Nanofiber Production Equipment| SCOF Hollow Fiber Constructing Ion Selective Conduction Nano-Pipeline Network for Vanadium Redox Flow Batteries

Hollow covalent organic frameworks (COFs) can reduce proton conduction resistance, but existing hollow microspheres or nanorods have low aspect ratios and only serve as dispersed phases in membranes, limiting the full utilization of their intrinsic properties. To address this, this study first synthesized sulfonated covalent organic framework (HF-SCOF) materials with hollow fiber morphology. By controlling the dissolution-diffusion behavior of monomer-containing electrospun fibers in different COF synthesis solvents, the hollow structure of SCOF fibers was effectively regulated.

The resulting HF-SCOF fiber membrane exhibits a long-range oriented structure, where the hollow cavities absorb water/acid, and the outer walls are rich in sulfonic acid groups and protonated secondary amine groups. This constructs a continuous H⁺/Vⁿ⁺ ion-selective conductive nano-pipeline network, maximizing the intrinsic structural advantages of SCOF materials. After densification with sulfonated polybenzimidazole, the composite membrane demonstrated outstanding performance in vanadium redox flow batteries (VRFBs): achieving 81.9% energy efficiency at 200 mA cm⁻² and maintaining stability over 1000 charge-discharge cycles, surpassing most reported COF-based ion-conductive membranes.

Recently, Prof. He Gaohong, Prof. Wu Xuemei, and Engineer Cui Fujun from Dalian University of Technology published their latest research, "SCOF Hollow Fiber Constructing Ion Selective Conduction Nano-Pipeline Network for Vanadium Redox Flow Batteries," in Advanced Energy Materials. The study reveals how the hollow fiber SCOF morphology and its continuous nano-pipeline network regulate H⁺/Vⁿ⁺ ion selectivity, leading to superior battery performance.

1.HF-SCOF synthesis, morphology, and H⁺/Vⁿ⁺ selectivity mechanism.

By adjusting the dissolution-diffusion behavior of monomer-containing electrospun fibers in different solvents, hollow fiber SCOF was synthesized for the first time. When water was used as the solvent (where neither PAN electrospun fibers nor monomer Tp dissolve), only water and DABA monomers diffused outward, forming hollow fibers with tunable wall thickness. In methanol (which dissolves PAN and Tp), bidirectional diffusion created SCOF short nanowires on the outer walls.

The HF-SCOF membrane’s nano-pipeline network breaks the H⁺/Vⁿ⁺ conduction trade-off. The hollow cavities absorb water/acid, while the sulfonic acid-rich walls and protonated secondary amine Donnan exclusion effect synergistically establish fast proton channels and a vanadium ion barrier. The H⁺/Vⁿ⁺ selectivity reached 9.9 × 10⁹ mS·s·cm⁻³, 5.5× higher than Nafion 212 (1.8 × 10⁹).

2.HF-SCOF membrane properties: (a) water uptake/swelling, (b) proton conductivity/area resistance, (c) V²⁺ permeability/H⁺/Vⁿ⁺ selectivity, (d) tensile strength/elongation.

The superior H⁺/Vⁿ⁺ ion selectivity of hollow fiber SCOF significantly enhances VRFB performance. The continuous HF-SCOF nano-pipeline network maximizes the intrinsic structural advantages of SCOF materials. The composite membrane densified with sulfonated polybenzimidazole achieves an outstanding energy efficiency of 81.9% at 200 mA·cm⁻², significantly surpassing Nafion 212 (74.9%) and SPBI (77.5%). At 180 mA·cm⁻², the capacity decay rate is only 0.13% per cycle, far lower than Nafion 212's 0.37%. After 1000 charge-discharge cycles, the battery maintains stable performance with intact membrane chemical structure.

3.VRFB performance comparison: (a) Coulombic efficiency, (b) voltage efficiency, (c) energy efficiency, (d) cycling stability at 180 mA cm⁻², (e) benchmark against literature.

Paper link: https://doi.org/10.1002/aenm.202500523