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TECHNICAL PAPERS

Strain Accumulation in Polymer Electrolyte Membrane and Membrane Electrode Assembly Materials During a Single Hydration/Dehydration Cycle

[+] Author and Article Information
Louis G. Hector, Michael J. Lukitsch

Materials and Processes Lab, General Motors R & D Center, Warren, MI 48090-9055

Yeh-Hung Lai

 GM Fuel Cell Activities, Honeoye Falls, NY 14472-0603

Wei Tong

Department of Mechanical Engineering, Southern Methodist University, Dallas, TX 75275-0337

J. Fuel Cell Sci. Technol 4(1), 19-28 (Feb 08, 2006) (10 pages) doi:10.1115/1.2393302 History: Received September 16, 2005; Revised February 08, 2006

A digital image correlation technique was used to measure drying-induced strains in perfluorosulfonic acid (PFSA) membrane and membrane electrode assembly (MEA) materials which are key components of fuel cells. Circular coupons of each material were constrained in a stainless-steel drum fixture and hydration was achieved by immersing the drum/coupon assembly in 80°C water for 5min. During air drying at 25°C and 50% relative humidity, a series of 1280×960 digital images of each coupon surface was recorded with a digital camera and image acquisition system for drying periods up to 26h. Cumulative correlation of an initial image recorded prior to hot water immersion and the final image at the end of drying produced in-plane strain contour maps. Incremental correlation was employed to track strain evolution at the coupon centers and at peripheral points. During the earliest drying stages, where both materials exhibited viscoelastic behavior, peak radial strains of 9.50% and 2.50% were measured in the PFSA membrane and MEA materials, respectively. At longer drying times, peak radial strains reached constant values of 5.70% and 1.25% in the PFSA membrane and MEA materials, respectively. Strain accumulation in the MEA tends to be less uniform than that in the membrane, and corresponding peak strain values in the MEA tend to be less than those in the membrane independent of drying duration. Reinforcement from the electrode layers lowers strain accumulation in the MEA, but it does not preclude crack nucleation in these layers as drying proceeds.

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Copyright © 2007 by American Society of Mechanical Engineers
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Figures

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Figure 1

Sample MEA (black) coupons (75mm diameter), disassembled stainless steel drum halves (top half on the left, bottom half on the right) with 56.9 and 101.6mm inner and outer diameters, respectively, rubber seals (black rings just inside bolt circles) and mounting disk (center).

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Figure 2

Experimental configuration showing digital camera with telecentric lens, fiber optic light source, and drum/coupon assembly. The light-diffusing curtain is not shown. The fiber optic light source is positioned for the top-illumination mode.

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Figure 3

Schematic of polar coordinate system (r,θ) on a digital grid in a Cartesian (X,Y) system superimposed on a PFSA membrane image. Solid circle represents r=0, θ=0, i.e., center grid point in each coupon image.

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Figure 4

Digital images of PFSA membrane surface (mounted in the drum fixture) with contrast pattern in test D1: (a) prior to immersion; (b) with crease after 1.8min drying; (c) after 3.5min drying; (d) after 12min drying; (e) digital grid (periodic black dots) on image in (a); (f) digital grid on final image recorded at the end of drying (26h). Maximum radial field of view is 28.5mm.

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Figure 5

Cumulative strain maps from correlation of initial and final PFSA membrane digital images in test D1 (see Figs.  44) over a 26h drying time: (a) contours (Err) of radial strain εrr; (b) contours (Ett) of hoop strain εθθ; (c) contours (Ert) of shear strain εrθ. The filled white circle (superimposed) in each image represents the origin of the polar coordinate system (see Fig. 3) for strain measurement.

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Figure 6

Evolution of true strain at the membrane center (r=0,θ=0deg) and a peripheral point (r=20mm,θ=180deg) over first 1.4h of drying (from incremental analysis). Strain contour values represented by Err and Ett, respectively, following the strain map keys in Fig. 5. (a) radial strain contours Err from D1; (b) shear strain contours Ett from D1; (c) radial strain contours Err from D3; (d) hoop strain contours Ett from D3.

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Figure 7

Digital image of MEA coupon T1 (with white dot contrast pattern) in drum fixture. Note that portions of the digital grid superimposed on the coupon image appear as small black dots on the white dots.

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Figure 8

Cumulative strain maps comparing initial and final digital images from MEA test T1 over a 26h drying period. (a) contours (Err) of radial strain εrr; (b) contours (Ett) of hoop strain εθθ; (c) contours (Ert) of shear strain εrθ. The filled white circle (superimposed) in each image represents the origin of the polar coordinate system (see Fig. 3) for strain measurement.

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Figure 9

Evolution of strain at the membrane center (r=0,θ=0deg) and peripheral point (r=20mm,θ=180deg) over first 1.4h of drying (from incremental analysis). Strain contour values represented by Err and Ett, respectively, following the strain map keys in Fig. 8. (a) Radial strain contours Err from T1; (b) hoop strain contours Ett from T1; (c) radial strain contours Err from T3; (d) hoop strain contours Ett from T3.

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Figure 10

Digital images of B1 MEA coupon with bottom illumination (i.e., fiber optic ring in Fig. 2 placed beneath the drum/coupon assembly): (a) initial field of view in dry state (i.e., prior to hot water immersion showing “stars-at-night” (white dots) pattern; (b) after 26h drying time show increased density in pattern. Maximum radial field of view is 28.5mm.

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