Solar-driven Hydrogen Production Research Articles

Crystal Facet Engineering of Photoelectrodes for Photoelectrochemical Water Splitting.

Photoelectrochemical (PEC) water splitting is a promising approach for solar-driven hydrogen production with zero emissions, and it has been intensively studied over the past decades. However, the solar-to-hydrogen (STH) efficiencies of the current PEC systems are still far from the 10% target needed for practical application. The development of efficient photoelectrodes in PEC systems holds the key to achieving high STH efficiencies. In recent years, crystal facet engineering has emerged as an important strategy in designing efficient photoelectrodes for PEC water splitting, which has yet to be comprehensively reviewed and is the main focus of this article. After the Introduction, the second section of this review concisely introduces the mechanisms of crystal facet engineering. The subsequent section provides a snapshot of the unique facet-dependent properties of some semiconductor crystals including surface electronic structures, redox reaction sites, surface built-in electric fields, molecular adsorption, photoreaction activity, photocorrosion resistance, and electrical conductivity. Then, the methods for fabricating photoelectrodes with faceted semiconductor crystals are reviewed, with a focus on the preparation processes. In addition, the notable advantages of the crystal facet engineering of photoelectrodes in terms of light harvesting, charge separation and transfer, and surface reactions are critically discussed. This is followed by a systematic overview of the modification strategies of faceted photoelectrodes to further enhance the PEC performance. The last section summarizes the major challenges and some invigorating perspectives for future research on crystal facet engineered photoelectrodes, which are believed to play a vital role in promoting the development of this important research field.

Enzymatic synthesis of supported CdS quantum dot/reduced graphene oxide photocatalysts

Biomineralization provides a route towards scalable, aqueous phase synthesis of heterostructured photocatalysts for solar driven hydrogen production.

FexNi9-xS8 (x = 3-6) as potential photocatalysts for solar-driven hydrogen production?

The efficient reduction of protons by non-noble metals under mild conditions is a challenge for our modern society. Nature utilises hydrogenases, enzymatic machineries that comprise iron- and nickel- containing active sites, to perform the conversion of protons to hydrogen. We herein report a straightforward synthetic pathway towards well-defined particles of the bio-inspired material FexNi9-xS8, a structural and functional analogue of hydrogenase metal sulfur clusters. Moreover, the potential of pentlandites to serve as photocatalysts for solar-driven H2-production is assessed for the first time. The FexNi9-xS8 materials are visible light responsive (band gaps between 2.02 and 2.49 eV, depending on the pentlandite's Fe : Ni content) and display a conduction band energy close to the thermodynamic potential for proton reduction. Despite the limited driving force, a modest activity for photocatalytic H2 has been observed. Our observations show the potential for the future development of pentlandites as photocatalysts. This work provides a basis to explore powerful synergies between biomimetic chemistry and material design to unlock novel applications in solar energy conversion.

Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production

The present study develops a new solar and geothermal based integrated system, comprising absorption cooling system, organic Rankine cycle (ORC), a solar-driven system and hydrogen production units. The system is designed to generate six outputs namely, power, cooling, heating, drying air, hydrogen and domestic hot water. Geothermal power plants emit high amount of hydrogen sulfide (H2S). The presence of H2S in the air, water, soils and vegetation is one of the main environmental concerns for geothermal fields. In this paper, AMIS(AMIS® - acronym for “Abatement of Mercury and Hydrogen Sulphide” in Italian language) technology is used for abatement of mercury and producing of hydrogen from H2S. The present system is assessed both energetically and exergetically. In addition, the energetic and exergetic efficiencies and exergy destruction rates for the whole system and its parts are defined. The highest overall energy and exergy efficiencies are calculated to be 78.37% and 58.40% in the storing period, respectively. Furthermore, the effects of changing various system parameters on the energy and exergy efficiencies of the overall system and its subsystems are examined accordingly.

Solar-driven high temperature hydrogen production via integrated spectrally split concentrated photovoltaics (SSCPV) and solar power tower

Hydrogen production can be achieved via combined concentrated photovoltaic (CPV) and concentrated solar power (CSP) in which concentrated radiation is spectrally split and then converted in a photovoltaic receiver and a thermal absorber. This study thus proposes an innovative solar process design integrating both thermal and quantum components of solar energy while providing a complete assessment of its global performance to demonstrate its practical interest. A stand-alone solar-to-hydrogen path was modeled and numerically simulated, which was both electrically and thermally supplied by a solar power generation unit to feed the electrolyzer power utilization unit with enhanced solar-to-hydrogen conversion efficiency. Following balance of plant (BoP), the heliostat field and cavity receiver were designed to match the entire system in which the receiver only intercepts a definite range of infrared wavelength while the rest is converted by separately insulated PV cells. Moreover, dichroic reflectors and optimum cutoff wavelength were applied to fulfill separate optimization and heat load reduction of each solar cell. Finally, the solid oxide electrolysis cell (SOEC) was designed to utilize the generated thermal and electrical power appropriately. In best case scenario, a solar-to-hydrogen conversion efficiency of 36.5% was achieved under 899 W/m2 direct normal irradiance (DNI) and 1000 suns concentration. The solar plant outputs at this operating point were 850 g/h H2 and 6754 g/h O2. Further improvement in efficiency can be achieved through alignment in regard to the site location and annual insolation variation.

Photocatalysis with Reduced TiO2: From Black TiO2 to Cocatalyst-Free Hydrogen Production.

Black TiO2 nanomaterials have recently emerged as promising candidates for solar-driven photocatalytic hydrogen production. Despite the great efforts to synthesize highly reduced TiO2, it is apparent that intermediate degree of reduction (namely, gray titania) brings about the formation of peculiar defective catalytic sites enabling cocatalyst-free hydrogen generation. A precise understanding of the structural and electronic nature of these catalytically active sites is still elusive, as well as the fundamental structure–activity relationships that govern formation of crystal defects, increased light absorption, charge separation, and photocatalytic activity. In this Review, we discuss the basic concepts that underlie an effective design of reduced TiO2 photocatalysts for hydrogen production such as (i) defects formation in reduced TiO2, (ii) analysis of structure deformation and presence of unpaired electrons through electron paramagnetic resonance spectroscopy, (iii) insights from surface science on electronic singularities due to defects, and (iv) the key differences between black and gray titania, that is, photocatalysts that require Pt-modification and cocatalyst-free photocatalytic hydrogen generation. Finally, future directions to improve the performance of reduced TiO2 photocatalysts are outlined.

Pyrene-Based Covalent Organic Polymers for Enhanced Photovoltaic Performance and Solar-Driven Hydrogen Production

Photovoltaic solar-driven hydrogen production is promising but challenging. Herein, we report that pyrene-based covalent-organic-polymer-modified (COP-modified) TiO2 leads to composite materials with high power conversion efficiency (PCE). The suitable band position, inherent optimized pore size, large specific surface area, and wide light absorption region of COP-64 are beneficial for the photo-to-electron conversion and electron transfer efficiency. As a result, the PCE of the modified photoanode-assembled dye-sensitized solar cell (DSSC, N719 as the dye) achieved as high as 10.0% with the optimal COP-64 addition of 0.6 wt %, exhibiting a more than 20% promotion compared with the pristine TiO2 (7.9%). Moreover, the as-obtained integrated tandem DSSCs exhibited a high solar-to-hydrogen efficiency of 7.5% for the water splitting, as well as high stability and hydrogen production performance. Therefore, the COP-modified TiO2 photoanode assembled in DSSCs paved a new effective pathway to improve power conve...

Exploration of CeO2-CuO Quantum Dots in Situ Grown on Graphene under Hypha Assistance for Highly Efficient Solar-Driven Hydrogen Production.

The development of an artificial model of photoinduced hydrogen production system requires efficient, long-term stability and cost-competitive photocatalysts to store solar energy in chemical bonds. However, the existing photocatalysts still suffer from the high cost, high recombination rate of photoexcited electron-hole pairs, and poor photostability. Herein, we demonstrate the synthesis of a p-type CuO/n-type CeO2 heterojunction in situ grown on graphene via a hypha assistance process. Amazingly, optical and photoelectrochemical measurements show the superiority of this hierarchically biomorphic structure. The observed H2 evolution rate of the CeO2-CuO quantum dots/graphene has reached 2481 μmol·h-1·g-1 and remains unchanged in four hydrogen production cycles. Considering the convenience of microbial culture, this heterostructure system has great potential as a photocatalyst for solar-fuel conversion.

Charge Transfer and Recombination Dynamics at Inkjet-Printed CuBi2O4 Electrodes for Photoelectrochemical Water Splitting

One of the principal challenges for solar-driven hydrogen production via water splitting is to improve the solar-to-hydrogen conversion efficiency. We have employed combinatorial chemistry using a materials inkjet printer, and selected CuBi2O4 as a promising p-type material. The steady-state photocurrent corresponding to water reduction for a 280 nm film at 0.2 V (RHE) was about 0.12 mA cm–2, significantly lower than that attainable for a 2 eV band gap semiconductor. We have applied intensity-modulated photocurrent spectroscopy (IMPS) to distinguish between the photoelectrochemical processes involved and to determine the associated time constants, in order to gain insight into the loss processes responsible for the low efficiency. The charge separation efficiency reaches up to 0.66 at sufficiently negative potential, however, the recombination rate constant is larger than that corresponding to electron transfer to the solution. This results in a relative charge transfer efficiency of 0.2–0.4, explaining t...

Drastic Improvement of 1D-CdS Solar-Driven Photocatalytic Hydrogen Evolution Rate by Integrating with NiFe Layered Double Hydroxide Nanosheets Synthesized by Liquid-Phase Pulsed-Laser Ablation

Solar-driven semiconductor-based molecular hydrogen production is an ideal protocol for converting abundant solar energy to green fuel. However, this process suffers from costly semiconductor nanostructures, low efficiency, and poor stability. Here, we design a noble-metal-free photocatalyst, CdS-NiFe layered double hydroxide (LDH) nanocomposite, which is synthesized using the liquid-phase pulsed-laser ablation and hydrothermal method. The nanocomposite has a unique morphology of 2D-NiFe LDH nanosheets on 1D-CdS nanorods. The interfacial contact of heterostructures allows the efficient carrier transport and migration due to the appropriate potentials, which greatly reduce the recombination of carriers. It also provides a significant number of catalytically active sites for the hydrogen evolution reaction due to its thin and flexible nature and high specific surface area. The CdS/NiFe nanocomposite exhibits a hydrogen evolution rate of 72 mmol g–1 h–1, which is higher than reported nanocomposites of CdS-ba...

Fabrication of dual direct Z-scheme g-C3N4/MoS2/Ag3PO4 photocatalyst and its oxygen evolution performance

Semiconductor-based photocatalytic materials have emerged as promising candidates for solar-driven hydrogen production and oxygen evolution reactions. Direct Z-scheme photocatalysts offer competitive advantages that are superior to single-component or intensively studied heterojunction photocatalysts in photocatalytic water splitting. The development of high-performance direct Z-scheme photocatalysts is crucial to improving solar-driven water splitting efficiency. Herein, we report the fabrication of a novel g-C3N4/MoS2/Ag3PO4 ternary composite and its application in photocatalytic oxygen evolution under white light LED illumination. As-exfoliated, highly conductive two-dimensional molybdenum disulfide (2D MoS2) nanoflakes and modified graphitic carbon nitride (g-C3N4) nanosheets were employed simultaneously to couple with oxygen-evolving silver orthophosphate (Ag3PO4), forming a dual direct Z-scheme g-C3N4/MoS2/Ag3PO4 (CMA) composite photocatalytic system for highly improved oxygen evolution from water splitting. The optimal CAM-20 exhibits the fastest oxygen-producing rate of 232.1 μmol L-1 g-1 h-1, which is 5 times higher than that of bulk Ag3PO4. The enhancement in the photocatalytic oxygen evolution can be ascribed to synergistic effects of improved visible light absorption, more efficient separation of photoexcited electron-hole pairs and a specific charge transfer pathway of tandem dual direct Z-scheme configuration under light illumination. This work paves the way for the construction of direct Z-scheme composite photocatalytic systems in water splitting.

Uniform Pt quantum dots-decorated porous g-C3N4 nanosheets for efficient separation of electron-hole and enhanced solar-driven photocatalytic performance

Uniform Pt quantum dots-decorated porous g-C3N4 nanosheets (Pt/CN) are fabricated by a facile impregnation-ultrasonic-calcination method, using melamine as precursor. The as-prepared samples are evidently investigated by X-ray diffraction, UV–vis diffuse reflection spectra, N2 adsorption, transmission electron microscope, surface photovoltage spectroscopy and photoluminescence. The deposited Pt quantum dots with particle size of ∼5 nm are decorated on the surface of porous g-C3N4 nanosheets uniformly. The Pt/CN nanosheets show conspicuous solar-driven photocatalytic activity for splitting water to produce H2. The solar-driven photocatalytic hydrogen production rate of Pt/CN is up to ∼107 μmol h−1 g−1, which is about 5 times higher than that of pristine g-C3N4. The enhancement can be attributed to the porous structure offering adequate surface active sites and the efficient decoration of uniform Pt quantum dots on g-C3N4 nanosheets facilitating the separation of photogenerated electron-hole pairs, which is confirmed by surface photovoltage spectroscopy and photoluminescence. The strategy for fabricating Pt quantum dots-decorated g-C3N4 nanosheets offers new insights for constructing other high-performance quantum dot-semiconductor photocatalytic materials.

H-doped TiO2-x prepared with MgH2 for highly efficient solar-driven hydrogen production

Efficient utilization of visible light with high stability remains a critical challenge for solar-driven photochemical generation of hydrogen (H2) using particulate photocatalysts. Black TiO2 was introduced with remarkable enhancement of visible light absorption, but its efficiency in the visible light has not reached the desired level for real-world applications. Here we report a gold-colored H-doped TiO2-x (H:TiO2-x) nanoparticles prepared by controlled reduction via simultaneous presence of [Mg] and [H], which are obtained from the decomposition of MgH2. The H-doped TiO2-x exhibits a significant activity (16.1 mmolg−1h−1) and remarkable stability after Pt deposition for solar-driven H2 generation from methanol-water. The excellent photoactivity of H-doped TiO2 can be attributed to oxygen vacancies and H doping at the reduced TiO2-x surface generated by [Mg] and [H]. The H-doped TiO2 is also producing H2 from methanol-seawater with a rate of 6.1 mmolg−1h−1 under simulated sunlight.

Development and Characterization of Titanium Dioxide Gel with Encapsulated Bacteriorhodopsin for Hydrogen Production.

We study bacteriorhodopsin (BR) in its native purple membrane encapsulated within amorphous titanium dioxide, or titania, gels and in the presence of titania sol particles to explore this system for hydrogen production. Förster resonance energy transfer between BR and titanium dioxide sol particles was used to conclude that there is nanometer-scale proximity of bacteriorhodopsin to the titanium dioxide. The detection of BR-titania sol aggregates by fluorescence anisotropy and particle sizing indicated the affinity amorphous titania has for BR without the use of additional cross-linkers. UV-vis spectroscopy of BR-titania gels shows that methanol addition did not denature BR at a 25 mM concentration presence as a sacrificial electron donor. Additionally, confinement of BR in the gels significantly limited protein denaturation at higher concentration of added methanol or ethanol. Subsequently, titania gels fabricated through the sol-gel process using a titanium ethoxide precursor, water, and the addition of 25 mM methanol were used to encapsulate BR and a platinum reduction catalyst for the production of hydrogen gas under white light irradiation. The inclusion of 5 μM bacteriorhodopsin resulted in a hydrogen production rate of about 3.8 μmol hydrogen mL-1 h-1, an increase of 52% compared to gels containing no protein. Electron transfer and proton pumping by BR in close proximity to the titania gel surface are feasible explanations for the enhanced production of hydrogen without the need to cross-link BR to the titania gel. This work sets the stage for further developments of amorphous, rather than crystalline, titania-encapsulated bacteriorhodopsin for solar-driven hydrogen production through water splitting.

Vanadium disulfide decorated graphitic carbon nitride for super-efficient solar-driven hydrogen evolution

Highly efficient, earth-abundant, and low-cost photocatalysts are widely pursued for solar-driven hydrogen generation from water. Herein, we first report vanadium disulfide (VS2) with high hydrogen evolution reaction (HER) activity both in basal and edges to be the co-catalyst of graphitic carbon nitride (g-C3N4) for ultrahigh solar-driven hydrogen production. VS2-decorated g-C3N4 shows an impressing photocatalytic hydrogen evolution with a rate of 87.4 μmol/h, 26 times higher than pristine g-C3N4. Our combined experimental and computational studies reveal that the excellent efficiency of the composite is attributed to: (1) effective electron-hole separation and electron transfer from g-C3N4 to VS2, resulting from the optimal band alignment between VS2 and g-C3N4 and metallic characteristic of VS2; (2) fast hydrogen generation on the surface due to the high surface area and excellent HER activity of VS2. Our findings demonstrate that VS2/g-C3N4 may be applicable in solar-driven water splitting, and the design principle can be applied to search for novel photocatalysts.

Black TiO2 synthesized via magnesiothermic reduction for enhanced photocatalytic activity

Utilizing solar energy for hydrogen evolution is a great challenge for its insufficient visible-light power conversion. In this paper, we report a facile magnesiothermic reduction of commercial TiO2 nanoparticles under Ar atmosphere and at 550 °C followed by acid treatment to synthesize reduced black TiO2 powders, which possesses a unique crystalline core–amorphous shell structure composed of disordered surface and oxygen vacancies and shows significantly improved optical absorption in the visible region. The unique core–shell structure and high absorption enable the reduced black TiO2 powders to exhibit enhanced photocatalytic activity, including splitting of water in the presence of Pt as a cocatalyst and degradation of methyl blue (MB) under visible light irradiation. Photocatalytic evaluations indicate that the oxygen vacancies play key roles in the catalytic process. The maximum hydrogen production rates are 16.1 and 163 μmol h−1 g−1 under the full solar wavelength range of light and visible light, respectively. This facile and versatile method could be potentially used for large scale production of colored TiO2 with remarkable enhancement in the visible light absorption and solar-driven hydrogen production.

Engineering oxygen vacancy on rutile TiO2 for efficient electron-hole separation and high solar-driven photocatalytic hydrogen evolution

Oxygen vacancy (VO) plays a vital role in semiconductor photocatalysis. Rutile TiO2 nanomaterials with controllable contents of VO (0–2.18%) are fabricated via an in-situ solid-state chemical reduction strategy, with color from white to black. The bandgap of the resultant rutile TiO2 is reduced from 3.0 to 2.56 eV, indicating the enhanced visible light absorption. The resultant rutile TiO2 with optimal contents of VO (~2.07%) exhibits a high solar-driven photocatalytic hydrogen production rate of 734 μmol h−1, which is about four times as high as that of the pristine one (185 μmol h−1). The presence of VO elevates the apparent Fermi level of rutile TiO2 and promotes the efficient electron-hole separation obviously, which favor the escape of photogenerated electrons and prolong the life-time (7.6×103 ns) of photogenerated charge carriers, confirmed by scanning Kelvin probe microscopy, surface photovoltage spectroscopy and transient-state fluorescence. VO-mediated efficient photogenerated electron-hole separation strategy may provide new insight for fabricating other high-performance semiconductor oxide photocatalysts.

Ultrathin MoS2-coated Ag@Si nanosphere arrays as an efficient and stable photocathode for solar-driven hydrogen production

Solar-driven photoelectrochemical (PEC) water splitting has attracted a great deal of attention recently. Silicon (Si) is an ideal light absorber for solar energy conversion. However, the poor stability and inefficient surface catalysis of Si photocathodes for the hydrogen evolution reaction (HER) have remained key challenges. Alternatively, MoS2 has been reported to exhibit excellent catalysis performance if sufficient active sites for the HER are available. Here, ultrathin MoS2 nanoflakes are directly synthesized to coat arrays of Ag-core Si-shell nanospheres (Ag@Si NSs) by using chemical vapor deposition. Due to the high surface area ratio and large curvature of these NSs, the as-grown MoS2 nanoflakes can accommodate more active sites. In addition, the high-quality coating of MoS2 nanoflakes on the Ag@Si NSs protects the photocathode from damage during the PEC reaction. An photocurrent density of 33.3 mA cm−2 at a voltage of −0.4 V is obtained versus the reversible hydrogen electrode. The as-prepared nanostructure as a hydrogen photocathode is evidenced to have high stability over 12 h PEC performance. This work opens up opportunities for composite photocathodes with high activity and stability using cheap and stable co-catalysts.

One-step chemical vapor deposition of MoS2 nanosheets on SiNWs as photocathodes for efficient and stable solar-driven hydrogen production

Silicon nanowires (SiNWs) are widely used as photocathodes because of their large electrochemically available surface-area density and inherent ability to decouple light absorption from the transport of minority carriers. In order to minimize overpotential for solar-driven hydrogen (H2) production, a combination of an ultrathin molybdenum disulfide (MoS2) layer with SiNWs as photocathode has attracted much attention. Herein, for the first time, this study presents the synthesis of a composite photocathode via direct growth of ultrathin MoS2 nanosheets on SiNWs (referred to as SiNWs/MoS2) by one-step chemical vapor deposition (CVD). Due to the high surface-area density of the arrays of SiNWs, the discontinuous MoS2 nanosheets grown on the SiNWs achieved a much higher density of active sites. Moreover, the coating of MoS2 on the SiNWs was found to protect the photocathode during the photoelectrochemical (PEC) reaction. A high efficiency with photocurrent jsc of 16.5 mA cm-2 (at 0 V vs. reversible hydrogen electrode) and an excellent stability over 48 h of PEC operation were achieved under a simulated 1 sun irradiation.

Significantly enhanced photocatalytic hydrogen generation over graphitic carbon nitride with carefully modified intralayer structures

Graphitic carbon nitride has been widely investigated as promising photocatalyst for solar-driven photocatalytic hydrogen production. In this study, the intralayer properties of graphitic carbon nitride was modified by a facile hydrothermal treatment with appropriate amount of ammonium hydroxide to promote its photocatalytic H2 production performance. Transition from intralayer modification to bulk structural transformation occurred once the concentration of NH3·H2O is over certain concentration. The destruction of heptazine units resulted in the partial oxidation of C atoms in g-C3N4 during the hydrothermal treatment process and the introduction of OH and CO can effectively enhance the interaction between g-C3N4 with reactants. Band structure measurement revealed that the thus obtained g-C3N4 possessed elevated conduction band bottom indicative of improved reduction capacities. The significantly enhanced redox ability on the surface reaction sites overcame the deceased light absorption and higher charge recombination efficiency of charge carriers, and finally induced a significantly improved photocatalytic H2 evolution (707.58μmol·h−1·g−1). The highest visible light photocatalytic H2 evolution rate was 7.5 times higher than that of pristine graphitic carbon nitride (94.46μmol·h−1·g−1), with an apparent quantum yield of 5.18% at 420nm. This study demonstrates a facile and effective strategy to tune the electronic and photocatalytic properties of carbon nitride for improved solar energy conversion efficiency which are expected to be extended to the modifications of other photocatalytic materials.