Hydrogen production performance of active Ce/N co-doped SrTiO3 for photocatalytic water splitting

An efficient visible light responsive photocatalyst Ce/N co-doped SrTiO₃ was prepared via a hydrothermal method for hydrogen production. The phase structure, morphology, contents and valence states of the dopant elements, specific surface area, optical properties, and photocatalytic activity of the samples were characterized. The transient photocurrent response and electrochemical impedance spectra under visible light illumination indicated that Ce/N co-doped SrTiO₃ possessed a more intense photo-current response and lower surface resistance than N–SrTiO₃ and Ce–SrTiO₃. The water splitting rate of Ce/N-co-doped SrTiO₃ is 4.28 mmol/g/h, which is 84.49 times higher than that of pure SrTiO₃. The enhanced photocatalytic performance is due to the narrowing of the band gap of SrTiO₃ by Ce ion and N ion impurities.     

Hydrogen production by water splitting on manganese ferrite-sodium carbonate mixture: Feasibility tests in a packed bed solar reactor-receiver

The sodium manganese mixed ferrite thermochemical cycle Na(Mn_1/3Fe_2/3)O₂/(MnFe₂O₄ + Na₂CO₃) for sustainable hydrogen production has been implemented in a solar reactor-receiver, packed with indirectly heated MnFe₂O₄/Na₂CO₃ mixture pellets, with the aim of verifying its feasibility and of determining the critical aspects of the process. The reactor operates at nearly constant temperature in the range 750–800 °C; the shift between the hydrogen-producing and regeneration steps is obtained by switching the reactive gas from water to carbon dioxide. Hydrogen produced during 1-h operation of the reactor is in the range of 130–460 μmol/g of mixture, depending on experimental conditions. Compared to other existing prototypes, the implemented process obtains comparable production efficiencies while operating at lower temperature both in the hydrogen production and regeneration phases.     

Photocatalytic water splitting for hydrogen production on Au/KTiNbO5

Gold nanoparticles were deposited on potassium titanoniobate, KTiNbO₅ using deposition-precipitation (DP), conventional impregnation (IMP) and photodeposition method in order to improve photocatalytic hydrogen production from water splitting. The effect of synthesis pH value of a HAuCl₄ aqueous solution used in the DP process on the morphology of gold nanoparticles, optical property and photocatalytic activity of water splitting under UV light irradiation was investigated. These catalysts were characterized by powder X-ray diffraction patterns (XRD), inductively coupled plasma mass spectrometry (ICP-MS), UV–visible spectroscopy (UV–vis), and Transmission Electron Microscopy (TEM). The Au/KTiNbO₅ catalysts prepared by the DP method consisted of a good metal–semiconductor interface which allowed for a much higher efficient electron-hole separation. The 0.63 wt% Au/KTiNbO₅ catalyst prepared by the DP method at pH = 10 showed a uniform dispersion of gold nanoparticles with an average gold particle size of 4.2 nm and exhibited an ultra-high photocatalytic water splitting activity (3522 μmol g⁻¹ h⁻¹), about 47 times higher than that exhibited by the KTiNbO₅ photocatalyst.     

Synthesis of carbon-doped SnO2 nanostructures for visible-light-driven photocatalytic hydrogen production from water splitting

Environmental issues: global warming, organic pollution, CO₂ emission, energy shortage, and fossil fuel depletion have become severe threats to the future development of humans. In this context, hydrogen production from water using solar light by photocatalytic/photoelectrochemical technologies, which results in zero CO₂ emission, has received considerable attention due to the abundance of solar radiation and water. Herein, a single-step thermal decomposition procedure to produce carbon-doped SnO₂ nanostructures (C–SnO₂) for photocatalytic applications is proposed. The visible-light-driven photocatalytic performance of the as-prepared materials is evaluated by photocatalytic hydrogen generation experiments. The bandgaps of the photocatalysts are determined by ultraviolet–visible diffused reflectance spectroscopy. The crystallinity, morphological features (size and shape), and chemical composition and elemental oxidation states of the samples are investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. The proposed simple thermal decomposition method has significant potential for producing nanostructures for metal-free photocatalysis.     

Hydrogen production from water splitting over Eosin Y-sensitized mesoporous-assembled perovskite titanate nanocrystal photocatalysts under visible light irradiation

The photocatalytic water splitting is a promising process for producing H₂ from two abundant renewable sources of water and solar light, with the aid of a suitable photocatalyst. In this work, a combination of sensitizer addition and noble metal loading was employed to modify perovskite photocatalysts in order to achieve the enhancement of photocatalytic H₂ production under visible light irradiation. The dependence of the H₂ production on type of mesoporous-assembled perovskite titanate nanocrystal photocatalysts (MgTiO₃, CaTiO₃, and SrTiO₃), calcination temperature of photocatalyst, Pt loading, type and concentration of electron donor (diethanolamine, DEA; and triethanolamine, TEA), concentration of sensitizer (Eosin Y, E.Y.), photocatalyst dosage, and initial solution pH, was systematically studied. The experimental results showed that the 0.5 wt.% Pt-loaded mesoporous-assembled SrTiO₃ nanocrystal synthesized by a single-step sol-gel method and calcined at 650 °C exhibited the highest photocatalytic H₂ production activity from a 15 vol% DEA aqueous solution with dissolved 0.5 mM E.Y. Moreover, the optimum photocatalyst dosage and initial solution pH for the maximum photocatalytic H₂ production activity were found to be 6 g/l and 11.6, respectively.     

MoS2/CdS rod-like nanocomposites as high-performance visible light photocatalyst for water splitting photocatalytic hydrogen production

This study demonstrates a high-performance visible-light-driven photocatalyst for water splitting H₂ production. CdS nanorods (30 nm in diameters) with shorter radial transfer paths and fewer defects were prepared by a solvothermal method. To mitigate the recombination of electrons and holes, MoS₂ nanosheets with rich active sites were modified on the surface of CdS nanorods by a room-temperature sonication treatment. The photocatalytic water splitting tests show that the MoS₂/CdS nanocomposites exhibit excellent H₂ evolution rates. The highest H₂ evolution rates (63.71 and 71.24 mmol g^?1h^?1 in visible light and simulated solar light irradiation) was found at the 6% MoS₂/CdS nanocomposites, which was 14.61 times and 13.39 times higher than those of the corresponding pristine CdS nanorods in visible light and simulate solar light irradiation, respectively. The apparent quantum efficiency (AQE) of the 6% MoS₂/CdS nanocomposites at 420 nm was calculated to be 33.62%. The electrochemistry tests reveal that the enhanced photocatalytic activity is a result of extra photogenerated charge carries, greatly enhanced charge separation and transfer ability of the MoS₂/CdS composites. This study may give new insights for the rational design and facile synthesis of high-performance and cost-effective bimetallic sulfide photocatalysts for solar-hydrogen energy conversion.     

Titanium dioxide-coated copper electrodes for hydrogen production by water splitting

Pt is the most commonly used electrode and catalyst materials for H₂ production via water splitting as it provides the highest Gibbs free energy of H₂ adsorption (ΔG_H) an d overpotential. However, as Pt catalysts are expensive and difficult to mass-produce, several efforts have been made to identify suitable substitutes. Although Cu provides lower ΔG_H and overpotential than Pt, it exhibits better catalytic performance than other catalysts and is suitable for H₂ production. However, corrosion of Cu may affect its stability of Cu electrode. To overcome this limitation, we have coated a layer of carbon on the copper electrode and then synthesized titanium dioxide-(TiO₂-) on the C/Cu electrode for water splitting application. Carbon black (CB) has excellent electrical conductivity and stable resistance for effective working as an electrochemical catalyst, and TiO₂ has diverse applications because of its low-cost, non-toxic, and corrosion-resistant characteristics. In this study, TiO₂ was synthesized on C/Cu electrodes under UV irradiation for different durations. The optimum irradiation duration was determined to be 15 min via surface and electrochemical analyses. To identify the potential applications of this TiO₂–C/Cu electrode, we used artificial wastewater as the electrolyte. The synthesized TiO₂–C/Cu electrode exhibited better stability than C/Cu electrode. Further, H₂ production with TiO₂–C/Cu electrode was higher than that with C/Cu electrode at the same current density. We also investigated the effect of TiO₂–C/Cu electrode on decomposition of formaldehyde.     

Synthesis conditions effect on the of photocatalytic properties of MnWO4 for hydrogen production by water splitting

This work aims to study the synthesis conditions effect on the photocatalytic properties of manganese tungstate (MnWO₄) for H₂ production by the water splitting reaction under visible light irradiation. This is achieved by relating the materials characterization and photocatalytic evaluation of MnWO₄ at different synthesis conditions. MnWO₄ was synthesized through a precipitation reaction between Mn²⁺ and (WO₄)^2- ionic species, while adding oleic acid (OA) as surfactant at two concentrations (0.1% and 1% V) and using three different stirring methods: magnetic stirring (AM), ultrasound (US) and high-shear stirring (UT). Characterization was carried out by TGA, XRD, BET surface area, UV–Vis spectroscopy and FESEM. XRD patterns confirm the wolframite structure of MnWO₄. BET surface area increased by using UT stirring. UV–Vis spectroscopy results revealed indirect transition Eg values of ≈2 eV, favorable for the MnWO₄ photoactivation under visible light irradiation. During the photocatalytic evaluation, sample 1%-UT produced the highest H₂ amount among all samples with a value of 72 μmolH₂g⁻¹, which was far higher compared to WO₃, which was taken as a reference photocatalyst.     

Hydrogen energy production using manganese/semiconductor system inspired by photosynthesis

Hydrogen is the most efficient and the cleanest fuel. Inspired by photosynthesis, which is able to catalyze water-splitting reaction to achieve energy storage on the large scale at room temperature and neutral pH in nature, artificial systems and technology have been increasing advanced and great progresses have been made over the past decades. The three-dimensional structure of photosystem II with oxygen-evolving activity has been determined at an atomic level, which provides a thorough image with the specific position of each atom in the Mn₄CaO₅ cluster. These advancements have significantly enhanced our understanding of the mechanisms of water splitting in photosynthesis and offered a unique opportunity for hydrogen fuel production. In this review, the recent efforts in the area of manganese/semiconductor system for hydrogen generation were summarized with examples and discussed to highlight the operation mechanisms of this kind of catalytic configuration. A robust PS II mimic containing manganese/semiconductor nanostructure to accomplish the photo water splitting chemistry was evaluated. The development, mechanism, and future improvement of the manganese/semiconductor system were presented. The manganese/semiconductor system is highly likely to offer novel technology for hydrogen energy production.     

Nanostructured hematite thin films produced by spin-coating deposition solution: Application in water splitting

Doped and undoped hematite films for photoelectrochemical hydrogen production were prepared by spin-coating deposition solution (SCDS). To understand the influence of the Si-doping and identify the critical parameters of the proposed SCDS method an extensive characterization was conducted. The Si-doped hematite exhibited higher photocurrent response when compared with undoped films. We have shown that the crystallographic orientation degree of the films appears to be a dominant factor affecting the photocurrent. The performance of our hematite electrodes is well below the maximum theoretical efficiency and the conceivable explanation could be given by the high value of recombination phenomena (electron/hole pair).     

Towards hydrogen production using a breathable electrode structure to directly separate gases in the water splitting reaction

A novel electrode design to directly separate the gases and improve the efficiency of the water splitting reaction is described. In this work, platinum was used as a model catalyst, deposited on porous membranes with different pore size and shape. The O₂ evolution rate was monitored at the gaseous side of these breathable electrodes. We show that the hydrophobic Goretex^® membrane electrodes provide a highly efficient removal of the gases, breathing out 92% of expected O₂ during water splitting, and thereby also largely avoiding the well known migration of oxygen to the cathode in the absence of a separator in the cell. The breathable structure is also shown to operate as a hydrogen electrode. The ability to separate the two gases, without the need for a separator, decreases gas cross-over and thereby enhances the coloumbic efficiency. Merging this approach with catalysts and photocatalysts of a variety of types e.g. non-precious metal and metal oxides will allow fabrication of cost efficient and straightforward water splitting devices.     

Enhanced photoelectrochemical water splitting via engineered surface defects of BiPO4 nanorod photoanodes

Herein, we report on the defect engineering of BiPO₄ nanorods (NRs) via a facile room-temperature template-free co-precipitation method, followed by hydrogen treatment. The hydrogen treatment temperature determined the type of induced defects in the fabricated BiPO₄ NRs and consequently their photocatalytic performance. Upon varying the annealing temperature, the x-ray diffraction (XRD) analysis showed phase transformation and x-ray photoelectron spectroscopy (XPS) analysis revealed variation in the oxygen vacancy content. At moderate treatment temperatures (200–300 °C), shallow defects were predominant, which extended the optical activity of the material to the visible region and increased the photocurrent 3 times when compared to that of bare BiPO₄ NRs. However, treatment at higher temperatures completely altered the crystalline structure, destructed the morphology of the BiPO₄ NRs, and severely affected the photoelectrochemical performance.     

H2 production by thermochemical water splitting with reticulated porous structures of ceria-based mixed oxide materials

In this work, we present for the first time the preparation and evaluation of Ceria-based mixed oxides reticulated porous ceramic (RPC) structures for H₂ production by thermochemical water splitting. After appropriate screening of the powder materials, ceria-based materials modified with Co, Mn and Zr were discarded due to their low cyclability and/or hydrogen productivity, derived from segregation of active phases or sintering during the thermal reduction and reoxidation. Sponge replica method has been optimized to allow obtaining a Ce_0.9Fe_0.1O_y RPC sponge structure with an outstanding hydrogen production of 15 STPcm³/g_material·cycle at a maximum temperature of 1300 °C. This better performance, comparing to the powder, can be attributed to the open macroporosity of the reticulated porous structure which enhances both heat and mass transfer. The H₂ production is maintained along several consecutive cycles without loss of activity, reinforcing the favorable prospects for large-scale hydrogen production.     

Manganese compounds as water oxidizing catalysts for hydrogen production via water splitting: From manganese complexes to nano-sized manganese oxides

For hydrogen production by water splitting, the water oxidation half reaction is overwhelmingly rate limiting and needs high over-voltage (∼1 V), which results in low conversion efficiencies when working at current densities required. At this high voltage, other chemicals will be also oxidized and this would be environmentally unacceptable for large-scale H₂ production.     

Zn-doped hematite modified by graphene-like WS2: A p-type semiconductor hybrid photocathode for water splitting to produce hydrogen

A p-type Zn-doped hematite (α-Fe₂O₃(Zn)) in spindle-shape with an acceptor density of ca. 4.21 × 10¹⁸ cm^?3 were synthesized by a facile hydrothermal method. After α-Fe₂O₃(Zn) was modified with graphene-like WS₂ (α-Fe₂O₃(Zn)/WS₂), the photoelectrochemical performances of the composite can be further enhanced. A photocell composed of the p-type α-Fe₂O₃(Zn)/WS₂ nanocomposite as photocathode and n-type α-Fe₂O₃ as photoanode was assembled to estimate the photocatalytic activity of α-Fe₂O₃(Zn)/WS₂. The amount of the hydrogen and oxygen produced from this tandem cell with the optimal electrodes under 2 h simulated solar light irradiation is 12.5 μmol and 4.3 μmol, respectively.     

Photocatalytic water splitting hydrogen production via environmental benign carbon based nanomaterials

Environmentally benign hydrogen production via photochemical and photoelectrochemical processes by water splitting using carbon-based nanomaterials utilizes sunlight as the source of energy. Owing to their large surface area, pore volume, chemical and thermal stability, and favorable morphology, the carbon-based nanomaterials are quite effective in photocatalytic water splitting. The present review elucidates the photocatalytic nature of carbon materials such as graphene, graphene oxide, carbon nanotubes, graphitic carbon nitride, and fullerenes as they have the tendency to narrow the band gap, allocate electrons, and act as semiconductors, co-catalysts, photosensitizers, and support materials. The production methods, advantages as well as shortcomings of carbon-based materials and their applications in hydrogen production are critically discussed.     

Photocatalytic water splitting on Au/HTiNbO5 nanosheets

The layered potassium titanium niobate, KTiNbO₅, is known as a photocatalyst for hydrogen production from water splitting under UV light. Here we show that titanium niobate nanosheets with a slit like framework can be obtained by exfoliation of KTiNbO₅ followed by proton exchange. Gold nanoparticles were deposited on the titanium niobate nanosheets using deposition-precipitation (DP), photo-deposition (PD) and impregnation (IMP) method in order to improve photocatalytic hydrogen production from water splitting.     

Water splitting for hydrogen production with ferrites

The water splitting reaction by a thermo-chemical cycle using ferrites was investigated for H₂ production. In the first step (activation step), ferrites were thermally reduced at 1200 °C to form an oxygen-deficient ferrite. In the second step (water splitting step), the activated ferrites were oxidized by water at 800 °C to produce hydrogen. Among the prepared ferrites, Ni-ferrite was found to be the most suitable for H₂ production. NiFe₂O₄ produced an average of 0.442 cm³/g cycle of H₂. The H₂ productivity of the Ni-ferrite was much higher than that of the other ferrites at the same temperature. XRD showed that the crystal structure of NiFe₂O₄ during the redox reaction was not changed during the repeated cycles, indicating that NiFe₂O₄ was an excellent material in terms of structural stability and durability.