R. S. Jayashree

Electrochemical synthesis of α-cobalt hydroxide

Cathodic reduction of an aqueous solution of cobalt nitrate at low pH, high Co(II) ion concentration (≥1 M) and low current densities (<0.3 mA cm –2 ) leads to the formation of a novel layered hydroxide of Co(II) with an interlayer spacing of 8.93 A. This hydroxy deficient phase is structurally and compositionally related to α-nickel hydroxide, but likely contains Co(II) ions in a mixed octahedral/tetrahedral coordination. Under other deposition conditions, the better known β-cobalt hydroxide (a=3.17±0.01 A, c=4.61±0.02 A) is obtained.

The Effect of Crystallinity on the Reversible Discharge Capacity of Nickel Hydroxide

The higher reversible discharge capacity exhibited by (bc: badly crystalline)‐nickel hydroxide compared to that of β‐nickel hydroxide can be attributed to the presence of regions characterized by turbostratic disorder in the former. These regions comprise the α‐modification of nickel hydroxide existing in an interstratified form within the sample. The α‐motifs in the are anion‐free, but hydrated, by virtue of which hydroxide exhibits a higher water content (11% by weight) compared to the β‐form (<1%).

DISORDER IN LAYERED HYDROXIDES: DIFFaX SIMULATION OF THE X-RAY POWDER DIFFRACTION PATTERNS OF NICKEL HYDROXIDE

Layered metal hydroxides exhibit non-uniform broadening of lines in their X-ray powder diffraction (XRPD) patterns, which cannot always be explained on the basis of crystallite size effects. In the case of hexagonal solids such as nickel hydroxide, DIFFaX simulations of the XRPD patterns show that: (1) stacking faults and turbostratic disorder at low (<30%) incidence selectively broaden the h0l reflections; (2) turbostratic disorder at high (>40%) incidence causes asymmetric broadening of the hk0 reflections and a complete extinction of the hkl reflections while leaving 00l unchanged; (3) interstratification selectively broadens the non-hk0 reflections; and (4) cation vacancies reduce the relative intensity of the 100 reflection. In contrast, a reduction in the thickness of the crystallites along the stacking direction of the layers selectively broadens the 00l reflections while a reduction in the disc diameter causes the progressive broadening and extinction of the hk0 reflections. Comparison with experimental data shows that several kinds of disorders have to be invoked to account for the observed broadening. DIFFaX simulations enable the quantification of the different kinds of disorder.

Factors governing the electrochemical synthesis of α-nickel (ii) hydroxide

The electrodeposition of α-nickel hydroxide is promoted by the simultaneous chemical corrosion of the electrode by an acidic nitrate bath. Chemical corrosion results in the formation of a poorly ordered layered phase which is structurally similar to α-nickel hydroxide and provides nucleation sites for the deposition of the latter. Therefore under conditions which enhance corrosion rates such as low current density (<1.3 mA cm−2), high temperature (60 ∘C), high nickel nitrate concentration (≥ 1M) and the resultant low pH (∼1.7), α-nickel hydroxide electrodeposition is observed, while β-nickel hydroxide forms under other conditions. Further, α-nickel hydroxide deposition is more facile on an iron electrode compared to nickel or platinum.

Suppression of the α → β-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 α → β-nickel hydroxide transformation. The uptake of Al (10 mol%) and Zn (30 mol%) exhibited by the active material likely stabilizes the α-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.

Electrochemically impregnated aluminum-stabilized alpha-nickel hydroxide electrodes

Nickel-positive electrodes obtained by electrochemical impregnation of aluminum-substituted alpha-nickel hydroxide are found to deliver a reversible discharge capacity of ca. 450 mAh/g. This is much higher than the capacity of beta-nickel hydroxide electrodes [200 mAh/g: this work; 225 mAh/g: Dixit et al., J. Power Sources, 63, 167 (1996)] prepared under identical conditions and pasted electrodes comprising cobalt-doped nickel hydroxide [345 mAh/g: Faure et al., J. Power Sources, 36, 497 (1991)]. These observations suggest that the theoretical target-capacity for high-performance nickel-positive electrodes must be revised from the currently accepted value of 289 mAh/g (le exchange) to 491 mAh/g [1.7e exchange: Corrigan and Knight, J. Electrochem. Soc., 136, 613 (1989)]

Nickel hydroxide electrodeposition from nickel nitrate solutions: mechanistic studies

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

Effect of lightweight supports on specific discharge capacity of β-nickel hydroxide

While the performance of βbc (bc: badly crystalline)-nickel hydroxide is relatively unaffected by the use of different lightweight supports, the specific discharge capacity of crystalline β-Ni(OH)2 doubles to approximately 355 mAh g–1 Ni (theoretical, 456 mAh g−1) when pasted to a fibre support compared with 170 mAh g–1 Ni when pasted to a nickel foam. Consequently, the fibre is a superior support as it is unaffected by the quality of the active material and extracts a consistently high performance irrespective of the degree of crystallinity, moisture content, morphology and composition of the active material. Electrochemical impedance measurements indicate that a lower charge-transfer resistance at low states-of-charge is responsible for the superior performance of fibre supported β-Ni(OH)2 electrodes.

The Effect of the Moisture Content on the Reversible Discharge Capacity of Nickel Hydroxide

The moisture content of I²-nickel hydroxide samples obtained by strong alkali precipitation from nickel salt solutions varies as a function of the pH at precipitation. A low (7-9) pH at precipitation yields nickel hydroxides with 5-6 moisture content, whereas a high (>13) pH at precipitation yields a moisture content of nearly 12. In all cases, the moisture content is lost at moderate (65-125°C) temperatures but is restored on cooling and equilibration (24 h) in the ambient. The restoration of the original moisture content in its entirely in all samples shows that the nickel hydroxide phases obtained at low and high pH are distinct from each other. DlFFaX simulations show that the phases with different moisture contents differ in the degree of turbostratic disorder and the proportion of I±-phase interstratified by them; the high moisture content phase has a higher proportion of the I±-motifs. This is reflected in the characteristic broadening of lines in the power X-ray diffraction patterns of the two phases. The high moisture content hydroxide delivers a higher (385 mAh g-1 of Ni) (0.85 e- exchange) reversible discharge capacity compared to the low moisture content sample (185 mAh g-1) (0.4 e- exchange) in foam pasted electrodes. These observations highlight (i) the decisive role of moisture content in high performance electrode materials and (ii) the role of the pH during precipitation as the sole determining factor that yields high moisture content.

Modified Nickel Hydroxide Electrodes Effect of Cobalt Metal on the Different Polymorphic Modifications

Inclusion of Co metal as an additive during the preparation of pasted electrodes enhances the reversible discharge capacity of β-nickel hydroxide from 220 mAh g -1 of Ni to 400 mAh g -1 (theoretical, 456 mAh g -1 Ni). In contrast, the performance of β bc (bc badly crystalline) and α-nickel hydroxide is relatively unaffected by addition of Co. In electrodes comprising the crystalline-layered double hydroxide of Ni with Al, Co addition not only results in enhancement of the capacity from 375 to 585 mAh g -1 but also increases capacity retention.