UC Berkeley

In the push to decarbonize the energy sector, hydrogen technologies have garnered significant interest. Fueled by (clean) hydrogen, proton-exchange-membrane fuel cells (PEMFCs) have potential to replace internal combustion engines and batteries in vehicles, especially for heavy-duty applications. These devices rely on ion-conducting polymers (ionomers) to facilitate mass transport of reactive species, which directly influences device performance and efficiency. However, PEMFC performance is limited by transport resistances through ionomer thin films in the device electrodes (catalyst layers, CLs, comprised of catalyst and ionomer). To better understand and design ionomers in PEMFCs and other electrochemical energy conversion devices, this dissertation focuses on ionomer thin films and the phenomena governing their structure-property relationships.

In CLs, ionomers act as the catalyst-binding electrolyte and are confined to a nanoscale thin film surrounding the catalyst particles. Reactive species must move through the ionomer to reach catalyst sites. However, due to confinement, CL ionomer properties deviate from bulk properties (e.g., that of membranes), including greater mass transport resistances. PEMFC performance is limited by low water uptake at low humidities, ionomer adsorption to catalyst sites, and poor oxygen permeability in the CL ionomer. To add further complications, CL ionomer properties also depend on the processing conditions during CL fabrication and device assembly. Supported thin films have been widely used as a model system to gain insight into CL ionomer behavior.

The most widely used ionomer is perfluoro sulfonic acid (PFSA) ionomer. PFSA is a random copolymer with a hydrophobic backbone and pendant sidechains terminating in hydrophilic sulfonic acid groups. The backbone lends chemical and mechanical integrity while the acid groups enable ionic conductivity. Due to the dissimilarity between the hydrophobic backbone and hydrophilic sulfonic acid groups, the ionomer exhibits nanophase separation into hydrophobic and hydrophilic domains. Upon hydration, the hydrophilic domain coalesces into a network forming pathways for ion conduction.

While recent studies provided insights into the origins of performance limitations caused by the CL ionomer, limited improvements demonstrate the need for new ionomers and integration strategies designed to improve film function. Rational design requires a deeper understanding of how chemistry impacts ionomer thin film structure-property relationships. Furthermore, ionomers are sensitive to processing conditions, and consideration for processing effects is necessary when evaluating potential CL ionomers. This dissertation focuses on two central ideas to control CL ionomer function: rational ionomer design and structure-property modulation via processing effects. PFSA sidechain and backbone modification strategies are investigated through systematic thin film characterization to elucidate chemistry-structure-property relationships. The impacts of dispersion solvent and thermal treatment temperature are explored to assess ionomer response to processing conditions.

To begin, two sidechain modifications to PFSA with potential to improve water uptake and/or mitigate sulfonate adsorption are explored. Application-relevant properties (i.e., water uptake and thermal transition temperature) are characterized via in situ spectroscopic ellipsometry (SE), and nanomorphology is probed using environmentally-controlled grazing-incidence small- and wide-angle x-ray scattering (GISAXS/GIWAXS). In addition, film function is assessed via proton conductivity measurements. Introducing multiple acid groups to sidechains increases water uptake and improves phase separation while reducing chain mobility. Replacing sulfonic acid end groups with increasingly hydrophobic end groups leads to the opposite trends. Moreover, results indicated that ionomer chemistry has more impact on morphology and water uptake than ionomer-substrate interactions. Importantly, proton conductivity—measured via potentiostatic electrochemical impedance spectroscopy (PEIS)—is strongly correlated with film water volume fraction. These findings provide a deeper understanding of how two ionomer modification strategies impact thin film behavior.

With an understanding of the structure-property relationships for sidechain-modified PFSA-based ionomers, the impact of dispersion solvent on ionomer structure is investigated next, using select ionomers from the two modification strategies explored. Using solvents relevant to CL fabrication, water:n-propanol (nPA) mixtures are used to disperse ionomers and cast thin films on supports. Ionomer structure in dispersion is characterized via pH measurements and transmission small-angle x-ray scattering (SAXS), and thin film structure is characterized via GISAXS. Additionally, the impact of dispersion water content on proton conductivity is assessed using PEIS. Importantly, the relationship between dispersions (ionomer + solvent) and thin films is also discussed. Ionomer aggregation/conformation in dispersion is primarily driven by ionomer chemistry, but all ionomers exhibit similar structural evolution with increasing dispersion water content. These differences in dispersion translate to differences in thin film hydrophilic domain spacing and orientation, ultimately impacting proton conductivity. Thus, while ionomer design has more influence on ionomer structure and properties, dispersion water content offers a method to modulate ionomer behavior.

In the final study, these findings are extended to a backbone-modified PFSA-based ionomer designed to improve thin film oxygen permeability, referred to as a high-oxygen-permeability ionomer (HOPI). HOPI membrane and thin film morphology, water uptake, and proton conductivity are characterized to assess confinement effects and compared to PFSA. Then, two processing effects are evaluated and discussed: processing solvent and thermal treatment temperature. When cast from water-rich dispersion, HOPI thin films exhibit greater proton conductivity despite reduced water uptake and a lack of observable morphological differences in the hydrophilic domain. Elevating thermal treatment temperature after film casting leads to lower water uptake and proton conductivity. Importantly, HOPI is more sensitive to confinement effects and processing conditions than PFSA. Careful control of processing conditions can enable water uptake and proton conductivity in HOPI that is comparable or greater than that of PFSA. Thus, processing conditions are important to consider when evaluating potential CL ionomers. Moreover, thin film properties can be modulated via processing.

The chemistry-structure-property relationships elucidated in this dissertation guide ionomer design and integration strategies to overcome ionomer functional limitations. Additionally, these findings inform processing conditions used in device fabrication for performance and efficiency improvements. The insight gained into the relationship between chemistry, processing conditions, and ionomer functionality enables modulation of ionomer behavior for PEMFCs and can be extended to other energy conversion devices utilizing electrochemically-active interfaces (e.g., water splitting electrolyzers, carbon dioxide reduction technologies).