Highly Active NiO Photocathodes for H₂O₂ Production Enabled via Outer-Sphere Electron Transfer

Area of Science:

• Materials Science
• Electrochemistry
• Photochemistry

Background:

• Tandem dye-sensitized photoelectrosynthesis cells are key for solar fuel production.
• Low photocathode efficiency limits current densities in these systems.
• Developing efficient photocathodes is crucial for advancing solar fuel technologies.

Purpose of the Study:

• To report a new design principle for highly active photocathodes.
• To demonstrate the production of hydrogen peroxide (H2O2) using dye-sensitized nickel oxide (NiO) photocathodes.
• To investigate the mechanism of outer-sphere reduction for enhanced photoelectrosynthesis.

Main Methods:

• Dye-sensitization of nickel oxide (NiO) with porphyrin, coumarin, and ruthenium dyes.
• Fabrication of NiO photocathodes for photoelectrosynthesis.
• Electrochemical measurements under visible light irradiation in near-neutral water.
• Bulk photoelectrolysis experiments over 24 hours.

Main Results:

• Dye-sensitized NiO photocathodes achieved large photocurrents (up to 400 μA/cm²) for H2O2 production.
• Photoelectrosynthesis operated near the thermodynamic potential for O2/H2O2 conversion.
• Bulk electrolysis yielded millimolar H2O2 concentrations with 100% faradaic efficiency.
• The mechanism involves superoxide formation and disproportionation.

Conclusions:

• The developed NiO photocathodes are among the most active reported for multiproton/multielectron transformations.
• The outer-sphere reduction strategy circumvents electron-hole pair separation challenges.
• This work enables new applications for photoelectrosynthesis cells in solar fuel production.