The Coupling of Structural and Solution Fe to Active Surface Fe in OER Catalysis at NiFe-Phosphides

There exist inconsistencies in the elusive role of Fe in Ni-based oxygen evolution reaction (OER) catalysts that are related to the influence of structural Fe (Febulk) vs electrolyte Fe (Fesolution). This work decouples their influences and interrogates their coupling with the OER active surface site (Fesurface) in a NiFe-phosphide precatalyst. Ni-phosphide (NixPy) amorphous precatalysts were cathodically deposited undoped or doped with Fe at 6:4 Ni/Fe (Ni0.6xFe0.4xPy) in different loadings and were anodically conditioned in 1 M KOH with Fe traces (KOH) or in purified 1 M KOH (Fe-free KOH) to form the oxyhydroxide layer with and without Fesurface. The electrochemical behavior and OER kinetics were examined in combinations of inclusion and exclusion of Febulk and Fesurface, with and without Fesolution. OER catalysis had the same figures of merit (Tafel slope, rate per redox active Ni) at Ni0.6xFe0.4xPy/Fe-free KOH and NixPy-Fesurface/KOH. This indicates the same mechanism by inclusion of Febulk or Fesurface; therefore, the identity of the active site is the same. We explored two hypotheses: that this electrodeposition yields an activating and replenishing Febulk → Fesurface pathway or that leaching under operating bias yields sufficient Fesolution to activate the surface via a Febulk → Fesolution → Fesurface pathway. While OER electrokinetics and electrochemical impedance spectroscopy at NixPy in an Fe-leached KOH in which Ni0.6xFe0.4xPy was previously held at anodic bias show kinetic facility attributed to leached Fesolution, this path did not yield ideal OER catalysis. Therefore, the Febulk → Fesurface pathway is involved in forming the active site in Ni0.6xFe0.4xPy. We recorded a difference in the catalytic stability at Fesurface with different initial states. The Fesurface-activated NixPy did not retain its ideal catalysis without solution Fe, while Ni0.6xFe0.4xPy-Fesurface leads to stable catalysis in Fe-free KOH. Electrochemical data show the influence of the catalyst and active site preparation and loading; and the coupling between the three Fe species with their different electrical-chemical environments in forming the active site and modulating its dynamics for efficient and stable catalysis.