Anion Exchange Membranes for High Performance Neutral Aqueous Organic Redox Flow Battery

Abstract

Neutral pH aqueous organic redox flow batteries (AORFB) employing water-soluble dimethyl viologen (MV) and N,N,N-2,2,6,6-heptamethylpiperi-dinyl oxy-4-ammonium chloride (TMA-TEMPO) molecules have been demonstrated as a high-performing RFB1,2. As a key component, the membrane allows the transport of counter ions while preventing the intermixing of the redox-active molecules in the electrolyte solutions, despite of which, appropriate membranes designed for the system are usually overlooked in many studies3. Therefore, synthesis of a suitable membrane, which shows a high (chloride) ion conductivity and selectivity, optimized water uptake, adapted mechanical strength, is required 4.In this work, first, in-depth correlations between polymer structure - membrane properties and TMA-TEMPO/MV–based AORFB cell cycling stability and performance were investigated. Four anion-exchange membranes (AEMs) based on poly (phenylene oxide) (PPO) grafted with various amounts of poly(diallylpiperidinium chloride) cations were fabricated via a rapid UV-irradiation method and tested in a 5 cm2 TMA-TEMPO/MV-based AORFB. The trend in available capacity after 100 cycles at 80 mA/cm2 was closely related to the initial resistance of the membranes and their corresponding degree of cross-contamination. Membranes with moderate membrane resistance and low crossover was found to display the best overall cell performance. Our results show quite a promising understanding on the correlations between the different membrane properties and their corresponding cell performance and stability.Furthermore, a series of flexible and conductive anion-exchange membranes (AEMs) composed of PPO with six carbon side chain ended with quaternary ammonium (Diazabicyclo[2.2.2]octane)-DABCO or Trimethylamine - TMA) were fabricated via casting and tested in the TMA-TEMPO/MV–based AORFB single cell. The PPO-DABCO AEM with low water uptake and high chloride conductivity exhibited much higher peak power density than a well-performing commercial membrane (FAA-3-50®Fumatech) (388 vs 244 mW/cm2) (Figure 1). Moreover, our membrane permitted to obtain excellent coulombic efficiency (>99%) and a high energy efficiency (80% vs. 78%) than the commercial membrane. Our results show an excellent membrane candidate to improve the cell performance of AORFBs.