Copper Silver Thin Films with Metastable Miscibility for Oxygen Reduction Electrocatalysis in Alkaline Electrolytes

Increasing the activity
of Ag-based catalysts for the oxygen reduction reaction (ORR) is important
for improving the performance and economic outlook of alkaline-based
fuel cell and metal–air battery technologies. In this work,
we prepare CuAg thin films with controllable compositions using electron
beam physical vapor deposition. X-ray diffraction analysis indicates
that this fabrication route yields metastable miscibility between
these two thermodynamically immiscible metals, with the thin films
consisting of a Ag-rich and a Cu-rich phase. Electrochemical testing
in 0.1 M potassium hydroxide showed significant ORR activity improvements
for the CuAg films. On a geometric basis, the most active thin film
(Cu70Ag30) demonstrated a 4-fold activity improvement
vs pure Ag at 0.8 V vs the reversible hydrogen electrode. Furthermore,
enhanced ORR kinetics for Cu-rich (>50 at. % Cu) thin films was
demonstrated by a decrease in Tafel slope from 90 mV/dec, a commonly
observed value for Ag catalysts, to 45 mV/dec. Surface enrichment
of the Ag-rich phase after ORR testing was indicated by X-ray photoelectron
spectroscopy and grazing incidence synchrotron X-ray diffraction measurements.
By correlating density functional theory with experimental measurements,
we postulate that the activity enhancement of the Cu-rich CuAg thin
films arises due to the non-equilibrium miscibility of Cu atoms in
the Ag-rich phase, which favorably tunes the surface electronic structure
and binding energies of reaction species.