Investigation of a novel protonic/electronic ceramic composite material as a candidate for hydrogen separation membranes

Abstract

A novel ceramic protonic/electronic conductor composite BaCe_0.2 Zr_0.7 Y_0.1 O_3-[delta] / Sr_0.95 Ti_0.9 Nb_0.1 O_3-[delta] (BCZY27/STN95: BS27) has been synthesized, and its electrical properties and hydrogen permeability have been investigated. The volume ratio of the STN95 phase was varied from 50 - 70 % to test the effects on conductivity and hydrogen permeability. BCZY27 and STN95 powders were prepared by solid-state reaction, and membrane samples were fabricated through conventional and spark plasma sintering techniques. The phase composition, density, and microstructure were compared between the sintering methodologies. Total conductivities of 0.01 - 0.06 S[middle dot]cm[superscript -1] were obtained in wet (+1 % H2O) dilute H2/(N2, He, Ar) from 600 - 800 degrees C for 50 volume % STN95. With increasing STN content (60 and 70 volume %), conductivity generally increased, though remained lower than predicted by standard effective medium models, even at 70 volume % STN95. A new effective medium model was proposed, which accounted for an interfacial resistance term associated with the heterojunctions formed between the BCZY27 and STN95 phases. Better fits for the measured data were achieved with this new method, although some effects remain unexplained. Discrepancies between the model and experiment were attributed to space charge effects, grain boundary resistances, and insulating impurity phase formation during synthesis. Dense BS27 samples were tested for high-temperature hydrogen permeation and a measured flux of 0.006 [mu]mol[middle dot]cm[superscript -2][middle dot]s[superscript -1] was recorded for a 50 volume % STN95 sample at 700 degrees C, using dry argon as a sweep gas. This value represents a modest improvement on other ceramic composite membranes, but remains short of targets for commercialization. Persistent leaks in the flux experiments generated a shallower hydrogen gradient across the samples, although this p(H2) on the sweep side simultaneously decreased the oxygen partial pressure gradient across the sample and preserved the reduced state of the membrane. The addition of thin palladium layers (100 nm) to another 50 volume % STN95 sample increased the flux five-fold to 0.026 [mu]mol[middle dot]cm[superscript -2][middle dot]s[superscript -1] at 700 degrees C. Experiments on 60 and 70 volume % STN95 samples revealed no measurable hydrogen flux, which was attributed to the proton-conducting BCZY27 phase being non-percolating for these compositions.