Dynamics and Control of Liquid Water Movement in PEM Fuel Cells

Princeton University, Princeton NJ

Investigators

Abstract

CBET-0754715 Benziger Model Polymer Electrolyte Membrane (PEM) fuel cell reactors are employed to elucidate the key physics of water transport to develop improved dynamic models and control systems. Previous models of PEM fuel cells have missed essential physics of the necessary hydraulic pressure to drive water transport through the gas diffusion layer (GDL), which results in water slugs blocking the gas flow channels and exhaust manifolds. Simplified model PEM fuel cells have helped identify the essential physics that govern the water transport dynamics in PEM fuel cells. New experiments are planned employing simplified model reactors to quantify the effects of surface tension, gravity, and viscosity for water transport in the GDL, the gas flow channels and the exhaust manifold. Flow visualization coupled with local current density measurements to identify better control systems to improve fuel utilization with minimal fuel recycle. The use of simplified model reactors can vastly improve our understanding of reaction and transport in fuel cells, assisting in their improved design, operation and control. Intellectual Merit: A new methodology for analyzing fuel cell operation, focusing on the fuel cell as a chemical reactor will be used. The PI's previous studies of the dynamics of a one-dimensional Stirred Tank Reactor (STR) PEM fuel cell unearthed complex behavior not previously recognized in fuel cell operation. The dynamic response of the STR PEM fuel cell showed that the positive feedback between water produced and proton conductivity in the membrane resulted in current ignition/extinction and steady state multiplicity. Past studies were with a model 2-dimensional segmented anode parallel channel fuel cell that demonstrated current ignition coupled with diffusive water flow in the polymer membrane resulted in current density fronts propagating along the flow channel. The results with the SAPC fuel cell demonstrate flooding in PEM fuel cells occurs due to water slug formation in the gas flow channels which blocks reactant supplies. This work shows how slug motion in the gas flow channels is correlated with local current density fluctuations. By improving knowledge of the basic physics in fuel cells it is possible to develop the channel-less self-draining PEM fuel cell that operates with dry feeds to temperatures of 130ºC with current densities >1 A/cm2. The self-draining fuel cell design has also led to the development of a variable area fuel cell that follows power demands with 100% fuel utilization and is insensitive to temperature. This project is to show the importance that GDL pore size, gravity, and gas velocity have in liquid droplet motion in fuel cell operation, and how that alters the current and power output. Experiments with model micro-fluidic systems to elucidate the dynamics of liquid motion in 2-phase micro-fluidic systems that can identify the necessary feed control to match variable power loads for PEM fuel cells will be done. The approach of employing model reactor systems to identify the essential physics of PEM fuel cell operation is unique. The complex reactor configurations employed in fuel cell development and research has provided integrated responses where the key physics is obscured. Only by the careful design of experimental systems to can isolate the essential physics of reaction and transport on a system scale will it be possible to design PEM fuel cells that optimize performance. Broader Impact: Fuel cells have been identified as an essential element of the hydrogen economy to reduce demand for fossil fuels and improve the environment. Water management in PEM fuel cells is a major stumbling block to robust and simplified operation that is essential for consumer acceptance. The fundamentals of fuel cell dynamics and control addressed here are critical to the engineering design of efficient fuel cells. The data and models developed from this work will serve as a basis for systems engineering of PEM fuel cell systems to make them an economically viable technology. These fundamental studies have also provided an outstanding forum for educational outreach. Past work engaged more than ten undergraduates and three high school teachers in this research. Experiments and teaching modules have been put in place for grades 9-12 in three local high schools and these will be expanded to introduce energy technology to new students