Impermeable Separator

AIRFLOW-FIELD VII INNER SEAL (Flew Inward)

H2 FLOW-FIELD Yll OUTER SEAL (Flow Outwart)

Figure 1. Schematic of the components in a unit cell of the air-breather fuel cell stack.

AIRFLOW-FIELD VII INNER SEAL (Flew Inward)

H2 FLOW-FIELD Yll OUTER SEAL (Flow Outwart)

Figure 1. Schematic of the components in a unit cell of the air-breather fuel cell stack.

Air-Breather Stack

Figure 1 depicts the key components of a unit cell for an air-breather stack. Most stacks are 2" (5.1 cm) in diameter with cell active areas of about 13 cm2, although 1.5" (3.8 cm) diameter stacks with 6 cm2 active area cells have also been investigated. End-plates (not shown in the figure) compress the collection of unit cells together with the use of a tie-bolt projecting up through the middle. Around the tie-bolt is a porous, channeled sleeve that provides alignment for the unit cell components and a conduit for the hydrogen to reach the inner edge of the hydrogen flow-fields. Hydrogen feed is dead-ended, although provision is made for an initial purge. The reactant flow-fields are typically reinforced carbon paper (e.g. Spectracarb, from Spectracorp,

Lawrence, MA). Seals are located at the inner edge of the air flow-field and the outer edge of the hydrogen flow-field. The flow-fields bracket the membrane/electrode assembly (MEA), which consists of a catalyzed polymer electrolyte membrane sandwiched between two gas diffusion backings from E-TEK (Natick, MA). Stainless steel foil separators, that are typically 0.010" (0.25 mm) thick, prevent the reactants in the back-to-back flow-fields from mixing. In multi-cell stacks, the odd separators are of a larger diameter to provide cooling fins, which gives the stack the appearance of a finned tube.

Figure 2. Polarization curves of 8-cell air-breather stacks using a series of different MEAs. Results

A series of 13 cm2 active area, 8-cell stacks using the 2" (5 cm) diameter elements and 2.5" (6.4 cm) diameter cooling fins/separators every two cells were assembled and tested using various combinations of flow-fields, MEAs and backings at ambient pressures of 0.75 atm (the laboratory is at an altitude of 7,200 ft or 2,200 m). Not surprisingly, cell performance is substantially affected by the thickness of the cathode flow-field because oxygen must diffuse in from the periphery through this structure. However, while the performance of a single cell air-breather increases with flow-field thickness, more modest thicknesses provide the best performances in multi-cell stacks. Since under continuous operation a multi-cell stack naturally runs warmer than a single cell (ca. 60*C vs. ca. 30°C), too thick of a flow-field allows the cell to dry out excessively. Modeling suggests that the cells suffer once the water partial pressure throughout the flow-field drops below the saturated vapor pressure [3]. A cathode flow-field thickness of about 3 mm is optimal for continuous operation with the 2" diameter 8-cell stacks.

Figure 2. Polarization curves of 8-cell air-breather stacks using a series of different MEAs. Results

A series of 13 cm2 active area, 8-cell stacks using the 2" (5 cm) diameter elements and 2.5" (6.4 cm) diameter cooling fins/separators every two cells were assembled and tested using various combinations of flow-fields, MEAs and backings at ambient pressures of 0.75 atm (the laboratory is at an altitude of 7,200 ft or 2,200 m). Not surprisingly, cell performance is substantially affected by the thickness of the cathode flow-field because oxygen must diffuse in from the periphery through this structure. However, while the performance of a single cell air-breather increases with flow-field thickness, more modest thicknesses provide the best performances in multi-cell stacks. Since under continuous operation a multi-cell stack naturally runs warmer than a single cell (ca. 60*C vs. ca. 30°C), too thick of a flow-field allows the cell to dry out excessively. Modeling suggests that the cells suffer once the water partial pressure throughout the flow-field drops below the saturated vapor pressure [3]. A cathode flow-field thickness of about 3 mm is optimal for continuous operation with the 2" diameter 8-cell stacks.

Figure 2 depicts polarization curves for a series of continuously operating 8-cell stacks using different MEAs, one a W. L. Gore catalyzed composite membrane product [4], and the other two MEAs are commercially supplied membranes catalyzed by a process developed at Los Alamos (LANL) [5]. It is readily evident that the W. L. Gore MEA enjoys more than a 100 mV advantage over the other two MEAs, most likely due to an electrode kinetic advantage realized by attaining a higher water content in the catalyst layer. Since the LANL catalyzation process "toughens" the ionomer in the catalyst layer such that it can withstand the rigors of high current density operation, it apparently does not adsorb an appreciable amount of water under the air-breather conditions. Not only is the W. L. Gore MEA quite thin (ca. 20 Jim), but it also appears to accommodate substantially more water in the catalyst layer, the combination of which results in improved hydration and performance under these conditions.

Another key component is the gas diffusion backing. 8-cell stacks were operated with and without E-TEK backings and with and without a Teflon treatment. The cells using the backings were clearly superior, as flooding was apparent in the cells with the MEA directly against the carbon paper. The need for a "microporous" backing structure to control flooding has been previously observed in high performance cells [5].

Figure 3 depicts the voltage output of an 8-cell air-breathing stack operating on a portable metal hydride canister at a constant current of 1 A. After coming up quickly, the fluctuations about 5 V are quite small for the first 5-6 hours, at which point the hydrogen becomes depleted and some of the cells become starved for hydrogen.

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