Mechanics and multi-physics deformation behavior of polymer electrolyte membranes

Abstract

Fuel cells are a developing technology within the energy sector that offer both efficiency and environmental advantages over traditional combustion processes. In particular, proton exchange membrane fuel cells (PEMFC) are promising for transportation and portable devices due to their low operating temperature, reduced C02 emissions, and scalability. A central component is the polymer electrolyte membrane (PEM) which conducts protons from the fuel source (typically either hydrogen or methanol) at the anode to the cathode where it reacts with oxygen while preventing the transport of either electrons or the fuel itself. Historically membranes have been designed primarily in terms of maximizing proton conductivity, but it is also important that they prevent fuel crossover and have minimal chemical and mechanical degradation over the target lifetime of the fuel cell. Membrane mechanical integrity is thus a critical concern for commercial distribution of PEMFC technology. This thesis has two primary focus areas: (1) characterization and modeling of Nafion, the benchmark PEM, in order to understand hygro-thermal loading in the existing technology and (2) mechanical characterization and modeling toward the development of an alternative polymer electrolyte membrane. These two areas are linked by the common technological application of low temperature fuel cells and can also be placed more broadly in the field of microstructurally and micromechanically informed constitutive modeling. The presulfonated polytetrafluoroethylene membrane Nafion is perhaps the most commercially prominent and widely studied polymer electrolyte membrane (PEM). Here Nafion is experimentally characterized first under monotonic and cyclic uniaxial tensile loading as a function of rate, temperature, and hydration. The data is used to develop a microstructurally motivated three-dimensional constitutive model. The Nafion model is validated under uniaxial tension for monotonic, cyclic, stress relaxation, and creep loading at various environmental conditions. Small and wide angle x-ray scattering characterization is then performed during uniaxial tensile testing in order to assign a microstructural interpretation to the mechanical behavior. The model is then validated for loading conditions which are expected to occur in the fuel cell, specifically, biaxial tension in the membrane plane and constrained swelling. Biaxial characterization is conducted via in-plane tensile testing of cruciform shaped specimens. The biaxial response is found to be qualitatively similar to the uniaxial response with the stiffness and strength in a given direction dependent on the degree of biaxiality. The constitutive model was shown to well predict this complex multiaxial deformation response when the model is implemented in the experimental geometry and reduced by the same methods as the experimental results. Biniaterial strip swelling of Nafion and typical gas diffusion layer material (GDL) is used to probe the partially constrained swelling behavior of Nafion. When the strip is hydrated the membrane swells causing the strip to curl with the membrane on the convex side until the force from the membrane is balanced by a moment in the GDL. Upon drying, plastic deformation that occurred during hydration induces a residual curvature of the opposite convexity. The hydrated and dried radii are found to agree with the finite element simulation predictions for two thicknesses of Nafion to within experimental error. Finally, the Nafion constitutive model is used to simulate a simplified fuel cell cycle. A negative hydrostatic pressure develops in the membrane upon drying, suggesting a driving force for cavitation or crazing. A study of the effect of ramp rate and hold time reveal a significant time dependence of the pressure, which is not surprising given the significant rate dependence observed for Nafion under uniaxial mechanical loading. Simulations of this nature are useful in guiding startup and shutdown procedures for fuel cells, for designing/validating potential procedures for accelerated lifetime testing, and for designing alternative fuel cell geometries. Focus is then shifted to the design of new polymer electrolyte membranes for direct methanol fuel cells (DMFC) which are a special case of PEMFC. DMFC operate under the same principal as PEMFC., however the fuel is liquid methanol rather than hydrogen. The high energy density of methanol makes DMFC particularly promising for portable applications where they could replace Li-ion batteries. In contrast to PEMFC, fuel crossover is a major design concern even when the membrane is fully intact. Given the multi-functionality of a DMFC PEM, it is natural to look to a composite solution. For the proton transport and fuel crossover resistance we use a chemistry and synthesis technique developed in the Hammond lab at MIT. This membrane is itself a composite of sulfonated Poly(2,6-dimethyl 1,4-phenylene oxide) (sPPO) and poly(diallyl dimethyl ammonium chloride) (PDAC) assembled via layer-by-layer (LBL) deposition. Unfortunately these films tear easily under dry conditions and are almost fluid like under hydrated conditions. The PDAC/sPPO membrane must therefore be combined with a mechanical support component. Here we use a highly porous and mechanically robust mat produced by electrospinning polyamide (EFM). In this thesis, focus is on the mechanical aspects of the design. A model for the mechanical behavior of the composite is developed based on experiments and models of the component materials. Uniaxial tensile tests are conducted on each of the materials (LBL, EFM, and LBL coated EFM) and the material morphology is examined via scanning electron microscopy where appropriate (EFM and LBL coated EFM). A micromechanically motivated constitutive model is then developed separately for the LBL and the EFM. The LBL model is a single mechanism elastic-plastic model that is highly hydration sensitive. The EFM structure is idealized as a layered triangulated network of elastic-plastic fibers. The behavior of the constituent fibers is taken to be elastic-plastic accounting for stretching and bending of the fibers when subjected to end tensile and compressive loads; the bending of the fibers when a fiber is locally under compression is found to be the key mechanism enabling the mat to consolidate during tensile loading. The layers of triangles impose mutual kinematic constraints emulating the layered structure of real mats, providing greater isotropy to the yield and post-yield behavior. A composite model is then developed as the superposition of the two materials. It is found that a composite model consisting of a weighted summation of the two component behaviors can capture the dry behavior. but not the hydrated behavior. In the hydrated state, the LBL, which is itself quite compliant under uniaxial loading, is found to inhibit fiber bending, thereby lending initial elastic stiffness and reducing post-yield hardening in a non-additive manner.