Amino–Functionalized Multi–walled Carbon Nanotube/SPEEK Hybrid Proton Exchange Membrane for Iron–chromium Redox Flow Battery**

Abstract

In the field of Fe‐Cr redox flow batteries (ICRFBs), the optimal doping ratio of the amino‐functionalized multi‐walled carbon nanotube (MWCNT‐NH2) with SPEEK ion exchange membrane (IEM) is determined to be 2% by series tests. The ICRFB single‐cell containing the S/MWCNT‐NH2‐2 IEM was tested and compared with the Nafion 212 and original SPEEK IEMs. It is worth noting that the single‐cell containing S/MWCNT‐NH2‐2 exhibits a significantly higher coulombic efficiency (CE:96.06%) and energy efficiency (EE:88.68%) under 40 mA cm‐2 than that of the Nafion 212 (EE: 82.83%, CE:88.9%), and different from the original SPEEK IEM, it also exhibits a higher EE (82.04%) than the Nafion 212 (81.09%) at 100 mA cm‐2. During the self‐discharge test, the lower permeability of iron/chromium ions through S/MWCNT‐NH2‐2 IEM also allows for a longer self‐discharge time than the other two single‐cells. Additionally, the single‐cell containing S/MWCNT‐NH2‐2 showed a smaller decay range of EE (from 82.67% to 77.36%) and more discharge capacity retention (48%) than that of the Nafion 212 (EE: from 82.64% to 75.01%, discharge capacity decay: 18%) after 100 cycles, which suggests that the hybrid SPEEK IEM with optimal content of MWCNT‐NH2 has a great potential to replace Nafion IEMs in the large‐scale commercialization of ICRFB.