高效分子离子传递膜(英文版)

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