Comparative structural and electrochemical study of high density spherical and non-spherical Ni(OH)2 as cathode materials for Ni-metal hydride batteries

In this paper we compare the behavior of non-spherical and spherical β-Ni(OH)₂ as cathode materials for Ni-MH batteries in an attempt to explore the effect of microstructure and surface properties of β-Ni(OH)₂ on their electrochemical performances. Non-spherical β-Ni(OH)₂ powders with a high-density are synthesized using a simple polyacrylamide (PAM) assisted two-step drying method. X-ray diffraction (XRD), infrared spectroscopy (IR), scanning electron microscopy (SEM), thermogravimetric/differential thermal analysis (TG-DTA), Brunauer-Emmett-Teller (BET) testing, laser particle size analysis, and tap-density testing are used to characterize the physical properties of the synthesized products. Electrochemical characterization, including cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and a charge/discharge test, is also performed. The results show that the non-spherical β-Ni(OH)₂ materials exhibit an irregular tabular shape and a dense solid structure, which contains many overlapped sheet nano crystalline grains, and have a high density of structural disorder and a large specific surface area. Compared with the spherical β-Ni(OH)₂, the non-spherical β-Ni(OH)₂ materials have an enhanced discharge capacity, higher discharge potential plateau and superior cycle stability. This performance improvement can be attributable to a higher proton diffusion coefficient (4.26 × 10⁻⁹ cm² s⁻¹), better reaction reversibility, and lower electrochemical impedance of the synthesized material.     

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.     

Cation ordering in the layered Li1+x(Ni0.425Mn0.425Co0.15)1 − xO2 materials (x = 0 and 0.12)

The layered Li_1+x(Ni_0.425Mn_0.425Co_0.15)_1 − xO₂ (x = 0 and 0.12) materials were prepared by a coprecipitation method. Their structure was investigated using the combination of X-ray and electron diffraction experiments. For both materials (x = 0 and 0.12), the electron diffraction patterns revealed an in-plane √3a_hex. × √3a_hex. superstructure in agreement with the ordering of the Li⁺, Ni²⁺, Ni³⁺, Mn⁴⁺ and Co³⁺ ions in the transition metal layers. The stoichiometry of these materials was not in agreement with an ideal ordering: the possible presence of point defects or of a domain microstructure was thus discussed. Electron diffraction also revealed that these ordered layers were slightly correlated along the c_hex. axis for both materials.     

Combustion synthesized Nd2−xCexCuO4 (x = 0-0.25) cathode materials for intermediate temperature solid oxide fuel cell applications

It is found that the solid solubility of Ce in Nd_2−xCe_xCuO_4±δ is limited up to x = 0.2. A semiconductor to metallic transition is observed at 600 °C in d.c. conductivity data, which coincides with a transition in temperature-dependent area-specific resistance (ASR). Nd_1.8Ce_0.2CuO_4±δ is thermodynamically and chemically stable against gadolinia-doped ceria (GDC) up to 1200 °C. On the other hand, it reacts with a yttria-stabilized zirconia electrolyte to form Nd₂Zr₂O₇. At 700 °C, the ASR of a Nd_1.8Ce_0.2CuO_4±δ/GDC/Nd_1.8Ce_0.2CuO_4±δ cell sintered at 800 °C is 0.13 ohm cm², and the ASR proportionally improves with increase in the sintering temperature of the electrochemical cell. The improved ASR and electrochemical performance are attributed to the nanocrystalline nature of the cathode material.     

The influence of compositional change of 0.3Li2MnO3·0.7LiMn1−xNiyCo0.1O2 (0.2 ≤ x ≤ 0.5, y = x − 0.1) cathode materials prepared by co-precipitation

Cathode materials prepared by a co-precipitation are 0.3Li₂MnO₃·0.7LiMn_1−xNi_yCo_0.1O₂ (0.2 ≤ x ≤ 0.4) cathode materials with a layered-spinel structure. In the voltage range of 2.0-4.6 V, the cathodes show more than one redox reaction peak during its cyclic voltammogram. The Li/0.3Li₂MnO₃·0.7LiMn_1−xNi_yCo_0.1O₂ (x = 0.3, y = 0.2) cell shows the initial discharge capacity of about 200 mAh g⁻¹. However, when x = 0.2 and y = 0.1, the cell exhibits a rapid decrease in discharge capacity and poor cycle life.     

La1−xSrxCoO3 (x = 0.1-0.5) as the cathode catalyst for a direct borohydride fuel cell

Direct borohydride fuel cells (DBFCs), with a series of perovskite-type oxides La_1−xSr_xCoO₃ (x = 0.1-0.5) as the cathode catalysts and a hydrogen storage alloy as the anode catalyst, are studied in this paper. The structures of the perovskite-type catalysts are mainly La_1−xSr_xCoO₃ (_x = 0.1-0.5) oxides phases. However, with the increase of strontium content, the intensities of the X-ray diffraction peaks of the impure phases La₂Sr₂O₅ and SrLaCoO₄ are gradually enhanced. Without using any precious metals or expensive ion exchange membranes, a maximum current density of 275 mA cm⁻² and a power density of 109 mW cm⁻² are obtained with the Sr content of x = 0.2 at 60 °C for this novel type of fuel cell.     

Effects of different Ni(OH)2 precursors on the structure and electrochemical properties of NiOOH

Nickel oxyhydroxide (NiOOH), an active material for alkaline Zn-NiOOH batteries, was synthesized by electrolysis oxidation of different Ni(OH)₂ precursors, which were prepared by three methods: polyacrylamide (PAM) assisted two-step drying (PTSD), conventional co-precipitation (CCP), and “controlled crystallization” (CC). The NiOOH samples were characterized and tested using X-ray diffraction (XRD), infrared spectroscopy (IR), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) testing, laser particle size analysis, tap-density testing, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and a discharge test. The results demonstrate that the physical and electrochemical properties of NiOOH are strongly dependent on the properties of the Ni(OH)₂ precursor, such as its morphology, microstructure, tap density, and specific surface area. The results of the electrochemical studies also show that the sample prepared by the PTSD method is superior to the others in electrochemical performance. The as-prepared, high-density, non-spherical NiOOH is a promising active material for the positive electrode in Zn-NiOOH batteries.     

Oxidation of methanol on perovskite-type La2-xSrxNiO4 (0 ≤ x ≤ 1) film electrodes modified by dispersed nickel in 1 M KOH

Finely-dispersed nickel particles are electrodeposited on high surface-area perovskite-type La_2-xSr_xNiO₄ (0 ≤ x ≤ 1) electrodes for possible use in a direct methanol fuel cell (DMFC). The study is conducted by cyclic voltammetry, chronoamperometry, impedance spectroscopy and anodic Tafel polarization techniques. The results show that the apparent electrocatalytic activities of the modified oxide electrodes are much higher than those of unmodified electrodes under similar experimental conditions; the observed activity is the greatest with the modified La_1.5Sr_0.5NiO₄ electrode. At 0.550 V (vs. Hg|HgO) in 1 M KOH + 1 M CH₃OH at 25 °C, the latter electrode delivers a current density of over 200 mA cm⁻², whereas other electrodes of the series produce relatively low values (65–117 mA cm⁻²). To our knowledge, such high methanol oxidation current densities have not been reported on any other non-platinum electrode in alkaline solution. Further, the modified electrodes are not poisoned by methanol oxidation intermediates/products.     

Electrochemical behavior of Li3−xM′xV2−yM″y(PO4)3 (M′ = K, M″ = Sc, Mg + Ti)/C composite cathode material for lithium-ion batteries

Composites of monoclinic Li_3−xM′_xV_2−yM″_2y(PO₄)₃ (M′ = K, M″ = Sc, Mg + Ti) with carbon were synthesized by solid-state reaction using oxalic acid or 6% H₂/Ar gas mixture as reducing agents at sintering temperature of 850 °C. The samples were characterized by X-ray diffraction (XRD), voltammetry and electrochemical galvanostatic cycling. The capacity of Li₃V₂(PO₄)₃ synthesized using hydrogen as the reducing agent was 127 mA h g⁻¹ and decreased to 120 mA h g⁻¹ after 20 charge-discharge cycles. The substitution of lithium and vanadium for other ions did not result in the improvement of the electrochemical characteristics of the samples.     

Mechanical behavior and electrical conductivity of La1−xCaxCoO3 (x = 0, 0.2, 0.4, 0.55) perovskites

This paper compares the important mechanical properties and the electrical conductivities from room temperature to 800 °C of four LaCoO₃ based cobaltite compositions with 0, 20, 40 and 55% Ca²⁺ ions substituted on the A site of the perovskite structure respectively. Ca²⁺ doped lanthanum cobaltite materials are strong candidates for use as cathodes in lower temperature solid oxide fuel cells operating at or below 800 °C. Among these four cobaltite compositions, two (LaCoO₃ and La_0.8Ca_0.2CoO₃) were found to be phase pure materials, whereas the remaining two compositions (La_0.6Ca_0.4CoO₃ and La_0.45Ca_0.55CoO₃) contained precipitation of secondary phases such as CaO and Co₃O₄. The mechanical properties of the four compositions, in terms of Young's modulus, four-point bending strength and fracture toughness measurements, were measured at both room temperature and 800 °C. At room temperature, doping with Ca²⁺ was found to substantially increase the mechanical properties of the cobaltites, whereas at 800 °C the pure LaCoO₃ composition exhibited higher modulus and strength values than La_0.8Ca_0.2CoO₃. All of the four compositions exhibited ferroelastic behavior, as shown by the hysteresis loops generated during uniaxial load-unload compression tests. Electrical conductivity measurements showed the La_0.8Ca_0.2CoO₃ composition to have the highest conductivity among the four compositions.     

Bulk conduction and relaxation in [(ZrO2)1−x(CeO2)x]0.92(Y2O3)0.08 (0 ≤ x ≤ 1) solid solutions at intermediate temperatures

Bulk conduction and relaxation of the [(ZrO₂)_1−x(CeO₂)_x]_0.92(Y₂O₃)_0.08 (0 ≤ x ≤ 1) solid solutions were studied using impedance spectroscopy at intermediate temperatures (200-500 °C). The bulk conductivity as a function of x shows a “V-shape” variation which is a competitive effect of the defect associates and the lattice parameter. In the ZrO₂-rich region (x < 0.5) CeO₂ doping increases the concentration of defect associates which limits the mobility of the oxygen vacancies; in the CeO₂-rich region (x > 0.5) the increase of x increases the lattice parameter which enlarges the free channel for oxygen vacancy migration. Further analysis indicates the ionic radius of the tetravalent dopant determines the composition dependence of the ionic conductivity of the solid solutions. When doping YSZ with other tetravalent dopant with similar ionic radius with Zr⁴⁺, e.g., Hf⁴⁺, such “V-shape” composition dependence of the bulk conductivity cannot be observed.     

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.     

Structural characteristics and hydrogen storage properties of Ca3.0−xMgxNi9 (x = 0.5, 1.0, 1.5 and 2.0) alloys

The effect of Mg content on the structural characteristics and hydrogen storage properties of the Ca_3.0−xMg_xNi₉ (x = 0.5, 1.0, 1.5 and 2.0) alloys was investigated. The lattice parameters and unit cell volume of the PuNi₃-type (Ca, Mg)Ni₃ main phase decreased with increasing Mg content. The 6c site of PuNi₃-type structure was occupied by both Ca and Mg atoms. Moreover, the occupation factor of Ca on the 6c site decreased with the increase of Mg content. The hydrogen absorption capacity of the alloys decreased due to higher Mg content. However, the thermodynamic properties of hydrogen absorption and desorption were improved and the plateau pressures were increased. When x = 1.5–2.0, the Ca_3.0−xMg_xNi₉ alloys had favorable enthalpy (ΔH) and entropy (ΔS) of hydride formation.     

Microscopic magnetism in lithium insertion materials of LiNi1−xCoxO2 (x = 0, 1/4, 1/2, 3/4, and 1)

The magnetic nature of lithium insertion materials of LiNi_1−xCo_xO₂ (x = 0, 1/4, 1/2, 3/4, and 1) were investigated by means of positive muon-spin rotation/relaxation (μ⁺SR) spectroscopy combined with X-ray diffraction (XRD) analyses and susceptibility measurements. Zero field μ⁺SR spectra for all the samples below 300 K were well fitted by a dynamic Kubo–Toyabe function, indicating the existence of randomly oriented magnetic moments even at 2 K, i.e., disordered state. The field distribution width Δ due to magnetic Ni³⁺ ions decreases exponentially with increasing x, suggesting that the Co substitution is likely to simply dilute Ni moments. This also supports that cobalt and nickel ions are homogeneously distributed in a solid matrix even in a muon-scale (microscopically), which is consistent with the results of macroscopic measurements.     

Effect of oxygen non-stoichiometry and temperature on cation ordering in LiMn2−xNixO4 (0.50 ≥ x ≥ 0.36) spinels

The effect of oxygen stoichiometry on the transition metal ordering and electrochemical activity of LiMn_2−xNi_xO₄ solid solutions was investigated. Temperature–oxygen-partial-pressure–composition (pO₂–T–x) diagrams of ordered and disordered phases of LiMn_2−xNi_xO₄ (0.50 ≥ x ≥ 0.36) in the vicinity of order–disorder transition temperature (T_c) was constructed by means of infrared spectroscopy, thermogravimetric analysis and galvanostatic measurements. Despite their simplicity and limitations over traditional diffraction techniques, all three techniques offered near excellent capability to distinguish ordered and disordered phases. The effect of oxygen-partial-pressure (pO₂) in the annealing atmosphere and nickel content of the spinel on T_c was studied. The transition temperature increased with pO₂ and nickel content, except in oxygen-rich (pO₂ = 1) atmosphere for the maximum nickel content spinel of LiMn_1.5Ni_0.5O₄. An explanation for the dependence of the transition temperature on the two variables and changes induced by the post-fabrication heat treatments is provided.     

Li1+xFePO4 (0 ≤ x ≤ 3) as anode material for lithium ion batteries: From ab initio studies

Li_1+xFePO₄ (0 ≤ x ≤ 3) as anode material for lithium ion batteries has been studied using ab initio calculations. Results show that large amount of lithium ions can be intercalated into LiFePO₄ host. The structure changes continuously when the first two Moles of lithium ions (x ≤ 2) are intercalated into the LiFePO₄ host, accompanied by large volume expansion (37.4% and 25.4% for the first and second Mole). The final product of Li₃FePO₄ possesses a stable chained structure, which is favorable for storing even more lithium. In the same time, lithium ion diffuses in a three-dimension pathway within the chained structure. The unit cell volume increases only by 4.9% from Li₃FePO₄ to Li₄FePO₄, and the chained structure keeps unchanged.     

Ce0.9Sr0.1VOx (x = 3, 4) as anode materials for H2S-containing CH4 fueled solid oxide fuel cells

The stability and activity in 0.5% H₂S-CH₄ of Ce_0.9Sr_0.1VO₃ and Ce_0.9Sr_0.1VO₄ anode materials for H₂S-containing CH₄ fueled SOFCs have been determined. XRD showed that Ce_0.9Sr_0.1VO₄ was reduced when the fuel gas was 0.5% H₂S-CH₄, while Ce_0.9Sr_0.1VO₃ remained stable over 24 h at 950 °C. Electrochemical tests in 0.5% H₂S-CH₄ showed stable performance at 950 and 800 °C for cells comprising Ce_0.9Sr_0.1VO₃|YSZ|Pt. Comparison of fuel cell performances using 0.5% H₂S-CH₄, 0.5% H₂S-N₂ and 5% H₂S-N₂ as feeds showed that Ce_0.9Sr_0.1VO₃ was not active for oxidation of methane, but highly active for conversion of H₂S. Electrochemical impedance results were consistent with the finding that the anode was activated only in an environment that contained H₂S. Conductivity measurements showed there was an increase in conductivity in H₂S-containing environments, and that this increase resulted from a change in composition and structure from the oxide to monoclinic Ce_0.9Sr_0.1V(O,S)₃, as evidenced by XPS and XRD analyses.     

Electrochemical and structural studies of the carbon-coated Li[CrxLi(1/3−x/3)Ti(2/3−2x/3)]O2 (x = 0.3, 0.35, 0.4, 0.45)

A series of carbon-coated layered structured Li[Cr_xLi_(1/3−x/3)Ti_(2/3−2x/3)]O₂ samples (0.3 ≤ x ≤ 0.45) were prepared. Among them, the sample of x = 0.4 shows the highest initial reversible capacity of 207 mAh g⁻¹ at 30 mA g⁻¹ in 2.5–4.4 V. The reversible Li-storage capacities for the samples with high x values (x = 0.4, 0.45) faded slightly while the samples with low Cr content (x = 0.3 and 0.35) showed a capacity increase upon cycling. It was found that the relative intensity ratio of (0 0 3) peak to (1 0 4) peak (R_(0 0 3) = I_(0 0 3)/I_(1 0 4)) is influenced strongly by x value in as-prepared samples. The samples of x = 0.35 and 0.4 turn to a similar structure with low R_(0 0 3) value during cycling. These phenomena indicate that the cation mixing of Cr³⁺ in the lithium layer occurs in as-prepared samples and became more significant upon delithiation and lithiation. This is supposed being a necessary process for Cr-based layered structure materials possessing electrochemical reactivates. The occurrence of the cation mixing is beneficial from the local lattice distortion caused by the short-range ordering between Ti and Li. This is supposed to be helpful for the migration of Cr⁶⁺ and Cr³⁺ at tetrahedral and octahedral sites. Different from the case of LiNiO₂, the cation mixing is essential for the transport and storage of lithium in the carbon-coated Li–Cr–Ti–O layered compounds.     

Influence of annealing treatment on the hydrogen storage properties of La2−xTixMgNi9 (x = 0.2, 0.3) alloys

La_2−xTi_xMgNi₉ (x = 0.2, 0.3) alloys have been prepared by magnetic levitation melting under an Argon atmosphere, and the as-cast alloys were annealed at 800 °C, 900 °C for 10 h under vacuum. The effects of annealing on the hydrogen storage properties of the alloys were investigated systematically by XRD, PCT and electrochemical measurements. For the La_2−xTi_xMgNi₉ (x = 0.2, 0.3) alloys, LaNi₅, LaMg₂Ni₉ and LaNi₃ are the main phases and a Ti₂Ni phase appears at 900 °C. The effective hydrogen storage capacity increases from 1.10, 1.10 wt.% (as-cast) to 1.22, 1.16 wt.% (annealed 800 °C) and 1.31, 1.27 wt.% (annealed 900 °C), respectively. The annealing not only improves the hydrogen absorption/desorption kinetics but also increases the maximum discharge capacity and enhances the cycling stability. The La_1.8Ti_0.2MgNi₉ alloy annealed at 900 °C exhibits good electrochemical properties, and the discharge capacities decrease from 366.1 mA h/g to 219.6 mA h/g after 177 charge-discharge cycles.