# Graphene-Based Membranes for CO2/CH4 Separation: Key Challenges and Perspectives
## 2 Nanoporous Singe-Layer Graphene
### Perspectives  
Ensuring a uniform pore size generation and narrow pore size distribution appears to be the bottleneck towards synthesizing functional nanoporous single-layer graphene membrane for CO2/CH4 separation. This is because all the etching techniques that have been developed so far occur in a random manner with no means to provide atomic-level control of where the etching takes place. Hence, future research direction should gear towards finding more controllable means of creating nanopores on single-layer graphene. Alternatively, rather than a top-down effort, a bottom-up approach can also be used to generate uniform nanopores through structural manipulations. For example, it is possible to synthesize graphene-like nanosheets, such as graphyne and graphdiyne materials, for gas separation applications [26]. Graphyne and graphdiyne are carbon allotropes comprising both hexagonal rings and acetylenic linkages made up of sp- and sp2-hybridized carbon atoms [27]. Like graphene-based materials, they possess a 2D morphological network. Yet, the key attribute is not in the 2D network but their uniformly distributed pores (Figure 2b,c), which is something lacking in pristine graphene nanosheets [27; 28]. Owing to these intrinsic pores, \(\gamma\)-graphdiyne is regarded as highly attractive as it has been shown through molecular dynamic simulations to possess gas separation properties that are way better than graphene [28; 29]. However, one potential trade-off is the challenge in synthesizing monolayer highly-crystalline \(\gamma\)-graphdiyne with a reasonably large surface area for evaluation by gas separation microdevices. This feat is to date more challenging to achieve than single-layer graphene, especially since the growth mechanism of graphyne and graphdiyne materials has yet to be fully understood [28].  
Comparatively, the progress made in large-scale synthesis and defect-free transfer of single-layer graphene appears to show greater promise. Roll-to-roll-based production of graphene synthesis and transfer had already been demonstrated, which was able to give up to 30-inch of high-quality single-layer graphene film on a polyethylene terephthalate (PET) substrate (Figure 2d) [30]. Recently, nondestructive large-area single-layer graphene transfer had also advanced to a stage where it can be done using a simple hot-roll laminator, making the overall transfer more effective, less complex, and easily adoptable for industrial applications (Figure 2e) [31]. Still, the question remains on how to incorporate the prior steps of defects and grain boundary sealing and nanopores generation before this graphene transfer step (Figure 2a) to realize a single and continuous line that is more suited for industrial production.  
Figure 2: **(a)** Schematic showing a general approach to synthesizing nanoporous single-layer nanocomposite graphene membrane. Chemical structures of **(b)**\(\gamma\)- and **(c)**\(\beta\)-graphdiyne. **(d)** A typical roll-to-roll-based production of large-area single-layer graphene on a target substrate. Adapted from Reference [30]. **(e)** A typical large-area single-layer graphene transfer using a hot-roll laminate: (1) copper foil oxidized in deionized water for overnight, (2) the oxidized foil laminated with a commercial polyvinyl alcohol (PVA) film using a hot-roll laminator, (3) the PVA film mechanically delaminated from the copper foil after baking at 110 \({}^{\circ}\)C, and (4) the ensuing large-area single-layer graphene transferred on the PVA film. Adapted from Reference [31]. Creative Commons license CC BY 4.0.