OH- Conductivity and Water Uptake of Anion Exchange Thin Films Under Humidity Control

Abstract

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. Figure 1