(785e) Supported Molten Metal Membranes for Hydrogen Separation: Promise and Pitfalls

Palladium hydrogen separation membrane are being developed for reformers, as they are more compact, energy efficient, and selective as compared to the traditional technologies, i.e., absorption, pressure-swing adsorption, and cryogenic distillation. However, they are expensive, sensitive to poisons, and lack adequate durability. Therefore, the search for novel and cheaper materials to replace palladium is ongoing. We are, thus, developing entirely novel supported molten metal membranes (SMMMs) based on low melting metals and their alloys because of their higher diffusivity and greater potential robustness to poisons, and for their inherent ability to avoid sintering, hydrogen embrittlement, and thermal mismatch between the membrane and the support. In this presentation, we will describe the promise of SMMM and the pitfalls we have so far encountered in this quest.

Development of thin and stable SMMMs requires a basic understanding of the wettability and reactivity between porous support and the molten metals. This is a fine balance since some chemical interaction between the liquid metal and the support is necessary for adequate wetting, while too much of it dooms the membrane. Therefore, a goal of our work was to find a porous support material with both good wettability and stability. A thermodynamic assessment of molten Ga with various ceramic materials was performed to estimate the stability and wettability. Coupon test of selected material was also performed for comparison. It was found that materials with good wetting tend to be unstable and materials with good stability tend to have bad wetting with few exceptions among tested materials, most notable silicon carbide (SiC). A further complication is that the wetting changes with the gas composition. Such issues are incompletely understood and are being investigated. Hydrogen permeance experiments with dense gallium molten metal membrane showed that the measured hydrogen permeance of Ga is comparable to Pd membrane at 500-550 , which makes molten Ga a potential material as hydrogen separation membrane.