Revealing the Internal Architecture of Alkaline Fuel Cell Membranes with Cryo-4D-STEM and Cryo-STEM-EELS

Characterization of the internal architecture of polymer materials plays an important role in the development of energy devices such as fuel cells. The beam sensitivity of polymer materials coupled with their inherent low contrast makes imaging using conventional TEM and STEM techniques a challenge. Here, we demonstrate robust mapping of the internal architecture of fuel cell polymer membranes using cryo-4D-STEM and cryo-STEM-EELS. We will focus on characterization of alkaline anion exchange membranes (AAEMs) used in AAEM fuel cells, a cheaper alternative to current fuel cell technologies as they allow for platinum group metal-free catalysts [1]. In fuel cell polymer membranes, the internal architecture is critical in performance metrics such as conductivity, mechanical durability, and chemical stability. Semicrystalline polymers such as hydrogenated polynorbornene (hPN) are a common backbone choice for AAEMs as the crystalline domains can act as cross-linkers, increasing the mechanical integrity of the membrane [1]. Fig. 1 demonstrates the use of cryogenic four-dimensional scanning transmission electron microscopy (cryo-4D-STEM) to map the crystalline architecture of a series of phosphonium-functionalized hPN membranes. We find that variations in polymer synthesis components such as molecular weight and thermal history directly affect the material’s crystalline domain size and distribution. These effects, combined with the mechanical performances of the membranes lend new insights into the design of more durable AAEMs. Further information about the internal organization of the membranes beyond their crystalline domain architecture can be extracted from the cryo-4D-STEM data by examining the diffraction signal at scattering angles higher than those attributed to the crystalline reflection (Fig. 1). Crystallinity in polymers has been considered in the past and is believed to be critical for traditional proton exchange membranes (PEMs) commonly used in commercial fuel cells and particularly in Nafion, which is often considered a performance benchmark [2]. Surprisingly, we show with our technique that the crystallinity in Nafion is lower than previously expected and may not be the sole reason for Nafion’s superior mechanical integrity. The use of the second-generation electron pixel array detector (EMPAD-G2) allowed the acquisition of this extremely low-dose 4D-STEM data with 10 kHz framerates and negligible dead time [3]. Dose rates were limited to a critical dose of 10 e-/Å2 determined for these materials [4]. The use of amorphous polymer backbones with high glass transition temperatures (Tg) such as vinyl-addition hexyl-norbornene (HNB) is an alternative method that can achieve mechanically robust AAEMs [5]. In the case of amorphous AAEMs, we don’t have crystalline domains which are identifiable with 4D-STEM. Electron energy loss spectroscopy (EELS) performed in the STEM, however, allows identification of amorphous, phase-separated regions of the copolymer. Fig. 2 demonstrates the investigation of the hydrophobic-hydrophilic microphase-separated architecture of an amorphous, ammonium-functionalized HNB membrane with Cl- counter-ion using cryo-STEM-EELS. Elemental maps displaying the chlorine and carbon signals demonstrate the location of hydrophilic domains more heavily populated with the cationic groups bonded to the Cl- counter-ion and hydrophobic domains more densely populated with the amorphous carbon backbone. The use of the Gatan K2 Summit direct electron detector provided a sufficient SNR to detect the location of even the weakly scattering Cl- signal [6]. In EELS measurements, the damage metric is dictated by changes in the bonding state. Here, the data were limited to a total dose of ∼3000 e-/Å2, above which fine structure damage was observed in the carbon K-edge. Our results demonstrate the use of cryo-4D-STEM and cryo-STEM-EELS in spatially mapping the architecture of polymer membranes including the distribution of the hydrophobic-hydrophilic domain structure, as well as nanometer-size crystalline domains that act as cross-linkers in the membranes [7]. Mapping polymer architecture using cryo-4D-STEM. Top row: Virtual ADF-STEM images constructed from an annulus corresponding to the location of the crystalline reflections (Virtual Detector 1) and from higher scattering angles explicitly excluding the crystalline reflection (Virtual Detector 2). Virtual Detector 1 consequently displays the location of crystalline domains while Virtual Detector 2 displays the location of scattering at higher angles, attributed to the amorphous backbone distribution. We attribute the regions with higher amorphous scattering to those populated with densely packed backbone, while the lower scattering regions result from domains with a higher concentration of the bulky cationic groups. Bottom row: Crystallinity maps display the change in crystalline morphology of phosphonium-functionalized hPN membranes as a result of variations in the polymer synthesis and compared to a commercial Nafion 212 membrane. Elemental maps and simultaneous ADF-STEM image of an ammonium-functionalized HNB membrane with Cl- counter-ion. The Cl-L2,3 signal corresponds to the hydrophilic domains, more heavily populated with ionic channels as the Cl- counter-ions are bonded to the cationic groups. The C-K signal corresponds to regions with more densely-packed backbone making up the hydrophobic domains of the material. Spectral profiles for both the C-K and Cl-L2,3 edges from the highlighted area on the elemental maps highlight the disassociation between chlorine-dense and carbon-dense domains.