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Chapter 1 Introduction to Membrane Jingtao Wang and Wenjia Wu In the past decades�� membrane technology has been widely utilized in various separation processes�� because of their low-energy consumption�� low-cost�� reliability�� and scalability when compared with conventional separation processes like distillation�� extraction�� or crystallization [1��2]. In order to further increase the competitiveness�� intensive efforts have been made from improving the separation efficiency of existing membrane processes to exploring new applications. As the core part�� membrane materials with high permeability�� high selectivity�� and high stability are extremely desired since they can significantly accelerate the practical application of membrane technology [3�� 4]. To date�� plenty of membranes with different pore sizes have been developed�� such as polymer membrane�� ceramic membrane�� two-dimensional (2D) lamellar membrane�� molecule sieving membrane�� hybrid membrane�� and composite membrane [5-10]. These membranes have been widely used for different separation processes including�� microfiltration�� ultrafiltration�� nanofiltration�� reverse osmosis�� gas separation�� and proton/ion conduction�� etc. [11�� 12]. For each category of membrane�� the physical and chemical environments of transfer channels are of great importance in manipulating the comprehensive properties. The physical environments are dictated by the connectivity�� tortuosity�� and size of transfer channels�� while the chemical environments are dictated by the type�� amount�� and distribution of functional groups within transfer channels [13]. Generally�� ideal transfer channels should integrate the following attributes: (i) they should be short with appropriate transfer environment to endow membranes with high permeability�� (ii) the channel size distribution should be narrow to endow membranes with high selectivity�� and (iii) the chemical and mechanical stability should be high to endow membranes with long-term operation stability [14]. Currently�� polymers are the dominant membrane materials�� due to their easy processability and high scale-up capability. For conventional polymer membranes�� breaking the permeability-selec-tivity or permeability-stability trade-off remains a challenge. The great progress in polymer membranes over the past decades has brought about the booming of novel kinds of structured membranes including�� hybrid membrane�� composite membrane�� and phase-separated membrane�� which push the separation performances of polymer membranes to new records [15-18]. Hybrid membrane is an intricately structured membrane configuration�� owing to its merit of coupling the good flexibility and processability of polymers with the regular topological structure as well as the tunable functionality of fillers [19�� 20]. Impermeable fillers such as silica particles�� graphene oxide (GO) nanosheets�� and organic/inorganic nanorods can induce a distortion of chain alignment to improve the free volume property or induce the construction of long-range�� ordered transfer channels in membrane [21��22]. Permeable fillers such as metal-organic frameworks (MOFs)�� covalent organic frameworks (COFs)�� and zeolite can afford additional transfer pathways and mechanisms to membrane including�� molecule sieving�� and selective adsorption [23�� 24]. Composite membrane for molecule transfer is generally a heterogeneous membrane with dense separation layer and porous support layer�� where the separation layer and the support layer can be separately optimized to achieve simultaneously high separation performance and stability [25��26]. Particularly�� the fabrication of composite membrane with an ultrathin separation layer is deemed as a delicate strategy to achieve highly permeable membrane�� which is one of the most important pursuits for membrane technology [27�� 28]. At present�� researches related to composite membranes mainly focus on the precise manipulation of physical structure and chemical component of separation layer; however�� these remain challenging due to the pursuit of ultrathin thickness. For proton/ion separation�� electrospinning is increasingly recognized as a powerful mean for introducing unique phase-separated architectures into composite membranes [29]. Indeed�� it allows the elaboration of composite membranes with a rather facile mean to control of the long-range organization/distribution/percolation ofhydrophilic and hydrophobic domains of the ionomer by adjusting the type of electrospun material�� the volume fraction of nanofibers�� and the experimental conditions [30]. Moreover�� electrospinning can impart uniaxial alignment of polymer chains within nanofibers�� resulting in enhanced mechanical properties. Importantly�� it can promote the formation of interconnected transfer channels�� which facilitate the improvement in proton/ion conduction [31]. In recent years�� 2D nanosheets�� with a thickness of one to a few atoms�� have become the promising building blocks for ad