Water flipping and the oxygen evolution reaction on Fe₂O₃ nanolayers

Area of Science:

• Electrochemistry
• Materials Science
• Surface Science

Background:

• Hematite photoanodes are crucial for the oxygen evolution reaction (OER) but suffer from high overpotentials and low photocurrent, hindering economic viability.
• The energetic cost of orienting water molecules at the electrode-electrolyte interface, particularly water dipoles, is a potential, overlooked factor contributing to the OER overpotential.

Purpose of the Study:

• To quantify the number of aligned water molecules in the Stern layer and the work required for water flipping on hematite surfaces.
• To investigate the relationship between water molecule orientation, applied potential, pH, and the overpotential in the oxygen evolution reaction.

Main Methods:

• Utilized second harmonic amplitude and phase measurements to probe water molecule orientation at the hematite surface.
• Investigated pH-dependent potentials for Stern layer water molecule flipping under varying applied biases.

Main Results:

• Stern layer water molecule flipping at zero applied bias showed Nernstian behavior dependent on pH.
• At pH 13 and positive potentials, 1-2 monolayers of water molecules oriented oxygen atoms towards the electrode, coinciding with the highest OER current density.
• The work for water flipping correlated with OER current density, being lowest (44 kJ/mol) at high current and highest (100 kJ/mol) at negligible current (pH 5).

Conclusions:

• A causal link is suggested between the energetic cost of Stern layer water molecule flipping and the observed OER overpotential on hematite.
• Findings may guide strategies to reduce the OER overpotential by controlling water molecule orientation at the electrode surface.