High Mass-Transport, Low Pt Loading Fuel Cell Electrodes

Abstract

The loading of Pt-group metals (PGMs) in PEMFCs has been lowered considerably since the first usesof PEMFCs in the 1960s, by up to two orders of magnitude. The PGM loading still needs to be reducedto a level comparable to that in current model catalytic converters required for IC engine cars in mostmodern industrial states [1]. The U.S. Department of Energy has set a target for PGM loading inPEMFCs of 125 μg kW-1 and a total PGM loading of at 125 μgPt cm-2 to be reached by 2020 [2]. Previousapproaches to lower Pt loadings to this level have involved sputtering, electrodeposition, ion-beamtechniques and electro-spray techniques [3]. Here we report a novel method of filter-deposition of highly efficient and high power density Pt/Ccatalyst-coated membrane (CCM) layers using commercially available catalysts. Previous efforts in ourresearch group have demonstrated that using this electrode deposition method in combination withthe floating electrode technique allows kinetic measurements of hydrogen oxidation and oxygenreduction across the full fuel cell potential window without a mass-transport limitation [4]. We showthat using a modified form of this filter-deposition method we can fabricate low-loading electrodeswith high Pt utilization compared to commercial CCMs and other literature reports. Electrodes were deposited with loadings between 20 μgPt cm-2 and 100 μgPt cm-2 by filtration of catalystinks (60 wt.% Pt/C) through a track-etched polycarbonate (PCTE) membrane, hot-pressing to a Nafionmembrane, dissolution of the PCTE membrane in alkaline solution and finally re-acidification of theNafion membrane in 1 M H2SO4. A new method of electrode break-in was developed which allowedincreases in mass activity of up to three times compared to the more commonly used DoE method.PEMFC single cell tests showed that the lowest loading electrodes had performance exceeding that ofDoE targets with 82 μgPt kW-1 with a total loading of 20 μgPt cm-2. Increasing the total electrode loadingto 100 μgPt cm-2 gave a catalyst to power ratio of 128 μg kW-1, almost matching the DoE requirementwithout the need for high performance alloy catalysts. This method of CCM manufacture seems very promising at creating thin, evenly-distributed, highperformance catalyst layers in a simple and reproducible manner and has potential to gain even moreperformance with optimised catalysts and ionomer/carbon ratios.