Multiple Reaction Pathways for Oxygen Evolution as Key Factor for the Catalytic Activity of Nickel-Iron (Oxy)Hydroxides

We present the results of a comprehensive theoretical investigation, based on state-of-the-art density functional theory simulations, of the structural and electrochemical properties of amorphous pristine and iron-doped nickel-(oxy)hydroxide catalyst films for water oxidation in alkaline solution, hereafter referred to as NiCat and Fe:NiCat. In the case of the structural properties, our simulations accurately reproduce the structural changes occurring in locally ordered units, reported by X-ray absorption spectroscopy measurements when the catalyst films are activated by exposition to a positive potential. We have highlighted the crucial role in this process of a series of proton- coupled electron transfer events in the reversible oxidation of Ni(II) to Ni(III). Once assessed structural models of NiCat and Fe:NiCat in close agreement with experimental results, we used them to investigate the oxygen evolution reaction (OER) atomistic mechanism, activated when the applied potential exceeds the overpotential required to oxidize water and produce molecular oxygen. We have quantitatively compared seven different pathways for the OER enrolled on both the proposed families of reaction mechanisms, namely the adsorbate evolution mechanism (AEM) and the lattice-oxygen mediate mechanism (LOM), and we have rationalized the effect of iron in the huge enhancement of catalytic activity of Fe:NiCat with respect to NiCat. Regarding the competition between AEM and LOM mechanisms, our results support the idea that simple metal- oxygen-metal atomistic motifs, ubiquitous on the surface of all kinds of crystalline and amorphous metal (oxy)hydroxide catalyst films, are able to promote different mechanisms of both types, all compatible with the application of an external positive potential in the range of those used in real electrochemical devices performing the OER. Finally, our results suggest that the elusive role of iron is related to the significant difference between Ni(IV)-O and Fe(IV)-O bonds in two crucial reaction intermediates immediately preceding the formation of the O-O bond, with Fe ions able to lower the potential required to form such intermediates along most of the investigated reaction paths.