Photocatalytic solar hydrogen production from water on a 100-m2 scale.

The unprecedented impact of human activity on Earth's climate and the ongoing increase in global energy demand have made the development of carbon-neutral energy sources ever more important. Hydrogen is an attractive and versatile energy carrier (and important and widely used chemical) obtainable from water through photocatalysis using sunlight, and through electrolysis driven by solar or wind energy1,2. The most efficient solar hydrogen production schemes, which couple solar cells to electrolysis systems, reach solar-to-hydrogen (STH) energy conversion efficiencies of 30% at a laboratory scale3. Photocatalytic water splitting reaches notably lower conversion efficiencies of only around 1%, but the system design is much simpler and cheaper and more amenable to scale-up1,2-provided the moist, stoichiometric hydrogen and oxygen product mixture can be handled safely in a field environment and the hydrogen recovered. Extending our earlier demonstration of a 1-m2 panel reactor system based on a modified, aluminium-doped strontium titanate particulate photocatalyst4, we here report safe operation of a 100-m2 array of panel reactors over several months with autonomous recovery of hydrogen from the moist gas product mixture using a commercial polyimide membrane5. The system, optimized for safety and durability, and remaining undamaged on intentional ignition of recovered hydrogen, reaches a maximum STH of 0.76%. While the hydrogen production is inefficient and energy negative overall, our findings demonstrate that safe, large-scale photocatalytic water splitting, and gas collection and separation are possible. To make the technology economically viable and practically useful, essential next steps are reactor and process optimization to substantially reduce costs and improve STH efficiency, photocatalyst stability and gas separation efficiency.

Surface Modifications of (ZnSe)0.5(CuGa2.5Se4.25)0.5 to Promote Photocatalytic Z-Scheme Overall Water Splitting.

Charge separation is crucial for an efficient artificial photosynthetic process, especially for narrow-bandgap metal sulfides/selenides. The present study demonstrates the application of a p-n junction to particulate metal selenides to enhance photocatalytic Z-scheme overall water splitting (OWS). The constructed p-n junction of CdS-(ZnSe)0.5(CuGa2.5Se4.25)0.5 significantly boosted charge separation. A thin TiO2 coating layer also was introduced to inhibit photocorrosion of CdS and suppress the backward reaction of water formation from hydrogen and oxygen. By employing Pt-loaded TiO2/CdS-(ZnSe)0.5(CuGa2.5Se4.25)0.5 as a hydrogen evolution photocatalyst (HEP), we assembled a Z-scheme OWS system, together with BiVO4:Mo and Au as an oxygen evolution photocatalyst and electron mediator, respectively. An apparent quantum yield of 1.5% at 420 nm was achieved, which is by far the highest among reported particulate photocatalytic Z-scheme OWS systems with metal sulfides/selenides as HEPs. The present work demonstrates that a well-tailored p-n junction structure is effective for promoting charge separation in photocatalysis and opens new pathways for the development of efficient artificial photosynthesis systems involving narrow bandgap photocatalysts.

Neutron diffraction experiment with the Y116S variant of transthyretin using iBIX at J-PARC: application of a new integration method.

Transthyretin (TTR) is one of more than 30 amyloidogenic proteins, and the amyloid fibrils found in patients afflicted with ATTR amyloidosis are composed of this protein. Wild-type TTR amyloids accumulate in the heart in senile systemic amyloidosis (SSA). ATTR amyloidosis occurs at a much younger age than SSA, and the affected individuals carry a TTR mutant. The naturally occurring amyloidogenic Y116S TTR variant forms more amyloid fibrils than wild-type TTR. Thus, the Y116S mutation reduces the stability of the TTR structure. A neutron diffraction experiment on Y116S TTR was performed to elucidate the mechanism of the changes in structural stability between Y116S variant and wild-type TTR through structural comparison. Large crystals of the Y116S variant were grown under optimal crystallization conditions, and a single 2.4 mm3 crystal was ultimately obtained. This crystal was subjected to time-of-flight (TOF) neutron diffraction using the IBARAKI biological crystal diffractometer (iBIX) at the Japan Proton Accelerator Research Complex, Tokai, Japan (J-PARC). A full data set for neutron structure analysis was obtained in 14 days at an operational accelerator power of 500 kW. A new integration method was developed and showed improved data statistics; the new method was applied to the reduction of the TOF diffraction data from the Y116S variant. Data reduction was completed and the integrated intensities of the Bragg reflections were obtained at 1.9 Šresolution for structure refinement. Moreover, X-ray diffraction data at 1.4 Šresolution were obtained for joint neutron-X-ray refinement.

Z-Scheme Water Splitting under Near-Ambient Pressure using a Zirconium Oxide Coating on Printable Photocatalyst Sheets.

Sunlight-driven water splitting systems operating under ambient pressure are essential for practical renewable hydrogen production. Printable photocatalyst sheets, composed of a hydrogen evolution photocatalyst (HEP), an oxygen evolution photocatalyst (OEP), and conductive metal nanoparticles, are cost-effective and scalable systems. However, the decrease in water splitting activity under ambient pressure due to reverse reactions hampers their practical application. In this study, coating zirconium oxide (ZrOx ) by facile drop-casting onto a printed photocatalyst sheet, which consists of SrTiO3 : Rh, BiVO4 : Mo, and Au nanocolloids as the HEP, OEP, and electron mediator, respectively, effectively maintains the water splitting activity at elevated pressure. The ZrOx -coated photocatalyst sheet retains 90 % and 84 % of the base performance (the pristine sheet at 10 kPa) at 50 and 90 kPa, respectively. Achieving efficient water splitting at the ambient pressure by inexpensive and extensible processes is an important step toward solar hydrogen production.

Neutron crystallography of copper amine oxidase reveals keto/enolate interconversion of the quinone cofactor and unusual proton sharing.

Recent advances in neutron crystallographic studies have provided structural bases for quantum behaviors of protons observed in enzymatic reactions. Thus, we resolved the neutron crystal structure of a bacterial copper (Cu) amine oxidase (CAO), which contains a prosthetic Cu ion and a protein-derived redox cofactor, topa quinone (TPQ). We solved hitherto unknown structures of the active site, including a keto/enolate equilibrium of the cofactor with a nonplanar quinone ring, unusual proton sharing between the cofactor and the catalytic base, and metal-induced deprotonation of a histidine residue that coordinates to the Cu. Our findings show a refined active-site structure that gives detailed information on the protonation state of dissociable groups, such as the quinone cofactor, which are critical for catalytic reactions.

Efficient Water Oxidation Using Ta3 N5 Thin Film Photoelectrodes Prepared on Insulating Transparent Substrates.

Photoelectrochemical (PEC) water splitting using visible-light-responsive photoelectrodes is the preferred approach to converting solar energy into hydrogen as a renewable energy source. A transparent Ta3 N5 photoanode embedded within a PEC cell having a tandem configuration is a promising configuration that may provide a high solar-to-hydrogen energy conversion efficiency. Ta3 N5 thin films are typically prepared by heating precursor films in an NH3 flow at high temperatures, which tends to degrade the transparent conductive layer, such that producing efficient Ta3 N5 transparent photoanodes is challenging. Herein, the direct preparation of transparent Ta3 N5 photoanodes on insulating quartz substrates was demonstrated without the insertion of a transparent conductive layer. The resulting devices generated a photocurrent of 6.0 mA cm-2 at 1.23 V vs. a reversible hydrogen electrode under simulated sunlight. This study provides a new strategy for the preparation of transparent photoelectrodes that mitigates current challenges.

Current status and near future plan of neutron protein crystallography at J-PARC.

The IBARAKI Biological Crystal Diffractometer (iBIX) has been available for use at MLF (Material and Life Science Facility) in J-PARC (Japan Proton Accelerator Research Complex) since 2008. The development in state-of-the-art detector systems could enable iBIX to become one of the highest-performance neutron single-crystal diffractometers in the world. Here, together with other various developments, such as data reduction software, crystal growth, and new techniques in measurement coupled analysis, we provided new hydrogen and water structural data of several proteins and macromolecules. Although the proton power at MLF has not yet reached its planned maximum (1MW), a more powerful neutron source will be soon needed for neutron protein crystallography. A future idea is also proposed and discussed in this article.

Efficient photocatalytic hydrogen evolution on single-crystalline metal selenide particles with suitable cocatalysts.

It is important to improve the apparent quantum yields (AQYs) of narrow bandgap photocatalysts to achieve efficient H2 production. The present work demonstrates a particulate solid solution of zinc selenide and copper gallium selenide (denoted as ZnSe:CGSe) that evolves H2 efficiently and is responsive to visible light up to 725 nm. This material was synthesized using a flux-assisted method and was found to comprise single-crystalline tetrahedral particles. The coloading of Ni and Rh, Pt, Pd or Ru as cocatalysts further improved the photocatalytic H2 evolution rate over this photocatalyst. With the optimal coloading of a Ni-Ru composite cocatalyst, an AQY of 13.7% was obtained at 420 nm during a sacrificial H2 evolution reaction, representing the highest value yet reported for a photocatalyst with an absorption edge longer than 700 nm. The present study demonstrates that the preparation of single-crystalline particles and the rational assembly of composite cocatalysts are effective strategies that allow the efficient utilization of long wavelengths by metal selenide photocatalysts for solar fuel production.

Self-activated Rh-Zr mixed oxide as a nonhazardous cocatalyst for photocatalytic hydrogen evolution.

Efficient, robust and environmentally friendly cocatalysts for photocatalysts are important for large-scale solar hydrogen production. Herein, we demonstrate that a Rh-Zr mixed oxide is an efficient cocatalyst for hydrogen evolution. Impregnation of Zr and Rh precursors (Zr/Rh = 5 wt/wt%) formed RhZrO x cocatalyst particles on Al-doped SrTiO3, which exhibited 31× higher photocatalytic water-splitting activity than a RhO x cocatalyst. X-ray photoelectron spectroscopy proved that the dissociation of Cl- ions from preformed Rh-Cl-Zr-O solid led to formation of the active phase of RhZrO x , in which the Zr/Rh ratio was critical to high catalytic activity. Additional CoO x loading as an oxygen evolution cocatalyst further improved the activity by 120%, resulting in an apparent quantum yield of 33 (±4)% at 365 nm and a long durability of 60 h. Our discovery could help scale up photocatalytic hydrogen production.

Evaluation of permeability applicability based on continuum mechanics law in fluid flow through graphene membrane.

Graphene is expected to be used in separation applications such as desalination. However, it is difficult to predict the flow phenomena at the nanoscale using the conventional continuum law. Particularly at a Knudsen number (Kn) of >0.1, which is applied in filtration, it has been reported that not even slip boundary conditions can be applied. In this study, to identify the parameters that affect the applicability of the continuum law, we conducted a fluid permeation simulation using graphene. The deviation of the permeability from that of the continuum model was calculated by changing the channel width, fluid temperature, and fluid type. The result showed that the channel width has the largest influence among the three factors, and that the magnitude of the divergence is sorted out based on the Knudsen number. Therefore, the permeability can be predicted even at the nanoscale where the continuum law cannot be applied.

Oxysulfide photocatalyst for visible-light-driven overall water splitting.

Oxysulfide semiconductors have narrow bandgaps suitable for water splitting under visible-light irradiation, because the electronegative sulfide ions negatively shift the valence band edges of the corresponding oxides1,2. However, the instability of sulfide ions during the water oxidation is a critical obstacle to simultaneous evolution of hydrogen and oxygen3. Here, we demonstrate the activation and stabilization of Y2Ti2O5S2, with a bandgap of 1.9 eV, as a photocatalyst for overall water splitting. On loading of IrO2 and Rh/Cr2O3 as oxygen and hydrogen evolution co-catalysts, respectively, and fine-tuning of the reaction conditions, simultaneous production of stoichiometric amounts of hydrogen and oxygen was achieved on Y2Ti2O5S2 during a 20 h reaction. The discovery of the overall water splitting capabilities of Y2Ti2O5S2 extends the range of promising materials for solar hydrogen production.

An Al-doped SrTiO3 photocatalyst maintaining sunlight-driven overall water splitting activity for over 1000 h of constant illumination.

Photocatalytic water splitting is a viable approach to the large-scale production of renewable solar hydrogen. The apparent quantum yield for this reaction has been improved, but the lifespan of photocatalysts functioning under sunlight at ambient pressure have rarely been examined, despite the critical importance of this factor in practical applications. Herein, we show that Al-doped SrTiO3 (SrTiO3:Al) loaded with a RhCrO x (rhodium chromium oxide) cocatalyst splits water with an apparent quantum yield greater than 50% at 365 nm. Moreover, following the photodeposition of CoOOH and TiO2, this material maintains 80% of its initial activity and a solar-to-hydrogen energy conversion efficiency greater than or equal to 0.3% over a span of 1300 h under constant illumination by simulated sunlight at ambient pressure. This result is attributed to reduced dissolution of Cr in the cocatalyst following the oxidative photodeposition of CoOOH. The photodeposition of TiO2 further improves the durability of this photocatalyst. This work demonstrates a concept that could allow the design of long-term, large-scale photocatalyst systems for practical sunlight-driven water splitting.

Phase segregated Cu2-x Se/Ni3Se4 bimetallic selenide nanocrystals formed through the cation exchange reaction for active water oxidation precatalysts.

Control over the composition and nanostructure of solid electrocatalysts is quite important for drastic improvement of their performance. The cation exchange reaction of nanocrystals (NCs) has been reported as the way to provide metastable crystal structures and complicated functional nanostructures that are not accessible by conventional synthetic methods. Herein we demonstrate the cation exchange-derived formation of metastable spinel Ni3Se4 NCs (sp-Ni3Se4) and phase segregated berzelianite Cu2-x Se (ber-Cu2-x Se)/sp-Ni3Se4 heterostructured NCs as active oxygen evolution reaction (OER) catalysts. A rare sp-Ni3Se4 phase was formed by cation exchange of ber-Cu2-x Se NCs with Ni2+ ions, because both phases have the face-centered cubic (fcc) Se anion sublattice. Tuning the Ni : Cu molar ratio leads to the formation of Janus-type ber-Cu2-x Se/sp-Ni3Se4 heterostructured NCs. The NCs of sp-Ni3Se4 and ber-Cu2-x Se/sp-Ni3Se4 heterostructures exhibited high catalytic activities in the OER with small overpotentials of 250 and 230 mV at 10 mA cm-2 in 0.1 M KOH, respectively. They were electrochemically oxidized during the OER to give hydroxides as the real active species. We anticipate that the cation exchange reaction could have enormous potential for the creation of novel heterostructured NCs showing superior catalytic performance.

Transparent Ta3 N5 Photoanodes for Efficient Oxygen Evolution toward the Development of Tandem Cells.

Photoelectrochemical water splitting is regarded as a promising approach to the production of hydrogen, and the development of efficient photoelectrodes is one aspect of realizing practical systems. In this work, transparent Ta3 N5 photoanodes were fabricated on n-type GaN/sapphire substrates to promote O2 evolution in tandem with a photocathode, to realize overall water splitting. Following the incorporation of an underlying GaN layer, a photocurrent of 6.3 mA cm-2 was achieved at 1.23 V vs. a reversible hydrogen electrode. The transparency of Ta3 N5 to wavelengths longer than 600 nm allowed incoming solar light to be transmitted to a CuInSe2 (CIS), which absorbs up to 1100 nm. A stand-alone tandem cell with a serially-connected dual-CIS unit terminated with a Pt/Ni electrode was thus constructed for H2 evolution. This tandem cell exhibited a solar-to-hydrogen energy conversion efficiency greater than 7 % at the initial stage of the reaction.

Efficient hydrogen evolution on (CuInS2)x(ZnS)1-x solid solution-based photocathodes under simulated sunlight.

A thin film of (CuInS2)x(ZnS)1-x has been developed as a photocathode for solar water splitting for the first time. A superior photoelectrochemical performance has been achieved, mainly attributed to the formation of a solid solution by CuInS2 and ZnS, which proved to be an effective strategy to improve the onset potential and efficiency of CuInS2 photocathodes.

Status of the neutron time-of-flight single-crystal diffraction data-processing software STARGazer.

The STARGazer data-processing software is used for neutron time-of-flight (TOF) single-crystal diffraction data collected using the IBARAKI Biological Crystal Diffractometer (iBIX) at the Japan Proton Accelerator Research Complex (J-PARC). This software creates hkl intensity data from three-dimensional (x, y, TOF) diffraction data. STARGazer is composed of a data-processing component and a data-visualization component. The former is used to calculate the hkl intensity data. The latter displays the three-dimensional diffraction data with searched or predicted peak positions and is used to determine and confirm integration regions. STARGazer has been developed to make it easier to use and to obtain more accurate intensity data. For example, a profile-fitting method for peak integration was developed and the data statistics were improved. STARGazer and its manual, containing installation and data-processing components, have been prepared and provided to iBIX users. This article describes the status of the STARGazer data-processing software and its data-processing algorithms.

Phase-segregated NiP x @FeP y O z core@shell nanoparticles: ready-to-use nanocatalysts for electro- and photo-catalytic water oxidation through in situ activation by structural transformation and spontaneous ligand removal.

The high overpotential of the oxygen evolution reaction is a critical issue to be overcome to realize efficient overall water splitting and enable hydrogen generation powered by sunlight. Homogeneous and stable nanoparticles (NPs) dispersed in solvents are useful as both electrocatalysts and cocatalysts of photocatalysts for the electro- and photo-catalytic oxygen evolution reaction, respectively, through their adsorption on various electrode substrates. Here, phase-segregated NiP x @FeP y O z core@shell NPs are selectively synthesized by the reaction of Fe(CO)5 with amorphous NiP x seed-NPs. The NiP x @FeP y O z NPs on conductive substrates exhibit higher electrocatalytic activity in the oxygen evolution reaction than those of other metal phosphide-based catalysts. The NiP x @FeP y O z NPs can also be used as a cocatalyst of an anodic BiVO4 photocatalyst to boost the photocatalytic water oxidation reaction. The excellent catalytic activity and high stability of the NiP x @FeP y O z NPs without any post-treatments are derived from in situ activation through both the structural transformation of NiP x @FeP y O z into mixed hydroxide species, (Ni, Fe)O x H y , and the spontaneous removal of the insulating organic ligands from NPs to form a smooth and robust (Ni, Fe)O x H y /substrate heterointerface during the oxygen evolution reaction.

Effects of slit width on water permeation through graphene membrane by molecular dynamics simulations.

Graphene membranes can be used for nanoscale filtration to remove atoms and are expected to be used for separation. To realize high-permeability and high-filtration performance, we must understand the flow configuration in the nanochannels. In this study, we investigated the applicability of continuum-dynamics laws to water flow through a graphene slit. We calculated the permeability of the flow through a slit using classical molecular dynamics (MD) and compared the MD simulation results for different Knudsen numbers (Kn) to predictions based on the no-slip model and slip model. Consequently, the flow through the graphene nanoslit was treated as slip flow only in the range of Kn < 0.375. This study provides guidelines for the development of graphene filtration membranes.

Visible-Light-Responsive Photoanodes for Highly Active, Stable Water Oxidation.

Solar energy is a natural and effectively permanent resource and so the conversion of solar radiation into chemical or electrical energy is an attractive, although challenging, prospect. Photo-electrochemical (PEC) water splitting is a key aspect of producing hydrogen from solar power. However, practical water oxidation over photoanodes (in combination with water reduction at a photocathode) in PEC cells is currently difficult to achieve because of the large overpotentials in the reaction kinetics and the inefficient photoactivity of the semiconductors. The development of semiconductors that allow high solar-to-hydrogen conversion efficiencies and the utilization of these materials in photoanodes will be a necessary aspect of achieving efficient, stable water oxidation. This Review discusses advances in water oxidation activity over photoanodes of n-type visible-light-responsive (oxy)nitrides and oxides.

Overall water splitting by photoelectrochemical cells consisting of (ZnSe)0.85(CuIn0.7Ga0.3Se2)0.15 photocathodes and BiVO4 photoanodes.

The design of photoelectrochemical (PEC) cell structures that mitigate challenges related to mass transfer is the key to achieving efficient hydrogen production. A PEC cell based on an integrated photoelectrode incorporating (ZnSe)0.85(CuIn0.7Ga0.3Se2)0.15 strips and BiVO4 strips exhibited 1.0% solar-to-hydrogen energy conversion efficiency even without stirring the electrolyte solution.