Strain Relaxation and Bonding Asymmetry of Nanoparticle Electrocatalysts

The fundamental science of catalysis revolves around the understanding of both surface and bulk structures. For electrocatalysts, this understanding is often complicated by oxide layers on the surface. Here we employ electron ptychography to resolve the positions of Mn, O, and N atoms and obtain 3D structural information of MnN@Mn₃O₄ core-shell nanoparticle electrocatalyst.^1,2 Our findings reveal the exposure of (001) and (101) Mn₃O₄ facets and their terminations, along with the mapping of a 2.3% tensile strain at the interface of the (100) facet, which relaxes from 1.6% to 2.2% toward subsurface to the surface. Additionally, we identify the asymmetry in Mn-Mn and Mn-O bonds at the surface and interface. This structural insight forms the basis for a DFT model, which explains how strain relaxation in the (001) facet and the reduction in its surface area led to a decline in electrochemical reactivity, despite an overall increase in surface area due to the growth of the inactive (101) facet.

Reference:

(1) Chen, Z.; Electron Ptychography Achieves Atomic-Resolution Limits Set by Lattice Vibrations. Science 2021, 372 (6544), 826–831.

(2) Zeng, R.; Origins of Enhanced Oxygen Reduction Activity of Transition Metal Nitrides. Nat. Mater. 2024, 1–9.