Demonstration and Characterization of Ladder-Type Conjugated Polymer Photoanode for Direct Light-Driven Water Oxidation

Energy production is not only a scientific challenge. It also has implications on human and animal health, global environment, and international politics and economics. In order to reduce these effects, the development of a new clean and renewable method is essential. A novel alternative is the application of photoelectrochemical (PEC) water splitting cells in which a photoanode and a photocathode can achieve the water hydrolysis with solar illumination as energy input. With this technology, solar energy can be converted into a chemical fuel, hydrogen, stored and distributed offering a decentralized and constant energy flux without daily or seasonal variation. In this work, we demonstrate the feasibility of such a device using all metal oxide photoelectrodes. With state-of-the-art photoelectrodes coupled with water oxidation and reduction catalysts an unassisted solar photocurrent density on the order of 1 mA/cm2 is obtained. We also present the use of a organic material for the water photooxidation. This solution opens a wide range of new materials which can potentially reduce the production prices. We have chosen the Poly-(benzimidazobenzophenanthroline) (BBL) for its exceptional stability and electronic properties. In aqueous electrolyte with a sacrificial hole acceptor, BBL-covered photoelectrodes show a morphology-dependent performance. Films prepared by a dispersion-spray method with a nanostructured surface gave photocurrents up to 0.23 mA/cm2 at 1.23 V vs the RHE reference under standard simulated solar illumination. The solar water oxidation photocurrent with bare BBL electrodes is found to increase with increasing pH, and no evidence of semiconductor oxidation was observed during operation. Characterization of the evolved products demonstrated the formation of hydroxyl radicals and pretreatment with TiO2 followed by a nickel-cobalt catalyst attachment gave solar photocurrents of 20-30 microA/cm2, corresponding with oxygen evolution. Further characterization of these electrodes was realized with various electrochemical techniques. They revealed the formation of a capacitive layer at the film surface in response to the applied potential. The layer formation corresponds to the onset of charge extraction determined by electrochemical impedance spectroscopy. We propose that this process is essential for the charge extraction under illumination and corresponds to a change in charge transport process in the film.