Renata Solarska

Metal oxide photoanodes for solar hydrogen production

The development of sustainable hydrogen production is a key target in the further facilitation of a hydrogen economy. Solar hydrogen generation through the photolytic splitting of water sensitised by semiconductor materials is attractive as it is both renewable and does not lead to problematic by-products, unlike current hydrogen sources such as natural gas. Consequently, the development of these semiconductor materials has undergone considerable research since their discovery over 30 years ago and it would seem prescient to review the more practical results of this research. Among the critical factors influencing the choice of semiconductor material for photoelectrolysis of water are the band-gap energies, flat band potentials and stability towards photocorrosion; the latter of these points directs us to focus on metal oxides. Careful design of thin films of photocatalyst material can eliminate potential routes of losses in performance, i.e., recombination at grain boundaries. Methods to overcome these problems are discussed such as coupling a photoanode for photolysis of water to a photovoltaic cell in a “tandem cell” device.

Metal oxide photoanodes for water splitting.

Solar hydrogen production through photocatalytically assisted water splitting has attracted a great deal of attention since its first discovery almost 30 years ago. The publication of investigations into the use of TiO₂ photoanodes has continued apace since and a critical review of current trends is reported herein. Recent advances in the understanding of the behaviour of nanoparticulate TiO₂ films is summarized along with a balanced report into the utility and nature of titania films doped with non-metallic elements and ordered, nanostructured films such as those consisting of nanotubes. Both of these are areas that have generated a not insignificant degree of activity. One goal of doping TiO₂ has been to extend the photoresponse of the material to visible light. A similar goal has seen a resurgence in interest in Fe₂O₃ photoanodes. Herein, the influence of dopants on the photocurrent density observed at Fe₂O₃ photoanodes and, in this regard, the role of silicon has attracted much attention, and a little debate. Finally, we look beyond the binary oxides. Photoanodes made from new materials such as mixed metal oxides, perovskite structured semiconductors, metal (oxy)nitrides or composite electrodes offer the potential to either tailor the optical band gap or tune the conduction or valence band energetics. Recent work in this area is detailed here.

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.

Nanostructured thin-film tungsten trioxide photoanodes for solar water and sea-water splitting

About 3 μm thick tungsten trioxide film electrodes consisting of partly sintered, 40-80 nm in diameter, particles deposited on conducting glass substrates exhibit high photon-to-current conversion efficiencies for the photooxidation of water, exceeding 70% at 400 nm. This is facilitated by a ca. 40% film porosity resulting in high contact area with the electrolyte. It is shown that the activity of the WO3 electrodes towards photooxidation of water is enhanced by addition of even small amounts of halide (Cl-, Br-) ions to the acidic electrolyte. Photoelectrolysis experiments performed either in acidic electrolytes containing chloride or bromide anions or in a 0.5 M NaCl solution, under simulated 1.5 AM solar illumination, demonstrated long term stability of the photocurrents. Oxygen remains the main product of the photoanodic reaction even in a 0.5 M NaCl solution, a composition close to the sea water, with chlorine accounting for ca. 20% of current efficiency.

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.

To what extent do the nanostructured photoelectrodes perform better than their macrocrystalline counterparts

A look at recent literature dealing with photoelectrochemical (PEC) water splitting might suggest using nanostructured semiconductors to build the photoelectrodes as being a kind of panacea to enhance their energy conversion efficiency. Taking example of the most frequently investigated photoanode materials, we show the actual situation as being largely contrasted going from a substantial improvement to a spectacular drop of the PEC performance.