Electronic Structure of the (Undoped and Fe-Doped) NiOOH O₂ Evolution Electrocatalyst

Abstract

A DFT study was carried out on the atomic and electronic structure of bulk β-NiOOH, with and without substitution of Ni by Fe, examining different stackings and proton distributions in the constituent sheets. The energies of different NiOOH configurations, evaluated at the DFT + U level including dispersion interactions, have very similar values for different stackings and proton distributions, justifying the observed fact that NiOOH is usually obtained in highly disordered form and supporting the assumption that local structures might depend on the synthesis method. Ni ions seem to prefer centrosymmetric coordinations, even if this implies inhomogeneous numbers of OH groups around them; these inhomogeneities can even induce disproportionation of Ni 3+ into Ni 2+ and Ni 4+ . The electronic structure, computed with a hybrid DFT functional able to give accurate band gaps for different materials, always shows semiconducting character, with band gaps having widths varying in the 0.8−1.5 eV range and edges formed by Ni 3d states. A metallic character, anticipated by some earlier DFT calculations, is never found. Substitution of Ni by Fe seems to be preferred at sites coordinated with lower numbers of OH groups; this can induce electron transfer from Fe to Ni, that is, formation of Fe 4+ and Ni 2+ , accompanied in some cases by proton jumps across the intersheet space. Some local-level environments, the presence of which might depend on the material synthesis method used, can however stabilize Fe 3+ , explaining conflicting literature data on the preferred redox state of Fe when included in NiOOH. Overall, the system seems to facilitate electron and proton movement across the material, which can help electrocatalytic O 2 evolution processes.

Characterization of NiFe oxyhydroxide electrocatalysts by integrated electronic structure calculations and spectroelectrochemistry

“…We used the site-specific magnetizations, namely the difference in spin-up and spin-down densities localized on Ni and Fe, as signatures of different oxidation states. Magnetizations have also been used recently to analyze the 3D periodic solid β-NiOOH (19). Whereas other quantitative methods for inferring integer metal oxidation states in the solid state have been developed (38,39), the use of In the reactant and products, nH denotes the stoichiometry of the film.…”

“…The formation or catalytic role of Fe 4+ or other high-valent Fe species was disfavored based on the experimental and computational data. A subsequent operando Mössbauer spectroscopic study, however, showed that significant quantities of Fe 4+ are generated in NiFe-oxyhydroxide catalysts during electrocatalytic water oxidation (18), and the accessibility of Fe 4+ was supported by an independent computational study of such materials (19). These different, and sometimes conflicting, observations highlight the need for an improved understanding of complex materials of this type, ideally drawing upon synergistic contributions from experimental and computational approaches.…”

Exceptional electrocatalytic oxygen evolution via tunable charge transfer interactions in La0.5Sr1.5Ni1−xFexO4±δ Ruddlesden-Popper oxides

“…While precious metals such as Ir and Ru have become standard OER catalysts when working at low pH, alkaline electrolytes allow for the use of abundant, less expensive metals. For example, Ni–M oxyhydroxides such as Ni 1− x Fe x OOH are known to be very active for the OER; however, lack of long-range order and uncontrolled electronic structure stemming from different synthetic methods 2 complicates elucidation of the mechanism(s) by which the OER activity is improved 3–5 . In fact, recent reports question whether Fe is part of the catalytic cycle or if it promotes partial charge transfer between Ni and Fe metal centers 2,4,6 .…”

“…For example, Ni–M oxyhydroxides such as Ni 1− x Fe x OOH are known to be very active for the OER; however, lack of long-range order and uncontrolled electronic structure stemming from different synthetic methods 2 complicates elucidation of the mechanism(s) by which the OER activity is improved 3–5 . In fact, recent reports question whether Fe is part of the catalytic cycle or if it promotes partial charge transfer between Ni and Fe metal centers 2,4,6 . Additionally, recent work on Ni–Fe oxyhydroxides demonstrated that a significant portion of the measured OER current may be due to other processes and highlighted the need for careful electrochemical analysis of what reactions are contributing to the high activities reported 7 .…”

“…Additionally, recent work on Ni–Fe oxyhydroxides demonstrated that a significant portion of the measured OER current may be due to other processes and highlighted the need for careful electrochemical analysis of what reactions are contributing to the high activities reported 7 . Collectively this means that while Ni 1− x Fe x OOH materials have been reported as highly active catalysts, the large variations in electronic configuration and the resulting catalytic activity in these studies complicate establishment of precise structure–property correlations for Ni–Fe oxyhydroxides 2,5,8 .…”