Amino‐Functionalized Multi‐walled Carbon Nanotube/sulfonated PEEK Hybrid Ion‐Exchange Membrane for Fe‐Cr Redox Flow Battery *

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.