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

Abstract

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.