High-Temperature PEM Fuel Cells: Towards Decreasing the Pt Loadings

Abstract

The commercialization of high temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) has been limited by (1) their considerable platinum (Pt) loadings, and (2) durability, compared to low temperature PEMFCs (LT-PEMFC). However, the elevated temperature in HT-PEMFC (140 – 160 °C) ensures resilience towards catalytic poisoning from CO (<3%) and H2S (<20 ppm) as fuel impurities and SO2 (<40 ppm) as air pollution. This enables the use of methanol, via reforming, as an affordable and environmentally benign fuel.[1] These temperature-driven advantages have been the force behind the development of the HT-PEMFC technology.We have been optimizing methods and materials to enhance Pt utilization and cell lifetime. Recent results showed cell peak performance >400 mW/cm2 using Pt loadings as low as 0.1 mgPt/cm2, however, with lab scale MEAs operating with pure hydrogen. [1,2] Herein we present efforts to decrease Pt loading while retaining or improving the cell performance on industrial scale (humidified reformate feed). These efforts can be split in two different approaches: (1) the MEA materials optimizations, and (2) MEA fabrication process optimizations. The former focused on studying the MEA onset voltage after the 24 – 48 h cell activation interval, versus the optimal MEA Pt loadings. Efforts to decrease the Pt amounts were expanded to comparing the performances of pure Pt and Pt/Co (Cobalt) -alloyed catalysts. The Pt/Co catalyst was superior towards the oxygen reduction reaction (ORR) as compared to its monometallic counterpart. Additionally, the Pt loading in the MEA was lowered by diluting the catalyst with pristine carbon support particles. The goal was to homogeneously distribute Pt towards the bulk of the catalytic layer (CL) so the phosphoric acid (PA) front, originating from the pressed PA-doped polybenzimidazole (PBI) membrane, does not significantly flood Pt nanoparticles. Increased mass activity was demonstrated for cells in which the Pt/C catalyst was “diluted” by additional carbon particles. The effect is explained by adaptation to a dynamic flooding effect. Furthermore, we demonstrate that a pressure increase of the in-going gases (1.5 BarA) enhanced the operating voltage and the cell showed 500 mW/cm2 at 900 mA/cm2. This entails that for the same performance, the Pt MEA content could be reduced.Additionally, the MEA fabrication processes were optimized with the focus on fine-tuning the (1) hot-pressing conditions and (2) the thermal treatment of the electrodes. The parameter changes affected the cell performance favorably, both qualitatively and quantitatively. By optimizing the MEA fabrication processes, the cells exhibited positive activation trends and up to 50 mV voltage increase.Several demonstration projects have been made for automotive and stationary applications to showcase a new generation of economically more efficient MEAs.