Communication—Sulfonated Poly (ether ether ketone) as Cation Exchange Membrane for Alkaline Redox Flow Batteries

Abstract

A sulfonated poly (ether ether ketone) (sPEEK) was tested as the separator in a full alkaline flow battery with 2,6-dihydroxyanthraquinone-ferro/ferricyanide, DHAQ-FeCy, redox couples. Cell performance was compared to that of an identical cell utilizing a perfluorosulfonic acid (PFSA) membrane. Replacement of the PFSA membrane with sPEEK resulted in a 10% power density increase, a 40% decrease in capacity loss per day and an 85-fold decrease in ferricyanide permeation. Though long-term stability of sPEEK in alkaline media requires improvement, these results highlight the potential to produce non-fluorinated membranes with better performance in organic redox flow batteries than the commercially available PFSAs. Aqueous organic redox flow batteries (AORFBs) constitute one of the most attractive alternatives to solve the storage problem associated with environmentally friendly, yet intermittent, power sources. 1,2-9Promising electrolyte compositions have been reported for AORFBs using commercially available cation exchange membranes (CEMs) as the electrode separator. The typical materials of choice for these membranes are perfluorinated sulfonic acids (PFSAs), such as Nafion. One barrier that hinders their widespread adoption is their relatively high cost.10 Replacement of PFSAs with a non-fluorinated membrane could reduce the cost of the films by a factor of 2.5 and facilitate the commercial implementation of AORFBs.11 Though inspiring new AORFBs operating at neutral pH have recently been reported, 8,[12][13][14] the majority of the systems developed to date require relatively harsh operation conditions (pH>13 or pH<0). These operating parameters limit the polymer chemistry choices that warrant further exploration. One promising starting point is the sulfonated version of poly ether ether ketone (sPEEK). This material has been previously tested as the CEM in fuel cells and vanadium redox flow batteries.15,16 Herein we compare the performance of a sPEEK membrane with a 50% degree of sulfonation with that of a commercially available perfluorinated alternative, Nafion NR212. The sPEEK membranes exhibited higher ionic conductivity, higher peak power density, lower capacity loss per cycle, and lower crossover of electroactive species than the PFSA membranes. Though the long-term stability of these nonfluorinated polymers needs to be improved, these results illustrate the potential for producing non-fluorinated membranes with comparable or better performance than the commercially available PFSAs, if the hydrolysis-prone heteroatoms are eliminated from the polymer backbone. ExperimentalThe procedures used for membrane preparation and characterization, as well as cell assembly and testing, are included in the supplemental information (SI). Results and DiscussionMembrane properties relevant for flow cell performance are presented in Table I. The higher conductivity observed for sPEEK is ascribable to its higher ion exchange capacity and higher water uptake. The water uptake differences between sPEEK and Nafion in their cor...

Sulfonated Microporous Polymer Membranes with Fast and Selective Ion Transport for Electrochemical Energy Conversion and Storage

“…This value is much greater than that from the otherwise identical cell using Nafion 117 as membrane (ca. 75 %, Figure S19), that reported for as ulfonated poly(ether ether ketone) membrane (66.2 %) [23] or for aNafion 212 membrane (59.2 %) [23] and even higher than that from am uch thinner Nafion 211 membrane (25.4 mmthick, 84 %). [2c] TheSPX-BP-0.95 cell exhibited ac apacity retention rate of 99.98 %p er cycle,o ratemporal retention rate of 99.95 %p er hour (Figure 5j).…”

Near Neutral pH Redox Flow Battery with Low Permeability and Long‐Lifetime Phosphonated Viologen Active Species

“…[ 17,19 ] The permeability of ferricyanide, the more permeating species of the pair, is 4.4 × 10 −12 cm 2 s −1 across E620(K) [ 17 ] and 4.5 × 10 −9 cm 2 s −1 across Nafion 212. [ 30 ] The permeability of BPP−Vi through E620(K) is 1.4 × 10 −12 cm 2 s −1 at pH = 7 and 5.4 × 10 –13 cm 2 s −1 at pH = 9, which are both slower than the permeability through Nafion 212 (Figure S11, Supporting Information) (Table 1). With the lowest permeability value (5.4 × 10 −13 cm 2 s −1 ), it should take 910 and 6000 years to lose 10% and 50% capacity, respectively, due to BPP−Vi crossover (Table S1), assuming exponential behavior.…”

Design Rules for Membranes from Polymers of Intrinsic Microporosity for Crossover-free Aqueous Electrochemical Devices

“…The ionic conductivity of AquaPIMs 1-3 in aqueous alkaline electrolyte is on par or higher when compared to other polymer membranes, however, most of these membranes have been developed as anion-exchange membranes for fuel cells and other devices 18,[63][64][65][66][67][68][69][70] The intentional design of AquaPIMs as cation exchange membranes begins to resolve technology gaps for polymer membranes suitable for use in aqueous alkaline electrochemical devices, where the working ions may be cations rather than hydroxide or chloride. This led us to further consider the foundations for ion transport by AquaPIM 1 in other aqueous electrolytes, by varying the chemical identity of the supporting salt as well as pH, all of which influence the extent of ionization of the amidoxime and in turn the structure and dynamic properties of water and ions confined to amidoxime-lined pores at the sub-nm scale.…”