(201d) Three-Dimensional Atomic Structure and Local Chemical Order of Medium- and High-Entropy Alloy Electrocatalysts

Medium- and high-entropy alloys (M/HEAs) represent a landscape-changing strategy for designing materials with enhanced properties, including electrocatalysts with entropic stabilization and near-continuum surface adsorption energies. Unlike conventional catalysts, M/HEAs confine different elements to the same lattice, which distorts the lattice structure and induces strain. The lattice distortion and surface strain along with the chemical diversity of adsorption sites increase the activity, selectivity, and durability of M/HEA catalysts, with performance improvements shown over conventional alloys for various multi-step reactions including ammonia oxidation and decomposition, carbon dioxide reduction, and methane combustion. Yet, our current understanding of the chemical and structural properties of M/HEA catalysts remains limited due to the dearth of 3D space and atomic-scale information from diffraction, spectroscopy, electron microscopy, and atomistic simulations. Unlike in cryogenic electron microscopy of macromolecules where averaging over identical unit cells (e.g., molecules or proteins) can solve their 3D structure, imaging M/HEA-based electrocatalysts (and other physical materials) requires a direct method to investigate the unique arrangements of atoms that would not be captured by averaging, such as defects, dopants, and chemical short-range order (CSRO).

We have recently determined [1] the 3D atomic structures of catalytically-active NiPdPt-based M/HEA nanoparticles using atomic electron tomography (AET) with a 19.5 pm precision. AET utilizes sub-Angstrom real-space imaging and advanced reconstruction algorithms to measure atomic coordinates and identify the chemical identities of atoms in materials without averaging nor assuming crystallinity. From these measurements, we have quantified the local lattice distortion, strain tensor, and CSRO, as well as twin boundaries and dislocation cores. We observed a causal relationship between chemical order and structural defects which represents, to our knowledge, the first experimental demonstration of such correlation in any material. Determining the 3D atomic structure of M/HEA catalysts and measuring their 3D local lattice distortion and strain could pave the way for their rational design in a largely untapped range of compositions and structures. The present case study of NiPdPt-based M/HEA nanoparticles provides insights into the heterogeneous distribution of strain and CSRO and represents an important step in this direction. Looking ahead, correlative studies with experimentally-measured 3D atomic structures used as input to density functional calculations and combined with machine learning are expected to enable the discovery of yet-unknown attributes of M/HEA-based electrocatalysts.

1. S. Moniri et al., Nature 624, 564 (2023).

Figure caption: Experimental atomic structure of an HEA nanoparticle, where elements are colored differently and the yellow circles represent the atoms along the twin boundaries.