Abstract Pseudo‐nanophase separation enabled by supramolecular‐interaction‐grafted sidechains proves a promising alternative for constructing high‐performance commercially viable membranes with quick ion transport, excellent chemical stability, and simplified membrane manufacturing. Nonetheless, the concept of pseudo‐nanophase separation is still in nuce, and determinants for controlling pseudo‐nanophase separation remain somewhat opaque. In this contribution, supramolecular sidechain topology is found critical to engineering pseudo‐nanophase separation. Three supramolecular sidechain topological (viz. linear, branched, and cyclic) structures are investigated using experimental and theoretical protocols, and the underlying mechanisms by which supramolecular sidechain topology alters the microstructure and ion‐conducting behaviors of the membranes are proposed. Consequently, the cyclic sidechain‐mediated membrane achieves the highest proton conductivity with an area resistance as low as 0.10 Ω cm 2 . The resulting membrane endows an acidic aqueous redox flow battery with an energy efficiency of up to 80.7% even at high current densities of 220 mA cm −2 , breaking the record set by the pseudo‐nanophase separation strategy constructed membranes and ranking among the highest values ever documented. This study advances the understanding of supramolecular sidechain topology for the design and preparation of high‐performance membranes via pseudo‐nanophase separation engineering for flow batteries and beyond.
Abstract Non‐fluorinated polymer membranes offer a commercially feasible solution for redox flow batteries (RFBs), yet their practical applications have been hampered by inherent challenges such as chemical instability and low ionic conductivity. In this study, the development of a series of ether‐bond‐free poly(aryl piperidine) membranes that address these limitations by introducing enhanced disorder in polymer chain packing through supramolecular interactions with organic acids, is presented. These interactions effectively disrupt densely packed polymer chains, transforming proton‐inaccessible crystalline regions into accessible amorphous ones. By eliminating chemically unstable aryl ether bonds and avoiding additional chemical modifications, these membranes exhibit remarkable long‐term chemical stability. The presence of abundant interchain gaps further facilitates rapid proton‐selective transport. As a result, the engineered membranes demonstrate sustained performance in vanadium RFBs, maintaining stable operation for over 1000 charge/discharge cycles, and achieving an impressive energy efficiency of 80% at a high current density of 280 mA cm − 2 . The combination of experimental data and theoretical modeling suggests that the membrane's outstanding performance arises from the interconnected and widely distributed interchain gaps, which exhibit a pore‐limiting diameter of ≈8 Å. These findings offer a robust design strategy for developing chemically stable, high‐performance non‐fluorinated membranes for RFBs and related energy conversion devices.
Abstract Nonaqueous redox flow batteries (RFBs) have received significant research interest, but the lack of promising separators with advanced performance seriously hinders the development of nonaqueous RFBs. Here, a robust yet flexible membrane with enhanced selectivity for nonaqueous RFBs is designed via in situ synthesis of metal–organic frameworks (MOFs) in a porous polymeric membrane (Celgard) with a gradient density. The crossover of active species is mitigated by the reduced effective pore size while high ionic conductivity is maintained, which is attributed to the 3D channel structure of MOFs and their gradient distribution in the membrane. A Li/ferrocene RFB with the MOF‐imbedded membrane delivers an excellent high‐rate capability and enhanced cycling stability. The discharge capacity reaches as high as ≈94% of theoretical value at a current density of 4 mA cm −2 , and maintains 76% even at 12 mA cm −2 . Moreover, a much slower capacity decay rate is achieved (0.09% per cycle over 300 cycles) by using the composite membrane compared with the pristine Celgard membrane (0.24% per cycle). The demonstrated strategy provides new insight into rational design and fabrication of size‐sieving separators for RFBs and can promote further research of MOFs' capability in energy storage.
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