Identification of non-traditional coordination environments for iron ions in nickel hydroxide lattices

Abstract

The electrochemical behavior of iron-doped nickel hydroxide is remarkably consistent across the diverse fabrication protocols employed throughout the literature, which contrasts with variations reported in composition-dependent changes in structure and...

Linking Lattice Strain and Electron Transfer Kinetics in Crystalline Layered Double Hydroxides

“…Past results indicate that the Fe 2 peak arises in disordered iron−nickel oxyhydroxides due to a second coordination environment for Fe(III) in Ni(OH) 2 samples; this coordination environment sits atop the two-dimensional Ni(OH) 2 sheets. 53 In agreement with the smaller-thanexpected lattice contraction observed in XRD, the intensity of this Fe 2 Raman peak is significantly greater at low Fecontent values here than previously reported. These secondary sites thus appear more favored for samples prepared by hydrothermal synthesis than for previous alkaline precipitation methods.…”

“…600 cm –1 ( Fe 2 ; Figure A) is intense immediately upon addition of Fe to the samples. Past results indicate that the Fe 2 peak arises in disordered iron–nickel oxyhydroxides due to a second coordination environment for Fe­(III) in Ni­(OH) 2 samples; this coordination environment sits atop the two-dimensional Ni­(OH) 2 sheets . In agreement with the smaller-than-expected lattice contraction observed in XRD, the intensity of this Fe 2 Raman peak is significantly greater at low Fe-content values here than previously reported.…”

“…This third peak has been variably assigned as a second-harmonic of the E g mode at ca. 300 cm –1 , to the presence of cation vacancies, and to Fe­(III) ions residing in a coordination environment atop the two-dimensional sheets within Ni­(OH) 2 . Assignment as a second-harmonic is unlikely here because the peak is an order of magnitude more intense than the fundamental E g mode and increases in intensity with Fe-content and associated structural disorder.…”

“…Cyclic voltammetry experiments were recorded between 0.9 and 1.7 V versus the RHE electrode at a 10 mV s –1 scan rate with 10 cycles, and the last cycle of each sample was compared. All currents were normalized by the charge passed under the cathodic redox peak in a cyclic voltammogram. , This normalizes the electrochemical results relative to the number of electroactive nickel centers, which accommodates unavoidable variations in sample loading, Ohmic contact between the sample and the electrode, changes in sample morphology, or changes in accessibility of redox active sites to the electrolyte solution on the timescale of the experiments. Cyclic voltammetric results showing normalization by the geometric surface area are provided (Figure S4).…”

“…This risk is sufficiently high that phase contamination of layered double hydroxides that are not based on amphoteric ions such as Al­(III) has been suggested to be prolific yet underreported in the literature . Phase-pure layered double hydroxides can be prepared with the assistance of organic surfactants , or hydrogen bonding additives such as formamide, , but this leads to lower crystallinity. A desire to study highly crystalline layered double hydroxides necessitates the use of hydrothermal techniques and the associated risk of phase impurity.…”