Ultrahigh Permeance Metal-Coated Graphene Membranes for Efficient Gas Separation Applications

Graphene is considered as the ideal membrane due to its atomic thickness. Indeed, a million times higher permeance values were realized using porous graphene compared to the traditional membranes. However, the selectivity values of these membranes were rather low, in the limits of Knudsen selectivity. In order to achieve high selectivity on the graphene membranes, a large number of pores below 3 nm are required to operate in the molecular sieving regime without sacrificing the permeance. However, this is highly challenging, and currently, existing pore generation techniques result either in high permeance-low selectivity or vice-versa. Herein, in order to overcome this challenge, we have developed a novel adsorptive membrane approach, in which one of the gases interacts with the adsorbent surface and is retained while the non-interacting one passes through the membrane. By doing so, we achieved complete separation of the helium and hydrogen mixture, which is not possible using conventional approaches moreover, using graphene as a membrane support allowed us to obtain permeance values in the range of 10⁷ GPU (1 GPU = 3.35x10^-10 mol s^-1 m^-2 Pa^-1).<br/>In another approach, we also developed a controlled pore-tuning method to create a large number of pores below 3 nm. We deposited the gold layer in a stepwise manner and studied molecular transport properties. Initially, pristine graphene showed Knudsen selectivity, however, upon increasing the thickness of the gold layer, the gas transport started to shift to surface diffusion where hydrogen was favored over other gases. Eventually, the deposition of more gold led to the molecular sieving and we achieved a record high H₂/CO₂ selectivity of 31.3 at an H2 permeance of 2.23x10⁵ GPU.<br/><br/>References<br/>T. Ashirov & A. Coskun. <u><i>Chem</i></u>, <b>7</b> (2021) 2385-2394<br/>T. Ashirov, AO. Yazaydin, A. Coskun, <u><i>Adv. Mater</i></u>., <b>34</b> (2022) 2106785<br/>K. Celebi et. al, <u><i>Science</i></u>, <b>344 </b>(2014) 289-292