Unveiling the heterogeneous structure of FeCoNiPdCu High-Entropy Alloy: A case study on electrochemical nitrate reduction

Recently, high-entropy nanoalloys (HEA) had attracted significant interest in various potential catalytic applications due to their unique high-entropy effect. However, in most previous studies, the successful preparation of single-phase HEA has been identified primarily through X-ray diffraction (XRD) and inductively coupled plasma mass spectrometry, often neglecting the presence of minor secondary phases owing to the spatially averaged information provided by XRD. In this work, using FeCoNiPdCu HEA applied to an electrochemical nitrate reduction reaction (NO₃RR) as an example, we confirmed that trace amounts of oxygen induced the formation of a FeCoNiPdCu high-entropy oxide/FeCo oxide@NiPdCu alloy/FeCoNiPdCu HEA three-layer heterogeneous structure, achieving a Faradaic efficiency of 95.16% at an applied potential of −0.7 V vs. RHE. Moreover, by modifying the oxygen content, the element distribution across the entire HEA can be tuned owing to differences in oxide formation enthalpy, further affecting catalytic activity and selectivity. Our work deepens the understanding of HEA structures and provides insights into the active sites and corresponding catalytic mechanisms. © 2025 Elsevier B.V.