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Liquid water transport and removal from the gas diffusion layer ͑GDL͒ and gas channel of a polymer electrolyte fuel cell ͑PEFC͒ are studied experimentally and theoretically. In situ observations of the liquid water distribution on the GDL surface and inside the gas channel were made in an operating transparent PEFC. Liquid droplet formation and emergence from the GDL surface are characterized and two modes of liquid water removal from the GDL surface identified: one through droplet detachment by the shear force of the core gas flow followed by a mist flow in the gas channel, and the other by capillary wicking onto the more hydrophilic channel walls followed by the annular film flow and/or liquid slug flow in the channel. In the former regime, typical of high gas flow rates, the droplet detachment diameter is correlated well with the mean gas velocity in the channel. In the latter regime characteristic of low gas flow rates, liquid spreading over hydrophilic channel surfaces and drainage via corner flow were observed and analyzed. A theory is developed to determine what operating parameters and channel surface contact angles lead to sufficient liquid drainage from the fuel cell via corner flow. Under these conditions, the fuel cell could operate stably under a low flow rate ͑or stoichiometry͒ with only a minimum pressure drop required to drive the oxidizer flow. However, when the corner flow is insufficient to remove liquid water from the gas channel, it was observed that the annular film flow occurs, often followed by film instability and channel clogging. Channel clogging shuts down an entire channel and hence reduces the cell's active area and overall performance.Polymer electrolyte fuel cells ͑PEFCs͒ are presently regarded as a promising energy conversion system for future automobiles and stationary applications. A significant technical challenge in a PEFC is that the cell is prone to excess liquid water formation due to water production from oxygen reduction reaction ͑ORR͒ at the cathode. Liquid water may fill open pores of a gas diffusion layer ͑GDL͒, thereby blocking the transport of oxygen into a catalyst layer ͑CL͒, and may further cover the catalyst sites in the CL, rendering them electrochemically inactive. This is known as "GDL/CL flooding." Liquid water formation and subsequent flooding may also occur at low current densities under certain operating conditions, such as low temperatures and low gas flow rates, due to faster saturation of the gas phase with water vapor. If liquid water accumulation becomes excessive in a PEFC, a water lens or water band may form inside the gas channel, thereby clogging and shutting down the oxidizer flow. This latter condition is referred to as "channel flooding and clogging." In the presence of either GDL/CL flooding or channel flooding, the cell performance decreases and the longevity of PEFC materials and components suffers. Therefore, liquid water removal from a PEFC is of paramount importance for improving PEFC performance and durability.The need for modeling liqui...

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