Atomically Dispersed Pt and Fe Sites and Pt-Fe Nanoparticles for Durable Proton Exchange Membrane Fuel Cells

Abstract

Proton exchange membrane fuel cells (PEMFCs) as a promising clean energy conversion technology have gained considerable attention. However, the high cost and low durability of Pt-based nanocatalysts for the cathodic oxygen reduction reaction (ORR) hinder the wide adoption of this technology.1 Despite great efforts on the development of advanced Pt-based catalysts to improve the Pt utilization and mass activity toward ORR, 2-3 their high activities/durability measured in liquid cells have rarely been realized in fuel cells. On the other hand, carbon-based Pt group metal (PGM)-free ORR electrocatalysts consisting of highly dispersed transition-metal single atoms in nitrogen-coordinated carbon surfaces (Me-N-C) are promising candidates to replace Pt.4 Unfortunately, the poor durability of Me-N-C has limited their practical applications.5 Some early studies6-7 shown that Me-N-C as a support for Pt-based electrocatalyst could improve the stability of the latter.Herein, we report a hybrid electrocatalyst (denoted as Pt-Fe-N-C) consisting of Pt-Fe alloy nanoparticles on highly dispersed Pt and Fe single atoms in a nitrogen-doped carbon support (Figure 1a). The multiple types of active sites result in not only a 3.7 times higher Pt mass activity, but also excellent durability. The performance loss is negligible even after 100,000 potential cycles (Figure 1b), and no current drop is observed at 0.6 V in a fuel cell test with an ultra-low Pt loading (0.015 mgPt cm−2) in the cathode. The morphology and structure of Pt-Fe-N-C catalyst after 100,000 cycles in a fuel cell were further analyzed. Abundant single atoms were still uniformly distributed on the carbon support. No noticeable aggregation of Pt-Fe nanoparticles was observed. Most of particle size was smaller than 4 nm, a solid PtFe@Pt core-shell structure was formed. Around 7% large size particles formed a percolated structure.This work highlights the importance of the synergistic effects among different active sites in hybrid electrocatalysts and provide an alternative way to design more active and durable low-PGM electrocatalysts for fuel cells and other electrochemical devices.This work was supported by the Research Grant Council (HKUST C6011-20G) and Shenzhen Science and Technology Innovation Committee (SGDX2019081623340748).Figure 1. (a) Scanning transmission electron microscope image (the spots in the blue and red dashed circles are ascribed to the Fe and Pt single atoms, respectively). (b) H2/O2 fuel cell polarization (solid symbols and left axis) and power density (hollow symbols and right axis) plots of the Pt-Fe-N-C cathode before cycling (blue) and after 100,000 potential cycles (red) between 0.6 and 0.95 V. Figure 1