Enhanced activity and durability for the electroreduction of oxygen at a chemically ordered intermetallic PtFeCo catalyst

Abstract

We have designed a chemically ordered face-centred tetragonal intermetallic PtFeCo (trimetallic) (fct-TM) alloy catalyst using a simple solid-state impregnation method for the oxygen–reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFCs). The fct-TM catalyst has demonstrated both enhanced activity and durability, unlike many Pt alloys. The chemical ordering of the fct-TM was verified by high-angle annular dark-field scanning transmission electron microscopy. The ORR activity of fct-TM was examined using the rotating-disk electrode (RDE) technique and the results are compared with those for a chemically disordered face-centred cubic (fcc), fcc-TM catalyst, and a commercial catalyst from Tanaka Kikinzoku Kogyo, TKK-PtC. The fct-TM displayed superior catalytic (mass) activity relative to disordered fcc-TM and TKK-PtC. The mass activity of fct-TM (0.505 A mg_Pt⁻¹) is 2.5 times higher than that of TKK-PtC (0.23 A mg_Pt⁻¹). The durability of these catalysts was evaluated over 5000 (5k) potential cycles in the lifetime regime. The fct-TM retained 80% of its initial mass activity and electrochemically active surface area (ECSA); however, fcc-TM and TKK-PtC maintained about 50% and 70% activity, respectively. The fct-TM also retained the chemically ordered structure after 5k durability cycles. This was confirmed using selected-area electron-diffraction (SAED) patterns. Furthermore, scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDX) line scans of the fct-TM catalysts after 5k durability cycles revealed that Fe and Co were found similar to as before cycling, which signifies that the dissolution of Fe and Co was impeded by the fct-TM catalysts. The observed enhancement in durability might be due to the ordered arrangement of Pt and Fe/Co within the alloy.