Enhanced Photoelectrochemical Water Oxidation from CdTe Photoanodes Annealed with CdCl2.

Most CdTe photoanodes and photocathodes show positive and negative photocurrent onset potentials for water oxidation and reduction, respectively, and are thus unable to drive photoelectrochemical (PEC) water splitting without external applied biases. Herein, the activity of a CdTe photoanode having an internal p-n junction during PEC water oxidation was enhanced by applying a CdCl2 annealing treatment together with surface modifications. The resulting CdTe photoanode generated photocurrents of 1.8 and 5.4 mA cm-2 at 0.6 and 1.2 VRHE , respectively, with a photoanodic current onset potential of 0.22 VRHE under simulated sunlight (AM 1.5G). The CdCl2 annealing increased the grain sizes and lowered the density of grain boundaries, allowing more efficient charge separation. Consequently, a two-electrode tandem PEC cell comprising a CdTe-based photoanode and photocathode split water without any external bias at a solar-to-hydrogen conversion efficiency of 0.51 % at the beginning of the reaction.

Suppression of poisoning of photocathode catalysts in photoelectrochemical cells for highly stable sunlight-driven overall water splitting.

A photoelectrochemical (PEC) cell composed of two semiconductor electrodes, a photocathode, and a photoanode is a potentially effective means of obtaining hydrogen through spontaneous overall water splitting under light irradiation. However, the long-term stability (that is, operation for more than one day) of a PEC cell has not yet been demonstrated. In addition to the corrosion of both photoelectrodes, the gradual migration of heavy metal cations from the photoanode into the electrolyte can also result in degradation of the cell by contamination of the photocathode surface. In the present work, BiVO4-based photoanodes were used in conjunction with two different modifications: dispersion of a chelating resin in the electrolyte and coating of the photoanode surface with an anion-conducting ionomer. The chelating resin was found to capture Bi3+ cations in the electrolyte before they became deposited on the cathode surface. Consequently, a PEC cell incorporating a BiVO4-based photoanode and a (ZnSe)0.85(CuIn0.7Ga0.3Se2)0.15-based photocathode showed stable overall water splitting over a span of two days under simulated sunlight. To the best of our knowledge, this represents the longest period over which stable PEC cell performance has been established. A considerable decrease in the performance of the BiVO4-based photoanode was still observed due to the continuous dissolution of Bi species, but surface coating of the photoanode with an anion-conducting ionomer prevented the movement of Bi3+ ions into the electrolyte because of the selective conduction of ions. The coating also served as a protective layer that improved the durability of the photoanode. This study therefore suggests a simple yet effective method for the construction of stable PEC cells using semiconductor photoelectrodes.

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.

Recent Progress in the Surface Modification of Photoelectrodes toward Efficient and Stable Overall Water Splitting.

Photoelectrochemical (PEC) water splitting using a combination of a photocathode and photoanode is one of the most promising methods of producing hydrogen from water employing sunlight. Recent reports have shown that surface modification of the photoelectrodes dramatically improves their PEC performance. Bare photoelectrodes often exhibit insufficient depletion regions, undesired surface states and/or degradation due to photocorrosion. It has been demonstrated that surface modifications can tune the flat-band potentials, band-edge potentials, surface states and chemical stabilities of these electrodes and thus improve quantum efficiency, onset potential and durability. This review describes in detail the various surface modification materials that have been developed to date, and the functions of these modifiers. This information is expected to provide guidelines for the future development of photoelectrodes capable of highly efficient and stable PEC water splitting.

La5 Ti2 Cu0.9 Ag0.1 S5 O7 Modified with a Molecular Ni Catalyst for Photoelectrochemical H2 Generation.

The stable and efficient integration of molecular catalysts into p-type semiconductor materials is a contemporary challenge in photoelectrochemical fuel synthesis. Here, we report the combination of a phosphonated molecular Ni catalyst with a TiO2 -coated La5 Ti2 Cu0.9 Ag0.1 S5 O7 photocathode for visible light driven H2 production. This hybrid assembly provides a positive onset potential, large photocurrents, and high Faradaic yield for more than three hours. A decisive feature of the hybrid electrode is the TiO2 interlayer, which stabilizes the oxysulfide semiconductor and allows for robust attachment of the phosphonated molecular catalyst. This demonstration of an oxysulfide-molecular catalyst photocathode provides a novel platform for integrating molecular catalysts into photocathodes and the large photovoltage of the presented system makes it ideal for pairing with photoanodes.

Particulate Photocatalyst Sheets Based on Carbon Conductor Layer for Efficient Z-Scheme Pure-Water Splitting at Ambient Pressure.

Development of sunlight-driven water splitting systems with high efficiency, scalability, and cost-competitiveness is a central issue for mass production of solar hydrogen as a renewable and storable energy carrier. Photocatalyst sheets comprising a particulate hydrogen evolution photocatalyst (HEP) and an oxygen evolution photocatalyst (OEP) embedded in a conductive thin film can realize efficient and scalable solar hydrogen production using Z-scheme water splitting. However, the use of expensive precious metal thin films that also promote reverse reactions is a major obstacle to developing a cost-effective process at ambient pressure. In this study, we present a standalone particulate photocatalyst sheet based on an earth-abundant, relatively inert, and conductive carbon film for efficient Z-scheme water splitting at ambient pressure. A SrTiO3:La,Rh/C/BiVO4:Mo sheet is shown to achieve unassisted pure-water (pH 6.8) splitting with a solar-to-hydrogen energy conversion efficiency (STH) of 1.2% at 331 K and 10 kPa, while retaining 80% of this efficiency at 91 kPa. The STH value of 1.0% is the highest among Z-scheme pure water splitting operating at ambient pressure. The working mechanism of the photocatalyst sheet is discussed on the basis of band diagram simulation. In addition, the photocatalyst sheet split pure water more efficiently than conventional powder suspension systems and photoelectrochemical parallel cells because H+ and OH- concentration overpotentials and an IR drop between the HEP and OEP were effectively suppressed. The proposed carbon-based photocatalyst sheet, which can be used at ambient pressure, is an important alternative to (photo)electrochemical systems for practical solar hydrogen production.

Particulate photocatalyst sheets for Z-scheme water splitting: advantages over powder suspension and photoelectrochemical systems and future challenges.

Water splitting using semiconductor photocatalysts has been attracting growing interest as a means of solar energy based conversion of water to hydrogen, a clean and renewable fuel. Z-scheme photocatalytic water splitting based on the two-step excitation of an oxygen evolution photocatalyst (OEP) and a hydrogen evolution photocatalyst (HEP) is a promising approach toward the utilisation of visible light. In particular, a photocatalyst sheet system consisting of HEP and OEP particles embedded in a conductive layer has been recently proposed as a new means of obtaining efficient and scalable redox mediator-free Z-scheme solar water splitting. In this paper, we discuss the advantages and disadvantages of the photocatalyst sheet approach compared to conventional photocatalyst powder suspension and photoelectrochemical systems through an examination of the water splitting activity of Z-scheme systems based on SrTiO3:La,Rh as the HEP and BiVO4:Mo as the OEP. This photocatalyst sheet was found to split pure water much more efficiently than the powder suspension and photoelectrochemical systems, because the underlying metal layer efficiently transfers electrons from the OEP to the HEP. The photocatalyst sheet also outperformed a photoelectrochemical parallel cell during pure water splitting. The effects of H+/OH- concentration overpotentials and of the IR drop are reduced in the case of the photocatalyst sheet compared to photoelectrochemical systems, because the HEP and OEP are situated in close proximity to one another. Therefore, the photocatalyst sheet design is well-suited to efficient large-scale applications. Nevertheless, it is also noted that the photocatalytic activity of these sheets drops markedly with increasing background pressure because of reverse reactions involving molecular oxygen under illumination as well as delays in gas bubble desorption. It is shown that appropriate surface modifications allow the photocatalyst sheet to maintain its water splitting activity at elevated pressure. Accordingly, we conclude that the photocatalyst sheet system is a viable option for the realisation of efficient solar fuel production.

Highly Efficient Water Oxidation Photoanode Made of Surface Modified LaTiO2 N Particles.

An improved variation of highly active/durable O2 -evolving LaTiO2 N powder-based photoelectrode has been fabricated by pre-cleaning the powder with mild polysulfonic acid and by homogeneous deposition of CoOx co-catalyst aided by microwave annealing. The treatment in aqueous solution of poly(4-styrene sulfonic acid) results in removal of surface LaTiO2 N layers, forming fine pores in the crystallites. The CoOx co-catalyst by microwave deposition in Co(NH3 )6 Cl3 /ethylene glycol homogeneously covers the particle surface. The LaTiO2 N powder is fabricated into particle-transferred electrodes on Ti thin film supported on solid substrate. The modified LaTiO2 N grains on the electrode serve as a highly active O2 -evolving photoanode achieving 8.9 mA cm-2 of the photocurrent density at 1.23 V versus reversible hydrogen electrode (RHE) in 0.1 m NaOH (pH 13) under solar-simulator irradiation Airmass 1.5 Global (AM 1.5G). The activity has been much improved, compared with conventional LaTiO2 N treated in mineral acid or with CoOx deposited by impregnation. The new electrode also exhibits better durability in fixed-potential chronoamperometric tests under AM 1.5G irradiation.

Crystal Structure, Electronic Structure, and Photocatalytic Activity of Oxysulfides: La2Ta2ZrS2O8, La2Ta2TiS2O8, and La2Nb2TiS2O8.

The novel oxysulfides La2Ta2ZrS2O8 (LTZSO), La2Ta2TiS2O8 (LTTSO), and La2Nb2TiS2O8 (LNTSO) were synthesized, and their crystal structures, electronic structures, and photocatalytic activities for water splitting under visible light were investigated. Density functional theory calculations showed that these compounds are direct-band-gap semiconductors. Close to the conduction band minimum, the main contribution to the band structure comes from the d orbitals of Zr or Ti ions, while the region near the valence band maximum is associated with the 3p orbitals of S ions. The absorption-edge wavelength was determined to be 540 nm for LTZSO and 700 nm for LTTSO and LNTSO. An analysis of the crystal structure using synchrotron X-ray diffraction revealed that these compounds contained antisite defects at transition metal ion sites, and these were considered to be the origin of the broad absorption at wavelengths longer than that corresponding to band-gap excitation. LTZSO was revealed to be active in the oxygen evolution reaction from aqueous solution containing a sacrificial electron acceptor under visible-light illumination. This result was supported by the band alignment and flat-band potential determined by photoelectron spectroscopy and Mott-Schottky plots.

Enhanced Hydrogen Evolution under Simulated Sunlight from Neutral Electrolytes on (ZnSe)0.85 (CuIn0.7 Ga0.3 Se2 )0.15 Photocathodes Prepared by a Bilayer Method.

A (ZnSe)0.85 (CuIn0.7 Ga0.3 Se2 )0.15 photocathode with a bilayer structure was fabricated and found to exhibit a photocurrent almost twice that of a photocathode with a monolayer structure during hydrogen evolution from water. The cathodic photocurrent reached maximum values of 12 and 4.9 mA cm-2 at 0 and 0.6 VRHE in a neutral phosphate buffer under simulated sunlight, while the half-cell solar-to-hydrogen conversion efficiency was 3.0 % at 0.6 VRHE , with a maximum value of 3.6 % at 0.45 VRHE . Cross-sectional mapping of the electron-beam-induced current established that the increased photocurrent can be attributed to improved uniformity at the solid-liquid junction in the bilayer sample, which results in enhanced carrier collection.

Pt/In2S3/CdS/Cu2ZnSnS4 Thin Film as an Efficient and Stable Photocathode for Water Reduction under Sunlight Radiation.

An electrodeposited Cu2ZnSnS4 (CZTS) compact thin film modified with an In2S3/CdS double layer and Pt deposits (Pt/In2S3/CdS/CZTS) was used as a photocathode for water splitting of hydrogen production under simulated sunlight (AM 1.5G) radiation. Compared to platinized electrodes based on a bare CZTS film (Pt/CZTS) and a CZTS film modified with a CdS single layer (Pt/CdS/CZTS), the Pt/In2S3/CdS/CZTS electrode exhibited a significantly high cathodic photocurrent. Moreover, the coverage of the In2S3 layer was found to be effective for stabilization against degradation induced by photocorrosion of the CdS layer. Bias-free water splitting with a power conversion efficiency of 0.28% was achieved by using a simple two-electrode cell consisting of the Pt/In2S3/CdS/CZTS photocathode and a BiVO4 photoanode.

Mg-Zr Cosubstituted Ta3N5 Photoanode for Lower-Onset-Potential Solar-Driven Photoelectrochemical Water Splitting.

In p/n photoelectrochemical (PEC) cell systems, a low onset potential for the photoanode, as well as a high photocurrent, are critical for efficient water splitting. Here, we report a Mg-Zr cosubstituted Ta3N5 (Ta3N5:Mg+Zr) photoanode, designed to provide a more negative onset potential for PEC water splitting. The anodic photocurrent onset on Ta3N5:Mg+Zr was 0.55 V(RHE) under AM 1.5G-simulated sunlight, which represented a negative shift from the ca. 0.8 V(RHE) for pure Ta3N5. This negative shift in the onset potential of PEC water splitting was attributed to the change in the bandgap potential due to partial substitution by the foreign ions Mg(2+) and/or Zr(4+).

Photoelectrochemical oxidation of water using BaTaO2N photoanodes prepared by particle transfer method.

A photoanode of particulate BaTaO2N fabricated by the particle transfer method and modified with a Co cocatalyst generated a photocurrent of 4.2 mA cm(-2) at 1.2 V(RHE) in the photoelectrochemical water oxidation reaction under simulated sunlight (AM1.5G). The half-cell solar-to-hydrogen conversion efficiency (HC-STH) of the photoanode reached 0.7% at 1.0 V(RHE), which was an order of magnitude higher than the previously reported photoanode made from the same material. The faradaic efficiency for oxygen evolution from water was virtually 100% during the reaction for 6 h, attesting to the robustness of the oxynitride.

Surface Modification of CoO(x) Loaded BiVO4 Photoanodes with Ultrathin p-Type NiO Layers for Improved Solar Water Oxidation.

Photoelectrochemical (PEC) devices that use semiconductors to absorb solar light for water splitting offer a promising way toward the future scalable production of renewable hydrogen fuels. However, the charge recombination in the photoanode/electrolyte (solid/liquid) junction is a major energy loss and hampers the PEC performance from being efficient. Here, we show that this problem is addressed by the conformal deposition of an ultrathin p-type NiO layer on the photoanode to create a buried p/n junction as well as to reduce the charge recombination at the surface trapping states for the enlarged surface band bending. Further, the in situ formed hydroxyl-rich and hydroxyl-ion-permeable NiOOH enables the dual catalysts of CoO(x) and NiOOH for the improved water oxidation activity. Compared to the CoO(x) loaded BiVO4 (CoO(x)/BiVO4) photoanode, the ∼6 nm NiO deposited NiO/CoO(x)/BiVO4 photoanode triples the photocurrent density at 0.6 V(RHE) under AM 1.5G illumination and enables a 1.5% half-cell solar-to-hydrogen efficiency. Stoichiometric oxygen and hydrogen are generated with Faraday efficiency of unity over 12 h. This strategy could be applied to other narrow band gap semiconducting photoanodes toward the low-cost solar fuel generation devices.

A Photoelectrochemical Solar Cell Consisting of a Cadmium Sulfide Photoanode and a Ruthenium-2,2'-Bipyridine Redox Shuttle in a Non-aqueous Electrolyte.

A photoelectrochemical (PEC) cell consisting of an n-type CdS single-crystal electrode and a Pt counter electrode with the ruthenium-2,2'-bipyridine complex [Ru(bpy)3](2+/3+) as the redox shuttle in a non-aqueous electrolyte was studied to obtain a higher open-circuit voltage (V(OC)) than the onset voltage for water splitting. A V(OC) of 1.48 V and a short-circuit current (I(SC)) of 3.88 mA cm(-2) were obtained under irradiation by a 300 W Xe lamp with 420-800 nm visible light. This relatively high voltage was presumably due to the difference between the Fermi level of photo-irradiated n-type CdS and the redox potential of the Ru complex at the Pt electrode. The smooth redox reaction of the Ru complex with one-electron transfer was thought to have contributed to the high V(OC) and I(SC). The obtained V(OC) was more than the onset voltage of water electrolysis for hydrogen and oxygen generation, suggesting prospects for application in water electrolysis.

Trapped state sensitive kinetics in LaTiO2N solid photocatalyst with and without cocatalyst loading.

In addition to the process of photogeneration of electrons and holes in photocatalyst materials, the competitive process of trapping of these charge carriers by existing defects, which can both enhance the photocatalytic activity by promoting electron-hole separation or can deteriorate the activity by serving as recombination centers, is also very crucial to the overall performance of the photocatalyst. In this work, using femtosecond diffuse reflectance spectroscopy we have provided evidence for the existence of energetically distributed trapped states in visible-light responsive solid photocatalyst powder material LaTiO2N (LTON). We observe trapped state sensitive kinetics in bare-LTON. CoOx cocatalyst loading (2 wt % CoOx-LTON) shows effect on the kinetics only when presence of excess energy (for above bandgap excitation) results in the generation of surface carriers. Thus, the kinetics show appreciable excitation wavelength dependence, and the experimental results obtained for different λexc have been rationalized on this basis. In an earlier work by Domen and co-workers, the optimized CoOx/LTON has been reported to exhibit a high quantum efficiency of 27.1 ± 2.6% at 440 nm, the highest reported for this class of photocatalysts (J. Am. Chem. Soc. 2012, 134, 8348-8351). In the present work, the mechanism is addressed in terms of picosecond charge carrier dynamics.

Photoelectrochemical properties of SrNbO2N photoanodes for water oxidation fabricated by the particle transfer method.

SrNbO(2)N photoanodes, which have a band gap energy of 1.8 eV, were tested for photoelectrochemical water oxidation in water splitting. The photoanodes were fabricated either by a particle transfer method or an electrophoretic deposition method. The effects of the precursors, fabrication method, and CoOx catalyst loading were studied in order to identify the shortcomings of the photoanodes and improve their photoelectrochemical properties for water oxidation. SrNbO(2)N photoanodes fabricated by particle transfer generated a photocurrent that was one order of magnitude higher than that of photoanodes prepared via electrophoretic deposition. The stoichiometric oxide precursor (Sr(2)Nb(2)O(7)) was found to be preferable to the Sr-rich oxide precursor (Sr(5)Nb(4)O(15)). CoOx increased the photoanodic current on SrNbO(2)N photoelectrodes. Nevertheless, the incident photon-to-current efficiency was still limited to 10% at most. Potential problems with SrNbO(2)N photoanodes were discussed.

Hydrogen evolution from water using Ag(x)Cu(1-x)GaSe2 photocathodes under visible light.

Photoelectrochemical (PEC) water splitting using CuGaSe2 (CGSe) thin film photocathodes modified by partial substitution of Cu with Ag was investigated. The AgxCu1-xGaSe2 (ACGSe) thin films were deposited onto Mo-coated soda-lime glass substrates by means of co-evaporation using a molecular beam epitaxy (MBE) system. The valence band maximum (VBM) potential of ACGSe is deeper than that of CGSe, and its grain size is greatly increased compared to that of CGSe. A Pt and CdS modified ACGSe electrode (Pt/CdS/ACGSe) with a Ag/(Cu + Ag) ratio of about 5% showed a cathodic photocurrent of 8.1 mA cm(-2) at 0 VRHE and an onset potential of 0.70 VRHE (defined as a cathodic photocurrent of 0.05 mA cm(-2)) under simulated sunlight in a 0.1 M Na2SO4 solution (pH 9.5). Moreover, Pt/CdS/ACGSe exhibited a stable cathodic photocurrent for over 55 h, with no clear decrease.

Enhancement of solar hydrogen evolution from water by surface modification with CdS and TiO2 on porous CuInS2 photocathodes prepared by an electrodeposition-sulfurization method.

Porous films of p-type CuInS2, prepared by sulfurization of electrodeposited metals, are surface-modified with thin layers of CdS and TiO2. This specific porous electrode evolved H2 from photoelectrochemical water reduction under simulated sunlight. Modification with thin n-type CdS and TiO2 layers significantly increased the cathodic photocurrent and onset potential through the formation of a p-n junction on the surface. The modified photocathodes showed a relatively high efficiency and stable H2 production under the present reaction conditions.

Introduction of a Conductive Layer into Flood-Resistant Gas Diffusion Electrodes with Polymer Substrate for an Efficient Electrochemical CO2 Reduction with Copper Oxide.

Conversion of atmospheric carbon dioxide (CO2) into valuable feedstocks is a crucial technology, and electrochemical reduction of CO2 is a promising approach that can provide a useful source of ethylene (C2H4). Gas diffusion electrodes (GDEs) placed at the interface of the CO2 gas and electrolyte can achieve high current density through a sufficient supply of dissolved CO2 to the reaction site, making them indispensable in industrial applications. However, conventional GDEs with carbon substrate have suffered from electrolyte flooding and consequent loss of efficiency, posing an obstacle for practical application. While flood-resistant GDEs with hydrophobic polymer substrate have been proposed recently, only conductive materials can be employed as electrocatalysts because of their insulative properties, despite the high activities of oxide materials such as copper oxide. Here, we introduce an aluminum conductive layer in GDE with polymer substrate to enable the use of electrically resistive catalysts. Cuprous oxide (Cu2O) with silver particles was tested as a model material and has shown prolonged stability (>17 h) with high C2H4 Faraday efficiency (>50%) while suppressing flooding. A thorough characterization revealed that the conductive layer makes Cu2O an efficient electrocatalyst, even on the polymer substrate, by providing sufficient electrons through its conduction path. This research significantly expands the scope of electrode design by enabling the incorporation of a wide range of nonelectrically conductive materials on GDEs with polymer substrate.