Synthesis of nickel hydroxide: Effect of precipitation conditions on phase selectivity and structural disorder

The synthesis of nickel hydroxide by precipitation from a suitable solution containing dissolved Ni2+ ions is described. A large matrix of precipitation conditions is explored to generate a wide range of nickel hydroxide samples. While most precipitation reactions result in the formation of β-nickel hydroxide, the samples differ from one another in the degree of structural disorder as reflected by the differences in their powder X-ray diffraction (PXRD) patterns. A correlation of the long-range structure as deduced from diffraction studies with the short-range structure as deduced from infrared (IR) spectral studies is performed. The aim is to control the synthesis conditions to obtain materials with 'tailor-made' structural disorders. © 2005 Elsevier B.V. All rights reserved

Synthesis and characterization of arsenate-intercalated layered double hydroxides (LDHs): Prospects for arsenic mineralization

The arsenate-intercalated layered double hydroxide (LDH) of Mg and Al is synthesized by coprecipitation. The higher thermodynamic stability and the consequent lower solubility of the unitary arsenates preclude the formation of arsenate-intercalated LDHs of other metals directly from solution. However other M/Al-AsO4 (M = Co, Ni, Zn) LDHs could be prepared by anion exchange, showing that arsenate intercalation proceeds topotactically. The intercalation of various species of As(V) into the interlayer of LDHs and the subsequent arsenate carrying capacity are dependent upon the pH of the solution. Upon thermal decomposition, the intercalated arsenate ion undergoes reductive deintercalation to give a mixture of As(III) and As(V) oxides. The product oxides revert back to the LDH upon soaking in water on account of the compositional and morphological metastability of the former. This is in contrast with the phosphate-intercalated LDHs, in which the reversibility is suppressed, consequent to the formation of stable metal phosphates. © 2008 Elsevier Inc. All rights reserved

Suppression of the alpha -> beta-nickel hydroxide transformation in concentrated alkali: Role of dissolved cations

The presence of dissolved cations such as Al and Zn in alkaline electrolyte (6 M KOH) suppresses the alpha --> beta-nickel hydroxide transformation. The uptake of Al (10 mol%) and Zn (30 mol%) exhibited by the active material likely stabilizes the alpha-phase. Dissolved Al is deleterious to the performance of the nickel hydroxide electrode, whereas, dissolved Zn enhances the specific discharge capacity of nickel hydroxide by approximately 25% showing that the mode of metal uptake is different in the two cases

Electrodeposition of dicalcium phosphate dihydrate coatings on stainless steel substrates

Cathodic reduction of an aqueous solution containing dissolved calcium and phosphate ions results in the deposition of micrometer thick CaHPO 4·2H2O (dicalcium phosphate dihydrate) coatings on stainless steel substrates. The coating obtained at a low deposition current (8 mA cm-2) comprises lath-like crystallites oriented along 020. The 020 crystal planes are non-polar and have a low surface energy. At a high deposition current (12 mA cm-2), platelets oriented along 121Ì are deposited. CaHPO4·2H2O is an important precursor to the nucleation of hydroxyapatite, the inorganic component of bones. Differently oriented CaHPO4·2H2O coatings transform to hydroxyapatite with different kinetics, the transformation being more facile when the coating is oriented along 121Ì�. These observations have implications for the development of electrodeposited biocompatible coatings for metal endoprostheses for medical applications. © Indian Academy of Sciences

Nickel Oxyhydroxide/manganese Dioxide Composite as a Candidate Electrode Material for Alkaline Secondary Cells

Nickel hydroxide and manganese dioxide are used in alkaline cells as positive electrode materials. Positive electrodes comprising a nickel oxyhydroxide/manganese dioxide composite, with modification by Bi2O3, deliver a combined reversible discharge capacity of 2.25e per metal atom (650 mAh g−1 metal content), which is higher than that realized from electrodes of either component taken singly. The composite discharges with two potential plateaux, the first appearing at 325 mV corresponds to the discharge of the nickel component, whereas the second at −600 mV is due to the manganese component. Composites of NiO(OH)/MnO2 can be used as a new electrode material with higher discharge capacity than conventional electrodes

The effect of cobalt on the electrochemical performance of β-nickel hydroxide electrodes

Crystalline β-nickel hydroxide comprises of a periodic stacking of charge neutral nickel hydroxide layers. Translation or rotation of nickel hydroxide layers with respect to each other generates stacking faults while an intergrowth of one polymorphic modification in the other generates interstratification. These changes generate structural disorder within the sample and the phases are designated as βbc (bc-badly crystalline) nickel hydroxide. The structure, composition and morphology of these phases differ significantly compared to highly ordered crystalline β-nickel hydroxide. Crystalline β-nickel hydroxide exchanges 0.3e/Ni whereas stacking faulted β-nickel hydroxide and βbc-nickel hydroxide exchanges 0.8-0.9e/Ni. Inclusion of cobalt metal as a conducting additive during the electrode fabrication of pasted electrodes is expected to enhance the electrochemical performance of nickel hydroxide. In contrast to the literature reports, partial substitution of cobalt for graphite to highly ordered crystalline phase of β-nickel hydroxide does not show any improvement in their electrochemical activity. Stacking faulted β-nickel hydroxide, βbc-nickel hydroxide and chemically substituted nickel hydroxide samples also does not show any enhancement in their reversible discharge capacity on inclusion of cobalt. This clearly demonstrates that the electrochemical activity is mainly dictated by the structural disorders at 25-30 °C. © 2008 Elsevier Ltd. All rights reserved

Nickel hydroxide electrodeposition from nickel nitrate solutions: Mechanistic studies

Nickel hydroxide electrodeposition by cathodic reduction of nitrate ions follows an electrochemical (EC) reaction followed by an irreversible chemical reaction mechanism. On subsequent cycling, the electrodeposited nickel hydroxide undergoes a reversible redox reaction. The mechanistic behavior of the nickel hydroxide electrodeposition from nickel nitrate solutions is investigated

Bi2O3 modified cobalt hydroxide as an electrode for alkaline batteries

Manganese dioxide electrode shows reversible charge storage capacity, if the charge-discharge process is limited to 0.3e- exchange. Addition of small amount of Bi2O3 to manganese dioxide induces reversibility with an exchange of 2e-/Mn. Nickel hydroxide is known to reversibly exchange 1e-. In spite of isostructural relationship between the cobalt hydroxide, nickel hydroxide and manganese dioxide, cobalt hydroxide does not show any electrochemical activity. Bi2O3 modified cobalt hydroxide electrodes exchanges 0.3-0.5e-/Co during the charge discharge process. The oxidation-reduction process in cobalt hydroxide and Bi2O3 modified cobalt hydroxide electrodes were monitored using the PXRD patterns. © 2008 Elsevier Ltd. All rights reserved

Anionic clay-like behaviour of alpha-nickel hydroxide: chromate sorption studies

While the sorption capacity of alpha-nickel hydroxide for the chromate ion is comparable with that of hydrotalcite-like anionic clays, the control samples of beta- and beta(bc)(bc: badly crystalline)-nickel hydroxide show negligible sorption. This provides direct evidence for the clay-like nature of alpha-nickel hydroxide. (C) 2002 Elsevier Science B.V. All rights reserved

Structural synthon approach to predict the possible polytypes of layered double hydroxides

The complete universe of possible polytypes of layered double hydroxides (LDH) is predicted on the basis of symmetry arguments. A single MX 2 (X = OH) layer, also defined as a structural synthon, belongs to the layer group P$\bar{3}$2/m1. These layers can be stacked in such a way as to conserve the unique 3-axis of the layer in the resultant crystal. The different stacking sequences that facilitate symmetry conservation, yield the different possible polytypes of rhombohedral and hexagonal symmetries. More polytypes can be envisaged by including stacking sequences that systematically destroy the principal symmetry elements of the structural synthon. Thereby, stacking sequences that destroy the 3-axis, while retaining the 2-axis, yield possible polytypes of monoclinic symmetry. The nitrate-containing LDH of zinc and aluminum crystallizes in a faulted structure in which, the planar faults are shown to arise on account of stacking sequences whose local symmetry is monoclinic. This approach to polytype prediction expands on an earlier reported method by Bookin and Drits and is very general with important implications for other classes of layered materials. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim