Poly(ionic liquid)s (PIL)s, as a subclass of polyelectrolytes, are composed of polymeric backbones with ionic liquid (IL)-based species in each repeating unit. Recent studies have deepened the understanding of the PIL concept in terms of characteristics, functions and applications in comparison to classical ILs and traditional polyelectrolytes. During the past two decades, PILs have developed themselves into an interdisciplinary subject among various research areas such as polymer science, materials science, catalysis, separation and sensing. Currently, the chemistry and applications of conventional polyelectrolytes are being expanded forward by the PIL concept. 

This PhD thesis deals with PIL-based porous hybrid and composite membranes. It is motivated by the growing demand on functional porous polymer membranes, in particularly, porous polyelectrolyte membranes in both industry and academia. By applying PILs as building blocks in membranes, the as-prepared porous PIL membranes combine certain desirable properties of ILs and common polymers with a wider potential to satisfy this demand. As a step further, the incorporation of functional guest substances on a molecular or nanoscale can enable new functionalities of porous membranes and broaden their application scope. 

The aim of this thesis is to develop synthetic approaches to fabricate porous PIL-based membranes based on hybridization and composition of a cationic PIL and a guest substance, and explore their diverse functions. Herein, fabrication methods based on two mechanisms were proposed and investigated. First, electrostatic complexation between a cationic hydrophobic PIL and a weak poly-/multi-acid. Second, ice-assisted phase separation of a hydrophobic PIL in water when in contact with a multi-acid compound as an ionic crosslinker. In following, task-specific functions were built up in porous PIL membranes via addition of specific metal-containing substances. This thesis content is inherently interdisciplinary, as it combines polymer chemistry and processing, membrane fabrication and materials science to secure its success in implementation, and this thesis advances the design and application scope of porous polyelectrolyte membranes.

Controlled synthesis of polymer-based porous membranes via innovative methods is of considerable interest, yet it remains a challenge. Herein, we established a general approach to fabricate porous polyelectrolyte composite membranes (PPCMs) from poly-(ionic liquid) (PIL) and MXene via an ice-assisted method. This process enabled the formation of a uniformly distributed macroporous structure within the membrane. The unique characteristics of the as-produced composite membranes display significant light-to-heat conversion and excellent performance for solar-driven water vapor generation. This facile synthetic strategy breaks new ground for developing composite porous membranes as high-performance solar steam generators for clean water production.

Herein, we established a synthetic route towards MXene/poly(ionic liquid) (PIL) composite porous membranes as a new platform of solar-thermal conversion materials. These membranes were made by a base-triggered ionic crosslinking process between a cationic PIL and a weak polyacid in solution in the presence of dispersed MXene nanosheets. A three-dimensionally interconnected porous architecture was formed with MXene nanosheets uniformly distributed within it. The unique characteristics of the as-produced composite membranes displays significant light-to-heat conversion and excellent performance for solar-driven water vapor generation. This facile synthetic strategy opens a new avenue for developing composite porous membranes as solar absorbers for the solar-driven water production from natural resources.

High-frequency responsive capacitors with lightweight, flexibility, and miniaturization are among the most vital circuit components because they can be readily incorporated into various portable devices to smooth out the ripples for circuits. Electrode materials no doubt are at the heart of such devices. Despite tremendous efforts and recent advances, the development of flexible and scalable high-frequency responsive capacitor electrodes with superior performance remains a great challenge. Herein, a straightforward and technologically relevant method is reported to manufacture a carbon fabric membrane “glued” by nitrogen-doped nanoporous carbons produced through a polyelectrolyte complexation-induced phase separation strategy. The as-obtained flexible carbon fabric bearing a unique hierarchical porous structure, and high conductivity as well as robust mechanical properties, serves as the free-standing electrode materials of electrochemical capacitors. It delivers an ultrahigh specific areal capacitance of 2632 µF cm⁻² at 120 Hz with an excellent alternating current line filtering performance, fairly higher than the state-of-the-art commercial ones. Together, this system offers the potential electrode material to be scaled up for AC line-filtering capacitors at industrial levels. 

Two-dimensional (2D) MXene-based lamellar membranes bearing interlayers of tunable hydrophilicity are promising for high-performance water purification. The current challenge lies in how to engineer the pore wall's surface properties in the subnano-confinement environment while ensuring its high selectivity. Herein, poly(ionic liquid)s, equipped with readily exchangeable counter anions, succeeded as a hydrophilicity modifier in addressing this issue. Lamellar membranes bearing nanochannels of tailorable hydrophilicity are constructed via assembly of poly(ionic liquid)-armored MXene nanosheets. By shifting the interlayer galleries from being hydrophilic to more hydrophobic via simple anion exchange, the MXene membrane performs drastically better for both the permeance (by two-fold improvement) and rejection (≈99 %). This facile method opens up a new avenue for building 2D material-based membranes of enhancing molecular transport and sieving effect.

Herein, we synthesized a series of siloxane-based poly(ionic liquid)s (PILs) with imidazolium-type species in the main chain via the multicomponent Debus–Radziszewski reaction. We employed oligodimethylsiloxane diamine precursors to integrate flexible spacers in the polymer backbone and ultimately succeeded in obtaining main-chain PILs with low glass transition temperatures (T_gs) in the range of −40 to −18 °C. Such PILs were combined with conventional hydrophobic vinylimidazolium-based PILs for the fabrication of porous membranes via interpolyelectrolyte complexation with poly(acrylic acid), which leads to enhanced mechanical performance in the tensile testing measurements. This study will enrich the structure library of main-chain PILs and open up more opportunities for potential industrial applications of porous imidazolium-based membranes.

A series of organic phosphonic acids (OPAs) were applied as multifunctional spacers to enlarge the inner space of carbide MXene (Ti3C2Tx) laminates. A synergistic improvement in permeance, rejection and stability is achieved via introducing OPA to create pillared laminates. This strategy provides a universal way to regulate transport channels of MXene-based membranes.

Herein, the fabrication of iron-containing porous polyelectrolyte membranes (PPMs) via ionic complexation between an imidazolium-based poly(ionic liquid) (PIL) and 1,1-ferrocenedicarboxylic acid is reported. The key parameters to control the microstructure of porous hybrid membranes are investigated in detail. Further aerobic pyrolysis of such porous hybrid membranes at 900 °C can transfer the ferrocene-containing PPMs into freestanding porous iron oxide films. This process points out a sacrificial template function of porous poly(ionic liquid) membranes in the fabrication of porous metal oxide films.

In this communication, a one-step synthetic route is reported toward free-standing metal-nanoparticle-functionalized gradient porous polyelectrolyte membranes (PPMs). The membranes are produced by soaking a glass-plate-supported blend film that consists of a hydrophobic poly(ionic liquid) (PIL), poly(acrylic acid), and a metal salt, into an aqueous hydrazine solution. Upon diffusion of water and hydrazine molecules into the blend film, a phase separation process of the hydrophobic PIL and an ionic crosslinking reaction via interpolyelectrolyte complexation occur side by side to form the PPM. Simultaneously, due to the reductive nature of hydrazine, the metal salt inside the polymer blend film is reduced in situ by hydrazine into metal nanoparticles that anchor onto the PPM. The as-obtained hybrid porous membrane is proven functional in the catalytic reduction of p-nitrophenol. This one-step method to grow metal nanoparticles and gradient porous membranes can simplify future fabrication processes of multifunctional PPMs.

Assembling two-dimensional (2D) materials by polyelectrolyte often suffers from inhomogeneous microstructures due to the conventional mixing-and-simultaneous-complexation procedure (mix-and-complex) in aqueous solution. Herein a mix-then-on-demand-complex concept via on-demand in situ cascade anionization and ionic complexation of 2D materials is raised that drastically improves structural order in 2D assemblies, as exemplified by classical graphene oxide (GO)-based ultrathin membranes. Specifically, in dimethyl sulfoxide, the carboxylic acid-functionalized GO sheets (COOH-GOs) were mixed evenly with a cationic poly(ionic liquid) (PIL) and upon filtration formed a well-ordered layered composite membrane with homogeneous distribution of PIL chains in it; next, whenever needed, it was alkali-treated to convert COOH-GO in situ into its anionized state COO--GO that immediately complexed ionically with the surrounding cationic PIL chains. This mix-then-on-demand-complex concept separates the ionic complexation of GO and polyelectrolytes from their mixing step. By synergistically combining the PIL-induced hydrophobic confinement effect and supramolecular interactions, the as-fabricated nanofiltration membranes carry interface transport nanochannels between GO and PIL, reaching a high water permeability of 96.38 L m(-2) h(-1) bar(-1) at a maintained excellent dye rejection 99.79% for 150 h, exceeding the state-of-the-art GO-based hybrid membranes. The molecular dynamics simulations support the experimental data, confirming the interface spacing between GO and PIL as the water transport channels.