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...

Using an optical H 2 /air polymer electrolyte fuel cell ͑PEFC͒, the mechanics of liquid water transport, starting from droplet emergence on the gas diffusion layer ͑GDL͒ surface, droplet growth and departure, to the two-phase flow in gas channels, is characterized under automotive conditions of 0.82 A/cm 2 , 70°C, and 2 atm. It is observed that water droplets emerge from the GDL surface under oversaturation of water vapor in the gas phase, appear only at preferential locations, and can grow to a size comparable to the channel dimension under the influence of surface adhesion. Liquid film formation on more hydrophilic channel walls and channel clogging are also revealed and analyzed.Water management that balances membrane dehydration with electrode flooding is critical to achieve high performance and longevity of polymer electrolyte fuel cells ͑PEFCs͒. At high current density and/or low flow stoichiometry, PEFC is prone to flooding; that is, there is an excessive amount of water accumulated in the cell. If pores in the catalyst layer and gas diffusion layer ͑GDL͒ are filled with liquid water, or if the gas channels are clogged by liquid water to such an extent that the transport of reactant gases to the electrodes is hindered, substantially deteriorated cell performance results and mass transport limitation due to flooding occurs. The GDL, either nonwoven carbon paper or woven carbon cloth, is highly porous ͑Ͼ70% with pore sizes in the range of 10-30 m͒, electrically conductive, and hydrophobic. In addition, a microporous layer ͑MPL͒ ͑e.g., 30 m thick͒, consisting of carbon particles mixed with the PTFE binder, is usually applied onto the side of the GDL facing the catalyst layer. The MPL features a finer pore structure with a pore size on the order of 0.1-0.5 m. The MPL is intended to provide wicking of liquid water into the GDL by creating a gradient in liquid water pressure and minimize electric contact resistance with the adjacent catalyst layer. Wilson et al. 1 speculated that droplets of water generated at the interface of MPL and catalyst layer are in some form proportional in size to the diameter of MPL pores.Understanding liquid water transport and distribution in a PEFC is a key to unraveling the origin and development of flooding. Prior experimental efforts to probe the water distribution in an operating PEFC have included neutron radiography 2 and gas chromatography ͑GC͒ 3,4 measurements. The in situ method using neutron radiography was reported to investigate the two-phase flow pattern in the flowfield of both hydrogen and methanol PEFCs. Neutron beams can penetrate through a metal fuel cell to image the real-time liquid water profiles along the large-scale flowfield. However, the neutron radiographic imaging is currently limited in both spatial ͑e.g. Ͼ150 m͒ and temporal resolution ͑e.g. Ͻ30 Hz͒, making it difficult to capture two-phase flow phenomena in PEFC that is transient in nature and controlled by surface forces. Our previous work 3,4 on water distribution measurement by using a Micro GC provi...

Interest in the low-cost production of clean hydrogen is growing. Anion exchange membrane water electrolyzers (AEMWEs) are considered one of the most promising sustainable hydrogen production technologies because of their...