HYDROGEN-WATER FLOW REGIME TRANSITIONS APPLIED TO ANODE FLOW PHENOMENA IN A PEMFC

ABSTRACT Although water is produced on the cathode side of a polymer electrolyte membrane fuel cell (PEMFC), it is known to migrate to the anode, where the two-phase hydrogen-water interactions become critical to keep the channels clear for effective reactant delivery. Hydrogen-water flow regime transitions were experimentally investigated and compared to air-water transitions in a 1.0 mm square channel. Gas superficial velocities were evaluated for an anode stoichiometric ratio of 2.0 over a current density range from 0.1 A/cm 2 to 2.0 A/cm 2 . Liquid superficial velocities were controlled at the corresponding cathodic water production rate. It is shown that the annular transition in a hydrogen system occurs at as much as twice the gas velocity required for the same transition in an air system. At the low liquid flux expected in the anode channels of a PEMFC, a transition from slug to annular two-phase flow will occur at an unobtainable velocity for efficient fuel cell operation

Visualization of Fuel Cell Water Transport and Performance Characterization under Freezing Conditions

In this program, Rochester Institute of Technology (RIT), General Motors (GM) and Michigan Technological University (MTU) have focused on fundamental studies that address water transport, accumulation and mitigation processes in the gas diffusion layer and flow field channels of the bipolar plate. These studies have been conducted with a particular emphasis on understanding the key transport phenomena which control fuel cell operation under freezing conditions. Technical accomplishments are listed below: • Demonstrated that shutdown air purge is controlled predominantly by the water carrying capacity of the purge stream and the most practical means of reducing the purge time and energy is to reduce the volume of liquid water present in the fuel cell at shutdown. The GDL thermal conductivity has been identified as an important parameter to dictate water accumulation within a GDL. • Found that under the normal shutdown conditions most of the GDL-level water accumulation occurs on the anode side and that the mass transport resistance of the membrane electrode assembly (MEA) thus plays a critically important role in understanding and optimizing purge. • Identified two-phase flow patterns (slug, film and mist flow) in flow field channel, established the features of each pattern, and created a flow pattern map to characterize the two-phase flow in GDL/channel combination. • Implemented changes to the baseline channel surface energy and GDL materials and evaluated their performance with the ex situ multi-channel experiments. It was found that the hydrophilic channel (contact angle   10⁰) facilitates the removal of liquid water by capillary effects and by reducing water accumulation at the channel exit. It was also found that GDL without MPL promotes film flow and shifts the slug-to-film flow transition to lower air flow rates, compared with the case of GDL with MPL. • Identified a new mechanism of water transport through GDLs based on Haines jump mechanism. The breakdown and redevelopment of the water paths in GDLs lead to an intermittent water drainage behavior, which is characterized by dynamic capillary pressure and changing of breakthrough location. MPL was found to not only limit the number of water entry locations into the GDL (thus drastically reducing water saturation), but also stabilizes the water paths (or morphology). • Simultaneously visualized the water transport on cathode and anode channels of an operating fuel cell. It was found that under relatively dry hydrogen/air conditions at lower temperatures, the cathode channels display a similar flow pattern map to the ex-situ experiments under similar conditions. Liquid water on the anode side is more likely formed via condensation of water vapor which is transported through the anode GDL. • Investigated the water percolation through the GDL with pseudo-Hele-Shaw experiments and simulated the capillary-driven two-phase flow inside gas diffusion media, with the pore size distributions being modeled by using Weibull distribution functions. The effect of the inclusion of the microporous layer in the fuel cell assembly was explored numerically. • Developed and validated a simple, reliable computational tool for predicting liquid water transport in GDLs. • Developed a new method of determining the pore size distribution in GDL using scanning electron microscope (SEM) image processing, which allows for separate characterization of GDL wetting properties and pore size distribution. • Determined the effect of surface wettability and channel cross section and bend dihedral on liquid holdup in fuel cell flow channels. A major thrust of this research program has been the development of an optimal combination of materials, design features and cell operating conditions that achieve a water management strategy which facilitates fuel cell operation under freezing conditions. Based on our various findings, we have made the final recommendation relative to GDL materials, bipolar design and surface properties, and the combination of materials, design features and operating conditions: • GDL materials: use lower thermal conductivity cathode GDL and decrease the anode GDL thickness. • Bipolar plate design: use a channel geometry that can be produced using a high-speed manufacturing process, with a hydrophilic coating. • Shutdown and gas purge protocol: incorporate above findings in developing cost effective and energy efficient shutdown purge protocol. It should be noted that a comprehensive fuel cell operating strategy must consider the entire range of operating conditions under which the system needs to perform. Although the recommendations above will benefit fuel cell performance under conditions where liquid water is expected to be present, they must also be fully assessed to understand their impact under relatively dry conditions

Transparent PEM Fuel Cells for Direct Visualization Experiments

This paper reviews some of the previous research works on direct visualization of water behavior inside proton exchange membrane (PEM) fuel cells using a transparent single cell. Several papers which have employed the method have been selected and summarized, and a comparison between the design of the cell, materials, methods, and visual results are presented. The important aspects, advantages of the method, and a summary on the previous investigations are discussed. Some initial works on transparent PEM fuel cell design using a single serpentine flow-field pattern are described. The results show that the direct visualization via transparent PEM fuel cells could be one potential technique for investigating the water behavior inside the channels and a very promising way forward to provide useful data for validation in PEM fuel cell modeling and simulation