Solid State Transport and Hydrogen Permeation in the System Nd_5.5W_1–<i>x</i>Re_<i>x</i>O_11.25−δ

Nd_5.5WO_11.25−δ is a mixed proton–electron conducting oxide, which shows an important mixed conductivity and stability in moist CO₂ environments. However, the H₂ fluxes obtained with this material are not high enough in order to apply them as H₂ separation membranes in industrial applications. Re⁶⁺ cation presents similar ionic radii than W⁶⁺ and Re⁶⁺ can be reduced to different oxidation states under the operating conditions typical for hydrogen membrane separation. This fact leads to the improvement of the electronic conductivity and produce the generation of oxygen vacancies with the subsequent increase in the ionic conductivity. This work presents the synthesis as nanosized powders as well as the structural and electrochemical characterization of mixed conducting materials based on the system Nd_5.5W_1–<i>x</i>Re_<i>x</i>O_11.25−δ where <i>x</i> = 0, 0.1, 0.5 and 1. The evolution of the crystalline structure and the shrinkage behavior are studied as a function of the sintering temperature. Total conductivity in reducing and oxidizing environments is studied systematically for samples sintered at 1350 °C. The H/D isotopic effect and the hydration influence are also analyzed by means of DC-electrochemical measurements. H₂ permeation is carried out for the selected compound, Nd_5.5W_0.5Re_0.5O_11.25−δ, in the range of 700–1000 °C, obtaining a peak H₂ flux value of 0.08 mL·min^–1·cm^–2. The reduction of the Re cation in this compound under reducing conditions is investigated by TPR and XPS. Finally, the stability of this material under CO₂-rich gas stream was evaluated by measuring H₂ permeation using a CO₂ containing atmosphere as sweep gas

Particular Transport Properties of NiFe₂O₄ Thin Films at High Temperatures

NiFe₂O₄ (NFO) thin films were deposited on quartz substrates by rf magnetron sputtering, and the influence of the deposition conditions on their physic-chemical properties was studied. The films structure and the high temperature transport properties were analyzed as a function of the deposition temperature. The analysis of the total conductivity up to 800 °C in different <i>p</i>O₂ containing atmospheres showed a distinct electronic behavior of the films with regard to the bulk NFO material. Indeed, the thin films exhibit p-type electronic conductivity, while the bulk material is known to be a prevailing n-type electronic conductor. This difference is ascribed to the dissimilar concentration of Ni³⁺ in the thin films, as revealed by XPS analysis at room temperature. The bulk material with a low concentration of Ni³⁺ (Ni³⁺/Ni²⁺ ratio of 0.20) shows the expected n-type electronic conduction via electron hopping between Fe³⁺–Fe²⁺. On the other hand, the NFO thin films annealed at 800 °C exhibit a Ni³⁺/Ni²⁺ ratio of 0.42 and show p<i>-</i>type conduction via hole hopping between Ni³⁺–Ni²⁺