  Electrochemical water splitting is an environmentally friendly technology to
store renewable energy in the form of chemical fuels. Among the earth-abundant
first-row transition metal-based catalysts, mixed Ni-Fe oxides have shown
promising performance for effective and low-cost catalysis of the oxygen
evolution reaction (OER) in alkaline media, but the synergistic roles of Fe and
Ni cations in the OER mechanism remain unclear. In this work, we report how
addition of Ni changes the reactivity of a model iron oxide catalyst, based on
Ni deposited on and incorporated in a magnetite Fe3O4 (001) single crystal,
using a combination of surface science techniques in ultra-high-vacuum such as
low energy electron diffraction (LEED), x-rays photoelectron spectroscopy
(XPS), low energy ion scattering (LEIS), and scanning tunneling microscopy
(STM), as well as atomic force microscopy (AFM) in air, and electrochemical
methods such cyclic voltammetry (CV) and electrochemical impedance spectroscopy
(EIS) in alkaline media. A significant improvement in the OER activity is
observed when the top surface presents an Fe:Ni composition ratio in the range
20-40%, which is in good agreement with what has been observed for powder
catalysts. Furthermore, a decrease in the OER overpotential is observed
following surface aging in electrolyte for three days. At higher Ni load, AFM
shows the growth of a new phase attributed to an (oxy)-hydroxide phase which,
according to CV measurements, does not seem to correlate with the surface
activity towards OER. EIS suggests that the OER precursor species observed on
the clean and Ni-modified surfaces are similar and Fe-centered, but form at
lower overpotentials when the surface Fe:Ni ratio is optimized.
