Enhanced oxygen evolution on NiOOH through Fe-promoted transformation of β to γ phase

It is widely agreed that Fe doping boosts the oxygen evolution reaction (OER) on NiOOH, but the mechanism and the active phase responsible for this enhancement are still under debate. Here we systematically investigate the atomic structure, electronic properties and OER activity of NiOOH using a hybrid density functional theory (DFT) method augmented with dispersion corrections. Our approach is validated by the good agreement between calculated properties and experiments. Chemical-bonding and phase transition energy analyses indicate that Fe doping weakens the adjacent O-H bonds and promotes the transformation from β-NiOOH to γ-NiOOH. The computed overpotentials of 0.68 V for β-NiOOH and 0.45 V for Fe-doped γ-NiOOH agree well with experiments. Our work reveals that the phase transition from β- to γ-NiOOH induced by Fe doping is the key factor to the enhanced OER activity of Fe-doped NiOOH relative to undoped NiOOH, which finally elucidates the long-standing debate on the OER mechanism and active phase of this material. The OER mechanism, oxidation state, chemical bonding, and phase transition energy of pure and Fe-doped NiOOH were systemically studied by the HSE06+D3 method. We find Fe doping promotes the transformation from β-NiOOH to γ-NiOOH, with the β-NiOOH and Fe-doped γ-NiOOH as the OER active phase for the pure and Fe-doped NiOOH.