Operation of polymer electrolyte membrane fuel cells with dry feeds: Design and operating strategies

Hogarth, Warren H.J.; Benziger, J.

doi:10.1016/j.jpowsour.2005.11.079

Cited by 51 publications

(38 citation statements)

References 26 publications

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“…Our group introduced the self-draining channel-less PEM fuel cell that could operate with Nafion membranes utilizing dry feeds. 51 By removing the flow channels we could permit liquid water exiting the GDL to drain by gravity. We found no evidence of flooding in the self-draining fuel cell to current densities [ 2 A/cm 2 at temperatures of 25-808C with H 2 /O 2 feeds at exact stoichiometry of hydrogen and stoichiometric excess of 30% oxygen.…”

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Drops, slugs, and flooding in polymer electrolyte membrane fuel cells

et al. 2008

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in Wiley InterScience (www.interscience.wiley.com).The process of flooding has been examined with a single-channel fuel cell that permits direct observation of liquid water motion and local current density. As product water flows through the largest pores in the hydrophobic GDL, drops detach from the surface, aggregate, and form slugs. Flooding in polymer electrolyte membrane (PEM) fuel cells occurs when liquid water slugs accumulate in the gas flow channel, inhibiting reactant transport. Because of the importance of gravity, we observe different characteristics with different orientations of the flow channels. Liquid water may fall away from the GDL and be pushed out with minimal effect on the local current density, accumulate on the GDL surface and cause local fluctuations, or become a pulsating flow of liquid slugs and cause periodic oscillations. We show that flooding in PEM fuel cells is gravity-dependent and the local current densities depend on dynamics of liquid slugs moving through the flow channels. 2008 American Institute of Chemical Engineers AIChE J, 54: [1313][1314][1315][1316][1317][1318][1319][1320][1321][1322][1323][1324][1325][1326][1327][1328][1329][1330][1331][1332] 2008 Keywords: multi-phase flow, fuel cells, porous media, transport IntroductionPerhaps the greatest challenge facing fuel cells is the difficulty in maintaining stable operation and control due to flooding by liquid water. The build up of water produced at the membrane/cathode interface is known to limit the current output from PEM fuel cells. To describe the effects of flooding, several models have been proposed. [1][2][3][4][5][6] Most of these hypothesize that liquid water condenses in the pores of the gas diffusion layer (GDL) creating a mass transfer resistance for oxygen to get to the membrane/electrode interface as illustrated in Figure 1.Our group recently examined water permeation through the GDL and obtained results that contradicted the previous hypotheses about liquid flooding. 7 The GDL is a woven cloth or paper of carbon fibers that is usually treated with Teflon 1 to increase its hydrophobicity. We showed that water does not enter the GDL until a sufficient hydraulic pressure is applied to overcome the repulsive surface energy. The largest pores in the GDL are the first to permit water penetration, and once water penetrates the pores it can freely drain. Our results suggested a two-highway system for liquid and gas transport through the GDL, as illustrated in Figure 2. Liquid is driven by a hydraulic pressure from the membrane/cathode interface through the largest pores while gas moves from the gas flow channel to the membrane/cathode interface through smaller, but more plentiful, pores. Results that support these conclusions have also been reported using florescence microscopy to view the ex-situ transport of water through carbon paper. 8 Recently, water intrusion has been used to determine the capillary pressure vs. liquid saturation curves for different GDL materials 9,10 , providing pore volume distributions...

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Drops, slugs, and flooding in polymer electrolyte membrane fuel cells

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“…Honda has simply used parallel straight channels in a vertical orientation with the feeds at the top so that the liquid water is removed by gravity [29], similar to how we most easily obtain stable operation with our SAPC design and low flow rates. Hogarth and Benziger introduced the concept of gravity assisted draining with a channel-less flow design to achieve improved PEM performance with dry feeds at elevated temperatures [13]. Woo and Benziger demonstrated that gravity draining could be employed to change the active area of a PEM fuel cell for load following control [30].…”

Section: Discussionmentioning

confidence: 99%

“…In order to develop better, more efficient designs, it is necessary to first understand the physics that control water generation and motion in PEM fuel cells and identify the system parameters that can be engineered into the system. With this goal, earlier work in our group first showed that the water produced by the electrochemical reaction is more than sufficient for hydrating the membrane; it is the small flow channels and high gas velocities that reduce the back mixing of water in the fuel cell that allows for autohumidified operation with dry feeds [13]. Next, we provided evidence for the importance of gravity for liquid water motion and the influence on local current density; liquid water droplets emerging from the GDL either detach and fall when gravity overcomes the surface tension holding the drop to the pore or accumulate and block the pore openings, depending on the orientation of the flow channels [14].…”

Section: Introductionmentioning

confidence: 99%

Effects of GDL Structure with an Efficient Approach to the Management of Liquid water in PEM Fuel Cells

2010

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A model fuel cell with a single transparent straight flow channel and segmented anode was constructed to measure the direct correlation of liquid water movement with the local currents along the flow channel. Water drops emerge through the largest pores of the GDL with the size of the droplets that emerge on the surface determined by the size of the pore and its location under the gas flow channel or under the land. Gravity, surface tension, and the shearing force from the gas flow control the movement of liquid in the gas flow channel. By creating a single large diameter pore in the GDL, liquid water flow emergent from the GDL was forced to be in specific locations along the length of the channel and either under the land or under the channel. The effects of gravity were amplified when the large pore was under the channel, but diminished with the large pore under the land. Current fluctuations were minimised when the dominant water transport from the GDL pore was near the cathode outlet. The results show that it is possible to engineer the water distribution in PEM fuel cells by modifying the pore sizes in the GDL.

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“…Besides, Hogarth and Benziger [212] demonstrated an auto-humidified operation of a channel-less, self-draining fuel cell based on a stirred tank reactor. The new design offers substantial benefits for simplicity of operation and control including: the ability to self-drain reducing flooding, the ability to uniformly disperse water removing current gradients and the ability to operate on dry feeds eliminating the need for humidifiers.…”

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confidence: 99%

A Review of Water Management in Polymer Electrolyte Membrane Fuel Cells

2009

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At present, despite the great advances in polymer electrolyte membrane fuel cell (PEMFC) technology over the past two decades through intensive research and development activities, their large-scale commercialization is still hampered by their higher materials cost and lower reliability and durability. In this review, water management is given special consideration. Water management is of vital importance to achieve maximum performance and durability from PEMFCs. On the one hand, to maintain good proton conductivity, the relative humidity of inlet gases is typically held at a large value to ensure that the membrane remains fully hydrated. On the other hand, the pores of the catalyst layer (CL) and the gas diffusion layer (GDL) are frequently flooded by excessive liquid water, resulting in a higher mass transport resistance. Thus, a subtle equilibrium has to be maintained between membrane drying and liquid water flooding to prevent fuel cell degradation and guarantee a high performance level, which is the essential problem of water management. This paper presents a comprehensive review of the state-of-the-art studies of water management, including the experimental methods and modeling and simulation for the characterization of water management and the water management strategies. As one important aspect of water management, water flooding has been extensively studied during the last two decades. Herein, the causes, detection, effects on cell performance and mitigation strategies of water flooding are overviewed in detail. In the end of the paper the emphasis is given to: (i) the delicate equilibrium of membrane drying vs. water flooding in water management; (ii) determining which phenomenon is principally responsible for the deterioration of the PEMFC performance, the flooding of the porous electrode or the gas

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Operation of polymer electrolyte membrane fuel cells with dry feeds: Design and operating strategies

Cited by 51 publications

References 26 publications

Drops, slugs, and flooding in polymer electrolyte membrane fuel cells

Drops, slugs, and flooding in polymer electrolyte membrane fuel cells

Effects of GDL Structure with an Efficient Approach to the Management of Liquid water in PEM Fuel Cells

A Review of Water Management in Polymer Electrolyte Membrane Fuel Cells

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