Fabrication of Anion Exchange Membrane with Excellent Selectivity and Chemical Stability for Vanadium Redox Flow Batteries

Abstract

Vanadium redox flow batteries (VRFBs) have recently been revitalized as one of the most promising candidates for large scale electrical energy storage due to their significant merits, including the minimization of crossover contamination, excellent electrochemical reversibility and high efficiency. [1] As a key component of VRFBs, the membrane plays the role of preventing the migration of vanadium ions and transporting ions to complete the internal circuit. [2] Hence, the fabrication of novel membranes with the high chemical stability, low vanadium permeability, acceptable ionic conductivity and mechanical properties is one of the critical research objectives for the VRFB research community. [3, 4] In most reported works, the sulfonated polymers, i.e., proton exchange membrane (PEMs), have been widely researched for VRFBs. Although the ionic conductivity is acceptable, yet the energy efficiency and capacity retention are always plagued by the severe migration of vanadium ions. To address this issue, anion exchange membranes (AEMs) have been gradually employed in VRFBs due to the Donnan repulsion effect that can largely decrease the crossover of vanadium ions. But the chemical stability of functional groups is the critical issue for AEMs in the harsh chemical environments (high concentration of supporting electrolyte (H2SO4) and high oxidative reactive species (VO2 +)). Hence, the fabrication of AEMs with excellent chemical stability is highly required. In this regard, we here developed a novel cross-linked quaternary ammonia poly (2, 6-dimethyl-1, 4-phenylene oxide) (xQAPPO). The xQAPPO was prepared in four steps: bromomethylation, partial hydrophysis, quaternization and a final cross-linking process, as illustrated in Figure 1. Contrary to the conventional AEMs, not only the functional groups were grafted away from the polymer backbones, but also the side chain were cross-linked by glutaraldehyde. The cross-linked process can derive the aggregation of the backbones and side-chains and repel the functional groups, which enable to form a well-established hydrophilic /hydrophobic microstructure. Therefore, the polymer backbones could be well protected due to the reduced attack by the oxidative species. Meanwhile, the conductivity of the as-prepared AEMs enable to be readily tailored by the degree of bromo-methylation (x). The bromomethylated PPO with the x ranging from 40% to 80% was synthesized by controlling the ratio between PPO and the brominating agent. Our preliminary result showed that an ionic conductivity of 30.4 mS cm-1 was achieved at room temperature. More importantly, xQAPPO with the x value of 40% possesses a permeability of VO2+ ions of 0.15 × 10-7 cm2 min-1, which is largely lower than that of Nafion 212 (5.25 × 10-7 cm2 min-1). Moreover, the ex situ immersion test by immersing the membrane samples in 0.1 M VO2 + with 5 M H2SO4 was performed to determined the chemical stability of the as-prepared xQAPPO. It was shown that the weight loss of xQAPPO was as low as 6 wt.% for as long as 60 days. Further characterizations, including the water uptake, swelling ratio, mechanical properties, will be presented at the meeting. Meanwhile, the cell performance of single-cell VRFB respectively assembled with xQAPPO will also be presented at the meeting.