A Double-ligand Chelating Strategy to Iron Complex Anolytes with Ultrahigh Cyclability for Aqueous Iron Flow Batteries

Aqueous all-iron flow batteries (AIFBs) are attractive for large-scale and long-term energy storage due to their extremely low cost and safety features. To accelerate commercial application, a long cyclable and reversible iron anolyte is expected to address the critical barriers, namely iron dendrite growth and hydrogen evolution reaction (HER). Herein, we report a robust iron complex with triethanolamine (TEA) and 2-methylimidazole (MM) double ligands. By introducing two ligands into one iron center, the binding energy of the complex increases, making it more stable in the charge-discharge reactions. The Fe(TEA)MM complex achieves reversible and stable redox between Fe3+ and Fe2+ , without metallic iron growth and HER. AIFBs based on this anolyte perform a high energy efficiency of 80.5 % at 80 mA cm-2 and exhibit a record durability among reported AIFBs. The efficiency and capacity retain nearly 100 % after 1,400 cycles. The capital cost of this AIFB is $ 33.2 kWh-1 (e.g., 20 h duration), cheaper than Li-ion battery and vanadium flow battery. This double-ligand chelating strategy not only solves the current problems faced by AIFBs, but also provides an insight for further improving the cycling stability of other flow batteries.

Phase Evolution in the CaZrTi2O7-Dy2Ti2O7 System: A Potential Host Phase for Minor Actinide Immobilization

Zirconolite is considered to be a suitable wasteform material for the immobilization of Pu and other minor actinide species produced through advanced nuclear separations. Here, we present a comprehensive investigation of Dy³⁺ incorporation within the self-charge balancing zirconolite Ca_1–xZr_1–xDy_2xTi₂O₇ solid solution, with the view to simulate trivalent minor actinide immobilization. Compositions in the substitution range 0.10 ≤ x ≤ 1.00 (Δx = 0.10) were fabricated by a conventional mixed oxide synthesis, with a two-step sintering regime at 1400 °C in air for 48 h. Three distinct coexisting phase fields were identified, with single-phase zirconolite-2M identified only for x = 0.10. A structural transformation from zirconolite-2M to zirconolite-4M occurred in the range 0.20 ≤ x ≤ 0.30, while a mixed-phase assemblage of zirconolite-4M and cubic pyrochlore was evident at Dy concentrations 0.40 ≤ x ≤ 0.50. Compositions for which x ≥ 0.60 were consistent with single-phase pyrochlore. The formation of zirconolite-4M and pyrochlore polytype phases, with increasing Dy content, was confirmed by high-resolution transmission electron microscopy, coupled with selected area electron diffraction. Analysis of the Dy L₃-edge XANES region confirmed that Dy was present uniformly as Dy³⁺, remaining analogous to Am³⁺. Fitting of the EXAFS region was consistent with Dy³⁺ cations distributed across both Ca²⁺ and Zr⁴⁺ sites in both zirconolite-2M and 4M, in agreement with the targeted self-compensating substitution scheme, whereas Dy³⁺ was 8-fold coordinated in the pyrochlore structure. The observed phase fields were contextualized within the existing literature, demonstrating that phase transitions in CaZrTi₂O₇–REE³⁺Ti₂O₇ binary solid solutions are fundamentally controlled by the ratio of ionic radius of REE³⁺ cations.

Zirconolite (CaZrTi₂O₇) ceramics are candidate wasteform materials for Pu and other minor actinides. Herein, the Ca_1−xZr_1−xDy_2xTi₂O₇ solid solution was fabricated by a conventional mixed oxide synthesis, with Dy³⁺ included as a structural simulant for Am³⁺. A phase transformation from zirconolite-2M to zirconolite-4M was observed at low Dy concentrations (0.20 ≤ x ≤ 0.30) after which cubic pyrochlore was stabilized as the dominant phase. Observations and interpretations were supported by electron diffraction and X-ray absorption spectroscopic methods.

Investigation of Factors That Affect the Oxidation State of Ce in the Garnet-Type Structure

Oxide materials that adopt the garnet-type structure (X₃A₂B₃O₁₂) have received attention for a wide variety of applications, one of which is as potential wasteforms for the sequestration of radioactive actinide elements. The actinides are able to be accommodated in the eight-coordinate X site of the garnet structure. This study focuses on the investigation of Ce substitution into the X site as a surrogate for Pu because of their similar chemical properties. This is accomplished through analysis of the Y_{3- z}Ce _zAlFe₄O₁₂ (0.05 ⩽ z ⩽ 0.20) materials. The effects of the Ce concentration, oxidation state of the Ce in the starting materials, annealing environment, and cooling rate on the local structure and Ce and Fe oxidation states were investigated through analysis of the powder X-ray diffraction patterns and Ce L₃-edge and Fe K-edge X-ray absorption spectroscopy (XANES) spectra. Analysis of Ce L₃-edge XANES spectra indicated that Ce was present as both 3+ and 4+ oxidation states, the ratios of which depended on the synthetic conditions. The largest concentration of Ce⁴⁺ was observed when the materials were postannealed at 800 °C following synthesis of the materials at 1400 °C. Variations in the Ce oxidation state are the result of the temperature-dependent Ce³⁺/Ce⁴⁺ redox couple, with Ce⁴⁺ being favored at lower temperatures. Analysis of the Fe K-edge spectra indicated that Fe was only present in the 3+ oxidation state and the Fe coordination number increased with increasing concentration of Ce⁴⁺, which is necessary to charge balance the system. The materials in this study can be described as an oxygen-intercalated garnet-type structure with the formula Y_{3- z}Ce _zAlFe₄O_12+δ.

Effect of Synthetic Method and Annealing Temperature on the Structure of Hollandite-Type Oxides

Hollandite is a class of metal oxide material with the general formula A₂B₈O₁₆. Several methods have been used in the synthesis of this type of metal oxide, and the synthetic methods reported have typically employed high annealing temperatures between 1200 and 1300 °C. Appropriate synthetic methods must be employed to successfully synthesize these hollandite-type oxides at lower annealing temperatures. Hollandite compounds have been synthesized using ceramic (high annealing temperature only) and coprecipitation (high and low annealing temperatures) methods. Annealing temperatures ranging from 1200 to 700 °C have been employed in the thermal treatment process. Powder X-ray diffraction and X-ray absorption near-edge spectroscopy (XANES) were conducted on hollandite-type oxides (Ba _xAl_{2 x}Ti_{8-2 x}O_16-δ; x = 1.2; and Ba _xAl _xFe _xTi_{8-2 x}O_16-δ, Ba _xFe_{2 x}Ti_{8-2 x}O_16-δ; x = 1.16). Structural comparisons between materials annealed in the temperature range from 1200 to 800 °C were made, and an examination of the XANES spectra and powder X-ray diffraction patterns has provided confirmation of the absence of significant structural changes in these hollandite materials. This study has shown that hollandite-type materials can be formed using annealing temperatures as low as 700-800 °C when a coprecipitation method is used, with little change to the local and long-range structures being detected.

A one-step synthesis of rare-earth phosphate–borosilicate glass composites

REPO₄–BG composites synthesized by a new 1-step method were investigated and were found to be similar to the composite made by the traditional 2-step method.

Investigation of CeTi2O6- and CaZrTi2O7-containing glass–ceramic composite materials

Glass–ceramic composite materials are being investigated for numerous applications (i.e., textile, energy storage, nuclear waste immobilization applications, etc.) due to the chemical durability and flexibility of these materials. Borosilicate and Fe–Al–borosilicate glass–ceramic composites containing brannerite (CeTi2O6) or zirconolite (CaZrTi2O7) crystallites were synthesized at different annealing temperatures. The objective of this study was to understand the interaction of brannerite or zirconolite-type crystallites within the glass matrix and to investigate how the local structure of these composite materials changed with changing synthesis conditions. Powder X-ray diffraction (XRD) and Backscattered electron (BSE) microprobe images have been used to study how the ceramic crystallites dispersed in the glass matrix. X-ray absorption near edge spectroscopy (XANES) spectra were also collected from all glass–ceramic composite materials. Examination of Ti K-, Ce L3-, Zr K-, Si L2,3-, Fe K-, and Al L2,3-edge XANES spectra from the glass–ceramic composites have shown that the annealing temperature, glass composition, and the loading of the ceramic crystallites in the glass matrix can affect the local environment of the glass–ceramic composite materials. A comparison of the glass–ceramic composites containing brannerite or zirconolite crystallites has shown that similar changes in the long range and local structure of these composite materials occur when the synthesis conditions to form these materials or the composition are changed.