Water Sorption, Viscoelastic, and Optical Properties of Thin Nafion® Films

Open Access
Author:
Petrina, Stephanie Ann
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 28, 2013
Committee Members:
  • Michael Anthony Hickner, Dissertation Advisor
  • Ralph H Colby, Committee Member
  • David Lawrence Allara, Committee Member
  • Enrique Daniel Gomez, Committee Member
  • Nikolas Podraza, Special Member
Keywords:
  • Thin polymer films
  • Nafion
  • ionomer
  • water sorption
  • fuel cells
  • spectroscopic ellipsometry
  • quartz crystal microbalance
Abstract:
The hydrogen fuel cell industry continues to make strides in terms of improving device efficiency and performance, yet ion transport within the catalyst layer is not well understood. Thin ionomer films coating the catalytic particles are responsible for proton transport throughout the catalytic layer, yet the basic physical properties of these thin films, which interact with the catalyst surface, are widely unknown. Fundamentally, the material properties of thin polymer films are known to deviate from thick, free-standing membranes composed of the same material based on their interfacial interactions. The work in this dissertation seeks to uncover the properties of thin Nafion® films to begin to understand their role in catalyst layer performance. By identifying the influence of processing conditions, polymer–substrate interaction, and thickness on water uptake characteristics of thin Nafion® films, the proton and oxygen transport parameters that are most relevant to performance in the catalyst layer can be understood. Since the hydration of Nafion® is relevant for its proton conduction and performance in a fuel cell, water sorption of substrate–supported Nafion® thin films was characterized via the change in sample mass and thickness as the relative humidity (RH) of the sample environment was varied. Monolithic thin Nafion® films were characterized for a variety of sample preparation conditions and substrates to identify how processing conditions and other sample parameters may affect water uptake. Spin cast Nafion® films exhibited low density and refractive index for very thin films due to the higher relative void fraction induced by rapid film formation. The density of hydrated films was observed to decrease beyond the volume additivity limit as RH increased, and the relationship between density and refractive index was confirmed with the Lorentz-Lorenz relationship. The complex refractive indices (N = n + ik) of substrate–supported Nafion® films were identified for spin cast films 10 to 1000 nm in thickness. A sharp decline in n was observed as film thickness decreased below 50 nm, and n was found to be influenced more by thickness (i.e. processing conditions) than the underlying substrate (Au or SiO2). The spin casting process caused polymer chains to preferentially align parallel to the substrate, leading to an increase in anisotropy of the refractive index as measured by variable angle spectroscopic ellipsometry. The incremental change in both thickness and refractive index was observed to increase more with greater humidity, especially above 75 % RH. Chain anisotropy decreased as birefringence was eliminated when Nafion® was solvent annealed at high RH. Spin cast films exhibited hysteresis in mass uptake and swelling, confirming that some degree of polymer relaxation occurs as Nafion® is swollen. Thin Nafion® films on Au exhibited a lower percentage change in mass compared to thicker films and Nafion® on SiO2. Nafion® has been shown to form a hydrated layer at the interface of native oxide coated silicon which may account for the increased water sorption on SiO2 compared to Au. Additionally, there may be significant interaction between the sulfonate groups on Nafion and the Au surface. Mass uptake as a function of relative humidity was evaluated for Nafion® films 80 to 1200 nm in thickness. By using two models to estimate mass uptake, the magnitude of viscoelastic losses were evaluated as a function of thickness and relative humidity. As film thickness increased above 600 nm, the divergence in mass predicted by the Voigt and Sauerbrey equations indicated that thick films exhibited significant viscoelastic loss. The significance of processing conditions in determining water uptake characteristics was evaluated by comparing adsorbed and spin cast substrate supported films. Adsorbed Nafion® films exhibited different water uptake characteristics than spin cast polymer films, likely due to the structural differences induced in the film formation process. Confinement at an interface constrained morphological organization for very thin films, which resulted in significant swelling compared to thicker films due to the lack of hydrophobic domains that reinforce the film and limit water uptake. In summary, the properties of ionomer thin films are determined by the interplay between surface bonding and hydration, morphological organization, and mechanical properties. The basic properties of Nafion® thin films discussed in this dissertation provide a basis for further studies of proton and oxygen transport in the ionomer phase in a fuel cell catalyst layer.