Oxidation State Of Cobalt Ions Research Articles

3DOM Fe/Co-containing perovskites as bifunctional ORR/OER electrocatalysts for a photo-rechargeable zinc battery

Three-dimensionally ordered mesoporous perovskites (La0.8Ba0.2Fe1–xCoxO3–δ (x = 0, 0.2, 0.4)), which are integrated with a silicon photovoltaic cell are prepared. Their overall solar energy conversion and storage efficiency in zinc-ion-based batteries was assessed. The electrocatalytic activities were measured in OER/ORR, while the galvanostatic charge/discharge processes were applied to study battery performance under 1 sun irradiation. The electronic properties of the electrocatalysts were determined by X-ray absorption and UV photoelectron spectroscopies. The importance of controlling the oxidation state of cobalt ions to increase the specific capacity and energy density of the batteries was shown. The ORR activity depends on oxygen vacancies, inferred from the XANES and EXAFS spectra analysis. Their presence is associated with the Co2+/Co3+ ratio which controls the beneficial eg orbitals occupancy via the high spin d7 electron configuration. Herein, we report an overall energy conversion efficiency of 12.6 % for the PV-integrated La0.8Ba0.2Fe0.8Co0.2O3–δ photo-rechargeable battery cell.

NEXAFS and XPS Studies of Co Doped Bismuth Magnesium Tantalate Pyrochlores

Co doped bismuth magnesium tantalate with a pyrochlore structure (sp. gr. Fd-3m) was synthesized for the first time using the standard ceramic method. Single phase Bi2Mg1−xCoxTa2O9 samples were found to be formed when x < 0.7 in the X-ray phase analysis. However, with a higher cobalt content in the samples, the impurity phase β-BiTaO4 (sp. gr. P-1) is detected, and its amount is proportional to the degree of cobalt doping. The formation of solid solutions is evidenced by a uniform increase in the unit cell parameter of the Co,Mg co doped bismuth tantalate phase with an increase in the content of cobalt ions in the samples from 10.5412(8) (x = 0.3) to 10.5499(8) Å (x = 0.7). The samples exhibit a porous microstructure consisting of chaotically oriented and partially fused elongated grains measuring 1–2 μm. The dependence of the ceramic grain size on the n(Mg)/n(Co) ratio was not determined. X-ray spectroscopy (ear dge X-ray bsorption ine tructure (NEXAFS) and X-ray photoelectron spectroscopy (XPS)) was used to study the charge state of ions in Bi2Mg1−xCoxTa2O9. The NEXAFS and XPS data showed that doping with cobalt and magnesium did not change the bismuth and tantalum oxidation states in pyrochlore; in particular, the ions maintained their oxidation states of Bi(+3), Mg(+2) and Ta(+5). The energy position of the peaks of the Ta4f-, Ta5p-, Ta4d spectra had a characteristic shift towards lower energies compared to the binding energy in pentavalent tantalum oxide Ta2O5. A shift towards lower energies is characteristic of a decrease in the effective positive charge; in particular, for the Ta4f and Ta4d spectra we presented, this energy shift was ΔE = 0.65 eV, and in the region of the Ta4d edge—0.55 eV. This in turn allowed for us to assume that tantalum atoms have the same effective charge +(5-δ). The oxidation state of cobalt ions was predominantly 2+ and partially 3+, according to NEXAFS spectroscopy data.

Unveiling the effect of the structural transformation of CoZn-MOF on BiVO4 photoanode for efficient photoelectrochemical water oxidation

Photoelectrochemical (PEC) water splitting has been widely investigated for solar-to-hydrogen conversion. However, issues like high charge recombination rate and slow surface water oxidation kinetics severely hinder its (PEC) conversion efficiency. Herein, we constructed MOF-derived CoOOH cocatalyst on BiVO4 photoanode, using a feasible electrochemical activation strategy. The BiVO4-based photoanode obtained shows a high photocurrent density of 3.15 mA/cm2 at 1.23 VRHE and low onset potential. Detailed experiments and theoretical calculations show that during the activation of CoZn-MOFs, there was a partial breakage of 2-methylimidazole (mIM) linker, an increase in the oxidation state of Cobalt ion (Co), and increased O2–. The high PEC performance is mainly attributed to the MOF-derived CoOOH, which provides rich active sites for hole extraction and reduces the overpotential for oxygen evolution reaction. Furthermore, when CoZnNiFe-LDHs were decorated on BiVO4 using the ions exchange method, the photocurrent density of BiVO4/CoZnNiFe-LDHs photoanode got to 4.0 mA/cm2 at 1.23 VRHE, accompanied with high stability. This study provides insights into understanding the key role played by the structural transformation of MOF cocatalyst in PEC water splitting processes.

Qualitative study of structural phase transition in nickel doped La2CoTi(1−x)NixO6 double perovskite

La2CoTi(1−x)NixO6 (x = 0, 0.2 and 0.6) (A2BB′(1−x)B′′(x)O6) double perovskite samples in bulk pellet form were prepared successfully by using sol-gel method at a sintering temperature of 1350 °C. From the Rietveld refinement approach, crystal structures of prepared samples were investigated using X-ray powder diffraction data, wherein, a structural phase transition occurred from monoclinic (P21/n) to orthorhombic (Pbnm) to rhombohedral (R3̅c) as the nickel content is increased. X-ray photoelectron spectroscopy study reveals the presence of mixed oxidation state of cobalt ions (Co2+/Co3+) which changes significantly as the nickel concentration is increased to 20% and hence affects the bond length with the structure distortion from monoclinic to orthorhombic with symmetry Pbnm. In 60% nickel doped compound, there occurs a random distribution of Co2+/Co3+ and Ni3+ among the B′ and B′′ site with added repulsions between trivalent cobalt cations seeming in the structure due to Ti4+ located at antisite B site. Consequently, to maintain charge neutrality, distorted rhombohedral structure was observed. Tolerance factor was also estimated for all the prepared bulk compounds to further confirm the formation and structural transitions in the prepared compounds.

Oxo‐bridged Tri‐ and Tetra‐nuclear Cobalt Complexes Supported with Amide‐Based Nitrogen Donor Ligands

AbstractThis work presents two mixed‐valent μ3‐oxo‐bridged trinuclear [(Co2+)2Co3+] complexes 1 and 2 and a μ4‐oxo‐bridged tetranuclear [Co2+4] complex 3. Crystal structures of 1 and 2 showed the presence of triangular complexes having two Co2+ and one Co3+ centers bridged by a μ3‐oxo group. In contrast, complex 3 exhibited a tetrahedrally arranged μ4‐oxo‐bridged [Co2+4] core. A significant feature is the stabilization of unusual μ‐oxo‐bridged complexes exclusively supported with all nitrogen donor ligands. The X‐ray photoelectron spectroscopy confirmed the multiple oxidation states of cobalt ions in these complexes. Such oxo‐bridged cobalt complexes were investigated for their catalytic activity towards H2O2‐mediated oxidation of assorted benzyl alcohols.

Electrochemical supercapacitor application of CoFe2O4 nanoparticles decorated over graphitic carbon nitride

Fast-growing energy demands for the next generation quest for the development of engineered nanostructured electrode materials. In this direction, we report the performance and design of well crystalline cobalt ferrite nanoparticles (NPs) decorated on graphitic carbon nitride (CoFe2O4/GCN composite) for supercapacitor. The glycol functionalised composite material that was synthesized by a solvothermal process exhibits high specific capacitance (Csp) with long cyclic stability. The superior electrochemical properties are attributed to the presence of multiple oxidation states of cobalt ions, high surface area, nitrogen-rich structure, and porous nature that enables fast ion diffusion. Moreover, polyol functionalization increases hydrophilicity and improves the wettability of the composite which enhances the utilization rate of specific surface area. The composite based electrode exhibited a high Csp of 524 F/g @ 2 mV/s with superior capacitance retention (~ 98%) even after 5000 cycles at 8 A/g in 2 M KOH aqueous electrolytes. The power law and Trasatti method provide a qualitative analysis of the diffusivity of the NPs at the electrode/electrolyte interface. Presence of an ample number of electrochemically active sites and defect sites in the nanosheet structures of GCN modulate the electrochemical properties. The results evident that GCN composite can be one of the economical and active candidates as energy storage electrode materials.

The effect of non-bridging oxygen on the electrical transport of some lead borate glasses containing cobalt

Abstract The effect of reducing oxygen in glass network on the electrical conductivity of system 50 B2O3 − 20 Pb3O4 − 10 Co3O4 − (20 − x)CaO − xCaCl2 (0 ≤ x ≤ 20 mol%) has thoroughly been investigated. This reduction was created by substituting oxygen ions with chloride ions. The measurements were conducted in the temperature range 320–560 K for fixed frequencies 0.1, 1, 10 and 100 kHz. It was found that at low temperatures, the dc conductivity (σ dc) is lower than the measured ac conductivity σ(ω), whereas σ(ω) and σ dc became equal at high temperature for all frequencies. The ac, dc conductivity as well as dc activation energies decrease with the gradual increase of CaCl2 content. The ac conductivity and the frequency exponent data showed that the correlated barrier hopping of electrons between both of oxidation states of cobalt ions (Co2+ and Co3+) is the most probable mechanism. The dielectric constant and the dielectric loss of the present glass system can be fitted to the Cole–Cole equation for all frequencies and temperatures.

Electrochemical properties of CoFe2O4 nanoparticles and its rGO composite for supercapacitor

Cost-effective solutions for improved performance, activity, and stability are the main concern for the development of energy-storage devices. Generally, reduced graphene oxide (rGO) supported cobalt ferrite (CoF) based electrodes possesses enhanced electrochemical properties in supercapacitor applications such as high specific capacitance (CS) and long cyclic stability due to the presence of multiple oxidation states of cobalt ions, high surface area, and porous nanostructure that enables fast ion-diffusion. Herein, we report a one-pot solvothermal synthesis of glycol functionalised CoF nanoparticles and the composite with rGO for the supercapacitor electrode. Further, bare CoF was annealed at 300 and 500 °C to evaluate the dependency glycol functionalization on the electrochemical behaviour. The CoF-rGO nanocomposites in alkaline (2 M KOH) electrolyte exhibit better electrochemical behaviour compared to bare and annealed CoF. CoF-rGO exhibits high CS of 551 F/g at a scan rate of 2 mV/s with 98% capacitance retention after 2000 cycles. The power law and Trasatti method provide a qualitative analysis of the diffusivity of the nanoparticles at the electrode/electrolyte interface.

Investigation on catecholase activity of salen Co(III) octahedral complexes

We have synthesized three octahedral Co(III) compounds in order to study their catalytic efficiency for the oxidation of 3,5-di-tertiarybutylcatechol (3,5-DTBC) into the corresponding quinone. Among the tested compounds, only one compound, 3 shows the catecholase activity. The catalytic efficiency of this compound arises from its capability to stabilise two different oxidation states of the metal centre. The initial oxidation state of cobalt ion (Co(III)) changes into Co(II) in order to accommodate the substrate. The kinetic parameters were calculated using the Michaelis-Menten equation. The catalytic loop was proposed and the byproduct formed in the cycle was experimentally determined.

Highly Active Oxygen Evolution on Carbon Fiber Paper Coated with Atomic-Layer-Deposited Cobalt Oxide.

In this work, we evaluated the oxygen evolution performance of cobalt oxide (CoO x)-coated carbon fiber paper in electrochemical water splitting. For a uniform coating of CoO x layers along the carbon fiber paper, the atomic layer deposition (ALD) technique was applied. We achieved a uniform and conformal coating of atomic-layer-deposited CoO x (ALD-CoO x) on the carbon fiber paper. The overpotential for oxygen evolution measured for the optimized ALD-coated carbon fiber paper was as low as 343 mV at 10 mA cm-2, which is competitive with the activity of state-of-the-art CoO x prepared on electrodes with large surface areas. Oxygen evolution is not enhanced after a critical thickness, about 28 nm in our study, is reached. The optimal thickness of the ALD-CoO x film is dependent on two competing effects: the high oxidation state of cobalt ions in thicker CoO x helps the oxygen evolution, whereas the introduction of a thick oxide coating decelerates the rate of charge transfer at the surface.

Enhancement of oxygen evolution reaction activity and durability of Ba0.5Sr0.5Co0.8Fe0.2O3-δ by CO2 thermal treatment

This work demonstrates that in situ formation of carbonate layer on the surface of Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) obtained by exposure to CO2 during heating between 500 °C and 700 °C can provide enhanced oxygen evolution reaction (OER) performance and durability in an alkaline solution relative to the original BSCF. Three temperatures, i.e., 500 °C, 600 °C, and 700 °C were chosen to perform the CO2 thermal treatment, resulting into BSCF-500, BSCF-600, and BSCF-700 samples. The OER was enhanced in the order of BSCF-500 < BSCF-700 < BSCF-600. BSCF-600 showed the best OER performance, i.e., a low overpotential of 0.36 V required to attain 10 mA cm−2 current density as well as a mass activity of 74.14 Agcat-1 and a specific activity of 5.04 mA cmcat-2 at an overpotential of 0.4 V. The OER performance durability of BSCF-600 was highlighted by its ability to maintain a stable potential of around 1.61 V vs. RHE (RHE: reversible hydrogen electrode) when charged at a constant current density of 10 mA cm-2 throughout the 800 min continuous chronopotentiometry test. The enhanced OER performance for BSCF-600 relative to the original BSCF is attributed to three factors: (i) higher electrochemically active surface area; (ii) faster charge transfer rate and higher electrical conductivity; and (iii) modified oxidation state of cobalt ions. The formation of thin carbonate layer in BSCF-600 appears to suppress the durability issue observed in BSCF.

Single molecule force spectroscopy reveals that the oxidation state of cobalt ions plays an important role in enhancing the mechanical stability of proteins.

Engineered bi-histidine (biHis)-based metal chelation is a general and robust method to enhance the mechanical stability of proteins. Here we used single molecule force spectroscopy techniques to investigate the effect of binding of Co2+/Co3+ on the mechanical stability of an engineered biHis mutant of protein GB1, G6-53. We found that the binding of Co2+/Co3+ can lead to an enhancement of the mechanical stability of G6-53, but the degree of enhancement is drastically different. The binding of Co2+ can only lead to marginal enhancement of G6-53's mechanical stability, while Co3+ has a much stronger effect. This large difference is likely due to the large difference in thermodynamic stability and kinetic lability of Co2+ and Co3+ complexes. These results opened up new avenues towards fine tuning the mechanical properties of proteins.

Ultrathin LiCoO2 Nanosheets: An Efficient Water-Oxidation Catalyst.

Ultrathin cation-exchanged layered metal oxides are promising for many applications, while such substances are barely successfully synthesized to show several atomic layer thickness, owing to the strong electrostatic force between the adjacent layers. Herein, we took LiCoO2, a prototype cation-exchanged layered metal oxide, as an example to study. By developing a simple synthetic route, we synthesized LiCoO2 nanosheets with 5-6 cobalt oxide layers, which are the thinnest ever reported. Ultrathin nanosheets thus prepared showed a surprising coexistence of increased oxidation state of cobalt ions and oxygen vacancy, as demonstrated by magnetic susceptibility, X-ray photoelectron, electron paramagnetic resonance, and X-ray absorption fine spectra. This unique feature enables a higher electronic conduction and electrophilicity to the adsorbed oxygen than the bulk. Consequently ultrathin LiCoO2 nanosheets provided a current density of 10 mA cm-2 at a small overpotential of a mere 0.41 V and a small Tafel slope of ∼88 mV/decade, which is strikingly followed by an excellent cycle life. The findings reported in this work suggest that ultrathin cation-exchanged layered metal oxides could be a next generation of advanced catalysts for oxygen evolution reaction.

Comparative Studies of SrCo1−xTaxO3−δ (x=0.05–0.4) Oxides as Cathodes for Low‐Temperature Solid‐Oxide Fuel Cells

AbstractWays to address the sluggish ORR kinetics of cathodes have become an area of great interest and significance for the deployment of solid‐oxide fuel cells (SOFCs) that operate at lower temperatures. A series of Sr(Co,Ta)O3−δ oxides with multilevel substitutions of tantalum (Ta5+) ions have been studied as cathodes for SOFCs in terms of their crystal structures, electrochemical properties, and durability. The effects of doping tantalum on the oxygen reduction reaction (ORR) have been explored and discussed. Not only can the crystallite structure of SrCoO3−δ be stabilized by incorporating Ta5+ at elevated temperatures, but the activity towards ORR can also be significantly enhanced by doping relatively small amounts of Ta5+ (≤20 mol %), which is probably due to the improved oxygen surface exchange that arises from the positive effects of Ta5+ dopants on the oxidation states of cobalt ions.

Effect of calcination temperature on oxidation state of cobalt in calcium cobaltite and relevant performance as intermediate-temperature solid oxide fuel cell cathodes

Calcium cobaltite materials are synthesized by calcining the mixture of CaCO3 and Co3O4 with the Ca: Co ratio of 3:4. The reactivity of CaCO3 with Co3O4 is evaluated by thermogravimetric analysis (TGA), X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Thermal expansion coefficient (TEC), electrical conductivity and electrochemical performance as intermediate-temperature solid oxide fuel cells (IT-SOFCs) cathode of as-prepared materials are characterized. The experiment results show that simultaneous decomposition of CaCO3 with calcium cobaltite formation occurs at 650–900 °C. The average valence for Co ions of calcium cobaltite increases with temperature in the range of 750–900 °C, involved in the formation of the compounds Ca3Co4O9 and Ca9Co12O28 at 800 and 900 °C, respectively. The performance of calcium cobaltite cathodes applied in IT-SOFCs is significantly effected by the oxidation state of cobalt ions. As a result, Ca9Co12O28 cathode has a lower area specific resistance (e.g. 41.8% lower at 800 °C) and higher peak power density (e.g. 45.0% higher at 800 °C) than the cathode of Ca3Co4O9.

Study of the oxygen desorption from GdBa1−xSrxCo2O5+δ (x = 0, 0.25, 0.5 and 1): Effect of the Sr-content on the oxidation state of cobalt ions

In this work, the crystal structure at room temperature of GdBa1−xSrxCo2O5+δ (x=0.0, 0.25, 0.5 and 1.0), prepared by a modified sol–gel route, has been investigated. In addition, total conductivity was determined by the Van der Pauw method and the existence of structural phase transitions has been investigated by differential scanning calorimetry. The latter show us that reported structural phase transitions at ∼75°C and ∼470°C for GdBaCo2O5+δ are not revealed when Ba2+ is partially substituted by Sr2+. This can be explained regarding a complete stabilization of the high temperature tetragonal symmetry at room temperature. Moreover, the effect of the increase of Sr-content on the oxidation state of cobalt ions has been studied by means of the oxygen temperature programmed desorption (O2-TPD) method.

Synthesis and Properties Investigation of Non-equivalent Substituted W-Type Hexaferrite

A novel composed W-type hexaferrite Ba1−x La x Co2Fe16O27 was rapidly synthesized via a sol–gel self-combustion reaction. The effects of lanthanum ions on the oxidation state of iron ions and cobalt ions in hexaferrite were explored by X-ray photoelectron spectroscopy. The changes of the Fe 2p X-ray absorption spectra indicated that the nonequivalent substitution can lead to the transition Fe3+→ Fe2+ in Ba1−x La x Co2Fe16O27. However, the oxidation state of cobalt ions was maintained as Co2+. Moreover, the effects of La content on the phase composition, structural parameters, morphology, and static magnetic properties were also investigated in detail by using the X-ray diffractometer, scanning electron microscope, and vibrating sample magnetometer. The results indicated that the structural parameters decreased regularly with increasing the La content, and the magnetic properties were enhanced after substitution, which is beneficial for their application in various electrical devices employed for industrial and military applications.

Stability of Co–Ce–Mn mixed-oxide catalysts for CO preferential oxidation in H 2-rich gases

The stability of Co–Ce–Mn mixed-oxide catalysts for CO preferential oxidation in H 2-rich gases was investigated. The deactivation mechanism was explored using Fourier-transform infrared spectroscopy (FTIR), temperature-programmed desorption (TPD), and reduction and oxidation techniques. The deactivation of the catalysts was observed in reaction gas streams containing CO 2. FTIR and CO 2-TPD revealed carbonate formation in the deactivated samples. The activity of the deactivated catalysts could be partially regenerated by CO 2 desorption and fully regenerated by oxidation treatment which led to a change in the oxidation state of cobalt ions. It is proposed that both carbonate accumulation and the change in the oxidation state of cobalt ions during CO preferential oxidation are responsible for the deactivation of Co–Ce–Mn catalysts.