Tuning the spin state of Fe single atoms by Pd nanoclusters enables robust oxygen reduction with dissociative pathway

The development of Fe single-atom catalysts is greatly impeded by their lower oxygen reduction reaction (ORR) performance in acid electrolytes when compared with Pt/C catalyst. Herein, we report an unprecedented ORR catalyst consisting of Pd nanoclusters (PdNC) and Fe single atoms (Fe–N–C/PdNC). Experimental investigations and theoretical calculations indicate that the enhanced ORR activity results from the PdNC-induced spin-state transition of Fe(II) from low spin to intermediate spin, activating O–O bond through the side-on overlapping and achieving the pathway transition from associative to dissociative process. The over-binding of OH∗ can be facilely released to suppress the site-blocking effect and accelerate the removal of OH∗. In acid conditions, Fe–N–C/PdNC exhibits excellent ORR performance with impressive half-wave potential (0.87 V), insignificant decay after 30,000 cycles, and a high maximum power density of 920 mW/cm2 in fuel cell, superior to Pt/C catalyst. This work provides an efficient pathway to design non-Pt catalysts through the spin-regulation strategy.