## 6 Membranes with Modified Surface
An additional improvement in the properties of such membranes can be achieved by modifying the surface coating with inorganic oxide nanoparticles [280]. It can also be noted that membranes with a deposited surface layer can regulate the transfer rates of ions with different charge sizes [56], since multicharged ions predominantly adsorbed in the membrane pores limit the transfer of monovalent ones [281, 282]. Bipolar membranes containing two layers (cation-and anion-exchange) with functional groups of the opposite charge are one of the important types of ion-exchange membranes. Due to their high selectivity, they have been successfully used in a number of applications. Very interesting is the recently discovered possibility to convert sunlight into ionic electricity with bipolar membranes [283] and to apply this energy for CO\({}_{2}\) reduction [284] and for water electrolysis [285, 286, 287, 288, 289, 290, 291, 292]. The use of bipolar  
Figure 7: Optical image of the surface of the swollen MA-41 heterogeneous membrane; \({}^{\prime\prime}\)PE\({}^{\prime\prime}\) is the polyethylene binder, \({}^{\prime\prime}\)resin\({}^{\prime\prime}\) is the ion-exchange resin particle protruding over the polyethylene film (redrawn from [36]). membranes is also advantageous in reverse electrodialysis [293] and in storage of electrical energy using flow batteries [294]. There is another effective way to improve the selectivity of membranes by coating layers with fixed groups of opposite charge. For example, the surface of cation exchange membranes can be modified with polyethyleneimine [295; 296; 297; 298; 299] or polyaniline [234; 300; 301; 302; 303]. It should be noted that such membranes also differ in the asymmetry of ion transport and, in particular, in diffusion permeability [304]. This effect is determined by the ion concentration gradient, which complicates their transfer in one of the directions [305]. This phenomenon can be used to improve the selectivity of electrodialysis [306]. Surface modification by deposition of layers containing fixed ions of opposite charge is very often used to increase the selectivity of membranes to mono- and multicharged ions [300; 302; 306; 307; 308; 309; 310]. For this purpose, surface properties such as hydrophilicity, porosity, nature or concentration of functional groups can be changed [311; 312; 313; 314]. However, the layer-by-layer (LBL) modification, described in detail in recent reviews [55; 56], increasing the selectivity of transport of ions with different charge sizes, has been considered the most promising. In recent years, this approach has been widely used to create membranes for selective separation of a number of singly and doubly charged ions [309; 315; 316; 317; 318; 319; 320; 321; 322; 323; 324]. Their use permits to achieve outstanding separation ratios and developing highly efficient electrodialysis. For example, the modification of Nafion membranes made it possible to achieve a separation coefficient of K\({}^{+}\) and Mg\({}^{2+}\) ions in electrodialysis plants exceeding 1000 [320]. However, in this case, the membrane resistance increases by a factor of 20, and the limiting current drops by more than four times. Coating of the layers on the surface of AEM by electric-pulse deposition method increased the separation efficiency of Cl\({}^{-}\)/SO\({}_{4}\)\({}^{2-}\) from 8.93 to 94.4%, while the permselectivity increased from 0.81 to 47. However, a simultaneous increase in membrane resistance should be noted [325]. Nafion\({}^{\circledR}\) membranes coated with polystyrene sulfate and polyaniline showed a very high selectivity for the electrodialysis separation of Li\({}^{+}\)/Co\({}^{2+}\) and K\({}^{+}\)/La\({}^{3+}\) pairs; the separation coefficients exceeded 5000 with a decrease in the limiting current by several hundred times [326]. This approach can also be used to increase the selectivity of separation of ions of the same charge with the use of complexation. Thus, [PERSON] et al.