The effect of microporous layer (MPL) pore properties and carbon fiber paper (CFP) thermal properties on high current density fuel cell performance is investigated in this work. Two MPL inks were made using the same ink formulation but with two types of carbon black, Denka Li-400 and Li-435, and both inks were then coated on CFP substrates with different thermal properties. The Li-400 carbon black produces an MPL with a significantly larger pore size distribution compared to the MPL comprised of Li-435. Mercury porosimetry analysis shows that the peak pore size for the Li-400 MPLs is at 300 nm, consistent with recent MPL studies using Li-400 in the MPL [1]. The peak pore size of the Li-435 MPL is expected to be around 60-80 nm due to differences in the primary particle diameter (Li-400 = 48 nm, Li-435 = 23 nm) and specific surface area (Li-400 = 39 m²/g, Li-435 = 133 m²/g). The two MPL inks were coated on commercially available Freudenberg carbon paper substrates with wetproofing. The resulting GDLs (CFP + MPL) have different through-plane thermal conductivities (at 2 MPa compression) as measured. The difference in thermal conductivity was about 20%, 0.138 W/mK vs. 0.166 w/mK.

Single cell membrane electrode assemblies (MEAs) were made in a catalyst-coated membrane (CCM) configuration (5 cm² active area) and were tested under differential flow conditions. The cathode and anode Pt loadings are 0.10 and 0.025 mg/cm², respectively. Polarization curves at different operating conditions were obtained in addition to limiting current experiments to determine the total oxygen transport resistance. The ECAs and Mass Activities for all the CCM MEAs were measured to be comparable, hence cell performance trends can be attributed to the MPL/carbon paper differences.

At dry operating conditions (T_cell = 80°C, RH = 65%, p_abs = 150 kPa), the cell performance of all four (4) samples (small vs. large MPL pores coated on carbon fiber substrates with varying thermal properties) had no significant difference. Under wet operating conditions (T_cell = 80°C, RH = 100%, p_abs = 150 kPa), however, it was found that the thermal conductivity (and not the MPL pore size) had a strong influence on performance – GDLs with higher thermal conductivity showed lower performance. These results reiterate the importance of GDL through-plane thermal conductivity as the main GDL property that influences water management and controls cell performance at high current densities (> 2.0 A/cm²) [2].

References

1. C Simon, H Gasteiger et. al., Impact of Microporous Layer Pore Properties on Liquid Water Transport in PEM Fue Cells: Carbon Black Type and Perforation, Electrochem. Soc., 2017, 164 (14), F1697- F1711
2. J Owejan, M Mathias et. al., Water Transport Mechanisms in PEMFC Gas Diffusion Layers, Electrochem. Soc., 2010, 157 (10), B1456- B1464