Positron annihilation spectroscopic studies on Nd-doped ceria

Surface Activation by Single Ru Atoms for Enhanced High-Temperature CO2 Electrolysis

Cathodic CO2 adsorption and activation is essential for high-temperature CO2 electrolysis in solid oxide electrolysis cells (SOECs). However, the component of oxygen ionic conductor in the cathode displays limited electrocatalytic activity. Herein, stable single Ruthenium (Ru) atoms are anchored on the surface of oxygen ionic conductor (Ce0.8 Sm0.2 O2-δ , SDC) via the strong covalent metal-support interaction, which evidently modifies the electronic structure of SDC surface for favorable oxygen vacancy formation and enhanced CO2 adsorption and activation, finally evoking the electrocatalytic activity of SDC for high-temperature CO2 electrolysis. Experimentally, SOEC with the Ru1 /SDC-La0.6 Sr0.4 Co0.2 Fe0.8 O3-δ cathode exhibits a current density as high as 2.39 A cm-2 at 1.6 V and 800 °C. This work expands the application of single atom catalyst to the high-temperature electrocatalytic reaction in SOEC and provides an efficient strategy to tailor the electronic structure and electrocatalytic activity of SOEC cathode at the atomic scale.

Oxygen vacancy and valence engineering in CeO2 through distinct sized ion doping and their impact on oxygen reduction reaction catalysis

Defect tuning in ceria to enhance its catalytic properties is a subject of great interest for the scientific community owing to the growing demand for catalytic materials in drug, automobile and chemical industries. Doping induced defect engineering was found to be one of the most sought out strategies particularly in oxides for achieving multifunctionality. Here, in this study, we have doped ceria with distinct sized trivalent rare-earth ions, namely, Y³⁺, Eu³⁺ and La³⁺, using combustion techniques. Positron annihilation lifetime spectroscopy (PALS) suggested enhanced defect density with doping in general and higher concentration of oxygen vacancies in La³⁺ doped ceria compared to Y³⁺ and Eu³⁺ counterparts. X-ray photoelectron spectroscopy (XPS) suggested the existence of both Ce³⁺ and Ce⁴⁺, with the former having higher fraction in CeO₂:La³⁺ compared to CeO₂:Y³⁺. The electron transfer resistance (R_ct) reduced in all the doped samples when compared to undoped ceria and they demonstrated improved catalytic activity towards the oxygen reduction reaction (ORR). The highest reduction in R_ct was seen in the 5% La doped sample owing to the very high concentration of oxygen vacancies and Ce³⁺/Ce⁴⁺ ratio and CeO₂:5.0% La³⁺ showed the best performance towards ORR electrocatalysis. The studies are expected to help in further tuning the catalysts in terms of dopant concentrations, and in future work, the strategy will be to control the Ce³⁺/Ce⁴⁺ ratio and see its implication in both catalytic and magnetic applications.

Defect engineering in trivalent ion doped ceria through vanadium assisted charge compensation: insight using photoluminescence, positron annihilation and electron spin resonance spectroscopy

Pair matching charge compensation with trivalent and pentavalent dopants in ceria was found to be an attractive strategy in engineering defects with minimal distortions in the lattice and obtaining enhanced catalytic properties. In the present study, charge compensation with a vanadium codopant in trivalent ion doped ceria is studied. Defect evolution in the trivalent ion doped ceria with vanadium codoping has been studied in CeO₂:Eu³⁺, CeO₂:La³⁺,Eu³⁺ and CeO₂:Y³⁺,Eu³⁺ systems and the choices of the dopant and co-dopant are triggered by their ionic radius. Eu³⁺ photoluminescence (PL) is used as a spectroscopic probe to monitor local structural changes around the dopants. Positron lifetime studies showed that oxygen vacancies formed due to trivalent ion doping are weakly associated when larger ions are doped and result in the formation of vacancy aggregates. Positron lifetime studies along with XRD studies show that vanadium codoping effectively removes the vacancies but the distortions are significant when the size mismatch between the pair match used for charge compensation is higher. Photoluminescence demonstrated that the oxygen vacancies associated with Eu are more effectively removed in the case of Y codoped samples. Electron Spin Resonance (ESR) studies suggested that vanadium in excess over the stoichiometric concentration of the trivalent ion can lead to additional defects. These studies are expected to help in tuning the vacancy concentrations as well as controlling the lattice distortions for technological applications such as catalysis, ionic conductivity, etc.

Green Synthetic Fuels: Renewable Routes for the Conversion of Non-Fossil Feedstocks into Gaseous Fuels and Their End Uses

Innovative renewable routes are potentially able to sustain the transition to a decarbonized energy economy. Green synthetic fuels, including hydrogen and natural gas, are considered viable alternatives to fossil fuels. Indeed, they play a fundamental role in those sectors that are difficult to electrify (e.g., road mobility or high-heat industrial processes), are capable of mitigating problems related to flexibility and instantaneous balance of the electric grid, are suitable for large-size and long-term storage and can be transported through the gas network. This article is an overview of the overall supply chain, including production, transport, storage and end uses. Available fuel conversion technologies use renewable energy for the catalytic conversion of non-fossil feedstocks into hydrogen and syngas. We will show how relevant technologies involve thermochemical, electrochemical and photochemical processes. The syngas quality can be improved by catalytic CO and CO2 methanation reactions for the generation of synthetic natural gas. Finally, the produced gaseous fuels could follow several pathways for transport and lead to different final uses. Therefore, storage alternatives and gas interchangeability requirements for the safe injection of green fuels in the natural gas network and fuel cells are outlined. Nevertheless, the effects of gas quality on combustion emissions and safety are considered.

Defect evolution in Eu3+, Nb5+ doped and co-doped CeO2: X-ray diffraction, positron annihilation lifetime and photoluminescence studies

Defects and their influence on light emitting properties were explored in CeO₂:Eu,Nb using Rietveld refinement, positron annihilation and luminescence spectroscopy.

Proposed parameter-free model for interpreting the measured positron annihilation spectra of materials using a generalized gradient approximation

Positron annihilation spectroscopy is often used to analyze the local electronic structure of materials of technological interest. Reliable theoretical tools are crucial to interpret the measured spectra. Here, we propose a parameter-free gradient correction scheme for a local-density approximation obtained from high-quality quantum Monte Carlo data. The results of our calculations compare favorably with positron affinity and lifetime measurements, opening new avenues for highly precise and advanced positron characterization of materials.

Tuning the relative concentration ratio of bulk defects to surface defects in TiO2 nanocrystals leads to high photocatalytic efficiency

TiO(2) nanocrystals with tunable bulk/surface defects were synthesized and characterized with TEM, XRD, BET, positron annihilation, and photocurrent measurements. The effect of defects on photocatalytic activity was studied. It was found for the first time that decreasing the relative concentration ratio of bulk defects to surface defects in TiO(2) nanocrystals could significantly improve the separation efficiency of photogenerated electrons and holes, thus significantly enhancing the photocatalytic efficiency.