Structural and Electrochemical Analyses on the Transformation of CaFe₂O₄‑Type LiMn₂O₄ from Spinel-Type LiMn₂O₄

Lithium manganese oxides have received much attention as positive electrode materials for lithium-ion batteries. In this study, a post-spinel material, CaFe2O4-type LiMn2O4 (CF-LMO), was synthesized at high pressures above 6 GPa, and its crystal structure and electrochemical properties were examined. CF-LMO exhibits a one-dimensional (1D) conduction pathway for Li ions, which is predicted to be superior to the three-dimensional conduction pathway for these ions. The stoichiometric LiMn2O4 spinel (SP-LMO) was decomposed into three phases of Li2MnO3, MnO2, and Mn2O3 at 600 °C and then started to transform into the CF-LMO structure above 800 °C. The rechargeable capacity (Qrecha) of the sample synthesized at 1000 °C was limited to ∼40 mA h·g–1 in the voltage range between 1.5 and 5.3 V because of the presence of a small amount of Li2MnO3 phase in the sample (=9.1 wt %). In addition, the Li-rich spinels, Li­[LixMn2–x]­O4 with x = 0.1, 0.2, and 0.333, were also employed for the synthesis of CF-LMO. The sample prepared from x = 0.2 exhibited a Qrecha value exceeding 120 mA h·g–1 with a stable cycling performance, despite the presence of large amounts of the phases Li2MnO3, MnO2, and Mn2O3. Details of the structural transformation from SP-LMO to CF-LMO and the effect of Mn ions on the 1D conduction pathway are discussed

ZIF-Derived Co_9–<i>x</i>Ni_<i>x</i>S₈ Nanoparticles Immobilized on N‑Doped Carbons as Efficient Catalysts for High-Performance Zinc–Air Batteries

Bimetallic sulfides have been attracting considerable attention because of their high catalytic activities for oxygen reduction reaction (ORR) and oxygen evolution reaction; thus, they are considered efficient catalysts for important energy conversion devices such as fuel cells and metal–air batteries. Here, the catalytic activity of a novel catalyst composed of Co9–xNixS8 nanoparticles immobilized on N-doped carbons (Co9–xNixS8/NC) is reported. The catalyst is synthesized using a Ni-adsorbed Co–Zn zeolitic imidazolate framework (ZIF) precursor (NiCoZn-ZIF). Because of the porous structure of ZIF and the high intrinsic activity of the bimetallic sulfide nanoparticles, the Co9–xNixS8/NC catalyst exhibits high half-wave potential 0.86 V versus reversible hydrogen electrode for ORR and outstanding bifunctional catalytic performance. When Co9–xNixS8/NC is applied as a cathode catalyst in zinc–air batteries, considerably higher power density of about 75 mW cm–2 and discharge voltage are achieved compared to those of batteries with commercial Pt/C and other ZIF-derived catalysts. The zinc–air battery with the Co9–xNixS8/NC catalyst shows a high cyclability more than 170 cycles for 60 h with almost negligible decline at 10 mA cm–2. Our work provides a new insight into the design of bimetallic sulfide composites with high catalytic activities

Cation Dimerization in a 3d¹ Honeycomb Lattice System

In one-dimensional systems with partially filled valence bands, simultaneous changes occur in the electronic states and crystal structures. This is known as the Peierls transition. The Peierls transition (cation dimerization) in VO2, which has a quasi-one-dimensional structure, is well-known, and its mechanism has been extensively discussed. Honeycomb lattices exhibit the Peierls instability owing to their low dimensionality. However, cation dimerization is rare in the 3d1 honeycomb lattice system. Here, we perform an in-depth examination of the V–V dimerization (formation of V–V direct bond) in ilmenite-type MgVO3, which is a 3d1 honeycomb lattice system. A ladderlike pattern was observed in the V–V dimers through synchrotron X-ray experiments at temperatures below 500 K. This dimerization was accompanied by a magnetic-to-nonmagnetic transition. Moreover, a valence bond liquid phase may exist at 500–600 K. Our results reveal the behavior of the valence electrons in the 3d1 honeycomb lattice system

Positive and Negative Synergistic Effects of Fe–Co Mixing on the Oxygen and Hydrogen Evolution Reaction Activities of the Quadruple Perovskite CaCu₃Fe_4–<i>x</i>Co_<i>x</i>O₁₂

Highly active and earth-abundant catalysts for the oxygen and hydrogen evolution reactions (OER and HER) are crucial for attaining a sustainable society. Perovskite-related oxides containing Fe and Co ions, Ba0.5Sr0.5Co0.8Fe0.2O3−δ, Ca2FeCoO5, and CaFe0.5Co0.5O3, have been reported as potential OER catalysts, and the Fe–Co mixing in these oxides has been found to synergistically enhance OER activity. However, this effect has not been verified in quadruple perovskite oxides, including CaCu3Fe4O12, which exhibits the highest OER catalytic activity among perovskite oxide catalysts. We therefore synthesized CaCu3Fe4–xCoxO12 with 0 ≤ x ≤ 4 using a high-pressure/high-temperature method and investigated their catalytic properties for the OER and HER. Rietveld analyses based on synchrotron X-ray diffraction measurements revealed that all the CaCu3Fe4–xCoxO12 samples crystallized into almost single-phase solid solutions between x = 0 and 4 with a homogeneous distribution of Fe/Co cations over the entire x range. At x > 0.5, Fe–Co mixing improved the overpotentials and specific activity for both the OER and HER relative to those of the parent compounds, reaching broad maxima at 2 ≤ x ≤ 3, thus resulting in the first appearance of synergistic effects on the catalytic activities of quadruple perovskite catalysts. In contrast, at x ≤ 0.5, this positive synergistic effect disappeared for the OER and even became negative for the HER. Bond valence sum analyses and X-ray absorption spectroscopy clarified that the positive effects were caused by the isovalent substitution of Co∼3+ by Fe∼3+ on the CaCu3Co4O12 side at x > 0.5, whereas the negative effect was triggered by the aliovalent substitution of Fe∼4+ by Co∼3+ on the CaCu3Fe4O12 side at x ≤ 0.5. This information provides an effective strategy for further enhancing the catalytic activities of complex transition metal oxides in multiple valence-variable elements

Inverse Charge Transfer in the Quadruple Perovskite CaCu₃Fe₄O₁₂

Structural and spectroscopic analyses revealed that the quadruple perovskite CaCu₃Fe₄O₁₂ undergoes an “inverse” electron charge transfer in which valence electrons move from B-site Fe to A′-site Cu ions (∼3Cu^∼2.4+ + 4Fe^∼3.65+ → ∼3Cu^∼2.2+ + 4Fe^∼3.8+) simultaneously with a charge disproportionation transition (4Fe^∼3.8+ → ∼2.4Fe³⁺ + ∼1.6Fe⁵⁺), on cooling below 210 K. The direction of the charge transfer for CaCu₃Fe₄O₁₂ is opposite to those reported for other perovskite oxides such as BiNiO₃ and ACu₃Fe₄O₁₂ (A = Sr²⁺ or the large trivalent rare-earth metal ions), in which the electrons move from A/A′-site to B-site ions. This finding sheds a light on a new aspect in intermetallic phenomena for complex transition metal compounds

Multiple Factors on Catalytic Activity for Oxygen Evolution Reaction in Magnetoplumbite Fe–Co Oxide BaFe_{12–<i>x</i>}Co_<i>x</i>O₁₉

Complex metal oxides consisting of multiple transition-metal elements exhibit a higher electrocatalytic performance than simple transition-metal oxides. In this study, we demonstrate the physical and electrochemical properties of BaFe12–xCoxO19 (x = 0–12), a solid solution of magnetoplumbite-structured BaFe12O19 and BaCo12O19. Single-phase samples in the whole composition range are successfully obtained by using high-pressure and high-temperature conditions of 6.5 GPa and 1373 K. Co- and Fe-doping into BaFe12O19 and BaCo12O19, respectively, efficiently increases the catalytic activity for the oxygen evolution reaction (OER). The OER activity of BaFe12–xCoxO19 is maximized at two compositions of x = 5 and 10, whereas the nonsystematic change in the OER activity for the intermediate compositions between x = 5 and 10 indicates a competition of positive and negative factors on the OER activity such as the dopant as an active site, electrical conductivity, spin polarization, and charge-transfer resistance. We eventually found a characteristic relationship between specific activity and charge-transfer resistance reflecting the mechanistic difference for BaFe12–xCoxO19

Inverse Charge Transfer in the Quadruple Perovskite CaCu₃Fe₄O₁₂

Structural and spectroscopic analyses revealed that the quadruple perovskite CaCu₃Fe₄O₁₂ undergoes an “inverse” electron charge transfer in which valence electrons move from B-site Fe to A′-site Cu ions (∼3Cu^∼2.4+ + 4Fe^∼3.65+ → ∼3Cu^∼2.2+ + 4Fe^∼3.8+) simultaneously with a charge disproportionation transition (4Fe^∼3.8+ → ∼2.4Fe³⁺ + ∼1.6Fe⁵⁺), on cooling below 210 K. The direction of the charge transfer for CaCu₃Fe₄O₁₂ is opposite to those reported for other perovskite oxides such as BiNiO₃ and ACu₃Fe₄O₁₂ (A = Sr²⁺ or the large trivalent rare-earth metal ions), in which the electrons move from A/A′-site to B-site ions. This finding sheds a light on a new aspect in intermetallic phenomena for complex transition metal compounds

Room-Temperature Pressure-Induced Nanostructural CuInTe₂ Thermoelectric Material with Low Thermal Conductivity

A room-temperature high-pressure synthesis method is proposed as an alternative way to induce nanoscale structural disorder in the bulk thermoelectric CuInTe₂ matrix. This disorder stems from the coexistence of distinct domains with different degrees and geometries of disorder at Cu/In cation sites. The lattice thermal conductivity of high-pressure-treated CuInTe₂ is substantially less than that of hot-pressed CuInTe₂. The Debye–Callaway model reveals that the reduced lattice thermal conductivity is mainly attributed to disorder at the Cu/In cation sites and stacking faults, which were probably created during formation of the high-pressure-treated phases. This study demonstrates that room-temperature high-pressure synthesis can produce a radical change in the crystal structure and physical properties of conventional thermoelectric materials