Deciphering the Interaction of Single-Phase La0.3Sr0.7Fe0.7Cr0.3O3-δ with CO2/CO Environments for Application in Reversible Solid Oxide Cells.

A detailed study aimed at understanding and confirming the reported highly promising performance of a La0.3Sr0.7Fe0.7Cr0.3O3-δ (LSFCr) perovskite catalyst in CO2/CO mixtures, for use in reversible solid oxide fuel cells (RSOFCs), is reported in this work, with an emphasis on chemical and performance stability. This work includes an X-ray diffraction (XRD), thermogravimetric analysis (TGA), and electrochemical study in a range of pO2 atmospheres (pure CO2, CO alone (balance N2), and a 90-70% CO2/10-30% CO containing mixture), related to the different conditions that could be encountered during CO2 reduction at the cathode. Powdered LSFCr remains structurally stable in 20-100% CO2 (balance N2, pO2 = 10-11-10-12 atm) without any decomposition. However, in 30% CO (balance N2, pO2 ∼ 10-26 atm), a Ruddlesden-Popper phase, Fe nanoparticles, and potentially some coke are observed to form at 800 °C. However, this can be reversed and the original perovskite can be recovered by heat treatment in air at 800 °C. While no evidence for coke formation is obtained in 90-70% CO2/10-30% CO (pO2 = 10-17-10-18 atm) mixtures at 800 °C, in 70 CO2/30 CO, minor impurities of SrCO3 and Fe nanoparticles were observed, with the latter potentially beneficial to the electrochemical activity of the perovskite. Consistent with prior work, symmetrical two-electrode full cells (LSFCr used at both electrodes), fed with the various CO2/CO gas mixtures at one electrode and air at the other, showed excellent electrochemical performance at 800 °C, both in the SOFC and in SOEC modes. Also, LSFCr exhibits excellent stability during CO2 electrolysis in medium-term potentiostatic tests in all gas mixtures, indicative of its excellent promise as an electrode material for use in symmetrical solid oxide cells.

Water-splitting photoelectrodes consisting of heterojunctions of carbon nitride with ap-type low bandgap double perovskite oxide.

Quinary and senary non-stoichiometric double perovskites such as Ba2Ca0.66Nb1.34-xFexO6-δ(BCNF) have been utilized for gas sensing, solid oxide fuel cells and thermochemical CO2reduction. Herein, we examined their potential as narrow bandgap semiconductors for use in solar energy harvesting. A cobalt co-doped BCNF, Ba2Ca0.66Nb0.68Fe0.33Co0.33O6-δ(BCNFCo), exhibited an optical absorption edge at ∼800 nm,p-type conduction and a distinct photoresponse up to 640 nm while demonstrating high thermochemical stability. A nanocomposite of BCNFCo and g-C3N4(CN) was prepared via a facile solvent-assisted exfoliation/blending approach using dichlorobenzene and glycerol at a moderate temperature. The exfoliation of g-C3N4followed by wrapping on perovskite established an effective heterojunction between the materials for charge separation. The conjugated 2D sheets of CN enabled better charge migration resulting in increased photoelectrochemical performance. A blend composed of 40 wt% perovskites and CN performed optimally, whilst achieving a photocurrent density as high as 1.5 mA cm-2for sunlight-driven water-splitting with a Faradaic efficiency as high as ∼88%.

A perovskite-type Nd0.75Sr0.25Co0.8Fe0.2O3-δ cathode for advanced solid oxide fuel cells.

Perovskite-type Nd0.75Sr0.25Co0.8Fe0.2O3-δ (NSCF) has shown excellent oxygen reduction reaction (ORR) properties (an area specific polarization resistance of 0.1 Ω cm2 at 700 °C) as a composite cathode (30 wt% La0.8Sr0.2Ga0.8Mg0.2O3-δ (LSGM)) with remarkable chemical stability under CO2. The mechanism for the ORR was established employing pO2 and temperature dependent studies.

Transformation of Structure, Electrical Conductivity, and Magnetism in AA'Fe2O6-δ, A = Sr, Ca and A' = Sr.

The ability to control electrical properties and magnetism by varying the crystal structure using the effect of the A-site cation in oxygen-deficient perovskites has been studied in AA'Fe2O6-δ, where A = Sr, Ca and A' = Sr. The structure of Sr2Fe2O6-δ, synthesized at 1250 °C in air, contains dimeric units of FeO5 square pyramids separated by FeO6 octahedra. Here we show that this ordering scheme can be transformed by changing the A-site cations from Sr to Ca. This leads to a structure where layers of corner-sharing FeO6 octahedra are separated by chains of FeO4 tetrahedra. Through systematic variation of the A-site cations, we have determined the average ionic radius required for this conversion to be ∼1.41 Å. We have demonstrated that the magnetic structure is also transformed. The Sr2 compound has an incommensurate magnetic structure, where magnetic moments are in spin-density wave state, aligning perpendicular to the body diagonal of the unit cell. With the aid of neutron diffraction experiments at 10 and 300 K, we have shown that the magnetic structure is converted into a long-range G-type antiferromagnetic system when one Sr is replaced by Ca. In this G-type ordering scheme, the magnetic moments align in the 001 direction, antiparallel to their nearest neighbors. We have also performed variable-temperature electrical conductivity studies on these materials in the temperature range 298-1073 K. These studies have revealed the transformation of charge transport properties, where the metallic behavior of the Sr2 compound is converted into semiconductivity in the CaSr material. The trend of conductivity as a function of temperature is reversed upon changing the A-site cation. The conductivity of the Sr2 compound shows a downturn, while the conductivity of the CaSr material increases as a function of temperature. We have also shown that the CaSr compound exhibits temperature-dependent behavior typical of a mixed ionic-electronic conducting system.

Structure, Ionic Conductivity, and Dielectric Properties of Li-Rich Garnet-type Li5+2xLa3Ta2-xSmxO12 (0 ≤ x ≤ 0.55) and Their Chemical Stability.

Lithium garnet oxides are considered as very promising solid electrolyte candidates for all-solid-state lithium ion batteries (SSLiBs). In this work, we present a cubic garnet-type Li5+2xLa3Ta2-xSmxO12 (0 ≤ x ≤ 0.55) system as a potential electrolyte for SSLiBs. Powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM) were employed to investigate the structural stability of Li5+2xLa3Ta2-xSmxO12. The results from PXRD and SEM suggested structural and morphological transformation as a function of dopant concentration. In addition to Li-ion transport in Li5+2xLa3Ta2-xSmxO12, the dielectric properties were also investigated in the light of electron energy loss functions, which showed some surface energy loss and negligible volume energy loss for the studied garnets. Surface and volume energy loss functions of a mixed conducting LiCoO2 was studied for comparison. The long-term chemical stability of one of members, Li5.3La3Ta1.85Sm0.15O12, was performed on aged sample using PXRD, SEM, and thermogravimetric analysis.

Correction: Synthesis and characterization of novel Li-stuffed garnet-like Li5+2xLa3Ta2-xGdxO12 (0 ≤ x ≤ 0.55): structure-property relationships.

Correction for 'Synthesis and characterization of novel Li-stuffed garnet-like Li5+2xLa3Ta2-xGdxO12 (0 ≤ x ≤ 0.55): structure-property relationships' by Dalia M. Abdel Basset, et al., Dalton Trans., 2017, 46, 933-946.

Electrochemical regeneration of a reduced graphene oxide/magnetite composite adsorbent loaded with methylene blue.

In this work, two different reduced graphene oxide/iron oxide (rGO-IO) nanocomposites with different iron oxide loadings were fabricated using a one-step solvothermal method. The structure, properties and applications of the synthesized nanocomposites were evaluated with Raman spectroscopy, attenuated total reflectance Fourier transform infrared spectroscopy, thermogravimetric analysis, X-ray diffraction, electron microscopy, and energy-dispersive X-ray spectroscopy. The iron oxide is in the form of magnetite (Fe3O4), so that the resultant adsorbent can readily be separated from the treated water using a magnetic field. The ability of the nanocomposites to remove methylene blue (MB) from water by adsorption was investigated. The highest adsorptive capacity observed was 39 mg g-1, for the composite containing 60 wt% iron oxide. The adsorptive capacity of the rGO-IO decreased to 26 mg g-1 when the mass fraction of iron oxide was increased to 75 wt%. Electrochemical regeneration of MB loaded rGO-IO was also investigated. The electrochemical regeneration was found to be rapid and with low electrical energy consumption relative to conventional adsorbents, due to the high electrical conductivity and nonporous surface of the rGO. A regeneration efficiency of 100% was obtained after 30 min of electrochemical treatment using a 2 mm thick bed of rGO-IO loaded with 39 mg g-1 MB, using a current density of 10 mA cm-2. Multiple adsorption-electrochemical regeneration cycles demonstrated that the surface of the rGO was modified leading to increase in the adsorptive capacity to around 80 mg g-1 after the second regeneration cycle. The morphology of the rGO was observed to change significantly after electrochemical regeneration, suggesting that the rGO based adsorbent materials could only be used for a few cycles.

Synthesis and characterization of novel Li-stuffed garnet-like Li5+2xLa3Ta2-xGdxO12 (0 ≤ x ≤ 0.55): structure-property relationships.

In this article, we report the preparation and characterization of novel Li-stuffed garnets Li5+2xLa3Ta2-xGdxO12 (0 ≤ x ≤ 0.55) for all-solid-state Li ion batteries. The conventional solid-state method was used to prepare Li5La3Ta2O12 in air at 1200 °C and Li5+2xLa3Ta2-xGdxO12 at 1150 °C. Rietveld refinements for the powder X-ray diffraction (PXRD) patterns confirmed the formation of a cubic garnet-like structure (space group Ia3[combining macron]d) with cell constant increased from 12.8176(4) Å (x = 0) to 12.9372(2) Å (x = 0.55). However, small amounts of second phases were observed for higher Gd-doped samples. Scanning electron microscopy revealed that Li5.7La3Ta1.65Gd0.35O12 exhibits the highest density among all investigated samples in this study. The AC impedance spectroscopy data of the samples have been analyzed in relation to ionic conductivity, dielectric constants, and loss tangent. Among the investigated electrolytes, the Li5.7La3Ta1.65Gd0.35O12 composition demonstrated the highest bulk ionic conductivity of 8.18 × 10-5 S cm-1 at 25 °C, which is significantly higher than that of the parent garnet Li5La3Ta2O12 (1.65 × 10-5 S cm-1 at 25 °C). The appearance of a relaxation peak in the loss tangent plots for all samples seems to be due to the dipolar rotations of Li+ ions in Li-stuffed garnets.

Magnetically aligned iron oxide/gold nanoparticle-decorated carbon nanotube hybrid structure as a humidity sensor.

Functionalized carbon nanotubes (f-CNTs), particularly CNTs decorated with nanoparticles (NPs), are of great interest because of their synergic effects, such as surface-enhanced Raman scattering, plasmonic resonance energy transfer, magnetoplasmonic, magnetoelectric, and magnetooptical effects. In general, research has focused on a single type of NP, such as a metal or metal oxide, that has been modified on a CNT surface. In this study, however, a new strategy is introduced for the decoration of two different NP types on CNTs. In order to improve the functionality of modified CNTs, we successfully prepared binary NP-decorated CNTs, namely, iron oxide/gold (Au) NP-decorated CNTs (IA-CNTs), which were created through two simple reactions in deionized water, without high temperature, high pressure, or harsh reducing agents. The physicochemical properties of IA-CNTs were characterized by ultraviolet/visible spectroscopy, Fourier transform infrared spectroscopy, a superconducting quantum interference device, scanning electron microscopy, and transmission electron microscopy. In this study, IA-CNTs were utilized to detect humidity. Magnetic IA-CNTs were aligned on interdigitated platinum electrodes under external magnetic fields to create a humidity-sensing channel, and its electrical conductivity was monitored. As the humidity increased, the electrical resistance of the sensor also increased. In comparison with various gases, for example, H2, O2, CO, CO2, SO2, and dry air, the IA-CNT-based humidity sensor exhibited high-selectivity performances. IA-CNTs also responded to heavy water (D2O), and it was established that the humidity detection mechanism had D2O-sensing capabilities. Further, the humidity from human out-breathing was also successfully detected by this system. In conclusion, these unique IA-CNTs exhibited potential application as gas detection materials.