Effects of Ink Formulation on the Structure and Performance of PGM-Free Catalyst Layer in PEMFCs

Abstract

Platinum group metal (PGM) catalysts are the major electrocatalysts for oxygen reduction reaction (ORR) in the polymer electrolyte membrane fuel cells (PEMFCs). However, the cost of PGM catalysts is very high. Particular, the cost becomes unaffordable if the PEMFC is in massive application. In order to remove this cost obstacle of fuel cell commercialization, PGM-free catalysts have been considered as the replacement of PGM catalysts for ORR because of the low cost and the reasonable performance. Fe-C-N complex is the one of the most active centers in PGM-Free catalyst groups. This type of catalyst shows very promising activity in rotation disk electrode (RDE) testing. The half wave potential could reach 0.91 V versus standard hydrogen electrode (SHE). However, in a membrane electrode assembly (MEA), the performance of PGM-Free catalysts is not good enough to replace the PGM catalysts. Since the PGM-free catalysts are so different from the PGM catalysts in terms of catalytic activity, stability, surface conditions, particle size etc, the fabrication of PGM-Free catalyst MEA cannot simply copy the method of making PGM MEA. In addition, the thicknesses of catalyst layers of PFM-free are significantly thicker than that of PGM, for example, 10 times. We proposed a novel method of fabricating PGM-Free catalyst MEA, so that the intrinsic catalyst activity from RDE can be translated into MEA performance. The method is based on the catalyst coated membrane (CCM) method using optimized ionomer to carbon (I/C) ratio and solvent mixture of catalyst ink. Using this method, the PGM-free catalyst MEA achieved the current density 44.9 mA cm-2 at 0.9 ViR-free in H2/O2 and 150 mA cm-2 at 0.8 V in H2/air, which surpassed the performance targets of US Department of Energy (DOE) for PGM-Free catalyst MEA. The property (solvent composition, dispersion of catalyst and ionomer in an ink), structure (pore structure) and the MEA performance have been characterized using ultra-small angle x-ray scattering (USAXS), cyro-TEM, mercury intrusion porosimetry (MIP), SEM/EDAX, RDE and MEA testing. A property-structure-performance relationship has been established.