The tungstates Ln₆WO₁₂ are proton conducting crystalline materials exhibiting sufficient mixed conductivity (protonic and electronic) and remarkable stability to consider them as potential candidates for the separation of hydrogen at high temperature. Membranes made of this type of materials can be applied for industrial gas separations such as hydrogen separation from hydrogen containing gases, for the implementation of pre-combustion strategies in power plants, and for the application in ceramic proton conducting fuel cells. This contribution summarizes the recent developments achieved on ceramic hydrogen-permeable membranes in the frame of the MEM-BRAIN Alliance. It is shown the preparation of nanocrystalline mixed conducting materials based in the system Ln₆WO₁₂ using a sol-gel complexation synthesis method. This method is based on the citrate-complexation route modified to stabilize W- and Ln-containing ions. The influence of the lanthanide (Ln) nature and the partial substitution in the system Ln_6-xA_xWO₁₂ (Ln= La and Nd; A= Ce, Eu, Gd, La, Nd, Pr, Sm, Tb, Yb; and x =1 and 3) on the total conductivity and crystalline structure was studied systematically. The electrical conductivity measurements were conducted by standard four-point DC technique on the sintered rectangular bars and silver paste and wire were used for contacting. The measurements were carried out under different atmospheres, i.e., helium and H₂/D₂ saturated with H₂O/D₂O at 20ºC. For selected samples, it was studied the water up-take process followed by thermogravimetry, XRD and Raman spectroscopy. Hydrogen permeation was studied for different disk-shaped Ln₆WO₁₂ membranes in the range of 700-1000ºC. The membrane sides were coated by screen printing with a 20 μm layer of a Pt ink aiming to improve surface hydrogen exchange. Furthermore, stability of these materials at 700 and 800ºC has been evaluated in a CO₂-rich gas stream, dry and saturated with water at room temperature. Materials remained unchanged after the treatments, irrespective of its original sintering temperature and lanthanide nature. The experimental work has been supported by ab-initio calculations aiming to give more insight into the fundamentals of protonic conduction through the crystalline lattice.

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