Unassisted Water Splitting Research Articles

Si/TiO2 Tandem-Junction Microwire Arrays for Unassisted Solar-Driven Water Splitting

Tandem-junction microwire array photoelectrodes have been fabricated by coating np+-Si radial homojunction microwire arrays sequentially with fluorine-doped tin oxide (FTO) and titanium dioxide (TiO2). These photoelectrodes effected unassisted water splitting under simulated 1 Sun conditions with an open-circuit potential (Eoc) of −1.5 V vs the formal potential for oxygen evolution, E0′(OH−/O2), a current density at E = E0′(OH−/O2) of 0.78 mA cm−2, a fill factor ( ff ) = 0.51, and a photovoltaic-biased photoelectrochemical ideal regenerative cell efficiency of 0.6%.

An Optically Transparent Iron Nickel Oxide Catalyst for Solar Water Splitting.

Sunlight-driven water splitting to produce hydrogen fuel is an attractive method for renewable energy conversion. Tandem photoelectrochemical water splitting devices utilize two photoabsorbers to harvest the sunlight and drive the water splitting reaction. The absorption of sunlight by electrocatalysts is a severe problem for tandem water splitting devices where light needs to be transmitted through the larger bandgap component to illuminate the smaller bandgap component. Herein, we describe a novel method for the deposition of an optically transparent amorphous iron nickel oxide oxygen evolution electrocatalyst. The catalyst was deposited on both thin film and high-aspect ratio nanostructured hematite photoanodes. The low catalyst loading combined with its high activity at low overpotential results in significant improvement on the onset potential for photoelectrochemical water oxidation. This transparent catalyst further enables the preparation of a stable hematite/perovskite solar cell tandem device, which performs unassisted water splitting.

Water Splitting Semiconductor Photoanodes - a Comparative Study

The long term stability is the critical issue that conditions application of semiconductor materials in photo-electrochemical water splitting devices. This is the reason why the choice of the semiconductors intended to operate as water oxidation photo-anodes is actually restricted to a few metal oxides. However, none among those materials, able to absorb a significant portion of solar light, exhibits band-edge energy levels that match those of hydrogen and oxygen evolution reactions. Neither tungsten trioxide, nor bismuth vanadate and still less hematite ferric oxide have the positions of the conduction band edges more negative than the H2 evolution potential, a condition necessary to perform unassisted water splitting. Consequently, continuing efforts are devoted to minimize the bias voltage required to perform visible light-driven photo-oxidation of water. These efforts include, among others, development of new electrocatalysts and incorporation into photo-anodes of plasmonic metal nanostructures that allow incoupling of light into the semiconductor films, by scattering from plasmonic nanostructures and/or resonant coupling of the plasmonic electromagnetic near field to the semiconductor. Recent advances regarding the most investigated photoanode materials: WO3 (Eg = 2.5 eV); BiVO4 (Eg = 2.4 eV) and hematite Fe2O3 (Eg = 2.2 eV) will be discussed.

Perovskite-Hematite Tandem Cells for Efficient Overall Solar Driven Water Splitting.

Photoelectrochemical water splitting half reactions on semiconducting photoelectrodes have received much attention but efficient overall water splitting driven by a single photoelectrode has remained elusive due to stringent electronic and thermodynamic property requirements. Utilizing a tandem configuration wherein the total photovoltage is generated by complementary optical absorption across different semiconducting electrodes is a possible pathway to unassisted overall light-induced water splitting. Because of the low photovoltages generated by conventional photovoltaic materials (e.g., Si, CIGS), such systems typically consist of triple junction design that increases the complexity due to optoelectrical trade-offs and are also not cost-effective. Here, we show that a single solution processed organic-inorganic halide perovskite (CH3NH3PbI3) solar cell in tandem with a Fe2O3 photoanode can achieve overall unassisted water splitting with a solar-to-hydrogen conversion efficiency of 2.4%. Systematic electro-optical studies were performed to investigate the performance of tandem device. It was found that the overall efficiency was limited by the hematite's photocurrent and onset potential. To understand these limitations, we have estimated the intrinsic solar to chemical conversion efficiency of the doped and undoped Fe2O3 photoanodes. The total photopotential generated by our tandem system (1.87 V) exceeds both the thermodynamic and kinetic requirements (1.6 V), resulting in overall water splitting without the assistance of an electrical bias.

Modeling, simulation, and fabrication of a fully integrated, acid-stable, scalable solar-driven water-splitting system.

A fully integrated solar-driven water-splitting system comprised of WO3 /FTO/p(+) n Si as the photoanode, Pt/TiO2 /Ti/n(+) p Si as the photocathode, and Nafion as the membrane separator, was simulated, assembled, operated in 1.0 M HClO4 , and evaluated for performance and safety characteristics under dual side illumination. A multi-physics model that accounted for the performance of the photoabsorbers and electrocatalysts, ion transport in the solution electrolyte, and gaseous product crossover was first used to define the optimal geometric design space for the system. The photoelectrodes and the membrane separators were then interconnected in a louvered design system configuration, for which the light-absorbing area and the solution-transport pathways were simultaneously optimized. The performance of the photocathode and the photoanode were separately evaluated in a traditional three-electrode photoelectrochemical cell configuration. The photocathode and photoanode were then assembled back-to-back in a tandem configuration to provide sufficient photovoltage to sustain solar-driven unassisted water-splitting. The current-voltage characteristics of the photoelectrodes showed that the low photocurrent density of the photoanode limited the overall solar-to-hydrogen (STH) conversion efficiency due to the large band gap of WO3 . A hydrogen-production rate of 0.17 mL hr(-1) and a STH conversion efficiency of 0.24 % was observed in a full cell configuration for >20 h with minimal product crossover in the fully operational, intrinsically safe, solar-driven water-splitting system. The solar-to-hydrogen conversion efficiency, ηSTH , calculated using the multiphysics numerical simulation was in excellent agreement with the experimental behavior of the system. The value of ηSTH was entirely limited by the performance of the photoelectrochemical assemblies employed in this study. The louvered design provides a robust platform for implementation of various types of photoelectrochemical assemblies, and can provide an approach to significantly higher solar conversion efficiencies as new and improved materials become available.

A monolithic device for solar water splitting based on series interconnected thin film absorbers reaching over 10% solar-to-hydrogen efficiency

Efficient production of hydrogen from solar energy is anticipated to be an important component in a future sustainable post-carbon energy system. Here we demonstrate that series interconnected absorbers in a PV-electrolysis configuration based on the compound semiconductor CIGS, CuInxGa1−xSe2, are a highly interesting concept for solar water splitting applications. The band gap energy of CIGS can be adjusted to a value close to optimum for efficient absorption of the solar spectrum, but is too low to drive overall water splitting. Therefore we connect three cells in series, into a monolithic device, which provides sufficient driving force for the full reaction. Integrated with a catalyst this forms a stable PV/photo-electrochemical device, which when immersed in water reaches over 10% solar-to-hydrogen efficiency for unassisted water splitting. The results show that series interconnected device concepts, which enable use of a substantial part of the solar spectrum, provide a simple route towards highly efficient water splitting and could be used also for other solar absorbers with similar electro-optical properties. We discuss how the efficiency could be increased for this particular device, as well as the general applicability of the concepts used in this work. We also briefly discuss advantages and disadvantages of photo-electrochemical cells in relation to PV-electrolysis with respect to our results.

Photoelectrochemical cell/dye-sensitized solar cell tandem water splitting systems with transparent and vertically aligned quantum dot sensitized TiO2 nanorod arrays

The present work reports fabrication of vertically aligned CdS sensitized TiO2 nanorod arrays grown on transparent conducting oxide substrate with high transparency as a photoanode in photoelectrochemical cell for water splitting. To realize an unassisted water splitting system, the photoanode and dye-sensitized solar cell tandem structures are tried and their electrochemical behaviors are also investigated. The hydrothermally grown TiO2 nanorod arrays followed by CdS nanoparticle decoration can improve the light absorption of long wavelength light resulting in increased photocurrent density. Two different techniques (electrodeposition and spray pyrolysis deposition) of CdS nanoparticle sensitization are carried out and their water splitting behaviors in the tandem cell are compared.

Synthesis of transparent mesoporous tungsten trioxide films with enhanced photoelectrochemical response: application to unassisted solar water splitting

Tungsten trioxide (WO3) films with a mesoporous morphology, high transparency, and monoclinic phase crystallinity were prepared using polyethyleneglycol (PEG) as a surfactant and their photoelectrochemical properties were measured. By controlling the weight ratio of the tungsten precursor to PEG, a sphere-like WO3 nanoparticle film with high transparency can be synthesized. The photocurrent responses of the films under 1 sun solar light illumination were measured. Due to the high transparency of the WO3 photoanode, it is possible to fabricate a tandem cell composed of a WO3/Pt bipolar electrode connected with a dye-sensitized solar cell. Unassisted water splitting from the tandem cell was demonstrated but the maximum current density was exhibited at around +0.4 V (vs.Pt).

Photoelectrolysis of water using thin copper gallium diselenide electrodes

Thin CuGaSe 2 films were deposited by vacuum co-evaporation and characterized for their structure, properties and performance as hydrogen-evolving photoelectrodes. The 0.9 μm thick films were nearly stoichiometric with very slight copper deficiency and showed polycrystalline structure with grain sizes of tens of nanometers. An electrode based on such a film was demonstrated operating with outdoor 1-sun photocurrent of up to 13 mA/cm 2. Spectral response data show significant incident-photon-to-current efficiency throughout the visible spectrum, peaking at 63% at 640 nm. Photocurrent output under simulated 1-sun Air Mass 1.5 light was stable over 4 h. Unassisted water-splitting is not possible due to high band edge positions, but operation in tandem configuration with a suitable bottom junction is feasible.