CuO nanosheets grown on cupper foil as the catalyst for H2O2 electroreduction in alkaline medium

The growth of CuO nanosheet arrays on Cu foil was demonstrated. The morphology and structure of the CuO were examined by scanning electron microscopy and X-ray diffraction spectroscopy. The catalytic performance of the obtained CuO/Cu electrode for hydrogen peroxide electroreduction in 3.0 mol dm⁻³ KOH was evaluated by means of cyclic voltammetry and chronoamperometry. The CuO/Cu electrode shows an onset potential for H₂O₂ electroreduction comparable to Co₃O₄ nanowire arrays grown on Ni foam and around 100 mV more negative than precious metal catalysts, such as Pt and Pd, demonstrating its good catalytic activity for H₂O₂ electroreduction. The stabilized mass current density for H₂O₂ electroreduction on the CuO/Cu electrode at −0.3 V reached about 57% of that on Co₃O₄ nanowire arrays grown on nickel foam. Compared to conventional fuel cell electrodes fabricated by mixing active materials with conducting agents and polymer binders, this electrode of CuO nanosheet arrays directly grown on Cu has superior mass transport property, which combining with its low-cost and facile preparation, make it a promising electrode for fuel cell using H₂O₂ as the oxidant.     

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 properties of Li(Ni0.4Co0.3Mn0.3)O2 cathode by surface modification using Li3PO4-based materials

The surface of a commercial Li[Ni_0.4Co_0.3Mn_0.3]O₂ cathode is modified using Li₃PO₄-based coating materials. The electrochemical properties of the coated materials are investigated as a function of the pH value of the coating solution and the composition of coating materials. The Li₃PO₄ coating solution with pH 2 is found to be favorable for the formation of stable coating layers having enhanced electrochemical properties. The Li₃PO₄, Li_1.5PO₄, and PO₄ coating layers are formed as amorphous phases. However, the Li_3−xNi_x/2PO₄ coating layers are composed of small particles with a crystalline phase covered with an amorphous phase. Li₃PO₄ and Li_1.5PO₄ coatings considerably enhance the rate capability of the Li[Ni_0.4Co_0.3Mn_0.3]O₂ electrode. In contrast, the Li_3−xNi_x/2PO₄ coating material, which contained Ni, has an inferior rate capability compared to the Li_xPO₄ series (x = 1.5 and 3), although the LiNiPO₄-coated electrode shows a better rate capability than a pristine one. Li₃PO₄-based coating materials are effective at enhancing the cyclic performance of the electrode in the voltage range of 3.0-4.8 V. DSC analysis also confirms the improved thermal stability attained by coating the cathode with Li₃PO₄-based materials.     

AlF3-coated LiCoO2 and Li[Ni1/3Co1/3Mn1/3]O2 blend composite cathode for lithium ion batteries

Surface modifications of electrode materials can improve the electrochemical and thermal properties of cathodes for use in lithium batteries. In this study, AlF₃-coated LiCoO₂ and AlF₃-coated Li[Ni_1/3Co_1/3Mn_1/3]O₂ cathode materials are blended, as both have the same crystal structure and exhibit similar electrochemical properties. The composite electrodes exhibit high discharge capacities of 180-188 mAh g⁻¹ in a voltage range of 3.0-4.5 V at room temperature. The capacity retention of the composite electrode is greater than 95% of the initial capacity after 50 cycles. The thermal stability of these composite electrodes is greatly improved because of the superior thermal stability of AlF₃-coated Li[Ni_1/3Co_1/3Mn_1/3]O₂. The blended AlF₃-coated LiCoO₂ and AlF₃-coated Li[Ni_1/3Co_1/3Mn_1/3]O₂ electrode shows two exothermic peaks, one at 227 °C from AlF₃-coated LiCoO₂ and another at 277 °C from AlF₃-coated Li[Ni_1/3Co_1/3Mn_1/3]O₂, accompanied by significantly reduced exothermic heat generation.     

Electrochemical deposition of porous Co3O4 nanostructured thin film for lithium-ion battery

Porous Co₃O₄ nanostructured thin films are electrodeposited by controlling the concentration of Co(NO₃)₂ aqueous solution on nickel sheets, and then sintered at 300 °C for 3 h. The as-prepared thin films are characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The electrochemical measurements show that the highly porous Co₃O₄ thin film with the highest electrochemically active specific surface area (68.64 m² g⁻¹) yields the best electrochemical performance compared with another, less-porous film and with a non-porous film. The highest specific capacity (513 mAh g⁻¹ after 50 cycles) is obtained from the thinnest film with Co₃O₄ loaded at rate of 0.05 mg cm⁻². The present research demonstrates that electrode morphology is one of the crucial factors that affect the electrochemical properties of electrodes.     

Synthesis and electrochemical properties of Li[Ni0.45Co0.1Mn0.45−xZrx]O2 (x = 0, 0.02) via co-precipitation method

Li[Ni_0.45Co_0.1Mn_0.45−xZr_x]O₂ (x = 0, 0.02) was synthesized via co-precipitation method. Partial Zr doping on the host structure of Li[Ni_0.45Co_0.1Mn_0.45]O₂ was carried out to improve the electrochemical properties. The Zr-doped Li[Ni_0.45Co_0.1Mn_0.43Zr_0.02]O₂ was evaluated in terms of specific discharge capacity, cycling performance and thermal stability. The Zr-doped Li[Ni_0.45Co_0.1Mn_0.45−xZr_0.02]O₂ shows the improved cycling performance and stable thermal stability. The major exothermic reaction was delayed from 252.1 °C to 289.4 °C.     

Differences in the effects of Co and CoO on the performance of Ni(OH)2 electrode in Ni/MH power battery

The effects of metallic cobalt (Co) and cobalt monoxide (CoO), as additives in positive electrodes, on the electrochemical performance of nickel/metal hydride (Ni/MH) power batteries are studied. Commercial Co and CoO are charged at 50 °C in 6 M KOH solution. The oxidation mechanism of cobalt materials is investigated by observing structural and morphological evolutions during charging. A pure Co₃O₄-type phase is formed when the starting material is CoO. When Co is used, a cobalt oxyhydroxide (CoOOH) phase is present, together with a tricobalt tetroxide (Co₃O₄) phase. In both cases, the cobalt concentration in the electrolyte decreases during oxidation. The final product is dependent on the solubility of cobalt and the kinetics of the reaction that consumes cobalt tetrahydroxide [Co(OH)₄]²⁻. The highly compact CoOOH phase, which works well between the nickel foam frame and nickel hydroxide [Ni(OH)₂] particles, enhances the power performance of Ni/MH power battery. The Co₃O₄ phase, which works well in connecting Ni(OH)₂ particles, improves the capacitive performance of Ni/MH power battery.     

Electrode properties of Co-doped Ca2Fe2O5 as new cathode materials for intermediate-temperature SOFCs

Brownmillerite oxide Ca₂Fe_2−xCo_xO₅ (x = 0.2, 0.4, 0.6) was characterized by XRD, SEM and electrochemical impedance spectrum (EIS), respectively. Ca₂Fe_2−xCo_xO₅ has no reaction with Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte at 1100 °C for 10 h in air. The thermal expansion coefficient (TEC) of Ca₂Fe_2−xCo_xO₅ increased with increasing Co content, and the TEC value was compatible with SDC. The electrode properties of Ca₂Fe_2−xCo_xO₅ were studied under various temperatures and oxygen partial pressures. The polarization resistance (R_p) of Ca₂Fe_2−xCo_xO₅ with x = 0.2, 0.4 and 0.6 are 0.23, 0.48 and 1.05 Ω cm² at 700 °C in air, respectively. The rate-limiting step for oxygen reduction reaction was the charge transfer process. Ca₂Fe_1.8Co_0.2O₅ cathode exhibits the lowest overpotential of about 50 mV at a current density of 70 mA cm⁻² at 700 °C in air.     

Potentiodynamic deposition of composition influenced Co1−xNix LDHs thin film electrode for redox supercapacitors

Current paper comprises the electrodeposition of nanostructured porous Co_1−xNi_x layered double hydroxide (Co_1−xNi_x LDHs) thin films on to stainless steel substrate by a potentiodynamic mode. The compositional impacts on the various properties of Co_1−xNi_x LDHs are examined via structural, morphological, surface wettability and electrochemical studies. The nanocrystalline Co_1−xNi_x LDHs thin films possess varying porous, nanoflake like morphology and superhydrophilic behavior by the composition influence. Electrochemical studies demonstrate the supercapacitive performance of Co_1−xNi_x LDHs thin film electrodes. The maximal specific capacitance for Co_1−xNi_x LDHs electrode is found to be ∼1213 F g⁻¹ for composition Co_0.66Ni_0.34 LDH in 2 M KOH electrolyte at 5 mV s⁻¹ scan rate owing specific energy of 104 Whkg⁻¹, specific power of 1.44 kW kg⁻¹ with ∼94% of coulomb efficiency and stability of electrode retained to 77% after 10,000th cycle. The high capacitance retention proposes the deposited Co_1−xNi_x LDHs thin film as promising contender for supercapacitor applications.     

Synthesis and characterization of LiNi0.6Mn0.4−xCoxO2 as cathode materials for Li-ion batteries

LiNi_0.6Co_xMn_0.4−xO₂ (x = 0.05, 0.10, 0.15, 0.2) cathode materials are prepared, and their structural and electrochemical properties are investigated using X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), differential scanning calorimetric (DSC) and charge–discharge test. The results show that well-ordering layered LiNi_0.6Co_xMn_0.4−xO₂ (x = 0.05, 0.10, 0.15, 0.2) cathode materials are successfully prepared in air at 850 °C. The increase of the Co content in LiNi_0.6Mn_0.4−xCo_xO₂ leads to the acceleration of the grain growth, the increase of the initial discharge capacity and the deterioration of the cycling performance of LiNi_0.6Mn_0.4−xCo_xO₂. It also leads to the enhancement of the ratio Ni³⁺/Ni²⁺ in LiNi_0.6Co_xMn_0.4−xO₂, which is approved by the XPS analysis, resulting in the increase of the phase transition during cycling. This is speculated to be main reason for the deteriotion of the cycling performance. All synthesized LiNi_0.6Co_xMn_0.4−xO₂ samples charged at 4.3 V show exothermic peaks with an onset temperature of larger than 255 °C, and give out less than 400 J g⁻¹ of total heat flow associated with the peaks in DSC analysis profile, exhibiting better thermal stability. LiNi_0.6Co_0.05Mn_0.35O₂ with low Co content and good thermal stability presents a capacity of 156.6 mAh g⁻¹ and 98.5% of initial capacity retention after 50 cycles, showing to be a promising cathode materials for Li-ion batteries.     

Physical and electrochemical properties of spherical Li1+x(Ni1/3Co1/3Mn1/3)1−xO2 cathode materials

A (Ni_1/3Co_1/3Mn_1/3)CO₃ precursor with an uniform, spherical morphology was prepared by coprecipitation using a continuously stirred tank reactor method. The as-prepared spherical (Ni_1/3Co_1/3Mn_1/3)CO₃ precursor served to produce dense, spherical Li_1+x(Ni_1/3Co_1/3Mn_1/3)_1−xO₂ (0 ≤ x ≤ 0.15) cathode materials. These Li-rich cathodes were also prepared by a second synthesis route that involved the use of an M₃O₄ (M = Ni_1/3Co_1/3Mn_1/3) spinel compound, itself obtained from the carbonate (Ni_1/3Co_1/3Mn_1/3)CO₃ precursor. In both cases, the final Li_1+x(Ni_1/3Co_1/3Mn_1/3)_1−xO₂ products were highly uniform, having a narrow particle size distribution (10-μm average particle size) as a result of the homogeneity and spherical morphology of the starting mixed-metal carbonate precursor. The rate capability of the Li_1+x(Ni_1/3Co_1/3Mn_1/3)_1−xO₂ electrode materials, which was significantly improved with increased lithium content, was found to be better in the case of the denser materials made from the spinel precursor compound. This result suggests that spherical morphology, high density, and increased lithium content were key factors in enabling the high rate capabilities, and hence the power performances, of the Li-rich Li_1+x(Ni_1/3Co_1/3Mn_1/3)_1−xO₂ cathodes.     

Potentiostatically deposited nanostructured CoxNi1−x layered double hydroxides as electrode materials for redox-supercapacitors

Cobalt–nickel layered double hydroxides (Co_xNi_1−x LDHs) were deposited onto stainless steel electrodes by the potentiostatic deposition method at −1.0 V vs. Ag/AgCl using various molar ratios of Co(NO₃)₂ and Ni(NO₃)₂ in distilled water. Their structure and surface morphology were studied by using X-ray diffraction analysis, energy dispersive X-ray spectroscopy and scanning electron microscopy. A network of Co_xNi_1−x LDH nanosheets was obtained. The nature of the cyclic voltammetry and charge–discharge curves suggested that the Co_xNi_1−x LDHs exist in the form of solid solutions. The capacitive characteristics of the Co_xNi_1−x LDHs in 1 M KOH electrolyte showed that Co_0.72Ni_0.28 LDHs had the highest specific capacitance value, 2104 F g⁻¹, which is also the highest yet reported value for oxide materials in general.     

Evolution of physicochemical and electrocatalytic properties of NiCo2O4 (AB2O4) spinel oxide with the effect of Fe substitution at the A site leading to efficient anodic O2 evolution in an alkaline environment

Within the framework of this work, spinel-type ternary transition metal oxides of nickel, cobalt and iron with the composition Fe_xNi_1−xCo₂O₄ (0 ≤ x ≤ 1) were prepared and tested as promising electrocatalysts for the oxygen evolution reaction (OER) in alkaline water electrolysis. The hydroxide precipitation method was used for the synthesis. The morphology, structure and specific surface area of the prepared electrocatalysts were determined by means of scanning electron microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy, the Brunauer Emmet Teller method and X-ray photo electron spectroscopy. The electrochemical properties were tested by thin-film technique on a rotating disk electrode and in a single-cell laboratory water electrolyzer coupled with electrochemical impedance spectroscopy. The OER studies indicate that substitution of Ni by Fe increases the electrocatalytic activity of the resulting material significantly. The highest activity was achieved for x = 0.1. Whereas the current density obtained using a pure nickel anode in the water electrolysis test was 54 mA cm⁻² at a cell voltage of 1.85 V, in the case of the anode modified with NiCo₂O₄ catalyst the value was 87 mA cm⁻². Using ternary transition metal oxides in the water electrolysis test and under identical conditions, the catalyst with the highest activity displayed a current density of 115 mA cm⁻².     

Interlayer-free electrodes for IT-SOFCs by applying Co3O4 as sintering aid

The influence of Co₃O₄ as a sintering aid for a series of cobalt-containing perovskite oxides on the microstructure and electrical properties have been investigated. X-ray diffraction and scanning electron microscopic results showed that well connected electrode particles with firm adhesion to the 8 mol% yttria-stabilized zirconia (YSZ) electrolyte surface were realized at a temperature free from interfacial phase reaction. Both ohmic and polarization resistances of symmetric cells by adopting YSZ electrolyte, measured by electrochemical impedance spectroscopy, were much lower than that without adding Co₃O₄. The peak power density of 1176 mW cm⁻² at 750 °C was achieved when La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ + Co₃O₄ was selected as a representative cobaltite cathode, which is much higher than a similar fuel cell with the cathode fabricated by a conventional way. Fabrication of interlayer-free electrodes by applying Co₃O₄ as a sintering aid is very simple and general, applicable for a wide range of cobalt-containing electrode materials.     

The electrochemical property of ZrFx-coated Li[Ni1/3Co1/3Mn1/3]O2 cathode material

Electrochemical and thermal properties of pristine and ZrF_x-coated Li[Ni_1/3Co_1/3Mn_1/3]O₂ cathode materials are compared. The hydrothermal method is introduced for the fabrication of a uniform coating layer. The formation of a compact coating layer on the surface of pristine powder is observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). From TEM-EDS and XPS analysis, it is inferred that the coating layer is ZrO_xF_y (zirconium oxyfluoride) form. The coated Li[Ni_1/3Co_1/3Mn_1/3]O₂ electrodes have better rate capability and cyclic performance at high temperature compared with the pristine electrode. The thermal stability of the Li[Ni_1/3Co_1/3Mn_1/3]O₂ electrode is also enhanced by the ZrF_x coating. Such enhancements are due to the presence of a stable coating layer, which effectively suppresses the chemical instability ascribed to surface reaction between electrode and electrolyte.     

Electrochemical performance of La2Cu1−xCoxO4 cathode materials for intermediate-temperature SOFCs

K₂NiF₄-type structure oxides La₂Cu_1−xCo_xO₄ (x = 0.1, 0.2, 0.3) are synthesized and evaluated as cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs). The materials are characterized by XRD, SEM and electrochemical impedance spectrum (EIS), respectively. The results show that no reaction occurs between La₂Cu_1−xCo_xO₄ electrode and Ce_0.9Gd_0.1O_1.95 (CGO) electrolyte at 1000 °C. The electrode forms good contact with the electrolyte after sintering at 800 °C for 4 h in air. The electrode properties of La₂Cu_1−xCo_xO₄ are studied under various temperatures and oxygen partial pressures. The optimum composition of La₂Cu_0.8Co_0.2O₄ results in 0.51 Ω cm² polarization resistance (R_p) at 700 °C in air. The rate limiting step for oxygen reduction reaction (ORR) is the charge transfer process. La₂Cu_0.8Co_0.2O₄ cathode exhibits the lowest overpotential of about 50 mV at a current density of 48 mA cm⁻² at 700 °C in air.     

Electrocatalysts for bifunctional oxygen/air electrodes

Oxygen reduction and evolution have been studied with respect to the development of bifunctional air/oxygen electrode (BFE). Three groups of catalysts have been prepared: (i) Cu_xCo_3−xO₄ by thermal decomposition of mixed nitrate and carbonate precursors; (ii) thin films of Co–Ni–Te–O and Co–Te–O were deposited by vacuum co-evaporation of Co, Ni and TeO₂ and (iii) Co_xO_v/ZrO₂ films were obtained by electrochemical deposition.     

Synthesis and electrochemical properties of lithium non-stoichiometric Li1+x(Ni1/3Co1/3Mn1/3)O2+δ prepared by a spray drying method

Lithium non-stoichiometric Li[Li_x(Ni_1/3Co_1/3Mn_1/3)_1−x]O₂ materials (0 ≤ x ≤ 0.17) were synthesized using a spray drying method. The electrochemical properties and structural stabilities of the synthesized materials were investigated. The synthesized materials exhibited a hexagonal structure in all the x-value and the lattice parameters of the materials were gradually decreased with increasing x-value due to an increasing amount of Ni³⁺ ions for charge compensation. The capacity retention ability and rate capability of the stoichiometric Li(Ni_1/3Co_1/3Mn_1/3)O₂ material were improved by increasing x-value, the so-called overlithiation. We found that the overlithiated materials could keep more structural integrity than the stoichiometric one during electrochemical cyclings, which could be one of reasons for a better electrochemical properties of the overlithiated materials.     

Improved electrochemical properties of amorphous Mg65Ni27La8 electrodes: Surface modification using graphite

Amorphous Mg₆₅Ni₂₇La₈ alloy is prepared by melt-spinning. The alloy surface is modified using different contents of graphite to improve the performances of the Mg₆₅Ni₂₇La₈ electrodes. In detail, the electrochemical properties of (Mg₆₅Ni₂₇La₈) + xC (x = 0–0.4) electrodes are studied systematically, where x is the mass ratio of graphite to alloy. Experimental results reveal that the discharge capacity, cycle life, discharge potential characteristics and electrochemical kinetics of the electrodes are all improved. The surface modification enhances the electrocatalytic activity of the alloy, reduces the contact resistance of the electrodes and obstructs the formation of Mg(OH)₂ on the alloy surface. An optimal content of graphite has been obtained. The (Mg₆₅Ni₂₇La₈) + 0.25 C electrode has the largest discharge capacity of 827 mA h g⁻¹, which is 1.47 times as large as that of the electrode without graphite, and the best electrochemical kinetics. Further increasing of graphite content will lead to the increase of contact resistance and activation energy for charge-transfer reaction of the electrode, resulting in the degradation of electrode performance.     

Minimization of the cation mixing in Li1+x(NMC)1−xO2 as cathode material

Li_1+x(Ni_1/3Mn_1/3Co_1/3)_1−xO₂ layered materials were synthesized by the co-precipitation method with different Li/M molar ratios (M = Ni + Mn + Co). Elemental titration evaluated by inductively coupled plasma spectrometry (ICP), structural properties studied by X-ray diffraction (XRD), Rietveld analysis of XRD data, scanning electron microscopy (SEM) and magnetic measurements carried out by superconducting quantum interference devices (SQUID) showed the well-defined α-NaFeO₂ structure with cationic distribution close to the nominal formula. The Li/Ni cation mixing on the 3b Wyckoff site of the interlayer space was consistent with the structural model [Li_1−yNi_y]_3b[Li_x+yNi_{(1−x)/3−y}Mn_(1−x)/3Co_(1−x)/3]_3aO₂ (x = 0.02, 0.04) and was very small. Both Rietveld refinements and magnetic measurements revealed a concentration of Ni²⁺-3b ions lower than 2%; moreover, for the optimized sample synthesized at Li/M = 1.10, only 1.43% of nickel ions were located into the Li sublattice. Electrochemical properties were investigated by galvanostatic charge-discharge cycling. Data obtained with Li_1+x(Ni_1/3Mn_1/3Co_1/3)_1−xO₂ reflected the high degree of sample optimization. An initial discharge capacity of 150 mAh g⁻¹ was delivered at 1 C-rate in the cut-off voltage of 3.0-4.3 V. More than 95% of its initial capacity was retained after 30 cycles at 1 C-rate. Finally, it is demonstrated that a cation mixing below 2% is considered as the threshold for which the electrochemical performance does not change for Li_1+x(Ni_1/3Mn_1/3Co_1/3)_1−xO₂.