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

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 (SO32-), photoelectrodes show a morphology-dependent performance. Films prepared by a dispersion-spray method with a nanostructured surface (feature size of similar to 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 center dot 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 mu A cm(-2), corresponding with 02 evolution. Limitations to photocurrent production are discussed

A Bottom-Up Approach toward All-Solution-Processed High-Efficiency Cu(In,Ga)S

The development of solution-processable routes to prepare efficient photoelectrodes for water splitting is highly desirable to reduce manufacturing costs. Recently, sulfide chalcopyrites (Cu(In,Ga)S-2) have attracted attention as photocathodes for hydrogen evolution owing to their outstanding optoelectronic properties and their band gap-wider than their selenide counterparts-which can potentially increase the attainable photovoltage. A straightforward and all-solution-processable approach for the fabrication of highly efficient photocathodes based on Cu(In,Ga)S-2 is reported for the first time. It is demonstrated that semiconductor nanocrystals can be successfully employed as building blocks to prepare phase-pure microcrystalline thin films by incorporating different additives (Sb, Bi, Mg) that promote the coalescence of the nanocrystals during annealing. Importantly, the grain size is directly correlated to improved charge transport for Sb and Bi additives, but it is shown that secondary effects can be detrimental to performance even with large grains (for Mg). For optimized electrodes, the sequential deposition of thin layers of n-type CdS and TiO2 by solution-based methods, and platinum as an electrocatalyst, leads to stable photocurrents saturating at 8.0 mA cm(-2) and onsetting at similar to 0.6 V versus RHE under AM 1.5G illumination for CuInS2 films. Electrodes prepared by our method rival the state-of-the-art performance for these materials

CuInGaS2 photocathodes treated with SbX3 (X = Cl, I): the effect of the halide on solar water splitting performance

The realization of photoelectrochemical tandem cells for efficient solar-to-hydrogen energy conversion is currently impeded by the lack of inexpensive, stable, and efficient photocathodes. The family of sulfide chalcopyrites (CuInxGa1-xS2) has recently demonstrated a remarkable stability and performance even when prepared by solution-based routes that potentially lower the cost of fabrication. However, the photovoltage delivered by the photocathodes is still well-below the attainable values, a classical limitation linked to a large density of surface states in these materials. In the present work, we show that the identity of halide present during the growth of the solution-processed CuIn0.3Ga0.7S2 (CIGS) thin-films governs the overall performance by directing the crystal growth and the passivation of surface states. Replacing chlorine by iodine leads to CIGS photocathodes that deliver photocurrents of 5 mA cm(-2) (at 0 V versus RHE) and a turn-on voltage of 0.5 V versus RHE without charge extracting overlayer nor any sign of deterioration during stability test

A bismuth vanadate–cuprous oxide tandem cell for overall solar water splitting

Through examination of the optoelectronic and photoelectrochemical properties of BiVO4 and Cu2O photoelectrodes, we evaluate the feasibility of a BiVO4/Cu2O photoanode/photocathode tandem cell for overall unassisted solar water splitting. Using state-of-the-art photoelectrodes we identify current-matching conditions by altering the photoanode active layer thickness. By further employing water oxidation and reduction catalysts (Co-Pi and RuOx, respectively) together with an operating point analysis, we show that an unassisted solar photocurrent density on the order of 1 mA cm–2 is possible in a tandem cell and moreover gain insight into routes for improvement. Finally, we demonstrate the unassisted 2-electrode operation of the tandem cell. Photocurrents corresponding to ca. 0.5% solar-to-hydrogen conversion efficiency were found to decay over the course of minutes because of the detachment of the Co-Pi catalyst. This aspect provides a fundamental challenge to the stable operation of the tandem cell with the currently employed catalysts