Anion exchange membrane fuel cells (AEMFCs) have attracted widespread attention because of the lower cost of using non-precious metal catalysts and high oxygen reduction reaction (ORR) kinetics in alkaline conditions.^[1] The previous research focused on the water uptake and OH^- conduction properties of anion exchange membranes (AEMs) as thick forms.^[2–5] The fuel cell reaction occurs at the triple-phase interface where the junction of ion-conductive ionomer, catalyst, and fuel/oxidant. The ionomer plays an important role to deliver OH^- ion between the thick membrane andto electrochemical catalysts in fuel cells. However, the investigation of OH^- ion conduction and hydration properties of thin ionomers is important but not sufficient.

This work demonstrated the relations between OH^- conductivity and water uptake of anion exchange thin films for the first time.^[6] The reported poly[(9,9-bis(6′-(N,N,N-trimethylammonium)-hexyl)-9H-fluorene)-alt-(1,4-benzene)] (PFB⁺)^[6] was synthesized as a model ionomer. We established in situ methods for measuring OH^- conductivity and water uptake of anion exchange thin films^[7] because OH^- ion easily exchanges for carbon dioxide in the air. The OH^- conductivity of 273 nm-thick PFB⁺ thin film form at 25 °C under 95 % relative humidity (RH) is comparable to the reported OH^- conductivity value of PFB⁺ bulk membrane. Reduced OH^- conductivity and water uptake were observed in 30 nm-thick PFB⁺ film compared to thicker 273 nm-thick PFB⁺ film. This reduced OH^- conductivity was caused by the decreased number of water molecules contained in thinner PFB⁺ films. Under the same number of water molecules contained, similar OH^- conductivity results can be obtained for both 273 and 30 nm-thick films as shown in Figure. Results show a different trend compared to the case of the proton conductive thin films.^[8]

References

[1] G. Merle, M. Wessling, K. Nijmeijer, J. Memb. Sci. 2011, 377, 1–35.

[2] J. Y. Jeon, S. Park, J. Han, S. Maurya, A. D. Mohanty, D. Tian, N. Saikia, M. A. Hickner, C. Y. Ryu, M. E. Tuckerman, S. J. Paddison, Y. S. Kim, C. Bae, Macromolecules 2019, 52, 2139–2147.

[3] J. Chen, M. Guan, K. Li, S. Tang, ACS Appl. Mater. Interfaces 2020, 12, 15138–15144.

[4] U. Salma, Y. Nagao, Polym. Degrad. Stab. 2020, 179, 109299.

[5] C. G. Arges, L. Zhang, ACS Appl. Energy Mater. 2018, 1, 2991–3012.

[6] W. H. Lee, A. D. Mohanty, C. Bae, ACS Macro Lett. 2015, 4, 453–457.

[7] F. Wang, D. Wang, Y. Nagao, ChemSusChem 2021, 14, 2694–2697.

[8] Y. Nagao, Sci. Tech. Adv. Mater. 2020, 21, 79–91.