Performance Of Transition Metal Oxides Research Articles

Stabilizing Sulfur Sites in Tetraoxygen Tetrahedral Coordination Structure for Efficient Electrochemical Water Oxidation.

Ion regulation strategy is regarded as a promising pathway for designing transition metal oxide-based electrocatalysts for oxygen evolution reaction (OER) with improved activity and stability. Precise anion conditioning can accurately change the anionic environment so that the acid radical ions (SO4 2- , PO3 2- , SeO4 2- , etc.), regardless of their state (inside the catalyst, on the catalyst surface, or in the electrolyte), can optimize the electronic structure of the cationic active site and further increase the catalytic activity. Herein, we report a new approach to encapsulate S atoms at the tetrahedral sites of the NaCl-type oxide NiO to form a tetraoxo-tetrahedral coordination structure (S-O4 ) inside the NiO (S-NiO -I). Density functional theory (DFT) calculations and operando vibrational spectroscopy proves that this kind of unique structure could achieve the S-O4 and Ni-S stable structure in S-NiO-I. Combining mass spectroscopy characterization, it could be confirmed that the S-O4 structure is the key factor for triggering the lattice oxygen exchange to participate in the OER process. This work demonstrates that the formation of tetraoxygen tetrahedral structure is a generalized key for boosting the OER performances of transition metal oxides.

Stabilizing Sulfur Sites in Tetraoxygen Tetrahedral Coordination Structure for Efficient Electrochemical Water Oxidation

AbstractIon regulation strategy is regarded as a promising pathway for designing transition metal oxide‐based electrocatalysts for oxygen evolution reaction (OER) with improved activity and stability. Precise anion conditioning can accurately change the anionic environment so that the acid radical ions (SO42−, PO32−, SeO42−, etc.), regardless of their state (inside the catalyst, on the catalyst surface, or in the electrolyte), can optimize the electronic structure of the cationic active site and further increase the catalytic activity. Herein, we report a new approach to encapsulate S atoms at the tetrahedral sites of the NaCl‐type oxide NiO to form a tetraoxo‐tetrahedral coordination structure (S‐O4) inside the NiO (S‐NiO ‐I). Density functional theory (DFT) calculations and operando vibrational spectroscopy proves that this kind of unique structure could achieve the S‐O4 and Ni‐S stable structure in S‐NiO‐I. Combining mass spectroscopy characterization, it could be confirmed that the S‐O4 structure is the key factor for triggering the lattice oxygen exchange to participate in the OER process. This work demonstrates that the formation of tetraoxygen tetrahedral structure is a generalized key for boosting the OER performances of transition metal oxides.

Surface engineering of 2D layered MnO2 with co-doping of Ni and Fe for rechargeable zinc-air battery

Surface engineering by doping has always been one of the well-applied effective strategies to improve the performance of transition metal oxides towards their application in energy conversion and storage devices. However, fewer studies have been conducted to understand the effect of co-doping of two or more transition metals in layered transition metal oxides. It is interesting to note how multiple metal ion doping could create a synergistic environment that modulates the electrocatalytic ability of the host lattice. In this work, we have sequentially introduced Fe, followed by Ni, into the δ-MnO2 lattice (NiFeMn) to occupy either interplanar spaces or to substitute the host lattice. Spectroscopic and high resolution microscopic characterizations suggested that the dopants have entered the in-planar voids, as well as in the inter-layer region. The electrochemical characterizations revealed the positive synergistic interaction of Fe and Ni in the MnO2 lattice improved overall bifunctional oxygen electrode activity parameters. In the case of a practical Zn-air battery, the electrode NiFeMn shows 2.4 times improved output power density, cyclic stability, and durability than FeMn, along with a round- trip efficiency of 51 %, and specific capacity of 589.96 mAh g−1 corresponding to energy density of 470.67 mWh g−1. Our studies reveal that both Fe and Ni have played a major role as dopants and their synergistic interaction has significantly boosted the overall electrocatalytic properties of δ-MnO2.

Design of chromium-doped spinel Mn3O4 modulated electronic structure as an efficient catalyst for Li-O2 batteries

Heteroatom doping is considered to be a crucial strategy for the enhancement of the electrocatalytic performance of transition metal oxides by effectively modulating the electronic structure of the catalysts. Herein, chromium-doped Mn3O4 electrocatalyst was systematically synthesized. Due to the doping of Cr ions, a large number of oxygen vacancies appear in the material to provide high catalytic activity. Remarkably, the modified material with a larger surface area can achieve more exposure active sites, which facilitate better electrochemical performance. The lithium-oxygen battery with optimized 15% Cr-Mn3O4 as the electrode exhibited a low overpotential (0.86 V), large discharge capacity (22,288 mAh g−1), excellent rate performance and stable cycle for 740 h. This work provides new insights into the synthesis and development of efficient and highly active Lithium-oxygen battery catalysts with regulating the surface chemical state of the material.

Transition metal oxide complexes as molecular catalysts for selective methane to methanol transformation: any prospects or time to retire?

Transition metal oxides have been extensively used in the literature for the conversion of methane to methanol. Despite the progress made over the past decades, no method with satisfactory performance or economic viability has been detected. The main bottleneck is that the produced methanol oxidizes further due to its weaker C-H bond than that of methane. Every improvement in the efficiency of a catalyst to activate methane leads to reduction of the selectivity towards methanol. Is it therefore prudent to keep studying (both theoretically and experimentally) metal oxides as catalysts for the quantitative conversion of methane to methanol? This perspective focuses on molecular metal oxide complexes and suggests strategies to bypass the current bottlenecks with higher weight on the computational chemistry side. We first discuss the electronic structure of metal oxides, followed by assessing the role of the ligands in the reactivity of the catalysts. For better selectivity, we propose that metal oxide anionic complexes should be explored further, while hydrophylic cavities in the vicinity of the metal oxide can perturb the transition-state structure for methanol increasing appreciably the activation barrier for methanol. We also emphasize that computational studies should target the activation reaction of methanol (and not only methane), the study of complete catalytic cycles (including the recombination and oxidation steps), and the use of molecular oxygen as an oxidant. The titled chemical conversion is an excellent challenge for theory and we believe that computational studies should lead the field in the future. It is finally shown that bottom-up approaches offer a systematic way for exploration of the chemical space and should still be applied in parallel with the recently popular machine learning techniques. To answer the question of the title, we believe that metal oxides should still be considered provided that we change our focus and perform more systematic investigations on the activation of methanol.

Synergistic effects boosting hydrogen evolution performance of transition metal oxides at ultralow Ru loading levels.

In this study, ultralow ruthenium nanoparticles on the nickel molybdate nanorods grown on nickel foam (Ru-NiMoO4-NF) were synthesized. The Ru-NiMoO4-NF exhibited outstanding hydrogen evolution reaction performances in alkaline with overpotential of 52 mV at the current density of 10 mA cm-2. And, it maintains excellent stability for 20 h at the current density of 20 mA cm-2. The mass activity of Ru-NiMoO4-NF is 0.21 A mgRu -1, which is higher than that of Pt/C. Lots of exposed heterojunction interfaces and synergistic effects between Ru nanoparticles and NiMoO4 nanorods were regarded as the reasons for excellent performance. This work provides an innovative route for developing low-cost catalysts based on the transition metal oxides and trace precious metal with unique heterostructures for hydrogen production through water splitting.

Magnetic Field-Enhanced Electrocatalytic Oxygen Evolution on a Mixed-Valent Cobalt-Modulated LaCoO3 Catalyst.

Extensive efforts to enhance the oxygen evolution reaction (OER) catalytic performance of transition metal oxides mainly concentrate on the extrinsic morphology tailoring, lattice doping, and electrode interface optimizing. Nevertheless, little room is left for performance improvement using these methods and an obvious gap still exists compared to the precious metal catalysts. In this work, a novel "mixed-valent cobalt modulation" strategy is presented to enhance the electrocatalytic OER of perovskite LaCoO3 (LCO) oxide. The valence transition of cobalt is realized by ethylenediamine post reduction procedure at room temperature, which further induces the variation of magnetic properties for LCO catalyst. The optimized LCO catalyst with Co2+ /Co3+ of 1.98 % exhibits the best OER activity, and the overpotential at 10 mA cm-2 current density is decreased by 170 mV compared pristine LCO. Impressively, the ferromagnetic LCO catalyst can perform magnetic OER enhancement. By application of an external magnetic field, the overpotential of LCO at 10 mA cm-2 can be further decreased by 20 mV compared to that of under zero magnetic field, which arises from the enhanced energy states of electrons and accelerated electron transfer process driven by magnetic field. Our findings may provide a promising strategy to break the bottleneck for further enhancement of OER performance.

A simple room–temperature acid capture strategy to controllably tune oxygen vacancy in Co3O4 for oxygen evolution reaction

Oxygen vacancy (Vo) engineering is considered as a general but effective way to improve the oxygen evolution reaction (OER) performance of transition metal oxides (TMOs). However, the deep understanding of Vo is still limited. In this work, a simple room–temperature acid capture strategy is employed to create Vo in TMOs. As discovered in the case of commercial Co3O4 microspheres, the Vo can be tuned by the acid capture time. As the acid capture time is growing, the concentration of Vo increases. Moderate Vo concentration created by controlled acid capture time (less than 32 h) can elevate the electrical conductivity and optimize the electronic structure. With overlong acid capture time (reaching 64 h), excessive Vo will present, leading to enlarged energy band gap, declined electrical conductivity as well as collapsed structure of Co3O4 microspheres, which is detrimental to the exposure of catalytic sites and accessibility of electrolyte. Consequently, the resultant Co3O4 −Vo− 32 (32 represents acid capture time) obtains a balanced Vo concentration while retaining structure integrity, thus displaying a better OER performance than the counterparts.

Defect and Interface Engineering of Three-Dimensional Open Nanonetcage Electrocatalysts for Advanced Electrocatalytic Oxygen Evolution Reaction.

Defect engineering and interface engineering are two efficient approaches to promote the electrocatalytic performance of transition metal oxides (TMOs) by modulating the local electronic structure and inducing a synergistic effect but usually require costly and complicated processes. Herein, a facile electrochemical etching method is proposed for the controllable tailoring of the defects in a three-dimensional (3D) open nanonetcage CoZnRuOx heterostructure via the in situ electrochemical etching to remove partial ZnO. The highly open 3D nanostructures, numerous defects, and multicomponent heterointerfaces endow the CoZnRuOx nanonetcages with more accessible active sites, moderated local electronic structure, and strong synergistic effect, thereby enabling them to not only deliver an ultralow overpotential (244 mV @ 10 mA cm-2) for oxygen evolution reaction (OER) but also high-performance overall water electrolysis by coupling with commercial Pt/C, with a potential of 1.52 V at 10 mA cm-2. Moreover, experiments and characterizations also reveal that the remaining Zn2+ can facilitate OH- adsorption and charge transfer, which also further improves the electrocatalytic OER performance. This work proposes a promising strategy for creating surface defects in heterostructured TMOs and provides insights to understand the defect- and interface-induced enhancement of OER electrocatalysis.

Ozone-activated CNTs to induce uniform coating of MnO2 as high-performance supercapacitor electrodes

Carbon nanotube (CNT), as a promising material to enhance electrical conductivity and ionic diffusivity, plays an indispensable role in improving the electrochemical performance of transition metal oxides. In this work, a novel core-shell structure of MnO2 coating on carbon nanotubes (MnO2@CNT) was successfully synthesized via a simple solvothermal method. The internal porous structure of CNT as core can facilitate the penetration of electrolyte while improving the electrical conductivity. The thin MnO2 nanosheets as shell can simultaneously achieve a large interaction area and rapid ion exchange. Benefitting from the synergistic effects, the MnO2@CNT shows an excellent capacitance of 386 F g−1 at 1 A g−1 with the content of MnO2 about 52 wt.%. In addition, the MnO2@CNT retains nearly 93.6% of its initial capacitance after 5000 cycles, indicating its excellent cycling performance.

Laser-synthesized ultrafine NiO nanoparticles with abundant oxygen vacancies for highly efficient oxygen evolution

Introducing oxygen vacancies and reducing size of NPs are two effective strategies to enhance the electrocatalytic performance of transition metal oxides for oxygen evolution reaction (OER). Herein, NiO NPs with ultrasmall size (≈2.1 nm) and rich oxygen vacancies (NiO-LFL) are prepared by laser fragmentation of bulk NiO NPs in water. The ultrasmall size of NPs enable more active sites to be exposed. And the abundant oxygen vacancies in NiO greatly improve the intrinsic activity and electrical conductivity. Benefit from these advantages, NiO-LFL NPs display a remarkable OER activity and stability in 1 M KOH, which is superior to the state-of-the-art RuO2 catalyst.

CeO2-NiO/N,O-rich porous carbon derived from covalent-organic framework for enhanced Li-storage

Heteroatom-doped porous carbon is crucial to improve energy storage performance of transition metal oxide (TMO)/carbon nanocomposites. However, their traditional template synthesis, and chemical or thermal activation methods are cumbersome and inefficient. Crystalline covalent-organic framework (COF) has customizable structure and abundant heteroatoms. Its abundant heteroatoms and ordered network structure make it to be promising precursors to prepare TMO/carbon nanocomposites. Herein, a new N,O-rich COFDHNDA-BTH with ribbon morphology was synthesized by amine-aldehyde condensation reaction. Using COFDHNDA-BTH as precursors, CeO2-doped NiO heterostructures/N,O-rich porous carbon (CeO2-NiO/NC) nanocomposites were synthesized and successfully applied for enhanced Li-storage. The N,O-rich COFDHNDA-BTH as precursors can not only avoid the cumbersome procedures and low efficiency of traditional template method but also provide more uniformly distributed active sites to anchor CeO2-NiO and hierarchical pores. Thanks to larger lattice space provided by doping of large radius cerium for Li+ insertion/de-insertion and uniformly distributed small CeO2-NiO nanoparticles with a diameter of 18 nm, the obtained 1/5CeO2-NiO/NC exhibits excellent Li-storage performance with a capacity of 852 mAh g−1 after 500 cycles at 1000 mA g−1. The simple yet efficient strategy provides a new guide to prepare CeO2-doped TMO heterostructures/NC nanocomposites for enhanced Li-storage.

Controlled fabrication of core–shell γ-Fe2O3@C–Reduced graphene oxide composites with tunable interfacial structure for highly efficient microwave absorption

The design of a high-performance microwave absorbing material is highly dependent on the synergistic structural design of heterostructure and the appropriate material compositions. Herein, a series of composites of reduced graphene oxide (RGO) and core–shell structured γ-Fe2O3@C nanoparticles have been achieved by a hydrothermal and in-situ chemical vapor deposition (CVD) method. In particular, the structure of the carbon layer, including its graphitization and thickness, can be controlled by optimizing the CVD conditions, which is beneficial to tailor the impedance matching and dielectric loss. The rationally designed RGO/γ-Fe2O3@C composite has multiple electromagnetic dissipation mechanisms. The effective absorption ranges of an optimal sample at a filling rate of 20% can cover 100% X-band and 98% Ku-band at thicknesses of 3.0 mm and 2.2 mm, respectively. This finding suggested that the controllable fabrication of core–shell heterostructures could be viable approach to upgrade the microwave absorption performance of transition metal oxides.

Nitrogen plasma activation of cactus-like MnO2 grown on carbon cloth for high-mass loading asymmetric supercapacitors

The electrochemical performance of high theoretical capacitance MnO2 is usually restricted to low mass loading electrode due to the slower electron/ion diffusion kinetics in dense electrodes. Defect engineering has been supposed to an effective way to improve the capacitive performance of transition metal oxides. Herein, cactus-like structure MnO2 was grown on carbon cloth (CC) and treated with N2-plasma to obtain the N-MnO2 electrode with high mass loading of 12 mg cm−2. Benefiting from the hierarchical structure and rich oxygen vacancies, the as-prepared N-MnO2 electrode delivered a high mass specific capacitance of 443F g−1 and a high areal capacitance of 5320 mF cm−2. Furthermore, the N-MnO2 electrode released an excellent rate capability with 71.8% capacitance retention at 20 mA cm−2 as well as gratifying cycling life with 96 % capacitance retention after 8 000 cycles at 20 mA cm−2. Moreover, an asymmetrical supercapacitor device assembled by N-MnO2 and activated CC can achieve a high energy density of 0.56 mWh cm−2 (46 Wh kg−1) at the power density of 5.14 mW cm−2 (420 W kg−1) with good cycle stability. Such facile N2-plasma treatment for defect engineering shows great potential in the rational design for advanced supercapacitor electrodes with high mass loading.

Amorphous Fe2O3 film-coated mesoporous Fe2O3 core-shell nanosphere prepared by quenching as a high-performance anode material for lithium-ion batteries

Amorphous Fe 2 O 3 films-coated mesoporous crystalline Fe 2 O 3 material was synthesized through an easy quenching strategy, demonstrating excellent electrochemical performance for LIBs. • Novel amorphous Fe 2 O 3 films coated mesoporous crystalline Fe 2 O 3 were synthesized. • Amorphous Fe 2 O 3 films provided more active sites and restrain the volume change during cycling. • AmFO@mFe 2 O 3 demonstrated high capacity and ideal cycling stability as anode for LIBs. • Iced water quenching method is low-cost, simplicity and suitable for scalable manufacture. Ferric oxide (Fe 2 O 3 ) is a promising anode material for lithium-ion batteries (LIBs) thanks to its high theoretical capacity and abundance. However, pure-phase Fe 2 O 3 falls far short of its theoretical specific capacity during application, and most of the modification methods are complex as well as pollute the environment. In this work, a novel amorphous Fe 2 O 3 film-coated mesoporous Fe 2 O 3 crystalline core-shell structure (amFO@mFe 2 O 3 ) with narrowed lattice and oxygen vacancy was synthesized through easy and operable annealing followed by a time-saving water quenching strategy. Benefiting from the special features after quenching, the optimized amFO@mFe 2 O 3 demonstrated exceptional electrochemical performance as the anode material for LIBs, achieving a specific capacity of ∼ 1000 mAh·g −1 after stabilization at 500 mA·g −1 , superior to the values of LIBs constructed by the bare crystalline Fe 2 O 3 . Excellent rate performance was exhibited up to 220 mAh·g −1 at a high current density of 20,000 mA·g −1 , and reversible capacity can be restored to approximately 1015 mAh·g −1 when the current density returns to 100 mA·g −1 . This protocol provides an ultra-simple method to enhance the electrochemical performance of transition metal oxides anode for LIBs and would propel the development of new functional materials for energy storage and conversion.

In situ exsolution of metallic Cu in mixed oxides as battery-type electrode for energy storage devices

• Atomic coupled metal/mixed oxide heterostructure is prepared by in situ exsolution method. • Exsolved Cu from mixed oxides promote the electrochemical performance. • Surface reconstruction of the heterostructure electrode leads to capacity recovery. The performances of transition-metal oxides as battery-type electrodes can be effectively enhanced by optimizing their intrinsic properties and integrating conductive components. Herein, an in situ exsolution strategy was used to prepare metallic Cu/mixed oxides heterostructured electrodes that containing multi-cations (Cu 2+ , Ni 2+ , and Co 2+ ). The synergetic effects of the uniformly distributed metallic Cu components, a compact metal/oxides interface, and a mesoporous structure ensures that the prepared heterostructured electrodes have enhanced redox reactivity. A Cu@CuNiCoO nanowire electrode gave a high specific capacity (1054 C g −1 ), and rate capacity, and had superior cycling stability. Our work broadens the scope of metal/oxides heterostructured electrodes for energy storage.

Plasma-assisted lattice oxygen vacancies engineering recipe for high-performing supercapacitors in a model of birnessite-MnO2

Defect engineering has been considered as an efficient strategy to enhance the electrochemical performance of transition metal oxides based energy storage devices. However, the electrochemical activity and stability were greatly determined by the defect located crystal and external environments, which dominate the electrochemical properties of its based electrode. Thus, regulating a defect engineering recipe becomes a vital and direct route to advance the performance of the electrode materials. Herein, a versatile recipe combining the mild H-plasma and O-plasma was demonstrated for prototypical birnessite-MnO2 to achieve the robust lattice oxygen vacancies in birnessite-MnO2 (LOV-MnO2), targeting to boost its electrochemical energy storage performance. Theoretical calculation reveals the facilitated ion intercalation and diffusion kinetics due to the lower energy barrier in the LOV-MnO2. The LOV-MnO2 demonstrates an exceptional electrochemical performance with a specific capacitance as high as 445.1 F g−1 (at the current density of 1 A g−1), and the diffusion-controlled capacitance contribution reaches unprecedented ~70% (at a scan rate of 5 mV s−1). Besides, the configured symmetrical supercapacitor device LOV-MnO2//LOV-MnO2 delivers remarkable performance with an energy density of 92.3 Wh kg−1 at a power density of 1100.3 W kg−1 with a widen working voltage of 2.2 V. An outstanding cyclic life of 92.2% capacitance retention was also achieved after 10,000 charge–discharge cycles. Such superior electrochemical performance suggests that the proposed defect engineering recipes here will aid in the future development of advanced electrode materials for electrochemical energy storage devices.

A universal electrochemical lithiation-delithiation method to prepare low-crystalline metal oxides for high-performance hybrid supercapacitors.

The electrochemical performance of transition metal oxides (TMOs) for hybrid supercapacitors has been optimized through various methods in previous reports. However, most previous research was mainly focused on well-crystalline TMOs. Herein, the electrochemical lithiation–delithiation method was performed to synthesise low-crystallinity TMOs for hybrid supercapacitors. It was found that the lithiation–delithiation process can significantly improve the electrochemical performance of “conversion-type” TMOs, such as CoO, NiO, etc. The as-prepared low-crystallinity CoO exhibits high specific capacitance of 2154.1 F g−1 (299.2 mA h g−1) at 0.8 A g−1, outstanding rate capacitance retention of 63.9% even at 22.4 A g−1 and excellent cycling stability with 90.5% retention even after 10 000 cycles. When assembled as hybrid supercapacitors using active carbon (AC) as the active material of the negative electrode, the devices show a high energy density of 50.9 W h kg−1 at 0.73 kW kg−1. Another low-crystallinity NiO prepared by the same method also possesses a much higher specific capacitance of 2317.6 F g−1 (302.6 mA h g−1) compared to that for pristine commercial NiO of 497.2 F g−1 at 1 A g−1. The improved energy storage performance of the low-crystallinity metal oxides can be ascribed to the disorder of as-prepared low-crystallinity metal oxides and interior 3D-connected channels originating from the lithiation–delithiation process. This method may open new opportunities for scalable and facile synthesis of low-crystallinity metal oxides for high-performance hybrid supercapacitors.

Tuning electromagnetic absorption properties of transition metal oxides by hydrogenation with nascent hydrogen

• Zn/HCl system was used for controlled hydrogenation of transition metal oxides. • Electrical conductivity and dielectric property correlate to hydrogenation degree. • The EMA performance of transition metal oxides can be tuned by proton hydrogenation. • Hydrogenation effect on conductivity was revealed by First-principle calculations. Transition metal oxides (TMO) show great potential in various applications, but are rarely be reported as good electromagnetic absorption (EMA) materials, owing to their low electrical conductivity. In this work, we present a modified Zn/HCl system for controlled hydrogenation of a series of TMO, and investigate the relationship between EMA property and electronic structure. It is discovered that the EMA performance of TMO is improved remarkably by this proton hydrogenation strategy. It is revealed that EMA property of TMO is closely associate with its hydrogenation degree, and can be efficiently tuned by hydrogenation controlling. First-principle calculations and experimental results suggest that the variation of band structure resulting in increased delocalized electrons, narrowed bandgap and elevated electrical conductivity is thought to be the main reason for the enhanced EMA performance of hydrogenated TMO. This research expands the application scope of hydrogenated TMO and provides a new idea for the design of high-performance EMA materials.

Three-dimensional hierarchical Ca3Co4O9 hollow fiber network as high performance anode material for lithium-ion battery

Transition metal oxides have high specific capacity as anode materials for lithium-ion battery. But aggregation of particles and volume expansion during lithiation/delithiation restrict their application. In this work, a three-dimensional hierarchical Ca3Co4O9 hollow fiber network assembled by nanosheets is prepared by a electrospinning combining with heat treatment method to overcome these issues and to boost its lithium storage performance. As-synthesized sample possesses excellent cyclic stability (578.6 mA h g−1 at 200 mA g−1 after 500 cycles) and rate performance (293.5 mA h g−1 at 5000 mA g−1), much better than those of commercial Co3O4. Furthermore, the fast kinetics of the three-dimensional Ca3Co4O9 hollow fiber network is also confirmed by the variable scan rates CV tests and the EIS measurements, which is dedicated to the specific hierarchical hollow fiber network structure that provides shorter ion transport distances and higher electrical conductivity. This work supplies a universal approach to improve the electrochemical performance of transition metal oxides for lithium ion batteries.