Functionalization Of Poly(Ether Ether Ketone)s For Ion-Exchanging Vanadium Redox Flow Battery Membranes

Since the vanadium redox flow battery (VRFB) was proposed by Skyllas-Kazacos and Sum in 1985 as candidates for stationary energy storage facilities, it has attracted extensive attention for its long cycle, flexibility in design, fast response time, deep-discharge capability and high energy efficiency. However, the commercial progress of VFBs has so far been limited by the high cost and low selectivity of the applied perfluorinated membranes. Thus, considerable efforts have been devoted to searching for a high performance membrane based on non-fluorinated polymers, on account of their attractive features like high selectivity, tunable ion conductivity and low cost. Among the alternative materials investigated, functionalized aromatic polymers have been considered as promising candidates as alternative membrane materials due to their high thermal and chemical stability, high proton conductivity, and low cost.In general, the ion exchange capacity (IEC) is closely related to the proton conductivity of PEMs because the acid functionalities. Beyond a certain sulfonation degree, PEMs tend to absorb too much water or are even soluble in water, which nega-tively affect their mechanical resistance and water resistance. Therefore, the improvement of proton conductivity and mechanical resistance simultaneously using aromatic polymers with clear microphase-seperated structre was investaged. A proton-conducting aromatic polymers carrying hypersulfophenylated side chains (HS-PEEK) was prepared by demethylation of PEEK-OM and subsequently copolymerization of PEEK-OH with sulfonated monomers. The obtained HS-PEEK copolymers had good overall properties as polymer electrolyte membrane materials, having high proton conductivity in the range of 50.3-89.4 mS cm-1 with excellent mechanical stabilities. HS3-PEEK (IEC=2.47 mmol g-1) showed higher proton conductivity while enhanced vanadium ion permeabilities than that of Nafion 117. Therefore, the open circuit voltage (OCV) of the VRFB with the resulting HS2-PEEK showed poorer performance than that with Nafion 117 membrane.The fluorinated aromatic backbone has been considered as a versatile structure in PEMs, by virtue of the excellent chemical stability and mechanical property. Herein, we constructed a partially fluorinated poly(aryl ether ketone) backbone with commercially available monomers. The obtained polymers were post-sulfonated using concentrated sulfuric acid to give highly proton conductive sulfonic fluorenyl groups. The synthesized membranes exhibit high ion-exchange capacities, excellent mechanical properties in hydrated state, thermal stability, as well as better oxidative stabilities at relatively lower IEC value. The as-made SF-PEEK membrane was investigated as the separator of VRFBs and compared with Nafion 117 membrane. The SF-PEEK membrane displays much lower vanadium ions permeability than Nafion 117. The self-discharge of VRFB single cell was effectively restrained using the SF-PEEK membrane. The coulombic efficiency (CE) of VRFB assembled with SF-PEEK membranes is generally higher than those of the VRFB with Nafion 117 membrane. The highest CE of VRB with SF-PEEK membrane is 77.0% while the highest CE of VRB with Nafion 117 membrane is 74.9% at the same testing condition. Therefore, it appears to be a promising separator for vanadium redox flow battery application.Owing to Donnan exclusion effect, anionic exchange membranes exhibit lower vanadium ion permeability and therefore higher coulombic efficiency in VRFB application. To develop high performance and cost-effective membranes with low permeability of vanadium ions for VRFB application, a series of poly(ether ether ketone) anion exchange membranes were prepared via the bromination of tetramethyl bisphenol-PEEKs and subsequently functionalized with pyridinium and ion exchange. The resulting pyridinium functionalized poly(ether ether ketone)s (Py-PEEK) membranes show much lower vanadium ions permeability (1.52 × 10-7?11.2 ? 10-7 cm2 min-1) than that of Nafion 117 membrane (70.4 × 10-7 cm2 min-1). As a result, the self-discharge duration of the VRFB cell with Py-PEEK membrane (97 h) is about two times longer than that of VRFB cell with Nafion 117 membrane (57 h). Furthermore, the VRFB cell with Py-PEEK membrane exhibits higher coulumbic efficiency (80.3%) compare with that of VRFB cell with Nafion 117 membrane (74.9%) at a current density of 40 mA cm-2. The results indicate that Py-PEEK membranes are high performance and low-cost alternative membrane for VRFB application.