La1 xSrxCoO3 Research Articles

Positive effects of the Sr doping on LaCoO3 perovskites for simultaneous catalytic oxidation performances of NO and Hg0

LaCoO3 perovskite, with a general formula of ABO3, have received significant attention as environmental functional catalysts for NO and Hg0 oxidation owing to their compositional adjustability and chemical stability. Structural modifications of the perovskite have been performed through the substitution of La atoms at the A-site with atoms of different ion radii to enhance the oxidation ability. In this study, herein, a series of La1–xSrxCoO3 (x = 0.1–0.5) perovskites were synthesized, resulting in a transition from the R–3C rhombohedral phase structure to the Pm-3m cubic phase. Sr doping was effective in improving the oxidation of NO and Hg0. The improved catalytic activity was attributable to the generation of more electronegative Co3+ and the higher content of surface-adsorbed oxygen species. Moreover, Sr doping enhanced the redox performance and reduced the reaction temperature. Notably, SrCO3 species were the main factors responsible for the reducing the activation of La1–xSrxCoO3 (x = 0.1–0.5) perovskites, which could be detected when the Sr doping content exceeded 0.4. This study provides a screening strategic idea for the designing highly active perovskite catalysts.

Spray-flame-synthesized Sr- and Fe-substituted LaCoO3 perovskite nanoparticles with enhanced OER activities

La1–xSrxCoO3 (x = 0, 0.1, 0.2) and LaCo0.8Fe0.2O3 perovskite nanoparticles were synthesized by spray-flame synthesis, and their electrocatalytic water oxidation activity was evaluated in the prepared state. Highly crystalline, rhombohedrally distorted cubic structures of the cobaltite perovskites were confirmed by STEM and XRD analyses. The decreased JT distortions were observed in the cobaltite perovskite structure upon substitution with Sr, while orthorhombic distortions in Fe-substituted perovskites could explain the changes in their Raman spectra. Debye–Waller factors and coordination numbers from fitted EXAFS data indicate more disordered crystalline structures upon Sr substitution and a lower Co–O coordination number at 20 at% Sr. Thermal characterization of the catalysts by STA coupled with QMS shows higher mass losses in Sr-substituted catalysts, and these results were associated with a higher concentration of carbonate species in these catalysts, which was also confirmed by XPS measurements. Both Fe and Sr substitution lead to higher catalytic OER activity of the cobaltite perovskites with lower overpotentials of about 30–50 mV. The cobaltite perovskite catalyst substituted with 20 at% Sr exhibited the highest OER activity and stable electrocatalytic performance at moderate conditions.

State of the Active Site in La1–xSrxCoO3−δ Under Oxygen Evolution Reaction Investigated by Total-Reflection Fluorescence X-Ray Absorption Spectroscopy

Recent developments in hydrogen energy devices have furthered the research on sustainable hydrogen production methods. Among these, the water splitting process has been considered a promising hydrogen production method, particularly, in alkaline media. The lack of information on the reaction active sites under the conditions of the oxygen evolution reaction (OER) hinders establishing guidelines for catalyst development. In the case of powder catalysts, many operando techniques also measure bulk information, and therefore, extracting information on the reaction active sites is challenging. Accordingly, film electrodes were used in this study, and the electrochemical performance and reaction kinetics of perovskite-type La1–xSrxCoO3−δ films as OER catalysts synthesized by pulsed laser deposition were investigated. By combining ex situ X-ray absorption spectroscopy (XAS) and operando total-reflection fluorescence X-ray absorption spectroscopy (TRF-XAS), we succeeded in observing a significant oxidation state change on the surface of La0.6Sr0.4CoO3−δ, which indicated that the active surface sites were formed upon applying the OER potential. This surface reconstruction resulted in numerous active sites at the reaction interface, thereby enhancing the OER activity. This study provides definitive evidence for the surface reconstruction of OER catalysts, which enhances the fundamental understanding of OER catalyst behaviors, and can inspire the design of active OER catalysts by suitable surface modulation.

Determining the Oxygen Stoichiometry of Cobaltite Thin Films

Transition metal oxides (TMOs) are promising materials to realize low-power neuromorphic devices. Their physical properties critically depend on their oxygen vacancy concentration, whose experimental determination remains a challenging task. Here, we focus on cobaltites, in particular La1–xSrxCoO3−δ (LSCO), and present a strategy to identify fingerprints of oxygen vacancies in X-ray absorption (XA) spectra. Using a combination of experiments and theory, we show that the variation of the oxygen vacancy concentration in the perovskite phase of LSCO is correlated with the change in the relative peak positions of the O K-edge XA spectra. We also identify an additional geometrical fingerprint that captures both the changes in the Co–O bond length and Co–O–Co bond angle in the material due to the presence of oxygen vacancies. Finally, we predict the oxygen vacancy concentration of experimental samples and show how the resistivity of the oxide material may be tuned as a function of the defect concentration, in the absence of any structural transformation. Our study shows that, in order to predict the complex transport properties of TMOs, it is crucial to gain a detailed understanding of their oxygen defect density.

Pt4+ as an Active Site for Oxygen Evolution Reaction in La1–xSrxCo1–yPtyO3

This study presents an investigation on the role of Pt4+ in Pt-doped La1–xSrxCoO3 (LSPtCO) perovskite for oxygen evolution reaction (OER) activity. LSPtCO with Pt in the +4 oxidation state shows th...

Confined Ir single sites with triggered lattice oxygen redox: Toward boosted and sustained water oxidation catalysis

Confined Ir single sites with triggered lattice oxygen redox: Toward boosted and sustained water oxidation catalysis

Correlating the electronic structure of perovskite La1−xSrxCoO3 with activity for the oxygen evolution reaction: The critical role of Co 3d hole state

Perovskite LaCoO3 is being increasingly explored as an effective low-cost electrocatalyst for the oxygen evolution reaction (OER). Sr doping in LaCoO3 (La1−xSrxCoO3) has been found to substantially increase its catalytic activity. In this work, we report a detailed study on the evolution of the electronic structure of La1−xSrxCoO3 with 0 ≤ x ≤ 1 and its correlation with electrocatalytic activity for the OER. A combination of X-ray photoemission spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) was used to unravel the electronic density of states (DOS) near the Fermi level (EF), which provide insights into the key electronic structure features for the enhanced OER catalytic activity. Detailed analysis on the Co L-edge XAS suggest that LaCoO3 has a low spin state with t2g6eg0 configuration at room temperature. This implies that the high OER catalytic activity of LaCoO3 should not be rationalized by the occupancy of eg = 1 descriptor. Substituting Sr2+ for La3+ in LaCoO3 induces Co4+ oxidation states and effectively dopes hole states into the top of valence band. A semiconductor-to-metal transition is observed for × ≥ 0.2, due to the hole-induced electronic DOS at the EF and increased hybridization between Co 3d and O 2p. Such an electronic modulation enhances the surface adsorption of the *OH intermediate and reduces the energy barrier for interfacial charge transfer, thus improving the OER catalytic activity in La1−xSrxCoO3. In addition, we found that the La1−xSrxCoO3 surface undergoes amorphization after certain period of OER measurement, leading to a partial deactivation of the electrocatalyst. High Sr doping levels accelerated the amorphization process.

Predicting the Onset of Metal–Insulator Transitions in Transition Metal Oxides—A First Step in Designing Neuromorphic Devices

Several transition metal oxides, including La1–xSrxCoO3, are promising materials to realize resistive switching devices for neuromorphic applications, as they can undergo a metal-to-insulator transition (MIT) upon small perturbations to their electronic structure. Here, we focus on the MIT induced by varying the oxygen vacancy concentration, and we propose a model to predict the required electrical bias, based on three fundamental quantities: the vacancy formation energy, the permanent polarization in the material, and its dielectric constant. We obtain these quantities using first-principles electronic structure calculations, and we discuss strategies to minimize the electrical bias which may be adopted experimentally to design and optimize neuromorphic devices.

Molten Salt Synthesized Submicron Perovskite La1–xSrxCoO3 Particles as Efficient Electrocatalyst for Water Electrolysis

Perovskite oxides are an important and effective class of mixed oxides which play significant role in the fields of energy storage and conversion systems. Here we present a series of cobaltite perovskite LaCoO3 particles which have been doped with 0, 5, 10, 20 and 30% of Sr2+ and have been synthesized by a combined sol-gel and molten-salt synthesis procedure, which provides regular morphology of the particles. These Sr2+ doped LaCoO3 particles have been characterized by powder X-ray diffraction, Raman spectroscopy, infrared spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy. Moreover, these Sr2+ doped LaCoO3 particles have been demonstrated as efficient catalyst for oxygen evolution reaction (OER) based on the measured specific capacitance, total charge, most accessible charge, electrochemically active surface area, and roughness factor using rotating disk and rotating ring-disk electrode techniques. The 30% Sr2+ doped LaCoO3 sample shows enhanced electrocatalytic OER activity in 0.5 M H2SO4 media compared to the LaCoO3 samples doped with 0, 5, 10, 20% Sr2+. Among all five LaCoO3 samples, the doped LaCoO3 samples demonstrate better OER activity than the undoped sample.

Surface Segregation Acts as Surface Engineering for the Oxygen Evolution Reaction on Perovskite Oxides in Alkaline Media

La1–xSrxCoO3-δ perovskites are potential catalysts for the anodic reaction of alkaline water electrolyzers, i.e., the oxygen evolution reaction (OER). It is well-known that La1–xSrxCoO3−δ perovskites can easily display strontium surface segregation, but how this influences the performance of La1–xSrxCoO3−δ perovskites as anodic electrode in alkaline water electrolyzers, particularly in terms of OER activity, has not been unveiled yet. This study focuses on La0.2Sr0.8CoO3−δ, which shows relatively high activity for the OER, and reveals the influence of the preparation temperature on the amount and morphology of segregated strontium-containing islands. Thin film samples were prepared at different temperatures by using pulsed laser deposition. Those samples were then characterized with synchrotron-based X-ray photoelectron spectroscopy “as prepared” and after being immersed in ultrapure water. We found that higher preparation temperatures enhance the segregation of strontium, which is then almost quantitatively removed by washing the samples with ultrapure water. After immersion in water, the samples expose a cobalt-rich surface. Investigating the OER activity as a function of the perovskite deposition temperature, it has been found that the higher the deposition temperature (i.e., the more extended the strontium segregation), the higher the OER activity. Such an effect has been linked to the higher amount of cobalt accessible after removing the strontium segregated islands.

Influence of strontium content on the oxygen surface exchange kinetics and oxygen diffusion in La1–xSrxCoO3–δ oxides

Oxygen surface exchange kinetics between the gas phase oxygen and La1–xSrxCoO3–δ (x = 0, 0.3, 0.4, 0.6) oxides at 0.67 kPa oxygen pressure in the 600–850°°С range were investigated. The mechanism within the two-step model including two consecutive stages, oxygen dissociative adsorption and oxygen adatom incorporation into the oxide lattice, was analysed. The oxygen diffusion coefficient (D), the rate of heterogeneous oxygen exchange (rH) and the elementary rates of this process — dissociative adsorption (ra) and incorporation (ri) — were determined. The effect of oxygen vacancy concentration, strontium segregation on the surface, temperature and oxygen pressure on rH, ra and ri are discussed.

Thermoelectric properties of strained, lightly-doped La1–x­SrxCoO3 thin films

We report on the thermoelectric properties of strained, highly-oriented, lightly-doped La1–xSrxCoO3 thin films, grown on MgO (100) substrates by pulsed laser deposition. The large lattice mismatch (∼9%), along with the different thermal expansion coefficients, between La1–xSrxCoO3 and MgO, result in the incoherent growth of thin films and the generation of a tensile thermal strain (∼1%). Room temperature resistivity values of the films are similar to those of bulk samples, while the temperature dependence of resistivity of the films does not seem to be affected by the thermal strain. The Seebeck coefficient values of all investigated films are positive but lower than those of their bulk counterparts, throughout the measured temperature range. The lower values of the films' Seebeck coefficient are attributed to the stabilization of mixed Co spin state configuration, which is induced by the thermal strain. As a result of the reduced Seebeck coefficient values, the thermoelectric power factor is suppressed, suggesting that the tensile thermal strain does not favor the thermoelectric behavior of the La1–xSrxCoO3.

Electrical properties of the LaLiyCo1 – yO3 – δ (0 ≤ y ≤ 0.10) oxides

The effect of the Li ion concentration on the phase composition, the electrical conductivity, and the thermoelectric power of the LaLi y Co1–yO3–δ (0 ≤ y ≤ 0.1) oxides synthesized by cocrystallization has been studied. It is found that the region of the perovskite-like solid solution LaLi y Co1–yO3–δ is no higher than y = 0.037. In the temperature range 300–1020 K, lithium alloying leads to an increase in the electrical conductivity and a decrease in the positive thermoelectric power of the single-phase samples compared to LaCoO3–δ. The results are discussed using the density of states model proposed by Senarus Rodriguez and Goodenough for LaCoO3–δ and La1–xSr x CoO3–δ and using the Mott theory of noncrystalline substances.

Defect Chemistry and Electrical Conductivity of Sm-Doped La1–xSrxCoO3−δ for Solid Oxide Fuel Cells

We have calculated the electrical conductivity of the solid oxide fuel cell (SOFC) cathode contact material La1–xSrxCoO3−δ at 900 K. Experimental trends in conductivity against x, and against δ for fixed x, are correctly reproduced for x ≲ 0.8. Furthermore, we have studied the chemistry of neutral and charged intrinsic and extrinsic defects (dopants) in La0.5Sr0.5CoO3 and have calculated the conductivity of the doped systems. In particular, we find that doping with Sm on the La site should enhance the conductivity, a prediction that is subsequently confirmed by electrical conductivity measurements.

Fabrication of La1-XSrxCoO3–Sc2O3–ZrO2 Composite Cathode By Electrochemically Assisted Deposition for Solid Oxide Fuel Cells

We developed a novel method for the fabrication of La1-xSrxCoO3–Sc2O3–ZrO2 (LSC–SSZ) composite cathode for solid oxide fuel cells (SOFC) via electrochemically assisted deposition of La1-xSrxCoO3 (LSC) into a porous Sc2O3–ZrO2 (SSZ) scaffold. To do so, a highly porous SSZ scaffold was fabricated by screen printing an SSZ ink containing 30 wt % PMMA pore-former onto a dense SSZ electrolyte layer of a NiO–SSZ|SSZ half-cell. The porous SSZ scaffold was made electrically conductive for the electrodeposition process by carbon deposition in a flowing N2/ethylene environment at 800 °C. Subsequently, lanthanum, strontium and cobalt were simultaneously deposited into the porous carbon coated SSZ scaffold by electrochemically assisted deposition involving nitrate reduction and hydroxide precipitation processes. To form an LSC perovskite phase, the button cells were heat treated in air at 800 °C for 5 h. The LSC–SSZ composite cathode functional layer was characterized by the SEM, EDS and XRD analysis. The results indicated a formation of LSC into the porous SSZ scaffold in the form of dendrites initially and as the deposition time was increased the LSC dendrites were thickened and finally formed a dense layer on the top of SSZ scaffold. The deposition was found to be uniform inside the SSZ scaffold up till the cathode/electrolyte interface. The electrochemical testing of button cells showed a high performance, which indicated that the developed technique is promising and it should be further explored to fabricate high performance SOFCs.

Molecular and Dissociative Adsorption of Oxygen on Solid Oxide Fuel Cell Cathode Materials of La1–xSrxCoO3, La1–xSrxCo1–yFeyO3, and La1–xSrxMnO3 Studied by Temperature-Programmed Desorption

Molecular and dissociative adsorption of oxygen on the surfaces of solid oxide fuel cell cathodes of La0.8Sr0.2CoO3, La0.6Sr0.4CoO3, La0.6Sr0.4Co0.2Fe0.8O3, and La0.8Sr0.2MnO3 were investigated using temperature-programmed desorption measurements for molecularly and atomically adsorbed oxygen. The desorption of molecularly adsorbed oxygen occurred at 140–150 K, and the adsorption sites for molecularly adsorbed oxygen were mostly oxygen vacancy sites that were formed by combinative desorption of surface oxygen atoms at 550–580 K. In the case of La1–xSrxCoO3, the recovery of oxygen vacancy sites required oxygen adsorption above 523 K, which indicates that the dissociative adsorption of oxygen at the oxygen vacancy sites was an activation process. Oxygen adsorption is the first step in the cathode reactions of fuel cells, and the oxygen adsorption properties of each cathode material are discussed with respect to the activity of the cathode.

Preparation of p-type semiconductor perovskite La1–xSrxCoO3 films and their p–n heterostructure devices

Semiconductor La0.67Sr0.33CoO3 thin films has been investigated as function of oxygen atmosphere during film growth in pulsed laser deposition. While amount of oxygen greatly depends on an oxygen atmosphere during film growth, in condition of oxygen atmosphere from 4Pa to 100Pa, the carrier in La0.67Sr0.33CoO3 thin films were found to be positive, exhibiting p-type conduction. Furthermore, we prepared (p)-La0.67Sr0.33CoO3/(i)-CeO2/(n)-InGaZnOx heterostructure devices and investigated electrical properties. These results showed rectifying behavior was clearly observed in heterostructure at room temperature. The sharp increase of current with voltage, when voltage exceeds diffusion potential, indicates the conductive resistance was small. p-type semiconductor La0.67Sr0.33CoO3 thin films and their p–n heterostructure devices with rectifying behavior may aid in the development of various semiconductor devices such as three terminal transistors, light emitted diode (LED) and solar cell.

First principles study of Cr poisoning in solid oxide fuel cell cathodes: Application to (La,Sr) CoO3

Solid oxide fuel cell (SOFC) systems provide efficient and clean power. Chromium poisoning is known to deteriorate the performance of the SOFC cathodes such as La1−xSrxCoO3 (LSCO) due to the formation of undesired products. We identify the reaction products when LSCO is in a CrO3 and O2 atmosphere using first principle thermodynamics, with the assumption that gas-phase CrO3 is the Cr source. Our results correctly identify SrCrO4 as the primary Cr poisoning product as observed in experiments and also provides temperature-pressure regimes under which Cr poisoning effects may be avoided.

Novel acetone sensing performance of La1-xSr xCoO3 nanoparticles at room temperature

La1-xSr xCoO3 (x=0, 0.1) nanoparticles were synthesized by the simple sol-gel technique. Based on characteristics by scanning electron microscope, transmission electron microscope and X-ray diffractometer, the nanoparticles with the size distributed ranging from 40 to 80nm all show a single phase with rhombohedral distortion perovskite structure. The sensors exhibit excellent acetone sensing performance at room temperature (∼25°C), especially showing a nearly linear relation with the acetone concentration between 10 and 100ppm. Sr2+ doping can bring holes in the p-type semiconductor, thus increasing the resistance variation range of LaCoO3 and enhancing the sensing performance. It suggests that the La1-xSr xCoO3 nanoparticles can be considered as a potential candidate for acetone detecting at room temperature.

Electrical conductivity and thermoelectric power of La1–x Li x CoO3–δ (0 ≤ x ≤ 0.1) oxides

The influence of the concentration of lithium ions on the phase composition, the electrical conductivity, and the thermoelectric power of La1–xLixCoO3–δ (0 ≤ x ≤ 0.1) oxides synthesized by the ceramic method has been investigated. It has been found that the region of the existence of perovskite-type La1–xLixCoO3–δ solid solutions does not exceed x = 0.05. The doping with lithium leads to an increase in the electrical conductivity of single-phase samples in comparison with that of the LaCoO3 compound. As the temperature increases from 300 to 400 K, the thermoelectric power of the LaCoO3 compound increases from the negative to positive values and then decreases, but remains positive in the temperature range from 400 to 1020 K. The thermoelectric power of the other samples has a positive sign. The results obtained have been discussed based on the models of the electron density of states in LaCoO3 and La1–xSrxCoO3–δ, proposed in the studies of Senaris-Rodriguez and Goodenough, as well as in the framework of the theory of non-crystalline materials, developed by Mott.