capacitive performance of BP emerges only at very low frequencies of charging/discharging process.     

Hierarchical nanostructured Au–SnO2 for enhanced energy storage performance

Electrode materials with high energy and power density are mostly essential to overcome traditional fossil fuel use. Herein, we demonstrate the synthesis of Au decorated self-assembled SnO₂ nanoflowers consisting of nanorods by a cost-effective and eco-friendly solvothermal process. The as-synthesized SnO₂ based materials were employed as electrode material in the energy storage system, which delivered considerably high specific capacitance of 634.3 F/g at a current density of 1 A/g. The electrode material also exhibits excellent cycle stability of 83.52% after 4000 galvanostatic charge–discharge (GCD) cycles. The high specific capacitive value is attributed to the hybrid performance of battery and supercapacitor, more active sites, and higher surface area. A solid state asymmetry device was fabricated using Au–SnO₂ and activated carbon (AC) as positive and negative electrodes. The asymmetry device shows an excellent energy density of 168.9 Wh/kg at a power density of 1 kW/kg with an applied current density of 1 A/g.     

Wide-temperature range operation supercapacitors from nanostructured activated carbon fabric

Electrochemical power sources that offer high energy and power densities and, can also withstand a harsh temperature range have become extremely desirable in applications ranging from civilian portable electronic devices to military weapons. In this report, we demonstrated a wide temperature withstanding supercapacitor which can be operated from 100 °C to −40 °C within a voltage window from −2 V to 2 V. The performance of the supercapacitor coin cells, assembled with nanostructured activated carbon fabric (ACF) as the electrode material and 1 M tetraethylammonium tetrafluoroborate (TEABF₄) in polypropylene carbonate (PC) solution as the electrolyte, was systematically studied within the set temperature window. The ACF supercapacitor yielded ideal rectangular shapes in cyclic voltammograms within 0-100 °C with an average mass capacitance of 90 F g⁻¹ and, 60 F g⁻¹ at −25 °C. The capacitance was still over 20 F g⁻¹ at the extremely low temperature of −40 °C. Another exciting feature of the ACF supercapacitors was that they resumed their room temperature capacitance when cooled from 100 °C and defrosted from −40 °C, demonstrating an excellent repeatability and stability. The charge-discharge behavior of the ACF supercapacitors showed long-cycle stability at extreme temperatures. These high electrochemical performances make this type of supercapacitors very promising in many practical applications.     

Tantalum oxide–ruthenium oxide hybrid(R) capacitors

A hybrid capacitor consisting of porous tantalum oxide anode electrode and ruthenium oxide cathode electrode was examined and characterized. The capacitor has a capacitance of 35 mF and an internal resistance of 45 mΩ. It was found that the capacitance was insensitive to current density up to 110 mA/cm², and temperature ranging from −70 to 50 °C. During dc charge and discharge cycles, the potential of the cathode electrode was within the electrochemical stability window. However, a sudden voltage-jump as high as 7.5 V could occur at the cathode electrode during a short circuit discharge. A simple model was established to describe the transient behavior of cathode and anode electrodes. It was found that the voltage-jump was proportional to the ratio of the internal resistance of the cathode electrode to the total resistance of the capacitor. The resistance distribution inside the capacitor was also determined to be 47, 28, and 25% from the cathode, anode, and electrolyte, respectively.     

Fabrication and electrochemical activity of Ni-attached carbon nanotube electrodes for hydrogen storage in alkali electrolyte

The electrochemical activity of an electrode of carbon nanotubes (CNTs) attached with Ni nanoparticles was investigated. A surface modification technique enabled different Ni particle densities to coat onto the CNT surface, which was chemically oxidized by nitric acid. It was found that each nickel nanoparticle has an average size of 30–50  nm, and the Ni-attached CNTs still possessed a similar pore size distribution. Cyclic voltammetry measurements in 6  M KOH showed that the electrochemical adsorption and desorption amount of hydrogen is a linearly increasing function of the Ni loading. This enhancement of electrochemical activity was ascribed to a fact that Ni particle acts as a redox site for hydrogen storage, thus leading to a greater specific peak current. According to our calculation, the electrochemical capacitance of nickel nanocatalyst in KOH electrolyte was estimated to be the value of 217  F/g. Charge/discharge cycling demonstrated that the Ni-attached CNT electrode maintains fairly good cycleability during 50 cycles.     

The capacitive characteristics of supercapacitors consisting of activated carbon fabric–polyaniline composites in NaNO3

A sandwich-type supercapacitor consisting of two similar activated carbon fabric–polyaniline (ACF–PANI) composite electrodes was demonstrated to exhibit excellent performance (i.e., highly reversibility and good stability) in NaNO₃. Polyaniline with the charge density of polymerization less than or equal to 9 C cm⁻² synthesized by means of a potentiostatic method showed a high specific capacitance of 300 F g⁻¹. Influences of the polymerization charge density (i.e., the polymer loading) on the capacitive characteristics of ACF–PANI composites were compared systematically. The capacity of an ACF–PANI electrode reach ca. 3.4 F cm⁻² (a 100% increase in total capacity) when the charge density of polymerization is equal to 9 C cm⁻². The surface morphology of these ACF–PANI composites was examined by a scanning electron microscope (SEM).     

Influence of carbon microstructure on the Li–O2 battery first‐discharge kinetics

Defects in the carbon microstructure have been reported to enhance the discharge performance of Li–O₂ battery. However, systematic studies correlating the presence of defects with the discharge kinetics have not addressed the variation of carbon electrode surface areas. In this work, carbon blacks and carbon nanofibers with different defect densities were investigated for their discharge properties. The electrolyte‐accessible areas of the carbon electrodes were obtained from Cyclic voltammetry measurements. The microstructure and surface areas of the carbons were characterized by Raman spectroscopy, electron microscopy, and N₂ isotherm. Linear sweep voltammetry and galvanostatic discharge experiments consistently demonstrated that graphitic carbons have more negative onset potentials and more negative discharge potentials at the same current density than defective carbons. The linear sweep voltammetry data were normalized to the carbon masses, Brunauer–Emmet–Teller surface areas, and double layer capacitance‐derived areas for comparison. Plot of inverse charge transfer resistance and double layer capacitance from electrochemical impedance spectroscopy measurements were used to extract current density values without knowledge of electrode areas. The current densities from impedance measurements exhibited good agreement with the data from linear sweep experiments. The electrochemical experiments conclusively showed that defects on the graphitic microstructure increase the discharge kinetics of the Li–O₂ battery. Copyright © 2016 John Wiley & Sons, Ltd.     

Facile SILAR preparation of Fe(OH)3/Ag/TiO2 nanotube arrays ternary hybrid for supercapacitor negative electrode

The ternary hybrid composite electrode of Fe(OH)₃/Ag/TNTA (where TNTA stands for TiO₂ nanotube arrays) was prepared by a simple successive ionic layer adsorption and reaction method. The effects of calcination temperature of Ag/TNTA, drying temperature of Fe(OH)₃/Ag/TNTA, and deposition amount of Ag and Fe(OH)₃ on the supercapacitor performance of the composite electrode were investigated, and the related reasons were discussed in detail. The results show that Ag modification can obviously improve the performance of Fe(OH)₃/TNTA composite electrode. Both the calcination temperature of Ag/TNTA and the deposition amount of Ag affect the particle size of Ag and the reaction resistance of the electrode. The deposition amount of Fe(OH)₃ also has influence on the reaction resistance of the electrode. Under the optimized conditions, the capacitance value of the Fe(OH)₃/Ag/TNTA composite electrode is as high as 84.67 mF cm^?2@5 mV s^?1（596.30 F g^?1@5 mV s^?1）, and the electrode has high rate performance and good cycle stability. The asymmetric supercapacitor assembled with Fe(OH)₃/Ag/TNTA as the negative electrode and activated carbon as the positive electrode can store energy stably under the potential window of 0–1.5 V. When the power density is 2.77 kW kg^?1 (50 mW cm^?3), the energy density can reach 18.34 Wh kg^?1 (0.33 mWh cm^?3).