Hierarchically SrTiO3@TiO2@Fe2O3 nanorod heterostructures for enhanced photoelectrochemical water splitting

In this particular work, the fabrication of SrTiO₃@TiO₂@ Fe₂O₃ nanorod heterostructure has been demonstrated via hydrothermal growth of SrTiO₃ cubic on the rutile TiO₂ nanorod as a template and later sensitized with Fe₂O₃ for photocatalytic solar hydrogen production in a tandem photoelectrochemical cell and dye-sensitized solar cell (DSSC) module. The photocatalytic solar hydrogen production of this heterostructure was optimized by controlling the amount of Sr and Fe on the surface of photocatalyst. The details of the influencing parameters on the physicochemical and photoelectrochemical properties are discussed. It was found that the morphology and quality of the fabricated materials were greatly manipulated by the concentration of Sr and Fe. The optimized 0.025 M SrTiO₃@TiO₂@ Fe₂O₃ heterostructure exhibited a higher photoconversion efficiency with a long electron lifetime, low charge transfer resistance and large donor density at the electrode and electrolyte interface. This composite has significantly improved the photocatalytic hydrogen production, yielding 716 μmol/cm² of maximum accumulative hydrogen. These results show that morphology rendering and manipulation of energy band alignment is crucial in creating efficient heterojunctions for excellent contributions in photocatalytic applications.     

Metal-organic-framework based catalyst for hydrogen production: Progress and perspectives

Fossil fuel shortage and global warming have inspired scientists to search for alternative energy sources which are green, renewable, and sustainable. Hydrogen formed from water splitting has been considered as one of the most promising candidates to replace traditional fuels due to its low production cost and zero-emission. Metal-organic frameworks (MOFs) have been considered as potential catalysts for hydrogen production from water splitting account for their flexible structure, ultra-large surface area, and chemical component diversification. This paper reviews different kinds of MOF-related electrocatalysts, involving metals, metal oxides, single atoms, metal phosphides, metal nitrides, and metal dichalcogenides for hydrogen production. Also, MOF-based photocatalysts consisting of pristine MOFs, MOFs as supporters, and MOF-derived heterojunction architectures are reviewed. The finding of MOF-based catalysts for hydrogen generation is summarized. The pros and cons of different MOF-based materials as catalysts for water splitting are discussed. Finally, current challenges and the potential developments of these unique materials as catalysts are also provided.     

Recent trends in MXene/Metal chalcogenides for electro-/photocatalytic hydrogen evolution reactions

Hydrogen due to high energy density and ecologically benign characteristics can become an excellent energy carrier for a sustainable energy economy and to appease the energy demand of humankind. Moreover, cost-effective and long-lasting photocatalysts can make the hydrogen generating process more economical and suitable. Recently, MXene have become one of the most sought-after composite materials for photocatalytic hydrogen generation. However, the photocstalytic performance can be further enhanced by doping with other semiconductor materials. Transition metal chalcogenides (Transition metals = Cu, Co, Ni, Zn, Cd, Mo, W)/MXene composites and mixed transition metal chalcogenide/MXene nanocomposites have been extensively investigated for the photocatalytic hydrogen generation. These materials possess unique two-dimensional layered structure that ameliorates the photocatalytic water splitting performance by increasing the light adsorption even at low photon flux density. The 2D design assists in reducing the distance necessary to transverse charge carriers to the surface. Because the layered structure tends to trap electrons in the ultrathin layers, 2D materials have unusual optoelectronic properties. In-plane covalent bonding assisted the creation of various heterojunctions and heterostructures in these 2D materials. Water splitting and hydrogen production are aided by the high surface area of these 2D materials. Due to its diverse elemental composition, unique 2D structure, good photoelectronic characteristics, large surface area, and many surface terminations. The design and production of many types of materials used as catalysts for the hydrogen evolution process are discussed in this article.     

H2O2-assisted preparation of microspherical C60–TiO2 photocatalyst and its hydrogen evolution mechanism

Photocatalytic water splitting to produce hydrogen is one of the promising methods to deal with energy shortage and environmental crisis. In this paper, n-type H₂O₂/C₆₀–TiO₂ photo-catalysts with excellent hydrogen production performance were prepared by simple hydrothermal method. The prepared catalysts were characterized by polycrystalline XRD, TEM, UV–Vis–NIR spectroscopy, X-ray photoelectron spectroscopy, FTIR spectroscopy, Raman spectroscopy, etc. The results showed that H₂O₂ can promote the formation of microspherical catalyst; meanwhile, fullerene can broaden the light response range, increase the separation ability of photogenerated carriers and catalyze the formation of molecular H₂ due to the formed superoxide radical. The water splitting experiments showed that the hydrogen evolution rate of H₂O₂/C₆₀–TiO₂ is up to 41.6 mmol?g^?1h^?1, 9.7 times of pure TiO₂. These results have important reference significance for the development of new photocatalysts for water splitting to produce hydrogen.     

TiO2/CeO2 core/shell heterojunction nanoarrays for highly efficient photoelectrochemical water splitting

In this paper, novel TiO₂/CeO₂ core/shell heterojunction nanorod (NR) arrays were synthesized as photoanode for photoelectrochemical (PEC) water splitting via a simple and facial two-step hydrothermal approach. This synthesis route can obtain different amount of CeO₂ nanoparticles by controlling the hydrothermal time and eventually achieve uniform TiO₂/CeO₂ core/shell nanostructures. The uniform TiO₂/CeO₂ core/shell heterojunction nanoarrays exhibit a markedly enhanced photocurrent density of 5.30 mA·cm^?2 compared to that of pristine TiO₂ NR 1.79 mA·cm^?2 at 1.23 V vs. RHE in 1 M KOH solution. The superior PEC performance of the TiO₂/CeO₂ core/shell heterojunction is primarily due to much enhanced visible light absorption and appropriate gradient energy gap structure. This work not only offers the synthesis route for the novel TiO₂/CeO₂ core/shell heterojunction, but also suggests that this new core/shell heterojunction has a great potential application for efficient PEC water splitting devices.     

Visible light‐driven metal‐oxide photocatalytic CO2 conversion

In this work, nine photocatalysts were prepared by a conventional solid‐state reaction method. The samples were characterized by X‐ray diffraction, UV–Vis diffuse reflectance spectroscopy, surface area measurements based on the Brunauer–Emmett–Teller theory, and scanning electron microscopy. The tested materials (BaBiO₃, Bi₂WO₆, SrTiO₃, KNbO₃, NaNbO₃, Sr₄Nb₂O₉, YInO₃, CaIn₂O₄, and YFeO₃) showed great potential for use as photocatalysts in the efficient reduction of CO₂ into a renewable hydrocarbon fuel as well as in water splitting. Our results showed that among the nine tested photocatalysts, three could generate CH₄. In particular, it was observed that KNbO₃, as a result of its high surface area and the suitable band gap, showed the highest CH₄ generation, (86.842 ppm g^?1 h^?1). Some of the tested photocatalysts could generate H₂ and O₂ at a very promising rate; Sr₄Nb₂O₉ and NaNbO₃ were the best two photocatalysts, with an average O₂ production rate of 69.476 ppm g^?1 h^?1 and 57.928 ppm g^?1 h^?1, respectively. Further, NaNbO₃ showed the highest H₂ production average with a rate of 220.128 ppm g^?1 h^?1. The photocatalysts presented herein represent a significant improvement because of the reactor type and the preparation techniques implemented in this study. Copyright © 2015 John Wiley & Sons, Ltd.     

CO2 aided H2 recovery from water splitting processes

Solar energy can be utilized to produce H₂ via photocatalytic water splitting. One major drawback of the one-step approach is the co-production of H₂ and O₂ in the same reactor environment creating a potentially hazardous scenario. This obstacle can be avoided by utilizing CO₂ as a flammability suppressant which is proven to be more effective than N₂. In this case study, several membrane cascade designs were implemented to recover the H₂ while maintaining compositions outside the flammability range. The optimizations are based on the use of commercially available composite polymer membranes from Membrane Technology and Research, Inc. (MTR) in the spiral wound architecture. Both the H₂-selective membrane (Proteus?) and CO₂-selective membrane (Polaris?) were explored in three layouts. The process optimization is solved by nonlinear programming. The aim of the study was to minimize the present value of all outgoing cash flow (no income) for the separation process and achieving 99% product (H₂) purity. Optimization results showed that utilizing three membrane units of Proteus? material with one recycle stream is the optimum layout over a wide range of recovery values. Incorporating the CO₂-selective membrane (Polaris?) leads to more expensive process due to higher recycle flow rates to compensate for the low selectivity of this material. Overall, the best economic results for this process were obtained at 85% recovery rate with 99% product purity at a cost of 6.40 $/kg. Comparing to our previous study using a N₂ diluent, higher purity product with lower specific cost can be achieved with CO₂ diluent system but with slight decrease in recovery rate. As a final element to this study, comparative simulations were executed to demonstrate the potentially added value of using hollow fiber membranes versus spiral wound for this separation process.     

Hydrogen generation through water splitting reaction using waste aluminum in presence of gallium

The in-situ hydrogen generation through water splitting reaction using waste aluminum wires has been studied in presence of room temperature liquid metal gallium and alkaline activator potassium hydroxide. Various proportions of gallium i.e. 50%, 75%, 90% and 95% (weight by weight of the total metal in reaction) were used in order to study the effect of gallium addition on the water splitting reaction. The effect of addition of gallium on the water splitting reaction was also studied and co-related with various concentrations of activator (0.5 N and 1.0 N aqueous KOH) and reaction temperature (50, 60 and 70 °C). The effect of gallium was found to be more prominent at 0.5 N and 50 °C as compare to the 1.0 N and higher temperatures of 50 and 60 °C. The 12 fold increase in hydrogen generation rate was observed for 0.5 N aqueous KOH at 90% gallium addition and 1.0 N aqueous KOH at 75% gallium addition. The added gallium was completely recovered from the reaction. The Shrinking Core Model has been applied to the experimental data for predicting the rate controlling mechanism. The diffusion was predicted as the rate controlling step for maximum cases.     

Electrodeposition of NiS/Ni2P nanoparticles embedded in amorphous Ni(OH)2 nanosheets as an efficient and durable dual-functional electrocatalyst for overall water splitting

To develop earth-abundant and cost-effective catalysts for overall water splitting is still a major challenge. Herein, a unique “raisins-on-bread” Ni–S–P electrocatalyst with NiS and Ni₂P nanoparticles embedded in amorphous Ni(OH)₂ nanosheets is fabricated on Ni foam by a facile and controllable electrodeposition approach. It only requires an overpotential of 120 mV for HER and 219 mV for OER to reach the current density of 10 mA cm⁻² in 1 M KOH solution. Employed as the anode and cathode, it demonstrates extraordinary electrocatalytic overall water splitting activity (cell voltage of only 1.58 V @ 10 mA cm⁻²) and ultra-stability (160 h @ 10 mA cm⁻² or 120 h @50 mA cm⁻²) in alkaline media. The synergetic electronic interactions, enhanced mass and charge transfers at the heterointerfaces facilitate HER and OER processes. Combined with a silicon PV cell, this Ni–S–P bifunctional catalyst also exhibits highly efficient solar-driven water splitting with a solar-to-hydrogen conversion efficiency of 12.5%.     

Recent developments in reduced graphene oxide nanocomposites for photoelectrochemical water-splitting applications

Graphene oxide (GO) sheets have extremely adjustable electronic characteristics due to their distinctive 2D carbon composition, allowing comprehensive surface modifications. Photodriven water splitting uses semiconducting materials that have water-decomposition electronic structures appropriate for electron and hole injection. Photoelectrochemical (PEC) is regarded as an extremely efficient energy conversion system for the manufacturing of clean solar fuel. There have been tremendous attempts to design and create feasible unassisted PEC systems that can effectively divide water to form hydrogen gas and oxygen with only solar energy input (sunlight) necessary. In particular, in the presence of a photocatalyst modified with an appropriate cocatalyst, overall PEC water splitting can be accomplished. For the development of PEC systems, the fundamental concept of PEC water splitting and enhanced energy-conversion efficiency are essential for solar fuel manufacturing. Therefore, this review paper provides a concise summary of unassisted PEC systems with state-of-the-art advancements towards effective PEC water-splitting equipment for the sustainable future use of solar energy.