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

Abstract

Nd5.5WO11.25−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 Nd5.5W1–xRexO11.25−δ where x = 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, Nd5.5W0.5Re0.5O11.25−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.