Dependence of oxygen evolution reaction catalysis at thin cathodically-deposited nickel-iron-selenide on Fe in the alkaline electrolyte vs. codeposited Fe

Abstract

This study interrogated the role of Fe co-deposited in films versus Fe included from traces in the electrolyte on the electrochemical dynamics and kinetics for the oxygen evolution reaction (OER) in alkaline at a nickel-iron selenide OER pre-catalyst. Iron was co-included in the bulk of nickel selenide (NixSey) during cathodic deposition and is termed bulk-Fe (Febulk), and/or was included on its surface post-deposition by conditioning at anodic bias in alkaline with Fe traces and is termed surface-Fe (Fesurface). The electrochemical dynamics, OER kinetics, and sustainability of catalysis were studied in 1 M KOH stripped of Fe (Fe-free KOH) or containing Fe traces (KOH). The selenides transformed into a selenide/hydroxide with a few monolayers of surface hydroxide during the hours of conditioning and electrokinetic measurements at anodic bias in both solutions. Bulk-Fe did not enhance OER catalysis at nickel selenide, while surface-Fe promoted OER catalysis. Tafel slopes of ca. 40 mV/dec were recorded at (NiFebulk)xSey-Fesurface and NixSey-Fesurface in KOH, while Tafel-slopes of ca. 100 mV/dec were recorded at (NiFebulk)xSey in Fe-free KOH. Tafel slopes of 60–80 mV/dec were recorded in Fe-free KOH at (NiFebulk)xSey-Fesurface and NixSey-Fesurface that had been conditioned in KOH. OER catalysis at Fesurface-activated electrocatalysts was not sustained in alkaline stripped of Fe, neither at anodic bias nor subjected to potential cycling. The electrochemical data is consistent with a mechanism involving a low-coordination surface-Fe in the thin hydroxide layer of not more than few monolayers formed in-situ on the selenide. Surface-Fe is pictured as part of a dynamic catalytic site whose equilibrium density, to ensure sustainable catalysis, requires a greater than a threshold activity of Fe ions and lower than a threshold of activity of Ni, otherwise Fe will be in an inactive high-coordination state.