Morphological, structural and compositional evolution of PtPdFeCoNi high-entropy alloy nanoparticles towards bifunctional oxygen electrocatalysis

Developing active and stable bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is essential for a wide range of applications of rechargeable air batteries, water electrolysers, and fuel cells. Here, we report that single-phase face-centred cubic structured PtPdFeCoNi high-entropy alloy (HEA) nanoparticles, synthesized <i>via</i> a facile colloidal synthesis approach, possess a good combination of activity and stability toward OER and ORR. Specifically, pristine PtPdFeCoNi HEA nanoparticles exhibit an overpotential of 306 mV at 10 mA cm^-2 for OER and a half-wave potential of 0.82 V <i>versus</i> RHE for ORR, with a narrow overvoltage (Δ<i>E</i>) of 0.71 V in alkaline media, outperforming commercial Pt/C and RuO₂ benchmark electrocatalysts. The OER and ORR activity of the HEA nanoparticles do not change significantly after prolonged electrochemical cycling (3000 cycles). Using X-ray photoelectron spectroscopy and transmission electron microscopy, we found no evident structural, morphological and compositional changes on the HEA nanoparticle surfaces after ORR cycling, explaining its high activity and stability. In contrast, after extended OER cycling, the PtPdFeCoNi nanoparticle surfaces transform into an amorphous layer embedded with Fe-, Co-, and Ni-rich oxyhydroxides, as well as Co-rich oxides, which likely promote activity. Additionally, the shell oxyhydroxide and oxide layer could prevent the continuous dissolution of Pt and Pd, providing long-term stability. Overall, this work underscores the importance of correlating morphological, structural, and compositional changes of HEA nanocatalysts with electrocatalytic performance, for understanding how individual elements behave toward bifunctional oxygen electrocatalysis.