Gradient‐Distributed Metal–Organic Framework–Based Porous Membranes for Nonaqueous Redox Flow Batteries

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.

Pathways to Widespread Applications: Development of Redox Flow Batteries Based on New Chemistries

“…18 Metal-organic frameworks (MOFs) consist of repeating well-patterned structures with metal ions coordinated to organic ligands, and the highly tunable interstitial channels within MOFs are capable of mitigating shuttle effects of redox species. 19 Moreover, some inorganic ceramics have been used as membranes for novel RFBs as well, and the high elastic moduli of ceramics are beneficial for applications in harsh environments. 20 These superionic conductors only allow for the transport of selected charge carriers, preventing crossover of any redox species and solvents.…”

Directional asymmetry over multiple length scales in reticular porous materials

“…A different method to achieve a gradient distribution of MOFs inside a composite membrane, via gravity-assisted solvent evaporation, was recently proposed by Peng et al 75 In their work, a commercial Celgard membrane (polypropylene, PP) was immersed into a Cu(NO 3 ) 2 and H 3 btc solution in DMF at 100 °C for a certain period of time to allow the diffusion of the precursors into the membrane. Then, the membrane was transferred to a glass plate and kept at 80 °C for solvent evaporation and growth of HKUST-1 seeds.…”

A Chemistry and Microstructure Perspective on Ion‐Conducting Membranes for Redox Flow Batteries

“…MOFs and inorganic materials with intrinsic sub-nanometer pores have been explored as membrane materials for RFB applications.V arious MOFs exhibit intrinsic 3D channels as well as high porosity,h olding great potential as membrane materials for RFBs.T he first proof-of-concept work was conducted by He,Y u, and co-workers. [131] A gradient-distributed MOF-based porous membrane was designed for NARFBs (Figure 14 a) and obtained via in situ growth of MOFs.I tw as expected that the selectivity of the membrane would be improved via the sieving effect of MOFs; meanwhile,h igh ionic conductivity could be achieved due to the 3D ion transport channels constructed by MOFs as well as the low effective thickness of the MOF layer. As expected, the as-prepared membrane exhibited amuch higher permeability to balancing ions (Li + )than to various potential active species in NARFB systems,and the selectivity of Li + to active species increased by nearly four times compared with that of the neat [131] with permission from John Wiley and Sons.…”

“…[131] A gradient-distributed MOF-based porous membrane was designed for NARFBs (Figure 14 a) and obtained via in situ growth of MOFs.I tw as expected that the selectivity of the membrane would be improved via the sieving effect of MOFs; meanwhile,h igh ionic conductivity could be achieved due to the 3D ion transport channels constructed by MOFs as well as the low effective thickness of the MOF layer. As expected, the as-prepared membrane exhibited amuch higher permeability to balancing ions (Li + )than to various potential active species in NARFB systems,and the selectivity of Li + to active species increased by nearly four times compared with that of the neat [131] with permission from John Wiley and Sons. b) Schematic of the permeation of balancingi ons and active species through a2D MOF layer in nonaqueous systems.…”