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

Abstract In the field of iron–chromium redox flow battery (ICRFB), the optimal doping ratio of the amino–functionalized multi–walled carbon nanotube (MWCNT−NH 2 ) blended with SPEEK as hybrid proton exchange membrane (PEM) is determined to be 2 wt % by series tests. The ICRFB single–cell containing the S/MWCNT−NH 2 −2 PEM was tested and compared with the Nafion 212 and original SPEEK PEMs. It is worth noting that the single–cell containing S/MWCNT−NH 2 –2 exhibits 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 %). Differing from the original SPEEK PEM, it also exhibits 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−NH 2 –2 PEM also allows for a longer self–discharge time than the other two single–cells. Additionally, the single–cell containing S/MWCNT−NH 2 –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 retention: 18 %) after 100 cycles, which suggests that the hybrid SPEEK PEM with optimal content of MWCNT−NH 2 has a great potential to replace Nafion PEMs in the large‐scale commercialization of ICRFB.