Extended X‐Ray Absorption Fine Structure Investigations of High and Medium Entropy Alloys for Electrocatalysis

Co–Ni–Cu medium entropy alloy (MEA) and Mn–Co–Ni–Cu–Zn high entropy alloy (HEA) nanoparticles with diameters of about 100 nm were prepared on nitrogen‐doped carbon for a potential use as an electrocatalyst. Structural engineering of these platinum‐free materials is expected to further boost their electrocatalytic performance, and thus, intense efforts on MEAs and HEAs are currently ongoing. Here we investigate the nanoparticle structure by extended X‐ray absorption fine structure spectroscopy (EXAFS) at the respective 3d transition metal K‐edges. A detailed quantitative analysis of the EXAFS shows that the MEA nanoparticles feature a face‐centered‐cubic structure, and the lattice sites are randomly occupied by the Co, Cu, and Ni atoms in a solid solution. Compared to the bulk metals, only a slightly increased disorder was found. For the HEA nanoparticles, the EXAFS data suggest a more complex core–shell structure. The metallic core predominantly consists of Co, Ni, and Cu, and only minor contributions of Mn and Zn were found. Again, the EXAFS at the Co, Ni, and Cu K‐edges could be well‐fitted employing a fcc solid solution. In contrast, the shell is only composed by Mn and Zn oxides, predominantly MnO, ZnO and Mn 2 ZnO 4 , and the EXAFS structures at the Mn and Zn K‐edges can sufficiently be described by linear combination fitting of these oxides.