Abstract
The concept of liquid metal membranes for hydrogen separation, based on gallium or indium, was recently introduced as an alternative to conventional palladium-based membranes. The potential of this class of gas separation materials was mainly attributed to the promise of higher hydrogen diffusivity. The postulated improvements are only beneficial to the flux if diffusion through the membrane is the rate-determining step in the permeation sequence. Whilst this is a valid assumption for hydrogen transport through palladium-based membranes, the relatively low adsorption energy of hydrogen on both liquid metals suggests that other phenomena may be relevant. In the current study, a microkinetic modeling approach is used to enable simulations based on a five-step permeation mechanism. The calculation results show that for the liquid metal membranes, the flux is limited by the dissociative adsorption over a large temperature range, and that the membrane flux is expected to be orders of magnitude lower compared to the membrane flux through pure palladium membranes. Even when accounting for the lower cost of the liquid metals compared to palladium, the latter still outperforms both gallium and indium in all realistic scenarios, in part due to the practical difficulties associated with making liquid metal thin films.
Highlights
Hydrogen selective membranes have the potential to enable efficient and low-cost hydrogen separation and purification
The objective of this study is to look into the differences between palladium-based membranes and liquid metal membranes and to be able to provide a basis for optimization of the latter
Based on the results from this microkinetic modeling study, it can be concluded that liquid metal membranes for hydrogen separation based on gallium or indium will likely operate in the adsorption-limited regime
Summary
Hydrogen selective membranes have the potential to enable efficient and low-cost hydrogen separation and purification. It was reported that the high surface tension of the liquid metals and complex wetting behavior introduces practical complications in the production of thin films and in the selection of an appropriate support material. These complications were addressed by the use of a sandwiched configuration with SiC porous supports and rather thick films (in the order of several hundred μm) [8]. Whilst the permeability is much higher compared to palladium, the flux in actual application is expected to be lower as palladium-based membranes with a thickness of several μm can be prepared
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