Synthesis of Nanocomposite Materials for Hydrogen Separation Membranes
Hydrogen separation membranes based on proton conducting oxides used to remove hydrogen from the products of fuels reforming now attract a great attention in the modern energy field. For design of asymmetric supported proton-conducting membranes the most promising materials are nanocomposites comprised of oxides with perovskite or fluorite structure such as lanthanide niobates and tungstates  combined with metals (Pd, Ni, Cu) or their alloys .
In our work lanthanum niobates La0.99Ca0.01NbO4 and LaNb3O9 were synthesized using a modified Pechini route, while molybdates – tungstates Nd5.5(Mo,W)O11.25δ were prepared by citrate or soft mechanochemistry routes. To prepare the powder of NiCu alloy, the mixture of NiO and CuO obtained by decomposition of the mixed nitrates was reduced in the flow of H2 at 350 C. Oxide-oxide (La0.99Ca0.01NbO4 + Nd5.5W0.5Mo0.5O11.2) and metal-oxide (Cu0.5Ni0.5+Nd5.5W0.5Mo0.5O11.2) nanocomposites were prepared either by treating the mixture of oxides calcined at 700ºC or oxide +NiCu powders in 1:1 ratio in the planetary ball mill or via their ultrasonic dispersion in i-PrOH. The powders of nanocomposites were pressed into pellets (d = 13 mm) and sintered using conventional thermal sintering as well as hot pressing under 50 MPa at 1100 °C for 15 min. All obtained materials were characterized using thermal analysis, XRD, TEM, IR and Raman spectroscopy. The oxygen and protonic mobility were studied by the oxygen 16O/18O (with С18O2) and H/D (with D2O) isotope heteroexchange and weight relaxation (after changing pH2O) techniques. The total as well as protonic conductivity were estimated by impedance spectroscopy.
According to XRD and Raman data, the main phase of complex oxides is formed already after calcination at 700°C. La0.99Ca0.01NbO4 is present in two modifications: the low-temperature monoclinic structure and the high- temperature tetragonal one. Both modifications of La0.99Ca0.01NbO4 contain isolated NbO4 tetrahedra (Nb-O IR band at 600 cm-1) bound with La cations in 8-fold oxygen coordination. Nd5.5WO11.25 and Nd5.5W0.5Mo0.5O11.25 are crystalized in a defect fluorite structure already at 700 °C. After sintering at 1300 °C the phase crystallinity improves, while additional weak XRD peaks appear evidencing tetragonal distortion of the cubic fluorite phase as well as admixture of the Nd2O3 phase. For Nd5.5W0.5Mo0.5O11.25 oxide sintered at high temperatures the most intense Raman peak at 791-815 cm-1 is shifted to the low frequency region as compared with that for Nd5.5WO11.25, due to a larger lattice parameter.
In nanocomposites sintered at high temperatures and reduced by hydrogen, the main phases are scheelite for La0.99Ca0.01NbO4, fluorite for Nd5.5(Mo,W)O11.25 σ, and NiCu alloy. Optimization of the synthesis and sintering procedures provides the nano-sized domains of component phases even in dense nanocomposite materials. Such a developed surface of the phase boundaries also creates fast oxygen and hydrogen diffusion pathways. The oxygen mobility studies revealed two types of bulk oxygen related to two phases in oxide nanocomposite samples. The total conductivity has been studied systematically in different environments (in dry/wet air and hydrogen). Addition of the metal component allowed to increase the total conductivity of nanocomposite up to 10-100 S/cm. After high-temperature sintering of nanocomposites based on proton conductors and NiCu alloy proton conductivity values ~ 10 4 S/cm at 400 °C were obtained close to those for pure oxides. D2O exchange studies demonstrated very fast protonic transport in samples with DH+ ~ 10 11 cm2/s at 400 °C. Weight loss experiments revealed Dchem ~ 10 -4 cm2/s at working temperatures, which is typical for proton conductors. Hence, protonic mobility in these nanocomposites is sufficiently high for the practical application.
The work was supported by the Russian Science Foundation (Project 16-13-00112).
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