Double-Layered Perovskite Oxyfluoride Cathodes with High Capacity Involving O-O Bond Formation for Fluoride-Ion Batteries.

Developing electrochemical high-energy storage systems is of crucial importance toward a green and sustainable energy supply. A promising candidate is fluoride-ion batteries (FIBs), which can deliver a much higher volumetric energy density than lithium-ion batteries. However, typical metal fluoride cathodes with conversion-type reactions cause a low-rate capability. Recently, layered perovskite oxides and oxyfluorides, such as LaSrMnO4 and Sr3Fe2O5F2, have been reported to exhibit relatively high rate performance and cycle stability compared to typical metal fluoride cathodes with conversion-type reactions, but their discharge capacities (∼118 mA h/g) are lower than those of typical cathodes used in lithium-ion batteries. Here, we show that double-layered perovskite oxyfluoride La1.2Sr1.8Mn2O7-δF2 exhibits (de) intercalation of two fluoride ions to rock-salt slabs and further (de) intercalation of excess fluoride ions to the perovskite layer, leading to a reversible capacity of 200 mA h/g. The additional fluoride-ion intercalation leads to the formation of O-O bond in the structure for charge compensation (i.e., anion redox). These results highlight the layered perovskite oxyfluorides as a new class of active materials for the construction of high-performance FIBs.

Lead(II)-Based Coordination Polymer Exhibiting Reversible Color Switching and Selective CO2 Photoreduction Properties.

Herein, we report a new photofunctional Pb-S-based coordination polymer (CP) with the formula [Pb(ATAT)(OAc)]n (ATAT = 3-amino-5-mercapto-1,2,4-triazole, OAc = acetate, CP1). Apart from its photoactive one-dimensional (1D) (-Pb-S-)n chain, CP1 is also composed of another 1D (-Pb-O-)n chain that originates from the coordination with acetate. The coordinated acetate can be exchanged with water (H2O) or dimethyl sulfoxide (DMSO), leading to the formation of a CP1-H2O or CP1-DMSO structure that exhibits a distinct change in optical properties, including a white-to-yellow color change. The structural transformation of CP1 to CP1-H2O and CP1-DMSO, and its subsequent recovery to the original CP1 structure could be controlled by the presence or absence of acetic acid vapor; the transformation was completely reversible. CP1 absorbed light with wavelengths shorter than 390 nm, with an estimated bandgap of 3.18 eV. Density functional theory calculations indicated that the valence band of CP1 is mainly formed by N and S orbitals originating from the ATAT unit, whereas the conduction band is composed of the Pb orbitals. Even without any modification, such as the incorporation of a molecular catalyst, CP1 reduced CO2 into formate under UV light with >99% selectivity.

Modification of Visible-Light-Responsive Pb2Ti2O5.4F1.2 with Metal Oxide Cocatalysts to Improve Photocatalytic O2 Evolution toward Z-Scheme Overall Water Splitting.

The development of a highly active photocatalyst for visible-light water splitting requires a high-quality semiconductor material and a cocatalyst, which promote both the migration of photogenerated charge carriers and surface redox reactions. In this work, a cocatalyst was loaded onto an oxyfluoride photocatalyst, Pb2Ti2O5.4F1.2, to improve the water oxidation activity. Among the metal oxides examined as cocatalysts, RuO2 was found to be the most suitable, and the O2 evolution activity depended on the preparation conditions for Ru/Pb2Ti2O5.4F1.2. The highest activity was obtained with RuCl3-impregnated Pb2Ti2O5.4F1.2 heated under a flow of H2 at 523 K. The H2-treated Ru/Pb2Ti2O5.4F1.2 showed an O2 evolution rate an order of magnitude higher than those for the analogues without the H2 treatment (e. g., RuO2/Pb2Ti2O5.4F1.2). Physicochemical analyses by X-ray absorption fine-structure spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and time-resolved microwave conductivity measurements indicated that the optimized photocatalyst contained partially reduced RuO2 species with a particle size of ~5 nm. These partially reduced species effectively trapped the photogenerated charge carriers and promoted the oxidation of water into O2. The optimized Ru/Pb2Ti2O5.4F1.2 could function as an O2-evolving photocatalyst in Z-scheme overall water splitting, in combination with an Ru-loaded, Rh-doped SrTiO3 photocatalyst.