High-current-density performance of polymer electrolyte fuel cells ͑PEFCs͒ is known to be limited by transport of reactants and products. In addition, at high current densities, excessive amount of water is generated and condenses, filling the pores of electrodes with liquid water, and hence limiting the reactant transport to active catalyst. This phenomenon known as ''flooding'' is an important limiting factor of PEFC performance. In this work, the governing physics of water transport in both hydrophilic and hydrophobic diffusion media is described along with one-dimensional analytical solutions of related transport processes. It is found that liquid water transport across the gas diffusion layer ͑GDL͒ is controlled by capillary forces resulting from the gradient in phase saturation. A one-dimensional analytical solution of liquid water transport across the GDL is derived, and liquid saturation in excess of 10% is predicted for a local current density of 1.4 A/cm 2 . Effect of GDL wettability on liquid water transport is explored in detail for the first time. Furthermore, the effect of flooding on oxygen transport and cell performance is investigated and it is seen that flooding diminishes the cell performance as a result of decreased oxygen transport and surface coverage of active catalyst by liquid water.

A general form of the thermal energy equation for a battery system is derived based on first principles using the volume-averaging technique. A thermal-electrochemical coupled modeling approach is presented to simultaneously predict battery electrochemical and thermal behaviors. This approach couples the thermal energy equation with the previous multiphase micro-macroscopic electrochemical model via the heat generation and temperature-dependent physicochemical properties. The thermal-electrochemical model is multidimensional and capable of predicting the average cell temperature as well as the temperature distribution inside a cell. Numerical simulations are performed on a Ni-MH battery to demonstrate the significance of thermal-electrochemical coupling and to investigate the effects of thermal environment on battery electrochemical and thermal behaviors under various charging conditions.

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...

A two-phase, multicomponent model has been developed for liquid-feed direct methanol fuel cells ͑DMFC͒. In addition to the anode and cathode electrochemical reactions, the model considers diffusion and convection of both gas and liquid phases in the backing layers and flow channels. In particular, the model fully accounts for the mixed potential effect of methanol oxidation at the cathode as a result of methanol crossover caused by diffusion, convection, and electro-osmosis. This comprehensive model is solved numerically using computational fluid dynamics. The transport phenomena and electrochemical kinetics in a liquid-feed DMFC are delineated and the effects of the methanol feed concentration on cell performance are explored. The model is validated against DMFC experimental data with reasonable agreement. The void fraction at the anode outlet is found to be as high as 95% at a cell current density of 0.45 A/cm 2 for a 7 cm long channel. Increase in methanol feed concentration leads to a slight decrease in cell voltage and a proportional increase in the mass-transport limiting current density for a methanol concentration below 1 M. However, when the methanol feed concentration is larger than 2 M, the cell voltage is greatly reduced by excessive methanol crossover and the maximum current density begins to be limited by the oxygen supply at the cathode. The oxygen depletion results from excessive parasitic oxygen consumption by methanol crossing over.

This paper seeks to gain a better understanding of the thermal behavior of Li-ion cells using a previously developed two-dimensional, first principles-based thermal-electrochemical modeling approach. The model incorporates the reversible, irreversible, and ohmic heats in the matrix and solution phases, and the temperature dependence of the various transport, kinetic, and mass-transfer parameters based on Arrhenius expressions. Experimental data on the entropic contribution for the manganese oxide spinal and carbon electrodes, recently published in the literature, are also incorporated into the model in order to gauge the importance of this term in the overall heat generation. Simulations were used to estimate the thermal and electrical energy and the active material utilization at various rates in order to understand the effect of temperature on the electrochemistry and vice versa. In addition, the methodology of using experimental data, instead of an electrochemical model, to determine the heat-generation rate is examined by considering the differences between the local and lumped thermal models, and the assumption of using heat generation rate determined at a particular thermal environment under other conditions. Model simulations are used to gain insight into the appropriateness of various approximations in developing comprehensive thermal models for Li-ion cells. © 2002 The Electrochemical Society. All rights reserved.

There has been much recent interest and development of methods to accurately measure the current distribution in an operating polymer electrolyte fuel cell ͑PEFC͒. This paper presents results from a novel technique that uses a segmented flow field with standard, nonaltered membrane electrode assemblies and gas diffusion layers. Multiple current measurements are taken simultaneously with a multichannel potentiostat, providing high-resolution temporal and spatial distribution data. Current distribution data are shown that display the distributed effects of cathode stoichiometry variation and transient flooding on local current density. It is shown that the time scale for liquid accumulation in gas diffusion layer pores is much greater than that of any electrochemical or gas-phase species transport process. In order to facilitate state-of-the-art PEFC model validation, an idealized single-pass serpentine flow field was used, and the exact geometry is presented.

This paper describes and demonstrates a new method for determination of current density distribution in an operating polymer electrolyte membrane ͑PEM͒ fuel cell. The technique is a modification of the current mapping technique that relies on an array of shunt resistors embedded within a current collecting plate. Standard, nonaltered membrane electrode assemblies are utilized with gas diffusion layers in direct contact with an electrically segmented current collector/flow field. Multiple current measurements are taken simultaneously, allowing transient distribution detection with a multichannel potentiostat. Both steady state and transient data are presented for an operating liquid fed direct methanol fuel cell. Cathode flooding is predicted, and shown to occur at relatively high cathode flow rates. This technique can contribute to knowledge and understanding of key phenomena including water management and species distribution in PEM fuel cells.