# Graphene-Based Membranes for CO2/CH4 Separation: Key Challenges and Perspectives
## 3 Few- to Multi-Layer Graphene-Based Stacked Laminates
### Perspectives  
To fill the technological gap, we contend that research efforts should focus on graphene-based stacked laminates that have greater potential for practical large-scale membrane applications. Specifically, graphene-based stacked laminates with good scalability and processability must be given closer attention. These include flat-sheet graphene-based stacked laminates that are prepared by shear-alignment (or commonly known as doctor-blade technique) and spray-coating techniques,  
Figure 3: Graphene-based stacked laminates as membranes using (**a**) pristine graphene oxide (GO) nanosheets with an extensive and tortuous diffusion pathway, (**b**) porous (holey) GO nanosheets with a more direct carbon dioxide (CO\({}_{2}\)) transport through the pores, (**c**) chemically functionalized GO nanosheets with a stronger affinity towards CO\({}_{2}\) molecules, leading to a facilitated transport, and (**d**) intercalated GO nanosheets with enlarged interlayer spacing and lower membrane resistance for CO\({}_{2}\) diffusion. as well as dip-coated hollow fiber graphene-based stacked laminates. As compared to vacuum- and pressure-assisted filtration techniques which are widely adopted by the academia, stacked laminates prepared from these techniques have greater relevance as they are more commonly used by the membrane industry. Hence, to ensure effective implementation of graphene-based stacked laminates, we propose to strengthen the efforts in the following five key initiatives. First, future endeavors should move from current lab-scale prototypes to large-scale demonstrations. In recent years, GO-based stacked laminates fabrications are progressively moving from tenth to thousandth of square centimeters by using the aforementioned techniques [11; 13; 38; 47]. This looks promising given that the current scale of effective membrane area is now able to support pilot-plant studies. Second, with successful large-scale demonstrations, the stacked laminate membrane design must then be optimized to maximize its CO\({}_{2}\)/CH\({}_{4}\) separation performances. This includes optimizing the lateral dimension of the graphene-based materials used, the thickness of the laminates, as well as the choice of substrates. Also, for laminates which utilized chemical functionalization and spacer intercalation to enhance the CO\({}_{2}\) permeability, it is necessary to optimize the extent of functionalization and loading of the intercalated spacers to tune the interlayer spacing between the graphene sheets, as well as for processability of the laminates and defect control. Third, creating a deeper understanding of the membrane stability and long-term separation performances of graphene-based stacked laminates is critical. In particular, biogas and natural gas contain water vapor and impurity gases, such as H\({}_{2}\)S [4; 48]. As such, we expect graphene-based stacked laminates, especially the GO-based ones, to show detrimental effects on the long-term stability given that the oxygen-containing functional groups can interact readily with the water vapor and H\({}_{2}\)S to induce competitive sorption and plasticization effect [16; 49]. Fourth, developing more effective techniques to realize hollow fiber graphene-based stacked laminates will be instrumental in improving the marketability and adoption of the membranes for CO\({}_{2}\)/CH\({}_{4}\) separation. Last but not least, there is a need to retrofit graphene-based stacked laminates processing into existing polymeric membrane fabrication manufacturing to produce a continuous production line. Without progress made in any of these initiatives, graphene-based stacked laminates for CO\({}_{2}\)/CH\({}_{4}\) separation is likely to receive poor commercial acceptance by the industry. Beyond the technological aspects, increasing efforts to provide comprehensive techno-economic analyses of graphene-based stacked laminates are absolutely essential.