Importance of Nickel Oxide Lattice Defects for Efficient Oxygen Evolution Reaction

Abstract

Nickel oxide/hydroxides [NiO x (OH) y ] are considered to be promising materials to replace noble metals for the oxygen evolution reaction in alkaline media. While several studies showed that iron impurities promote the activity of nickel-based catalysts, the effects of intrinsic nickel defects and the underlying mechanism remain unknown. In this work, X-ray photoelectron spectroscopy is combined with surface-enhanced Raman scattering to understand the reactivity of NiO x thin films, which were prepared at different temperatures and thus varied in their chemical composition and crystalline order. Raman spectroscopy was used to follow the characteristic oxidation of nickel species from Ni II (OH) 2 to Ni III OOH and Ni IV OO − under electrochemical conditions. A stronger oxide-to-hydroxide conversion, consistent with the post-electrochemistry study, was associated with the presence of initial Ni III impurities and oxygen vacancies and appears beneficial for the electrocatalytic activity.

In situ Raman spectroscopy reveals the structure evolution and lattice oxygen reaction pathway induced by the crystalline–amorphous heterojunction for water oxidation

“…16,17 The peaks at 481 cm À1 , 565 cm À1 and 692 cm À1 belong to the vibration peaks of NiO. 16,18 Specically, the Raman peak at 481 cm À1 represents the vibration peak of NiO defects, indicating that there are abundant oxygen defects in amorphous NiO. This result is also consistent with the EPR measurement.…”

Nickel Site Modification by High-Valence Doping: Effect of Tantalum Impurities on the Alkaline Water Electro-Oxidation by NiO Probed by Operando Raman Spectroscopy

“…Therefore, one can say that the result is consistent with the literature. 54 In addition, we agree that there might be a partial transformation of Ni 2+ to Ni 3+ for incorporation of Ta.…”

Optimized NiFe-Based Coordination Polymer Catalysts: Sulfur-Tuning and Operando Monitoring of Water Oxidation

“…The energy shifts in the rising absorption edge position of the Fe K -edge XANES spectra and the changes in the valence states of Fe ions show slight differences compared with those of the Ni K -edge (Figure c,d): (1) The noncatalytic region was defined for potentials below 1.0 V vs RHE; (2) the oxidation of Fe (denoted as Fe ox ) occurred in the range from 1.0 to 1.425 V vs RHE; (3) OER was activated along with the formation of Fe (3+σ)+ (0 < σ < 1) species above 1.425 V vs RHE; and (4) after the removal of the applied potentials, the valence states of Fe ions slightly recovered to a lower value (Figure S39b). Compared with the analyses of the Ni K -edge XANES spectra (Figure a,b), the Fe K -edge XANES spectra indicate an earlier trigger potential of the Fe ox region, suggesting that the Fe sites undergo an early deprotonation process (OH – → O* + H + + e – ). , In summary, investigations of the Ni and Fe K -edge XANES spectra of S-R-NiFe-CPs demonstrate that both Ni and Fe centers undergo oxidation to form high-valent species (Ni (3+σ)+ and Fe (3+σ)+ ), suggesting that they both play a pivotal role during the OER process. ,,, …”