Electrochemical behaviour of Ni(OH)2 ultrafine powder

Ultrafine Ni(OH)₂ powder is prepared by converting a Ni₂C₂O₄ precipitate in NaOH solution which contained Tween-80. The sample prepared by this method is β(II)-type phase and its particle size is about 30 nm. The electrochemical behaviour of nickel foam electrodes using this ultrafine Ni(OH)₂ powder as active material is studied and compared with micron-sized spherical Ni(OH)₂ by means of a galvanostatic charge–discharge method, cyclic voltamperommetry (CV) and electrochemical impedance spectroscopy (EIS). It is found that ultrafine Ni(OH)₂ powder has superior electrochemical properties, such as lower polarization, better reversibility, and smaller reaction resistance.     

Layered double (Ni,Fe) hydroxide grown on nickel foam and modified by nickel carbonyl powder and carbon black as an efficient electrode for water splitting

Developing novel oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) catalysts is vital for water splitting. Here, carbon black (CB) and nickel carbonyl powder (NCP) are used as components, and the nonionic surfactant polyvinyl pyrrolidone (PVP) is used as a shape-controlled capping agent to easily prepare layered double (Ni, Fe) hydroxide (NiFe-LDH) electrodes. Scanning electron microscopy observation and X-ray photoelectron spectra analysis show that the Fe²⁺-doped layered double hydroxide is grown in situ on nickel foam (NF). CB (XC-72) and NCP further improve the electrical conductivity. At 10 mA?cm^?2, the overpotentials of the OER and HER are 203 mV and 83 mV, respectively, in 1 M KOH. Only 1.48 V is needed when both electrodes are applied for water splitting, and it has a stability of more than 100 h. This work can be used as a medium to elevate the OER and HER performance of NiFe-LDH-based catalysts.     

Hybrid capacitor with activated carbon electrode,Ni(OH)2 electrode and polymer hydrogel electrolyte

A new hybrid capacitor (HC) cell was assembled using an activated carbon (AC) negative electrode, an Ni(OH)₂ positive electrode and a polymer hydrogel electrolyte prepared from crosslinked potassium poly(acrylate) (PAAK) and KOH aqueous solution. The HC cell was characterized compared with an electric double layer capacitor (EDLC) using two AC electrodes and the polymer hydrogel electrolyte. It was found that the HC cell successfully worked in the larger voltage range and exhibited ca. 2.4 times higher capacitance than the EDLC cell. High-rate dischargeability of the HC cell was also superior to that of the EDLC cell. These improved characteristics strongly suggest that the HC cell can be a promising system of capacitors with high energy and power densities.     

Fabrication and characterization of Ni modified TiO2 electrode as anode material for direct methanol fuel cell

Highly ordered porous titanium dioxide nanotube (TiO₂-NT) surfaces were prepared with anodization method to obtain a larger specific surface area that plays a very important role in methanol oxidation. In this regard, optimum conditions such as various anodization voltages and times were determined. The largest surface area of TiO₂ occurred at anodization voltage and time of 60 V and 2 h, respectively. After obtaining the high specific surface area, very small amounts of Nickel (Ni) nanoparticles were deposited on TiO₂-NT surface and their behaviors of methanol electro-oxidation were investigated by Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and chronoamperometry (CA) methods. Characterizations of the TiO₂-NT and Ni modified electrodes are exerted by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD). The average tube length and diameter are 36.32 μm and 93.6 nm according to SEM images. XRD results indicated the tetragonal structured anatase of TiO₂ and Ni (111) and (200). While methanol oxidation peak does not observe on TiO₂-NT surface, behaviors of methanol oxidation depend on the Ni content on TiO₂-NT surface. Oxidation response increases by the increasing amount of Ni nano-particles in the deposits. High surface coverage (Γ) with 3.87 × 10⁻⁹ mol cm⁻² and very low activation energy (E_a) with 11.0 kJ/mol are measured on Ni modified TiO₂-NT with the highest Ni content. Charge transfer resistance either reduced or provided long stability and durability with the deposition of Ni on TiO₂-NT. This may associate that TiO₂-NTs with the large surface areas may play a significant role in the methanol oxidation efficiency. Modification of TiO₂-NT surface with Ni particles is an effective plan for high-performance electrocatalysis. Besides, the strong electronic interaction between Ni and TiO₂ may facilitate the adsorption of methanol through the bi-functional mechanism on the electrode surface.     

Electrocatalytic hydrogen evolution properties of anionic NiS2-Ni(OH)2 nanosheets produced on the surface of nickel foam

To circumvent the expensive cost of precious metal electrocatalysts, a hydrothermal method and a plasma-enhanced chemical vapor deposition (PECVD) method have been used to prepareNiS₂-Ni(OH)₂/NF-PECVD (N-PECVD) with excellent properties. After XRD, SEM, XPS, and other characterization methods, it can be seen that after the hydrothermal reaction of Ni foam (NF), 50-60 nm Ni(OH)₂ nanosheets have grown on the NF frame. The subsequent PECVD reaction produces 90 nm NiS₂-Ni(OH)₂ nanosheets on the NF frame. The hydrogen release overpotentials in the acidic solution and alkaline solution are 90 and 95 mV (j = 10 mA/cm²), respectively. And the Tafel slopes reach 80 and 83.6 mV/dec, respectively. The chronopotential (CP) test shows that the material has good HER stability over 12 h. The results from the subsequent electric double layer capacitance and alternating current impedance (EIS) tests show that N-PECVD synthesized by PECVD has a relatively large active electrochemical surface area (ECSA) and a low electrochemical impedance. This study not only finds good inspiration for the synthesis of cheap HER catalysts but also provides a valuable reference for the synthesis of materials in other fields.     

Construction of Co(OH)F/Ni(OH)2@Fe(OH)3 core-shell heterojunction on nickel foam for efficient oxygen evolution

The sluggish anodic reaction (OER) kinetics hinder electrochemical water splitting at high energy densities, which can be solved by developing suitable catalysts. Herein, we report a novel Co(OH)F/Ni(OH)₂@Fe(OH)₃-D_x (D_x represents the hydrolysis time, X = 0.5, 1, 2 days) heterojunction grown on nickel foam, which was synthesized by the hydrolysis of Fe³⁺ on the Co(OH)F/Ni(OH)₂ surface at room temperature. Electrocatalytic oxygen evolution tests showed that the prepared Co(OH)F/Ni(OH)₂@Fe(OH)₃-D_x composites had better catalytic activity than pure Co(OH)F/Ni(OH)₂ in 1.0 M KOH, especially Co(OH)F/Ni(OH)₂@Fe(OH)₃-D₁. It only requires an ultra-low overpotential (η₁₀) of 270 mV, SEM and TEM showed that Co(OH)F/Ni(OH)₂@Fe(OH)₃-D₁ is a perfect all-encapsulated core-shell structure, which facilitates the exposure of active sites and electron transfer, and thus obviously improves oxygen evolution.     

Recycling of nickel from NiOOH/Ni(OH)2 electrodes of spent Ni–Cd batteries

In this work, nickel from the positive electrode of Ni–Cd batteries was recycled by chemical precipitation and electrodeposition. The structure of the material recovered by chemical precipitation is affected by temperature. Alfa nickel hydroxide is stable at low temperature but becomes beta nickel hydroxide with increasing of the synthesis temperature. Electrodeposition was accomplished by using the galvanostatic technique. The chronopotentiometric plots presented a plateau potential in the initial stage of the deposit growth due to the reduction of ionic nickel. Charge density of the plateau potential and charge efficiency decreased with increase in current density. Charge efficiency around 81.0% was the largest for current densities between 5.0 mA cm⁻² and 10.0 mA cm⁻² for q = 9.0 C cm⁻². As charge density increased to 90.0 C m⁻², the electrodeposition efficiency decreased. In this case, there is a second plateau potential at which the evolution of hydrogen on the nickel electrode, the principal reaction, is quickly reached. Charge density affects not only the reaction kinetics, but also the deposit morphology. A decrease in microporosity is observed with the increase in charge density. The microporosity increases as the current density increases for the same charge density.     

Effect of ZnO nanomaterials associated with Ca(OH)2 as anode material for Ni–Zn batteries

Prism ZnO nanomaterials coated with Ca(OH)₂ were prepared by direct precipitation. TEM micrographs showed that dendritic Ca(OH)₂ seemed to attach on the surface of nanosized ZnO. The XRD patterns indicated that the coating was Ca(OH)₂·2Zn(OH)₂·2H₂O. The nanosized ZnO coated with Ca(OH)₂ as the anode materials were investigated by the charge–discharge cycle measurement and EIS. The combination of ZnO nanomaterials and Ca(OH)₂ prevented the discharge product ZnO from dissolving in the electrolyte. Therefore, the Ca(OH)₂-coated ZnO nanomaterials exhibited higher electrochemical activity than the pure nanosized ZnO, including high the discharge capacity and discharge middle voltage, low the charge middle charge voltage. Although Ca(OH)₂ resulted in more difficult activation, EIS showed that the charge-transfer resistance was lower than that of the pure ZnO nanomaterials.     

Electrochemical properties of NiO–Ni nanocomposite as anode material for lithium ion batteries

NiO–Ni nanocomposite was prepared by calcining a mixture of Ni₂(OH)₂CO₃ and ethanol in a tube furnace at 700 °C for 45 min in air. The microstructure and morphology of the powders were characterized by means of X-ray diffraction (XRD) and transmission electron microscopy (TEM). In the composite, nanoscale Ni particles (<10 nm) were dispersed in the NiO matrix (about 100 nm). Electrochemical tests showed that the nanocomposite had higher initial and reversible capacity than pure NiO. The presence of the nanoscale Ni phase had improved both of the initial coulombic efficiency and the cycling performance, due to its catalytic activity, which would facilitate the decomposition of Li₂O and the SEI during the charge process.     

Three-dimensional Ni(OH)2 nanoflakes/graphene/nickel foam electrode with high rate capability for supercapacitor applications

Supercapacitor, known as an important energy storage device, is also a critical component for next generation of hydrogen fuel cell vehicles. In this study, we report a novel route for synthesis of three-dimensional Ni(OH)₂/graphene/nickel foam electrode by electrochemical depositing Ni(OH)₂ nanoflakes on graphene network grown on nickel foam current collector and explore its applications in supercapacitors. The resulting binder-free Ni(OH)₂/graphene/nickel foam electrode exhibits excellent supercapacitor performance with a specific capacitance of 2161 F/g at a current density of 3 A/g. Even as the current density reaches up to 60 A/g, it still remains a high capacitance of 1520 F/g, which is much higher than that of Ni(OH)₂/nickel foam electrode. The enhanced rate capability performance of Ni(OH)₂/graphene/nickel foam electrode is closely related to the presence of highly conductive graphene layer on nickel foam, which can remarkably boost the charge-transfer process at electrolyte–electrode interface. The three-dimensional graphene/nickel foam substrate also significantly improves the electrochemical cycling stability of the electrodeposited Ni(OH)₂ film because of the strong adhesion between graphene film and electrodeposited Ni(OH)₂ nanoflakes. Results of this study provide an alternative pathway to improve the rate capability and cycling stability of Ni(OH)₂ nanostructure electrode and offer a great promise for its applications in supercapacitors.     

Growth of nickel vacancy NiFe-LDHs on Ni(OH)2 nanosheets as highly efficient bifunctional electrocatalyst for overall water splitting

A bifunctional electrocatalyst was fabricated by in-situ vertical growth of Ni(OH)₂ nanosheets on nickel foam (NF), with subsequent accretion of nickel vacancy NiFe-LDHs (Ni^vacFe-LDHs) by two step hydrothermal method. It was exhibited to be a high-efficiency overall water splitting performance with good stability. The low over-potentials of 292, 330, and 376 mV were acquired when the current density was selected as 50, 100, and 200 mA/cm² for oxygen evolution reaction (OER) with a relatively low Tafel slope. It also achieved low over-potentials of 116 and 247 mV when the current densities were 10 and 200 mA/cm² for hydrogen evolution reaction (HER), and Tafel slope was estimated to be 95.87 mV/dec. For the overall water splitting, NF–Ni(OH)₂-Ni^vacFe-LDHs needed only a low overpotential (291 mV) to achieve 25 mA/cm² in 1 mol/L potassium hydroxide. The long-term testing of this electrode for 24 h chronopotentiometric test at 25 mA/cm² demonstrated very eminent stability.     

Hierarchitecture Co2(OH)3Cl@FeCo2O4 composite as a novel and high-performance electrode material applied in supercapacitor

One-step hydrothermal reaction has successfully been used to prepared three-dimensional hierarchitecture Co₂(OH)₃Cl@FeCo₂O₄ composite without any annealing treatment. The samples are investigated to confirm the crystal structure, elemental composition, morphology structure and electrochemical performance. The results show the sample has a three-dimensional hierarchitecture that nanoblocks are assembled with nanoparticles. And the specific surface area is 87.5 m² g⁻¹ and the total pore volume is 0.17 cm³ g⁻¹. Meanwhile, the composite shows a high specific capacitance of 1110.0 F·g⁻¹ at 1 A·g⁻¹ and great cycling stability with 98.8% capacitance retention after 3000 cycles. To evaluate the electrochemical performances, the results are used to compare with the Co₂(OH)₃Cl and FeCo₂O₄ nanomaterials, indicating a higher capacitance and longer cycle stability shown by the as-synthesized sample. The as-synthesized Co₂(OH)₃Cl@FeCo₂O₄ composite has an outstanding electrochemical performance, predicting an enormous potential and promising future as a novel electrode material applied in supercapacitor.     

Hierarchical design of NiCo-LDH NFs@Co(OH)2 nanosheets supercapacitor electrode material with boosted electrochemical performance

Layered double hydroxides (LDHs) have been considered as excellent pseudocapacitive electrode materials for supercapacitors because of their controllable layered structures and high theoretical specific capacitances. However, poor conductivity and shortage of effective active sites hinder their applications. Herein, in order to boost the capacitance performance, an electrode material composed of NiCo-LDH nanofibers with cobalt hydroxide (Co(OH)₂) nanosheets firmly anchored to or uniformly immobilized onto NiCo-LDH nanofibers surface is constructed. The material architecture takes advantage of superior capacitive activity from Co(OH)₂ with good electronic conductivity and dispersibility over the surface of NiCo-LDH nanofibers. The merits based on the elegant synergy between NiCo-LDH nanofibers and Co(OH)₂ nanosheets induce an outstanding specific capacitance of 858.9 F/g at a current density of 0.5 A/g. With the current density increasing from 0.5 to 10 A/g, the specific capacitance retention is 67.3%, indicating a good rate capability. Furthermore, the electrode material exhibits a capacitance retention of 83.7% after 2000 cycles at 10 A/g. These excellent electrochemical properties are due to the hierarchically designed structure and the cooperative contributions of NiCo-LDH nanofibers and Co(OH)₂ nanosheets, which shows great potential for energy storage applications.     

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.     

Effect of synthesis conditions on characteristics of the precursor material used in NiO·OH/Ni(OH)2 electrodes of alkaline batteries

The synthesis of nickel hydroxide occurs by many stages. When the precipitating reagent is NH₄OH solution, the precipitation of nickel hydroxide occurs between pH 8.0 and 8.6. For pH between 8.6 and 10.0, a soluble complex such as [Ni(NH₃)₆]²⁺ is formed. The precipitation of nickel hydroxide happens again after the pH equals 10.0. Finally, there occurs the ageing of α-Ni(OH)₂. A mixture of α-Ni(OH)₂ and β-Ni(OH)₂ phases is formed when the solid state reaction is not totally completed. One adsorbed layer becomes very hard with the exit of the water intercalated in the α-Ni(OH)₂. In presence of KOH solution occurs the formation and the ageing of α-Ni(OH)₂. Synthesis was characterized by the following techniques: X-ray diffraction, Fourier transform infrared spectroscopy (FT-IR), differential thermal analysis (DTA) and gravimetric thermal analysis (GTA), and specific surface area and UV–vis spectroscopy.     

Activation behaviour of the Ni/MH batteries electrodic material Ni(OH)2 by single particle microelectrode technique

The activation process of Ni(OH)₂ used as the positive electrode active material of Ni/MH batteries was studied by a single particle microelectrode method thanks to an improved apparatus. The images of the Ni(OH)₂ particle during the charge process were collected. The electrochemical properties of Ni(OH)₂ were studied by cyclic voltammetry and galvanostatic charge/discharge of a single particle. The charge efficiency (η) of the single particle was as high as 94%. The normalized output rate (NOR) was proposed as a parameter to evaluate the output performance of the electrode material. The NOR value varied with the electrode potential value. But the NOR value remained constant at fixed electrode potential value during the activation process. This implies that the activation process did not improve the reaction rate of the particle, although the capacity kept increasing during the activation process. The intrinsic nature of the activation of Ni(OH)₂ was deduced as the formation of dispersed Ni(III) in the active mass. The Ni(III) phase was formed during the charge process and some remained unreduced during the discharge process. The remaining Ni(III) resulted in a much higher electronic conductivity of Ni(OH)₂.     

Well-dispersed Pt nanodots interfaced with Ni(OH)2 on anodized nickel foam for efficient hydrogen evolution reaction

Hindered by price and scarcity, the exploitation of supported Pt-based electrocatalysts with Pt single atoms or Pt nanoclusters is an alternative way to decrease the dosage of Pt and improve the electrocatalytic performance for hydrogen evolution reaction (HER) of water splitting. The anodization technology is used to modify the surface of nickel foam (NF) to form the porous NiF₂ network structure. Then Pt nanodots interfaced with Ni(OH)₂ (Pt/Ni(OH)₂) hybrid on the anodized NF has been in-situ synthesized by a simple hydrothermal decomposition method. Results show that Pt nanodots on the substrate have good dispersion with the average size of 3 nm, and the Pt loading is only 0.229 mg cm⁻². The prepared electrode exhibits the low overpotentials of 25.9 mV and 211 mV at the current densities of 10 and 100 mA cm⁻², respectively, a small Tafel slope of 37.6 mV dec⁻¹, and the excellent durability for HER. The porous network nanostructure of Pt/Ni(OH)₂ hybrid, the large electrochemical surface area, the fast facilitated electron transport capability, and the firm adhesion of Pt nanodots with the anodized NF substrate contribute to the remarkable performance towards HER.     

Ni3S2@Co(OH)2 heterostructures grown on Ni foam as an efficient electrocatalyst for water oxidation

It is of high significance to design robust, low-cost and stable electrocatalysts for the oxygen evolution reaction (OER) under alkaline medium. In this communication, we present the exploitation of Ni₃S₂@Co(OH)₂ which directly grown on nickel foam (Ni₃S₂@Co(OH)₂/NF) as a robust and stable electrocatalyst for OER. Such Ni₃S₂@Co(OH)₂/NF-5h demanding overpotential of only 290 mV is less than that of Ni₃S₂@Co(OH)₂/NF-10h (310 mV), Ni₃S₂@Co(OH)₂/NF-2h (320 mV) and Ni₃S₂/NF(350 mV), respectively, to drive a geometrical catalytic current density of 35 mA cm⁻², which is also better than that of noble metal catalyst IrO₂/NF (320 mV). In addition, the Ni₃S₂@Co(OH)₂/NF-5h presents a superior long-term electrocatalytic stability, keeping its activity at 26 mA cm⁻² for 40 h.     

TiO2-coated Ni powder as a new cathode material for molten carbonate fuel cells

The properties of a new cathode material of nano-sized TiO₂-coated Ni powder for application in molten carbonate fuel cells (MCFCs) are investigated. The material is prepared by a sol–gel method. X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM) with energy dispersive X-ray analysis (EDAX), transmission electron microscopy (TEM) and Raman spectroscopy are employed to characterize the cathode material. The nano-sized TiO₂ particles are homogenously coated on the surface of the Ni powder. A stable LiTi_1−xNi_xO₂ phase is formed on the surface of the Ni powder during immersion in molten carbonate at 650 °C. The solubility of the TiO₂-coated Ni cathode is about 50% lower compare with that of a pure Ni cathode.