First‐Principles Modeling of Electrochemical Water Oxidation on MnO:ZnO(001)

Nickel oxyhydroxide (NiOOH) is known to increase the oxygen evolution reaction (OER) performance of hematite (Fe2O3) photoanodes. In recent experimental studies, it has been reported that the increased OER activity is related to the activation of the hematite (α-Fe2O3) surface by NiOOH rather than the activity of NiOOH itself. In this study, we investigate the reason behind the higher activity and the low overpotentials for NiOOH-Fe2O3 photoanodes using first principles calculations. To study the activity of possible catalytic sites, different geometries with NiOOH as a cluster and as a strip geometry on hematite (110) surfaces are studied. Density functional theory + U calculations are carried out to determine the OER activity at different sites of these structures. The geometry with a continuous strip of NiOOH on hematite is stable and is able to explain the activity. We found that the Ni atoms at the edge sites of the NiOOH cocatalyst are catalytically more active than Ni atoms on the basal plane of the cocatalyst; the calculated overpotentials are as low as 0.39 V.

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“…38 They are calculated for each step in eq 1-4 based on literature. 34,41 The equations are given as…”

Section: Methodology and Computational Detailsmentioning

confidence: 99%

Why does NiOOH cocatalyst increase the oxygen evolution activity of α-Fe2O3?

George

Zhang

Bieberle‐Hütter

2019

The Journal of Chemical Physics

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“…We use previously reported values for zero point energy (ZPE) correction and entropy contribution (T∆S), since they were previously found to be very similar between different oxide materials. 29,35 Kanan et al 57 where ∆G signifies the free energy with the free site as the reference. n is the number of reactions considered in the system and ∆Gn is the free energy step for a single reaction.…”

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confidence: 99%

Oxygen Evolution at Hematite Surfaces: The Impact of Structure and Oxygen Vacancies on Lowering the Overpotential

et al. 2016

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Simulations of the oxygen evolution reaction (OER) are essential for understanding the limitations of water splitting. Most research has focused so far on the OER at flat metal oxide surfaces. The structure sensitivity of the OER has, however, recently been highlighted as a promising research direction. To probe the structure sensitivity, we investigate the OER at eleven hematite (Fe2O3) surfaces with density functional theory + Hubbard U (DFT + U) calculations. The results show that the O-O coupling (O-O bond formation via two adjacent terminal Os at dual site) OER mechanism at the (110) surface is competing with the mechanism of OOH formation at single site. We study the effects of surface orientation (110 vs. 104), active surface sites (bridge vs. terminal site), presence of surface steps and oxygen vacancy concentration on the OER and explore strategies to reduce the OER overpotential. It is found that the oxygen vacancy concentration is the most effective parameter in reducing the overpotential. In particular, an overpotential of as low as 0.47 V is obtained for the (110) surface with an oxygen vacancy concentration of 1.26 vacancies/nm 2 .

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“…In the calculations of free energy changes, we use previously reported values used for Fe2O3 for ∆ZPE -T∆S [32] and keep them constant for different reaction sites, since they were found to vary little for different oxide materials (Fe2O3, TiO2, MnO:ZnO) as well as for different reaction sites (top and bridge sites of MnO:ZnO alloys) with a maximum differences of 0.05 eV [33]. The overpotential η for OER is defined as the extra potential needed to drive water splitting compared to the standard potential which is 1.23 V for water splitting.…”

Section: Computational Detailsmentioning

confidence: 99%