Mixed-valence Metal Oxides Research Articles

Mixed-valence cobalt oxides bifunctional electrocatalyst with rich oxygen vacancies for aqueous metal-air batteries

As the bifunctional oxygen electrocatalysts, the mixed-valence metal oxide with multiple oxidation state of metal ions usually gives analogous alloy effect. However, the irreversible surface reconstruction of the electrocatalyst during the electrochemical catalysis usually changes the oxidation state of metal ions in the near-face to higher oxidation state especially in the mixed-valence metal oxide. For the modulation equilibrium, we adopt a new approach to prepare the mixed-valence cobalt oxide with the regulatable ratio of Co2+/Co3+ by inducing air at a certain temperature during the cooling procedure. The obtained Co/CoO/Co3O4/NCS with hollow nanosphere possesses multiple heterointerfaces accompanying rich oxygen vacancies. Together with the contribution of the mixed valence and oxygen vacancies, Co/CoO/Co3O4/NCS exhibits an exceptional ORR activity with a half-wave potential of 0.844 V and superb OER activity with a low overpotential of 285 mV at 10 mA cm−2. The as-fabricated Zn-air batteries with Co/CoO/Co3O4/NCS catalyst exhibit a remarkable open-circuit voltage (1.47 V), a competitive power density of 146.5 mW cm−2, and good durability without nearly no performance decay for over 400 h at 5.0 mA cm−2. More attractively, when constructed into Mg-air batteries, Co/CoO/Co3O4/NCS cathode realizes an excellent stable discharge voltage over 90 h at 20 mA cm−2.

Gradient Design of Vacancies and Their Positive Correlation with Electrochemical Anticorrosion Protection

The contribution of defects to electrochemistry is a controversial but practically applicable subject. Meanwhile, it is challenging to obtain precisely a certain nonchemometric single phase in mixed-valence compounds. The precise design of nonchemometric single-phase WO3-x (x = 0, 0.1, 0.28, and 1) mixed-valence metal oxides (MVMOs) was achieved by the gradient intrinsic reduction method, and the correlation between oxygen vacancies and electrochemical anticorrosion protection was explored systematically. Then, the decisive role of periodic oxygen vacancies in electrochemical anticorrosion was confirmed. And the origin was the synergistic reaction of oxygen vacancy-upgraded photocathodic protection, vacancy-induced passivation, and mixed-valence reductive protection, which were brought about by the high oxygen vacancy concentration. Integrating the above three aspects, the WO2.72 MVMO showed the best electrochemical anticorrosion performance by increasing the resistance value to 7.67 times that of the epoxy resin coating. The establishment of a positive correlation between oxygen vacancy and corrosion protection in WO3-x (x = 0, 0.1, 0.28, and 1) materials can not only guide the design of MVMOs but also make an important contribution to the rapid precorrosion performance of the materials.

Strategies to Boost MFC Power By Supercapacitive Materials and Components

The low cost, the intrinsic robustness, the low environmental impact combined with the potential integration with the current production processes are some of the strength points that are pushing the research on Microbial Fuel Cells (MFCs) since the last thirty years. Still, to become a technology capable of incisively change the water and energetic landscape critical issues, MFCs needs to be improved for power output and quality. Here we report about three main strategies to address this challenge: i) the enhancement of the oxygen redox reaction at the cathode in the circum-neutral pH environment of MFCs by PGM-free catalysts based on Manganese and/or Iron oxides prepared by fast autocombustion from nitrates and glycols; ii) the exploitation of the inherent capacitive behaviour of the metal (Fe, Mn) oxides cathodes, iii) the smart integration of MFCs with commercial supercapacitors properly sized and connected in order to match short recharge time and high power output. Acknowledgments The research has been carried out under the Italy-South Africa joint Research Programme 2018-2020 Italian Ministers of Foreign Affairs and of the Environment. References Malaie, Keyvan, et al. "Simple preparation of carbon–bimetal oxide nanospinels for high-performance bifunctional oxygen electrocatalysts." New Journal of Chemistry 42.24 (2018): 20156-20162.Francesca Soavi, Keyvan Malaie and Mohammad Reza Ganjal, “Towards low-cost and sustainable supercapacitor electrode processing simultaneous carbon grafting and coating of mixed-valence metal oxides by fast annealing ”, Frontiers, Accepted manuscript

Toward Low-Cost and Sustainable Supercapacitor Electrode Processing: Simultaneous Carbon Grafting and Coating of Mixed-Valence Metal Oxides by Fast Annealing.

There is a rapid market growth for supercapacitors and batteries based on new materials and production strategies that minimize their cost, end-of-life environmental impact, and waste management. Herein, mixed-valence iron oxide (FeOx) and manganese oxide (Mn3O4) and FeOx-carbon black (FeOx-CB) electrodes with excellent pseudocapacitive behavior in 1 M Na2SO4 are produced by a one-step thermal annealing. Due to the in situ grafted carbon black, the FeOx-CB shows a high pseudocapacitance of 408 mF cm−2 (or 128 F g−1), and Mn3O4 after activation shows high pseudocapacitance of 480 mF cm−2 (192 F g−1). The asymmetric supercapacitor based on FeOx-CB and activated-Mn3O4 shows a capacitance of 260 mF cm−2 at 100 mHz and a cycling stability of 97.4% over 800 cycles. Furthermore, due to its facile redox reactions, the supercapacitor can be voltammetrically cycled up to a high rate of 2,000 mV s−1 without a significant distortion of the voltammograms. Overall, our data indicate the feasibility of developing high-performance supercapacitors based on mixed-valence iron and manganese oxide electrodes in a single step.

Hierarchical supercapacitor electrodes based on metallized glass fiber for ultrahigh areal capacitance

The limited charge storage of supercapacitors at the surface region results in its high power density but low energy density. It still remains a great challenge to realize supercapacitor electrodes with both bulk-like charge storage (high energy density) and surface-like fast electron/ion kinetics (high power density). Here we demonstrate scalable and hierarchical electrodes by metalizing 3D glass fiber (GF) frameworks and loading with 2D mixed-valence metal oxides. The resulting GF-Ni-Au@NiOx cathode and GF-Ni-Au@FeOx anode allow fast electron transportation through the conductive networks and unimpeded ion transport through the micrometer channels over long distance, provide large specific surface area with the hierarchical nanostructures, and exhibit ultrahigh areal capacitances (3.57 F cm-2 for cathode and 3.34 F cm-2 for anode at the current density of 3 mA cm-2). The asymmetric supercapacitor is assembled by employing industrial printed circuit board packaging techniques, showing high areal (1.67 F cm-2) and volumetric (13.92 F cm-3) capacitances. Remarkably, the device exhibits a maximum energy density of 6.19 mW h cm-3 and a maximum power density of 334.15 mW cm-3 based on the total packaged volume of the device. Furthermore, the device shows superior stability during a 400-h continuous test. This hierarchical electrode shows great potential for maintaining high capacity and fast kinetics simultaneously, and can be further extended to other electrochemical energy storage or conversion devices.

Synthesis of hierarchical NiCo2O4 hollow nanorods via sacrificial-template accelerate hydrolysis for electrochemical glucose oxidation.

Hierarchical NiCo2O4 hollow nanorods (HR) were directly grown on stainless steel via a sacrificial template accelerated hydrolysis and post calcination using ZnO nanorod as a template. The composition of the NiCo2O4 HR electrode was determined using X-ray diffraction and X-ray photoelectron spectroscopy. The morphology of the NiCo2O4 HR is comprised of nanoflakes that were characterized with scanning electron microscopy and transmission electron microscopy. The mixed-valence metal oxide and hollow structure provided high chemical reactivity and a large surface area for glucose oxidation in an alkaline solution. Under an optimal applied potential of +0.6 V, the developed NiCo2O4 HR electrode showed a broad detection range of 0.0003–1.0 mM, a sensitivity of 1685.1 μA mM−1 cm−2, and a low detection limit of 0.16 μM. These results represent a significant improvement over both NiO and Co3O4 HR. The developed NiCo2O4 HR electrode not only demonstrated excellent selectivity in the presence of several electro-active species, but also exhibited high stability following a 200 cycles voltammetry test.

Flow-injection and sequential injection determination of hydroxypurines on an electrode modified with mixed-valence ruthenium and iridium oxides

Mixed-valence ruthenium (RuOx) and iridium (IrOx) oxides and composites on their basis (RuOx-IrOx or IrOx-RuOx) electrodeposited onto the surface of a glassy carbon electrode exhibit a catalytic activity in the electrooxidation of uric acid, xanthine, and hypoxanthine. The transition from metal oxides to composites of two mixed-valence metal oxides leads to an increase in the catalytic effect of the oxidation of hydroxypurines. The IrOx-RuOx composite demonstrated the highest catalytic activity. Procedures for the amperometric detection of hydroxypurines on this composite electrode under the conditions of flow-injection analysis (FIA) and sequential injection analysis are proposed. A linear dependence of an analytical signal on the analyte concentration is observed in the range 1 × 10−6 to 5 × 10−3 M for uric acid and xanthine and 5 × 10−7 to 5 × 10−3 M for hypoxanthine under FIA conditions and from 5 × 10−7 to 5 × 10−3 M for uric acid and xanthine and 5 × 10−9 to 5 × 10−3 M for hypoxanthine in sequential injection analysis. Under FIA conditions, the sensitivity, rapidity, and performance of the analysis increase compared to the stationary conditions. The advantages of sequential injection analysis over FIA include a lower consumption of the supporting electrolyte; the absence of pumps and connections; and an increase in sensitivity, reproducibility, and rapidity.

Mixed-valent oxide-catalytic carbonization for synthesis of monodispersed nano sized carbon spheres

Abstract A mixed-valent oxide-catalytic carbonization process for synthesizing monodispersed carbon spheres at very low cost is reported for the first time. The carbon spheres are formed by catalytic carbonization of natural gas (primarily methane) with the assistance of mixed-valent metal oxides. The catalyst is reusable and the entire synthesis process produces no environmental waste. The product can be controlled by temperature to produce macroscopic quantities and a high percentage (>95%) of nano sized carbon spheres; thus, measurements of their physical properties can be performed easily. The carbon spheres are solid and comprise layered graphitic flakes. The sphere is nucleated from a pentagonal carbon ring, followed by a spiral shell growth. When the sphere grows larger, graphitic flakes of atomic thickness are nucleated on the surface owing to the nucleation of the paired pentagonal-heptagonal (P-H) carbon rings. The combination of the P-H carbon rings with the hexagonal networks produces eight ba...

Synthesis of Carbon Spheres/Tubes Using a Mixed-Valent Oxide-Catalytic Carbonization Process

ABSTRACTA mixed-valent oxide-catalytic carbonization (MVOCC) process is described for synthesizing monodispersed carbon spheres (or tubes) in macroscopic quantities at low cost. The technique uses natural gas and reusable catalysts and produces no environmental waste or pollution. The product is controlled to be either all of spheres (∼ 210 nm) or all of tubes with high purity (> 95 %). In spherical form, the product is solid and comprised of layered graphitic flakes containing paired pentagonal-heptagonal carbon-rings. In spiral tube form, the product is composed of twisted graphitic helix layers containing spherical nodes. A growth mechanism has been proposed, in which the pairing of pentagonal and heptagonal carbon-rings plays an important role. It is concluded that a change in fraction and nucleation rates of pentagonal, hexagonal and heptagonal carbon-rings results in the growth of different geometrical shapes. The success of using mixed-valence metal oxides as catalysts has opened a new field in catalysis research and applications.

Resistivity of multiphase high-T c superconductors

Resistivity versus temperature (ρ-T) behavior of high-Tc superconductors in the normal state was recently modeled semiempirically according to conventional electron hopping theory for mixed-valence metal oxides. One important feature of this work was the interpretation of a resistivity minimum (i.e., the transition between insulator and metal) in the temperature regime near Tc, based on the decrease of activation energies Ehop with increases in magnetic dilution reported in the NiO and CuO systems. Quantitative accuracy has now been established by fitting ρ-T data of the La2−xSrxCuO4 and YBa2Cu3O7 high-Tc compounds, which serve as the basis for interpreting the varying slopes of multiphase superconductors. Calculated curves fitted to reported experimental data show that the metallic slope (∂ρ/∂T≳0 above Tc) and the ρ-axis intercept of the linear extrapolation may be used to estimate the volume ratio of effective superconducting phase to normal phase within a particular specimen. From the application of this theory, it is determined that: (i) ρ-axis intercepts of single-phase superconductors are proportional to Ehop(1 − x)/x; (ii) normal phases cause increases in ρ, but decreases in slope of ρ(T)/ρ(300); and (iii) metallic slopes may be achieved with less than 10% effective volume of superconducting phase.