Research Papers

Optimization of Passive Air Breathing Fuel Cell Cathodes

[+] Author and Article Information
Bryan Babcock, A. J. Tupper, Dan Clark

Department of Engineering, Colorado School of Mines, Golden, CO 80401

Tibor Fabian

Department of Mechanical Engineering, Stanford University, Stanford, CA 94305

Ryan O’Hayre

Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO 80401

J. Fuel Cell Sci. Technol 7(2), 021017 (Jan 19, 2010) (11 pages) doi:10.1115/1.3177381 History: Received April 16, 2008; Revised October 22, 2008; Published January 19, 2010; Online January 19, 2010

Air-breathing polymer electrolyte membrane fuel cells (ABFCs) use free convection airflow to supply oxygen to their cathodes. These cells are typically characterized by low output power densities compared with forced-convection fuel cells. Because ABFC designs rely on natural convection air delivery, cathode performance is often the performance bottleneck. This paper specifically examines the tradeoff between mass transport losses and ohmic electrical resistance losses for an optimal ABFC cathode design. Optimization is nontrivial because the simultaneous requirements for excellent cell compression, current collection, and gas access are often in contradiction. Simple scaling analysis and experimental observations suggest that the tradeoff between lateral mass transport resistance losses and cathode/gas diffusion layer (GDL) contact resistance losses determines the optimal ABFC cathode design. In order to quantitatively study these effects, we have tested a series of different cathode geometries in a standardized ABFC. Using high frequency resistance measurements and fast-scan polarization measurements, we have been able to interrogate both the ohmic and mass transport losses associated with each cathode configuration. We have also used pressure sensitive foils to examine the pressure distribution for representative configurations, providing a quantitative link between pressure distribution and cell resistance. Finally, we have studied the effect of deploying a current collecting contact layer between the cathode and the GDL. Results indicate that the deployment of a sufficiently stiff yet highly porous contact layer significantly reduces contact resistance losses while imposing minimal additional mass transport losses. A stiff yet porous contact layer reduces the contact resistance losses by increasing the total contact surface area and providing a more even distribution of pressure across the face of the cell. By minimizing contact resistance losses, this strategy enables the deployment of ABFC cathode structures with greater than 90% open area, thereby leading to an enhanced ABFC performance, particularly at high current densities.

Copyright © 2010 by American Society of Mechanical Engineers
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Figure 1

(a) Scaling analysis geometry; (b) mean path lengths for the lateral resistance loss model

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Figure 2

Assembled and exploded cell hardware

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Figure 3

(ac), Digitized original, (d)–(f)color-mapped, and (g)–(i) contour-mapped images of the pressure distributions acquired from the 80% OA cathode with: no CL (a), (d), and (g); thick foil CL (b), (e), and (h); and thick mesh CL (c), (f), and (i). All cell configurations were compressed to 483 kPa nominal pressure and gaskets were adjusted to account for the different thickness of each CL configuration. Films measure 3×3 cm2.

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Figure 4

HFR as a function of nominal cell compression pressure for various CL types in conjunction with the 86% OA cathode

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Figure 5

HFR versus CL stiffness for the 86% OA cathode at a nominal cell compression of 483 kPa. The trend lines included on this graph should not be construed as expected behavior—although HFR resistance must level out at high stiffness due to finite, nonzero contributions to resistance from various cell components such as the leads, GDL and the membrane.

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Figure 6

HFR versus cathode superstructure %OA with either the stiff mesh CL or no CL at a nominal cell compression of 483 kPa

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Figure 7

(a) Polarization plots for the 53%, 80%, and 86% OA cathodes deployed without a CL at a nominal cell compression of 483 kPa; (b) polarization plots for the 53%, 80%, and 86% OA cathodes deployed in conjunction with the thick foil CL at a nominal cell compression of 483 kPa; (c) polarization plots for the 53%, 80%, and 86% OA cathodes deployed with the thick (stiff) mesh CL at a nominal cell compression of 483 kPa



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