Enhanced CO2 Electrolysis Through Mn Substitution Coupled with Ni Exsolution in Lanthanum Calcium Titanate Electrodes

In this study, perovskite oxides La0.3Ca0.6Ni0.05MnxTi0.95- xO3- γ (x = 0, 0.05, 0.10) are investigated as potential solid oxide electrolysis cell cathode materials. The catalytic activity of these cathodes toward CO2 reduction reaction is significantly enhanced through the exsolution of highly active Ni nanoparticles, driven by applying a current of 1.2 A in 97% CO2 - 3% H2O. The performance of La0.3Ca0.6Ni0.05Ti0.95O3-γ is notably improved by co-doping with Mn. Mn dopants enhance the reducibility of Ni, a crucial factor in promoting the in situ exsolution of metallic nanocatalysts in perovskite (ABO3) structures. This improvement is attributed to Mn dopants enabling more flexible coordination, resulting in higher oxygen vacancy concentration, and facilitating oxygen ion migration. Consequently, a higher density of Ni nanoparticles is formed. These oxygen vacancies also improve the adsorption, desorption, and dissociation of CO2 molecules. The dual doping strategy provides enhanced performance without degradation observed after 133 h of high-temperature operation, suggesting a reliable cathode material for CO2 electrolysis.

Laser Irradiation-Induced Nanoscale Surface Transformations in Strontium Titanate

We studied the structural transformations and atomic rearrangements in strontium titanate (SrTiO3) via nanosecond pulsed laser irradiation-induced melting and ultrafast quenching. Using scanning transmission electron microscopy, we determine that the laser-irradiated surface in single-crystalline SrTiO3 transforms into an amorphous phase with an interposing disordered crystalline region between amorphous and ordered phases. The formation of disordered phase is attributed to the rapid recrystallization of SrTiO3 from the melt phase constrained by an epitaxial relation with the pristine region, which eases up on the surface, leading to amorphous phase formation. With electron energy-loss spectroscopic analysis, we confirm the transformation of Ti+4 to Ti+3 due to oxygen vacancy formation as a result of laser irradiation. In the disordered region, the maximum transformation of Ti+4 is observed to be 16.2 ± 0.2%, whereas it is observed to be 20.2 ± 0.2% in the amorphous region. Finally, we deduce that the degree of the disorder increases from atomically disordered to amorphous transition in SrTiO3 under laser-irradiation. The signatures of short-range ordering remain similar, leading to a comparable fingerprint of electronic structure. With these results, this study addresses the gap in understanding the atomic and electronic structure modified by pulsed laser irradiation and functionalizing pristine SrTiO3 for electronic, magnetic, and optical applications.

Catalysts Based on Strontium Titanate Doped with Ni/Co/Cu for Dry Reforming of Methane

Two series of strontium titanates doped with Ni, Co, or Cu with general formula of SrTi_1-xMe_xO₃ for Sr-stoichiometric and Sr_0.95Ti_1-xMe_xO₃ for Sr-non-stoichiometric materials (where Me = Ni, Co or Cu and x were 0.02 and 0.06) were obtained by the wet chemical method. The samples were calcinated at 900, 950, and 1050 °C and characterized in terms of their structural properties (XRD), the possibility of undergoing the reduction and oxidation reactions (TPR/TPOx), and catalytic properties. All obtained materials were multiphase and although the XRD analysis does not confirm the presence of Ni, Co, and Cu oxides (with one exception for Cu-doped sample), the TPR/TPOx profiles show reduction peaks that can be attributed to the reduction of these oxides which may at first appear in an amorphous form. Catalytic tests in dry reforming of methane reaction showed that the highest catalytic activity was achieved for Ni-doped materials (up to 90% of CH₄ conversion) while Co and Cu-doped samples showed only a very slight catalytic effect. Additionally, the decrease in methane conversion with an increasing calcination temperature was observed for Ni-doped strontium titanates.

Is Reduced Strontium Titanate a Semiconductor or a Metal?

In recent decades, the behavior of SrTiO3 upon annealing in reducing conditions has been under intense academic scrutiny. Classically, its conductivity can be described using point defect chemistry and predicting n-type or p-type semiconducting behavior depending on oxygen activity. In contrast, many examples of metallic behavior induced by thermal reduction have recently appeared in the literature, challenging this established understanding. In this study, we aim to resolve this contradiction by demonstrating that an initially insulating, as-received SrTiO3 single crystal can indeed be reduced to a metallic state, and is even stable against room temperature reoxidation. However, once the sample has been oxidized at a high temperature, subsequent reduction can no longer be used to induce metallic behavior, but semiconducting behavior in agreement with the predictions of point defect chemistry is observed. Our results indicate that the dislocation-rich surface layer plays a decisive role and that its local chemical composition can be changed depending on annealing conditions. This reveals that the prediction of the macroscopic electronic properties of SrTiO3 is a highly complex task, and not only the current temperature and oxygen activity but also the redox history play an important role.

The analysis of charge transport mechanism in mixed ionic electronic conductor composite of Sr2TiCoO6double perovskite with yttria stabilized zirconia

In this investigation, the ionic conduction mechanism in mixed ionic electronic conductors composites of Sr₂TiCoO₆/YSZ has been studied with the help of universal dynamic response. 3 mol% and 8 mol% yttria stabilized ZrO₂have been mixed with Sr₂TiCoO₆(STC) double perovskite in 1:1 ratio to prepare STC/3YSZ and STC/8YSZ composites via solid-state reaction route. AC Impedance spectroscopy has been carried out to examine the charge transport mechanism, which has been modeled using the microstructural networks of resistors and capacitors. Grain boundaries are more resistive and capacitive compared to the bulk. Modulus spectroscopy analysis demonstrates the non-Debye character of conductivity relaxation with frequency. Complex frequency-dependent AC conductivity is found to obey Almond West power law and reveals that ion migration occurs through the correlated hopping mechanism. Further, the DC conductivity and relaxation time have been found to follow the Barton Nakajima and Namikawa relation, which is correlated with AC to DC conduction. The time-temperature superposition principle has been used to explain the conductivity scaling in the intermediate frequency range. At low temperatures, the ions are localized in the asymmetric potential well, while at high temperatures, hopping behavior starts dominating. Further Kramers-Kronig transformation connects the dielectric strength with conductivity relaxation and verifies the impedance data.

Surface Engineering Strategy Using Urea To Improve the Rate Performance of Na2 Ti3 O7 in Na-Ion Batteries

Na2 Ti3 O7 (NTO) is considered a promising anode material for Na-ion batteries due to its layered structure with an open framework and low and safe average operating voltage of 0.3 V vs. Na+ /Na. However, its poor electronic conductivity needs to be addressed to make this material attractive for practical applications among other anode choices. Here, we report a safe, controllable and affordable method using urea that significantly improves the rate performance of NTO by producing surface defects such as oxygen vacancies and hydroxyl groups, and the secondary phase Na2 Ti6 O13 . The enhanced electrochemical performance agrees with the higher Na+ ion diffusion coefficient, higher charge carrier density and reduced bandgap observed in these samples, without the need of nanosizing and/or complex synthetic strategies. A comprehensive study using a combination of diffraction, microscopic, spectroscopic and electrochemical techniques supported by computational studies based on DFT calculations, was carried out to understand the effects of this treatment on the surface, chemistry and electronic and charge storage properties of NTO. This study underscores the benefits of using urea as a strategy for enhancing the charge storage properties of NTO and thus, unfolding the potential of this material in practical energy storage applications.

Suitability of Sm3+-Substituted SrTiO3 as Anode Materials for Solid Oxide Fuel Cells: A Correlation between Structural and Electrical Properties

Perovskite anodes, nowadays, are used in any solid oxide fuel cell (SOFC) instead of conventional nickel/yttria-stabilized zirconia (Ni/YSZ) anodes due to their better redox and electrochemical stability. A few compositions of samarium-substituted strontium titanate perovskite, SmxSr1−xTiO3−δ (x = 0.00, 0.05, 0.10, 0.15, and 0.20), were synthesized via the citrate-nitrate auto-combustion route. The XRD patterns of these compositions confirm that the solid solubility limit of Sm in SrTiO3 is x < 0.15. The X-ray Rietveld refinement for all samples indicated the perovskite cubic structure with a P m 3 ¯ m space group at room temperature. The EDX mapping of the field emission scanning electron microscope (FESEM) micrographs of all compositions depicted a lower oxygen content in the specimens respect to the nominal value. This lower oxygen content in the samples were also confirmed via XPS study. The grain sizes of SmxSr1−xTiO3 samples were found to increase up to x = 0.10 and it decreases for the composition with x > 0.10. The AC conductivity spectra were fitted by Jonscher’s power law in the temperature range of 500–700 °C and scaled with the help of the Ghosh and Summerfield scaling model taking νH and σdc T as the scaling parameters. The scaling behaviour of the samples showed that the conduction mechanism depends on temperature at higher frequencies. Further, a study of the conduction mechanism unveiled that small polaron hopping occurred with the formation of electrons. The electrical conductivity, in the H2 atmosphere, of the Sm0.10Sr0.90TiO3 sample was found to be 2.7 × 10−1 S∙cm−1 at 650 °C, which is the highest among the other compositions. Hence, the composition Sm0.10Sr0.90TiO3 can be considered as a promising material for the application as the anode in SOFCs.

Applicability of a linear diffusion model to determination of the height of the potential barrier at the grain boundaries of Fe-doped SrTiO3

The potential barrier formed at the grain boundaries in Fe-doped SrTiO3 is reported to be one of the main reasons of the exceptionally large grain boundary resistivity of the material. Of particular interest is thus how to accurately quantify the potential barrier height, Ψgb, in such electronic conductors. This study aims to expand the applicability of a linear diffusion model (namely I-V model) to electronic conductors. The I-V model has previously proven its success in accurate determination of Ψgb in popular ionic conductors. By employing 1 mol% Fe-doped SrTiO3 as a model material, the current-voltage characteristics of the grain boundary investigated demonstrate the power law behavior predicted by the I-V model, verifying the applicability of this model. The Ψgb estimated from the I-V model at different temperatures are compared with those from the resistivity ratio of the grain boundary to the bulk. The resistivity ratio has been exclusively used to determine Ψgb in various conductors over several decades and yet has limitations in its accuracy. The Ψgb determined by the I-V model are found to be substantially lower than those from the resistivity ratio; such discrepancy implies that the potential barrier only partially contributes to the high grain boundary resistivity of a lightly doped electron-hole conducting SrTiO3.

Highly Efficient CO2 Electrolysis on Cathodes with Exsolved Alkaline Earth Oxide Nanostructures

The solid oxide CO₂ electrolyzer has the potential to provide storage solutions for intermittent renewable energy sources as well as to reduce greenhouse gas emissions. One of the key challenges remains the poor adsorption and activity toward CO₂ reduction on the electrolyzer cathode at typical operating conditions. Here, we show a novel approach in tailoring a perovskite titanate (La, Sr)TiO_3+δ cathode surface, by the in situ growing of SrO nanoislands from the host material through the control of perovskite nonstoichiometry. These nanoislands provide very enhanced CO₂ adsorption and activation, with stability up to 800 °C, which is shown to be in an intermediate form between carbonate ions and molecular CO₂. The activation of adsorbed CO₂ molecules results from the interaction of exsolved SrO nanoislands and the defected titanate surface as revealed by DFT calculations. These cathode surface modifications result in an exceptionally high direct CO₂ electrolysis performance with current efficiencies near 100%.

Concentration- and Temperature-Induced Phase Transitions in PrAlO3-SrTiO3 System

Single-phase mixed aluminates-titanates Pr1-x Sr x Al1-x Ti x O3 (x = 0.1, 0.2, 0.3, 0.5, 0.7) with rhombohedral perovskite structure were prepared by solid-state reaction technique at 1823-1873 K. Morphotropic rhombohedral-to-cubic phase transition in Pr1-x Sr x Al1-x Ti x O3 series is predicted to occur at x = 0.88. The temperature-induced structural phase transition R  [Formula: see text]  с - Pm [Formula: see text]  m in Pr0.5Sr0.5Al0.5Ti0.5O3, detected at 930 K by in situ high-temperature X-ray synchrotron powder diffraction, occurs at considerably lower temperature as the corresponding transformation in the parent compound PrAlO3 (1770 K). Such remarkable drop of the transition temperature is explained by gradual decrease of the perovskite structure deformation in the Pr1-x Sr x Al1-x Ti x O3 series with increasing Sr and Ti contents as a consequence of the increasing Goldschmidt tolerance factor. For Pr0.3Sr0.7Al0.3Ti0.7O3 phase, a sequence of the low-temperature phase transformation R  [Formula: see text]  с - Immb(C2/m) - I4/mcm was detected.

Strategies for Carbon and Sulfur Tolerant Solid Oxide Fuel Cell Materials, Incorporating Lessons from Heterogeneous Catalysis

Solid oxide fuel cells (SOFCs) are a rapidly emerging energy technology for a low carbon world, providing high efficiency, potential to use carbonaceous fuels, and compatibility with carbon capture and storage. However, current state-of-the-art materials have low tolerance to sulfur, a common contaminant of many fuels, and are vulnerable to deactivation due to carbon deposition when using carbon-containing compounds. In this review, we first study the theoretical basis behind carbon and sulfur poisoning, before examining the strategies toward carbon and sulfur tolerance used so far in the SOFC literature. We then study the more extensive relevant heterogeneous catalysis literature for strategies and materials which could be incorporated into carbon and sulfur tolerant fuel cells.

Promoting oxygen vacancy formation and p-type conductivity in SrTiO3via alkali metal doping: a first principles study

Strontium titanate (SrTiO₃, STO) is a prototypical perovskite oxide, widely exploited in many technological applications, from catalysis to energy conversion devices. In the context of solid-oxide fuel cells, STO has been recently applied as an epitaxial substrate for nano-sized layers of mixed ion-electron conductive catalysts with enhanced electrochemical performances. To extend the applications of such heterogeneous nano-cathodes in real devices, also the STO support should be active for both electron transport and oxide diffusion. To this end, we explored using first-principles calculations the strategy of doping of STO at the Sr site with sodium and potassium. These two ions fit in the perovskite structure and induce holes in the STO valence band, so as to obtain the desired p-type electronic conduction. At the same time, the doping with alkali ions also promotes the formation of oxygen vacancies in STO, a prerequisite for effective oxide diffusion. Analysis of electron density rearrangements upon defect formation allows relating the favorable vacancy formation energies to an improved electronic delocalization over the oxide sub-lattice, as observed in closely related materials (e.g. Sr₂Fe_1.5Mo_0.5O₆). Overall, our results suggest the alkali-doped STO as a new potential substrate material in nanoscale heterogeneous electrodes for solid oxide electrochemical cells.

Optimization of Sm0.5Sr0.5CoO3−δ-infiltrated YSZ electrodes for solid oxide fuel cell/electrolysis cell

The Sm_0.5Sr_0.5CoO_3−δ (SSC) oxygen electrode has attracted much interest due to its high electrical conductivity and electrochemical activity.

In situ study of copper reduction in SrTi1−xCuxO3 nanoparticles

Fourier transform magnitude of Cu K-edge absorption spectra before (300 K) and after (550 K) reduction.