Abstract
Highly conductive and low vanadium permeable crosslinked sulfonated poly(ether ether ketone) (cSPEEK) membranes were prepared by electrophilic aromatic substitution for a Vanadium/Air Redox Flow Battery (Vanadium/Air-RFB) application. Membranes were synthesized from ethanol solution and crosslinked under different temperatures with 1,4-benzenedimethanol and ZnCl2 via the Friedel–Crafts crosslinking route. The crosslinking mechanism under different temperatures indicated two crosslinking pathways: (a) crosslinking on the sulfonic acid groups; and (b) crosslinking on the backbone. It was observed that membranes crosslinked at a temperature of 150 °C lead to low proton conductive membranes, whereas an increase in crosslinking temperature and time would lead to high proton conductive membranes. High temperature crosslinking also resulted in an increase in anisotropy and water diffusion. Furthermore, the membranes were investigated for a Vanadium/Air Redox Flow Battery application. Membranes crosslinked at 200 °C for 30 min with a molar ratio between 2:1 (mol repeat unit:mol benzenedimethanol) showed a proton conductivity of 27.9 mS/cm and a 100 times lower VO2+ crossover compared to Nafion.
Highlights
Cation exchange membranes, such as Nafion and sulfonated polyarylethers, are frequently used in many electrochemical energy storage applications
Visual observation of the membrane before and after crosslinking at 150 and 200 °C were done (Figure 2). It can be noticed, that crosslinked sulfonated poly(ether ether ketone) (cSPEEK) appears in a different color than the non-crosslinked membrane
It was found that the ion exchange capacity (IEC) of the membrane after 60 min at 200 °C was close to the initial value, i.e., IEC for the non-crosslinked SPEEK (Table 2)
Summary
Cation exchange membranes, such as Nafion and sulfonated polyarylethers, are frequently used in many electrochemical energy storage applications. The Vanadium/Air-RFB is an electrochemical energy storage system engaging fuel cell and all vanadium redox flow battery technology [1,2,3,4] It consists of two electrochemical half-cells, separated by an ion exchange membrane. A perfluorosulfonated polymer, is one of the most stable and conductive membranes applied in electrochemical membrane reactors, such as fuel cells and all vanadium redox flow batteries It shows excellent chemical stability and proton conductivity, but suffers from high vanadium crossover, since it has very broad and well interconnected hydrophilic channels through which vanadium ions can be transported [7]. As was mentioned earlier, an increasing sulfonation degree will lead to a poor mechanical stability of the membrane, due to high swelling up to total dissolution in water This would make the membrane not suitable for any separation technique using protic and polar solvents. The membranes show high potentials for Vanadium/Air-RFB application, since the vanadium cross over through the membrane could be reduced by a factor of 100 compared with the Nafion membrane
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