# Graphene-Based Membranes for CO2/CH4 Separation: Key Challenges and Perspectives
## 1 Introduction  
The strong demand for energy to meet the increasing population and economic growth has motivated a shift towards alternative energy resources to supplement the depleting conventional fossil fuels [1]. Such a change is also driven by the escalating environmental and public health issues associated with the anthropogenic climate change [2, 3]. As a result, methane gas (CH4) attracts an immense amount of attention as it is a relatively cleaner form of fuel with a lower carbon per unit energy as compared to other fossil fuels. Moreover, there is an abundant amount of CH4 in the form of natural gas, and sustainable supplies of biogas also make CH4 a good choice of renewable energy [4, 5]. Typically, natural gas and biogas can contain up to 90% CH4 with up to 50% carbon dioxide gas (CO2) [4]. To increase the calorific value and marketability of natural gas and biogas, there is a need to sweeten and enrich the amount of CH4 in them. CO2/CH4 separation, therefore, becomes an important application.  
Conventionally, CO2/CH4 separation has been carried out using chemical and water scrubbing, adsorption processes and cryogenic distillation [6]. Membrane-based CO2/CH4 separation is a relatively newer technology which boasts higher energy-efficiency and cost-effectiveness, as well as a more robust design with a smaller plant footprint [5; 7]. Hence, membrane separation is a promising technology to realize high-performance CO2/CH4 separation. To achieve this vision, advanced membranes using state-of-the-art materials are necessary [8]. Recently, graphene-based materials have been showcased as an attractive material for membrane preparation, owing to their two-dimensional (2D) morphology and tunable physicochemical properties [9]. In particular, the monoatomic thickness of graphene-based materials gives theoretically the lowest transport resistance possible of a membrane [10] and their rich chemistry enables tailorable chemical functionalization to suit the needs of different membrane-based applications [9; 11]. Among the different graphene-based materials exploited thus far, graphene oxide (GO) shows exceptional hydrophilicity, as well as water solubility [12]. Thus, the competitive advantage of GO lies in its solution processability, which allows easy processing into continuous films [13], and seamless integration with existing membrane fabrication techniques [11]. For this reason, GO garners widespread attention from the membrane community and is one of the most extensively used graphene-based materials for designing membranes to date.  
In this paper, we do not seek to furnish the reader with a comprehensive overview of graphene-based membranes for CO2/CH4 separation since several high-quality reviews are already available in the literature [9; 11; 14; 15; 16; 17; 18]. Rather, we aim to arouse awareness of various challenges faced by current graphene-based membranes for CO2/CH4 separation. By doing so, we hope to kindle the interest of researchers by sharing solution-oriented perspectives that can potentially put together effective strategies for resolving these challenges. As such, we initiate our discussion from three mainstream graphene-based membrane designs, namely, (1) nanoporous single-layer graphene; (2) few- to multi-layer graphene-based stacked laminate and (3) mixed-matrix membrane (Figure 1). We first present a brief review summarizing the current progress to help the reader grasps the essential background of this research field, before proceeding to focus on several key challenges for each membrane design. The discussion then closes off with useful insights from positive implementation in practice or problem-solving strategies that can be prospective solutions to help advance graphene-based membranes for CO2/CH4 separation. Beyond CO2/CH4 separation, these insights are also of relevance to a wider community with interests in other gas separation and water treatment applications.