(340w) Ionomers: Leading the Way to Electrochemical Devices

Achieving energy security is one of the major challenges in the whole world. With time, the need for energy consumption is increasing worldwide resulting in pollution and the greenhouse effect, which is harmful to the atmosphere, human health, and natural habitat. Automobiles consume a major fraction of fossil fuels. Therefore, we need to switch to green energy technologies (e.g., fuel cells, batteries, etc.) that emit low-to-zero pollutants to the atmosphere. Ionomers (ion-conducting polymers) are one of the core components of these devices. Unraveling the ion transport limitations in < 1 µm-thick ionomer films is crucial for designing efficient ionomer-catalyst interfaces and improving redox efficiency in electrochemical devices. In my Ph.D., I am focusing on developing structure-property relationships of ionomers within bulk- (25-50 μm) and nanoscale (7-15 nm thick) materials by adopting multi-faceted material characterization approaches. My approach to understanding sub-µm thick ionomeric materials leverages the unique capabilities of fluorescence confocal laser scanning microscopy and spectroscopy, small-angle X-ray scattering, quartz crystal microbalance, scanning electron microscopy, transmission electron microscopy, and other electrochemical measurement techniques.

In a collaborative project with 3M (funded by National Science Foundation (NSF)), I investigated the local proton conduction environment within sub-µm thick films of three prominent fluorocarbon-based ionomers, 3M PFIA, 3M PFSA, and Nafion. By utilizing functional fluorescent photoacid probes, I have been able to show how the extent of ion transport and ionic domain characteristics change as a function of environmental humidity, film thickness, and interfacial interactions. Interestingly, we found that a high water uptake does not necessarily lead to high proton conductivity. Rather, the ionic domain size dominantly governs the ion conduction under thin-film confinement. Smaller, extremely acidic, and poorly phase segregated ionic domains with highly confined water molecules lead to lower proton conductivity in Nafion films, despite high water uptake. On the other hand, the relatively larger ionic domains made 3M PFIA the highest proton conductor of this ionomer series. This work was published at J. Phys. Chem. C (2019) and featured as a supplemental cover story.¹

Next, I developed an everyday-accessible, fluorescence confocal laser scanning microscopy-based strategy that can probe the distribution of mobility, ion conduction, and other properties across ionomer samples. This work (funded by NSF) has recently been published at ACS Macro Lett. (2021).² The analyzed depth profiles showed thickness- and interface-dependent proton conduction behavior in thin films. In films, proton conduction was weak over a region next to the substrate interface, then gradually increased till the air interface at high humidity. Conversely, consistently high proton conduction with no interface dependence was observed across ~35-50 µm-thick bulk, free-standing Nafion membranes. A hump-like mobility/stiffness distribution was observed across Nafion films. The proton conduction and mobility distribution were rationalized as a combinatorial effect of interfacial interaction, ionomer chain orientation, chain density, and ionic domain characteristics.

Furthermore, I utilized industrial/agricultural lignin-rich wastes as efficient, cost-effective materials for electrochemical devices, which can support both bio- and energy economies. The first work on this series (funded by Nebraska Center for Energy Science Research) was published in Frontiers in Chemistry in 2020 and featured in a special collection “Women in Science: Chemistry” under the “Green Chemistry” section.³ Here, I strategically sulfonated Kraft lignin to design ionomers with varied ion exchange capacities (IECs) that can potentially overcome the interfacial ion conduction limitation. The designed lignin sulfonate ionomers showed proton conductivity significantly higher than current state-of-the-art proton-conducting ionomer Nafion in < 1 µm thick films. Within the 3-dimensional, less dense, branched architecture of lignin sulfonate macromolecules larger ionic domain formation was facilitated. This work showed that lignin-based ionomers are potential candidates for low-temperature, water-mediated ion conduction under thin-film confinement and at catalyst interfaces of fuel cell electrodes.

Overall, my Ph.D. research aims to address some of the toughest scientific challenges of energy conversion and storage devices. The research experience I gathered through these works motivates me to build my future career in the field of polymeric materials in the industry as a research scientist, and find solutions to more real-life, materials-related problems.

References:

(1) Farzin, S.; Sarella, A.; Yandrasits, M. A.; Dishari, S. K. Fluorocarbon-Based Ionomers with Single Acid and Multiacid Side Chains at Nanothin Interfaces. J. Phys. Chem. C 2019, 123, 30871–30884. https://doi.org/10.1021/acs.jpcc.9b10015.

(2) Farzin, S.; Zamani, E.; Dishari, S. K. Unraveling Depth-Specific Ionic Conduction and Stiffness Behavior across Ionomer Thin Films and Bulk Membranes. ACS Macro Lett. 2021, 10, 791–798. https://doi.org/10.1021/acsmacrolett.1c00110.

(3) Farzin, S.; Johnson, T. J.; Chatterjee, S.; Zamani, E.; Dishari, S. K. Ionomers From Kraft Lignin for Renewable Energy Applications. Front. Chem. 2020, 8, 1–17. https://doi.org/10.3389/fchem.2020.00690.