Electrochemical energy-conversion devices represent a potentially significant contribution to the renewable-energy paradigm. A subclass of these devices (fuel cells, electrolyzers, flow batteries, CO2 reduction devices, etc.) rely on ion-conducting polymers (ionomers) as a critical component to transport reactants and products between and within the device’s electrodes. A major impediment towards widespread implementation of these devices is high transport resistance within the electrode. Previous research has correlated the resistance to the ionomer that exists as thin films in the electrodes, but the origin of the resistance remains unknown. To enable next-generation electrochemical energy-conversion devices, it is crucial to understand better the structure-property relationships of these ionomer thin films.
Perfluorosulfonic acid (PFSA) ionomers are heavily utilized for these technologies due to their excellent mechanical and electrochemical stability and superior ion transport. They are random copolymers with a hydrophobic PTFE backbone and pendant side-chains that are terminated with a sulfonic-acid group. Upon hydration, the acid-group proton dissociates and enables high ionic conductivity. While this class of materials has been heavily studied in bulk membrane form, the behavior of PFSAs as thin films in catalyst layers is poorly understood. Part of the reason for this are the difficulties in ionomer thin-film characterization.
Because of these challenges, the first half of this dissertation focuses on method development for thin-film characterization. The first and foremost characterization method covered is grazing-incidence x-ray scattering (GIXS). A general introduction is given and the differences between transmission and GIXS are highlighted. These concepts are built upon to construct a new method of data collection and analysis, termed electric field intensity (EFI)-modulated scattering. This technique allows high-resolution depth-profiling of thin film samples, which is not achievable with standard GIXS experiments.
A cantilever bending method for the mechanical characterization of thin films is described within polymer network theory and solid mechanics analysis, and validated with a model hydrogel system. It is shown that the osmotic pressure and equilibrium shear modulus can be measured in non-glassy materials. Characterizing PFSA thin-film mechanical properties is discussed in the context of these results, and the chapter concludes with a discussion on extending the technique to study thin films under different thermal conditions.
As PFSAs exist in catalyst layers, there is a need to study and understand their behavior under working conditions such as applied potentials. The fabrication of heterogeneous, planar electrode devices for operando characterization is discussed, and two different ionomer chemistries are used as case studies for the device. In the first case study, Nafion thin films show a change in through-plane density with applied potential. The second case study uses a perfluorinated anion exchange membrane, which shows a change in morphology from lamellae to a bicontinuous structure as the anionic form is reacted from carbonated species to hydroxide. Based on these findings, we discuss improvements to the cell design, as well as fabrication of interdigitated electrodes for detailed impedance measurements.
The second half of the dissertation focuses on studying the structure and physical properties of PFSA thin films. This starts with a study on the morphological evolution of PFSA thin-film formation. An in-situ slot die coater is used to cast ionomer dispersions and GIXS is used to investigate the dynamic behavior of ionomer morphology as it dries into thin films. Aggregate interactions in dispersion directly impact the hydrophilic-domain network of the cast film and the onset of crystallization occurs simultaneously with the solution-to-film transition but each length scale evolves on different time scales. In addition, confinement is shown to induce anisotropic morphology at multiple length scales. These results demonstrate promising avenues for tuning ionomer morphology during film formation and end-film functionality.
The inverse relationship between mechanical properties and water uptake is well established in PFSA membranes, but less so in thin films. This relationship is investigated in PFSA thin films and connected to the nanostructural morphology using GIXS and the cantilever bending method. GIXS was used to determine the relative degree of crystallinity for Nafion thin films at three different annealing temperatures and across a range of thicknesses. From these measurements, transitional points in confinement effects are identified and reveal a counterintuitive decrease in crystallinity with increasing annealing temperature. Films prepared under the same conditions were measured with the cantilever bending method. The same universal inverse relationship between mechanical properties and water uptake is observed in thin films prepared under these conditions. The connection between the crystallinity and macroscopic mechanical properties is less strong, which highlights network effects and the complex structure of PFSAs.
The final study looks at the structure and swelling behavior of Nafion and 3M PFSA ionomer thin films. Each of these chemistries were annealed across a range of temperatures and their structure was characterized using GISAXS and correlated to water uptake measurements. Domain correlations are oriented through-plane in films annealed below their thermal transition. Above this transition, the domain orientation becomes isotropic and there is a corresponding decrease in water uptake, which demonstrates the connection between the nanostructural morphology and resulting physical properties. This chapter ends with a discussion on investigating domain connectivity and tortuosity using the interdigitated electrodes developed in Chapter 4.
