Performance evaluation of CNT/polypyrrole/MnO2 composite electrodes for electrochemical capacitors

A ternary composite of CNT/polypyrrole/hydrous MnO₂ is prepared by in situ chemical method and its electrochemical performance is evaluated by using cyclic voltammetry (CV), impedance measurement and constant-current charge/discharge cycling techniques. For comparative purpose, binary composites such as CNT/hydrous MnO₂ and polypyrrole/hydrous MnO₂ are prepared and also investigated for their physical and electrochemical performances. The specific capacitance (SC) values of the ternary composite, CNT/hydrous MnO₂ and polypyrrole/hydrous MnO₂ binary composites estimated by CV technique in 1.0 M Na₂SO₄ electrolyte are 281, 150 and 35 F g⁻¹ at 20 mV s⁻¹ and 209, 75 and 7 F g⁻¹ at 200 mV s⁻¹, respectively. The electrochemical stability of ternary composite electrode is investigated by switching the electrode back and forth for 10,000 times between 0.1 and 0.9 V versus Ag/AgCl at 100 mV s⁻¹. The electrode exhibits good cycling stability, retaining up to 88% of its initial charge at 10,000th cycle. A full cell assembled with the ternary composite electrodes shows a SC value of 149 F g⁻¹ at a current loading of 1.0 mA cm⁻² during initial cycling, which decreased drastically to a value of 35 F g⁻¹ at 2000th cycle. Analytical techniques such as scanning electron microscopy (SEM), X-ray diffraction spectroscopy (XRD), Brunauer-Emmet-Teller (BET) surface area measurement and inductively coupled plasma-atomic emission spectrometry (ICP-AES) are also used to characterize the composite materials.     

Fast and reversible surface redox reaction of graphene-MnO2 composites as supercapacitor electrodes

We present a quick and easy method to synthesize graphene-MnO₂ composites through the self-limiting deposition of nanoscale MnO₂ on the surface of graphene under microwave irradiation. These nanostructured graphene-MnO₂ hybrid materials are used for investigation of electrochemical behaviors. Graphene-MnO₂ composite (78 wt.% MnO₂) displays the specific capacitance as high as 310 F g⁻¹ at 2 mV s⁻¹ (even 228 F g⁻¹ at 500 mV s⁻¹), which is almost three times higher than that of pure graphene (104 F g⁻¹) and birnessite-type MnO₂ (103 F g⁻¹). Interestingly, the capacitance retention ratio is highly kept over a wide range of scan rates (88% at 100 mV s⁻¹ and 74% at 500 mV s⁻¹). The improved high-rate electrochemical performance may be attributed to the increased electrode conductivity in the presence of graphene network, the increased effective interfacial area between MnO₂ and the electrolyte, as well as the contact area between MnO₂ and graphene.     

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.     

NiO-based composite electrode with RuO2 for electrochemical capacitors

NiO/RuO₂ composite materials were prepared for use in electrochemical capacitors (ECs) by co-precipitation method followed by heat treatment. X-ray diffraction (XRD) spectra indicated that no new structural materials were formed and ruthenium oxide particles were coated by NiO particles. RuO₂ partly introduced into NiO-based electrode had improved its electrochemical performance and capacitive properties by using electrochemical measurements. A maximum specific capacitance of 210 F/g was obtained for NiO-based composite electrode with 10 wt.% RuO₂ in the voltage range from −0.4 to 0.5 V in 1 mol/l KOH solution. By comparison of effect of modified modes on the specific capacitance, chemically modified composite electrodes had more stable cycling properties than those of physically modified electrodes. After 200 cycles, specific capacitance of NiO-based chemical composite electrode with 5 wt.% RuO₂ kept 95% above, while that of physical electrode was only 79% of initial specific capacitance.     

Rapid microwave-assisted synthesis of graphene nanosheet/Co3O4 composite for supercapacitors

Graphene nanosheet (GNS)/Co₃O₄ composite has been rapidly synthesized by microwave-assisted method. Field emission scanning electron microscopy and transmission electron microscopy observation reveals the homogeneous distribution of Co₃O₄ nanoparticles (3-5 nm in size) on graphene sheets. Electrochemical properties are characterized by cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy. A maximum specific capacitance of 243.2 F g⁻¹ has been obtained at a scan rate of 10 mV s⁻¹ in 6 M KOH aqueous solution for GNS/Co₃O₄ composite. Furthermore, the composite exhibits excellent long cycle life along with ∼95.6% specific capacitance retained after 2000 cycle tests.     

Supercapacitive properties of a nanowire-structured MnO2 electrode in the gel electrolyte containing silica

Nanowire-structured MnO₂ active materials were prepared by a chemical precipitation method and their supercapacitive properties for use in the electrodes of supercapacitors were investigated by means of cyclic voltammetry in an aqueous gel electrolytes consisting of 1 M Na₂SO₄ and fumed silica (SiO₂). The MnO₂ electrode showed a maximum specific capacitance of 151 F g⁻¹ after 1000 cycles at 100 mV s⁻¹ when using the gel electrolyte containing 3 wt.% of SiO₂, which is higher than 121 F g⁻¹ obtained when using the 1 M Na₂SO₄ liquid electrolyte alone.     

Factors influencing MnO2/multi-walled carbon nanotubes composite's electrochemical performance as supercapacitor electrode

Poor crystallined α-MnO₂ grown on multi-walled carbon nanotubes (MWCNTs) by reducing KMnO₄ in ethanol are characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and Brunauer-Emmett-Telle (BET) surface area measurement, which indicate that MWCNTs are wrapped up by poor crystalline MnO₂ and BET areas of the composites maintain the same level of 200 m² g⁻¹ as the content of MWCNTs in the range of 0-30%. The electrochemical performances of the MnO₂/MWCNTs composites as electrode materials for supercapacitor are evaluated by cyclic voltammetry (CV) and galvanostatic charge-discharge measurement in 1 M Na₂SO₄ solution. At a scan rate of 5 mV s⁻¹, rectangular shapes could only be observed for the composites with higher MWCNTs contents. The effect of additional conductive agent KS6 on the electrochemical behavior of the composites is also studied. With a fixed carbon content of 25% (MWCNTs included), MnO₂ with 20% MWCNTs and 5% KS6 has the highest specific capacitance, excellent cyclability and best rate capability, which gives the specific capacitance of 179 F g⁻¹ at a scan rate of 5 mV s⁻¹, and remains 114.6 F g⁻¹ at 100 mV s⁻¹.     

Effects of electrochemical-deposition method and microstructure on the capacitive characteristics of nano-sized manganese oxide

The amorphous nano-structured manganese oxide was electrochemically deposited onto a stainless-steel electrode. The structure and surface morphology of the obtained manganese oxide were studied by means of X-ray diffraction analysis and scanning electron microscopy. The capacitive characteristics of the manganese oxide electrodes were investigated by means of cyclic voltammetry and constant current charge-discharge cycling. The morphological and capacitive characteristics of the hydrous manganese oxide was found to be strongly influenced by the electrochemical deposition conditions. The highest specific capacitance value of ca. 410 F g⁻¹ and the specific power of ca. 54 kW kg⁻¹ were obtained at 400 mV s⁻¹ sweep rate of potentiodynamic deposition condition. The cyclic-life data showed that the specific capacitance was highly stable up to 10,000 cycles examined. This suggests the excellent cyclic stability of the obtained amorphous hydrous manganese oxide for supercapacitor application.     

Effects of substrates on the capacitive performance of RuOx·nH2O and activated carbon-RuOx electrodes for supercapacitors

The electrochemical energy storage and delivery on the electrodes composed of hydrous ruthenium oxide (RuO_x·nH₂O) or activated carbon-hydrous ruthenium oxide (AC-RuO_x) composites are found to strongly depend on the substrate employed. The contact resistance at the active material-graphite interface is much lower than that at the active material-stainless steel (SS) mesh interface. Thin films of gold plus RuO_x·nH₂O deposited on SS meshes (RuO_x/Au/SS) are found to greatly improve the poor contact between SS meshes and electrode materials. The maximum specific capacitance (C_S,RuOx) of RuO_x·nH₂O, 1580 F g⁻¹ (measured at 1 mV s⁻¹), very close to the theoretic value, was obtained from an AC-RuO_x/RuO_x/Au/SS electrode with 10 wt.% sol-gel-derived RuO_x·nH₂O annealed in air at 200 °C for 2 h. The highly electrochemical reversibility, high-power characteristics, good stability, and improved frequency response of this AC-RuO_x/RuO_x/Au/SS electrode demonstrate its promising application potential in supercapacitors. The ultrahigh specific capacitance of RuO_x·nH₂O probably results from the uniform size distribution of RuO_x·nH₂O nanoparticles, ranged from 1.5 to 3 nm which is clearly observed from the high-resolution transmission electron microscopy (HRTEM).     

Preparation of exfoliated graphite containing manganese oxides with high electrochemical capacitance by microwave irradiation

Exfoliated graphite (EG) containing manganese oxides was rapidly and efficiently prepared by microwave irradiation. The maximum expanded volume of EG was up to 317 mL g⁻¹ when the mass ratio of natural graphite and nitric acid to potassium permanganate was 1:2:2. EG showed specific capacitance of 97.5 F g⁻¹ at a scan rate of 1 mV s⁻¹ due to a pseudo-capacitance originated from the deposited manganese oxide particles on the surfaces of EG. After 400 cycles, the specific capacitance of EG increased by 21.2% of initial capacitance demonstrating good cycling stability.     

Metal-organic framework (MOF) as a template for syntheses of nanoporous carbons as electrode materials for supercapacitor

Five nanoporous carbons (NPCs) were prepared by polymerizing and then carbonizing carbon precursor of furfuryl alcohol accommodated in a porous metal-organic framework (MOF-5, [Zn₄O(bdc)₃], bdc = 1,4-benzenedicarboxylate) template. The Brunauer-Emmett-Teller (BET) surface areas for five NPC samples obtained by carbonizing at the temperatures from 530 to 1000 °C fall into the range from 1140 to 3040 m² g⁻¹ and the dependence of BET surface areas on carbonization temperatures shows a “V” shape. All the five NPC samples have a pore size distribution centered at about 3.9 nm. As electrode materials for supercapacitor, the NPC samples obtained at the temperatures higher than 600 °C display the ideal capacitor behaviors and give rise to almost constant specific capacitance (above 100 F g⁻¹ at 5 mV s⁻¹) at various sweep rates, which is associated with their mesoporous characteristics. However, the NPC sample with the highest BET surface area (3040 m² g⁻¹) obtained by carbonizing at 530 °C gives a unusually low capacitance (12 F g⁻¹ at 5 mV s⁻¹), which may be attributed to the poor conductivity of the carbon material due to the low carbonization temperature.     

Hydrated Mn(IV) oxide-exfoliated graphite composites for electrochemical capacitor

Commercially available low cost exfoliated graphite (EG, nominal diameter 130 μm) was used as a conductive substrate for electrochemical capacitor of hydrated Mn(IV) oxide, MnO₂·nH₂O. The MnO₂·nH₂O-EG composites were prepared by addition of EG to potassium permanganate solution, followed by 1 h stirring and then slow addition of manganese(II) acetate solution. By this procedure submicrometer or smaller sized MnO₂·nH₂O particles having mesopores of 6-12 nm in diameter were formed on the graphite sheets of EG. Although EG alone showed only about 2 F g⁻¹, the composites showed good rectangular cyclic voltammograms at 2-20 mV s⁻¹ in 1 mol L⁻¹ Na₂SO₄. The capacitance per net amount of MnO₂ increased proportionally with EG content, that is, utilization ratio of MnO₂ increased with EG content. The composites of MnO₂·nH₂O and smaller diameter of EG (nominal diameter 45 μm) or artificial graphite powder (average diameter 3.7 μm) showed fairly good performance at 2 mV s⁻¹, but with increasing potential scan rate the rectangular shape was distorted and capacitance decreased drastically. The results implies that sheet-like structure is more effective than small particles as conductive materials, when the formation procedure of composite is the same. Large sized EG may be a promising conductive material for electrochemical capacitors.     

Preparation and capacitive properties of cobalt-nickel oxides/carbon nanotube composites

In this paper, nickel-cobalt oxides/carbon nanotube (CNT) composites were prepared by adding and thermally decomposing nickel and cobalt nitrates directly onto the surface of carbon nanotube/graphite electrode to form nickel and cobalt oxides. Carbon nanotubes used in this paper were grown directly on graphite substrate by chemical vapor deposition technique. The capacitive behavior of nickel-cobalt oxides/CNT electrode was investigated by cyclic voltammetry and galvanostatic charge-discharge method in 1 M KOH aqueous solutions. The results show that nickel-cobalt oxides/CNT composite electrode has excellent charge-discharge cycle stability (0.2% and 3.6% losses of the specific capacitance are found at the 1000th and 2000th charge-discharge cycles, respectively) and good charge-discharge properties at high currrent density. Additionally, the effect of Ni/Co molar ratio on specific capacitance of the composite electrode was investigated and the highest specific capacitance (569 F g⁻¹ at 10 mA cm⁻²) is obtained at Ni/Co molar ratio = 1:1.     

Development of new nanocomposite based on nanosized-manganese oxide and carbon nanotubes for high performance electrochemical capacitors

We report the synthesis of a new composite electrode based on nanosized-manganese oxide and carbon nanotubes (CNTs) by electrophoretic deposition of CNTs on a stainless steel (SS) substrate followed by direct spontaneous reduction of MnO₄⁻ ions to MnO₂ to form the multi-scaled SS-CNT-MnO₂ electrode. The resulting material was characterized by scanning electron microscopy, energy dispersive X-ray analysis, cyclic voltammetry and galvanostatic charge-discharge in a 0.65 M K₂SO₄ aqueous solution. The binderless SS-CNT-MnO₂ nanocomposite electrode shows a very high specific capacitance of 869 F/g of CNT-MnO₂ and good stability during long galvanostatic charge-discharge cycling. To the best of our knowledge, this is one of the highest capacitance for manganese oxide electrode ever reported. In addition to its applicability in electrochemical capacitors, this methodology could be extended to develop other high performance nanocomposite material electrodes based on carbon nanotubes and metal oxide for the future generation of electrochemical power sources.     

Coating manganese oxide onto graphite electrodes by immersion for electrochemical capacitors

In this study, manganese oxide was coated on a graphite electrode by immersion. Durations for immersion were varied to control the amount of manganese oxide coated onto the electrode surface. Maximum capacitance of 556 mF cm⁻² was obtained in 0.5 M LiCl and with better/superior conditions (immersion time = 80 min and potential scan rate = 10 mV s⁻¹). In addition, cyclic voltammograms of the prepared electrode at different potential scan rates exhibited the approximately rectangular and symmetric current-potential characteristics of a capacitor. Furthermore, the chronopotentiometry (CP) charge-discharge curves of the electrode prepared at 80 min of immersion time with a constant current of 1 mA were symmetric and similar isosceles triangles, which demonstrate its high electrochemical reversibility and good stability. Finally, under scanning electron microscope (SEM), the surface of the electrode prepared at 80 min of immersion time and after 1500 cycles of potential cycling revealed that numerously three-dimensional network of macropores appeared on large spherical grains.     

Preparation of a graphene nanosheet/polyaniline composite with high specific capacitance

A graphene nanosheet (GNS)/polyaniline (PANI) composite was synthesized using in situ polymerization. The morphology and microstructure of samples were examined by scanning electron microscopy (SEM), transition electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy. Electrochemical properties were characterized by cyclic voltammetry (CV) and galvanostatic charge/discharge. GNS as a support material could provide more active sites for nucleation of PANI as well as excellent electron transfer path. The GNS was homogeneously coated on both surfaces with PANI nanoparticles (∼2 nm), and a high specific capacitance of 1046 F g⁻¹ (based on GNS/PANI composite) was obtained at a scan rate of 1 mV s⁻¹ compared to 115 F g⁻¹ for pure PANI. In addition, the energy density of GNS/PANI composite could reach 39 W h kg⁻¹ at a power density of 70 kW kg⁻¹.     

Seed-mediated growth of platinum nanoparticles on carbon nanotubes for the fabrication of electrochemical biosensors

Platinum nanoparticles (PtNPs) were prepared by seed-mediated growth method with Au nanoparticles (AuNPs) playing the role of seeds. Carbon nanotubes (CNTs) and AuNPs were first dropped onto the surface of glassy carbon (GC) electrode, and then the electrode was immersed into growth solution which contains H₂PtCl₆ and ascorbic acid. PtNPs were successfully grown onto the CNT surface due to the chemical reduction of Pt(IV). The electrode modified with AuNP_seed/PtNP/CNT film displayed excellent electrochemical response to H₂O₂ at 0.45 V versus saturated calomel electrode (SCE) with sensitivity much larger than that of PtNP/CNT and AuNP_seed/PtNP modified electrodes. Glucose oxidase was selected as a model enzyme and electrodeposited onto the AuNP_seed/PtNP/CNT modified electrode in the presence of a detergent. The resulting biosensor enabled selective determination of glucose with high sensitivity of 4.49 μA mM⁻¹, quick response time about 2 s, low-detection limit of 0.5 μM and wide linear range from 1 μM to 4 mM with a correlation coefficient 0.9998. Thus, the modified electrode proved to be a nice electrochemical biosensing platform for the fabrication of oxidase-based biosensors.     

Facile solvothermal synthesis of a graphene nanosheet-bismuth oxide composite and its electrochemical characteristics

This work demonstrates a novel and facile route for preparing graphene-based composites comprising of metal oxide nanoparticles and graphene. A graphene nanosheet-bismuth oxide composite as electrode materials of supercapacitors was firstly synthesized by thermally treating the graphene-bismuth composite, which was obtained through simultaneous solvothermal reduction of the colloidal dispersions of negatively charged graphene oxide sheets in N,N-dimethyl formamide (DMF) solution of bismuth cations at 180 °C. The morphology, composition, and microstructure of the composites together with pure graphite oxide, and graphene were characterized using powder X-ray diffraction (XRD), FT-IR, field emission scanning electron microscopy (FESEM), transmission electron microscope (TEM), thermogravimetry and differential thermogravimetry (TG-DTG). The electrochemical behaviors were measured by cyclic voltammogram (CV), galvanostatic charge-discharge and electrochemical impedance spectroscopy (EIS). The specific capacitance of 255 F g⁻¹ (based on composite) is obtained at a specific current of 1 A g⁻¹ as compared with 71 F g⁻¹ for pure graphene. The loaded-bismuth oxide achieves a specific capacitance as high as 757 F g⁻¹ even at 10 A g⁻¹. In addition, the graphene nanosheet-bismuth oxide composite electrode exhibits the excellent rate capability and well reversibility.     

Preparation and characterization of manganese oxide/CNT composites as supercapacitive materials

Manganese oxide was synthesized and dispersed on carbon nanotube (CNT) matrix by thermally decomposing manganese nitrates. CNTs used in this paper were grown directly on graphite disk by chemical vapor deposition technique. The capacitive behavior of manganese oxide/CNT composites was investigated by cyclic voltammetry and galvanostatic charge–discharge method in 1 M Na₂SO₄ aqueous solutions. When the loading mass of MnO₂ is 36.9 μg cm^− 2, the specific capacitance of manganese oxide/CNT composite (based on MnO₂) at the charge–discharge current density of 1 mA cm^− 2 equals 568 F g^− 1. Additionally, excellent charge–discharge cycle stability (ca. 88% value of specific capacitance remained after 2500 charge–discharge cycles) and power characteristics of the manganese oxide/CNT composite electrode can be observed. The effect of loading mass of MnO₂ on specific capacitance of the electrode has also been investigated.     

Synthesis and electrochemical performance of Li2FeSiO4/carbon/carbon nano-tubes for lithium ion battery

Li₂FeSiO₄/carbon/carbon nano-tubes (Li₂FeSiO₄/C/CNTs) and Li₂FeSiO₄/carbon (Li₂FeSiO₄/C) composites were synthesized by a traditional solid-state reaction method and characterized comparatively by X-ray diffraction, scanning electron microscopy, BET surface area measurement, galvanostatic charge-discharge and AC impedance spectroscopy, respectively. The results revealed that the Li₂FeSiO₄/C/CNT composite exhibited much better rate performance in comparison with the Li₂FeSiO₄/C composite. At 0.2 C, 5 C and 10 C, the former composite electrode delivered a discharge capacity of 142 mAh g⁻¹, 95 mAh g⁻¹, 80 mAh g⁻¹, respectively, and after 100 cycles at 1 C, the discharge capacity remained 95.1% of its initial value.