Anion Exchange Membranes for High Performance Neutral Aqueous Organic Redox Flow Battery
Authors: Tsehaye Misgina Tilahun , Yang Xian , Janoschka Tobias , Alloin Fannie , Iojoiu Cristina 
Published: December 2020

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.

References:

Janoschka, et al., Aqueous Redox-Flow Battery with High Capacity and Power: The TEMPTMA/MV System. Angew. Chem. Int. Ed., 55 (2016), pp. 14427-14430.

Hagemann, et al., An aqueous all-organic redox-flow battery employing a (2,2,6,6-tetramethylpiperidin-1-yl)oxyl-containing polymer as catholyte and dimethyl viologen dichloride as anolyte. J. Power Sources, 378 (2018), pp. 546-554.

Li, et al., Poly(phenylene oxide)-Based Ion-Exchange Membranes for Aqueous Organic Redox Flow Battery. Ind. Eng. Chem. Res. (2019), pp. 10707- 10712.

Wei, et al., Materials and Systems for Organic Redox Flow Batteries: Status and Challenges. ACS Energy Lett. 2017, 2, 9, 2187–2204).

Funding: This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Grant Agreement no. 765289.

Figure 1