Stable Transition Metal Research Articles

The surface states of transition metal X-ides under electrocatalytic conditions.

Due to conversion equilibrium between solvent and H- and O-containing adsorbates, the true surface state of a catalyst under a particular electrochemical condition is often overlooked in electrocatalysis research. Herein, by using surface Pourbaix analysis, we show that many electrocatalytically active transition metal X-ides (e.g., oxides, nitrides, carbides, and hydroxides) tend to possess the surface states different from their pristine stoichiometric forms under the pH and potential of interest due to water dissociation or generation. Summarizing the density functional theory calculated surface Pourbaix diagrams of 14 conditionally stable transition metal X-ide materials, we found that some of these surfaces tend to be covered by O-containing adsorbates at a moderate or high potential, while vacancies or H-covered surfaces may form at a low potential. These results suggest the possibility of poisoning or creation of surface sites beyond the pristine surface, implying that the surface state under reaction conditions (pH and potentials) needs to be considered before the identification and analysis of active sites of a transition metal X-ide catalyst. In addition, we provide an explanation of the observed theory and experiment discrepancy that some transition metal X-ides are "more stable in experiment than in theory." Based on our findings, we conclude that analyzing the surface state of transition metal X-ide electrocatalysts by theoretical calculations (e.g., surface Pourbaix diagram analysis), in situ/operando and post-reaction experiments are indispensable to accurately understand the underlying catalytic mechanisms.

Promotion effect of CO oxidation via activation of surface lattice oxygen by single atom Cu/MnO2 catalyst

The catalytic oxidation of CO by single-atom transition metal catalysts has always been the focus of researchers. Herein, we report a strategy for the preparation of Cu single-atom anchored on MnO2 by oxalate chelation-assisted hydrothermal method, and the obtained Cu/MnO2–0.25 catalyst with the optimal molar ratio achieved about 90% CO conversion at 80 °C. The strong electronic interaction between Cu and MnO2 not only effectively inhibits the aggregation of Cu single atoms, but also the formed Cu-O-Mn hybrid structure activates the adjacent surface lattice oxygen, further improving the catalytic oxidation performance of CO at low and medium temperatures. The elevated activity is achieved at the single-atom active site by the Mars-van Krevelen (MvK) mechanism. This work provides a simple and reliable method for the rational design of stable transition metal single-atom catalysts and deepens the understanding of metal-support strong interactions (SMSI).

In Situ Fabrication of Mn-Doped NiMoO4 Rod-like Arrays as High Performance OER Electrocatalyst

The slow kinetics of the oxygen evolution reaction (OER) is one of the significant reasons limiting the development of electrochemical hydrolysis. Doping metallic elements and building layered structures have been considered effective strategies for improving the electrocatalytic performance of the materials. Herein, we report flower-like nanosheet arrays of Mn-doped-NiMoO4/NF (where NF is nickel foam) on nickel foam by a two-step hydrothermal method and a one-step calcination method. The doping manganese metal ion not only modulated the morphologies of the nickel nanosheet but also altered the electronic structure of the nickel center, which could be the result of superior electrocatalytic performance. The Mn-doped-NiMoO4/NF electrocatalysts obtained at the optimum reaction time and the optimum Mn doping showed excellent OER activity, requiring overpotentials of 236 mV and 309 mV to drive 10 mA cm-2 (62 mV lower than the pure NiMoO4/NF) and 50 mA cm-2 current densities, respectively. Furthermore, the high catalytic activity was maintained after continuous operation at a current density of 10 mA cm-2 of 76 h in 1 M KOH. This work provides a new method to construct a high-efficiency, low-cost, stable transition metal electrocatalyst for OER electrocatalysts by using a heteroatom doping strategy.

Ultrasound assisted synthesis of cobalt tungstate decorated Poly(2,5-dimethoxyaniline) nanocomposite towards improved methanol electro-oxidation

In this study cobalt tungstate (CoWO4) nanoparticles decorated over PDMA (Poly 2,5 dimethoxy aniline) are utilized as a novel hybrid nanocomposite for methanol oxidation. The CoWO4 and PDMA were synthesized using an ultrasonication strategy. The physicochemical characterization such as SEM, HRTEM, XRD, and FT-IR affirmed the presence of CoWO4 over PDMA matrix. The electrocatalytic activity of the investigated CoWO4 decorated over the PDMA matrix towards methanol oxidation indicates good performances since the potential is lowered when compared to other materials that are reported. CoWO4@PDMA hybrid nanocomposite gave methanol oxidation at 0.80 V with a current density of 6.4 mA/cm2. The presence of CoWO4 offered good stability and retention for methanol oxidation reaction. On the whole, the present study will satisfy low-cost and stable transition metal–based nanocomposite for fuel cell applications.

Pomegranate-like Ni-doped cobalt boride implanted in B, N-doped carbon nanocages for enhanced electrochemical oxygen evolution

The development of efficient and stable transition metal boride electrocatalysts for oxygen evolution reaction (OER) is critical for energy conversion and environmental protection. Herein, we synthesized B, N-doped carbon layer encapsulated the Ni-doped CoxB nanocages electrocatalyst (denoted as Ni-CoxB@BNC) via a high-temperature boronizing, derived from Ni-doped cobalt-based zeolite imidazole frame (NiCo-ZIF), toward enhanced electrochemical alkaline oxygen evolution reaction. The Ni-CoxB@BNC electrocatalyst synthesized at 550 °C exhibits excellent OER activity with a low overpotential of 274 mV and a Tafel slope of 80 mV dec−1 at a current density of 10 mA cm−2, which is better than precious metal RuO2. The synergistic effect between B, N-doped carbon layer and Ni-doped CoxB in Ni-CoxB@BNC leads to higher OER catalytic activity. The B, N-doped carbon layer provides additional active sites, which accelerates charge transport and enhances the conductivity of Ni-CoxB@BNC during OER. In addition, it also protects the pomegranate seed-like Ni-CoxB nanoparticles inside, improving the stability of the Ni-CoxB@BNC material. This work unambiguously elucidates the design and preparation strategy of transition metal boride implanted B, N-doped carbon nanocage electrocatalysts derived from controlled bimetallic ZIF precursor.

Regulating Spin States in Oxygen Electrocatalysis.

Developing efficient and stable transition metal oxides catalysts for energy conversion processes such as oxygen evolution reaction and oxygen reduction reaction is one of the key measures to solve the problem of energy shortage. The spin state of transition metal oxides is strongly correlated with their catalytic activities. In an octahedral structure of transition metal oxides, the spin state of active centers could be regulated by adjusting the splitting energy and the electron pairing energy. Regulating spin state of active centers could directly modulate the d orbitals occupancy, which influence the strength of metal-ligand bonds and the adsorption behavior of the intermediates. In this review, we clarified the significance of regulating spin state of the active centers. Subsequently, we discussed several characterization technologies for spin state and some recent strategies to regulate the spin state of the active centers. Finally, we put forward some views on the future research direction of this vital field.

Regulating Spin States in Oxygen Electrocatalysis

AbstractDeveloping efficient and stable transition metal oxides catalysts for energy conversion processes such as oxygen evolution reaction and oxygen reduction reaction is one of the key measures to solve the problem of energy shortage. The spin state of transition metal oxides is strongly correlated with their catalytic activities. In an octahedral structure of transition metal oxides, the spin state of active centers could be regulated by adjusting the splitting energy and the electron pairing energy. Regulating spin state of active centers could directly modulate thedorbitals occupancy, which influence the strength of metal‐ligand bonds and the adsorption behavior of the intermediates. In this review, we clarified the significance of regulating spin state of the active centers. Subsequently, we discussed several characterization technologies for spin state and some recent strategies to regulate the spin state of the active centers. Finally, we put forward some views on the future research direction of this vital field.

A Bridge too Far? Comparison of Transition Metal Complexes of Dibenzyltetraazamacrocycles with and without Ethylene Cross-Bridges: X-ray Crystal Structures, Kinetic Stability, and Electronic Properties.

Tetraazamacrocycles, cyclic molecules with four nitrogen atoms, have long been known to produce highly stable transition metal complexes. Cross-bridging such molecules with two-carbon chains has been shown to enhance the stability of these complexes even further. This provides enough stability to use the resulting compounds in applications as diverse and demanding as aqueous, green oxidation catalysis all the way to drug molecules injected into humans. Although the stability of these compounds is believed to result from the increased rigidity and topological complexity imparted by the cross-bridge, there is insufficient experimental data to exclude other causes. In this study, standard organic and inorganic synthetic methods were used to produce unbridged dibenzyl tetraazamacrocycle complexes of Co, Ni, Cu, and Zn that are analogues of known cross-bridged tetraazamacrocycles and their transition metal complexes to allow direct comparison of molecules that are identical except for the cross-bridge. The syntheses of the known tetraazamacrocycles and the new transition metal complexes were successful with high yields and purity. Initial chemical characterization of the complexes was conducted by UV-Visible spectroscopy, while cyclic voltammetry showed more marked differences in electronic properties from bridged versions. Direct comparison studies of the unbridged and bridged compounds' kinetic stabilities, as demonstrated by decomposition using high acid concentration and elevated temperature, showed that the cyclen-based complex stability did not benefit from cross-bridging. This is likely due to poor complementarity with the Cu2+ ion while cyclam-based complexes benefited greatly. We conclude that ligand-metal complementarity must be maintained in order for the topological and rigidity constraints imparted by the cross-bridge to contribute significantly to complex robustness.

Potential rules for stable transition metal hexafluorides with high oxidation states under high pressures.

High pressure is a powerful tool in material sciences which can lead to the discovery of novel inorganic species in high oxidation states. Based on the prediction of the stability of PdF6 with a high Pd oxidation state of +6, we propose three potential guiding rules for finding stable transition metal (TM) fluorides with high +6 oxidation states: (1) the existence of a large (>7 eV) valence orbitals energy differences of atoms between the TM d orbital and the F 2p orbital; (2) an appropriate number of valence electrons within the range of 6-11; and (3) suitable electronegativity values less than 2.3 on the Pauli scale. More importantly, by synergistically invoking all of these rules, we predict, by combining a particle swarm optimization algorithm with first-principles calculation on the phase stabilities of the various TM-F compounds, a collection of new TMF6 species with the space group Pnma that have a +6 oxidation state. Subsequently, we develop an understanding of the high +6 oxidation state for the TM elements. These findings are expected to play a crucial role in the predictive discoveries of new fluorides with high oxidation states of +6.

Synthesis and Characterization of Late Transition Metal Complexes of Mono-Acetate Pendant Armed Ethylene Cross-Bridged Tetraazamacrocycles with Promise as Oxidation Catalysts for Dye Bleaching

Ethylene cross-bridged tetraazamacrocycles are known to produce kinetically stable transition metal complexes that can act as robust oxidation catalysts under harsh aqueous conditions. We have synthesized ligand analogs with single acetate pendant arms that act as pentadentate ligands to Mn, Fe, Co, Ni, Cu, and Zn. These complexes have been synthesized and characterized, including the structural characterization of four Co and Cu complexes. Cyclic voltammetry demonstrates that multiple oxidation states are stabilized by these rigid, bicyclic ligands. Yet, redox potentials of the metal complexes are modified compared to the "parent" ligands due to the pendant acetate arm. Similarly, gains in kinetic stability under harsh acidic conditions, compared to parent complexes without the pendant acetate arm, were demonstrated by a half-life seven times longer for the cyclam copper complex. Due to the reversible, high oxidation states available for the Mn and Fe complexes, the Mn and Fe complexes were examined as catalysts for the bleaching of three commonly used pollutant model dyes (methylene blue, methyl orange, and Rhodamine B) in water with hydrogen peroxide as oxidant. The efficient bleaching of these dyes was observed.

High-efficiency water splitting catalyzed by NiMoO4 nanorod arrays decorated with vacancy defect-rich NiTex and MoOy layers

Nickel molybdate is a promising non-noble metal candidate for alkaline hydrogen evolution reaction (HER), but its insufficient activity due to its low conductivity and slow reaction kinetics hinders its application for large-scale commercial hydrogen production. Here, we report an efficient self-supporting electrocatalyst consisting of NiMoO4 nanorods modified by MoOy with abundant internal oxygen vacancies and epitaxially grown NiTex with high specific surface area for alkaline HER. Benefiting from the structural advantages of nanorods/nanosheets integrated electrode, a larger specific surface area and abundant active centers can be provided for the HER. Such constructed NiTex/MoOy/NiMoO4 catalyst achieves a low overpotential of 59 mV to drive a current density of 10 mA cm−2 and a low Tafel slope of 32.8 mV dec−1 in 1.0 M KOH solution with sustainable durability. Additionally, a two-electrode electrolytic cell system assembled with NiTex/MoOy/NiMoO4 and NiMoO4 employed as cathode and anode, respectively, extraordinary performance with a voltage of 1.49 V at a current density of 10 mA cm−2 was achieved in alkaline electrolyte. This work demonstrates a novel idea for metalloid Te-modified transition metal oxides, which is expected to expand the family of stable and efficient transition metal oxide (such as CoMoO4, Co3O4, NiCo2O4, etc.)-based catalysts for energy electrocatalysis.

Highly Symmetrical Six‐Transition Metal Ring Units Promising High Air‐Stability of Layered Oxide Cathodes for Sodium‐Ion Batteries

AbstractLayered oxides are the most prevalent cathodes for sodium‐ion batteries (SIBs), but their poor air stability significantly limits their practical application owing to the rapid performance degradation of aged materials and the cost increase for material storage and transportation. Here, an effective strategy of constructing stable transition metal (TM) layers with a highly symmetrical six‐TM ring is suggested to enhance structure stability, thus hindering ambient air corrosion. The density functional theory calculations reveal that the higher symmetry ensures a higher thermodynamic energy for H2O insertion into Na layer. The combined analyses of selected area electron diffraction, transmission electron microscopy, X‐ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, thermogravimetric analysis, and chemical titration indicate that the six‐TM ring structure can effectively suppress the series of aging processes including water insertion, the spontaneous loss of lattice sodium, TM valence increment and residual alkali formation. Benefiting from the overall suppression of aging process, the strategy results in an excellent improvement in capacity retention after air exposure from 13.57% to 95.59%, and exhibits a good universality for both P2‐ and O3‐cathodes, which are the two most common structures of Na‐based layered oxides with different aging mechanism. These findings provide new insight to design high‐performance cathodes for SIBs.

Prediction of 2D ferromagnetic metal VNI monolayer with tunable topological properties

Two-dimensional ferromagnetic topological semi-metals have attracted much interest owing to their fascinating spintronic applications. Using first-principles calculations, we propose a stable transition metal nitrogen halide compound, namely, VNI monolayer, to display intrinsic ferromagnetism with high Curie temperature TC (∼510 K) and strong out-of-plane magnetism. It is found to be a topological Weyl nodal line material, which can be fully spin-polarized by controlling the on-site Coulomb interaction. Its nodal line can be destroyed by orienting the magnetization axis. Additionally, biaxial strain enables efficient tuning of the magnetic properties by switching the easy magnetic axis from an out-of-plane to in-plane direction with an enhanced TC to 540 K. These results highlight the great application potential of a VNI monolayer in low-dimensional topological magnets.

A KMnO4-Generated Colloidal Electrolyte for Redox Mediation and Anode Protection in a Li-Air Battery.

The rechargeable lithium-oxygen (Li-O2) battery has the highest theoretical specific energy density of any rechargeable batteries and could transform energy storage systems if a practical device could be attained. However, among numerous challenges, which are all interconnected, are polarization due to sluggish kinetics, low cycle life, small capacity, and slow rates. In this study, we report on use of KMnO4 to generate a colloidal electrolyte made up of MnO2 nanoparticles. The resulting electrolyte provides a redox mediator for reducing the charge potential and lithium anode protection to increase cycle life. This electrolyte in combination with a stable binary transition metal dichalcogenide alloy, Nb0.5Ta0.5S2, as the cathode enables the operation of a Li-O2 battery at a current density of 1 mA·cm-2 and specific capacity ranging from 1000 to 10 000 mA·h·g-1 (corresponding to 0.1-1 mA·h·cm-2) in a dry air environment with a cycle life of up to 150. This colloidal electrolyte provides a robust approach for advancing Li-air batteries.

A novel efficient dual-functional electrocatalyst for overall water splitting based on Ni0.85Se/RGO/CNTs nanocomposite synthesized via different nickel precursors

Synthesizing efficient and affordable electrocatalysts for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) remains a crucial problem on the way to practical applications for producing clean H2 fuel. Herein, high-efficiency and stable transition metal based electrocatalysts Ni0.85Se-1, Ni0.85Se-2 and Ni0.85Se-3 materials with different morphological characteristics were derived via a one-step hydrothermal route using the Ni(OH)2 and metal-organic framework (Ni-BDC and Ni-BTC) as precursors, respectively. The results showed that Ni0.85Se-2 exhibited excellent electrocatalytic activity. Subsequently, introducing carbon nanomaterials (RGO and CNTs) to form Ni0.85Se/RGO/CNTs nanocomposite material further improves the catalytic activity owing to high conductivity. The resulting Ni0.85Se/RGO/CNTs nanocomposites electrocatalyst showed a low overpotential of 232 mV and 165 mV and a low Tafel slope of 64 mV dec−1 and 98 mV dec−1 when the current density was 10 mA cm−2 for OER and HER, respectively. In addition, the Ni0.85Se/RGO/CNTs nanocomposites were used as an anode and cathode of the water electrolysis device and the overall water splitting performance was investigated. The results show just a voltage of 1.59 V was required when the current density was 10 mA cm−2 and good overall water splitting stability for 20 h. The outstanding electrocatalytic performance of Ni0.85Se/RGO/CNTs is mostly due to its noticeable porous structure, the high conductivity and the large surface area that came from RGO and CNTs.

(Invited) On the Structural and Mechanistic Studies of PGM-Free Oer Catalysts for PEM Electrolyzer

Low temperature water electrolysis represents a critical technology for green hydrogen production. Low temperature electrolysis can be operated using either proton exchange or alkaline membrane electrolyte. Compared to alkaline electrolyzer, proton exchange membrane (PEM) electrolyzer offers advantages of significantly higher current density and higher H2 purity, rendering it a preferred technology when high energy efficiency and low footprint are essential.Working in the oxidative and acidic environment under high polarization voltage, however, adds substantial demand to the electrode catalyst and the support. This is particularly the case at anode where the oxygen evolution reaction (OER) takes place. At present, the PGM materials such as Ir black or Ir oxide are catalysts of choice. Their high cost and limited reserve, however, adds a significant cost to PEM electrolyzer, which contributes to the overall expense of hydrogen production next only to the cost of electricity. Replacing Ir with earth-abundant transition metal oxides could help to reduce the electrolyzer system cost. These materials are known to be applicable in the alkaline electrolyte but not in acid due to dissolution. Since the traditional porous carbon cannot be used as the support under the oxidative potential during OER, their stand-alone conductivity represents another critical consideration.Argonne National Laboratory has recently designed and prepared a new class of PGM-free OER catalyst for PEM electrolyzer. The new catalysts are consisted of highly porous yet stable transition metal oxide derived from the metal-organic-frameworks (MOFs).Two catalyst series, ANL-Cat-A and ANL-Cat-B, were developed and investigated. The OER catalyst activity and durability were first measured by rotating disk electrode (RDE) method and in half-cell in the acidic media. Very promising OER activities were achieved. The catalyst durability was also measured through the multiple potential cycling from the voltage of 1.2 V to 2.0 V (vs. RHE) in the acidic electrolyte. Both ANL catalysts demonstrated promising activity and durability in the acidic medium. These PGM-free OER catalysts were also integrated into the membrane electrode assemblies and tested in PEM water electrolyzer under operating condition (60 °C to 80 °C and ambient pressure). Several MEAs demonstrated promising OER current density of 2000 mA/cm2 at 2.2 V iR-corrected .Extensive structural characterizations were carried out, both in static state and under the reaction condition using various tools such as high resolution electron microscopy and in situ X-ray absorption spectroscopy. Interesting correlation between the structure and property relationship was found. Computational modeling was also performed to understand the fundamental mechanism behind electron conductivity and the acid tolerance behind this new class of OER catalysts. Acknowledgement: This work is supported by U. S. Department of Energy, Hydrogen and Fuel Cell Technologies Office through Office of Energy Efficiency and Renewable Energy and by Office of Science, U.S. Department of Energy under Contract DE-AC02-06CH11357.

Two-dimensional transition metal triborides: Monolayers with robust intrinsic magnetism and high spin stability

Two-dimensional materials with intrinsic magnetism have provoked comprehensive attentions due to their potential application in spintronics. In this work, a class of stable transition metal triborides, $\mathrm{TM}{\mathrm{B}}_{3}$ ($\mathrm{TM}=\mathrm{Ti}$, V, Cr, Mn, Fe) with various electronic and magnetic features were predicted by density functional theory. Based on static energy, phonon spectrum, and $ab\phantom{\rule{0.16em}{0ex}}initio$ molecular dynamics simulations, all the $\mathrm{TM}{\mathrm{B}}_{3}$ monolayers have thermal and kinetic stability. Meanwhile, $\mathrm{TM}{\mathrm{B}}_{3}\mathrm{s}$ exhibit mechanical anisotropy reflected by Young's modulus and Poisson's ratio. Moreover, $\mathrm{Ti}{\mathrm{B}}_{3}$ and $\mathrm{Fe}{\mathrm{B}}_{3}$ are robust ferromagnetic metals with the respective Curie temperatures of 248 and 367 K. $\mathrm{Cr}{\mathrm{B}}_{3}$ is antiferromagnetic semiconductor with direct bandgap of 513.6 meV and magnetic transform temperatures of as high as 378 K. Finally, it is predicted that the magnetic properties can be further modulated by exerting external strains. Our results may provide new clues for searching and designing two-dimensional magnetic materials.

Theoretical study on CO2 reduction to methanol catalyzed by highly stable single-atom transition metal riveted bilayer 2D material

The main factor limiting the high activity of single-atom catalysts (SACs) is the structural instability, which is caused by atomic aggregation or attack of reaction intermediates. In this paper, the single-atom transition metal copper and palladium was riveted between two layers of the monolayer materials (carbon nitride and graphene) respectively to enhance the structural stability, and used C3N-TM-C3N and GR-TM-C3N (TM= Cu, Pd) in the theoretical study of CO2 reduction to methanol (CO2RR). It is found that a single TM atom can be firmly anchored between two C3N or GR-C3N layers, which can prevent the aggregation of TM atoms (TM= Cu, Pd) in SACs and inhibit the attack of reaction intermediates. The TM atom enhances the catalytic activity of the material system by providing electrons to the protective layer GR/C3N. And C3N-Cu-C3N material has the best performance and the lowest reaction energy barrier in CO2RR, which is only 2.094 eV. It provides a feasible method to improve the catalytic activity and maintain the stability for the practical application of SACs systems.

Hydrogenation-induced high-temperature superconductivity in two-dimensional molybdenum carbide Mo2C3

Two-dimensional (2D) superconductors have important potential applications in constructing nano superconducting devices, thus, 2D superconductors with high critical temperature (T c ) are highly desired. Till now, the T c of 2D superconductors are still relatively low. Based on density functional first-principles calculations, we predicted a new stable 2D transition metal molybdenum carbide 2H-Mo2C3. Based on the Eliashberg theory, we predict that it is not a superconductor. However, after hydrogenation, the hydrogen atoms increase the electronic density of states at the Fermi level, and strengthen the electron-phonon coupling. Accordingly, the hydrogenated 2H-Mo2C3, i.e., 2H-Mo2C3H2, is a 2D superconductor with T c of 53 K, exceeding the McMillan limit. Thus, hydrogenation is an important strategy for obtaining 2D high-temperature superconductors at atmosphere pressure. In view of the high superconducting T c of 2H-Mo2C3H2, it is highly expected to be synthesized in future experiments.

Screening transition metal-based polar pentagonal monolayers with large piezoelectricity and shift current

Two-dimensional (2D) materials entirely composed of pentagon motifs are of interest for their wide applications. Here, we demonstrate that in-plane polar symmetry can exist in ternary pentagonal monolayers, where the induced electric polarization is not associated with specific conditions, such as ferroelectric phase transition, strain gradient, and layer stacking, but is an intrinsic structural property coming from the orderly arranged polar bonds. Based on the high-throughput screening method and first-principles calculations, we find eight stable 2D polar transition metal compounds with a number of intriguing properties. In particular, their piezoelectric coefficients are three orders of magnitude larger than those of 2D elemental and binary pentagonal structures, and their bulk photovaltaic shift current can reach up to 300 μA V−2, superior to that of 2D conventional ferroelectric materials such as GeS. Our identified pentagonal monolayers not only expand the family of 2D pyroelectric materials, but also hold potential for energy conversions.