In-Situ Neutron Radiography of Water Freezing in a GDL

Abstract

Research in fuel cells has increased considerably in the past 10 years largely due to the desire to develop dependable, low emission, and inexpensive alternatives to internal combustion engines. Already deployed in industrial applications as backup power, the widespread utilization of polymer electrolyte membrane fuel cells (PEMFCs) in automobiles has long been a promising prospect. In order to make fuel cells a viable option for such an application, issues of water management must be addressed. Of particular importance in automobile applications is the problem of water within the cathode gas diffusion layer (GDL) and cathode channels freezing in cold temperatures while the car is off. Freezing not only damages the membrane electrode assembly (MEA), it also hinders normal startup and operation of the fuel cell. Neutron radiography has proven effective for imaging the water transport in a fuel cell. Neutrons can easily move through many metals but are heavily attenuated by the hydrogen content of water. This has been shown to be a valuable tool for detecting water in fuel cells. By using neutron radiography the dynamics of water production and movement within a fuel cell can be imaged during normal operation. In this research we take this approach one step further by taking advantage of the water phase density changes to image the freezing dynamics within a fuel cell using neutron radiography. This imaging will aid in the design of MEAs and bipolar plates to be used in cold temperatures. The nuclear research reactor at the University of Texas at Austin includes a neutron radiography system that uses a scintillator screen and a CCD camera. This system has been shown to have sufficient mass resolution to image the small density changes observed in water freezing within a thin GDL. A fuel cell mock-up capable of replicating water production and air usage in a fully functional PEMFC was developed and employed to observe the effects of freezing temperatures on the GDL and bipolar plate channels. Preliminary results show that neutron radiography is well suited for imaging freezing within a fuel cell. This methodology will be further employed in the study of water freezing within the GDL and cathode channels in a fully working PEMFC.