Michael A. Hickner

Relaxation of Proton Conductivity and Stress in Proton Exchange Membranes Under Strain

The stress relaxation and proton conductivity of Nafion 117 membrane (N117-H) and sulfonated poly(arylene ether sulfone) copolymer membrane with 35% sulfonation (BPSH35) in acid forms were investigated under uniaxial loading conditions. The results showed that when the membranes were stretched, their proton conductivities in the direction of the strain initially increased compared to the unstretched films. The absolute increases in proton conductivities were larger at higher temperatures. It was also observed that proton conductivities relaxed exponentially with time at 30°C. In addition, the stress relaxation of N117-H and BPSH35 films under both atmospheric and an immersed (in deionized water) condition was measured. The stresses were found to relax more rapidly than the proton conductivity at the same strains. An explanation for the above phenomena is developed based on speculated changes in the channel connectivity and length of proton conduction pathway in the hydrophilic channels, accompanied by the rotation, reorientation, and disentanglements of the polymer chains in the hydrophobic domains.

Experimental Studies of Liquid Water Droplet Growth and Instability at the Gas Diffusion Layer/Gas Flow Channel Interface

Experimental investigations were carried out to visualize the dynamic behavior (contact angle hysteresis and droplet shape) of liquid water droplets on carbon paper gas diffusion layers that are typically employed in proton exchange membrane fuel cells (PEMFCs). The experimental technique mimicks the generation of liquid water and formation of droplets in an air shear flow at the gas diffusion layer – gas flow channel interface of a simulated PEMFC cathode. Images obtained of growing liquid water droplets yield information on the contact angle hysteresis and droplet height, which were subsequently used to map droplet “instability” diagrams. These instability diagrams provide quantitative guidance on liquid water droplet removal at the gas diffusion layer/gas flow channel interface under the conditions of interest. The experimentally mapped droplet diagrams are compared with those predicted using a simplified model based on a macroscopic force balance and reasonably good agreement is obtained.Copyright

A New Constitutive Model for Predicting Proton Conductivity in Polymer Electrolytes

A new constitutive model relating proton conductivity to water content in a polymer electrolyte or membrane is presented. Our constitutive model is based on Faraday’s law and the Nernst-Einstein equation; and it depends on the molar volumes of dry membrane and water but otherwise requires no adjustable parameters. We derive our constitutive model in two different ways. Predictions of proton conductivity as a function of membrane water content computed from our constitutive model are compared with that from a representative correlation and other models as well as experimental data from the literature and those obtained in our laboratory using a 4-point probe.Copyright

Modeling PEM Fuel Cell Performance Using the Finite-Element Method and a Fully-Coupled Implicit Solution Scheme via Newton’s Technique

A numerical model that employs the finite-element method and a fully-coupled implicit solution scheme via Newton’s technique is presented for simulating the performance of polymer-electrolyte-membrane (PEM) fuel cells. With our model, solved are the multi-dimensional momentum, mass & species, and charge conservation equations that govern, respectively, pressure-gradient driven flows along the gas flow channels (GFCs) and within the gas diffusion layers (GDLs), species transport along GFCs and within GDLs, and proton and water transport within the membrane as well as the ButlerVolmer constitutive equations describing the hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR). For simplicity, the present version of our model considers PEM fuel cell operation as isothermal and water present as vapor, and treats the anode and cathode catalyst layers as respective interfaces at which HOR and ORR take place. With our numerical approach, all governing equations are solved simultaneously and quadratic convergence is ensured due to the use of Newton’s method with an analytical Jacobian. To demonstrate the utility of our computational approach, computed predictions of velocity field, contours of hydrodynamic pressure and molar concentrations of hydrogen, oxygen and water species, and current distribution and polarization (or I-V) curves from a two-dimensional case study of a simplified PEM fuel cell are presented. To help assess the validity of our PEM fuel cell model, measurements of current distribution and polarization curves were performed using a segmented PEM fuel cell, and the resultant experimental data as well as that from the literature are compared with computed predictions.Copyright

Bio micro fuel cell grand challenge final report.

Christopher Apblett, Kent Schubert, Bruce Kelley, Andrew Walker, Blake Simmons, Ted Borek, Stephen Meserole, Todd Alam, Brian Cherry, Greg Roberts, Jim Novak, Jim Hudgens, Dave Peterson, Jason Podgorski, Susan Brozik, Jeb Flemming, Stan Kravitz, David Ingersoll, Steve Eisenbies, Randy Shul, Sarah Rich, Carrie Schmidt, Mike Beggans, Jeanne Sergeant, Chris Cornelius, Cy Fujimoto, Micheal Hickner, Swapnil Chabra, Suzanne Ma, Joanne Volponi, Micheal Kelly, Kevin Zavadil, Chad Staiger, Patricia Dolan, Monica Manginell, Jason Harper, Dan Doughty, Steve Casalnuovo

Final report on LDRD project : elucidating performance of proton-exchange-membrane fuel cells via computational modeling with experimental discovery and validation.

In this report, we document the accomplishments in our Laboratory Directed Research and Development project in which we employed a technical approach of combining experiments with computational modeling and analyses to elucidate the performance of hydrogen-fed proton exchange membrane fuel cells (PEMFCs). In the first part of this report, we document our focused efforts on understanding water transport in and removal from a hydrogen-fed PEMFC. Using a transparent cell, we directly visualized the evolution and growth of liquid-water droplets at the gas diffusion layer (GDL)/gas flow channel (GFC) interface. We further carried out a detailed experimental study to observe, via direct visualization, the formation, growth, and instability of water droplets at the GDL/GFC interface using a specially-designed apparatus, which simulates the cathode operation of a PEMFC. We developed a simplified model, based on our experimental observation and data, for predicting the onset of water-droplet instability at the GDL/GFC interface. Using a state-of-the-art neutron imaging instrument available at NIST (National Institute of Standard and Technology), we probed liquid-water distribution inside an operating PEMFC under a variety of operating conditions and investigated effects of evaporation due to local heating by waste heat on water removal. Moreover, we developed computational models for analyzing the effects of micro-porous layer on net water transport across the membrane and GDL anisotropy on the temperature and water distributions in the cathode of a PEMFC. We further developed a two-phase model based on the multiphase mixture formulation for predicting the liquid saturation, pressure drop, and flow maldistribution across the PEMFC cathode channels. In the second part of this report, we document our efforts on modeling the electrochemical performance of PEMFCs. We developed a constitutive model for predicting proton conductivity in polymer electrolyte membranes and compared model prediction with experimental data obtained in our laboratory and from literature. Moreover, we developed a one-dimensional analytical model for predicting electrochemical performance of an idealized PEMFC with small surface over-potentials. Furthermore, we developed a multi-dimensional computer model, which is based on the finite-element method and a fully-coupled implicit solution scheme via Newtons technique, for simulating the performance of PEMFCs. We demonstrated utility of our finite-element model by comparing the computed current density distribution and overall polarization with those measured using a segmented cell. In the last part of this report, we document an exploratory experimental study on MEA (membrane electrode assembly) degradation.

Durability and Performance of Press Molded Polymer Composite Monopolar Plates

The electrical and mechanical properties of new lightweight graphite polymeric separator plates aged in a PEM fuel cell were investigated to assess their resistance to short-term durability. While the changes in electrical properties of great interest to the operation of the fuel cell, mechanical and dimensional stability over the life of the cell are critical. Thus, new polymeric based separator plates developed at Virginia Tech were aged under standard operating conditions in a PEM fuel cell over 300 hours at low pressure and 85°C. A comparison of conductivity, stiffness and strength of aged plates was made to as manufactured and unaged plates. Over the aging period, electrical conductivity did not decline even as the fuel cell performance showed some changes as evidenced by polarization curves. However, the mechanical strength of the monopolar plates was observed to declined less than 10% after 300 hours of fuel cell operation, due to the lack of stability of the polyester resin used to facilitate the rapid manufacturing of these new plates. These property changes were found to be independent of aging on the reduction and oxidation sides. Further work continues on plates formed through both fiber wet lay technology and those produced by compression molding of unique graphite filled epoxy systems, and to improve the electrochemical performance of cells fabricated using the resulting plates to levels comparable to those observed when using existing plate materials.© 2004 ASME