Preparation and characterization of turbostratic Ni/Al layered double hydroxides for nickel hydroxide electrode applications

Abstract

Replacement of nickel by aluminium in the brucite-type Ni(OH)₂ layers leads to layered double hydroxides (LDH), which can be used as active materials for nickel hydroxide electrodes. Ni/Al LDH compounds [0 < Al/(Ni + Al)⩽ 0.25] were synthesized by precipitation with ammonia from mixed Ni/Al nitrate solutions and characterized by X-ray powder diffraction, infrared spectroscopy, chemical analysis and transmission electron microscopy. X-Ray powder diffractograms of fresh precipitates are characteristic of turbostratic layered double hydroxides. A structural model describing the turbostratic layered double hydroxides is proposed on the basis of the experimental results. The brucite-type layers are non-stoichiometric and present hydroxide vacancies: their chemical composition can be written as [Ni_1–x²⁺ Al_x³⁺(OH^–)_{2–(y+2z–x)}] and the interlamellar layers as [(NO₃^–)_y(CO₃^2–)_z·nH₂O] with x⩽ 0.25. The interlamellar anions (NO₃^– or CO₃^2–) play two roles: those in D_3h symmetry, compensate for the positive-charge excess of Al³⁺ ions, while those in C_2v, symmetry, compensate for that of hydroxide vacancies. When aged, mixed turbostratic Ni/Al hydroxides with the higher aluminium content (x 0, 18) change into synthetic takovites (crystallized Ni–Al–CO₃ double hydroxides) through a dissolution–recrystallization process.