Tequila A. L. Harris

Development of a Casting Technique for Membrane Material Used in High-Temperature PEM Fuel Cells☆

This paper describes research work associated with development of a casting test bed for a polyphosphoric acid (PPA) based membrane material used in high-temperature PEM fuel cells with operating temperatures up to 160°C. Casting of this material is difficult due to its high viscosity, temperature-dependent rheological properties, curing tendencies, and corrosiveness at high temperatures. To design appropriate casting equipment, important physical and mechanical properties of this new membrane material have been characterized. Specifically, the rheological behavior of the solution at different concentrations of PPA was determined for use in casting system design. Based on these properties, various casting techniques other than the standard doctor blade film application used for laboratory work were investigated, including stencil printing on a substrate, stencil printing directly on a gas diffusion electrode, nip roller extrusion, and slot die extrusion. As a result, a manufacturing test bed that allows for temperature and humidity control during casting was designed and fabricated to conduct various experiments. Subsequent experiments show that slot die extrusion is the most promising casting method to cost effectively, continuously, and uniformly cast the membrane material.

Full-field optical thickness profilometry of semitransparent thin films with transmission densitometry

A novel bidirectional thickness profilometer based on transmission densitometry was designed to measure the localized thickness of semitransparent films on a dynamic manufacturing line. The densitometer model shows that, for materials with extinction coefficients between 0.3 and 2.9?D/mm, 100-500microm measurements can be recorded with less than +/-5% error at more than 10,000 locations in real time. As a demonstration application, the thickness profiles of 75mmx100?mm regions of polymer electrolyte membrane (PEM) were determined by converting the optical density of the sample to thickness with the Beer-Lambert law. The PEM extinction coefficient was determined to be 1.4D/mm, with an average thickness error of 4.7%.

Wetting behavior of the shear thinning power law fluids

The knowledge of the wetting characteristics of a coating solution is a prerequisite of assuring the final quality of thin films manufactured by the slot die coating process. Because the maximum coating speed is limited by defects, such as air entrainment, which are directly related to film quality, an understanding of the coating limits is critical. Despite the existence of a vast body of literature to understand the occurrence of defects and the coating limits to produce defect-free films, the mechanism of air entrainment is still not well understood, especially for various classes of solutions. In this study, the shape of the upstream meniscus of a mildly shear thinning non-Newtonian coating bead has been studied both numerically and experimentally and subsequently linked to the air entrainment threshold. It has been found that the dynamic contact angle reaches its maximum value at the air entrainment threshold, primarily as a function of the capillary number. In addition, this work suggests that the onset of the dripping and air entrainment boundaries of solutions that follow the power law can be predicted based on the dynamic contact angle as a function of the consistency and power law indices.

A simulation model to approximate penetration of a non-Newtonian fluid into a porous media during slot die coating

A computational fluid dynamics (CFD) model has been developed to predict the penetration depth of a non-Newtonian fluid as it is directly coated onto porous media by a slot die coating process. The model couples 1-D modified Blake–Kozeny equations and Navier–Stokes equations. Experiments of coating a non-Newtonian solution (black strap molasses) onto carbon paper (Toray 090) are conducted and the penetration depths are measured to validate the model. Preliminary results show that predicted and measured penetration depths follow the same trend; that is, as the coating speed increases the penetration depth decreases. However, the simulated penetration depths are found to be one to two times higher than measured values at low coating speeds. Even so, the results are considered reasonable, due to imposed simplifications and approximations of the CFD model and errors associated with the experiments and measurements.

Numerical Simulation of a High Temperature Polymer Electrolyte Membrane Fabrication Process

Cost, durability, and reliability are the major issues hindering the commercialization of polymer electrolyte membrane fuel cells. Electrolyte membranes present in the fuel cell fails under chemical, thermal, and mechanical influences, which, in turn, results in the overall fuel cell failure. In the present work, 2D studies are performed to understand the effect of manufacturing processing conditions and materials on the quality of the high-temperature membranes. Multiphase computational fluid dynamics models are used for solving the flow behavior of a shear-thinning non-Newtonian fluid. The viscosity and velocities were found to have a profound effect on the membrane structure.

Generalized periodic surface model and its application in designing fibrous porous media

Purpose – Fibrous porous media have a wide variety of applications in insulation, filtration, acoustics, sensing, and actuation. To design such materials, computational modeling methods are needed to engineer the properties systematically. There is a lack of efficient approaches to build and modify those complex structures in computers. The paper aims to discuss these issues. Design/methodology/approach – In this paper, the authors generalize a previously developed periodic surface (PS) model so that the detailed shapes of fibers in porous media can be modeled. Because of its periodic and implicit nature, the generalized PS model is able to efficiently construct the three-dimensional representative volume element (RVE) of randomly distributed fibers. A physics-based empirical force field method is also developed to model the fiber bending and deformation. Findings – Integrated with computational fluid dynamics (CFD) analysis tools, the proposed approach enables simulation-based design of fibrous porous me...

Manufacturing of High-Temperature Polymer Electrolyte Membranes—Part I: System Design and Modeling

The most important component of the fuel cell is the membrane electrolyte, having the fundamental responsibility of separating protons and electrons. Minor defects (e.g., pin holes) in the film will cause premature and/or catastrophic failure. As such, special attention should be given to the manufacturing of this fuel cell component. Increased interest in identifying and overcoming the technical and manufacturing challenges associated with fuel cells has surfaced over the past few years. To this end, a design methodology, the science, and the technology to manufacture unique high-temperature polymer electrolyte membranes in a uniform and continuous manner are presented, specifically focusing on system conceptualization, design, and modeling. It has been shown that an overall manufacturing system can be designed for a power-law fluid with time-temperature varying properties.

The effect of solidification on the casting window after bubble entrainment

The objective of this work is to investigate the feasibility of extending the velocity boundary during the slot die casting of a polymer film, by studying the dynamics of entrained bubbles as they transition from the fluid to the solid phase. The solution of interest is a relatively high viscosity non-Newtonian solution. A 2D computational fluid dynamics model, using ANSYS FLUENT 13.0 software, was developed to analyze this behavior. The volume of fluid method and enthalpy–porosity technique were used to track the air (i.e., entrained bubbles) and fluid phases and the solidification of the solution, respectively. It has been found that for this class of solution, bubbles that are entrained in the fluid phase will not diffuse out of the fluid due to the stresses formed during solidification. Thus, for relatively high viscosity non-Newtonian solution, the upper boundary of the casting window cannot be extended after a defect has originated in the film during the casting process.

Graduated Resistance to Gas Flow Through a GDL in an Unconventional PEM Stack

The goal of this study is to design a gas diffusion layer (GDL) for a polymer electrolyte membrane (PEM) fuel cell with a graduated permeability and thereby graduating the resistance to flow throughout the GDL. It has been shown that in using conventional materials, the GDL exhibits a higher resistance in the through-plane direction due to the orientation of the small carbon fibers that make up the carbon paper or carbon cloth. In this study, a GDL is designed for an unconventional PEM fuel cell stack where the reactant gases are supplied through the side of the GDL rather than through flow field channels machined into a bipolar plate. The effects of changing in-plane permeability, through-plane permeability, GDL thickness, and oxygen utilization on the expected current density distribution at the catalyst layer are studied. Three different thicknesses and three different utilizations are investigated. It has been found that a thinner GDL with a lower utilization yields a higher current density on the electrode. A quantitative metric to measure uniformity of reactant distribution and the ratio of the standard deviation of the current density to the average current density was introduced, and it was found that while the uniformity of the reactant distribution is independent of thickness of the GDL, it is inversely proportional to utilization.