Structural characterization of hydrogen separating membranes based on lanthanide-tungstates

The global energy supply is currently the most controversial issue discussed in our society. Despite the increasing importance of renewable energies, the largest portion of electrical energy has its origin in fossil fuels. CO2, emitted during combustion in power plants is known to be one of the greenhouse gases that contributes significantly to global climate change. The development of technologies for environmentally friendly power generation from coal and gas is an area of significant interest. One possibility is the capture and long-term storage of CO2 from the exhaust stream of fossil fuel power plants. In the pre-combustion process, CO2 and H2 can be separated after gasification of the fossil fuel. For this purpose gas-tight ceramic membranes with mixed electronic-protonic conductivity can be used. However, these materials have high requirements due to the extreme conditions in power plants. Mixed electronic-protonic conducting lanthanide tungstates (Ln6WO12 Ln = lanthanide or yttrium) are promising materials, which are stable in CO2-containing harsh environments. This work presents a study on structure-property relationships of Ln6WO12. The structural analysis was performed by the use of neutron and high-resolution X-ray diffraction methods for three exemplary systems (Ln = La, Nd, Y). Samples were prepared via solid state reaction (SSR), and also via a sol-gel approach (Pechini). For the systems LaWO and NdWO, new structural models were developed by combined Rietveld analysis and Fourier density maps. The latter was applied to determine the electron and nuclear density distribution. LaWO with a La/W ratio from 5.3 to 5.7 crystallizes with the space group F-43m and forms a superstructure due to a partially ordered arrangement of cations. On Wyckoff site 48h, up to 4.6 % W can be substituted by La. The oxygen atoms around tungsten are highly delocalized and 6 out of 24 possible split positions are occupied. Thus, W has an octahedral coordination. The oxygen vacancies are not ordered in this system. In the NdWO system, cations order at temperatures above 1300 C to form a superstructure. For Nd/W ∝ 5.6, these compounds crystallize with the tetragonal space group I-4. For a Nd/W ratio of 6, an additional Nd2O3 phase has been observed. For a Nd/W ratio of 5, the cations and the oxygen vacancies order completely, and the compound crystallizes in space group Pbcn with a composition of Nd10W2O21. In the YWO system, the previously known crystal structure, with ordered cations and oxygen vacancies in the space group R-3, has been confirmed. Single phase samples can only be synthesized with a precise Y/W ratio of 6. In addition to structural characterization, the water absorption and transport properties of the samples Ln2yW1-YO3 were investigated by thermogravimetry, and electrical conductivity was studied under reducing and oxidizing atmospheres. The water absorption and protonic conductivity of the samples is strongly dependent on the crystal structure, the Ln/W ratio and the phase composition. Among the materials studied here, La6WO12 showed high water absorption and the best protonic conductivity, mainly due to the disordered oxygen vacancies in the crystal structure. It was demonstrated that Ln6WO12 compounds, crystallizing in a cubic crystal structure with ordered cations and highly delocalized oxygen atom positions, show the highest water uptake and protonic conductivity. This connection has been developed and proven in this work.