An asymmetric supercapacitor using RuO2/TiO2 nanotube composite and activated carbon electrodes

We reported an asymmetric supercapacitor technology where RuO₂/TiO₂ nanotube composite was used as positive electrode and the activated carbon as negative electrode in 1 mol/L KOH electrolyte solution. The electrochemical capacitance performance of the asymmetric supercapacitor was tested by cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge-discharge tests. The results show that the asymmetric supercapacitor has electrochemical capacitance performance within potential range 0–1.4 V. A power density 1207 W/kg was obtained with an energy density of 5.7 W h/kg at a charge–discharge current density of 120 mA/cm². The supercapacitor also exhibits a good cycling performance and keep 90% of initial capacity over 1000 cycles.     

Flexible supercapacitor electrodes based on TiO2/rGO/TiO2 sandwich type hybrids

The flexible nanostructured supercapacitors have gained vast majority of interests during recent years. In this article, flexible supercapacitor electrode based on TiO₂/rGO/TiO₂ sandwich is fabricated through a facile low cost solution process method based on pre-synthesized vapor assisted GO paper and titania sol. The XRD and FTIR spectroscopy analyses confirm the in-situ reduction of GO paper when faced with the titania sol. The Raman spectroscopy shows the coexistence of titania anatase phase beside rGO layers. Moreover, FESEM analysis demonstrates that the sandwich electrodes are composed of titania and rGO layers with thickness of about 660 nm and 15 µm, respectively. The optimum parameter for film deposition is 0.17 M concentration, water to Ti precursor ratio of 4, acid catalyst to Ti precursor ratio of 0.5, and solvent of 1-propanol. The supercapacitor electrode based on this optimum deposited sandwich illustrates capacitance of 83.7 F/g at scan rate of 5 mV/s and appreciable charge-discharge behavior. These hybrid pseudo- and electric double layer capacitance behavior in this supercapacitor not only can dramatically improve the performance of the future energy storage devices but also can be applicable in cost-effective wearable electronics.     

Structural evolution of multi-walled carbon nanotube/MnO2 composites as supercapacitor electrodes

Multi-walled carbon nanotube (MWCNT)/MnO₂ supercapacitor electrodes containing MnO₂ nanoflakes in the MWCNT network are fabricated through the oxidation of manganese acetate with poly(4-styrenesulfonic acid) (PSS) dispersed MWCNTs. The structural evolution of the electrodes under charge/discharge (reduction/oxidation) cycles and its impact on the electrodes’ electrochemical properties are evaluated. Structural evolution involves the dissolution of MnO₂ upon reduction, the diffusion of the reduced Mn species from the MWCNT network toward the electrolyte solution, and the deposition of MnO₂ on the electrode surface upon oxidation. Electrode structural changes, including the electrode dissolution and the growth of the MnO₂ crystals, are scan rate dependent and have deteriorating effect on the electrode's electrochemical properties including the specific capacitance and cyclic stability.     

Nanodiamond particles/reduced graphene oxide composites as efficient supercapacitor electrodes

The paper reports on the preparation of reduced graphene oxide (rGO) modified with nanodiamond particles composites by a simple solution phase and their use as efficient electrode in electrochemical supercapacitors. The technique relies on heating aqueous solutions of graphene oxide (GO) and nanodiamond particles (NDs) at different ratios at 100 °C for 48 h. The morphological properties, chemical composition and electrochemical behavior of the resulting rGO/NDs nanocomposites were investigated using UV/vis spectrometry, Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, transmission electron microscopy (TEM) and electrochemical means. The electrochemical performance, including the capacitive behavior of the rGO/NDs composites were investigated by cyclic voltammetry and galvanostatic charge/discharge curves at 1 and 2 A g⁻¹ in 1 M H₂SO₄. The rGO/ND matrix with 10/1 ratio displayed the best performance with a specific capacitance of 186 ± 10 F g⁻¹ and excellent cycling stability.     

Rational design of binder-free ZnCo2O4 and Fe2O3 decorated porous 3D Ni as high-performance electrodes for asymmetric supercapacitor

The development of hierarchical, porous film based current collector has created huge interest in the area of energy storage, sensor, and electrocatalysis due to its higher surface area, good electrical conductivity and increased electrode-electrolyte interface. Here, we report a novel method to prepare a hierarchically ramified nanostructured porous thin film as a current collector by dynamic hydrogen bubble template electro-deposition method. At a first time, we report a porous 3D-Ni decorated with ZnCo₂O₄ and Fe₂O₃ by simple, low-cost electrochemical deposition method. The fabricated porous 3D-Ni based electrodes showed an excellent electrochemical property such as high specific capacitance, excellent rate capability, and good cycle stability. The asymmetric solid-state supercapacitor device was fabricated using porous, 3D Ni decorated with ZnCo₂O₄ and Fe₂O₃ as the positive and negative electrodes. The fabricated ZnCo₂O₄//Fe₂O₃ asymmetric device delivered an areal capacitance of 92?mF?cm^?2 at a current density of 0.5?mA?cm^?2 with a maximum areal power density of 3?W?cm^?2 and areal energy density of 28.8?mWh?cm^?2. The higher performances of porous, 3D current collector have a huge potential in the development of high performance supercapacitor.     

Self-discharge and potential recovery phenomena at thermally and electrochemically prepared RuO2 supercapacitor electrodes

Films of ruthenium oxide (RuO₂) exhibit large, almost constant capacitance, over a potential range of ˜ 1.4 V in aqueous acid solutions. This behaviour has led to their development as supercapacitor materials giving many Farads per gram. Applications of electrochemical capacitors require minimum self-discharge rates. In the present paper, the self-discharge kinetics of charged RuO₂ electrodes are studied and a remarkable phenomenon of successive potential recovery after sequential discharge transients is reported. The self-discharge and potential-recovery behaviour is analysed in terms of a process of diffusion of oxidation state involving proton and electron hopping, treated in a model of the RuO₂ film structure having three regions between which redistribution of oxidation states in the oxide film takes place on self-discharge and recovery.     

Hydrothermal synthesis of Polypyrrole/MoS2 intercalation composites for supercapacitor electrodes

As a pseudocapacitive electrode materials for supercapacitor, Polypyrrole (PPy) exhibit excellent theoretical specific capacitance. However, it suffers from a poor cycling stability due to structural instability during charge-discharge process. In this work, a novel and facile hydrothermal method has been developed for the intercalation composites of PPy/MoS₂ with multilayer three-dimensional structure. The report result shows that the as-prepared electrode possess a outstanding electrochemical properties with significantly specific capacitance of 895.6 F g⁻¹ at current density of 1 A g⁻¹, higher energy density (3.774 Wh kg⁻¹) at power density of 252.8 kW kg⁻¹, furthermore, it also achieve remarkable cycling stability (~98% capacitance retention after 10,000 cycles) which is attributed to the synergistic effect of PPy and MoS₂. This synthetic strategy integrates performance enables the multilayer PPy/MoS₂ composites to be a promising electrode for energy storage applications.     

Preparation of porous PbO2 electrodes by electrochemical deposition of composites

Porous PbO₂ electrodes have been prepared by anodic codeposition of PbO₂ particles with an electrochemically grown PbO₂ matrix, with the aim of illustrating how composite deposition may be a suitable method for obtaining electrodes of large and controlled surface roughness. By exploiting the different crystal structures of PbO₂, the composition of α-PbO₂+β-PbO₂ composites (i.e. composites with an α matrix and a β dispersed phase) has been determined by X-ray diffraction. It has been found that reliable indications on composition of deposits may be also obtained by measuring the current efficiency of the deposition process, which largely exceeds unity in the case of composites. This way, the dependence of the dispersed phase concentration in the deposits on current density and suspended particles concentration has been determined. The surface roughness of the composite electrodes has been estimated by electrochemical impedance spectroscopy. It was found to increase almost linearly with the deposition charge and reach limiting values as either the deposition current density or the suspended particles concentration are increased.     

Densification behavior of mullite-Al2TiO5 composites by reaction sintering of natural andalusite and TiO2

In this work, mullite-Al₂TiO₅ composites were fabricated by natural andalusite with TiO₂ as an additive. The densification characteristic, phase composition and mullitization process of andalusite with TiO₂ addition was investigated by the Archimedes’ principle, dilatometry, X-ray diffraction and scanning electron microscopy (SEM-EDS) techniques. The results showed that the incorporation of TiO₂ not only enhanced the thermal stability of in-situ Al₂TiO₅ in the silica liquid yielded from the mullitization of andalusite, but also accelerated andalusite decomposition and retarded mullite formation, which facilitated the sintering and densification of mullite-Al₂TiO₅ composites.     

Highly stable mesoporous CeO2/CeS2 nanocomposite as electrode material with improved supercapacitor electrochemical performance

The performance of a pseudocapacitor electrode relies largely on the conductivity, cyclic stability, specific surface area and the mesoporosity of the nanomaterials. The CeO₂ is highly stable oxide but poor conductor, on the other hand, CeS₂ is highly conductive but its stability is questionable. Herein, we report the synthesis of CeO₂/CeS₂ nanocomposite, and exploit the properties of both the constituent materials and demonstrates that CeO₂/CeS₂ nanocomposite electrode exhibits an improved capacitance and energy density than CeO₂ nanomaterial. It encompasses large number of pores with a mean size of ～17?nm. The mesoporous nature of the CeO₂/CeS₂ nanocomposite electrode increases its activity, rapid diffusion and transportation of ions and facilitates surface-dependent reversible redox reactions. The nanocomposite electrode demonstrates high stability and its specific capacitance increases almost linearly up to 1000 cyclic voltammetry (CV) cycles. At a current density of 1?A/g it achieves a specific capacitance of 420?F/g. These findings evidently suggest the practical use of CeO₂/CeS₂ nanocomposite as electrode material for future supercapacitors.     

Chlorine evolution and reduction on RuO2/TiO2 electrodes

The Cl₂ evolution reaction has been investigated on RuO₂/TiO₂ electrodes of various composition in 1 M HCl and 1 M KCl electrolyte by means of stationary and non stationary i-E curves and by ac impedance. A reaction mechanism is suggested. The qualitative features of Cl₂ reduction from saturated solutions in HCl and KCl are discussed.     

A consideration of the activation energy for the chlorine evolution reaction on RuO2 and IrO2 electrodes

The electrocatalysis for chlorine evolution reaction on RuO₂ and IrO₂ electrodes was discussed on the basis of the activation energy. The activated complex with the pentagonal bipyramid-type structure was speculated to be formed in the transition state for the Heyrovsky reaction. The activation energy for this reaction was evaluated by using the difference of the crystal field stabilization energy between the initial and transition states. The good agreement was obtained between the calculated and found values.     

TiO2 photocatalysts and diamond electrodes

Photocatalysis and electroanalysis are two seemingly disparate research areas, but they are linked by the fact that both involve the use of well-known materials, TiO₂ and diamond, respectively, in new ways in the service of both environmental and medical sciences. In the present article, recent developments in the area of TiO₂ photocatalysis and diamond electrochemistry are summarized, with emphasis on our findings at the University of Tokyo. In the photocatalysis section, we present the fundamental aspects of TiO₂ photocatalysis and its practical applications, including air purification, self-cleaning surfaces and transparent superhydrophilic coatings. The diamond electrochemistry section deals with the electrochemical characterization and applications of diamond electrodes, which exhibit high sensitivity and excellent stability for electroanalysis, in contrast to conventional electrode materials. A particularly interesting environmental application of diamond electrodes has been developed; this involves the trace analysis of lead without the use of mercury.     

Oxygen reduction on Ag-MnO2/SWNT and Ag-MnO2/AB electrodes

Bifunctional catalysts (Ag-MnO₂)/SWNT were prepared by using a simple chemical reduction route on single-walled carbon nanotubes (SWNT). Five weight percentage of Ag-MnO₂ on SWNT was found to be the best loading in terms of current density. The (Ag-MnO₂)/SWNT catalyst was characterized and the kinetics towards oxygen reduction reaction (ORR) was determined and compared with that of Ag-MnO₂ on acetylene black (AB) catalyst. The number of exchanged electrons for the ORR was found to be close to four on both (Ag-MnO₂)/SWNT and (Ag-MnO₂)/AB. The kinetic rate constant of catalytic reaction for the two catalysts was of the same order. The zinc-air batteries with both catalysts were fabricated and examined. A stable discharge potential plateau appeared at approximately 1.2 and 1.1 V with discharge capacities 261 and 190 mAh/g, respectively.     

Solid-state reaction of Cr2O3 and SnO2

Various mixtures of SnO₂---Cr₂O₃ and SnO₂---K₂Cr₂O₇, were subjected to different thermal treatments. The crystalline phases in the calcination products have been identified by X-ray diffraction. The non-formation of definite compounds was also proved by diffuse reflectance: The structural environment of the cation Cr(III) was also studied.     

Effects of surface modification on the cycling stability of LiNi0.8Co0.2O2 electrodes by CeO2 coating

A high-performance LiNi_0.8Co_0.2O₂ cathode was successfully fabricated by a sol-gel coating of CeO₂ to the surface of the LiNi_0.8Co_0.2O₂ powder and subsequent heat treatment at 700 °C for 5 h. The surface-modified and pristine LiNi_0.8Co_0.2O₂ powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), slow rate cyclic voltammogram (CV), and differential scanning calorimetry (DSC). Unlike pristine LiNi_0.8Co_0.2O₂, the CeO₂-coated LiNi_0.8Co_0.2O₂ cathode exhibits no decrease in its original specific capacity of 182 mAh/g (versus lithium metal) and excellent capacity retention (95% of its initial capacity) between 4.5 and 2.8 V after 55 cycles. The results indicate that the surface treatment should be an effective way to improve the comprehensive properties of the cathode materials for lithium ion batteries.     

Mechanical properties of Al2O3/ZrO2 composites

In the present study, both t-phase zirconia and m-phase zirconia particles are incorporated into an alumina matrix. Dense Al₂O₃/(t-ZrO₂+m-ZrO₂) composites were prepared by sintering pressurelessly at 1600 °C. The microstructure of the composites are characterized, the elastic modulus, strength and toughness determined. Because the ZrO₂ inclusions are close to each other in the Al₂O₃ matrix, the yttrium ion originally in t-ZrO₂ particles can diffuse to nearby m-ZrO₂ particles during sintering, and the m-phase zirconia is thus stabilized after sintering. The strength of the Al₂O₃/(t-ZrO₂+m-ZrO₂) composites after surface grinding can reach values as high as 940 MPa, which is roughly three times that of Al₂O₃ alone. The strengthening effect is contributed by microstructural refinement together with the surface compressive stresses induced by grinding. The toughness of alumina is also enhanced by adding both t-phase and m-phase zirconia, which can reach values as high as two times that of Al₂O₃ alone. The toughening effect is attributed mainly to the zirconia t–m phase transformation.     

Influence of crystal surface orientation on redox reactions at semiconducting MoS2

The redox system Cu(NH₃)^2+/1+_x, Fe(CN)^{$\text{3?}\text{4?}$}₆ and Fe^3+/2+ and oxygen reduction have been studied on MoS₂ surfaces normal to the C-axis and parallel to the C-axis, using rotating disc electrodes under potentiostatic conditions. Current—voltage curves were obtained and apparent exchange current densities (i₀) and rate constants (k⁰_app) have been evaluated. The i₀ and k^o_app values are one order of magnitude larger for the |-C surfaces than for the ⊥-C surfaces of MoS₂. Both forward and reverse reactions occurred readily in the case of Cu(NH₃)^2+/1+_x whereas the reverse reaction was partly or totally blocked in the case of Fe(CN)^{$\text{3?}\text{4?}$}₆ and Fe^{$\text{3+}\text{2+}$}, respectively. The results indicate that surface states and crystal imperfections play a very important role for all these redox reactions. These surface states seem to be connected with Mo-atoms at edges and steps on the surface where they are directly exposed to the electrolyte. The differences in the electron transfer rate between the two surfaces is attributed to the greater overlapping of the electron orbitals of the conduction band at the |-C surface of MoS₂ with the electron orbitals of the redox systems, than of those at the ⊥-C surface.     

Preparation and electrochemical capacitance of RuO2/TiO2 nanotubes composites

In this paper, RuO₂/TiO₂ nanotubes composites were synthesized by loading various amounts of RuO₂ on TiO₂ nanotubes. The symmetric supercapacitors based on these nanocomposites were fabricated by using gel polymer PVA-H₃PO₄-H₂O as electrolyte. The electrochemical capacitance performance of the nanocomposites in these supercapacitors was investigated by current-potential responses, galvanostatic charge-discharge tests and electrochemical impedance spectroscopy. The results show that the three dimensional nanotube network of TiO₂ offers a solid support structure for active materials RuO₂, allows the active material to be readily accessible (available) for electrochemical reactions, and improves the efficiency of the active materials. A maximum specific capacitance of 1263 F/g was obtained for the RuO₂ which was loading on TiO₂ nanotubes.