Preparation and Characterization of Nanocrystalline Mixed Proton−Electronic Conducting Materials Based on the System Ln₆WO₁₂

Abstract

The tungstates Ln6WO12 are proton-conducting materials exhibiting sufficient electronic conductivity to consider them as potential candidates for the separation of hydrogen at high temperature. Hydrogen-permeable membranes will find application in power plants applying precombustion strategies, process intensification using high-temperature catalytic membrane reactors, and in components for electrochemical systems as proton conducting fuel cells (PCFCs) and electrolyzers. This work presents the preparation and characterization of nanocrystalline mixed conducting materials with three different nominal compositions (Nd6WO12−Eu6WO12−Er6WO12) using a sol−gel complexation synthesis method. The evolution of the crystalline structure and crystallite size is studied as a function of the sintering temperature. Generally, the nanosized oxides show a (pseudo)-cubic crystalline fluorite structure which evolves into the most stable fluorite symmetry (tetragonal for Nd and rhombohedral for Er) with increasing sintering temperatures, i.e., crystallite sizes. Shrinkage behavior was analyzed for the three compositions in the range from 1000 to 1500 °C and the nanosized Nd-based oxide showed a very high sintering activity even at relatively low temperatures (1100−1200 °C). In addition, the total conductivity in different environments has been studied systematically for samples sintered at different temperatures and the highest total conductivity was obtained for the Eu-based compound having structure with tetragonal symmetry (0.009 S/cm at 850 °C). Hydrogen permeation was studied for a disk-shaped Nd6WO12 membrane in the range of 700−1000 °C. Finally, stability of these materials at 700 and 800 °C has been evaluated in contact with a CO2-rich gas stream (dry or humidified) as well as thermochemical compatibility with yttria-stabilized zirconia in the range 1200−1500 °C.