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

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 11 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/nm²

Schematic timeline of the experimental design.

<p>ASL: arterial spin labelling, WM: working memory.</p

Representative MEGA-PRESS spectra.

<p>(A) Averaged MEGA-PRESS spectra (averaged across the subject group) acquired at rest (left) and during the WM task. The GABA peak at 3.0 ppm appears to increase between the resting spectrum and the first WM spectrum, and then decrease during performance of the WM task (panels 2-5). The dashed line marks the resting state peak. Glx: glutamate + glutamine concentration, GABA: gamma-aminobutyric acid, NAA: N-acetylaspartate, IU: institutional units. (B) LCModel output for a single subject: the fit is shown in red, superimposed on the edited spectrum (in black). The top panel shows the residuals between the MRS data and the spectral fit.</p

Areas of significant perfusion change during the WM task (p<0.001, uncorrected, k = 150).

<p>The location of the left DLPFC voxel (white rectangle) is shown for comparison. Results are presented on an axial slice (MNI z-coordinate = 24) of a T1-weighted image from a single subject.</p

Resting perfusion map acquired from a single participant, shown in radiological orientation (scale: 0–90 ml/min/100ml).

<p>Resting perfusion map acquired from a single participant, shown in radiological orientation (scale: 0–90 ml/min/100ml).</p