Krzysztof Bienkowski

Enhanced water splitting at thin film tungsten trioxide photoanodes bearing plasmonic gold-polyoxometalate particles.

Tungsten trioxide (WO3) is one of a few stable semiconductor materials liable to produce solar fuel by photoelectrochemical water splitting. To enhance its visible light conversion efficiency, we incorporated plasmonic gold nanoparticles (Au NPs) derivatized with polyoxometalate (H3PMo12O40) species into WO3. The combined plasmonic and catalytic effect of Au NPs anchored to the WO3 surface resulted in a large increase of water photooxidation currents. Shielding the Au NPs with polyoxometalates appears to be an effective means to avoid formation of recombination centers at the photoanode surface.

Microwave-assisted nonaqueous synthesis of WO3 nanoparticles for crystallographically oriented photoanodes for water splitting

Nanostructured WO3 photoanodes with crystallographic orientation along the [001] direction were fabricated via doctor blading nanoparticles synthesized through a microwave-assisted nonaqueous sol–gel route. Monoclinic WO3 platelets with a size ranging from 20 to 40 nm and a thickness of 3 nm were obtained after a short reaction time of 10 minutes under microwave irradiation. The films consisted of a porous network of nanoparticles and their photoelectrochemical activity was tested. After cathodic polarization of the photoanodes in the dark which led to a significant increase of 65% of the photocurrent, the films exhibited initially a maximum photocurrent of 2.7 mA cm−2 at 1 V vs. reversible hydrogen electrode (RHE) in 3 M H2SO4 under simulated AM 1.5 G illumination (100 mW cm−2) comparable to the best photocurrents reported for WO3 photoanodes. However oxygen evolution measurements showed that the faradaic efficiency dropped after the cathodic polarization and other products than O2 might be formed. In comparison to the chemical solution growth of films from molecular precursors, the use of preformed nanoparticles in the form of powders is not only more robust and easier to up-scale, but also offers many opportunities to improve the photoelectrochemical performance by tailoring the nanoparticle size, the shape, and their arrangement on the substrate.

Solar-Driven Water Oxidation and Decoupled Hydrogen Production Mediated by an Electron-Coupled-Proton Buffer.

Solar-to-hydrogen photoelectrochemical cells (PECs) have been proposed as a means of converting sunlight into H2 fuel. However, in traditional PECs, the oxygen evolution reaction and the hydrogen evolution reaction are coupled, and so the rate of both of these is limited by the photocurrents that can be generated from the solar flux. This in turn leads to slow rates of gas evolution that favor crossover of H2 into the O2 stream and vice versa, even through ostensibly impermeable membranes such as Nafion. Herein, we show that the use of the electron-coupled-proton buffer (ECPB) H3PMo12O40 allows solar-driven O2 evolution from water to proceed at rates of over 1 mA cm–2 on WO3 photoanodes without the need for any additional electrochemical bias. No H2 is produced in the PEC, and instead H3PMo12O40 is reduced to H5PMo12O40. If the reduced ECPB is subjected to a separate electrochemical reoxidation, then H2 is produced with full overall Faradaic efficiency.

Nanoporous WO3–Fe2O3 films; structural and photo-electrochemical characterization

We investigated the structure and photo-electrochemical properties for water splitting of tungsten trioxide-ferric oxide thin films formed by spray pyrolysis. While annealing at 600°C produces films consisting of a mixture of monoclinic WO3 and hematite α-Fe2O3, the heating at a temperature above 1000°C affords formation of ferric tungstate Fe2WO6. Both kinds of films exhibit optical absorption range comparable or exceeding that of α-Fe2O3. Another important feature is a decreased rate of charge recombination of the mixed-oxide Fe2O3-WO3 with respect to the ferric oxide photo-anodes.