Thin Film and Bulk Ionomer Characterization
LaboratoryLawrence Berkeley National Laboratory (LBNL)
Capability ExpertAhmet Kusoglu
Node Readiness Category1: Low-Temperature Electrolysis (LTE)
1: Photoelectrochemical (PEC)
Previous capability name: "Ionomer Characterization and Understanding."
This node encompasses a myriad of advanced material characterization techniques for hydration behavior, transport properties, and thermal-mechanical responses suited for determining structure-functionality of ion-conductive thin film and bulk polymer electrolytes, which could be used to elucidate ionomer-film behavior in electrode structures and guide the relevant ionomer design efforts. LBNL has extensive experience and equipment dedicated to the characterization of ion-conductive thin film and bulk polymer electrolytes, as well as their interfaces, as found in various electrochemical devices and electrode structures. This includes both alkaline and proton-exchange ionomers from micrometer-thick membrane separators to nanometer-thick thin films on support materials as found in electrodes of devices such as fuel cells, photoelectrochemical (PEC) assemblies, as well as low temperature polymer-based electrolysis (LTE).
Bulk Ionomer Properties: Hydration, Transport, and Mechanics [LTE1, PEC1]
LBNL has capabilities suited for ionomer films, including bulk ionomers and catalysts and/or support containing ionomer layers, including their macroscopic solvent uptake, sorption kinetics, dimensional change, and swelling at various hydration levels (see Figure 1). Transport functionality of ionomers in bulk form can be characterized in terms of ion transport (conductivity with impedance both in thickness direction and in the plane), water or solvent transport, and gas permeation with single gas and mixtures. These properties could be measured with humidity and/or temperature control, and in some cases under liquid conditions. In addition, physical and mechanical properties of ionomers as membranes can be measured using stress-strain testing under tension and compression, dynamic mechanical analyzer (DMA) with environmental control to assess materials' thermal-mechanical stability, thermal gravimetric analysis (TGA) for weight loss and decomposition, and density measurements. Mechanical response and stability of polymer membranes in various device environments can be characterized under various stress-states such as high-pressure compression testing in liquid water or time-dependent creep or relaxation under controlled environment.
Ionomer Thin Film Characterization [LTE1]
LBNL capabilities include thin-film fabrication on supported substrates (e.g., spin casting, spray coating with a SONO-TEK Exacta Coat System), swelling and water-uptake measurements using environmental ellipsometry and quartz-crystal microbalance, respectively, mechanical properties (cantilever bending technique) as well as characterization of ionomer structure and interface using novel synchrotron X-ray techniques. Finally, LBNL ionomer labs also contain various equipment to understand the formation of these ionomers in solution form (see also Understanding Catalyst Inks and Ionomer Dispersions), such as dynamic light scattering (DLS), rheometry, and zeta potential. In accordance with the equipment, there is the associated expertise of using the equipment and analyzing and modeling the collected data.
Morphological Characterization [LTE1, PEC1]
LBNL has unique stages and cells for morphological characterization of ionomer membranes (bulk) using transmission small- and wide-angle X-ray scattering (SAXS/WAXS) at the Advanced Light Source (ALS). SAXS and WAXS experiments can be performed in situ, with capabilities to probe ionomer's morphological features and nanostructure in dry and hydrated conditions as a function of temperature (from 25°C to 90°C in liquid, and up to 200°C for dry environment). Structure of semi-crystalline polymeric materials are probed in a WAXS regime to analyze their crystalline features. These morphological investigations can be extended to thin-film regimes to investigate ionomer thin-films on support substrates, which could be accomplished in situ, using an environmental grazing-incidence X-ray scattering (GISAXS/GIWAXS).
Advanced Structural Characterization of Thin Films and Interfaces [LTE2]
To elucidate the chemical and nanostructural nature of ionomer/substrate interfaces, feasibility of novel soft X-ray spectroscopy tools for certain polymers and thin films could be explored. This will allow for the understanding of the effect of confinement and substrate interactions on the ionomer behavior, especially in the electrodes, and provide insight into the analysis and interpretation of the data measured in the abovementioned tasks. This node could also include exploration of casting parameters on the measured ionomer properties and structure as well as using a novel GISAXS setup to monitor the morphological evolution of films in situ during casting.
Depending on technique and properties, there are some limitations on sample size, film thickness, and casting. Almost all the systems are compatible with different sample geometries and suitable for both anion- and cation-exchange thin films within a thickness range of 10 nanometers (nm) to over 500 nm cast on Si wafers or other coated substrates. Most of these techniques have been demonstrated to be feasible for ionomers cast from alcohols and aqueous solutions and could be extended to other casting solutions. Case-by-case evaluations are necessary to assess how to best perform the experiment for some thin films or composite ionomer structures, especially in controlled environments.
Even though most of the equipment is available at other locations, the aggregation of the equipment and in particular the expertise in analyzing structure/property relationships of this class of materials—from anion- and cation-exchange bulk polymers to thin films—are unique, especially probing from small to large thickness ranges under a controlled environment.
Available and supported by other programs. Some of the synchrotron-based X-ray techniques at the Advanced Light Source (ALS) would be limited by availability of beam time of specific beamlines including the possible requirements for users to submit user proposals.
This capability and knowledge are central for evaluation of ion-conducting polymers as polymer-electrolyte separators and thin films to understand and improve their functionality, both as bulk ionomers and electrode-films and bulk ionomer layers, for alkaline and acid-based polymer electrolyzers as well as various PEC materials.
Figure 1. Comparison of techniques for bulk ionomers and thin-film ionomers (see  for details).
Representative images of data that can be collected in this node for bulk ionomers (see [2–6] for details). Note: These data are based on various PEMs and values are shown only to illustrate the node capabilities and should not be taken as a reference or baseline for other materials.
- A. Kusoglu, "Ionomer Thin Films in PEM Fuel Cells," In Encyclopedia of Sustainability Science and Technology, ed. R. A. Meyers, New York: Springer New York (2018): 1-23.
- A. Kusoglu, S. Savagatrup, K. T. Clark, A. Z. Weber, "Role of Mechanical Factors in Controlling the Structure–Function Relationship of PFSA Ionomers," Macromolecules 45, no. 18 (2012): 7467-7476.
- A. Kusoglu, T. J. Dursch, A. Z. Weber, "Nanostructure/Swelling Relationships of Bulk and Thin-Film PFSA Ionomers," Adv. Funct. Mater. 26, no. 27 (2016): 4961-4975.
- S. Shi, A. Z. Weber, A. Kusoglu, "Structure-Transport Relationship of Perfluorosulfonic-Acid Membranes in Different Cationic Forms," Electrochim. Acta 220 (2016): 517-528.
- S. W. Shi, A. Z. Weber, A. Kusoglu, "Structure/property relationship of Nafion XL composite membranes," J. Membr. Sci. 516 (2016): 123-134.
- A. Kusoglu, A. Z. Weber, "New Insights into Perfluorinated Sulfonic-Acid Ionomers," Chem. Rev. 117, no. 3 (2017): 987-1104.