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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 cm2. 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 FROM AUTHOR]

Redox flow batteries (RFBs) are among the most promising grid‐scale energy storage technologies. However, the development of RFBs with high round‐trip efficiency, high rate capability, and long cycle life for practical applications is highly restricted by the lack of appropriate ion‐conducting membranes. Promising RFB membranes should separate positive and negative species completely and conduct balancing ions smoothly. Specific systems must meet additional requirements, such as high chemical stability in corrosive electrolytes, good resistance to organic solvents in nonaqueous systems, and excellent mechanical strength and flexibility. These rigorous requirements put high demands on the membrane design, essentially the chemistry and microstructure associated with ion transport channels. In this Review, we summarize the design rationale of recently reported RFB membranes at the molecular level, with an emphasis on new chemistry, novel microstructures, and innovative fabrication strategies. Future challenges and potential research opportunities within this field are also discussed. [ABSTRACT FROM AUTHOR]

• Conductivity–selectivity dilemma in PEMs is disentangled by a morphology strategy. • Thin skinned asymmetric PEMs are fabricated by porogen-leaching-out method. • Low DS and defect-free skin layer fully guarantee low vanadium permeability. • Numerous interconnected pores are introduced to facilitate proton transfer. • APEM with both improved proton conductivity and ion selectivity is achieved. A novel integrally thin skinned asymmetric proton exchange membrane (ITSA-PEM) is proposed to disentangle the typical conductivity–selectivity dilemma in PEMs for vanadium flow batteries (VFBs). The membrane is successfully fabricated by a porogen-leaching-out method. It consists of a porous sublayer and an ultrathin skin layer, which is defect-free verified by the high H 2 /N 2 separation factor of 64.9. The degree of sulfonation (DS) of PEM is reduced to extremely low (DS = 36.3%) to suppress swelling, and numerous interconnected pores are introduced to facilitate proton transfer. Low swelling ratio and defect-free skin layer lead to undetectable vanadium permeation. Meanwhile, the area resistance of ITSA-PEM is dramatically lowered to 2.1 Ω cm2 from 5.4 Ω cm2 of the dense PEM. Therefore a membrane with both improved proton conductivity and ion selectivity is obtained. Low DS also equips the membrane with sufficient mechanical strength and enhanced thermal stability. The VFB assembled with ITSA-PEM displays high energy efficiencies (EE: 75.6–90.2%) over a current density of 20–80 mA cm−2, much superior to those of Nafion 211 (EE: 55.9–73.4%). It also shows favorable stability and slow capacity decay rate during cycling test over 50 cycles. [ABSTRACT FROM AUTHOR]

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. A gradient‐distributed metal–organic framework (MOF)–based porous membrane is designed for nonaqueous redox flow batteries (RFBs). Crossover is largely suppressed by reducing pore size while high ionic conductivity is maintained, thanks to the 3D channel structure of MOFs and unique gradient distribution in the membrane. A Li/ferrocene RFB with a designed membrane is demonstrated with excellent high‐rate capability and long‐term stability. [ABSTRACT FROM AUTHOR]

Pseudo-nanophase separation, enabled by noncovalently grafted sidechains, offers a promising approach for constructing high-performance membranes, featuring rapid ion transport, robust chemical stability, and simplified manufacturing. However, striking a balance between ionic conductivity and mechanical/chemical stability proves challenging since excessive hydrophilic grafting leads to overswelling and compromised integrity of the membranes, rendering them unsuitable for demanding applications like vanadium redox flow batteries (VRFBs). In this study, we describe a new approach for achieving high-performance VRFB membranes via employing polymer as supramolecular sidechains, rather than small molecules. This strategy achieves remarkable pseudo-nanophase separation while minimizing the utilization of functional (hydrophilic) sites. As a result, the resulting membranes exhibit exceptional robustness and proton conductivity, with an extraordinarily low area resistance of merely 0.11 Ω cm2, thus circumventing the prevailing trade-off between ionic conductivity and mechanical/chemical stability. Ultimately, VRFBs integrated with these membranes achieve energy efficiencies up to 80 % even at high current densities of 240 mA cm−2, accompanied by a remarkably low capacity decay rate of 0.064 % per cycle during long-cycle tests. This work not only achieves ultra-high conductivity with minimal functional groups, but also advances pseudo-nanophase separation strategies and provides valuable insights into optimized utilization of limited functional groups in membrane design. [Display omitted] • Polymers are proposed as supramolecular sidechains. • Underlying interaction mechanism is elucidated by experimental and simulation study. • A minimal number of hydrophilic sites enable noteworthy pseudo-nanophase separation. • Prevailing conductivity-mechanical/chemical stability trade-off is circumvented. • A VRFB with high efficiencies and slow capacity decay of 0.064 %/cycle is attained. [ABSTRACT FROM AUTHOR]

An interface-strengthened cross-linked graphene oxide/Nafion212 (CLGO/Nafion212) composite membrane was successfully fabricated and served as ion conducting membrane for vanadium flow batteries (VFBs). Firstly, the surface of Nafion212 was endowed with plenty of amino groups via in situ sol-gel chemistry approach of 3-aminopropyltriethoxysilane. Subsequently, a thin layer of m -xylylenediamino-crosslinked GO was formed using the spin-coating method. The composite membrane demonstrated much lower vanadium ion permeability (0.30 × 10−6 cm2 min−1) than pristine Nafion212 (1.88 × 10−6 cm2 min−1), mainly due to the presence of a cross-linked GO layer with angstrom-scale channels. The cell assembled with CLGO/Nafion212 exhibited enhanced coulombic efficiencies (CE s, 93.0–97.2% vs. 68.0–91.6%) and energy efficiencies (EE s, 88.9–85.2% vs. 65.7–82.2%) compared with the one with Nafion212 at 20–100 mA cm−2. At a constant current density of 80 mA cm−2, the long-term cycling tests showed that the discharge capacity fading of the flow cell equipped with CLGO/Nafion212 reached 0.25% per cycle, which was much smaller than that of Nafion212 (0.47% per cycle). The above findings proved that the cell equipped with CLGO/Nafion212 displayed superior electrochemical performances than that with Nafion212, therefore CLGO/Nafion212 composite membrane showed a great potential in VFBs. • An interface-strengthened cross-linked GO/Nafion212 membrane was prepared. • The membrane exhibited dramatically reduced vanadium-ion permeability. • The membrane showed good mechanical and chemical stabilities. • The cell with the membrane demonstrated improved efficiencies and capacity retention. [ABSTRACT FROM AUTHOR]

Widespread implementation of redox flow batteries (RFBs) is restricted by inefficient ion conducting membranes. It is especially challenging for membranes to achieve high ionic conductivity, high ion selectivity, and excellent chemical stability, simultaneously. Herein, we demonstrate a versatile solution to this toughest problem via designing multi-functional sidechain topology and manipulating their arrangement in ion conducting membranes. A series of comb-shaped ion conducting membranes with variable sidechains, along which the number of hydrophilic sites is different, are constructed, and their structure-performance relationships are comprehensively explored. Specifically, in the case of the same number of hydrophilic sites, the membranes with densely grafted short chains (the "stone") outperform those with loosely grafted long chains in terms of proton conductivity and ion selectivity (the first two "birds"), which is attributed primarily to the interconnected and size-constrained hydrophilic channels formed in the former, as demonstrated by an integrated experimental and simulation study. Furthermore, the former arrangement improves chemical stability of the membranes (the third "bird") since the size-constrained channels hamper the offensive active species from entering. These benefits result in higher energy efficiencies (EE: 92.7–71.7% at 40–200 mA cm−2) and slower capacity decay rate (CDR: 0.09% per cycle) of vanadium RFBs compared with the membranes with loosely grafted long chains (EE: 90.8–66.7% at 40–200 mA cm−2, CDR: 0.15% per cycle). This work sheds light on the path to comprehensively outstanding ion conducting membranes via molecular engineering, for energy-related devices and beyond. [Display omitted] • Sidechain topology & graft density have great impact on VRFB membrane. • Densely grafted short chain (DGSC) outperforms loosely grafted long chain (LGLC). • DGSC induced interconnected and size-constrained hydrophilic channels. • DGSC is advantageous for proton conductivity, ion selectivity & chemical stability. • DGSC enabled VRFB with high efficiencies and a slow capacity decay of 0.09%/cycle. [ABSTRACT FROM AUTHOR]