Valence States Of Transition Metals Research Articles

Modulating the valence states of transition metals in FeCoNiMnW high-entropy alloys for enhanced oxygen evolution reaction activity

Amorphous high-entropy alloys (HEA) are promising electrocatalysts with multi-element active sites and dense defect distribution, high valences of 3d transition metals (3d TMs) are beneficial for the oxygen evolution reaction (OER). However, it is still unknown how to enhance the 3d TMs valence state in amorphous HEA to improve OER catalytic activity. Herein, we successfully boosted the valence state of 3d TMs in HEA through electronegativity. The prepared FeCoNiMnW HEA exhibited significant amorphous characteristics. X-ray photoelectron spectroscopy analysis confirmed the formation of Ni3+ sites after adding electronegative W and the widespread occurrence of oxygen vacancies through amorphization. The enhanced oxidation state of Ni reduced the overpotential, while the abundant oxygen vacancies significantly facilitated electron transfer and thus boosting the rate of catalytic reactions. These characteristics lead to the amorphous FeCoNiMnW HEA exhibiting excellent OER performance. Amorphous FeCoNiMnW HEA films deposited on carbon cloth loaded with carbon nanotubes (FeCoNiMnW@CCC) only require an overpotential of 253 mV to generate a current density of 10 mA cm−2, which is much lower than RuO2 films (399 mV) and commercial RuO2 (309 mV), and shows excellent durability for 700 h at 20 mA cm−2 and 100 h at 200mA cm−2. This work provides a novel strategy for designing high-performance OER catalysts through structural and electronic modulation.

Synthesis of CuO/MnAl2O4 nanocomposite as electrocatalysts for robust oxygen evolution reaction

The primary focus of current research is the development of an affordable, effective and reliable electrocatalyst for electrochemical water splitting to generate clean energy. In this context, designing and manipulating the valence states of transition metals is a promising method for developing effective electrocatalysts for the splitting of water. Because increased valence metal sites can accelerate the reaction rates for oxygen evolution process (OER). Herein, the physiochemical features of CuO are modulated by the inclusion of spinel MnAl2O4 with high-valence metal sites and a porous framework comprised of linked tiny nanoparticles. Nanocomposite CuO/MnAl2O4 were fabricated through a sonication procedure in 1.0 M KOH for OER. The electrocatalytic measurement of CuO/MnAl2O4 showed outstanding oxygen activity for OER with reduced overpotential of 196 mV at current density (Cd:10 mA cm−2) and Tafel value (34 mV dec−1) and lower onset potential of 1.28V than pristine CuO. Also, the Nyquist plot illustrates the improved conductivity by illustrating a minor charge transfer resistance (Rct) value (0.13 Ω), moreover, nanocomposite shows stability for 50 h at 65 % of its current density.

Maxwell–Wagner Polarization and Mixed Ferromagnetic and Antiferromagnetic State in Eu2CoMnO6

Herein, the electrical and magnetic properties of polycrystalline Eu2CoMnO6 are explored. More specifically, based on transport, dielectric, and impedance measurements, a thermally activated dielectric polarization near room temperature is reported. The valence states of transition metals in the Eu2CoMnO6 sample are studied by measuring the X‐ray absorption spectra at both Co‐L2,3 and Mn‐L2,3 edges. Moreover, based on magnetization, X‐ray absorption spectroscopy, and X‐ray magnetic circular dichroism measurements, the coexistence of competing ferromagnetic and antiferromagnetic phases is probed. Dielectric measurement shows a large dielectric constant near room temperature along with a large frequency dispersion. The temperature‐dependent resistivity, Cole–Cole plot, and imaginary part of impedance confirm the semiconducting nature of Eu2CoMnO6. Temperature‐dependent magnetization shows a second‐order magnetic transition below 125 K. Isothermal magnetization versus magnetic field loop shows first‐order metamagnetic transition, driven by mixed magnetic state owing to the presence of antisite disorder and antiphase boundary.

Valence state effect of Cu on photocatalytic CO2 reduction

Copper (Cu) is extensively employed in photocatalytic CO2 reduction reactions for the production of high-value products. The valence state of transition metals plays a pivotal role in influencing the catalytic process. However, due to the complex valence state changes of Cu in the CO2 reduction reaction, research on its valence state effect is lacking. The current work is to prepare a series of TiO2/CuX with stable Cu valence composition using different copper halides (CuX and CuX2, X = Br or Cl) as precursors. The results show that the CuBr2 loading leads to Cu+/Cu2+ mixed cocatalyst and exhibits the highest activity for CO2 photoreduction. The CH4 evolution rate of the TiO2/CuBr2 catalyst is as high as 100.59 μmol h−1 g−1, which is 6.6 times that of pristine TiO2. The CH4 selectivity reaches 77%. The enhanced catalytic activity and selectivity can be ascribed to the efficient surface adsorption, activation, excellent carrier separation, and transfer of Cu+/Cu2+ mixed cocatalyst. Our findings provide a reference for designing highly active Cu-based photocatalysts.

Spintronic State‐Induced Interface Reconfiguration of 2H‐Perovskite Related Oxides for Pseudocapacitance Increase

AbstractPerovskite oxides are able to realize pseudocapacitive energy storage through oxygen anion intercalation. Herein, it is demonstrated that, based on the three 2H‐perovskite related oxides Sr6Co5O15‐x, Sr5Co4O12‐x, and Sr5Co3NiO12‐x as the research objects, metal oxyhydroxides generated through surface reconfiguration during electrochemical processes can contribute extra capacity besides the oxygen‐anion intercalation. This is because the electron spin state controlled by the transition metal valence state in the pseudo trigonal prisms for the 2H‐perovskite structure will affect the Jahn–Teller (JT) distortion. The strong JT distortion of Co2+ and Ni3+ would elongate metal‐O bonds, thereby helping the formation of metal oxyhydroxides on the surface of the pristine material. Among the three 2H‐perovskite related oxides, the specific capacity of Sr5Co3NiO12‐x having the most Co2+ and Ni3+ contents showed the highest capacity increase. This study will provide a promising pathway to develop advanced perovskite‐like oxide pseudocapacitive materials.

Ultrasensitive Electrochemiluminescence Biosensor with Silver Nanoclusters as a Novel Signal Probe and α-Fe2O3-Pt as an Efficient Co-reaction Accelerator for Procalcitonin Immunoassay.

Herein, a high-efficiency biosensor based on ternary electrochemiluminescence (ECL) system was constructed for procalcitonin (PCT) detection. Specifically, silver nanoclusters (Ag NCs) with stable luminescence properties were prepared with small-molecule lipoic acid (LA) as the ligand, and its ECL emission in persulfate (S2O82-) was first reported. Meanwhile, the prepared Ag NCs possessed ligand-to-metal charge-transfer characteristics, thus transferring energy from LA to Ag+ for luminescence. Based on the small particle size, good biocompatibility, and molecular binding ability, Ag NCs-LA was used as an ideal luminescent probe. In addition, α-Fe2O3-Pt was introduced to facilitate the activation of S2O82-, thereby generating more sulfate radicals to react with the free radicals of Ag NCs to enhance ECL emission. The synergistic effect of the variable valence state of transition metals and high catalytic activity of noble metals endows α-Fe2O3-Pt with excellent catalytic ability for S2O82-. Importantly, the sensing mechanism was systematically demonstrated by UV-vis, fluorescence, and ECL analysis, as well as density functional theory calculations. At last, NKFRGKYKC was designed for specific immobilization of antibodies, thus releasing the antigen binding sites to improve the antigen recognition efficiency. Based on this, the developed biosensor showed high sensitivity for PCT detection, with a wide linear range (10 fg/mL-100 ng/mL) and a low detection limit (3.56 fg/mL), which could be extended to clinical detection of multiple biomarkers.

Understanding mechanism of voltage decay and temperature sensitivity of Li-rich manganese-based cathode materials

Li-rich manganese-based (LRM) cathode materials are known as one of the most promising cathode materials for new-generation lithium-ion batteries. At present, exploring the complex voltage decay mechanism of LRM is the main task to promote its commercialization. Herein, the structural evolution and transition metal valence state change of LRM during different reaction stages under different temperatures are discussed, and the mechanism of voltage decay is finally determined based on the electrochemical properties. The results show the evolution of irreversible thermodynamic structure is the fundamental cause leading to voltage decay of LRM cathode, and it worsens with increasing temperature. The early activation of inert Mn, multiple phase transitions, migration of transition metals to the surface, anisotropy of internal valence states caused by partial valence failure and severe interfacial reactions are all strong proofs of the above views. In summary, the reason for voltage decay is revealed by investigating the sensitivity of the LRM cathode materials to temperature. This work not only provides strong evidence for the mechanism of the voltage decay, but also points out the direction to modification design for achieving future commercialization of LRM cathode materials.

Semi-Embedding Zn-Co3O4 Derived from Hybrid ZIFs into Wood-Derived Carbon for High-Performance Supercapacitors.

Transition metal oxides (TMOs) can provide high theoretical capacitance due to the change of multiple valence states of transition metals. However, their intrinsic drawbacks, including poor electrical conductivity, lower energy density, and huge volume expansion, will result in the pulverization of electrode materials and restricted electrochemical kinetics, thus leading to poor rate capability and rapid capacity fading. Composite electrodes based on transition metal oxides and carbon-based materials are considered to be promising candidates for overcoming these limitations. Herein, we reported a preparation method of hybrid ZIFs derived Zn-doped Co3O4/carbon (Zn-Co3O4/C-230) particles semi-embedded in wood-derived carbon skeleton for integrated electrodes. A large specific surface area, excellent conductivity, and electrochemical stability provide a larger electrochemical activity and potential window for the electrode. Prepared Zn-Co3O4@CW-230 electrode (0.6 mm thick) displays ultrahigh area specific capacitances of 7.83 and 6.46 F cm-2 at the current densities of 5 and 30 mA cm-2, respectively. Moreover, a symmetric supercapacitor assembled by two identical Zn-Co3O4@CW-230 electrodes delivers a superior area-specific capacitance of 2.61 F cm-2 at the current densities of 5 mA cm-2 and great energy densities of 0.36 mWh cm-2 (6.0 mWh cm-3) at 2.5 mW cm-2, while maintaining 97.3% of initial capacitance over 10,000 cycles. It notably outperforms those of most carbon-based metal oxides, endowing the Zn-Co3O4@CW-230 with extensive prospects for practical application.

Understanding the Aging Mechanism of Na-Based Layered Oxide Cathodes with Different Stacking Structures.

Manganese-based layered oxides are one of the most promising cathodes for Na-ion batteries, but the prospect of their practical application is challenged by high sensitivity to ambient air. The stacking structure of materials is critical to the aging mechanism between layered oxides and air, but there remains a lack of systematic study. Herein, comprehensive research on model materials P-type Na0.50MnO2 and O-type Na0.85MnO2 reveals that the O-phase displays a much higher dynamic affinity toward moisture air compared to P-type compounds. For air-exposed O-type material, Na+ ions are extracted from the crystal lattice to form alkaline species at the surface in contact with air, accompanying by the increase of the valence state of transition metals. The series of undesired reactions result in an increase of interfacial resistance and huge capacity loss. Comparatively, the insertion of H2O into the Na layer is the main reaction during air-exposure of P-type material, and the inserted H2O can be extracted by high-temperature treatment. The H2O de/insertion process not only causes no performance degradation but also can enlarge the interlayer distance. With these understandings, we further propose a washing-resintering strategy to recover the performance of aged O-type materials and an aging strategy to build high-performance P-type materials.

(Digital Presentation) The New 2H Hexagonal Double Perovskite Ba2CoMnO6 As a Superior OER Catalyst in Alkaline Media

Climate change has stimulated great interest in developing clean and renewable energy conversion and storage solutions. Emerging technologies for solutions, like unitized regenerative fuel cells, metal-air batteries, and water electrolyzers, depend on the reliable catalyst materials suited for long-term application in alkaline environments[1]. In particular, the demanding reaction in these systems is the oxygen evolution reaction involving complex multielectron/proton transfer processes and thus sluggish kinetic.[2] At present, the state-of-the-art OER catalysts are IrO2 and RuO2. However, the high cost and scarcity of these materials severely prevent the wide-scale application of these systems.[3] Perovskite oxide (e.g., single perovskite expressed as ABO3) electrocatalysts are particularly considered next-generation OER catalysts due to their adjustable physicochemical properties and, as a consequence, their catalytic properties by substitution of ions in the A and B sites.[4] Via doping both at A and B site, double perovskite oxides structure can form (AA′B2O5+δ), which were shown to have stable structure during the OER due to proper O p-band center position relative to the Fermi level. The previous research shows that hexagonal perovskites exhibit enhanced catalytic activity rather than cubic or tetragonal perovskites, which is associated with the face-sharing octahedral unit. [5,6] In this work, we systematically investigate doping of parent La2CoMnO6 with Ba to tune the crystal structure and electronic structure. A series of the double perovskites with the chemical formula of La2-xBaxCoMnO6 (x = 0, 0.5, 1.5, 2) were fabricated via the modified Sol-Gel Pechini method. According to Goldschmidt tolerance factor, it is expected that when Ba amount increases in the A site, the crystal structure changes from cubic to hexagonal, which can contribute to electrocatalytic OER activity based above discussions. Furthermore, it is anticipated that increasing Ba content at the A site should result in higher valence state of transition metals due to stabilizing charge balance of the structure. It is widely reported that presence of higher valence state of transition metals is beneficial for OER.The crystal structure was confirmed by combining powder X-ray diffraction (XRD) patterns and selected area electron diffraction (SAED) patterns. The two-layer hexagonal structure of Ba2CoMnO6 (BCM) was further characterized by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The results show doping of Ba into parent La2CoMnO6 resulted in the hexagonal phase formation and BCM achieves a current density of 10 mA cm–2 at a low overpotential of 288 mV and has the highest intrinsic activity in the series of BaxLa2-xCoMnO6 (x = 0, 0.5, 1, 1.5, 2). Furthermore, BCM exhibits outstanding stability of 12 h in a 0.1 M KOH electrolyte. The electronic structure and surface work function values of the catalysts were examined with X-ray photoelectron spectroscopy (XPS). XPS analysis revealed that valence state of Co and Mn increases through Ba amount in the structure, indicating the double perovskite oxide charge balance is maintained. In addition to valence state analysis, we performed work function analysis of the catalysts tested in this work. The result shows work function of the series of BaxLa2-xCoMnO6 (x = 0, 0.5, 1, 1.5, 2) substantially decreases with Ba content. Here, we show that the high intrinsic activity of BCM is not only related with the hexagonal structure and also may be related with exhibiting lower work function value in the series. In addition, our approach presents a new strategy, air quenching method, to synthesize a phase pure 2H-hexagonal double perovskite oxide. [1] Y. Zhou, X. Guan, H. Zhou, K. Ramadoss, S. Adam, H. Liu, S. Lee, H. Shi, M. Tsuchiya, D. Fong and S. Ramanathan, Nature, 534, 231-234 (2016).[2] X. Cui, P. Ren, D. Deng, J. Deng and X. Bao, Energy & Environmental Science, 9, 123-129 (2016).[3] X. Qiu, Y. Zhang, Y. Zhu, C. Long, L. Su, S. Liu and Z. Tang, Advanced Materials, 33, 2001731 (2020).[4] W. Yin, B. Weng, J. Ge, Q. Sun, Z. Li and Y. Yan, Energy & Environmental Science, 12, 442-462 (2019).[5] L. Tang, W. Zhang, D. Lin, Y. Ren, H. Zheng, Q. Luo, L. Wei, H. Liu, J. Chen and K. Tang, Inorganic Chemistry Frontiers, 7, 4488-4497 (2020).[6] C. Chen, G. King, R. M. Dickerson, P. A. Papin, S. Gupta, W. R. Kellogg and G. Wu, Nano Energy, 13, 423-432 (2015).

Deep insight of the influence of Cu valence states in co-catalyst on CO2 photoreduction

Cu is widely used to prepare high-value products in photocatalytic CO2 reduction reaction (CO2PR). The valence state of transition metals usually has a great impact on the catalytic process. However, the research on Cu valence in CO2PR is lack for its complex valence change. In current work, Cux/P25 with stable Cu valence composition in CO2PR is prepared. The results show that Cu is composed of Cu0 and Cu2O, and their proportions change regularly as Cu loading changing, which are linearly related to the selectivity of the corresponding products of CO2PR. Combined with thermodynamic and kinetic analysis, the CO adsorption and activation on the surface of Cu0 and Cu2O are considered to be the key to determine the product of CO2PR, which is further confirmed by DFT calculation. Cu0 and Cu2O are proved to be active site of producing CH4 and CO, respectively. In this case, a reference for the study of highly selectivity Cu based photocatalysts is provided.

Study on the thermal reduction effect of organic components in spent ternary lithium battery cathode active materials

To improve the adhesion between cathode materials and current collector, and increase the electronic conductivity among electroactive substances, a certain proportion of conductive agents (acetylene black) and agglomerant (PVDF) are usually added in the battery manufacturing process. However, these conductive agents have negative effects on the recovery of cathode materials by pyrolysis or calcination. Recognizing this issue, a method based on the concept of “treating spent with spent” was developed in this paper. Organic matters contained in cathode active materials functioned as the reduction reagents, which can reduce the valence state of transition metals, resulting in the breakdown of the strong chemical bond and the stable layered structure of cathode materials. In this study, the thermal reduction effect of different organic components on cathode active materials was analyzed respectively to evaluate the reduction function of each component. XRD, XPS and ICP-MS were used to compare and analyze changes of phase, element compound state and ion leaching efficiencies of different cathode materials before and after thermal reduction under different amounts of reducing agents. The results show that both PVDF and acetylene black reduced the high-valent metals to low-valent oxides or elemental substances, demonstrating their thermal reduction capabilities. Comparisons of the XRD, XPS analysis and ion leaching results of thermal reduced products suggest that acetylene black has a stronger thermal reduction ability than that of PVDF. The results also show that the reduction of the high nickel cathode material (NCM811) is easier than that of the low nickel cathode material (NCM111).

Cagelike CoSe2@N-Doped Carbon Aerogels with Pseudocapacitive Properties as Advanced Materials for Sodium-Ion Batteries with Excellent Rate Performance and Cyclic Stability.

Electrochemical conversion reaction based electrodes offer a high sodium storage capacity in rechargeable batteries by utilizing the variable valence states of transition metals. Thus, transition metal chalcogenides (TMCs) as such materials have been intensively investigated in recent years to explore the possibilities of practical application in rechargeable sodium-ion batteries; however, it is hindered by poor rate performance and a high-cost preparation method. In addition, some issues in regards to conversion reactions remain poorly understood, including incomplete reversible reaction processes, polarization, and hysteresis. Herein, a novel cagelike CoSe2@N-doped carbon aerogels hybrid composite was designed and prepared by a facile and high-efficiency sol-gel technology. Benefiting from the surface engineering optimization, high charge transfer, and low-energy diffusion barrier, the CoSe2@N-doped carbon aerogels exhibit a high pseudocapacitive property. Most importantly, the CoSe2 anode has been carefully investigated at different discharge/charge states by X-ray absorption near edge spectroscopy technologies and density functional theory (DFT) simulations, which deeply reveal the capacity fading mechanism and phase transition behavior.

Synergistic Na+ and F− co-doping modification strategy to improve the electrochemical performance of Li-rich Li1·20Mn0·54Ni0·13Co0·13O2 cathode

The development of the high-profile Li-rich Mn-based cathode materials is limited by the high irreversible capacity loss, low rate performance, fast voltage decay and poor cycle durability. A synergistic modification strategy of Na+ and F− co-doping is implemented via carbonate co-precipitation followed by high-temperature solid-state reaction to enhance electrochemical properties of Li1·2Mn0·54Ni0·13Co0·13O2. The co-doping changes the lattice constant and expands the lithium layer spacing while maintaining well layered structure of the cathode materials, which accelerate the Li+ diffusion. F doping reduces the ratios of Mn4+/Mn3+ and Co3+/Co2+ from the transition metal valence state analysis and alleviates the erosion of the cathode materials with the electrolyte, preventing the dissolution of the transition metal ions. Besides, the structure evolution reveals that the Na+ and F− co-doping inhibits the transition metal ions transfer, thus blocking the spinel phase formation and stabilizing the layered structure of the cathode materials. As a consequence, co-doped cathode materials exhibit the synergistic advantages of both Na+ and F− ions, including increased initial reversible capacity (291.3 mAh g−1@0.1 C), considerably superior rate capability (144.3 mAh g−1@5 C), and remarkable cycle durability (84.8% retention even after 300 cycles). This study provides an alternative modification strategy for Li-rich Mn-based cathode materials with outstanding electrochemical performances.

Enhancing the Electrocatalysis of LiNi0.5Co0.2Mn0.3O2 by Introducing Lithium Deficiency for Oxygen Evolution Reaction.

LiNi0.5Co0.2Mn0.3O2 (NCM523), as a cathode material for rechargeable lithium-ion batteries, has attracted considerable attention and been successfully commercialized for decades. NCM is also a promising electrocatalyst for the oxygen evolution reaction (OER), and the catalytic activity is highly correlated to its structure. In this paper, we successfully obtain NCM523 with three different structures: spinel NCM synthesized at low temperature (LT-NCM), disordered NCM (DO-NCM) with lithium deficiency obtained at high temperature, and layered hexagonal NCM at high temperature (HT-NCM). By introducing lithium deficiency to tune the valence state of transition metals in NCM from Ni2+ to Ni3+, DO-NCM exhibits the best catalytic activity with the lowest onset potential (∼1.48 V) and Tafel slope (∼85.6 mV dec-1), whereas HT-NCM exhibits the worst catalytic activity with the highest onset potential (∼1.63 V) and Tafel slope (∼241.8 mV dec-1).

Unexpected Molecular Structure of a Putative Rhenium-Dioxo-Benzocarbaporphyrin Complex. Implications for the Highest Transition Metal Valence in a Porphyrin-Type Ligand Environment.

A combination of quantum chemical calculations and synthetic studies was used to address the possibility of very high (>6) valence states of transition metals in porphyrin‐type complexes. With corrole as a supporting ligand, DFT calculations ruled out Re(VII) and Ir(VII) dioxo complexes as stable species. Attempted rhenium insertion into benzocarbaporphyrin (BCP) ligands on the other hand led to two products with different stoichiometries – Re[BCP]O and Re[BCP]O2. To our surprise, single‐crystal structure determination of one of the complexes of the latter type indicated an ReVO center with a second oxygen bridging the Re−C bond. In other words, although the monooxo complexes Re[BCP]O are oxophilic, the BCP ligand cannot sustain a trans‐ReVII(O)2 center. The search for metal valence states >6 in porphyrin‐type ligand environments must therefore continue.

In‐situ synthesis of NaP zeolite doped with transition metals using fly ash

AbstractHere, a novel approach has been proposed to synthesize NaP zeolite adsorbents doped with transition metals (M‐NaP) using the Na2SiO3 and NaAlO2 compounds extracted from fly ash via activation and stage treatment. The preparation process of M‐NaP employed the in‐situ synthesis in combination with organic complexation method. The effects of the addition amounts and valence state of transition metals on the zeolite products were investigated. The crystalline phase, morphology, and particle size of M‐NaP were characterized using X‐ray diffractometry (XRD), Scanning electron microscopy (SEM), and Laser particle size analyzer (PSD). The doping process was investigated using X‐ray photoelectron spectroscopy (XPS) and Materials Studio. The adsorption performance of M‐NaP was tested by the adsorption of Zn2+. The optimal molar ratio of n(Al2O3)/n(M) was identified as 8:1 for Co–NaP, Ni–NaP, and Fe–NaP, compared with 6:1 for Ti–NaP. The influence of transition metals on adsorption was: Co–NaP > Ni–NaP > Ti–NaP > Fe‐NaP. Due to largest surface area of Co–NaP (162.5 m2/g, five times than that of NaP‐RAW), the maximum removal rate of Zn2+ was above 99.50% at 25°C. The existence state of transition metals in M‐NaP primarily included: isomorphous substitution and balance of the skeletal charge, with the latter in majority. The simulation of Materials Studio indicates that Si at T4 was first substituted and then, Si at T3 opposite T4 was replaced when the replacement of Si in different sites occurred.

High-Temperature Ferrimagnetic Half Metallicity with Wide Spin-up Energy Gap in NaCu3Fe2Os2O12.

A new oxide NaCu3Fe2Os2O12 is synthesized using high pressure and temperature conditions. The Rietveld structural analysis shows that the compound possesses both A- and B-site ordered quadruple perovskite structure in Pn3̅ symmetry. The valence states of transition metals are confirmed to be Cu2+/Fe3+/Os5.5+. The three transition metals all take part in magnetic interactions and generate strong Cu2+(↑)Fe3+(↑)Os5.5+(↓) ferrimagnetic superexchange interactions with a high Curie temperature about 380 K. Electrical transport measurements suggest its half-metallic properties. The first-principles theoretical calculations demonstrate that the compound has a spin-down conducting band and a spin-up insulating band with a wide energy gap.

Conversion Chemistry of Cobalt Oxalate for Sodium Storage.

Conversion electrodes, which can realize high capacities by employing the wider valence states of transition metals, are investigated for sodium storage and applied for rechargeable sodium-ion batteries (SIBs). Importantly, this work is a first report for the sodium storage ability and related storage mechanism in oxalate compounds, specifically cobalt oxalate (CoC2O4) nanorods. The nanorods are intimately blended with acetylene black powders to achieve sufficient electrical conductivity (∼10-3 S cm-1). The resulting C-CoC2O4 electrode delivers an initial capacity of about 330 mA h (g-CoC2O4)-1 at a rate of 0.2 C (60 mA g-1) and preserves 75% of the initial capacity over 200 cycles. A high charge (oxidation) capacity, ∼111 mA h g-1, was achieved even at 30 C (9000 mA g-1). This remarkable electrode performance is reported for the first time for metal oxalate compounds tested for Na cells, to the best of our knowledge. X-ray diffraction, transmission electron microscopy, and time-of-flight secondary-ion mass spectroscopy analyses lead to the proposal of a new sodium storage mechanism. For this mechanism, CoC2O4 is converted into Co metal involving with the creation of Na2C2O4 on discharge (reduction), and the Co metal is recovered to CoC2O4 on charge. The employed electroconducting carbon is likely to provide good electron conduction paths, which enables fast conversion on both discharge and charge. A full cell comprised of the C-CoC2O4 anode and carbon-coated NaCrO2 cathode exhibits good retention capacity over prolonged cycling, with retention of about 84.7% of the first capacity [107 mA h (g-NaCrO2)-1] for 300 cycles, and is active at a rate of 5 C (550 mA g-1), with a capacity of 79.5 mA h g-1. This result demonstrates the potential of applying C-CoC2O4 as an anode material for rechargeable SIBs.

Tuning Spin State of Rock‐Salt‐Based Oxides by Manipulation of Crystallinity for Efficient Oxygen Electrocatalysis

AbstractThe spin state, specifically antibonding orbital (e g) occupancy, of transition‐metal ions is recognized as a descriptor for oxygen electrocatalysts with perovskite or spinel structures, and can be facilely adjusted by varying the valence states of transition metals. However, both perovskites and spinels show unsatisfactory performance even at the optimal spin states. In comparison, some oxides with a rock salt structure (e.g., NiIICoIIO2) exhibit higher activity than perovskites and spinels, nevertheless, the rock salt structure excludes valence changes of the transition metals, obstructing further e g optimization and performance enhancement. Herein, an innovative strategy is demonstrated to regulate the spin states of CoII in NiCoO2 via crystallinity manipulation, thus providing a new strategy for eg optimization. Remarkably, the catalyst (CNO‐8) with a moderate eg occupancy (≈1.2) achieves the best electrocatalytic activity, which is among the highest achieved by the state‐of‐the‐art electrocatalysts, namely, an overpotential of ≈269 mV at 10 mA cm−2 for oxygen evolution reaction and an onset potential of 935 mV for oxygen reduction reaction. As an efficient bifunctional catalyst for rechargeable Zn‐air batteries, CNO‐8 even outperforms the noble metal catalyst (Pt/C + RuO2), demonstrating high potential for practical applications in electrochemical energy conversion.