Electrophoretic deposition of graphene nanosheets on nickel foams for electrochemical capacitors

Graphene nanosheets are deposited on nickel foams with 3D porous structure by an electrophoretic deposition method using the colloids of graphene monolayers in ethanol as electrolytes. The high specific capacitance of 164 F g⁻¹ is obtained from cyclic voltammetry measurement at a scan rate of 10 mV s⁻¹. When the current densities are set as 3 and 6 A g⁻¹, the specific capacitance values still reach 139 and 100 F g⁻¹, respectively. The high capacitance is attributed to nitrogen atoms in oxidation product of p-phenylene diamine (OPPD) adsorbed on the surface of the graphene nanosheets. The comparable results suggest potential application to electrochemical capacitors based on the graphene nanosheets.     

Fabrication of porous nickel oxide film with open macropores by electrophoresis and electrodeposition for electrochemical capacitors

The electrophoretic deposition of polystyrene sphere monolayer as a template for anodic electrodeposition of interconnected nickel oxide nanoflakes is explored. Result indicates that a nickel oxide film with nanoflakes and open macropores has superior capacitive behavior. A nickel oxide film with interconnected nanoflakes is of great importance for electrochemical capacitors due to the high-specific surface area, fast redox reactions, and shortened diffusion path in solid phase. The open macropores may facilitate the electrolyte penetration and ion migration, therefore increasing the utilization of nickel oxide due to the increased surface area for electrochemical reactions. The specific capacitance of a nickel oxide film with open macropores at a scan rate of 10 mV s⁻¹ reaches as high as 351 F g⁻¹, which is 2.5 times higher than that of the bare nickel oxide film (140 F g⁻¹).     

Chemically grown, porous, nickel oxide thin-film for electrochemical supercapacitors

A porous nickel oxide film is successfully synthesized by means of a chemical bath deposition technique from an aqueous nickel nitrate solution. The formation of a rock salt NiO structure is confirmed with XRD measurements. The electrochemical supercapacitor properties of the nickel oxide film are examined using cyclic voltammetery (CV), galvanostatic and impedance measurements in two different electrolytes, namely, NaOH and KOH. A specific capacitance of ∼129.5 F g⁻¹ in the NaOH electrolyte and ∼69.8 F g⁻¹ in the KOH electrolyte is obtained from a cyclic voltammetery study. The electrochemical stability of the NiO electrode is observed for 1500 charge-discharge cycles. The capacitative behaviour of the NiO electrode is confirmed from electrochemical impedance measurements.     

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.     

Cathodic deposition and characterization of tin oxide coatings on graphite for electrochemical supercapacitors

Amorphous tin oxide (SnO_x) was cathodically deposited onto graphite electrode in a bath containing 0.1 M stannous chloride (SnCl₂), 0.5 M sodium nitrate (NaNO₃), and 0.4 M nitric acid (HNO₃) in an aqueous solution of 50% (v/v) ethanol. The SnO_x coatings grown on graphite were characterized as typical capacitive behaviors by cyclic voltammetry (CV), chronopotentiometric (CP) in 0.5 M KCl. Specific capacitance (in milli-farad per square centimeter, C_a) changes linearly with the deposition charge up to 4.5 C cm⁻², and a maximum of as high as 355 mF cm⁻² was obtained with the SnO_x coating grown at around 5 C cm⁻². For the SnO_x coating deposited at 0.2 C cm⁻², a maximum specific capacitance (in farad per gram, C_m) of 298 and 125 F g⁻¹ was achieved from CVs at a scan rate of 10, and 200 mV s⁻¹, respectively. The value of C_m significantly gets lower from 265 to around 95 F g⁻¹ when the deposition charge increases from 0.2 to around 6.0 C cm⁻². The long cycle-life and stability of the SnO_x coatings on graphite via the presented cathodic deposition were also demonstrated.     

Electrospun nickel oxide/polymer fibrous electrodes for electrochemical capacitors and effect of heat treatment process on their performance

Electrospinning is a versatile method for preparation of submicron-size fibers under ambient temperature. We demonstrate a new approach based on this method for preparing an electrode which consists of the fibers coated with nickel oxide (NiO) and acetylene black (AB) on their surfaces. The NiO/polymer fibrous electrodes show the electrochemical responses based on the electrochemical reaction of Ni(OH)₂ which is produced from NiO in alkaline aqueous solution. The capacitance of the test half cell with the as-prepared NiO/polymer fibrous electrode in 1 mol l⁻¹ KOH aqueous solution is 5.8 F g⁻¹ (per gram of NiO). Heat treatment (at 150 °C for 1 h in the air) of the NiO/polymer fibrous electrode increases the capacitance of the NiO/polymer fibrous electrode. The capacitance of the cell with the heat treated (HT) NiO/polymer fibrous electrode is 163 F g⁻¹ (per gram of NiO). SEM observation of the heat treated electrode suggests that partial melt of the fibers on the current collector forms the conducting passes and networks between the NiO particles and the collector and increases the specific capacity of the fibrous electrode.     

Anodic deposition of manganese oxide electrodes with rod-like structures for application as electrochemical capacitors

The electrochemical properties of nanocrystalline manganese oxide electrodes with rod-like structures were investigated to determine the effect of morphology, chemistry and crystal structure on the corresponding electrochemical behavior of manganese electrodes. Manganese oxide electrodes of high porosity composed of 1-1.5 μm diameter rods were electrochemically synthesized by anodic deposition from a dilute solution of Mn(CH₃COO)₂ (manganese acetate) onto Au coated Si substrates without any surfactants, catalysts or templates under galvanostatic control. The morphology of the electrodes depended on the deposition current density, which greatly influenced the electrochemical performance of the capacitor. Electrochemical property and microstructure analyses of the manganese oxide electrodes were conducted using cyclic voltammetry and microstructural techniques, such as scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The synthesized rod-like manganese oxide electrodes at low current densities exhibited a high specific capacitance due to their large surface areas. The largest value obtained was 185 F g⁻¹ for deposits produced at .5 mA cm⁻². Specific capacity retention for all deposits, after 250 charge-discharge cycles in an aqueous solution of 0.5 M Na₂SO₄, was about 75% of the initial capacity.     

Incorporation of polyaniline into macropores of three-dimensionally ordered macroporous carbon electrode for electrochemical capacitors

Three-dimensionally ordered macroporous (3DOM) carbons having walls composed of mesosized spherical pores were prepared by a colloidal crystal templating method. A composite electrode consisting of bimodal porous carbon and polyaniline (PAn) was prepared by electropolymerization of aniline within the macropores of the bimodal porous carbon. It was found that the deposition of PAn decreased the porosity and specific surface area (SSA) of the electrode. The electrochemical properties of the composite electrode were characterized in a mixed solution of ethylene carbonate (EC) and diethyl carbonate (DEC) containing 1 mol dm⁻³ LiPF₆. The discharge capacity of the carbon–PAn composite electrode was 111 mAh g_carbon–PAn⁻¹ in the potential range of 2.0–4.0 V vs. Li/Li⁺, which corresponded to a volumetric discharge capacity of 53 mAh cm⁻³. Both the double-layer capacity (30 mAh g⁻¹) and the redox capacity of PAn (81 mAh g⁻¹) contributed to the discharge capacity of the composite electrode. The carbon–PAn composite showed good rate capability, and the discharge capacity at a high current density of 6.0 A g⁻¹ was as high as 81 mAh g⁻¹.     

Evaluation of electrochemical hydrogen storage capability of graphene oxide multi-layer coating

Due to its unique physical and chemical properties, graphene oxide (GO) has excellent potential in energy-saving applications, especially in hydrogen storage materials such as bulk and layer coatings. A three-layer GO/Ni/GO coating was applied successfully on the Ni-foam through the hybrid coating process. GO was firstly synthesized by modified Hummers' method and then deposited on Ni-Foam using an electrophoretic deposition (EPD) method at 40, 60, and 80 V at various times (1 and 2 h). Furthermore, Ni-layer was applied on the first applied to GO layer using a Ni-electroplating bath at 3 V for 3 min. For evaluation of the electrochemical hydrogen storage performance, cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) were carried out in 1 M NaOH solution at room temperature. Obtained results showed a significant improvement in the amount of adsorbed hydrogen. The hydrogen storage capacity was increased from 88 to 741 F. g^?1, and load transfer resistance decreased from 260 to 35 Ω for uncoated and coated porous substrates, respectively, which is due to the achieved high specific area. Based on the hydrogen storage capacity, the optimum EPD method is 60 V and 2 h.     

Nickel hydroxide/activated carbon composite electrodes for electrochemical capacitors

In this paper, a nickel hydroxide/activated carbon (AC) composite electrode for use in an electrochemical capacitor was prepared by a simple chemical precipitation method. The structure and morphology of nickel hydroxide/AC were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that nano-sized nickel hydroxide was loading on the surface of activated carbon. Electrochemical performance of the composite electrodes with different loading amount was studied by cyclic voltammetry and galvanostatic charge/discharge measurements. It was demonstrated that the introduction of a small amount of nickel hydroxide to activated carbon could promote the specific capacitance of a composite electrode. The composite electrodes have good electrochemical performance and high charge–discharge properties. Moreover, when the loading amount of nickel hydroxide was 6 wt.%, the composite electrode showed a high specific capacitance of 314.5 F g⁻¹, which is 23.3% higher than pure activated carbon (255.1 F g⁻¹). Also, the composite electrochemical capacitor exhibits a stable cyclic life in the potential range of 0–1.0 V.     

Prediction of the self-discharge profile of an electrochemical capacitor electrode in the presence of both activation-controlled discharge and charge redistribution

In the porous electrodes typically used for electrochemical capacitors, the self-discharge profile may be affected by charge redistribution in the pores of the electrode and the kinetics of the Faradaic self-discharge. In this paper, the activation-controlled self-discharge of a porous electrode is modelled using a de Levie transmission line hardware circuit to model a pore and an activation-controlled discharge is applied at the mouth of this “pore”. The self-discharge profile exhibits three main regions. The first region is governed purely by the activation-controlled discharge as the charge redistribution has not yet begun. The second region combines both charge redistribution and the activation-controlled discharge, resulting in a shallower slope than expected for a purely activation-controlled discharge. Finally, charge redistribution ends and the profile reverts to the activation-controlled profile. These three regions are mirrored in experimental self-discharge profiles and show that the duration charge redistribution can be determined from the shape of the self-discharge profile.     

Electrophoretic deposition on non-conducting substrates: The case of YSZ film on NiO–YSZ composite substrates for solid oxide fuel cell application

This paper report the results of our investigation on electrophoretic deposition (EPD) of YSZ particles from its suspension in acetylacetone onto a non-conducting NiO–YSZ substrate. In principle, it is not possible to carry out electrophoretic deposition on non-conducting substrates. In this case, the EPD of YSZ particles on a NiO–YSZ substrate was made possible through the use of an adequately porous substrate. The continuous pores in the substrates, when saturated with the solvent, helped in establishing a “conductive path” between the electrode and the particles in suspension. Deposition rate was found to increase with increasing substrate porosity up to a certain value. The higher the applied voltage, the faster the deposition. For a given applied voltage, there exists a threshold porosity value below which EPD becomes practically impossible. An SOFC constructed on bi-layers of NiO–YSZ/YSZ with YSZ layer thickness of 40 μm exhibited an open circuit voltage (OCV) of 0.97 V at 650 °C and peak power density of 263.8 mW cm⁻² at 850 °C when tested with H₂ as fuel and ambient air as oxidant.     

A new approach for design of organic electrochromic devices with inter-digitated electrode structure

We present a new approach for design of organic electrochromic devices (ECD) with inter-digitated electrode (IDE) structure and three-electrode dynamic operation. The advantages of the IDE design include the ability to produce fast and homogenous color change over large areas. In addition, it enables fabrication of multi-color devices. Our method involves photolithographic etching of ITO followed by electrophoretic deposition (EPD) and mechanical compression of porous titania to produce finely patterned electrodes with high surface area. The titania layer is chemically modified by new stable and reversible electrochromic viologen derivatives involving phenylphosphonic acid anchoring moiety. The new device demonstrates reversible and strong color change from colorless to deep blue and yellow.     

Electrophoretic deposition of graphene oxide on carbon brush as bioanode for microbial fuel cell operated with real wastewater

Microbial fuel cell (MFC) is a promising technology for simultaneous wastewater treatment and energy harvesting. The properties of the anode material play a critical role in the performance of the MFC. In this study, graphene oxide was prepared by a modified hummer's method. A thin layer of graphene oxide was incorporated on the carbon brush using an electrophoretic technique. The deoxygenated graphene oxide formed on the surface of the carbon brush (RGO-CB) was investigated as a bio-anode in MFC operated with real wastewater. The performance of the MFC using the RGO-CB was compared with that using plain carbon brush anode (PCB). Results showed that electrophoretic deposition of graphene oxide on the surface of carbon brush significantly enhanced the performance of the MFC, where the power density increased more than 10 times (from 33 mWm^?2 to 381 mWm^?2). Although the COD removal was nearly similar for the two MFCs, i.e., with PCB and RGO-CB; the columbic efficiency significantly increased in the case of RGO-CB anode. The improved performance in the case of the modified electrode was related to the role of the graphene in improving the electron transfer from the microorganism to the anode surface, as confirmed from the electrochemical impedance spectroscopy measurements.     

Fabrication of nickel boride-coated carbon nanotube films by electrophoresis and electroless deposition for electrochemical hydrogen storage

Network-like carbon nanotube (CNT) films free of polymer binder were deposited directly onto a stainless steel substrate by electrophoresis. Results of experiments indicated that a CNT film with duplex surface treatment (nitric acid etching and nickel boride coating) provides satisfactory electrochemical hydrogen storage. Nitric acid treatment increased the hydrogen storage capacity of the CNT film due to the opening of CNTs, the formation of micropores in the CNT surface, and the improvement of surface wettability. Coating the CNT film with nickel boride (Ni₂B) greatly improved the electrochemical activity of the film and consequently increased the hydrogen storage capacity. The optimal amount of nickel boride coating on the CNT film was found to be about 45 wt%. Smaller amounts of nickel boride cannot provide enough electrochemical activity. Excessive nickel boride, on the other hand, leads to a decrease in the number of active sites on CNTs that are available for hydrogen storage.     

Mesoporous carbon-encapsulated NiO nanocomposite negative electrode materials for high-rate Li-ion battery

In this study, a novel mesoporous carbon-encapsulated NiO nanocomposite is proposed and demonstrated for Li-ion battery negative electrode. The nanostructure of the electrode composes of an ordered mesoporous CMK-3 as a 3D nanostructured current collector with micorporous channels for Li⁺ transportation. In addition, exclusive formation of NiO nanoparticles in the confined space of the ordered mesoporous carbon is achieved using the hydrophobic encapsulation route. The half-cell assembled with the synthesized NiO/CMK-3 nanocomposite is able to deliver a high charge capacity of 812 mAh g⁻¹ at the first cycle at a C-rate of 1000 mA g⁻¹ and retained throughout the test with only 0.236% decay per cycle. Even the C-rate as high as 3200 mA g⁻¹, a charge capacity of 808 mAh g⁻¹ contributed by the NiO nanoparticles in CMK-Ni is obtained, which shows excellent rate capability for NiO with utilization close to 100%. The result suggests fast kinetics of conversion reaction for NiO with Li⁺. It also indicates the blockage of the pore channels by NiO nanoparticles does not take place in the synthesized NiO/CMK-3.     

Poly(3-methylthiophene)/MnO2 composite electrodes as electrochemical capacitors

Composite electrodes prepared by electrodeposition of manganese oxide on titanium substrates modified with poly(3-methylthiophene) (PMeT) were investigated and compared with Ti/MnO₂ electrodes. The polymer films were prepared by galvanostatic deposition at 2 mA cm⁻² with different deposition charges (250 and 1500 mC cm⁻²). The electrodes were characterized by cyclic voltammetry in 1 mol L⁻¹ Na₂SO₄ and by scanning electron microscopy. The results show a very significant improvement in the specific capacitance of the oxide due the presence of the polymer coating. For Ti/MnO₂ the specific capacitance was of 122 F g⁻¹, while Ti/PMeT₂₅₀/MnO₂ and Ti/PMeT₁₅₀₀/MnO₂ displayed values of 218 and 66 F g⁻¹, respectively. If only oxide mass is considered, the capacitances of the composite electrode increases to 381 and 153 F g⁻¹, respectively. The micrographs of samples show that the polymer coating leads to very significant changes in the morphology of the oxide deposit, which in consequence, generate the improvement observed in the charge storage property.     

Electrode and electrolyte materials for electrochemical capacitors

Among different electric energy storage technologies electrochemical capacitors are used for energy storage applications when high power delivery or uptake is needed. Their energy and power densities, durability and efficiency are influenced by electrode and electrolyte materials however due to a high cost/performance ratio; their widespread use in energy storage systems has not been attained yet.Thanks to their properties such as high surface area, controllable pore size, low electrical resistance, good polarizability and inertness; activated carbons derived from polymeric precursors are the most used electrode materials in electrochemical capacitors at present. Other electrode materials such as shaped nano-carbons or metal oxides are also investigated as electrode materials in electrochemical capacitors, but only as useful research tools.Most commercially used electrochemical capacitors employ organic electrolytes when offering concomitant high energy and high power densities. The use of aqueous based electrolytes in electrochemical capacitor applications is mainly limited to research purposes as a result of their narrow operating voltage. Recent studies on room temperature ionic liquids to be employed as electrolyte for electrochemical capacitor applications are focused on fine tuning their physical and transport properties in order to bring the energy density of the device closer to that of batteries without compromising the power densities.In this paper a performance analysis, recent progress and the direction of future developments of various types of materials used in the fabrication of electrodes for electrochemical capacitors are presented. The influence of different types of electrolytes on the performance of electrochemical capacitors such as their output voltage and energy/power densities is also discussed.