OER Catalysis at Activated and Codeposited NiFe-Oxo/Hydroxide Thin Films Is Due to Postdeposition Surface-Fe and Is Not Sustainable without Fe in Solution

This work examines by electrochemical measurements a
hypothesis
that low-coordination Fe on the surface (surface-Fe) of NiFe-oxo/hydroxide
promotes catalysis for the oxygen evolution reaction (OER) rather
than Fe in the bulk structure (bulk-Fe) even in ultrathin films that
are mostly surface. The effect of method of incorporation of Fe in
Ni-oxo/hydroxide on the electrochemical behavior and OER activity
is interrogated, and the sustainability of OER catalysis at NiFe-oxo/hydroxide
is examined in the absence of Fe in solution. Ni­(Fe)-oxo/hydroxide
ultrathin films of a few monolayers and thicker films of tens of monolayers
of Ni­(OH)2 were deposited at anodic bias from potassium
borate buffer containing Ni nitrate or Ni and Fe nitrates at a 6:4
Ni:Fe ratio and were conditioned and studied in 1 M KOH containing
Fe or purified from Fe. Fe was incorporated in NiFe-oxo/hydroxide
during codeposition but removed from solution during conditioning
and catalysis, was included postdeposition during conditioning and
catalysis in Fe-containing solution, or was incorporated postdeposition
by conditioning in Fe-containing solution and then removed from solution
during catalysis. Ultrathin and thicker NiOxHy and Ni0.6Fe0.4OxHy films
exhibited high OER currents and low Tafel slopes in the range of 40
mV/dec in 1 M KOH after activation that included Fe from solution.
However, ultrathin and thicker codeposited Ni0.6Fe0.4OxHy films exhibited low OER currents in Fe-purified KOH, which further
decreased with the application of anodic bias, and exhibited high
Tafel slopes of ca. 100 mV/dec or higher, in a behavior similar to
that of NiOxHy in Fe-free KOH. Fe included postdeposition or surface-Fe is therefore
indicated to be responsible for high OER catalysis in ultrathin and
thicker NiFe-oxo/hydroxide films. The sustainability of OER catalysis
at postdeposition activated Ni­(Fe)-oxo/hydroxide still required the
presence of Fe in solution. NiOxHy films activated for OER postdeposition in
Fe-containing electrolyte did not sustain their high OER catalysis
in Fe-free KOH but were deactivated with potential cycling. An exchange
that causes surface-Fe to move into higher coordination bulk-Fe is
proposed to cause the loss of OER activity of activated NiFe-oxo/hydroxide
in Fe-free electrolyte.