Fabrication of Ni-BaZr0.8Y0.2O3-δ Composite Membrane for Hydrogen Separation

Hydrogen is an important raw material for production of ammonia, methanol, liquid hydrocarbons, etc. The application of membrane technology is expected to considerably reduce the capital and energy cost in hydrogen production. Composite membranes consisting of BaCeO₃-based proton conductor and electronic conductor (e.g. nickel) have been developed for this application. However, these membranes (e.g. Ni–BaZr_0.8–xCe_xY_0.2O_3–δ (Ni-BZCY), 0.4 ≤ x ≤ 0.8) suffered serious performance loss in CO₂-containing environment at 900 ^oC due to the reaction between BaCeO₃ and CO₂.^{[1, 2]} In order to avoid the chemical stability issue of BaCeO₃, CO₂-tolerant hydrogen membranes have been developed, e.g., RE₆WO_12-δ (RE: rare earth metal), Ca-doped LaNbO₄, Ce_0.8Sm_0.2O_2-δ, and Ni-La_0.4875Ca_0.0125Ce_0.5O_2-δ. However, the permeation fluxes of those membranes are significantly lower than that of Ni-BZCY membranes due to their low proton conductivity and/or electronic conductivity. Among the proton conductors that are tolerant to CO₂, BaZr_0.8Y_0.2O_3–δ (BZY) possesses the highest bulk proton conductivity.^[3]However, single phase BZY membrane shows very low hydrogen flux due to the relatively poor electronic conductivity. The low flux may be resolved in a similar way to Ni-BZCY membranes: combining BZY with highly electronic-conducting Ni to form Ni-BZY composite membrane. Ni-BZY membrane is expected to possess both high hydrogen permeation flux and chemical stability, which are the key factors for successful adoption of Ni-BZY hydrogen permeation membrane for practical applications. However, there has been no report on Ni-BZY composite membrane for hydrogen permeation study, probably due to the difficulty in obtaining dense membrane with large BZY grains.

 The sintering of Ni-BZY membrane needs to be performed below the melting point of Ni (~1453 ^oC). Unfortunately, due to the highly refractory nature, BZY samples prepared through the traditional solid state reaction method needs to be sintered at extremely high temperatures (1700-2100 ^oC) for a long time (24 h) to reach relatively high density. At a sintering temperature of 1400 ^oC, conventional solid state reaction method can only produce BZY with low relative density and small grain size. Moreover, BZY has a low grain boundary proton conductivity due to the blocking effect of space charge layer.^[4]Small grain size and large number of grain boundaries will greatly limit the total proton conductivity of BZY.

In this work, the sintering behavior, microstructure, and phase composition of the Ni-BZY from different methods were investigated. Dense Ni-BZY membranes with large BZY grains were successfully achieved through a two-step solid state reaction method.

Fig. 1 shows the surface and cross-section SEM images of sintered Ni-BZY membrane. The membrane is very dense. The size of BZY grains is about 1 μm. The size of Ni particles is rather large, ~ 5-10 μm. The membrane is conductive at room temperature, suggesting a connective network of Ni is formed. The flux of a 0.40-mm-thick Ni-BZY membrane at 900 ^oC in wet 20 and 40% H₂ are 3.4 and 4.3*10^-8 mol/cm²s, respectively. These values are highest among all non-BaCeO₃-based hydrogen membranes, suggesting the Ni-BZY is very promising in the application for hydrogen separation.

 Acknowledgements

We gratefully acknowledge the financial support from the HeteroFoaM Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DESC0001061, and the DOE Office of Nuclear Energy’s Nuclear Energy University Programs.

References

[1]           C. D. Zuo, T. H. Lee, S. J. Song, L. Chen, S. E. Dorris, U. Balachandran, M. L. Liu, Electrochem. Solid-State lett. 2005, 8, J35.

[2]           C. D. Zuo, S. E. Dorris, U. Balachandran, M. L. Liu, Chem. Mater. 2006, 18, 4647.

[3]           K. D. Kreuer, Ann. Rev. Mater. Res. 2003, 33, 333.

[4]           S. M. Haile, Y. Yamazaki, R. Hernandez-Sanchez, Chem. Mater. 2009, 21, 2755.

 Caption

Figure 1. (a) surface SEM image of sintered Ni-BZY membrane after thermal etching, (b) cross-section SEM image of sintered Ni-BZY membrane.