Hydrophilic Channel Alignment of Perfluoronated Sulfonic-Acid Ionomers for Vanadium Redox Flow Batteries

Abstract

It is known that uniaxially drawn perfluoronated sulfonic-acid ionomers (PFSAs) show diffusion anisotropy because of the aligned water channels along the deformation direction. We apply the uniaxially stretched membranes to vanadium redox flow batteries (VRFBs) to suppress the permeation of active species, vanadium ions through the transverse directions. The aligned water channels render much lower vanadium permeability, resulting in higher Coulombic efficiency (>98%) and longer self-discharge time (>250 h). Similar to vanadium ions, proton conduction through the membranes also decreases as the stretching ratio increases, but the thinned membranes show the enhanced voltage and energy efficiencies over the range of current density, 50-100 mA/cm. Hydrophilic channel alignment of PFSAs is also beneficial for long-term cycling of VRFBs in terms of capacity retention and cell performances. This simple pretreatment of membranes offers an effective and facile way to overcome high vanadium permeability of PFSAs for VRFBs.

Reprogrammable Three-Dimensional Configurations Using Ionomer Bilayers

“…PFSA is commercially available ionomer, for example, Nafion, Aquivion, and 3 M PFSA that strong hydrophilic sulfonic acid groups are well phase separated in nanoscale (3–5 nm diameter) from extremely hydrophobic polytetrafluorethylene-like domains. , The orientation of ion conducting hydrophilic channels have been studied to understand the nanostructures of PFSAs, − and to enhance the performance in energy-related applications of proton conducting membranes. − It has been reported that the cylindrical hydrophilic channels PFSAs are preferentially aligned along the machine direction (MD) of the extrusion-casted, and uniaxially drawn membranes. − The information on hydrophilic channels in PFSA membranes was obtained using SAXS. Figure a–d shows representative 2D SAXS patterns of N115 depending on λ measured using a synchrotron light source.…”

Geometry-Induced Asymmetric Vanadium-Ion Permeation of PVDF Membranes and Its Effect on the Performance of Vanadium Redox Flow Batteries

“…Figure shows the single-cell performances under various current densities from 50 to 100 mA/cm 2 . Voltage efficiency (VE) and Coulombic efficiency (CE) are proton and vanadium permeability-related efficiencies, respectively, under the similar membrane thickness . The membrane thicknesses for the cell performance were similar (115.6 ± 9.5 μm for PVDF30, and 116.0 ± 8.3 μm for PVDF40).…”

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

“…Auniaxial stretching method was applied to Nafion membranes by So,Hong et al to reduce vanadium crossover for VRFB (Figure 7a). [84] Theh ydrophilic channels could be aligned along the drawing direction, as evidenced by smallangle X-ray scattering (SAXS) data. Thet ortuosity of the channels along the drawing direction decreased, while that along the transverse directions increased.…”

“…Reproducedfrom ref. [84] with permission from the American Chemical Society.b)Molecular arrangement at the air/water interface. Reproducedf rom ref.…”