Amorphous Cobalt Boride Alloy Synthesized by Liquid Phase Methods as Electrode Materials for Electrochemical Capacitors

An amorphous cobalt boride alloy with high electronic conductivity is fabricated through the liquid-phase reduction method. Benefiting from large specific surface area and hierarchical pore structure, the as-synthesized Co-B nanoflakes expose substantial electrochemical active sites, promote the transfer of electrons and ions, and accelerate the redox kinetic process. The as-obtained amorphous Co-B alloy sample displays a specific capacitance of 411 F g⁻¹ at 0.5 A g⁻¹, and with the current density increased to 10 A g⁻¹, it maintains 69% of the initial capacitance. The as-assembled asymmetric supercapacitor device reveals electrochemical properties comprising an excellent specific capacitance of 64.3 F g⁻¹ at 0.25 A g⁻¹, superior cyclical stability of 105% after 20,000 cycles at 3 A g⁻¹, and maximum energy density of 22.9 Wh kg⁻¹ at a power density of 200.3 W kg⁻¹. This study demonstrates great potential in developing high-conductivity materials for an asymmetric supercapacitor through utilizing an amorphous cobalt boride alloy as a promising electrode material.     

Converting of the 2D graphene to its 3D by chicken red blood cells as sheets separator for construction supercapacitor electrode

Here, we report on a facile green and scalable method for the fabrication of porous 3D graphene as a well-known carbon-based material used in many energy storage devices. Chicken red blood cells were used as sheets spacer and heteroatom sources in the construction of 3D graphene. First, the red blood cells were separated from the blood and mixed with graphene oxide. Then, the mixture was freeze-dried and carbonized at 700 °C. The resulted 3D graphene containing heteroatoms was used as a supercapacitor electrode modifier on a glassy carbon electrode and tested with various electrochemical techniques. The supercapacitor electrode showed a specific capacitance of 330 F g⁻¹ at a current density of 1 A g⁻¹, maximum power density of 1958 W kg⁻¹, and maximum energy density of 85 Wh kg⁻¹. Furthermore, the supercapacitive performances were tested in a two-electrode symmetrical system which exhibited a specific capacitance of 238 F g⁻¹ for 1 A g⁻¹. It also showed a power density of 2200 W kg⁻¹ and an appreciable energy density of 160 Wh kg⁻¹. The excellent electrochemical behavior of 3D graphene indicates the promising abilities of the composite for other applications such as biosensors, batteries, electrocatalysts, etc.     

Effects of carbonization temperature on microstructure and electrochemical performances of phenolic resin-based carbon spheres

Activated phenol resin-based carbon spheres (APCS) electrodes with high double layer capacitance and good rate capability were prepared from phenol resin-based spheres (PS) at different carbonization temperatures prior to KOH activation. The carbonization temperature has a marked effect on both the pore structure and the electrochemical performances of the APCS in 6 M KOH electrolyte. APCS carbonized at 600 °C results in higher specific surface area and larger pore size, and hence higher capacitance and better rate capability. The specific capacitance of the APCS in 6 M KOH aqueous solution can be as high as 282 F g⁻¹. It remains 252 F g⁻¹ as the current density increases to 1000 mA g⁻¹.     

Study on the electrochemical behavior of vanadium nitride as a promising supercapacitor material

Vanadium nitride (VN) powder was synthesized by calcining V₂O₅ xerogel in a furnace under an anhydrous NH₃ atmosphere at 400 °C. The structure and surface morphology of the obtained VN powder were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The supercapacitive behavior of VN in 1 M KOH electrolyte was studied by means of cyclic voltammetry (CV), constant current charge-discharge cycling (CD) and electrochemical impedance spectroscopy (EIS). The XRD result indicates that the obtained VN belongs to the cubic crystal system (Fm3m [2 2 5]) with unit-cell parameter 4.15 Å. SEM images show the homogeneous surface of the obtained VN. The CV diagrams illustrate the existence of fast and reversible redox reactions on the surface of VN electrode. The specific capacitance of VN is 161 F g⁻¹ at 30 mV s⁻¹. Furthermore, the specific capacitance remains 70% of the original value when the scan rate increases from 30 to 300 mV s⁻¹. CD experiments show that VN is suitable for CD at high current density, and the slow and irreversible faradic reactions exist during the charge-discharge process of the VN electrode. The experimental results indicate that VN is a promising electrode material for electrochemical supercapacitors.     

Electric double layer capacitance of multi-walled carbon nanotubes and B-doping effect

The double layer capacitance properties of multi-walled carbon nanotubes (MWCNTs) prepared by CVD were investigated using propylene carbonate electrolytes. The original MWCNTsare typically entangled tubes with 10∼20 nm outer diameter, around 5 nm inner diameter, and around 220 m² g^-1 of BET specific surface area. The galvanostatic measurement (40 mA g^-1, 2∼4 V vs. Li/Li⁺) in a three-electrode system showed 15 F g^-1 of gravimetric capacitance and a good rate of this property. Due to the tips opening by thermal oxidation, the specific surface area and the capacitance increased. In contrast, heat-treatment at 3000 °C decreased the surface area and the capacitance of the MWCNT due to defect recovery. A small amount (<1 at. %) of boron was doped to the MWCNT by heat-treatment at 2200 °C or 2300 °C with a B-contained graphite crucible. The B-doping can improve the specific capacitance per surface area for the MWCNTswhile maintaining the tube morphology. This effect of B-doping on the capacitance can be explained by modification of the space charge layer in carbon. PACS 81.07.De; 81.65.Mq; 82.47.Uv; 61.72.Vv     

Synthesis and performance of nickel hydroxide nanodiscs for redox supercapacitors

Herein, we report the facile synthesis of β-Ni(OH)₂ nanodiscs by chemical precipitation method and their use in supercapacitors. β-Ni(OH)₂ nanodiscs are characterized by FTIR, XRD, FESEM, XPS and TGA analysis. Morphological analysis revealed the uniform nanodisc morphology of β-Ni(OH)₂. The supercapacitor behavior is evaluated by cyclic voltammetry, galvanostatic charge–discharge, and electrochemical impedance spectroscopy measurements in 1-M aqueous KOH solution with 0- to 0.6-V potential window. The specific capacitance of β-Ni(OH)₂ nanodiscs is found to be 400 F g⁻¹. The energy and power densities of the β-Ni(OH)₂ nanodiscs are found to be 7.15 W h kg⁻¹ and 1716 W kg⁻¹, respectively, at the current density of 1 A g⁻¹. The cycle life test shows the good stability of the electrode with 83 % retention capacitance even after 1500 cycles.

     

Free-standing 3D Co3O4@NF micro-flowers composed of porous ultra-long nanowires as an advanced cathode material for supercapacitor

The development of smart structured cathode materials for supercapacitors (SCs) has sparked tremendous interest. However, the appropriate design to achieve high capacitance and energy density-based cathode materials remains a major problem for energy storage systems. This article describes the effective synthesis of self-supported 3D micro-flowers composed of ultrathin nanowires array of Co₃O₄ on Ni foam (NF) using hydrothermal conditions (Co₃O₄@NF). The mesoporous Co₃O₄@NF with a high surface area, providing a rich active state for the Faraday redox reaction and increasing the diffusion rate of the electrolyte ions. The optimized Co₃O₄@NF-16h electrode exhibited supreme electrochemical performance by delivering a high specific capacitance of 1878, (1127) and 1200 (720 C g⁻¹) F g⁻¹ at 1.0 and 20 A g⁻¹, respectively. The Co₃O₄@NF electrode retained good capacitance stability of 91% over 10000 cycles at 20 A g⁻¹ with excellent rate-performance of 67% at 20 folded high current values. The obtained results for the Co₃O₄@NF electrode are presented the enhanced pseudocapacitive performance, indicating the substantial potential for high-performance supercapacitor applications.     

Electrodeposited ruthenium oxide thin films for supercapacitor: Effect of surface treatments

Amorphous and porous ruthenium oxide thin films have been deposited from aqueous Ru(III)Cl₃ solution on stainless steel substrates using electrodeposition method. Cyclic voltammetry study of a film showed a maximum specific capacitance of 650 F g⁻¹ in 0.5 M H₂SO₄ electrolyte. The surface treatments such as air annealing, anodization and ultrasonic weltering affected surface morphology. The supercapacitance of ruthenium oxide electrode is found to be dependent on the surface morphology.     

Cationic starch as a precursor to prepare porous activated carbon for application in supercapacitor electrodes

Three activated carbons (ACs) for the electrodes of supercapacitor were prepared from cationic starch using KOH, ZnCl₂ and ZnCl₂/CO₂ activation. The BET surface area, pore volume and pore size distribution of the ACs were evaluated using density functional theory method, based on N₂ adsorption isotherms at 77 K. The surface morphology was characterized with SEM. Their electrochemical performance in prototype capacitors was determined by galvanostatic charge/discharge characteristics and cyclic voltammetry, and compared with that of a commercial AC, which was especially prepared for use in supercapacitors. The KOH-activated starch AC presented higher BET surface area (3332 m² g⁻¹) and larger pore volume (1.585 cm³ g⁻¹) than those of the others, and had a different surface morphology. When used for the electrodes of supercapacitors, it exhibited excellent capacitance characteristics in 30 wt% KOH aqueous electrolytes and showed a high specific capacitance of 238 F g⁻¹ at 370 mA g⁻¹, which was nearly twice that of the commercial AC.     

Anchoring alpha-manganese oxide nanocrystallites on multi-walled carbon nanotubes as electrode materials for supercapacitor

The partial coverage of manganese oxide (MnO₂) particles was achieved on the surfaces of multi-walled carbon nanotubes (MWCNTs) through a facile hydrothermal process. These particles were demonstrated to be alpha-manganese dioxide (α-MnO₂) nanocrystallites, and exhibited the appearance of the whisker-shaped crystals with the length of 80–100 nm. In such a configuration, the uncovered CNTs in the nanocomposite acted as a good conductive pathway and the whisker-shaped MnO₂ nanocrystallites efficiently increased the contact of the electrolyte with the active materials. Thus, the highest specific capacitance of 550 F g⁻¹ was achieved using the resulting nanocomposites as the supercapacitor electrode. In addition, the enhancement of the capacity retention was observed, with the nanocomposite losing only 10% of the maximum capacity after 1,500 cycles.     

Bamboo-based activated carbon for supercapacitor applications

Bamboo-based activated carbon is synthesized by a simple heat treatment with or without KOH activation, and characterized for possible energy storage applications. The KOH activation introduces a very large surface area of more than 3000 m² g⁻¹ to the bamboo-based activated carbon, resulting in high specific capacitance, energy density, and power density in an aqueous electrolyte. The specific capacitance retention is more than 91% of the original capacitance after 3000 cycles, proving excellent cyclic stability for supercapacitor applications. Our results indicate that the natural resource of common bamboo could be an essential raw material for the energy storage devices.     

Template-free synthesis of MnO2 nanowires with secondary flower like structure: Characterization and supercapacitor behavior studies

Manganese dioxide (MnO₂) nanowires with diameter about 30-70 nm is achieved via a two-step process: first, template-free cathodic electrodeposition from aqueous solution of Mn(NO₃)₂ on steel substrate and followed by heat treatment. The temperature-annealed sample was studied by X-ray diffraction (XRD), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM) methods and Fourier transform infrared (FT-IR) spectroscopy. The electrochemical performance of the MnO₂ sample was studied by cyclic voltammetry (CV) and chronopotentiometry in Na₂SO₄ solutions. The sample showed excellent supercapacitive behavior. The specific capacitance (SC) of 237 F g⁻¹ in a potential window of 0-0.9V was obtained at the scan rate of 2 mV s⁻¹. The SC calculated from the chronopotentiometry data is about 246 F g⁻¹. The SC was decreased by 16% after 1000 cycles.     

Influence of carbon structure on performance of electrode material for electric double-layer capacitor

An isotropic pitch and an anisotropic pitch with similar softening point were chosen to be precursors for activated carbons (ISO and ANISO, respectively). Chemical activations with same conditions were carried out and the effects of microstructure of precursors on characteristics of activated carbons were discussed. Isotropic pitch with more noncrystallite carbon atoms or edge carbon atoms on the microstructural defects had more reactive ability and more pores were manufactured through sufficient chemical activation. Electric double-layer capacitors (EDLC) were made with the two activated carbons as electrode materials and 1 M Et₄NBF₄/PC as the electrolyte. The performance of EDLC with the ISO has higher specific capacitance (43.5 F g⁻¹) than the ANISO (21.3 F g⁻¹) and has better power performance and lower resistance than the latter.     

SnO2-decorated multiwalled carbon nanotubes and Vulcan carbon through a sonochemical approach for supercapacitor applications

Multiwalled carbon nanotubes (MWCNTs) and Vulcan carbon (VC) decorated with SnO₂ nanoparticles were synthesized using a facile and versatile sonochemical procedure. The as-prepared nanocomposites were characterized by means of transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Fourier transform infra red spectroscopy. It was evidenced that SnO₂ nanoparticles were uniformly distributed on both carbon surfaces, tightly decorating the MWCNTs and VC. The electrochemical performance of the nanocomposites was evaluated by cyclic voltammetry and galvanostatic charge/discharge cycling. The as-synthesized SnO₂/MWCNTs nanocomposites show a higher capacity than the SnO₂/VC nanocomposites. Concretely, the SnO₂/MWCNTs electrodes exhibit a specific capacitance of 133.33 F g⁻¹, whereas SnO₂/VC electrodes exhibit a specific capacitance of 112.14 F g⁻¹ measured at 0.5 mA cm⁻² in 1 M Na₂SO₄.     

Comparative study of electrochemical capacitance of multi-walled carbon nanotubes before and after chopping

In the work, short multi-walled carbon nanotubes (S-CNTs) were synthesized by chopping conventional μm-long multi-walled carbon nanotubes (L-CNTs) under ultrasonication in H₂SO₄/HNO₃ mixed acids. A comparative electrochemical investigation performed in 6 M KOH solution demonstrated that a specific capacitance (SC) of ca. 14.6 μF cm⁻² was delivered by the S-CNTs with the specific surface area (SSA) of 207 m² g⁻¹, much larger than that of ca. 10.1 μF cm⁻² for the L-CNTs with the SSA of 223 m² g⁻¹, the reason for which was that S-CNTs with two open ends, due to good ion penetrability, provided more entrances for electrolyte ions to access the inner surface easily through their shorter inner pathway so as to enhance their SSA utilization and geometric SC. The surface structure disruption of S-CNTs, owing to ultrasonication and oxidation during chopping process, deteriorated their electronic conductivity and resulted in an inferior power property in contrast to L-CNTs.     

Preparation of MnO<Subscript>2</Subscript>/WMNT composite and MnO<Subscript>2</Subscript>/AB composite by redox deposition method and its comparative study as supercapacitive materials

The manganese oxide/multi-walled carbon nanotube (MnO₂/MWNT) composite and the manganese oxide/acetylene black (MnO₂/AB) composite were prepared by translating potassium permanganate into MnO₂ which formed the above composite with residual carbon material using the redox deposition method and carbon as a reducer. The products were characterized by X-ray diffraction, Fourier transform infrared, and scanning electron microscope. Electrochemical properties of both the MnO₂/MWNT and MnO₂/AB electrodes were studied by using cyclic voltammetry, electrochemical impedance measurement, and galvanostatic charge/discharge tests. The results show that the MnO₂/MWNT electrode has better electrochemical capacitance performance than the MnO₂/AB electrode. The charge–discharge test showed the specific capacitance of 182.3 F·g⁻¹ for the MnO₂/MWNT electrode, and the specific capacitance of 127.2 F·g⁻¹ for the MnO₂/AB electrode had obtained, within potential range of 0–1 V at a charge/discharge current density of 200 mA·g⁻¹ in 0.5 mol·L⁻¹ potassium sulfate electrolyte solution in the first cycle. The specific capacitance of both the MnO₂/MWNT and MnO₂/AB electrodes were 141.2 F·g⁻¹ and 78.5 F·g⁻¹ after 1,200 cycles, respectively. The MnO₂/MWNT electrode has better cycling performance. The effect of different morphologies was investigated for both MnO₂/MWNT and MnO₂/AB composites.     

Preparation of activated carbons from cherry stones by activation with potassium hydroxide

Using cherry stones, the preparation of activated carbon has been undertaken in the present study by chemical activation with potassium hydroxide. A series of KOH-activated products was prepared by varying the carbonisation temperature in the 400-900 °C range. Such products were characterised texturally by gas adsorption (N₂, −196 °C), mercury porosimetry, and helium and mercury density measurements. FT-IR spectroscopy was also applied. The carbons prepared as a rule are microporous and macroporous solids. The degree of development of surface area and porosity increases with increasing carbonisation temperature. For the carbon heated at 900 °C the specific surface area (BET) is 1624 m² g⁻¹, the micropore volume is 0.67 cm³ g⁻¹, the mesopore volume is 0.28 cm³ g⁻¹, and the macropore volume is 1.84 cm³ g⁻¹.     

Mechanochemical synthesis of poly(2,5-dimethoxy aniline) nanobelts and its electrochemical performance in hybrid supercapacitors

A solvent-free mechanochemical route for the preparation of poly(2,5-dimethoxyaniline) hydrochloride nanostructures is developed and reported in the article. High conductivity, good crystallinity, and nanostructured morphology are observed for the prepared polymer. This polymeric powder is utilized as a cathode material in hybrid supercapacitor and its electrochemical performance is evaluated and discussed in this short report. The maximum specific capacitance of the poly(2,5-dimethoxyaniline) hydrochloride/activated carbon hybrid supercapacitor is found to be 125 F g⁻¹ at 1 mA cm⁻² current density. The cell delivers a specific energy as high as 50 Wh kg⁻¹ at a specific power of 97 W kg⁻¹ and also exhibits an excellent cycle performance with more than 99% coulombic efficiency and the maintenance of 85% of its initial capacitance after 1,000 cycles.     

Ultrasound-assisted facile one-pot synthesis of ternary MWCNT/MnO2/rGO nanocomposite for high performance supercapacitors with commercial-level mass loadings

Commercial application of supercapacitors (SCs) requires high mass loading electrodes simultaneously with high energy density and long cycle life. Herein, we have reported a ternary multi-walled carbon nanotube (MWCNT)/MnO₂/reduced graphene oxide (rGO) nanocomposite for SCs with commercial-level mass loadings. The ternary nanocomposite was synthesized using a facile ultrasound-assisted one-pot method. The symmetric SC fabricated with ternary MWCNT/MnO₂/rGO nanocomposite demonstrated marked enhancement in capacitive performance as compared to those with binary nanocomposites (MnO₂/rGO and MnO₂/MWCNT). The synergistic effect from simultaneous growth of MnO₂ on the graphene and MWCNTs under ultrasonic irradiation resulted in the formation of a porous ternary structure with efficient ion diffusion channels and high electrochemically active surface area. The symmetric SC with commercial-level mass loading electrodes (∼12 mg cm⁻²) offered a high specific capacitance (314.6 F g⁻¹) and energy density (21.1 W h kg⁻¹ at 150 W kg⁻¹) at a wide operating voltage of 1.5 V. Moreover, the SC exhibits no loss of capacitance after 5000 charge−discharge cycles showcasing excellent cycle life.     

Electrical studies on ionic liquid-based gel polymer electrolyte for its application in EDLCs

Ionic liquid-based gel polymer electrolyte (GPE) has been synthesized using standard solution cast technique. Different weight percent of ionic liquid, 1-Butyl-3-methylimidazolium chloride (BMIMCl) and liquid electrolyte, ethylene carbonate (EC)–propylene carbonate (PC)–tetra ethyl ammonium tetra fluoro borate (TEABF₄) was incorporated in polymer, poly(vinylidene fluoride-co-hexafluoro propylene (PVdF-HFP) to obtain mechanically stable gel polymer electrolyte film (GPE) having maximum conductivity of ~10⁻³ S cm⁻¹ at room temperature, which is acceptable from device fabrication point of view. Potential window and ionic transference number has been obtained to examine the potential limit and ionic characteristics of optimized GPE system. Temperature dependence behavior of electrical conductivity curve follows Arrhenius nature in the temperature range of 303–373 K. Pattern of dielectric constant and its loss as a function of frequency and temperature have been studied and is being explained on the basis of electrode interfacial polarization effect. Frequency-dependent conductivity spectra obey the Jonscher’s power law. Further, optimized composition of GPE has been tested successfully for its application in supercapacitor fabrication with activated charcoal as an electrode material. Maximum specific capacitance of 118.6 mF cm⁻² equivalent to single electrode specific capacitance of 61.7 F g⁻¹ have been observed for the optimized GPE film.