A relaxing time: A dramatic increase of the energy barrier (U_eff) in tetrairon(III) single-molecule magnets can be achieved by simple chemical modification. Site-specific replacement of the six methoxide bridges in [Fe₄(OMe)₆(dpm)₆] (Hdpm=dipivaloylmethane; U_eff/k_B=3.5 K) with two tripodal 1,1,1-tris(hydroxymethyl)ethane (H₃thme) ligands affords the compound [Fe₄(thme)₂(dpm)₆] (see figure) with U_eff/k_B=15.6(2) K and a magnetic relaxation time exceeding 1000 s below 0.2 K.

High-temperature proton conductors are promising as electrolytes for intermediate-temperature solid oxide fuel cells. Among them, BaCeO₃-based materials have high proton conductivity but rather poor chemical stability. In contrast, barium zirconates are rather stable, but have poorly reproducible densities and conductivities. In this study, the investigation of BaCe_1−x−yZr_xY_yO_3−δ solid solutions (x = 0, 0.10, 0.20, 0.30, 0.40; y = 0.15, 0.20) was undertaken, with the final aim of finding a composition having both high conductivity and good stability. The influence of the modified sol–gel Pechini synthetic approach on the powder morphology, and of a barium excess on the densification were demonstrated. Single-phase perovskite powders were prepared and high density pellets were obtained at temperatures lower than those commonly employed. Stability tests demonstrated that the Zr introduction into doped barium cerate greatly enhanced the chemical stability, particularly for Zr ≥ 20%. The proton conductivities, measured in a humidified H₂/Ar atmosphere by impedance spectroscopy, were only slightly influenced by the Zr amount. Overall, BaCe_1−x−yZr_xY_yO_3−δ solid solutions having Zr ≈ 20–40% and Y ≈ 15–20% showed good chemical stability and high conductivity.

Mixed proton and electron conductor ceramic composites were examined as hydrogen separation membranes at moderate temperatures (higher than 500 °C). In particular, dense ceramic composites of BaCe_0.65Zr_0.20Y_0.15O_3−δ (BCZ20Y15) and Ce_0.85M_0.15O_2−δ (M = Y and Gd, hereafter referred to as YDC15 and GDC15), as protonic and electronic conducting phases respectively, were successfully prepared and tested as hydrogen separation membranes. The mixture of these oxides improved both chemical and mechanical stability and increased the electronic conductivity in dual-phase ceramic membranes. The synthetic method and sintering conditions were optimized to obtain dense and crack free symmetric membranes. The addition of ZnO as a sintering aid allowed achieving robust and dense composites with homogeneous grain distribution. The chemical compatibility between the precursors and the influence of membrane composition on electrical properties and H₂ permeability performances were thoroughly investigated. The highest permeation flux was attained for the 50 : 50 volume ratio BCZ20Y15–GDC15 membrane when the feed and the sweep sides of the membrane were hydrated, reaching values of 0.27 mL min⁻¹ cm⁻² at 755 °C on a 0.65 mm thick membrane sample, currently one of the highest H₂ fluxes obtained for bulk mixed protonic–electronic membranes. Increasing the temperature to 1040 °C, increased the hydrogen flux up to 2.40 mL min⁻¹ cm⁻² when only the sweep side was hydrated. The H₂ separation process is attributed to two cooperative mechanisms, i.e. proton transport through the membrane and H₂ production via the water splitting reaction coupled with oxygen ion transport. Moreover, these composite systems demonstrated a very good chemical stability under a CO₂-rich atmosphere such as catalytic reactors for hydrogen generation.