Nickel–cobalt oxides/hydroxides have been considered as promising electrode materials for a high-performance supercapacitor. However, their energy density and cycle stability are still very poor at high current density. Moreover, there are few reports on the fabrication of mixed transition-metal oxides with multishelled hollow structures. Here, we demonstrate a new and flexible strategy for the preparation of hollow Ni–Co–O microspheres with optimized Ni/Co ratios, controlled shell porosity, shell numbers, and shell thickness. Owing to its high effective electrode area and electron transfer number (n3/2 A), mesoporous shells, and fast electron/ion transfer, the triple-shelled Ni–Co1.5–O electrode exhibits an ultrahigh capacitance (1884 F/g at 3A/g) and rate capability (77.7%, 3–30A/g). Moreover, the assembled sandwiched Ni–Co1.5–O//RGO@Fe3O4 asymmetric supercapacitor (ACS) retains 79.4% of its initial capacitance after 10 000 cycles and shows a high energy density of 41.5 W h kg–1 at 505 W kg–1. Importantly, the ACS device delivers a high energy density of 22.8 W h kg–1 even at 7600 W kg–1, which is superior to most of the reported asymmetric capacitors. This study has provided a facile and general approach to fabricate Ni/Co mixed transition-metal oxides for energy storage.
In this work, the structural, optical, and electronic properties of rare-earth perovskites of the general formula $R{\mathrm{CrO}}_{3}$, where $R$ represents the rare-earth Gd, Tb, Dy, Ho, Er, and Tm, have been studied in detail. These compounds were synthesized through a facile citrate route. X-ray diffraction, Raman spectroscopy, and UV-Visible spectroscopy were utilized to reveal the structural evolutions in $R{\mathrm{CrO}}_{3}$. The lattice parameters, ${\mathrm{Cr}}^{3+}--{\mathrm{O}}^{2}--{\mathrm{Cr}}^{3+}$ bond angle, and ${\mathrm{CrO}}_{6}$ octahedral distortions were found to strongly depend on the ionic radii of $R$. First-principles calculations based on density-functional theory within the generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) and strongly constrained and appropriately normed (SCAN) meta-GGA were also employed to calculate the structural and electronic properties of $R{\mathrm{CrO}}_{3}$. The ground-state energy, lattice constants, electronic structures, and density of states of $R{\mathrm{CrO}}_{3}$ were calculated. These provide some insights into the electronic characteristics of the $R{\mathrm{CrO}}_{3}$ compounds. The calculated values of lattice parameters and band gaps with Hubbard $U$ correction ($\mathrm{SCAN}+U$) agree well with values measured experimentally and show more accuracy in predicting the ground-state crystal structure and band structure compared to $\mathrm{PBE}+U$ approximation. The band gap of $R{\mathrm{CrO}}_{3}$ is found to be independent of the ionic radii of $R$ from both experiments and calculations.
Abstract The structure, magnetic, and magnetocaloric (MC) properties of orthorhombic nanocrystalline GdCrO 3 with six particle sizes: ⟨ d ⟩ = 87, 103, 145, 224, 318, and 352 nm are reported. The particle size was tailored by annealing under different temperatures and estimated by scanning electron microscopy. With increase in ⟨ d ⟩, Goldschmidt tolerance factor t , orthorhombic strain s , and out-of-plane Cr–O 1 –Cr bond angle first decrease, reaching minimum values for ⟨ d ⟩ = 224 nm, and then increase for sample with ⟨ d ⟩ = 318 nm and 352 nm, thus showing a V-shaped variation. Temperature dependence of the magnetization ( M ) reveals an antiferromagnetic transition at TNCr∼168 K for ⟨ d ⟩ ⩾ 224 nm and TNCr∼167 K for ⟨ d ⟩ < 224 nm and an essentially d -independent spin-reorientation at T SR = 9 K. M measured at 5 K and 7 T first increases with increase in ⟨ d ⟩, reaching maximum value for sample with ⟨ d ⟩ = 224 nm, and then decreases for samples with ⟨ d ⟩ = 318 nm and 352 nm, showing an inverted-V variation with ⟨ d ⟩. Similar ⟨ d ⟩-dependence is observed for the magnetic entropy change (MEC) and relative cooling power (RCP) showing a close relationship between the structural and magnetic properties of GdCrO 3 nanoparticles investigated here. The 224 nm sample with the minimum values of t , s , and Cr–O 1 –Cr bond angle exhibits the maximum value of MEC (−Δ S ) = 37.8 J kg −1 K −1 at 5 K under a field variation (Δ H ) of 7 T and its large estimated RCP of 623.6 J Kg −1 is comparable with those of typical MC materials. Both (−Δ S ) and RCP are shown to scale with the saturation magnetization M S , suggesting that M S is the crucial factor controlling their magnitudes. Assuming (−Δ S ) ∼ (Δ H ) n , the temperature dependence of n for the six samples are determined, n varying between 1.3 at 5 K to n = 2.2 at 130 K in line with its expected magnitudes based on mean-field theory. These results on structure-property correlations and scaling in GdCrO 3 suggest that its MC properties are tunable for potential low-temperature magnetic refrigeration applications.
Rare-earth chromites are a new type of magnetoelectric multiferroics. In this work, a Ho0.33Gd0.67CrO3 powder sample was synthesized via a citrate route, and the structural properties were characterized by X-ray diffraction, scanning electron microscopy, and the Raman technique. The UV-Visible optical absorbance spectra were also measured in the wavelength range of 200–800 nm. The valence state of Cr was found to be purely 3+ according to the X-ray photoelectron spectroscopy. The temperature-dependent dielectric constant and loss tangent data measured between the frequencies of 1 kHz and 1 MHz show no anomalies around the magnetic transition temperature of the material. The dc magnetization measurements show that the ordering temperature of Cr3+ (TNCr) is 155 K for Ho0.33Gd0.67CrO3, which is larger than 140 K for HoCrO3. The positive slope of the Arrott plots from 0 T to 7 T reveals that the antiferromagnetic-paramagnetic phase transition is second-order in nature. At a field of 7 T, the Ho0.33Gd0.67CrO3 sample showed a giant magnetocaloric entropy change, −ΔS, of ∼23.3 J/kg K at 5 K, and a refrigeration capacity of ∼481.2 J/kg, which are much higher than those of pure bulk HoCrO3. This renders this material prospective for magnetic refrigeration in the low temperature (<30 K) range.
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