Exaggerated capacitance using electrochemically active nickel foam as current collector in electrochemical measurement

In the past decades, nickel and cobalt oxide/hydroxide materials have been investigated intensively for supercapacitor applications. Some works report very high specific capacitance values, up to 3152 F g⁻¹, for these materials. By contrast, some other works report quite modest capacitance values, up to 380 F g⁻¹ for the same materials prepared using same strategy. It is found that most works reporting very high capacitance value applied nickel foam as current collector. In this paper, surface chemistry and electrochemical properties of nickel foam are investigated by XPS analysis, cyclic voltammetry and galvanostatic charge-discharge measurement. The results show that using nickel foam as current collector can bring about substantial errors to the specific capacitance values of electrode materials, especially when small amount of electrode active material is used in the measurement. It is suggested that an electrochemically inert current collector such as Ti or Pt film should be used for testing electrochemical properties of nickel and cobalt oxide/hydroxide positive electrode materials.     

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.     

A novel electrochemical process to prepare a high-porosity manganese oxide electrode with promising pseudocapacitive performance

A nanoporous nickel (Ni) substrate was successfully prepared by selective dissolution of copper (Cu) from a Ni–Cu alloy layer. It was noted that both the Cu etching and the Ni/Cu codeposition processes could be performed in the same solution. Afterwards, anodic deposition was carried out to disperse fibrous manganese (Mn) oxide onto the nanoporous Ni substrate. As a result, a novel oxide electrode with a high-porosity structure was fabricated by the totally electrochemical procedure, which is very simple and efficient. Pseudocapacitive performance of this oxide electrode was evaluated by cyclic voltammetry in 0.1 M Na₂SO₄ solution. The data indicated that specific capacitance of the Mn oxide was as high as 502 F g⁻¹, which was 85% higher than that deposited on a flat electrode. Capacitance retained ratio after 500 charge–discharge cycles of the Mn oxide was also significantly improved from 75 to 93% due to the use of the nanoporous substrate.     

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.     

Optical, structural and electrochromic properties of nickel oxide films produced by sol-gel technique

Nickel oxide films have been deposited from nickel acetate precursor using a sol-gel dip coating method, onto glass and conducting fluorine doped tin oxide (FTO) glass substrate. The direct energy gap (E_gd) values for the 2-10 layered films are in the range of 3.62 eV-3.72 eV. X-ray diffraction (XRD) analysis reveals that films consisting of 2-6 layers are amorphous, while films consisting of 8-10 layers are poly-crystalline with cubic grains of around 12 nm-20 nm and preferential growth along the (1 1 1) and (2 0 0) planes. Fourier Transform Infrared (FTIR) spectrum confirms the formation of Ni-O. Electrochromic properties of the nickel oxide coatings were studied using cyclic voltammetric (CV) technique. The 8 layered NiO films exhibit the anodic/cathodic diffusion coefficient of 16.7/5.73 × 10⁻¹³ cm²/s and the change in optical transmission is ΔT_630nm = 53% with a photopic contrast ratio of 2.87.     

Polypyrrole/carbon aerogel composite materials for supercapacitor

Polypyrrole (PPy)/carbon aerogel (CA) composite materials with different PPy contents are prepared by chemical oxidation polymerization through ultrasound irradiation and are used as active electrode material for supercapacitor. The morphology of PPy/CA composite is examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that PPy is deposited onto the surface of CA. As evidenced by cyclic voltammetry, galvanostatic charge/discharge test and EIS measurements, PPy/CA composites show superior capacitive performances to CA, moreover, the results based on cyclic voltammograms show that the composite material has a high specific capacitance of 433 F g⁻¹, while the capacitance of CA electrode is only 174 F g⁻¹. Although the supercapacitor used PPy/CA as active electrode material has an initial capacitance loss due to the instability of PPy, the specific capacitance after 500 cycles stabilizes nearly at a fixed value.     

Pseudo-capacitance of composite electrode of ruthenium oxide with porous carbon in non-aqueous electrolyte containing imidazolium salt

Pseudo-capacitance of composite materials where ruthenium oxide particles are loaded on activated carbon has been evaluated in the electrolyte of 1-ethyl-3-methyl imidazolium tetrafluoroborate dissolved in acetonitrile. The composite materials prepared by conventional a sol-gel method have dispersed structure of ruthenium oxide particle of tens nanometer diameter on the surface of activated carbon. The extent of the pseudo-capacitance of the composite electrodes in the imidazolium salt electrolyte, estimated by the comparison of the capacitance per surface area of electrode in different non-aqueous electrolyte, is ca. 3-5 μF cm⁻² in addition to the double-layer capacitance of ca. 6 μF cm⁻², depending on the loading status of ruthenium oxide. The symmetric cell consisting of the composite electrode containing 18 wt% of ruthenium oxide and the imidazolium salt electrolyte provides cell capacitance based on the pseudo-capacitance by a constant-current test.     

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.     

Enhanced electrochemical hydrogen storage capacity of activated mesoporous carbon materials containing nickel inclusions

Electrochemical properties of activated ordered mesoporous carbon (OMC) containing nickel inclusions are investigated using cyclic voltammetry and galvanostatic charge/discharge techniques. The hard-template-route prepared OMC is of structurally well-ordered two-dimensional hexagonal structure with a high specific surface area of 1896.95 cm² g⁻¹, a pore volume of 1.781 cm³ g⁻¹ and a pore size of 5.1 nm, respectively. It is shown that OMC/0.3Ni electrode displays the highest specific capacitance of 186.1 Fg⁻¹, almost 1.4 times higher than that of pure OMC electrode. The hydrogen storage capacity of pure OMC electrode is 87 mAh g⁻¹ and there exists no discharge platform. With the amount of nickel addition increasing, there appears a relatively stable discharge platform, and the discharge capacity reaches a maximum of 170 mAh g⁻¹ as the molar ratio of Ni:OMC is 0.3, almost two times higher than that of pure OMC electrode. The electrochemical properties of OMC can be greatly improved with incorporation of nickel powders. The Ni activated OMC electrodes display a high capacity retainability with strong resistance against oxidation and corrosion.     

Fabrication and electrochemical characterization of cobalt-based layered double hydroxide nanosheet thin-film electrodes

A continuous cobalt-based layered double hydroxide (LDH) nanosheet thin-film electrode has been fabricated by drying a nearly transparent colloidal solution of cobalt-based LDH nanosheets on an indium tin oxide (ITO)-coated glass plate substrate. The effects of varying the Al content, the film thickness, and the heating temperature on the electrochemical properties of the as-deposited thin-film electrode have been investigated. A thin-film electrode with a Co/Al molar ratio of 3:1, which has a large specific capacitance of 2500 F cm⁻³ (833 F g⁻¹) and a good high-rate capability, shows the best performance when used as an electrode in thin-film supercapacitors (TFSCs). As the thickness of the thin film was increased from 100 to 500 nm, the specific capacitance of the thin-film electrode remained essentially unchanged, which is due to the porous microstructure generated in the original electrochemical process and the low internal resistance of the thin-film electrode. The specific capacitance of the thin-film electrode showed no observable change after heating at 160 °C, but decreased on further heating to 200 °C, indicating that the electrochemically active Co sites inside the thin-film nanosheet electrode are already essentially fully exposed in the as-prepared material and hence cannot be further exposed through heating. Such a thin-film electrode made up of nanosheets may be a potential economical alternative electrode for use in TFSCs.     

Capacitance decay of nanoporous nickel hydroxide

Nanoporous nickel hydroxide Ni(OH)₂ coated on nickel foam by using a chemical bath deposition method shows a high specific capacitance of 2200 F g⁻¹ at a discharging current density of 1 Ag⁻¹. After 500 charge-discharge cycles, the specific capacitance is stabilized at 1470 Fg⁻¹, and there is only a 5% fall in specific capacitance during the following 1500 cycles. The relationship between the capacitance decay and changes in the microstructure and morphology of nanoporous Ni(OH)₂ is investigated. The results show that phase transformation and the growth of particle/crystal size, rather than the formerly proposed flaking off of Ni(OH)₂, are the major factors contributing to the capacitance decay.     

Electrochemical capacitance of Co3O4 nanowire arrays supported on nickel foam

Co₃O₄ nanowire arrays freely standing on nickel foam are prepared via template-free growth followed by thermal treatment at 300 °C in air. Their morphology is examined by scanning and transmission electron microscopy. The electrochemical capacitance behavior of the self-supported binderless nanowire array electrode is investigated by cyclic voltammetry, galvanostatic charge-discharge test and electrochemical impedance spectroscopy. The results show that nanowires are formed by nanoplatelets packed roughly layer by layer. They densely cover the nickel foam substrate and have diameters around 250 nm and the lengths up to around 15 μm. The Co₃O₄ nanowires display a specific capacitance of 746 F g⁻¹ at a current density of 5 mA cm⁻². The capacitance loss is less than 15% after 500 charge-discharge cycles. The columbic efficiency is higher than 93%.     

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).     

Evaluation of the effects of oxygen evolution on the capacity and cycle life of nickel hydroxide electrode materials

The effect of charge–discharge cycling on the capacity of surface-adhered nickel hydroxide (Ni(OH)₂) micro-particles is investigated in aqueous KOH by cyclic voltammetry, and compared with that for pasted nickel hydroxide electrodes. Cyclic voltammetry on adhered Ni(OH)₂ micro-particles enables rapid screening of four types of commercially available, battery-grade, nickel hydroxide samples and allows the separation of the oxidation process from the oxygen evolution reaction. With large pasted electrodes, due to their high uncompensated resistance (R_u), these processes are poorly resolved. Pasted β-nickel hydroxide electrodes with a specific capacity of between 190 and 210 mAh g⁻¹ are charged and discharged at constant currents greater than 15 C (18 mA cm⁻²). With no voltage limit in the charging profile, excess oxygen evolution occurs and capacity fading is observed within the first 50 cycles. Loss of capacity is attributed to the degradation of the electrode due to excess oxygen evolution at switching potentials greater than 0.55 V versus Hg/HgO (1 M KOH). X-ray diffraction (XRD) measurements confirm the formation of γ-NiOOH in these electrodes. Limiting the cell voltage to 1.5 V, and thereby minimizing oxygen evolution, results in no observed capacity loss within 100 cycles, and only β-Ni(OH)₂ can be detected by XRD phase analysis.     

Improved pseudocapacitive performance and cycle life of cobalt hydroxide on an electrochemically derived nano-porous Ni framework

The pseudocapacitance and morphology of an electrodeposited cobalt hydroxide (Co(OH)₂) significantly depends on the architecture of the electrode substrate. The nano-porous Ni framework, derived from the selective dissolution of Cu from a Ni-Cu alloy, effectively promotes the electrochemical utilization of deposited Co(OH)₂ even at a high loading amount condition. The great electronic and ionic conduction within the nano-structured electrode improves the energy storage performance of Co(OH)₂ as compared to that for a conventional flat Ni substrate. In this work, the Co(OH)₂ mass specific capacitance, evaluated using cyclic voltammetry (CV), only slightly decreases from 2650 to 2470 F g⁻¹ when the potential sweep rate is substantially increased from 5 to 200 mV s⁻¹. The developed Ni(OH)₂/NiOOH (from the nano-porous framework) incorporates with the deposited Co(OH)₂ upon CV cycling; the mixed hydroxide shows a noticeably synergistic capacitance. Furthermore, the dissolution of Co(OH)₂ in KOH electrolyte is greatly suppressed due to the incorporation of Ni(OH)₂/NiOOH, consequently prolonging the electrode cycle life.     

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.     

Supercapacitive properties of polyaniline/Nafion/hydrous RuO2 composite electrodes

Chemically prepared polyaniline is tested for its supercapacitive behaviour in an aqueous electrolyte of 1.0 M H₂SO₄. In order to improve the cycleability of the polyaniline electrode, it is made into a composite with Nafion. This composite electrode shows improved cycleability and higher specific capacitance compared with a pure polyaniline electrode. It is therefore used as a matrix for the electrochemical deposition of hydrous RuO₂. The resulting ternary composite electrode has a high specific capacitance of 475 F g⁻¹ at 100 mV s⁻¹ and 375 F g⁻¹ at 1000 mV s⁻¹ in the voltage range of −0.2 to 0.8 V versus Ag/AgCl. All three types of electrode are characterized by cyclic voltammetry and impedance anaylsis.     

Nanoporous H-sorbed carbon as anode of secondary cell

Nanoporous carbon is proposed as reversible hydrogen electrode for an electrochemical cell operating in aqueous KOH solution. Hydrogen is stored by water electroreduction on the carbon electrode during the charging step, while nickel gauze is used as auxiliary positive electrode. Then, electrical energy is harvested by using the charged electrode as anode associated to an air cathode. A specific capacity of 390–450 Ah kg⁻¹ of carbon material was measured. The system is technically more simple than a traditional fuel cell, since it does not require any hydrogen reservoir, membrane and noble metal for the hydrogen electrode.     

Preparation and electrochemical capacitance performances of super-hydrophilic conducting polyaniline

Super-hydrophilic conducting polyaniline was prepared by surface modification of polyaniline using tetraethyl orthosilicate in water/ethanol solution, whereas its conductivity was 4.16 S cm⁻¹ at 25 °C. And its electrochemical capacitance performances as an electrode material were evaluated by the cyclic voltammetry and galvanostatic charge/discharge test in 0.1 M H₂SO₄ aqueous solution. Its initial specific capacitance was 500 F g⁻¹ at a constant current density of 1.5 A g⁻¹, and the capacitance still reached about 400 F g⁻¹ after 5000 consecutive cycles. Moreover, its capacitance retention ratio was circa 70% with the growth of current densities from 1.5 to 20 A g⁻¹, indicating excellent rate capability. It would be a promising electrode material for aqueous redox supercapacitors.