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