Development of Mixed-Conducting Membranes for Hydrogen Separation

As the clean energy demands of the world continue to increase, so too will the use of hydrogen containing fuels. For harvesting “green” energy, hydrogen of sufficient purity will need to be extracted either from hydrogen-rich fuel sources or post-processing flue gases, requiring efficient separation of hydrogen from the many byproducts that arise from hydrogen production. This pure hydrogen can then be utilized for various power generation processes. While there are many different materials to be chosen as membrane constituents for this separation, proton-conducting ceramics represent a particularly appealing option for purification of hydrogen as a fuel source of solid oxide fuel cells. In these applications, the relatively high operating temperature of the fuel cell allows the integration of ceramic membranes with promising proton conductivity that is higher than or comparable to other hydrogen separation material systems. Additionally, these proton conductors need to attain high permeation rates with low fabrication costs[1]. Recently, a ceramic electrolyte composed of BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3-d (BZCYYb) has demonstrated high proton conductivity that has spurred research interest on its application in ceramic electrolyte and hydrogen membrane fields. [2]. In this work, we examine two main features of membrane performance, optimal hydrogen flux of the membrane and improved microstructure through scalable manufacturing techniques.

The membrane consists of a Ni-BZCYYb dense layer supported by a porous Ni-BZCYYb. The first part of the work identifies the optimal hydrogen flux through variation of the nickel content in an asymmetric dense Ni-BZCYYb pellet. The determination of the transference number for the ceramic phase allows us to optimize the electrical conductivity brought on by the nickel. The hydrogen flux through the Ni-BZCYYb was determined as a function of membrane thickness and hydrogen partial pressure. The hydrogen flux was determined by using a mass spectrometer, which correlates the measured hydrogen partial pressure to amount of hydrogen present in a known sweeping gas atmosphere. Results have shown a hydrogen flux of 0.714 ml/cm²min under 100% hydrogen at 750˚C.

The microstructure also plays an important role in the performance of an asymmetric permeation membrane. For dense ceramic membranes, the ohmic permeation resistance from the proton conduction is inversely proportional to the thickness of the membrane. By using a tape-cast method, the thickness of the dense layer can be tailored down to about 10 microns. The results demonstrate an optimized membrane in terms of nickel content and thickness of asymmetric dense/porous layers. This demonstrates the ability to fabricate a cermet hydrogen permeation membrane using a large-scale, industry method.

Further, various catalysts that may enhance hydrogen adsorption, dissociation, and oxidation on the surface of the hydrogen separation membranes are also explored and will discussed in the presentation.