Carbon-Free Platinum-Iron Nanonetworks with Chemically Ordered Structures as Durable Oxygen Reduction Electrocatalysts for Polymer Electrolyte Fuel Cells

Pt-based electrocatalysts for the oxygen reduction reaction (ORR) are required to exhibit not only high ORR activity but also high durability against both start/stop and load cycles to further expand the applications of polymer electrolyte fuel cells (PEFCs). Here, we developed a carbon-free connected Pt-Fe catalyst comprising a beaded nanonetwork, formed by the connection of Pt-Fe nanoparticles, and a highly chemically ordered face-centered tetragonal (fct) structure. Unlike the conventional catalysts of Pt nanoparticles supported on carbon black (Pt/C), this connected catalyst exhibits excellent durability against start/stop cycles, which is attributed to the elimination of carbon corrosion problems. In addition, a synthesis method, involving the combination of SiO2 coating and annealing, successfully prepared a connected Pt-Fe catalyst with a nanonetwork and high fct degree. This catalyst exhibited ca. 10 times higher ORR specific activity than the Pt/C catalyst. Furthermore, the connected Pt-Fe catalyst with the high fct degree suppressed the dissolution of metals from the catalyst during the load cycles, consequently increasing the retention of the electrochemical surface area and the ORR activity after 10 000 load cycles, in contrast to the catalyst with a low fct degree. In conclusion, this study demonstrates that a carbon-free connected Pt-Fe catalyst with an enhanced chemically ordered structure exhibits a high ORR activity as well as improved durability against both start/stop and load cycle operations, thus providing a great opportunity to realize high-performance ORR catalysts for PEFCs.