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.     

Pseudocapacitive properties of electrochemically prepared nickel oxides on 3-dimensional carbon nanotube film substrates

Nickel oxides on carbon nanotube electrodes (NiO_x/CNT electrodes) are prepared by depositing Ni(OH)₂ electrochemically onto carbon nanotube (CNT) film substrates with subsequent heating to 300 °C. Compared with the as deposited Ni(OH)₂ on CNT film substrates (Ni(OH)₂/CNT electrodes), the 300 °C heat treated electrode shows much high rate capability, which makes it suitable as an electrode in supercapacitor applications. X-ray photoelectron spectroscopy shows that the pseudocapacitance of the NiO_x/CNT electrodes in a 1 M KOH solution originates from redox reactions of NiO_x/NiO_xOH and Ni(OH)₂/NiOOH. The 8.9 wt.% NiO_x in the NiO_x/CNT electrode shows a NiO_x-normalized specific capacitance of 1701 F g⁻¹ with excellent high rate capability due to the 3-dimensional nanoporous network structure with an extremely thin NiO_x layer on the CNT film substrate. On the other hand, the 36.6 wt.% NiO_x/CNT electrode has a maximum geometric and volumetric capacitance of 127 mF cm⁻² and 254 F cc⁻¹, respectively, with a specific capacitance of 671 F g⁻¹, which is much lower than that of the 8.9% NiO_x electrode. This decrease in specific capacitance of the high wt.% NiO_x/CNT electrodes can be attributed to the dead volume of the oxides, high equivalent series resistance for a heavier deposit, and the ineffective ionic transportation caused by the destruction of the 3-dimensional network structure. Deconvolution analysis of the cyclic voltammograms reveals that the rate capability of the NiO_x/CNT electrodes is adversely affected by the redox reaction of Ni(OH)₂, while the adverse effects from the reaction of NiO_x is insignificant.     

Characterization of honeycomb-like “β-Ni(OH)2” thin films synthesized by chemical bath deposition method and their supercapacitor application

Nanostructured nickel hydroxide thin films are synthesized via a simple chemical bath deposition (CBD) method using nickel nitrate Ni(NO₃)₂ as the starting material. The deposition process is based on the thermal decomposition of ammonia-complexed nickel ions at 333 K. The structural, surface morphological, optical, electrical and electrochemical properties of the films are examined. The nanocrystalline “β” phase of Ni(OH)₂ is confirmed by the X-ray diffraction analysis. Scanning electron microscopy reveals a macroporous and interconnected honeycomb-like morphology. Optical absorption studies show that “β-Ni(OH)₂” has a wide optical band-gap of 3.95 eV. The negative temperature coefficient of the electrical resistance of “β-Ni(OH)₂”, is attributed to the semiconducting nature of the material. The electrochemical properties of “β-Ni(OH)₂” in KOH electrolyte are examined by cyclic voltammetric (CV) measurements. The scan-rate dependent voltammograms demonstrate pseudocapacitive behaviour when “β-Ni(OH)₂” is employed as a working electrode in a three-electrode electrochemical cell containing 2 M KOH electrolyte with a platinum counter electrode and a saturated calomel reference electrodes. A specific capacitance of ∼398 × 10³ F kg⁻¹ is obtained.     

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.     

High-rate discharge properties of nickel hydroxide/carbon composite as positive electrode for Ni/MH batteries

A chemical co-precipitation method was attempted to synthesize nickel hydroxide/carbon composite material for high-power Ni/MH batteries. The XRD analysis showed that there were a large amount of defects among the crystal lattice of the Ni(OH)₂/C composite, and the SEM investigation revealed that the as-synthesized spherical particles were composed of hundreds of nanometer crystals with a unique three-dimensional petal shape. Compared with pure Ni(OH)₂, the Ni(OH)₂/C composite showed improved electrochemical properties such as superior cycling stability, higher discharge capacity and higher mean voltage of discharge under high-rate discharge conditions, the discharge capacity and the mean discharge voltage of the Ni(OH)₂/C composite were about 281 mAh g⁻¹ and 0.303 V (vs. Hg/HgO) at 1 C-rate, 273 mAh g⁻¹ and 0.296 V at 5 C-rate, 250 mAh g⁻¹ and 0.292 V at 10 C-rate, respectively. The cyclic voltammetry (CV) tests showed that the Ni(OH)₂/C composite exhibited good electrochemical reversibility and the formation of γ-NiOOH during the charge–discharge processes was prevented. The existence of carbon in the Ni(OH)₂/C composite contributed great effect on the improvement of high-rate discharge performance.     

Effects of addition of different carbon materials on the electrochemical performance of nickel hydroxide electrode

Nickel hydroxide is used as an active material in positive electrodes of rechargeable alkaline batteries. The capacity of nickel-metal hydride (Ni-MH) batteries depends on the specific capacity of the positive electrode and utilization of the active material because of the Ni(OH)₂/NiOOH electrode capacity limitation. The practical capacity of the positive nickel electrode depends on the efficiency of the conductive network connecting the Ni(OH)₂ particle with the current collector. As β-Ni(OH)₂ is a kind of semiconductor, the additives are necessary to improve the conductivity between the active material and the current collector. In this study the effect of adding different carbon materials (flake graphite, multi-walled carbon nanotubes (MWNT)) on the electrochemical performance of pasted nickel-foam electrode was established. A method of production of MWNT special type of catalysts had an influence on the performance of the nickel electrodes. The electrochemical tests showed that the electrode with added MWNT (110-170 nm diameter) exhibited better electrochemical properties in the chargeability, specific discharge capacity, active material utilization, discharge voltage and cycling stability. The nickel electrodes with MWNT addition (110-170 nm diameter) have exhibited a specific capacity close to 280 mAh g⁻¹ of Ni(OH)₂, and the degree of active material utilization was ∼96%.     

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%.     

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.     

Electrochemical supercapacitor behavior of Ni3(Fe(CN)6)2(H2O) nanoparticles

Nanosized Ni₃(Fe(CN)₆)₂(H₂O) was prepared by a simple co-precipitation method. The electrochemical properties of the sample as the electrode material for supercapacitor were studied by cyclic voltammetry (CV), constant charge/discharge tests and electrochemical impedance spectroscopy (EIS). A specific capacitance of 574.7 F g⁻¹ was obtained at the current density of 0.2 A g⁻¹ in the potential range from 0.3 V to 0.6 V in 1 M KNO₃ electrolyte. Approximately 87.46% of specific discharge capacitance was remained at the current density of 1.4 A g⁻¹ after 1000 cycles.     

Synthesis and characterization of spherical nonstoichiometric Ni(OH)x (x = 2.03-2.10) as electrode materials

The spherical nonstoichiometric Ni(OH)_x (x = 2.03-2.10), as a new positive electrode material for Ni/MH batteries, are synthesized by spherical β-Ni(OH)₂ surface modified with chemically oxidized NiOOH nanoparticles. The average nickel oxidation state, microstructure and morphology of the spherical nonstoichiometric Ni(OH)_x are investigated by complexometric titration, X-ray diffraction (XRD), scamming electron microscopy (SEM) and transmission electron microscopy (TEM). It is demonstrated that the NiOOH with a flaky-like morphology are dispersed randomly on the surface of the spherical β-Ni(OH)₂. The effect of NiOOH on the electrochemical performance of spherical nonstoichiometric Ni(OH)_x is studied by galvanostatic charge-discharge experiments and cyclic voltammetry. Compared with the spherical β-Ni(OH)₂, the spherical nonstoichiometric Ni(OH)_x (x = 2.05) has an enhanced discharge capacity (300 mAh g⁻¹ at 0.2 C), higher discharge potential plateau and superior cycle stability. The existence of chemically oxidized NiOOH nanoparticles in the nickel electrode contributes great effect on the improvement of electrochemical performance.     

Electrochemical performance of carbon-coated lithium manganese silicate for asymmetric hybrid supercapacitors

Nanoscale carbon-coated Li₂MnSiO₄ powder is prepared using a conventional solid-state method and can be used as the negative electrode in a Li₂MnSiO₄/activated carbon (AC) hybrid supercapacitor. Carbon-coated Li₂MnSiO₄ material presents a well-developed orthorhombic crystal structure with a Pmn2₁ space group, although there is a small impurity of MnO. The maximum specific capacitance of the Li₂MnSiO₄/AC hybrid supercapacitor is 43.2 F g⁻¹ at 1 mA cm⁻² current density. The cell delivers a specific energy as high as 54 Wh kg⁻¹ at a specific power of 150 W kg⁻¹ and also exhibits an excellent cycle performance with more than 99% columbic efficiency and the maintenance of 85% of its initial capacitance after 1000 cycles.     

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.     

Supercapacitive properties of hybrid films of manganese dioxide and polyaniline based on active carbon in organic electrolyte

This is the first report about supercapacitive performance of hybrid film of manganese dioxide (MnO₂) and polyaniline (PANI) in an organic electrolyte (1.0 M LiClO₄ in acetonitrile). In this work, a high surface area and conductivity of active carbon (AC) electrode is used as a substrate for PANI/MnO₂ film electro-codeposition. The redox properties of the coated PANI/MnO₂ thin film exhibit ideal capacitive behaviour in 1 M LiClO₄/AN. The specific capacitance (SC) of PANI/MnO₂ hybrid film is as high as 1292 F g⁻¹ and maintains about 82% of the initial capacitance after 1500 cycles at a current density of 4.0 mA cm⁻², and the coulombic efficiency (η) is higher than 95%. An asymmetric capacitor has been developed with the PANI/MnO₂/AC positive and pure AC negative electrodes, which is able to deliver a specific energy as high as 61 Wh kg⁻¹ at a specific power of 172 W kg⁻¹ in the range of 0-2.0 V. These results indicate that the organic electrolyte is a promising candidate for PANI/MnO₂ material application in supercapacitors.     

Insights into the electrochemistry of layered double hydroxide containing cobalt and aluminum elements in lithium hydroxide aqueous solution

The electrochemical capacitive behavior of layered double hydroxide containing cobalt and aluminum (Co–Al LDH), synthesized by a “memory effect” route, was in detail evaluated by cyclic voltammetry (CV) and chronopotentiometry in 1 M LiOH aqueous electrolyte. A specific capacitance of 187 F g⁻¹ was obtained even after 1000 cycles at a current of 2 A g⁻¹. Moreover, it was found that Co–Al LDH undergoes two independent electrode processes in LiOH aqueous solution, involving the simultaneous intercalation of an ion-pair, i.e. lithium cation and hydroxyl group, which is different from the mechanisms in NaOH and KOH aqueous solutions. The possible reason is thought to be the selective intercalation into Co–Al LDH for alkali metal ions due to their respective ionic radius. Only the cation with appropriate size is suited for inserting into [Co(OH)₆] or [Al(OH)₆] octahedral vacancies.     

Effect and function mechanism of amorphous sulfur on the electrochemical properties of cobalt hydroxide electrode

S-Co(OH)₂ composite is prepared via a facile co-precipitation method and investigated as negative electrode of Ni/Co battery. The addition of amorphous S improves the electrochemical properties of Co(OH)₂ electrode. The discharge capacity of S-Co(OH)₂ electrode can reach 413.2 mAh g⁻¹ and still keep about 340 mAh g⁻¹ after 300 cycles, which is much higher than that of S-free Co(OH)₂ electrode. Amorphous S in S-Co(OH)₂ electrode shows two functions during the charge-discharge process. One is that the addition of amorphous S with high specific surface area improves the dispersion of Co(OH)₂ platelets. The other is that the dissolution of amorphous S in electrode brings the new interspaces among the Co(OH)₂ platelets, these two factors largely increase the interspaces among Co(OH)₂ platelets. More interspaces are correlated to larger contact area with alkaline solution, which is in favor of the surface electrochemical redox. Thus, the capacity utilization of Co(OH)₂ is enhanced.     

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.     

Electrochemical properties of nanosized hydrous manganese dioxide synthesized by a self-reacting microemulsion method

Manganese dioxide has been synthesized by a new simple self-reacting microemulsion method. The synthesized MnO₂ has been found to be amorphous structure containing a moderate amount of water by X-ray diffraction, Fourier transform infrared spectroscopy and thermogravimetric analysis. Particles in a spherical shape with about 4 nm in diameter have been observed by transmission electron microscopy. Cyclic voltammetic tests have been performed between −0.5 and 0.5 V versus Hg/Hg₂SO₄ in 1 mol L⁻¹ Na₂SO₄ solution at sweep rates up to 50 mV s⁻¹. A specific capacitance value as high as 246.2 F g⁻¹ was obtained, which was much higher than 146.5 F g⁻¹ of MnO₂ prepared by chemical co-precipitation. After 600 cycles, only 6% decrease of specific capacitance was measured which indicated that such a material possesses good cycling property.     

Preparation and supercapacitance of CuO nanosheet arrays grown on nickel foam

CuO nanosheet arrays freely standing on nickel foam are prepared via a template-free growth method. The morphology of CuO nanosheet arrays is examined by scanning and transmission electron microscopy and the phase structure of nanosheets is analyzed by X-ray diffraction spectroscopy. The supercapacitance of CuO nanosheet arrays is investigated by cyclic voltammetry, galvanostatic charge/discharge test and electrochemical impedance spectroscopy. The results show that the array of CuO nanosheets forms a uniform film of around 5 μm in thickness on nickel foam skeleton. The film is composed of clusters of arrays of nanosheets with a thickness up to around 150 nm. The CuO nanosheet arrays exhibit a specific capacitance of 569 F g⁻¹ at a current density of 5 mA cm⁻² in 6.0 mol dm⁻³ KOH electrolyte. The capacitance loss is less than 17.5% after 500 charge/discharge cycles at 10 mA cm⁻² and with columbic efficiency higher than 93%.     

Enhancement of the energy storage properties of supercapacitors using graphene nanosheets dispersed with metal oxide-loaded carbon nanotubes

Graphene nanosheets (GNs) dispersed with SnO₂ nanoparticles loaded multiwalled carbon nanotubes (SnO₂-MWCNTs) were investigated as electrode materials for supercapacitors. SnO₂-MWCNTs were obtained by a chemical method followed by calcination. GNs/SnO₂-MWCNTs nanocomposites were prepared by ultrasonication of the GNs and SnO₂-MWCNTs. Electrochemical double layer capacitors were fabricated using the composite as the electrode material and aqueous KOH as the electrolyte. Electrochemical performance of the composite electrodes were compared to that of pure GNs electrodes and the results are discussed. Electrochemical measurements show that the maximum specific capacitance, power density and energy density obtained for supercapacitor using GNs/SnO₂-MWCNTs nanocomposite electrodes were respectively 224 F g⁻¹, 17.6 kW kg⁻¹ and 31 Wh kg⁻¹. The fabricated supercapacitor device exhibited excellent cycle life with ∼81% of the initial specific capacitance retained after 6000 cycles. The results suggest that the hybrid composite is a promising supercapacitor electrode material.     

A doped activated carbon prepared from polyaniline for high performance supercapacitors

A novel doped activated carbon has been prepared from H₂SO₄-doped polyaniline which is prepared by the oxypolymerization of aniline. The morphology, surface chemical composition and surface area of the carbon have been investigated by scanning electron microscope, X-ray photoelectron spectroscopy and Brunaner-Emmett-Teller measurement, respectively. Electrochemical properties of the doped activated carbon have been studied by cyclic voltammograms, galvanostatic charge/discharge, and electrochemical impedance spectroscopy measurements in 6 mol l⁻¹ KOH. The specific capacitance of the carbon is as high as 235 F g⁻¹, the specific capacitance hardly decreases at a high current density 11 A g⁻¹ after 10,000 cycles, which indicates that the carbon possesses excellent cycle durability and may be a promising candidate for supercapacitors.