Highly Oxidation-Resistant Anion Exchange Membrane By Immobilized Radical Scavenger
Authors: Shi Wenjuan , Wang Junhua , Wang Lan , Hu Keda , Duan Hui , Weiss Catherine M. , Yan Yushan 
Published: December 2024

Hydroxide exchange membranes and ionomers (HEMs and HEIs) are crucial components to facilitate the flow of anion groups between the electrode in the electrochemical devices. HEMs consist of polymer backbones and cationic groups within the backbone chain or pendant side chain. The robustness of HEMs is critical to the long-term durability of devices. However, HEMs can be gradually degraded by the attack of hydroxide anion group and the oxidation reaction of radicals. In the past decades, many efforts have been made to mitigate the degradation by hydroxide attack with the utilization of ether-free backbones and stable cation groups. The radical attack to HEM is also detrimental to the cell durability, but rarely studied.

Superoxide (O2
•-), hydroperoxyl (HOO•), hydroxyl (HO•) and hydrogen (H•) radicals have been demonstrated to form in-situ in the fuel cells and water electrolyzers. The formation of reactive oxygen species (ROS) is a common phenomenon as radicals were found to be generated in the presence of both platinum group metal (PGM) and PGM-free oxygen reduction reaction (ORR) catalysts in the fuel cells. The rapid oxidation of the backbones and cation groups for both phenyl-based (poly(arylene piperidinium)) and phenyl-free-based (polynorbornene) ionomer was also observed in the durability test of HEM electrolyzers. These results indicated that it is imperative to enhance the oxidative stability of HEM and HEI due to their vulnerability to free radical attacks under challenging electrochemical operating conditions, which can result in material degradation, undermining the long-term stability of HEM.

In this work, an organic radical scavenger was incorporated into the polymer through a stable chemical bond. The radical scavenger can effectively capture radicals to inhibit the oxidation of the aryl ring to form the benzoate. The oxidative stability of designed HEM was enhanced, as 64.7% of strain, 64.2% of stress and 98.7% of conductivity were retained, while the membrane without radical scavenger moiety retained only 14.7% of strain, 46.7 of stress and 85.4% of conductivity.

References

Wang, J.; Zhao, Y.; Setzler, B. P.; Rojas-Carbonell, S.; Ben Yehuda, C.; Amel, A.; Page, M.; Wang, L.; Hu, K.; Shi, L.; Gottesfeld, S.; Xu, B.; Yan, Y. Poly(aryl piperidinium) membranes and ionomers for hydroxide exchange membrane fuel cells. Nat. Energy.
2019,
4, 392.

Parrondo, J.; Wang, Z.; Jung, M.-S. J.; Ramani, V. Reactive oxygen species accelerate degradation of anion exchange membranes based on polyphenylene oxide in alkaline environments. Phys. Chem. Chem. Phys.
2016,

18

, 19705.

Wierzbicki, S.; Douglin, J. C.; Singh, R. K.; Dekel, D. R.; Kruczała, K. Operando EPR Study of Radical Formation in Anion-Exchange Membrane Fuel Cells. ACS Catal.
2023, 13, 2744.

Lindquist G, Gaitor J, Thompson W, Brogden V, Noonan K, Boettcher S. Oxidative instability of ionomers in hydroxide-exchange-membrane electrolyzers. ChemRxiv. 2023; doi:10.26434/chemrxiv-2023-7w3rz This content is a preprint and has not been peer-reviewed.