## 1 Introduction  
Population growth and economic development have resulted in global water challenges. Already today, 50% of the world population are suffering medium water shortage, while 10% are undergoing extreme water problems [1]. Moreover, it is expected that the global population would grow by nearly 40% in the next forty years [2]. During the Global Risks 2015 Report of the World Economic Forum, water shortage had been identified as the most serious challenge for human beings in the next few decades [3]. The improved demand for water sources has posed a worldwide threat to water supply systems. More than seventy percent of the Earth's surface is covered by water, but the available freshwater only accounts for a tiny fraction of the earth's total water supply [4]. The available drinking water that is obtained from groundwater and lakes is limited because much of it is too deep to access or cannot be exploited in a sustainable way. Furthermore, severe ecosystem damage caused water depletion at a striking rate across the world [5]. Oceans, containing the most abundant water resources on the earth, can provide an inexhaustible, continuous, and high-quality water supply without damaging the original freshwater ecosystems [6]. Thus, developing advanced water treatment technologies for desalination is imperative. Over the past decades, membrane technology for water desalination has rapidly changed from an academic pursuit to commercial reality. When compared with reverse osmosis (RO), electrodialysis (ED) is the most commonly applied process for the production of drinking and process water. Electrodialysis, as an electrochemical separation process, is the most economic process using ion exchange membranes (IEMs) for producing drinking water when the salinity of target water is less than 5 g/L. In order to generate high quality water to meet the requirements of specific industrial processes, membranes with selectivity for given ions are critical. Monovalent selectivity is governed by the affinity towards the fixed charge groups and the migration speed within the membrane matrix [7]. Based on these effects, IEMs with selectivity for specific ions have been explored, by including a thin charged skin layer or by generating a compact functional layer on the surface of IEMs. Thin film composite (TFC) membranes, comprising an ultrathin separating barrier layer that was prepared by interfacial polymerization on top of a membrane, could be used in electrodialysis for the purpose of separating multivalent ions from a mixed solution containing monovalent and multivalent ions [8]. However, the inevitably higher area resistance that is caused by a surficial functional layer increases the energy consumption at the same time. In recent years, thin film nanocomposite (TFN) membrane emerged as a new type of composite membranes, have been widely studied and industrially applied. Nanoparticles are incorporated within the interfacial layer of the TFC membrane, with the aim of enhancing the properties of the surface layer, such as hydrophilicity, permeability, selectivity, stability, and surface charge density [9]. Currently, TFNs are widely used in forward osmosis (FO), reverse osmosis (RO), and nanofiltration (NF) [10; 11; 12]. However, they are seldom applied in electromembrane processes. Metal-organic frameworks (MOFs), as a class of hybrid inorganic-organic solid compounds, has gained interest due to their structural and functional tunability [13]. They can serve as porous materials similar to zeolites, while having a better affinity for the polymeric chains [14]. In addition, the flexibility in pore size of MOFs can be controlled by choosing appropriate organic ligands and inorganic secondary building units, which significantly broadens their application in molecular sieving. Zeolitic imidazolate frameworks (ZIFs) are a subclass of MOFs with large surface areas and pore volumes.