Spectroelectrochemical Analysis of the Water Oxidation Mechanism on Doped Nickel Oxides

Abstract

Metal oxides and oxyhydroxides exhibit state-of-the-art activity for the oxygen evolution reaction (OER); however, their reaction mechanism, particularly the relationship between charging of the oxide and OER kinetics, remains elusive. Here, we investigate a series of Mn-, Co-, Fe-, and Zn-doped nickel oxides using operando UV–vis spectroscopy coupled with time-resolved stepped potential spectroelectrochemistry. The Ni2+/Ni3+ redox peak potential is found to shift anodically from Mn- < Co- < Fe- < Zn-doped samples, suggesting a decrease in oxygen binding energetics from Mn- to Zn-doped samples. At OER-relevant potentials, using optical absorption spectroscopy, we quantitatively detect the subsequent oxidation of these redox centers. The OER kinetics was found to have a second-order dependence on the density of these oxidized species, suggesting a chemical rate-determining step involving coupling of two oxo species. The intrinsic turnover frequency per oxidized species exhibits a volcano trend with the binding energy of oxygen on the Ni site, having a maximum activity of ∼0.05 s–1 at 300 mV overpotential for the Fe-doped sample. Consequently, we propose that for Ni centers that bind oxygen too strongly (Mn- and Co-doped oxides), OER kinetics is limited by O–O coupling and oxygen desorption, while for Ni centers that bind oxygen too weakly (Zn-doped oxides), OER kinetics is limited by the formation of oxo groups. This study not only experimentally demonstrates the relation between electroadsorption free energy and intrinsic kinetics for OER on this class of materials but also highlights the critical role of oxidized species in facilitating OER kinetics.

The role of crystal facets and disorder on photo-electrosynthesis

“…52 However, kinetic analysis in Fe 2 O 3 containing different vacancy concentrations have shown that the OER rate determining step (RDS) is chemical in nature and is independent of the vacancy concentration. 53 Such discrepancy might be related to the chemical nature of the RDS 54,55 or to the difficulty in experimentally separating the contributions of vacancies within a large surface. As discussed below, further studies integrating modelling and experiments, are needed to unambiguously expose the role of defects and disorder in the catalytic mechanisms.…”

High-Density NiMnFe Hydroxide Hierarchical Nano-arrays for Industrial-level Electro-chemical Oxygen Evolution Reaction

“…8 Transition metals Ni-based catalysts have been extensively identified as a promising candidate for efficient OER owing to their intrinsic electron structure and high carrier mobility. [9][10][11][12] Various strategies have been well investigated for improving OER performance, including doping foreign metals, 13,14 creating heterojunction and defects, [15][16][17] morphology engineering, 18,19 and so on. Among these, introducing new transitional metal elements in Ni-based catalyst receives many attentions due to the capability of optimizing electronic structure.…”

Lewis acid Mg2+-doped cobalt phosphate nanosheets for enhanced electrocatalytic oxygen evolution reaction

“…7 The adsorption strength can be neither too strong nor too weak to attain the best activity, which can be described by the volcano-type activity plots. 8 According to the ''volcano'' model, the cost-effective cobalt-based materials have been considered as a group of promising candidates for the OER. 9 However, the catalytic efficiencies of cobalt-based electrocatalysts are still greatly limited by the kinetically sluggish surface adsorption and the formation of high valent Co active species.…”

“…Reported studies have demonstrated that the introduction of a heteroatom is an effective way to produce a favourable electronic configuration and tune the adsorption energy. 8,10 For instance, Durrant and co-workers reported that a series of Mn-, Co-, Fe-, and Zn-doped nickel oxides can decrease the oxygen binding energetics and modulate the chemical rate-determining step during the OER. 8 Their studies found that the Fe-doped sample exhibits optimal binding energetics to achieve the highest OER activity, which confirmed the Sabatier principle experimentally.…”

“…8,10 For instance, Durrant and co-workers reported that a series of Mn-, Co-, Fe-, and Zn-doped nickel oxides can decrease the oxygen binding energetics and modulate the chemical rate-determining step during the OER. 8 Their studies found that the Fe-doped sample exhibits optimal binding energetics to achieve the highest OER activity, which confirmed the Sabatier principle experimentally. Similarly, Qiao and co-workers demonstrated that Co-doped RuIr nanocrystals can modify the local electronic structure and binding strength of reaction intermediates to enhance the OER activity.…”