Engineering nanoporous and solid core-shell architectures of low-platinum alloy catalysts for high power density PEM fuel cells

Low-platinum (Pt) alloy catalysts hold promising application in oxygen reduction reaction (ORR) electrocatalysis of proton-exchange-membrane fuel cells (PEMFCs). Although significant progress has been made to boost the kinetic ORR mass activity at low current densities in liquid half-cells, little attention was paid to the performance of Pt-based catalysts in realistic PEMFCs particularly at high current densities for high power density, which remains poorly understood. In this paper, we show that, regardless of the kinetic mass activity at the low current density region, the high current density performance of the low-Pt alloy catalysts is dominantly controlled by the total Pt surface area, particularly in low-Pt-loading H2-air PEMFCs. To this end, we propose two different strategies to boost the specific Pt surface area, the post-15-nm dealloyed nanoporous architecture and the sub-5-nm solid core-shell nanoparticles (NPs) through fluidic-bed synthesis, both of which bring in comparably high mass activity and high Pt surface area for large-current-density performance. At medium current density, the dealloyed porous NPs provide substantially higher H2-air PEMFC performance compared to solid core-shell catalysts, despite their similar mass activity in liquid half-cells. Scanning transmission electron microscopy images combined with electron energy loss spectroscopic imaging evidence a previously unreported “semi-immersed nanoporous-Pt/ionomer” structure in contrast to a “fully-immersed core-shell-Pt/ionomer” structure, thus favoring O2 transport and improving the fuel cell performance. Our results provide new insights into the role of Pt nanostructures in concurrently optimizing the mass activity, Pt surface area and Pt/Nafion interface for high power density fuel cells.