Permselective anion exchange membranes (AEMs) are of great interest in electrochemical devices, allowing the selective transport of species of interest through the membrane while preventing the crossover of undesired substrates. However, porous and nonporous membranes have traditionally been developed and applied separately for these applications. In this study, we utilized the merits of polymerization-induced microphase separation to observe the transition from porous to nonporous membranes by controlling the etchable block length and the hydrophilic domain size, respectively. At a certain point, a transition from porous to nonporous permeation behavior was observed, where permeation no longer depended on the open pore size. For the fabrication of AEMs with well-defined and 3D continuous nanochannels, we integrated an etchable sacrificial block with a pre-functional block containing precursors for quaternary ammonium groups to create a pore decorated with positively charged dangling chains. Etching and amination yielded the target AEM with a controlled pore size and ammonium ion density. Increasing the length of the dangling chains resulted in nonporous AEMs, which allowed exclusive permeation of a negatively charged dye over a positive one. This provides strong evidence that the solution-diffusion model applies to nonporous membranes. Tunable permselectivity over a wide range, combined with stability in organic solvents and alkaline conditions, suggests that this methodology holds significant potential for the development of membranes for advanced electrochemical systems.

Despite extensive studies on nanoporous membranes for regulating lithium-ion (Li⁺) flux in lithium (Li)-metal batteries, the pore size design has largely focused on very small (< 5 nm) or extremely large (> 20 nm) dimensions, overlooking the intermediate pore size range. This gap, particularly between 5 and 15 nm, has limited exploration of critical Li⁺ transport phenomena and their impact on improving cell performance. Here, we developed robust and free-standing polymeric films with three-dimensional (3D) continuous nanoporous channels, precisely tuned to pore diameters ranging from 5 to 14 nm and immobilized sulfonate groups. Our systematic investigations revealed how pore size and immobilized anionic groups correlated with Li⁺ conductivity and battery performance. Notably, sulfonate-functionalized channels promoted Li⁺ conductivity significantly within this optimal pore range compared to non-functionalized counterparts. In an ether-based electrolyte with 1 M lithium bis(fluorosulfonyl)imide (LiFSI), the Li⁺ conductivity peaked at a pore diameter of 10 nm. Furthermore, the mobility of Li⁺ was approximately 4.4 times faster than FSI⁻, resulting in reducing interfacial resistance and promoting uniform Li deposition. The sulfonated nanoporous membrane in Li|LiFePO₄ full cells with an N/P ratio of 2.3 delivered excellent cycling stability over 1000 cycles while retaining approximately 80 % of the initial capacity.

We developed a block polymer-based synthetic route to sulfonated porous composites (SPCs) with precisely controlled nanopore size. By reducing the pore size to <4 nm and introducing a high density of surface sulfonic acid, the permeation of vanadium ions was effectively suppressed. We employed a polymerization-induced microphase separation (PIMS) process, in which a polyethylene fiber mat impregnated with a liquid polymerization mixture was spontaneously transformed into a fiber-reinforced and cross-linked block polymer membrane. Selective etching and sulfonation then produced the target composite membrane. In a vanadium redox flow battery (VRFB) cell, an SPC with 3.6 nm-sized mesopores, 109 m2 g–1 of specific surface area, and 0.3 mL g–1 of mesoporosity outperformed a Nafion 212 membrane of similar thickness, providing higher proton conductivity and much lower vanadium permeability. Thanks to the composite reinforcement, the membrane demonstrated remarkably enhanced mechanical stability. The SPC membrane could be successfully operated up to 300 cycles. Compared with Nafion 212, the SPC exhibited higher energy efficiencies (EEs) and higher discharge capacity retention. These results suggest the promise of block polymer-based permselective membranes in advanced battery applications.

We investigated proton conductivity and the permeability of monovalent cations across sulfonated mesoporous membranes (SMMs) prepared with well-defined pore sizes and adjustable sulfonic acid content. Mesoporous membranes with three-dimensionally continuous pore structure were produced by the polymerization-induced microphase separation (PIMS) process involving the reversible addition–fragmentation chain transfer (RAFT) copolymerization of styrene and divinylbenzene in the presence of a polylactide (PLA) macrochain transfer agent and subsequent PLA etching. This allowed us to control pore size by varying PLA molar mass. Postsulfonation of the mesoporous membranes yielded SMMs whose pore structure was retained. The sulfonic acid content was adjusted by reaction time. While proton conductivity increased with increasing ion exchange capacity (IEC) without noticeable dependence on the pore size, ion permeability was strongly influenced by the pore size and IEC values. Decreasing pore size and increasing IEC resulted in a decrease in ion permeability, suggesting that ions traverse across the membrane via the vehicular mechanism, through the mesoporous spaces filled with water. We further observed that the permeability of the vanadium oxide ion was dramatically suppressed by reducing the pore size below 4 nm, which was consistent with preliminary vanadium redox flow battery data. Our approach suggests a route to developing permselective membranes by decoupling proton conductivity and ion permeability, which could be useful for designing separator materials for next-generation battery systems.