Spinels with the formula of AB2O4 (where A and B are metal ions) and the properties of magnetism, optics, electricity, and catalysis have taken significant roles in applications of data storage, biotechnology, electronics, laser, sensor, conversion reaction, and energy storage/conversion, which largely depend on their precise structures and compositions. In this review, various spinels with controlled preparations and their applications in oxygen reduction/evolution reaction (ORR/OER) and beyond are summarized. First, the composition and structure of spinels are introduced. Then, recent advances in the preparation of spinels with solid-, solution-, and vapor-phase methods are summarized, and new methods are particularly highlighted. The physicochemical characteristics of spinels such as their compositions, structures, morphologies, defects, and substrates have been rationally regulated through various approaches. This regulation can yield spinels with improved ORR/OER catalytic activities, which can furth...

Spinels: Controlled Preparation, Oxygen Reduction/Evolution Reaction Application, and Beyond

Binary metal oxides have been regarded as ideal and potential anode materials, which can ameliorate and offset the electrochemical performance of the single metal oxides, such as reversible capacity, structural stability and electronic conductivity. In this work, monodisperse NiCo2O4 mesoporous microspheres are fabricated by a facile solvothermal method followed by pyrolysis of the Ni0.33Co0.67CO3 precursor. The Brunauer–Emmett–Teller (BET) surface area of NiCo2O4 mesoporous microspheres is determined to be about 40.58 m2 g–1 with dominant pore diameter of 14.5 nm and narrow size distribution of 10–20 nm. Our as-prepared NiCo2O4 products were evaluated as the anode material for the lithium-ion-battery (LIB) application. It is demonstrated that the special structural features of the NiCo2O4 microspheres including uniformity of the surface texture, the integrity and porosity exert significant effect on the electrochemical performances. The discharge capacity of NiCo2O4 microspheres could reach 1198 mA h g–1...

High electrochemical performance of monodisperse NiCo₂O₂ mesoporous microspheres as an anode material for Li-ion batteries

Here, the hybrid of NiCo2S4 nanoparticles grown on graphene in situ is first described as an effective bifunctional nonprecious electrocatalyst for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in the alkaline medium. NiCo2S4@N/S-rGO was synthesized by a one-pot solvothermal strategy using Co(OAc)2, Ni(OAc)2, thiourea, and graphene oxide as precursors and ethylene glycol as the dispersing agent; simultaneously, traces of nitrogen and sulfur were double-doped into the reduced graphene oxide (rGO) in the forms of pyrrolic-N, pyridinic-N, and thiophenic-S, which are often desirable for metal-free ORR catalysts. In comparison with commercial Pt/C catalyst, NiCo2S4@N/S-rGO shows less reduction activity, much better durability, and superior methanol tolerance toward ORR in 0.1 M KOH; it reveals higher activity toward OER in both KOH electrolyte and phosphate buffer at pH 7.0. NiCo2S4@graphene demonstrated excellent overall bicatalytic performance, and importantly, it suggests a novel kind ...

NiCo2S4@graphene as a bifunctional electrocatalyst for oxygen reduction and evolution reactions.

Development of stable bimetallic catalysts for carbon dioxide reforming of methane

Nickel cobaltite as an emerging material for supercapacitors: An overview

The compound NiCo2O4, with spinel-related structure, has been prepared by thermal decomposition of metal nitrates and its bulk structural properties examined by means of magnetic measurements, neutron diffraction, X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). The results suggest a delocalised electron distribution on the octahedral sites with average oxidation states of +3.5 and +2.5 for nickel and cobalt, respectively, and lead to a cation distribution for NiCo2O4 of {Ni3+0.1Co2+0.9}tet[Ni3.5+0.9Co2.5+1.1]octO4. This electronic configuration is consistent with magnetisation measurements if applied magnetic fields cause a charge redistribution on the octahedral sites to favour Co3+ and
Ni3+. The surface of NiCo2O4 was examined by X-ray photoelectron spectroscopy (XPS) and found to have a different composition containing Co2+, Co3+, Ni2+, Ni3+ and, probably, Ni4+.

Cation distribution and magnetic structure of the ferrimagnetic spinel NiCo2O4

Synthesis and characterization of the reduction properties of cobalt-substituted lanthanum orthoferrites

The Mn–Li ferrite system Li1−0.5xFe1.5x+1Mn1−xO4
(x
= 1, 0.8, 0.6, 0.4, 0.2) has been studied by means of X-ray diffraction, Mossbauer spectroscopy and Mn K- and Fe K-edge XANES/EXAFS. All the samples show the spinel-related structure with the lattice constant a gradually decreasing between a
= 0.8310 nm (x
= 1) and a
= 0.8301 nm (x
= 0.2). Mossbauer and Fe K-edge XANES/EXAFS results have shown that the oxidation state of Fe is +3 and that the fractions of Fe3+ ions occupying the octahedral and tetrahedral sublattices do not change significantly along the series. Mossbauer data recorded under applied magnetic field indicate the existence in the Mn-containing samples of canted spin structures for the Fe3+ ions on both the tetrahedral and octahedral sites, the canting angle being larger for the octahedral sublattice. Mn K-edge XANES and EXAFS have shown Mn to be present as Mn3+ and Mn4+ and to occupy only octahedral sites. The XANES/EXAFS results have also shown that the Mn4+/Mn3+ ratio increases with increasing Li/Mn content. The results indicate that as the extent of Li/Mn insertion increases the new inserted Li+ ions occupy tetrahedral sites and that some of the Li+ ions initially occupying octahedral sites are also driven into tetrahedral positions.

Characterization of the Mn–Li ferrite system Li1−0.5xFe1.5x+1Mn1−xO4 (0.2 ≤x≤ 1)

The oxidation states of titanium and niobium in compounds of composition SnxNbTiP3O12 (0 < x ⩽ 0.50) : an XPS study

Perovskite-related phases of the type LaFe1−x Co x O3 (x = 0 and 0.5) have been synthesised by milling techniques. The materials are of smaller particle size and more susceptible to reduction in a flowing hydrogen/nitrogen gas mixture than their counterparts made by conventional solid state calcination methods. The presence of cobalt enhances the reducibility of iron in the perovskite-related structure and, in contrast to LaFeO3 where the oxide structure is at least partially retained even after treatment in the reducing atmosphere at 1150°C, treatment of LaFe0.5Co0.5O3 in a reducing atmosphere leads to reduction of both Fe3+ and Co3+, their segregation from the oxide matrix, and the formation of an iron-cobalt alloy.

J. Ramón Gancedo

Papers

Cation distribution and magnetic structure of the ferrimagnetic spinel NiCo2O4

Synthesis and characterization of the reduction properties of cobalt-substituted lanthanum orthoferrites

Characterization of the Mn–Li ferrite system Li1−0.5xFe1.5x+1Mn1−xO4 (0.2 ≤x≤ 1)

The oxidation states of titanium and niobium in compounds of composition SnxNbTiP3O12 (0 < x ⩽ 0.50) : an XPS study

57 Fe Mössbauer Spectroscopy Study of LaFe 1-x Co x O 3 (x=0 and 0.5) Formed by Mechanical Milling