Startup characteristics of polymer electrolyte fuel cells under subzero temperatures

Cold start capability and survivability of polymer electrolyte fuel cells (PEFCs) in a subzero temperature environment remain a challenge for automotive applications. Fundamental mechanisms governing cold start are not fully understood, although it is recognized that product water becomes ice or frost upon startup when PEFC internal temperature is below the freezing point of water. If the local pore volume of the cathode catalyst layer (CL) is insufficient to contain all of the accumulated water before cell operating temperature rises above freezing, ice may plug the CL and stop the electrochemical reaction by starving it of the reactant gases.;In this dissertation the characteristics of PEFC cold start are experimentally studied in three parts, as follows. In part one the cold start capability and characteristics of the PEFC are investigated, introducing the concept of isothermal cold start. For this study two methods of gas purge are used to generate the initial conditions prior to cold start. The first method is to precisely control the cell internal condition to examine the intrinsic capability of the membrane and electrode assembly (MEA), although the purge procedure to generate this condition is not practical for vehicular application. The second is designed to simulate realistic operation in a fuel cell vehicle, but is less controllable, and less efficient with respect to cold start performance. In both cases, it is found that the membrane water absorption during cold start is important for better cold start performance, and therefore the initial condition with low membrane water content is crucial.;In part two, the gas purge process is elucidated. Gas purge is an integral part of the PEFC shutdown process, and this process determines the initial conditions of the eventual cold start. To investigate purge effectiveness, the experimental procedure to provide excellent repeatability is first developed. Introducing two characteristic parameters for purge effectiveness which represent the through-plane diffusive flux of water vapor from the CL to gas channels, and the in-plane convective flux of water vapor through the gas channels, respectively, it is found that the cell high frequency resistance after 60 second purge can be uniquely determined by these two parameters.;Part three provides a method to measure the time- and space-resolved water distribution along the gas channels. Purge effectiveness decreases as the purge gas flows down the channel because of the increasing water vapor pressure. Therefore, in order to develop a time-saving, energy-efficient purge protocol for vehicular applications, it becomes imperative to capture the spatial distribution of purge effectiveness. Measurement of local high frequency resistance (HFR) is non-trivial, because the imposed AC current can travel in-plane through the gas diffusion layers (GDLs) and pass across the membrane in the adjacent regions. In this study, we developed a novel method to estimate the local HFR during gas purge, and obtained the transient local HFR distribution data during gas purge.;These three sections form a comprehensive study of cold start capability and provide a route to prediction of cold start performance.