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Understanding the Impact of Confinement on Ionomer Thin Film Transport Properties

  • Author(s): Tesfaye, Meron
  • Advisor(s): Weber, Adam Z.
  • McCloskey, Bryan D.
  • et al.
Abstract

Existing state-of-the-art electrochemical devices like polymer-electrolyte fuel cells (PEFCs) and other growing applications like separation membranes, protective coatings, photonics, nanocomposites, and microelectronics utilize polymers of thicknesses less than 100 nm. These thin and ultra-thin polymer films provide ease of processing, application-specific tunable properties, and reduction in material cost. Nano-confined polymer thin films, however, often display surface and thickness-dependent behavior that results in deviation from well-characterized bulk properties. As a result, they pose significant challenges to predictability, optimization, and performance. In PEFCs, thin film ion-conducting polymers (ionomers) of thickness < 100 nm serve as functional binder in the catalyst layers (CLs), aiding proton conduction and extending the reaction zone of the CL porous electrode. Unfortunately, these same ionomer thin films contribute to large proton- and gas-transport losses in PEFCs, thereby considerably limiting commercialization of affordable, low precious-metal catalyst-loaded fuel cells. Remediation of this phenomenon requires fundamental understanding of thin film behavior near interfaces and surfaces and quantification of an ionomer’s thickness-dependent properties. Furthermore, design improvements circumventing the losses aforementioned will require establishment of correlations between tunable variables and ionomer thin-film properties under a range of operating conditions. This dissertation aims to carry this out in three interconnected ways. First, model systems are used to develop fundamental understanding of dimensional swellability and thermal relaxation of ionomer thin films proximal to supports and dynamic interfaces that mimic phenomena in CLs. Second, ionomer thin film structure-property relationships are established through exploring and exploiting ionomer counterion identity and thickness variation. Third, properties like ionomer thin-film gas transport are quantified as a function of thickness to create a direct link between losses observed in the CL to local alterations in thin-film properties.

Understanding pairwise interactions between gas/ionomer thin film, ionomer/substrate, and gas/substrate are critical for decoupling the impact of substrate and interface from intrinsic ionomer thin-film properties driven by a finite-size effect. Swelling and thermal relaxation of ionomer thin films on different supports are explored in Chapters 2 and 3 to provide meaningful insights that may also occur at interfaces in the electrode. Swelling behavior, morphology, and mechanical properties of ionomer thin films (~50 nm) spin coated onto the platinum (Pt) support were exposed to both H2 and Air as experimental systems that mimic anode and cathode CLs and their Pt/ionomer system. Findings indicate lower uptake, increased densification of ionomer matrix, and increased rate in relative humidity-induced aging in a reducing environment as compared to an oxidizing and/or inert environment. Intrinsic mobility of ionomer chains anchored by strong interaction with substrate were additionally explored via thermal relaxation. The change in thermal transition temperature is a key marker of stiffness and polymer-chain mobility and has direct implications on ease of water and gas transport through polymer films. An increase in thermal transition temperature in ionomers supported on silicon (Si/SiO2) is observed with decrease in thickness. Thermal relaxation dynamics observed result from the positive effect of increased chain mobility at the free surface and the hindered motion at the strongly interacting substrate interface. Monitoring of swellability in alternative gas-environment experiments demonstrated that substrate/ionomer interface can dictate water distribution through the thin film. Absent a dynamic interface and hydration, anchoring of ionomer chains at the substrate interface creates a distributed degree of chain mobility across the thickness that is overpowered by the influence of substrate interaction with further reduction in thickness.

Water uptake and chain mobility findings from these model systems have implications that translate to ion conductivity and gas transport in PEFCs. This correlation is established through structure-property relationships, notably altering cation as a proxy of tuning the nature of ionomer with no additional synthesis as explored in Chapter 4. Alteration of intermolecular forces in the conventional, proton conducting, acid-form ionomer was conducted via exchange with metal cations. Water-uptake capacity, mechanical property, and thermal transition were explored for these exchange ionomers. For monovalent cations with varying cation size and Lewis acid strength, water content showed an inverse relationship with the former and direct relationship with the latter. Hydration is reduced by the increase in mechanical strength upon cation exchange, which is a result of solvation and thermodynamic energy equilibrium in the ionomer matrix. Similar insight was gained for thermal expansivity; an increase in thermal transition temperature was observed in cation exchanged ionomers with a minimal dependence on ionomer thickness. Upon confinement, the interplay between chain mobility at the free surface and near the substrate is dominated by internal hindered motion of ionomer chains tethered to the strong cation. Findings also provide awareness to the impact of ionic contamination in operating PEFCs.

Lastly, links from property measurements in well-defined model systems to real PEFC performance metrics must be established. In this work, quantification of ionomer gas-transport property is carried out in three separate methods to account for variable pertinent conditions for operation (temperature, humidity, and potential). Constant-volume-permeation method showed that unsupported films under dry concentration gradients demonstrated a slight increase in gas permeability with reduction in thickness. However, this data suffered from various sources of inaccuracies due to the fragile nature of thin films under high concentration gradients. Oxygen permeation via luminescent quenching method overcomes this challenge by employing supported thin film system that maintains integrity of thin films during experiment. Additionally, humidity- and potential-dependent microelectrode method was also utilized. Both methods reflect an order of magnitude reduction in gas permeability for thin films of thickness 260 to 440 nm relative to the bulk ionomer membrane counterparts. However, more work is needed to ensure confidence in these findings and explore wide range of thicknesses under variable conditions essential to PEFC CLs.

Electrodes in electrochemical devices are somewhat of a black box with minimal direct insights and predictions. Improved understanding of confinement related performance losses at surfaces and interfaces can help expand ionomer thin-film functionality and affordability of energy-conversion devices by providing critical design metrics and research directions. It is therefore essential to employ model systems and investigate properties of ionomer thin films with perturbations similar to those in operating devices. Such insights are not only useful for PEFCs, but also extend fundamental insights into thin functional polymer films employed in various applications.

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