Direct Light-Driven Water Oxidation by a Ladder-Type Conjugated Polymer Photoanode

A conjugated polymer known for high stability (poly­[benzimidazobenzophenanthroline], coded as BBL) is examined as a photoanode for direct solar water oxidation. In aqueous electrolyte with a sacrificial hole acceptor (SO₃^2–), photoelectrodes show a morphology-dependent performance. Films prepared by a dispersion-spray method with a nanostructured surface (feature size of ∼20 nm) gave photocurrents up to 0.23 ± 0.02 mA cm^–2 at 1.23 V_RHE under standard simulated solar illumination. Electrochemical impedance spectroscopy reveals a constant flat-band potential over a wide pH range at +0.31 V_NHE. The solar water oxidation photocurrent with bare BBL electrodes is found to increase with increasing pH, and no evidence of semiconductor oxidation was observed over a 30 min testing time. Characterization of the photo-oxidation reaction suggests H₂O₂ or •OH production with the bare film, while functionalization of the interface with 1 nm of TiO₂ followed by a nickel–cobalt catalyst gave solar photocurrents of 20–30 μA cm^–2, corresponding with O₂ evolution. Limitations to photocurrent production are discussed.