Dopant Concentration-Porosity-Li-Ion Conductivity Relationship in Garnet-Type Li_5+2xLa₃Ta_2-xY_xO₁₂ (0.05 ≤ x ≤ 0.75) and Their Stability in Water and 1 M LiCl

Highly Li-ion conductive Y-doped garnet-type Li5+2xLa3Ta2-xYxO12 (0.05 ≤ x ≤ 0.75) were studied to understand the effects of yttrium- and lithium-doping on crystal structure, porosity, and Li-ion conductivity using 7Li MAS NMR, electrochemical ac impedance spectroscopy, and scanning electron microscopy (SEM), as well as ex situ and in situ powder X-ray diffraction (PXRD) to further explore the potential application of garnets in all-solid-state Li-ion batteries. Solid-state 7Li MAS NMR studies showed an increase in the Li-ion mobility as a function of Y- and Li-doping in Li5+2xLa3Ta2-xYxO12, which is consistent with the results from ac impedance spectroscopy. The SEM studies on sintered pellets indicated a systematic decrease in porosity and an increase in sinterability as the Y- and Li-doping levels increase in Li5+2xLa3Ta2-xYxO12. These results are consistent with the calculated porosity and densities using the Archimedes method. Using the variable-temperature in situ PXRD in the temperature range of 30-700 °C, a thermal expansion coefficient of 7.25 × 10-6 K-1 was observed for Li6La3Ta1.5Y0.5O12. To further explore the possibility of a new application for the Li-stuffed garnets, the stability of these materials in aqueous LiCl solution was also studied. A high degree of structural stability was observed in these materials upon 1 M LiCl treatment, making them suitable candidates for further studies as protective layers for lithium electrodes in aqueous lithium batteries. (Graph Presented).</p

Dopant Concentration - Porosity - Li-ion Conductivity Relationship in Garnet-Type Li5+2xLa3Ta2-xYxO12 (0.05 ≤ x ≤ 0.75) and Their Stability in Water and 1M LiCl

Ye

Isostructural Oxides Sr₃Ti_2−xM_xO_7−δ (M = Mn, Fe, Co; x = 0, 1) as Electrocatalysts for Water Splitting

The correlation of the electrocatalytic activity with electrical conductivity, oxygen-vacancies, and electronegativity have been studied in a series of isostructural oxides, having the so-called Ruddlesden-Popper structure. The structures of these materials comprise transition metals that are octahedrally coordinated to form a network of bilayer stacks. These materials are catalytically active for both half-reactions of water-splitting, namely oxygen-evolution reaction (OER) and hydrogen-evolution reaction (HER). They show a systematic increase in electrocatalytic activity in progression from Sr3Ti2O7 to Sr3TiMnO7, Sr3TiFeO7−δ, and Sr3TiCoO7−δ. The kinetic studies using the Tafel method indicate the same trend across the series, where the best catalyst also has the fastest kinetics for both HER and OER. In addition, the same progression is observed in the concentration of oxygen-vacancies, as well as the electrical conductivity in a wide range of temperatures, 25 °C–800 °C. The material that shows the best electrocatalytic activity, i.e., Sr3TiCoO7−δ, also has the highest electrical conductivity and the greatest concentration of oxygen vacancies in the series. The correlations observed in this work indicate that trends in electrocatalytic performance may be related to the systematic increase in electrical conductivity, electronegativity, and oxygen-vacancies, as well as the electron occupancy of eg orbitals, which can affect the strength of sigma interactions between the catalyst and reaction intermediates

Effect of sintering temperature on microstructure, chemical stability, and electrical properties of transition metal or Yb-doped BaZr _0.1 Ce _0.7 Y _0.1 M _0.1 O _3-δ (M = Fe, Ni, Co, and Yb)

Perovskite-type BaZr 0.1 Ce 0.7 Y 0.1 M 0.1 O 3-δ (M = Fe, Ni, Co, and Yb) (BZCY-M) oxides were synthesized using the conventional solid-state reaction method at 1350-1550°C in air in order to investigate the effect of dopants on sintering, crystal structure, chemical stability under CO2 and H 2 S, and electrical transport properties. The formation of the single-phase perovskite-type structure with an orthorhombic space group Imam was confirmed by Rietveld refinement using powder X-ray diffraction for the Fe, Co, Ni, and Yb-doped samples. The BZCY-Co and BZCY-Ni oxides show a total electrical conductivity of 0.01 and 8 × 10 -3 S cm -1 at 600°C in wet H 2 with an activation energy of 0.36 and 0.41 eV, respectively. Scanning electron microscope and energy-dispersive X-ray analysis revealed Ba and Co-rich secondary phase at the grain-boundaries, which may explain the enhancement in the total conductivity of the BZCY-Co. However, ex-solution of Ni at higher sintering temperatures, especially at 1550°C, decreases the total conductivity of the BZCY-Ni material. The Co and Ni dopants act as a sintering aid and form dense pellets at a lower sintering temperature of 1250°C. The Fe, Co, and Ni-doped BZCY-M samples synthesized at 1350°C show stability in 30 ppm H 2 S/H 2 at 800°C, and increasing the firing temperature to 1550°C, enhanced the chemical stability in CO 2 /N 2 (1:2) at 25-900°C. The BZCY-Co and BZCY-Ni compounds with high conductivity in wet H 2 could be considered as possible anodes for intermediate temperature solid oxide fuel cells.</p

Effect of sintering temperature on microstructure, chemical stability, and electrical properties of transition metal or Yb-doped BaZr0.1Ce0.7Y0.1M0.1O3?? (M = Fe, Ni, Co, and Yb)

Perovskite-type BaZr0.1Ce0.7Y0.1M0.1O3?? (M = Fe, Ni, Co, and Yb) (BZCY-M) oxides were synthesized using the conventional solid-state reaction method at 1350–1550°C in air in order to investigate the effect of dopants on sintering, crystal structure, chemical stability under CO2 and H2S, and electrical transport properties. The formation of the single-phase perovskite-type structure with an orthorhombic space group Imam was confirmed by Rietveld refinement using powder X-ray diffraction for the Fe, Co, Ni, and Yb-doped samples. The BZCY-Co and BZCY-Ni oxides show a total electrical conductivity of 0.01 and 8 × 10?3 S cm?1 at 600°C in wet H2 with an activation energy of 0.36 and 0.41 eV, respectively. Scanning electron microscope and energy-dispersive X-ray analysis revealed Ba and Co-rich secondary phase at the grain-boundaries, which may explain the enhancement in the total conductivity of the BZCY-Co. However, ex-solution of Ni at higher sintering temperatures, especially at 1550°C, decreases the total conductivity of the BZCY-Ni material. The Co and Ni dopants act as a sintering aid and form dense pellets at a lower sintering temperature of 1250°C. The Fe, Co, and Ni-doped BZCY-M samples synthesized at 1350°C show stability in 30 ppm H2S/H2 at 800°C, and increasing the firing temperature to 1550°C, enhanced the chemical stability in CO2/N2 (1:2) at 25–900°C. The BZCY-Co and BZCY-Ni compounds with high conductivity in wet H2 could be considered as possible anodes for intermediate temperature solid oxide fuel cells

Enhancing Li Ion Conductivity of Garnet-Type Li₅La₃Nb₂O₁₂ by Y- and Li-Codoping: Synthesis, Structure, Chemical Stability, and Transport Properties

Novel Li-stuffed garnet-like “Li_5+2<i>x</i>La₃Nb_2–<i>x</i>Y_<i>x</i>O₁₂” (0.05 ≤ <i>x</i> ≤ 0.75) was prepared via solid-state reaction in air and characterized using <i>ex situ</i> and<i> in situ</i> powder X-ray diffraction (PXRD), ⁷Li and ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR), scanning electron microscopy (SEM), thermo-gravimetric analysis (TGA), and AC impedance spectroscopy. Rietveld refinement with the PXRD data confirmed the formation of a cubic, garnet-like <i>Ia</i>-3<i>d</i> structure. ⁷Li MAS NMR showed a single sharp peak close to 0 ppm as the usual trend for fast Li ion conducting garnets. Among the materials studied, “Li_6.5La₃Nb_1.25Y_0.75O₁₂” showed a very high bulk ionic conductivity of 2.7 × 10^–4 S cm^–1 at 25 °C, which is the highest value found in garnet-type compounds, and is only reported for Li₇La₃Zr₂O₁₂. The <i>in situ</i> PXRD measurements revealed structural stability up to 400–600 °C after water treatment as well as chemical compatibility with high voltage Li cathodes Li₂MMn₃O₈ (M = Co, Fe). The current work demonstrates that slight Al contamination from the Al₂O₃ crucible, which is commonly observed in this class of materials and was detected by ²⁷Al MAS NMR, does not affect the Li ionic conductivity and chemical stability of “Li_5+2<i>x</i>La₃Nb_2–<i>x</i>Y_<i>x</i>O₁₂” garnets. It also shows that Li stuffing is critical to improve further the ionic conductivity of parent compound Li₅La₃Nb₂O₁₂. Y³⁺ seems to be a very efficient dopant to improve the ionic conductivity of garnets compared to other investigated dopants that include M²⁺ (M = alkaline earth metals), In³⁺, and Zr⁴⁺

Interstitial oxygens and cation deficiency in Mo-doped ceria, an anode material for SOFCs

Density functional theory studies show that it is energetically possible for oxygen atoms to reside on the interstitial sites in Mo-doped ceria. This is also studied experimentally using neutron diffraction experiments that show small concentration of oxygens on two intestinal sites, as well as cation deficiency in the Ce1-xMoxO2+δ (x ∼ 0.07) lattice.</p

Carbon Formation on Stainless Steel 304H in the Convection Section of an Ethane Cracking Plant

Ye