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.     

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.     

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.     

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.     

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%.     

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.     

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.     

Preparation and characterization of polypyrrole films for three-dimensional micro supercapacitor

As electro-active electrodes for supercapacitors, micro polypyrrole (PPy) films doping with ClO₄⁻ (PPyClO₄) and Cl⁻ (PPy_Cl) are prepared on Ni layers modified three-dimensional (3D) structures in Si substrates. The key process to fabricate the 3D structures is high-aspect-ratio deep reactive ion etching, which result in significant increase of available surface area. Homogeneous conformal Ni layers and PPy films are deposited on the 3D structures by electroless plating and electropolymerization, respectively. The supercapacitor properties of PPy films are investigated by using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge/discharge with three-electrode system in NaCl solution. It is shown that doping with ClO₄⁻ results in ideal supercapacitor behaviors with rectangle-like CV shapes at scan rates from 5 to 200 mV s⁻¹, linear galvanostatic charge/discharge curves at current loads from 0.5 to 2 mA and stable cyclic property. However, doping with Cl⁻ gives rise to non-ideal properties of supercapacitor. SEM of the PPyClO₄ shows that the surface of the PPyClO₄ electrode is smooth and the thickness of the PPyClO₄ film is about 2.5 μm. The geometric capacitance of PPyClO₄ is calculated as 0.030 F cm⁻² from CV at scan rate of 100 mV s⁻¹, 0.023 F cm⁻² from EIS and 0.027 F cm⁻² from galvanostatic discharge at 1 mA cm⁻² current density.     

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.     

Size effect investigation on battery performance: Comparison between micro- and nano-particles of β-Ni(OH)2 as nickel battery cathode material

In this work we report on the comparison between nano- and micro-particles of β-Ni(OH)₂ as cathode material of Ni battery. The synthesis of nano- and micro-particles of nickel hydroxide is done by two different procedures: sonication process and stirrer. Nano-particles of β-Ni(OH)₂ are synthesized by chemical precipitation from a solution containing NiCl₂·6H₂O and surfactant under ultrasonic irradiation. Micro-particles of β-Ni(OH)₂ are synthesized by a similar procedure while applying magnetic stirring instead of ultrasonic. The products are characterized by scanning electron microscopy and X-ray powder diffraction. Under the optimized conditions nickel hydroxide nano-particles, with an average particle size of 18 nm, are obtained. Cyclic voltammetric (CV) studies show a pair of well-defined peaks for Ni(OH)₂/NiOOH redox reaction, along with faster proton diffusion coefficient and higher oxygen evolution potential for nano-particles of nickel hydroxide compared to that of micro-particles. Electrochemical impedance spectroscopy (EIS) studies of Ni(OH)₂ electrodes show that the reaction occurring at the nickel hydroxide is controlled by charge transfer and Warburg diffusion. The β-Ni(OH)₂ nano-particles are found to exhibit a superior cycling reversibility and improved capacity when they are used as positive electrode materials of alkaline rechargeable batteries.     

Electrodeposition of mesoporous manganese dioxide supercapacitor electrodes through self-assembled triblock copolymer templates

Mesoporous manganese dioxide supercapcitor electrode materials were electrochemically deposited onto silicon substrates coated with Pt using triblock copolymer species (Pluronic P123 and F127) as the structure-directing agents. Deposited electrodes of manganese dioxide film were physically characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and were electrochemically characterized by cyclic voltammetry (CV) in 0.5 M Na₂SO₄ electrolyte. Maximum specific capacitance (SC) values of 449 F g⁻¹ was obtained at a scan rate of 10 mV s⁻¹ from F127 templated mesoporous MnO₂.     

High-temperature characteristics of advanced Ni-MH batteries using nickel electrodes containing CaF2

The high-temperature charge acceptance of Ni-MH batteries has been improved through the addition of calcium fluoride to the pasted nickel hydroxide electrode made using spherical Co(OH)₂-coated nickel hydroxide powder. The charge acceptance of the Ni-MH battery at 60 °C is over 95% at 1 C charge/discharge rates. The charge acceptance at 60 °C remains at over 90% through 10 cycles. The use of Co(OH)₂-coated Ni(OH)₂ plus a CaF₂ addition to the positive electrode also significantly improved the high-temperature stability in terms of reduced gas evolution.     

Influence of the mesoporous structure on capacitance of the RuO2 electrode

The difference in capacitive performance between high and low surface area RuO₂ electrodes, synthesized with and without a mesoporous silica template, respectively, was investigated in aqueous solutions of sulfuric acid and sulfates by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). RuO₂ synthesized with the template was crystalline and the formation of the mesoporous structure with a 6.5 nm diameter was confirmed using a transmission electron microscope and the nitrogen adsorption and desorption isotherm. From the CV at the scan rate of 1 mV s⁻¹, the specific capacitance of the high surface area electrode in H₂SO₄(aq) was determined to be 200 F g⁻¹. The high surface area RuO₂ has a three times higher BET specific surface area (140 m² g⁻¹) than the low surface area sample (39 m² g⁻¹). Introducing the mesoporous structure was proved effective for increasing the capacitance per mass of the RuO₂, though not all the surface functions as a capacitor. Both the CV and EIS suggest that by increasing the charging rate or frequency, the mesoporous structure of the electrode leads to a lower capacitance decrease (higher capacitance retention) than the low surface area electrode. The EIS also indicates that the response time of the capacitor is hardly influenced by the presence of the mesoporous structure.     

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.     

Binderless fabrication of amorphous RuO2 electrode for electrochemical capacitor using spark plasma sintering technique

The spark plasma sintering (SPS) technique was successfully used to mold a hydrous amorphous RuO₂electrode without any additives and binders. At the cyclic voltammetry (CV) scan rate of 1 mV s⁻¹, the electrochemical capacitances of the RuO₂ electrodes are 600-700 F g⁻¹ for the entire electrode. An increase in the SPS current during the compaction led to the crystallization and dehydration of RuO₂, which in turn, resulted in a significant decrease in its capacitance. There is room to improve the rate properties as we observed a steep drop in the capacitance when the CV scan rate was raised.     

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.     

A study of the catalysis of cobalt hydroxide towards the oxygen reduction in alkaline media

A cobalt hydroxide modified glassy carbon (Co(OH)₂/GC) electrode has been fabricated by a galvanostatic electrodeposition method. The catalytic activity for the oxygen (O₂) reduction reaction (ORR) of this electrode in alkaline media is studied by cyclic voltammetry, rotating disk electrode voltammetry, and rotating ring-disk electrode voltammetry. The O₂ reduction at the Co(OH)₂/GC disk electrode has been found to undergo an electrochemical process followed by sequential disproportionation of the electrochemical reduction intermediates, i.e., superoxide anion (O₂⁻) and hydrogen peroxide anion (HO₂⁻) in 0.1 M KOH solution. The Co(OH)₂ is first found to possess an excellent catalytic activity not only for the disproportionation of the O₂⁻ produced into O₂ and HO₂⁻ but also for that of the HO₂⁻ produced, combined with electrochemical reduction of O₂ mediated by surface functional groups at the carbon electrode surface. The Co(OH)₂ is a potential electrode material for the ORR in alkaline fuel cells and metal-air batteries.     

High-temperature electrochemical performance of Al-α-nickel hydroxides modified by metallic cobalt or Y(OH)3

Al-α-Ni(OH)₂ microspheres are modified with metallic Co and Y(OH)₃, respectively, in order to improve the high-temperature electrochemical performance. The microstructure, morphology, and surface chemical state of the as-prepared and the modified Al-α-Ni(OH)₂ microspheres are investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS), respectively. Metallic cobalt nanoparticles are distributed on the nanosheets of the microsphere edges. The existence of metallic Co and Y(OH)₃ can be further verified from ICP and XPS results. The effect of metallic Co or Y(OH)₃ on high-temperature performance of the Al-α-Ni(OH)₂ microspheres is measured by galvanostatic charge–discharge experiments and cyclic voltammetric (CV) measurements. The discharge capacities of the Al-α-Ni(OH)₂ microspheres, with optimized 5 wt% Co and 1 wt% Y(OH)₃, are 283.5 mAh g⁻¹ and 315 mAh g⁻¹, respectively, much higher than that of the as-prepared Al-α-Ni(OH)₂ (226.8 mAh g⁻¹) at 0.2 C and 60 °C. Furthermore, the high-rate discharge capability at high temperature can be also improved for both the modified samples.     

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 of Co(OH)2/graphene/Ni foam nano-electrodes with excellent pseudocapacitive behavior and high cycling stability for supercapacitors

Vertically aligned graphene nanosheets have been synthesized by radio-frequency plasma-enhanced chemical vapor deposition on nickel-foam current collectors and that have been used as substrates for cathodic electrodeposition of cobalt hydroxide nanosheets in Co(NO₃)₂ aqueous solution. Raman spectrum exhibits that high-quality graphene nanosheets have been synthesized. The composites have been characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, cyclic voltammetry and galvanostatic charge/discharge. It indicates that hexagonal Co(OH)₂ has a network microstructure, consisting of interlaced sheets with the thickness of 12 nm coated on the graphene nanosheets. The binder-free nano-electrode exhibits excellent pseudocapacitive behavior with pseudocapacitances of 693.8 and 506.2 Fg⁻¹ at current density of 2 and 32 Ag⁻¹, respectively, in a potential range of −0.1–0.45 V. The capacitance can retain about 91.9% after 3000 charge–discharge cycles at 40 Ag⁻¹, which is higher than that of Co(OH)₂/Ni foam (after 2000 cycles, 75.5% of initial capacitance remains). The introduction of graphene between Co(OH)₂ and Ni foam demonstrates an enhancement of electrochemical stability of the nano-electrodes.