(459f) Gas Mixture Separation through Nanoporous Graphene Membranes

Graphene sheets perforated with nanometer-scale pores have the potential to become high performance gas mixture separation membranes. Gas molecule translocation through molecularly-sized graphene pores can be an activated process, and differences in activation energy between different gases could give rise to high separation factors. These membranes could also achieve extremely high permeance due to graphene’s single-atom thickness. Finally, graphene’s outstanding chemical, thermal, and mechanical stability could enable these membranes to withstand the pressure differentials and elevated temperatures necessary for gas separations.

To date, very few experimental measurements of gas mixture permeation through graphene membranes have been reported. Some prior experimental works have investigated the transport of single gases through graphene membranes with nanometer-scale pores,¹ larger rips and tears,² or capillaries between multilayered graphene sheets.^{3, 4} In these studies, gas separation factors were estimated from single gas permeation measurements, which may not accurately account for phenomena such as competitive adsorption and diffusion that could influence mixture separations with these membranes.^5-8 Park and coworkers provided the first demonstration of gas mixture separation through graphene membranes with pores of 7.6 nm to 1 μm in diameter.⁹ While these studies have provided highly valuable insights into of the properties of graphene membranes, further direct measurements of gas mixture separations are critical for understanding graphene membrane separation performance.

For the first time, we have measured the temperature dependence of H₂, He, CH₄, CO₂, and SF₆ permeance from 22 – 208 °C through intrinsic defects in three suspended graphene membranes. These measurements reveal surprising insights about the mechanisms of gas permeation through the graphene membranes. The three membranes we tested show substantially different behavior. Membrane A exhibits permeance on the order of 10^-5 to 10^-4 mol m^-2 s^-1 Pa^-1 with minimal variation as a function of temperature and decreasing flux with increased gas molecular weight, suggesting that transport through this membrane is likely dominated by the direct gas impingement pathway. Membrane B shows complex behavior, with the permeance increasing as a function of temperature from 27 to 200 °C, and a CO₂/SF₆ separation factor of 12 at 200 °C, likely indicating a mix of several gas permeation mechanisms, potentially including activated translocation through molecularly sized nanopores. Membrane C shows no measureable permeance of any gas above the detection limit of our technique, indicating that it may be a molecularly impermeable barrier. Overall, this work demonstrates that graphene membranes can provide high mixture separation selectivity, and temperature-dependent permeance measurements are an important strategy for understanding complex physical phenomena involved in these separation processes.

References

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2. M. S. H. Boutilier, et al., ACS Nano, 8, 841 (2014).

3. R. K. Joshi, et al., Science, 343, 752 (2014).

4. H. W. Kim, et al., Science, 342, 91 (2013).

5. L. W. Drahushuk, et al., Langmuir, 28, 16671 (2012).

6. D. E. Jiang, et al., Nano Letters, 9, 4019 (2009).

7. J. Schrier, ACS Applied Materials & Interfaces, 3, 4451 (2011).

8. H. J. Liu, et al., Nanoscale, 5, 9984 (2013).

9. K. Celebi, et al., Science, 344, 289 (2014).