Operando Spectroscopic Analysis of Photovoltage Generation in Hematite Photoanodes

Area of Science:

• Materials Science
• Electrochemistry
• Photocatalysis

Background:

• Solar water splitting is crucial for renewable hydrogen production.
• Optimizing photoelectrode performance requires understanding photovoltage generation under operating conditions.
• Investigating photoanodes for water oxidation presents challenges due to slow multiredox reactions.

Purpose of the Study:

• To measure the hole quasi-Fermi level (E_F,p) in hematite photoanodes under operando conditions.
• To determine the origin of photovoltage in photoelectrodes.
• To identify factors limiting photovoltage generation and correlate them with water oxidation kinetics.

Main Methods:

• Operando optical spectroscopy was used to monitor the hole quasi-Fermi level (E_F,p) in hematite photoanodes.
• Measurements were performed as a function of applied bias and light intensity.
• Quasi-Fermi level splitting was compared with directly measured photovoltages.

Main Results:

• The quasi-Fermi level splitting accurately reflected the measured photovoltages, indicating an electrochemical origin.
• Hole trap states were identified approximately 0.2 eV above the valence band edge, causing E_F,p pinning at low light intensities.
• Saturation of these trap states at higher light intensities correlated with the onset of molecular oxygen production.

Conclusions:

• Photovoltage in hematite photoanodes is primarily electrochemical.
• Hole trap states play a significant role in limiting photoanode performance and water oxidation kinetics.
• Minimizing these trap states through materials processing is a promising strategy for enhancing solar water splitting efficiency.