# Graphene-Based Membranes for CO2/CH4 Separation: Key Challenges and Perspectives
## Mixed-Matrix Membranes
### Challenges  
Compared to the other membrane designed discussed in this article, one of the competitive advantages of graphene-based mixed-matrix membranes is the easy retrofitting and more scalable fabrications given minimal adaptation needed for existing polymer membrane production. Also, mixed-matrix membranes are more cost-competitive as compared to graphene-based stacked laminates due to the smaller amount of graphene-based materials needed, as well as greater reliability in terms of mechanical properties and membrane performances given that the membranes comprise primarily of polymeric materials. Despite that, there are some uncertainties regarding the prospects of graphene-based mixed-matrix membranes for CO\({}_{2}\)/CH\({}_{4}\) separation. First, there are limited number of studies on the fabrication of graphene-based hollow fiber mixed-matrix membranes. Among the 32 graphene-based mixed-matrix membranes reviewed thus far (Table 1), only one study by [PERSON] and coworkers was reported in hollow fiber configurations [63]. Hence, the impact of the graphene-based mixed-matrix membrane design on real industrial application remains unknown. Second, graphene-based materials employed in mixed-matrix membranes appear more useful in elevating the CO\({}_{2}\)/CH\({}_{4}\) selectivity of the membranes rather than enhancing the CO\({}_{2}\) permeability. Statistically speaking, among the reviewed membranes, only 66% of them exhibited a positive permeability enhancement, while 81% of them gave a positive selectivity enhancement (Table 1). In addition, there exist conflicting results in the separation performances. For example, [PERSON] et al. demonstrated a 107% enhancement in the CO\({}_{2}\)/CH\({}_{4}\) selectivity when 10 wt% of GO was loaded into a Matrimid\({}^{\textregistered}\) matrix [58]. However, [PERSON] and coworkers showed otherwise with a 36% decrease in the selectivity when 8 wt% loading of GO was used instead (Table 1) [64]. Hence, there is a need to better understand the role of graphene-based materials and the desirable attributes which they can offer. At this point, we would like to highlight that performance enhancements greater than 100% are not uncommon in the literature. This simply means that the performances are enhanced greater than two times as compared to the unfilled membranes. Third, there is a lack of effective benchmarking of the filler materials for mixed-matrix membranes. Currently, the Robeson upper bound is the benchmark used to evaluate membrane performances (Figure 4a). There are other self-proclaimed upper bounds that were redefined by next-generation materials, composite and mixed-matrix membranes [65]. However, to date, the 2008 upper bound for polymeric membranes remains the most robustly examined upper bound and so is officially recognized as the gold standard. Exceeding the upper bound has since become the holy grail of the academia. This upper bound was initially defined for polymeric membranes only and, except for the seemingly good separation performances, transcending the upper bound does not really tell more about the quality of the filler materials [5]. Hence, we contend that there is a need to supplement the Robeson upper bound with a more specific benchmark that is targeted at mixed-matrix membranes (see reference [5] for our previous arguments). \begin{table}
\begin{tabular}{l l c c c c c c c c c} \hline \hline  & & \multicolumn{1}{c}{\multirow{2}{*}{**Epither Loading (tr/s)**}} & \multicolumn{1}{c}{**Filter**} & \multicolumn{1}{c}{**Testing**} & \multicolumn{1}{c}{**Spearning**} & \multicolumn{1}{c}{**Spearning**} & \multicolumn{1}{c}{**Spearning**} & \multicolumn{1}{c}{**Spearning**} & \multicolumn{1}{c}{**Ref. **} \\  & & **(tr/s)** & & **Predes. ** & **Temp.