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 NiO x H y and Ni 0.6 Fe 0.4 O x H y 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 Ni 0.6 Fe 0.4 O x H y 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 NiO x H y 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. NiO x H y 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.

Adding Fe3+ or Al3+ to the electrolyte resulted in fast promotion or poisoning, respectively, of catalysis for oxygen evolution at nickel–borate, and both effects were accompanied with anodic shifts in the redox peaks with potential scanning.