Revealing Dynamic\nSurface and Subsurface Reconstruction\nof High-Entropy Alloy Electrocatalysts during the Oxygen Evolution\nReaction at the Atomic Scale

High-entropy alloys (HEAs) represent a promising material\nsystems\nin the search for next-generation high-performance oxygen evolution\nreaction (OER) electrocatalysts. Developing HEA electrocatalysts requires\na thorough understanding of surface and subsurface reconstruction\nduring the OER and their effects on activity and stability. However,\nit is difficult for most characterization techniques to resolve the\natomic-scale elemental distribution of multiple elements and their\nsurface composition. Herein, we combine atom probe tomography and\ntransmission electron microscopy with online and offline inductively\ncoupled plasma mass spectrometry to unveil the surface and subsurface\nreconstructions of a model CrMnFeCoNi Cantor alloy electrocatalyst\nduring the OER. We reveal that the Cantor alloy suffers from dissolution\nonce in contact with the electrolyte, whereby the surface is deficit\nin Cr, Mn, and Co before the OER. At the onset of the OER cycling,\nthe Cantor alloy surface is activated by forming an ∼5 nm amorphous\nNiFe-rich (oxy)hydroxide. Below the surface (oxy)hydroxide, an ∼3\nnm Cr-rich oxide layer and an oxygen-rich layer (∼3–5\nnm) are formed in the subsurfaces, which impede the outward diffusion\nof Mn, Fe, Co, and Ni to the surface, retarding their continuous dissolution\nfrom the bulk, while Cr dissolution occurs steadily. As the OER proceeds,\nthe Cr-rich oxide layer collapses due to steady Cr dissolution, which\nresults in the exfoliation of the amorphous (oxy)hydroxide layer.\nSimultaneously, the fastest diffusing Mn segregates to the surface,\nforming a Mn-rich oxide, which leads to a deterioration in the OER\nactivity. Our atomic-scale data advance the fundamental understanding\nof how concerted thermodynamically and kinetically driven elementary\nprocesses occur for different elements in HEAs during the OER. More\nimportantly, our study highlights the importance of establishing the\nstructure–activity–stability correlations of HEA electrocatalysts\nduring the OER in order to validate the hypothesis of synergistic\neffects in enhancing their activity and stability.