The Role of Compressive Stress on Gas Diffusion Media Morphology and Fuel Cell Performance

Understanding the respective morphology changes with compression of the gas diffusion layer (GDL) and microporous layer (MPL) in unitized gas diffusion media (GDM) is critical for polymer electrolyte fuel cell (PEFC) high-power performance, as the compression affects the ohmic resistance and the porosity that influences mass-transport resistance. We present a comprehensive study of morphology of two types of GDM (paper-type SGL 29BC and felt-type Freudenberg H2315 C2) under varied levels of compression using X-ray computed tomography (CT) to link GDM microstructure to fuel cell performance. The SGL 29BC morphology evolves more significantly with compression in ways that we expect to occlude oxygen diffusivity, while transitions in the Freudenberg H2315 C2 are more gradual. Upon compression by 0-34% its initial thickness, the 29BC pore-size distribution (PSD) shifts from bimodal (12.6 and 34.9 mu m average pore radii) to unimodal (9.67 mu m), extensive MPL surface cracks decrease in surface area and depth (5-2.2% crack surface area), and void volume fraction decreases from 0.45 to 0.18. Freudenberg H2315 C2 GDM maintains a unimodal PSD (10.5 to 8.33 mu m average pore radii), has minimal surface cracking in its discrete MPL layer, and maintains a larger void volume (0.54 to 0.35) upon compression from 0 to 28% its initial thickness. As a result, PEFCs operated in hot and humid conditions (80 degrees C, 100% RH) with SGL 29BC applied as cathode GDM lose performance beyond 14% compression; the current density at 0.6 V decreases from 827.8 to 795.9 mA cm(-2) as 29BC compression increases from 14 to 28% the uncompressed GDM thickness. Alternatively, PEFCs with Freudenberg H2315 C2 GDM at the cathode increase in current density at 0.6 V as compression increases from 14 to 28% (1007 to 1098 mA cm(-2)).