Elucidating Cathodic Corrosion Mechanisms with Operando Electrochemical Transmission Electron Microscopy

Cathodic corrosion represents an enigmatic electrochemical
process
in which metallic electrodes corrode under sufficiently reducing potentials.
Although discovered by Fritz Haber in the 19th century, only recently
has progress been made in beginning to understand the atomistic mechanisms
of corroding bulk electrodes. The creation of nanoparticles as the
end-product of the corrosion process suggests an additional length
scale of complexity. Here, we studied the dynamic evolution of morphology,
composition, and crystallographic structural information of nanocrystal
corrosion products by analytical and four-dimensional electrochemical
liquid-cell scanning transmission electron microscopy (EC-STEM). Our
operando/in situ electron microscopy revealed, in real-time, at the
nanometer scale, that cathodic corrosion yields significantly higher
levels of structural degradation for heterogeneous nanocrystals than
bulk electrodes. In particular, the cathodic corrosion of Au nanocubes
on bulk Pt electrodes led to the unexpected formation of thermodynamically
immiscible Au–Pt alloy nanoparticles. The highly kinetically
driven corrosion process is evidenced by the successive anisotropic
transition from stable Pt(111) bulk single-crystal surfaces evolving
to energetically less-stable (100) and (110) steps. The motifs identified
in this microscopy study of cathodic corrosion of nanocrystals are
likely to underlie the structural evolution of nanoscale electrocatalysts
during many electrochemical reactions under highly reducing potentials,
such as CO2 and N2 reduction.