(Invited) Nano-Characterization of the Internal Morphology of Polymer Electrolyte Membranes with Cryogenic Scanning Transmission Electron Microscopy
Authors: Markovich Danielle , Colletta Michael , Treichel Megan , Pivovar Bryan S. , Noonan Kevin J. T. , Muller David A. , Kourkoutis Lena F. 
Published: August 2024

Polymer membranes play important roles in electrochemical devices such as polymer electrolyte fuel cells. In recent years, there has been a growing interest in alkaline anion exchange membrane (AAEM) fuel cells as the alkaline environment offers a potential low-cost alternative to commercial proton exchange membrane (PEM) fuel cells [1]. Nonetheless, optimization of polymer properties such as ion conductivity and mechanical durability is still necessary to achieve commercially viable AAEM based fuel cells. Often, semicrystalline polymer backbones are selected for AAEM design, aiming to replicate the exceptional performance of Nafion in PEM fuel cells where the crystalline domains are believed to act as cross-linkers, mitigating excessive water uptake and mechanical degradation [2,3].

Characterizing the local polymer morphology at the nanoscale is a crucial step in understanding how to optimize the design of AAEMs with enhanced durability and ion conductivity. Imaging polymers using conventional transmission electron microscopy (TEM) techniques is challenging however, due to their inherently-high beam sensitivity and low image contrast. Here, we demonstrate nanometer-resolution mapping of the ordering in polymer electrolyte membranes using cryogenic four-dimensional scanning transmission electron microscopy (cryo-4D-STEM). Conventionally used for mapping inorganic crystalline materials, the 4D-STEM technique has more recently also found applications in mapping semicrystalline, soft materials as well [4,5]. Here, using a model system of semicrystalline hydrogenated polynorbornene (hPN) AAEMs [6], we demonstrate the effects of polymer synthesis including molecular weight and thermal history on crystalline morphology and in turn on the polymer performance. Further, we compare the crystalline architecture of the hPN copolymers to that of Nafion 212 (Figure 1). Our results suggest that polymers with homogeneously distributed, nanometer-size crystalline domains lead to improved water management, a property directly related to the mechanical integrity of the polymer, with possible impacts on ion conductivity. More broadly, the results give new insights into how we can tune AAEM design in order to improve their performance.

[1] Dekel, Journal of Power Sources, 375, 158-169. (2018).

[2] Yang, et al. Chemical Reviews, 122(6), 6117-6321. (2022).

[3] Mauritz & Moore, Chemical reviews, 104(10), 4535-4586. (2004).

[4] Panova, et al. Nature materials, 18(8), 860-865. (2019).

[5] Bustillo, et al. Accounts of chemical research, 54(11), 2543-2551. (2021).

[6] Treichel, et al. Macromolecules, 53(19), 8509-8518. (2020).

Figure 1: Cryo-4D-STEM crystallinity maps of hPN-co-iPrMe copolymers with varying syntheses and Nafion 212. We observe improvement in the water management of the copolymer membranes as the crystalline domains are reduced in volume and distributed homogeneously. The chemical structures of the polymers are listed below the crystallinity maps.

Figure 1