Oxidation State Of Co Research Articles

Nanocoral CuCo2S4 thiospinels: Oxygen evolution reaction via redox interaction of metal ions

Understanding the electrocatalytic activity and redox mechanism in mixed metal sulfides has steered the present work. Mixed metal sulfide, CuCo2S4, synthesised by hydrothermal method, reveals the presence of highly crystalline nanocoral sheets of CuCo2S4 with mixed oxidation states of Co and Cu ions. Electrochemical measurements show superior oxygen evolution reaction (OER) activity and high exchange current density over CuCo2S4 in comparison with pristine (Co3S4 and Cu2S) and sintered CuCo2S4. In 0.5 M KOH, the nanocoral CuCo2S4 catalyst and its sintered form show a current density of 10 mA cm−2 at overpotential of 368 mV and 511 mV, respectively. A close inspection of the electrocatalytic performance in correlation with XPS spectra reveals the synergistic effect of Co and Cu ions through a redox interaction. Density Functional Theory calculations also indicate strong dependence of redox activity on redox couples as accompanied with changes in the partial charges of different constituent ions in the catalyst upon adsorption of hydroxyl ions which is a key step in OER. The study not only details sulfide-based catalytic compound for OER but also provides conclusive insights into the surface redox processes thereby providing an alternative direction to the search on novel OER catalysts.

Influence of Oxygen Vacancy on Pseudocapacitance of La0.8Sr0.2CoO3-δ

Since the discovery of the charge storage mechanism involving a charge compensation by anion that can happen for some pseudocapacitive materials, oxides from the perovskite family have been intensively studied during the past five years, such as LaMO3-δ (M = Mn, Fe, Co, Ni). The oxygen vacancy present in some perovskites naturally becomes the center of this new mechanism. Indeed, the vacancies are suggested to host oxygen/hydroxyl ions to compensate the oxidation of the transition metal, and to release these ions during the reduction. Interestingly, there has not been any report dedicated to reveal the correlation between the oxygen vacancy with the pseudocapacitance of these materials.We propose here the study of the influence of the oxygen stoichiometry of the perovskite La0.8Sr0.2CoO3-δ, written as LSC82, used as pseudocapacitive electrode material. LSC82 has been prepared by a urea-nitrate combustion method, and then calcined at 850°C. Materials having different oxygen stoichiometries were obtained by different cooling rates. Electrochemical tests were performed on LSC82 based electrodes in 5M LiNO3 electrolyte to reveal the correlation between the oxygen stoichiometry and pseudocapacitve performance. The oxidation states of Co and the oxygen vacancies content in the various prepared LSC82 have been determined by various technics such iodometric titration, X-ray Absorption Spectroscopy and X-ray Photoelectron Spectroscopy. By associating electrochemical tests with spectroscopic technics, it is possible to connect oxygen vacancies content with electrochemical performances.Understanding the role of oxygen vacancies could give new lead to improving the performances of such materials.

Structural evolution of ZIF-67-derived catalysts for furfural hydrogenation

Zeolitic imidazolate framework-67 (ZIF-67) can be converted to metallic Co nanoparticles supported on N-doped carbon (Co/NC) through reduction. However, its unique properties, including extremely high surface area, isoreticular pore structure, and regular metal–organic network, disappear after high-temperature (>500 °C) reduction. Aggregated CoOx particles reduce the number of surface-active sites, resulting in poor catalytic activity. If the original ZIF-67 structure is maintained after the high-temperature reduction, promoting the uniform distribution of active sites in the porous carbon, the catalytic performance can be further improved. Herein, the correlation between the catalytic furfural hydrogenation performance, Co/NC morphology, and oxidation state of Co was investigated as a function of the H2 reduction temperature and time. The reduction of ZIF-67 at 400 °C for 6 h yields a highly dispersed Co/NC catalyst, while preserving the overall morphology. The resulting Co/NC-400-6 catalyst exhibits the highest activity, promoting high selectivity toward 2-methylfuran. The product selectivity can be further altered by incorporating Cu into ZIF-67 to produce furfuryl alcohol. With proper H2 treatment to minimize the damage to the intrinsic surface area and pore structure, metal–organic frameworks can be utilized as high-performance heterogeneous catalysts by maximizing the distribution of active sites.

PtNi bimetallic structure supported on UiO-67 metal-organic framework (MOF) during CO oxidation

Supported bimetallic nanoparticles (BNPs) are promising catalysts, but study on their compositional and structural changes under reaction conditions remains a challenge. In this work, the structure of PtNi BNPs supported on UiO-67 metal–organic framework (MOF) catalyst (i.e., PtNi@UiO-67) was investigated by in situ by near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). The results showed differences in the reduction behaviour of Ni species in PtNi BNPs and monometallic Ni supported on UiO-67 catalysts (i.e., PtNi@UiO-67 and Ni@UiO-67), suggesting charge transfer between metallic Pt and Ni oxides in PtNi@UiO-67. Under CO oxidation conditions, Ni oxides segregated to the outer surface of the BNPs forming a thin layer of NiOx on top of the metallic Pt (i.e., a NiOx-on-Pt structure). This resulted in a core-shell structure which was confirmed by high-resolution scanning transmission electron microscopy (HR-STEM). Accordingly, the layer of NiOx on PtNi BNPs, which is stabilised by charge transfer from metallic Pt, was proposed as the possible active phase for CO oxidation, being responsible for the enhanced catalytic activity observed in the bimetallic PtNi@UiO-67 catalyst.

Regulating Photocatalysis by Spin-State Manipulation of Cobalt in Covalent Organic Frameworks.

While catalysis is highly dependent on the electronic structure of the catalyst, the understanding of catalytic performance affected by electron spin regulation remains challenging and rare. Herein, we have developed a facile strategy to the manipulation of the cobalt spin state over covalent organic frameworks (COFs), COF-367-Co, by simply changing the oxidation state of Co centered in the porphyrin. Density functional theory (DFT) calculations together with experimental results confirm that CoII and CoIII are embedded in COF-367 with S = 1/2 and 0 spin ground states, respectively. Remarkably, photocatalytic CO2 reduction results indicate that COF-367-CoIII exhibits favorable activity and significantly enhanced selectivity to HCOOH, accordingly much reduced activity and selectivity to CO and CH4, in sharp contrast to COF-367-CoII. The results highlight that the spin-state transition of cobalt greatly regulates photocatalytic performance. Theoretical calculations further disclose that the presence of CoIII in COF-367-Co is preferable to the formation of HCOOH but detrimental to its further conversion, which clearly accounts for its distinctly different photocatalysis over COF-367-CoII. To the best of our knowledge, this is the first report on regulating photocatalysis by spin state manipulation in COFs.

Metal-Specific Reactivity in Single-Atom Catalysts: CO Oxidation on 4d and 5d Transition Metals Atomically Dispersed on MgO.

Understanding and tuning the catalytic properties of metals atomically dispersed on oxides are major stepping-stones toward a rational development of single-atom catalysts (SACs). Beyond individual showcase studies, the design and synthesis of structurally regular series of SACs opens the door to systematic experimental investigations of performance as a function of metal identity. Herein, a series of single-atom catalysts based on various 4d (Ru, Rh, Pd) and 5d (Ir, Pt) transition metals has been synthesized on a common MgO carrier. Complementary experimental (X-ray absorption spectroscopy) and theoretical (Density Functional Theory) studies reveal that, regardless of the metal identity, metal cations occupy preferably octahedral coordination MgO lattice positions under step-edges, hence highly confined by the oxide support. Upon exposure to O2-lean CO oxidation conditions, FTIR spectroscopy indicates the partial deconfinement of the monatomic metal centers driven by CO at precatalysis temperatures, followed by the development of surface carbonate species under steady-state conditions. These findings are supported by DFT calculations, which show the driving force and final structure for the surface metal protrusion to be metal-dependent, but point to an equivalent octahedral-coordinated M4+ carbonate species as the resting state in all cases. Experimentally, apparent reaction activation energies in the range of 96 ± 19 kJ/mol are determined, with Pt leading to the lowest energy barrier. The results indicate that, for monatomic sites in SACs, differences in CO oxidation reactivity enforceable via metal selection are of lower magnitude than those evidenced previously through the mechanistic involvement of adjacent redox centers on the oxide carrier, suggesting that tuning of the oxide surface chemistry is as relevant as the selection of the supported metal.

Investigation on electronic structure and magnetic properties of Co and Mn incorporated nanoscale SnO2

The effect of Co and Mn co-doping and annealing on electronic structure and magnetic properties of nanoscale SnO2 were investigated using complementary experimental techniques. The high resolution transmission electron microscope (HRTEM) image showed spherical-like morphology and signalled the nanoscale regime of investigated samples. The results of X-ray photoelectron spectroscopy (XPS) analysis indicated the oxidation state of Co and Mn mostly as 2+. Moreover, XPS and Raman analysis corroborated the formation of oxygen vacancies as result of substitution tetravalent Sn with divalent Co and Mn. The XPS valence band spectra unveiled the formation of occupied states above the top of the valence band in as-synthesized Co and Mn co-doped SnO2 samples. Magnetic measurements disclosed the paramagnetic nature for the as-synthesized Co and Mn co-doped SnO2 samples and also demonstrated the dominating antiferromagnetic exchange interactions between the magnetic moments of Co and Mn. Interestingly, the annealed samples (Sn0.96Co0.02Mn0.02O2, Sn0.94Co0.02Mn0.04O2 and Sn0.94Co0.04Mn0.02O2) exhibited weak room temperature ferromagnetism (RTFM). The observed RTFM was explained based on the bound magnetic polaron (BMP) model. The comprehensive experimental investigations provide an interesting insight into the magnetic behaviour within the co-doped system.

On the surface-dependent oxidation of Cu2O during CO oxidation: Cu2+ is more active than Cu+

Solid materials can exhibit very different surface catalytic properties depending on the exposed surface. Different from the previous finding by Bao et al. (Angew. Chem. Int. Ed. 50 (2011) 12294–12298) that Cu2O octahedra are more active than Cu2O cubes in CO oxidation, we found that Cu2O cubes exposing {100} surface showed significantly higher activity than Cu2O octahedra exposing {111} surface in the reaction. The Cu2O(100) surface was found to be oxidized during CO oxidation, leading to the formation of a CuO surface layer. On the other hand, Cu2O(111) strongly resisted such surface oxidation under CO oxidation conditions. Density functional theory calculations demonstrate that CO oxidation follows a Mars-Van Krevelen mechanism on Cu2O(111) with a high activation barrier of 1.36 eV. In contrast, the overall activation barrier is only 0.58 eV for CuO(111). These differences explain well the higher activity of oxidized Cu2O cubes.

The efficacy of Co3O4 loaded WO3 sheets for the enhanced photocatalytic removal of 2,4,6-trichlorophenol in natural sunlight exposure

Owing to the promising photocatalytic performance, the sheet-like WO3 was modified by depositing nanostructured Co3O4 at the surface. The appearance of the varying dual absorption edges in the optical analysis exposed the composite nature of the synthesized materials. The structural analysis revealed the deposition of Co3O4 particles at the surface without altering the lattice of WO3 however, during the processing the cracking of disc was also evidenced. The FESEM and HRTEM analysis corroborated the uniform surface dispersion of Co3O4 nanoparticles. The co-existence of 2+ and 3+ oxidation states of Co in the deposited Co3O4 was examined by XPS analysis. The efficient trapping of excitons by Co3O4 surface entities was witnessed in the emission measurements whereas the same was authenticated by the photo-electrochemical chronopotentiometry. The Co3O4 loaded sheets exhibited substantially enhanced activity for the removal of 2,4,6-trichlorophenol as compared to pure WO3 in the complete spectrum and visible region of natural sunlight exposure. The progress of the degradation process was monitored by HPLC whereas the degradation products were identified by GC–MS. The measurement and identification of the ion released during the photocatalytic process facilitated the estimation of the probable route and role of reactive oxygen species involved in the removal process. Although, the careful analysis of the findings from the analytical tools revealed the major involvement of hydroxyl radicals however, the role of superoxide anions was also exposed. An alternative mechanism of the generation of the superoxide radical involving the 2+ and 3+ oxidation states of Co was also proposed. The effect of the pH and the added concentration of H2O2 on the ease of removal process was also investigated.

Synthesis, structure, and properties of the multinuclear cobalt core POM Na14[Co3O(H2O)(A-α-PW9O34)2]·~29.5H2O

A new trimeric Co(II) containing Keggin trilacunary polyoxometalate (POM) sandwich complex, Na14[Co3O(H2O)(A-α-PW9O34)2]·~29·.5H2O (Na14-1) was synthesized by the reaction of [Co2(μ- OH2)(O2CCMe3)4(HO2CCMe3)4] and Na9[A-α-PW9O34]·7H2O. The compound has been characterized by elemental analysis, FTIR, TGA and single crystal XRD (Triclinic space group P- 1, a = 12.2204(8) Å, b = 16.7035(11) Å, c = 23.3466(16) Å, α = 78.448(2)°, β = 80.551(2)°, γ = 78.448(2)° and Z = 2). The POM was electrochemically active and its behavior was studied by cyclic voltammetry at different scan rates. Bond valence sum (BVS) calculation was used to determine the oxidation state of Co. The magnetic susceptibility study indicated antiferromagnetic interactions between Co(II) ions and at lower temperature ferromagnetic behavior dominant in the structure.

Nanostructured calcium cobalt oxide Ca3Co4O9 as thermoelectric material. Effect of nanostructure on local coordination, Co charge state and thermoelectric properties

The preparation of pure Ca3Co4O9 materials in the form of dense bodies made of nano-sized grains was investigated by combining a sol-gel route, different thermal cycles, a ball-milling post-treatment and final densification with HP-FAST (High Pressure-Field Assisted Sintering). We found that an effective way for obtaining nano-sized compact bodies with only a marginal increase of the particle sizes of the original powder was the operation of HP-FAST at extremely high pressures (up to 430 MPa) and comparatively low temperatures. A ball-milling treatment before HP-FAST compaction was then required to hinder the large plastic deformation occurring when using these pressures. In contrast, un-milled powders could be densified by HP-FAST only at lower pressure and therefore required higher temperatures, thus losing their nanostructure.For all powders, X-ray absorption spectroscopy assessed a mean Co oxidation state slightly higher than III, coupled to absence of localized Co(II) and presence of localized Co(IV). Ball milling induced a decrease of the mean oxidation state of Co coupled to an increased disorder. The latter effect was confirmed by X-ray Absorption Fine Structure.Nanostructure had a complex effect on the different properties of compact bodies. According to the different conditions of HP-FAST, densities of 75–98% were achieved; the decrease in thermal conductivity was of a factor 2–3, while the electronic transport properties – in particular electrical conductivity – of nano-sized compact bodies were reduced to a larger extent with respect to their micro-sized counterparts. This demonstrates that, for bulk Ca3Co4O9 materials, nanostructure was not an effective approach toward a performing thermoelectric material.

Template-free Synthesis of Mesoporous and Crystalline Transition Metal Oxide Nanoplates with Abundant Surface Defects

Summary Mesoporous transition metal oxides with crystalline walls show promising applications in catalysis and energy storage. Syntheses of those crystalline mesoporous oxides, however, currently remain as an unmet challenge because of the large volume shrinkage during crystallization in a sol-gel process. Here, we demonstrate a facile and general template-free method to prepare six mesoporous transition metal oxides, e.g., CuO, CoO, and spinel MCo2O4 (M = Co, Cu, Mn, Zn), with highly crystalline walls and continuous mesopores in the forms of two-dimensional nanoplates or nanosheets. The method is based on the thermal crystalline transformation from basic carbonates to mesoporous metal oxides. The crystal interconversion not only endows the crystallinity and mesoporosity of oxides but also generates abundant surface-step defects that show remarkable enhancement of the catalytic activity of porous oxides. Our method likely provides an alternative paradigm for cost-effective and universal synthesis of crystalline mesoporous oxides beyond the traditional templating methods.

Electronic and magnetic properties of structural defects in SrTiO3(Co)

The synthesis conditions of SrTiO3(Co) samples in which cobalt predominantly enters the A or B sites of the perovskite ABO3 structure are found. EXAFS studies show that the Co impurity at the A site is off-center and is displaced from the site by 1.0 Å. XANES studies reveal two predominant oxidation states of Co: Co2+ at the A site and Co3+ at the B site. First-principles calculations of a number of possible cobalt-containing structural defects reveal defects whose properties are compatible with the experimentally observed Co oxidation state, its local structure, magnetic, electrical, and optical properties.

Novel nano-architectured water splitting photoanodes based on TiO2-nanorod mats surface sensitized by ZIF-67 coatings

We report on the synthesis, characterization and application to water photo-dissociation of micrometer-long self-ordered TiO2 nanorod (TDNR) mats, surface sensitized by deposition of cobalt dodecahedral Zeolitic Imidazolate Framework (ZIF-67) coatings. TDNR mats have been grown over glass/FTO substrates from titanium (IV) butoxide, by using a solvothermal procedure at 150 °C. Homogeneous micrometer-thick mats of ~120 nm wide TDNRs (surface density ~15–20 nanorods per μm2) have been obtained. The TDNR mats have been impregnated by crystalline coatings of ZIF-67 MOF. Four different photoelectrodes, containing different amounts of ZIF-67, have been prepared and characterized. X-ray photoelectron spectroscopy analysis revealed that the oxidation state of Co is CoII. The presence of MOF at the surface of TDNRs has been found to greatly impact the optical properties of TiO2: light absorption is shifted to the Vis region. The apparent band-gaps of the composites were determined from Tauc's plots. The photoelectrodes have been used as photoanodes for water photooxidation in Na2SO4 aqueous solutions. I–V curves have been measured under chopped illumination conditions by linear sweep voltammetry (LSV), under Vis and UV–Vis irradiation. The dynamics of charge transfer at the interface has been investigated by photoelectrochemical impedance spectroscopy (PEIS). In the dark, all photoelectrodes present a capacitive behavior with a high impedance. Under Vis irradiation, only one time constant is observed, and all photoelectrodes impedances are found in the 105–106 Ω cm2 range. Such values are consistent with the observed photocurrents. Under UV–Vis light, PEIS spectra appeared flattened and therefore the data have been fitted with two time constants, with a model considering the presence of surface states at the interface. The photoanode presenting the highest photocurrent (16 h of MOF growth) also displays the lowest value for the charge transfer resistance from trap states, clearly evidencing the role of the MOF as a surface co-catalyst with a beneficial effect for water splitting. The presence of MOF at the surface has been also found to increase the rate of charge recombination. The observed photocurrent has been found to be a result of the dynamic interplay between charge transfer and recombination microscopic processes.

Influence of CO oxidation conditions on the Mn-Zr oxide catalyst structure: In situ XRD and MS study

Abstract Mn-Zr oxide catalyst was studied by X-ray powder diffraction and mass-spectrometry under CO oxidation reaction conditions with different CO/O2 ratio. Mn-Zr catalyst consists of manganese oxides and MnyZr1–yO2–x solid solution and was found to be stable in the environment with stoichiometric ratio of CO/O2 = 2:1 and with excess of oxygen. When the catalyst is treated in the reducing environment (pure CO and CO/O2 = 4:1) the transformation of manganese oxides to MnO and reduction of manganese cations in the structure of MnyZr1–yO2–x are observed. This process is reversible and reoxidation leads to the formation of the catalyst similar to the initial.

Investigation of the oxygen storage capacity behaviour of three way catalysts using spatio-temporal analysis

A transient lean-rich switching method has been developed in conjunction with the spatially resolved capillary inlet mass spectrometry (SpaciMS) technique using CO oxidation as a probe reaction. This has allowed in-situ spatiotemporal mapping of gaseous species and temperature profiles inside the channels of commercial three way 1%Pd/0.07%Rh/Al2O3/CexZr1-xO2 and 3%Pd/Al2O3 monolith catalysts. The identification of the spatiotemporal reactant conversion, intermediate and product formation, as well as the associated heat evolution, has helped in the elucidation of reaction sequences and furthermore, the postulation of reaction mechanisms, both axially and temporally inside the monolith catalyst. Intriguingly in this work, the SpaciMS technique has provided previously unobserved insights into an apparent excess OSC-like behaviour of the 3%Pd/Al2O3 catalyst, where excess CO conversion is detected under rich CO oxidation conditions in the presence of water. It has been demonstrated that the water gas shift reaction is insufficient to account for the excess CO conversion in this work, therefore a Pd(OH)x surface species is hypothesised to be acting in the same way as an oxygen storage component under rich conditions.

Hydration of gadolinium oxide ( GdOx ) and its effect on voltage-induced Co oxidation in a Pt/Co/GdOx/Au heterostructure

Magneto-ionic control of magnetism has garnered great interest in recent years due to the large magnetic changes that can be induced using a relatively small voltage. One model structure for this is $\mathrm{Pt}/\mathrm{Co}/\mathrm{Gd}{\mathrm{O}}_{x}/\mathrm{Au}$, where Co is the magnetic layer and $\mathrm{Gd}{\mathrm{O}}_{x}$ is the ionic conductor, with the magnetic properties dependent on the oxidation state of Co. While this structure is commonly used, there is limited understanding of the effect of $\mathrm{Gd}{\mathrm{O}}_{x}$ properties on voltage-induced magnetic changes. In this work, we show that hydration of $\mathrm{G}{\mathrm{d}}_{2}{\mathrm{O}}_{3}$ to form $\mathrm{Gd}{(\mathrm{OH})}_{3}$ is crucial for voltage-induced Co oxidation in a $\mathrm{Pt}/\mathrm{Co}/\mathrm{Gd}{\mathrm{O}}_{x}/\mathrm{Au}$ device. By examining the rate of Co oxidation in nonhydrated and hydrated devices, we conclude that ${\mathrm{H}}_{2}\mathrm{O}$ in the $\mathrm{Gd}{\mathrm{O}}_{x}$ layer acts as an oxidant during the voltage-induced Co oxidation process. Co oxidation through this interfacial reaction process is confirmed by in situ x-ray absorption spectroscopy.

Enhancement of thermoelectric power factor by inducing octahedral ordering in La2−xSrxCoFeO6 double perovskites

Among the various oxide thermoelectric materials, double perovskites provide more flexibility to maneuver interdependent thermoelectric parameters to achieve enhanced thermoelectric figure of merit (ZT), as octahedral ordering, i.e., arrangement of ${B}^{\ensuremath{'}}{\mathrm{O}}_{6}$ and ${B}^{\ensuremath{''}}{\mathrm{O}}_{6}$ octahedra, present in the ${A}_{2}{B}^{\ensuremath{'}}{B}^{\ensuremath{''}}{\mathrm{O}}_{6}$ structure is impacted by cation doping. In this work, we investigated the role of octahedral distortion on thermoelectric properties of $\mathrm{L}{\mathrm{a}}_{2\ensuremath{-}x}\mathrm{S}{\mathrm{r}}_{x}\mathrm{CoFe}{\mathrm{O}}_{6}$ (LSCF) double perovskites with $0.0\ensuremath{\le}x\ensuremath{\le}1.0$, synthesized by the autocombustion route. Rietveld refinement of x-ray diffraction data revealed the phase transition with increasing Sr concentration ($x$) in LSCF from rhombohedral crystal structure with R-3c space group $(x\ensuremath{\le}0.6)$ to monoclinic $P{2}_{1}/n$ ($0.8\ensuremath{\le}x\ensuremath{\le}1.0$) space group. X-ray photoemission spectroscopy analysis further confirmed the presence of multiple oxidation states of Co and Fe, and shifts in oxidation states population driven by Sr content. These multivalent cations participated in the charge transport mechanism, which was explained by the small polaron hopping conduction model in these double perovskites. The electrical conductivity at room temperature was found to be increased by more than ${10}^{7}$ times in LSCF due to Sr doping, causing a large enhancement in the thermoelectric power factor. Gradual decrease in the octahedral tilt angle with increasing Sr content in LSCF, leading toward the change of crystal structure from disordered (R-3c) to rock-salt-ordered ($P{2}_{1}/n$) double perovskites, was found to be responsible for the large decrease in activation energy barrier for small polaron hopping conduction in the LSCF system, resulting in the phenomenal increase in electrical conductivity. Maximum thermoelectric figure of merit, $\mathrm{ZT}=0.11$ was obtained at 723 K for $\mathrm{L}{\mathrm{a}}_{2\ensuremath{-}x}\mathrm{S}{\mathrm{r}}_{x}\mathrm{CoFe}{\mathrm{O}}_{6}$ with $x=0.2$ composition.

Promoting hydrogen-evolution activity and stability of perovskite oxides via effectively lattice doping of molybdenum

Electrocatalysts are the most compelling objectives in realizing highly efficient renewable energy conversion and storage applications. Rational doping is an effective strategy for the development of cost-effective perovskite oxides with high electrochemical performance. In this study, we report facilely prepared molybdenum (Mo)-doped SrCo0.70Fe0.30O3-δ perovskites such as SrCo0.7Fe0.25Mo0.05O3-δ (SCFM0.05) and SrCo0.7Fe0.20Mo0.10O3-δ (SCFM0.10) for boosting the hydrogen evolution reaction (HER) activity and stability. Among them, SCFM0.05 delivers a promising overpotential of ∼323 mV at the current density of 10 mA cmdisk-2 and keeps almost stable for 5 h and after accelerated 1000 cycles. The promoted HER activity of SCFM0.05 regarding the decreased overpotential, increased catalytic current density, and improved charge transfer kinetics, might originate from the combined effects of distortion of octahedral coordination, low oxygen vacancy/high oxidation state of Co, abundant lattice oxygen and highly oxidative oxygen species, long B–O length, and strong OH− adsorption compared to the un-doped counterpart. We ascribe the enhanced operational stability to the formation of a low concentration of oxygen vacancy that stabilizes the crystal structure of Mo-doped SrCo0.7Fe0.3O3-δ and prevents the surface from Sr leaching/surface amorphization. These findings suggest that tuning perovskite oxide using a redox-inactive dopant featured with high valence state may provide further avenues to HER optimization.

Structure and thermoelectric properties of calcium doped Sr2TiCoO6 double perovskites

Present work investigates the effect of calcium doping on the structure and thermoelectric properties of Sr2TiCoO6 (STC) double perovskite. Environmentally friendly rare earth free CaxSr2−xTiCoO6 (0.0 ≤ x ≤ 0.3) (CSTC) ceramics were synthesized using solid state synthesis route. CSTC ceramics were found to possess a cubic crystal structure with Pm3¯m space group as confirmed by Rietveld refinement of XRD data. Microstructure analysis was carried out by SEM and energy dispersive x-ray spectroscopy. The temperature dependent electrical conductivity and thermopower of these oxides showed the semiconductor to metal transition at around 700 K. X-ray photoelectron spectroscopy (XPS) confirmed the presence of multiples oxidation states of Co and Ti creating defect sites in these oxides. CSTC ceramic samples exhibited positive Seebeck coefficient implying p-type behavior. Charge transport mechanism of all the CSTC samples was found to be governed by small polaron hopping model.