# Graphene-Based Membranes for CO2/CH4 Separation: Key Challenges and Perspectives
## 3 Few- to Multi-Layer Graphene-Based Stacked Laminates  
Relative to the single-layer graphene membrane, the few- to multi-layer stacked graphene-based laminates, with more than one layer of nanosheet, can easily form a continuous film. The requirement on the quality and integrity of the nanosheets is, therefore, less stringent for this type of membrane design. Hence, there is no need for an elaborated bottom-up CVD synthesis to obtain high-quality graphene. Instead, membranes with few- to multi-layer stacked graphene-based laminates are usually prepared from a top-down approach, mostly using graphene oxide (GO) nanosheets that are derived from the oxidation of graphite. GO nanosheets are by far the most commonly used graphene-based material for preparing stacked laminates, owing to their facile synthesis and rich surface chemistry arising from oxygen-containing functional groups, such as the hydroxyl, carboxyl, and epoxy groups [5; 9]. More importantly, GO nanosheets are highly water-soluble, making them solution processable and easy for membrane fabrications [11]. To date, GO-based stacked laminates are generally fabricated from a myriad of techniques, such as vacuum-assisted filtration [32; 33], pressure-assisted filtration [13; 34], spin-coating [35; 36], spray-coating [37], dip-coating [12], shear-alignment [38], and layer-by-layer (LbL) techniques [39]. GO-based stacked laminates can range from a thickness of 1.8 nm (\(\sim\)3 layers of GO nanosheets) to several micrometers (multi-layered) [13; 32]. The ideal thickness of the laminates depends on the gas pairs to be separated and the target applications. For example, [PERSON] et al. prepared an ultrathin GO laminated membrane of 1.8 nm in thickness for selective hydrogen separation. The study, however, demonstrated a rather anomalous result where CO\({}_{2}\) permeance was reportedly lower than that of CH\({}_{4}\), making the membrane irrelevant for CO\({}_{2}\)/CH\({}_{4}\) separation [32]. In another work, [PERSON] et al. synthesized a slightly thicker membrane at \(\sim\)3 nm for CO\({}_{2}\) separation. The results showed CO\({}_{2}\) as the more permeable gas with an intrinsic permeability of \(\sim\)8500 barrer and CO\({}_{2}\)/CH\({}_{4}\) selectivity of \(\sim\)10 [36]. As the thickness continues to increase to the micrometer scale, GO laminates also evolve from a selective membrane into a gas-diffusion barrier where O\({}_{2}\) permeability can even reach lower than 0.01 barrer [19]. Such gas-diffusion barriers usually find applications in food packaging and electronic devices [13; 19]. The transport mechanism behind these stacked laminates stems from the tortuous diffusion pathways that gas molecules have to take in order to permeate through the laminates (Figure 3a). Undoubtedly, the most immediate impact is on the gas permeability. This is because the transport mechanism through the laminate is more often than not governed by a solution-diffusion mechanism where permeability is a function of solubility and diffusivity. As the thickness of the laminates increases, the diffusion pathway gets longer and more extended. For this reason, CO\({}_{2}\) diffusivity decreases and the difference in solubility and diffusivity between CO\({}_{2}\) and CH\({}_{4}\) is magnified. The CO\({}_{2}\)/CH\({}_{4}\) selectivity, therefore, gets elevated but at the expense of the CO\({}_{2}\) permeability. The thickness of GO-based stacked laminates is one of the most crucial criteria for making a defect-free continuous film. In this regard, thicker laminates are better as they are not only more processable from the membrane fabrication standpoint but defects arising from incomplete coverage of GO nanosheets can be rectified by increasing the thickness. Also, thicker laminates are easier to handle and manage. Hence, GO-based stacked laminates can be readily adopted as barrier films. For application as selective membranes, the use of thicker laminates is usually accompanied by a compromised gas permeability, including CO\({}_{2}\).