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.     

Direct solar thermal splitting of water and on-site separation of the products—II. Experimental feasibility study

The development of a process of hydrogen production by solar thermal water splitting (HSTWS) presents a formidable technological task. The process has, however, great potential from the thermodynamic point of view and, when combined with fuel cell technology, it can lead to efficient conversion of solar energy to power. In the process under development at the Weizmann Institute of Science, water vapor is partially dissociated in a solar reactor at temperatures approaching 2500 K. Hydrogen is separated from the hot mixture of water splitting products by gas diffusion through a porous ceramic membrane.The paper describes the problems encountered during the development of the HSTWS process. The following topics are discussed in some detail: (a) achievement of very high solar hydrogen reactor temperatures by secondary concentration of solar energy; (b) materials problems encountered in the manufacture of the solar reactor; (c) development of special porous ceramic membranes that resist clogging by sintering at very high temperatures.     

State-of-the-art progress in overall water splitting of carbon nitride based photocatalysts

Converting solar energy into hydrogen (H₂) by photocatalytic water splitting is a promising approach to simultaneously address the increasing energy demand and environmental issues. Half decade has passed since the discovery of photo-induced water splitting phenomenon on TiO₂ photoanode, while the solar to H₂ efficiency is still around 1%, far below the least industrial requirement. Therefore, developing efficient photocatalyst with a high energy conversion efficiency is still one of the main tasks to be overcome. Graphitic carbon nitride (g-C₃N₄) is just such an emerging and potential semiconductor. Therefore, in this review, the state-of-the-art advances in g-C₃N₄ based photocatalysts for overall water splitting were summarized in three sections according to the strategies used, and future challenges and new directions were discussed.     

Boosting photoelectrochemical performance of GaN nanowall network photoanode with bacteriorhodopsin

The ever-increasing demand for renewable and clean energy sources has prompted the development of novel materials for photoelectrochemical (PEC) water splitting, but efficient solar to hydrogen conversion remains a big challenge. In this work, we report a bio-nanohybrid strategy in a photo-system to simultaneously enhance the charge separation and water splitting efficiency of photoanode (PA) by introducing Bacteriorhodopsin (bR), a natural proton pumping photosystem and GaN nanowall network (NWN), a direct band gap and corrosion-resistant semiconductor. The experimental study reveals that this combination of bR and GaN NWN has huge potential as a light-activated sensitizer as well as proton pumping source to achieve enhance photocurrent density in hydrogen evolution reaction (HER). Consequently, this synergistic effect in bR/GaN NWN PA gives rise to largely enhanced applied bias photon-to-current efficiency (ABPE) ~7.8% and photocurrent density (28.74 mA/cm² at 1.0 V vs RHE). It is worth mentioning that the photocurrent density of bR/GaN NWN, to the best of our knowledge, is superior to previously reported bR-based PAs and bio-photoelectric devices reported till today for solar-to-hydrogen fuel generation.     

Comparison of thermochemical,electrolytic, photoelectrolytic and photochemical solar-to-hydrogen production technologies

Hydrogen produced from solar energy is one of the most promising solar energy technologies that can significantly contribute to a sustainable energy supply in the future. This paper discusses the unique advantages of using solar energy over other forms of energy to produce hydrogen. Then it examines the latest research and development progress of various solar-to-hydrogen production technologies based on thermal, electrical, and photon energy. Comparisons are made to include water splitting methods, solar energy forms, energy efficiency, basic components needed by the processes, and engineering systems, among others. The definitions of overall solar-to-hydrogen production efficiencies and the categorization criteria for various methods are examined and discussed. The examined methods include thermochemical water splitting, water electrolysis, photoelectrochemical, and photochemical methods, among others. It is concluded that large production scales are more suitable for thermochemical cycles in order to minimize the energy losses caused by high temperature requirements or multiple chemical reactions and auxiliary processes. Water electrolysis powered by solar generated electricity is currently more mature than other technologies. The solar-to-electricity conversion efficiency is the main limitation in the improvement of the overall hydrogen production efficiency. By comparison, solar powered electrolysis, photoelectrochemical and photochemical technologies can be more advantageous for hydrogen fueling stations because fewer processes are needed, external power sources can be avoided, and extra hydrogen distribution systems can be avoided as well. The narrow wavelength ranges of photosensitive materials limit the efficiencies of solar photovoltaic panels, photoelectrodes, and photocatalysts, hence limit the solar-to-hydrogen efficiencies of solar based water electrolysis, photoelectrochemical and photochemical technologies. Extension of the working wavelength of the materials is an important future research direction to improve the solar-to-hydrogen efficiency.     

Advancement in solar photovoltaic/thermal (PV/T) hybrid collector technology

This article presents an overview on the research and development and application aspects for the hybrid photovoltaic/thermal (PV/T) collector systems. A major research and development work on the photovoltaic/thermal (PVT) hybrid technology has been done since last 30 years. Different types of solar thermal collector and new materials for PV cells have been developed for efficient solar energy utilization. The solar energy conversion into electricity and heat with a single device (called hybrid photovoltaic thermal (PV/T) collector) is a good advancement for future energy demand. This review presents the trend of research and development of technological advancement in photovoltaic thermal (PV/T) solar collectors and its useful applications like as solar heating, water desalination, solar greenhouse, solar still, photovoltaic-thermal solar heat pump/air-conditioning system, building integrated photovoltaic/thermal (BIPVT) and solar power co-generation.     

光化学合成太阳能燃料的研究进展

光电转换、光热转换和光化学转换是太阳能利用的三种主要途径。近年来,太阳能燃料的研究已引起了人们的广泛关注。本文针对光化学合成太阳能燃料,简要综述了光解水制H₂及CO₂光化学还原为CO等燃料的研究进展,并展望了基于利用太阳能制取的H₂和CO进一步光费托合成碳氢燃料的前景。     

Photocatalytic hydrogen production: A solar energy conversion alternative?

Since the beginning of earth, ultra-violet rays from the sun have been splitting water molecules in the upper reaches of the atmosphere, producing hydrogen and oxygen. At the same time, photosynthesis has been sustaining life on earth by utilizing the longer wavelengths of sunlight to split carbon and water molecules forming carbohydrates and oxygen. Attempts are being made to imitate these processes for the purpose of harnessing solar energy for man's use.This paper presents a state-of-the-art review of photolysis of water by sunlight with consequent energy storage in the molecular bonds of hydrogen and oxygen. The photocatalytic solar energy conversion methods reviewed are classified according to the photosensitizer type. That includes catalysts such as compound salts, compound semi-conductor crystals and photosynthetic dyes. An assessment of these methods concludes that photolysis of water as a solar energy conversion process does not seem feasible at present or for the immediate future. On the other hand, efficiency estimates indicate that with research emphasis this unconventional energy conversion method may prove to be a long-term alternative in harnessing solar energy for man's use.     

Theoretical investigation on new materials for raising efficiency of energy conversion in PEC solar cells

In order to raise the conversion efficiency of solar energy into electric or chemical energy in solar cells, we have to developed new types of composite semiconductor materials. A composite structure with three layers has been designed. The design of the composite photoanode for water splitting in a photoelectrochemical (PEC) solar cell can be considered a breakthrough in principle. The energy correlations in the dark and under illumination have been analysed.     

Development and test of a solar reactor for decomposition of sulphuric acid in thermochemical hydrogen production

Decomposition of sulphuric acid is a key step of sulphur based thermochemical cycles for hydrogen production by thermal splitting of water. The Hybrid Sulphur Cycle (HyS) consisting of two reaction steps is considered as one of the most promising cycles: firstly, sulphuric acid is decomposed by high temperature heat of 800–1200 °C forming sulphur dioxide, which in a second step is used to electrochemically split water. Compared to conventional water electrolysis only about a tenth of the theoretical voltage is required making the HyS one of the most efficient processes to produce hydrogen by concentrated solar radiation. As a result, this thermochemical cycle has the potential to significantly reduce the amount of energy required for water splitting and to efficiently generate hydrogen free of carbon dioxide emissions. The European research project HycycleS aims at a technical realisation of the HyS. One objective of the project is to develop and qualify a solar interface, meaning a device to couple concentrated solar radiation into the endothermal steps of the chemical process. Therefore, a test reactor for decomposition of sulphuric acid by concentrated solar radiation was developed and tested in the solar furnace of DLR in Cologne. Tests in concentrated solar radiation were carried out for temperatures of the honeycomb up to 950 °C decomposing sulphuric acid of 50 and 96 weight-percent. Mass and energy flow of the process were calculated in order to determine energy efficiency and chemical conversion. The influence of process parameters like temperature, flow rates and space velocity on chemical conversion and reactor efficiency was analysed in detail. If catalysts like iron oxide (Fe₂O₃) and mixed oxides (i.e. CuFe₂O₄) were used a conversion of SO₃ to SO₂ of more than 80% at a thermal efficiency of over 25% could be reached.     

A comparative thermodynamic analysis of samarium and erbium oxide based solar thermochemical water splitting cycles

This paper reports a thermodynamic comparison between the samarium and erbium oxide based solar thermochemical water splitting cycles. These cycles are a two-step process in which the metal oxide is first thermally reduced into the pure metal, and the produced metal can be used to split water to produce H₂. The metal oxides can be reused for multiple cycles without consumption. The effect of water splitting temperature on various thermodynamic parameters which are essential to design the solar reactor system for the production of H₂ via water splitting reaction using the samarium and erbium oxides is studied in detail. The total amount of solar energy needed for the thermal reduction of samarium and erbium oxides is estimated. The amount of heat energy released by the water splitting reactor is calculated. Also, the cycle and solar-to-fuel energy conversion efficiency for both cycles are determined by employing heat recuperation. Obtained results indicate that the efficiencies associated with these cycles are comparable to the previously studies thermochemical cycles. It is observed that higher water splitting temperature favors towards higher efficiencies. At constant thermal reduction temperature = 2280 K, by employing 50% heat recuperation, the solar-to-fuel energy conversion efficiency for the samarium cycle (30.98%) is observed to be higher than erbium cycle (28.19%).     

Recent progress in photocatalysts for overall water splitting

Photocatalytic splitting of water with solar energy is considered as the most promising approach for the production of hydrogen fuel. However, its solar to hydrogen conversion efficiency is much below the industrial requirement (10%). This situation has stimulated intensive efforts to improve photocatalytic overall water splitting (namely, simultaneously providing unassisted oxidation and reduction of water), leading to the invention of novel catalysts in the recent years. The evaluation of these recent progresses constitutes this review article, with emphasis on the strategies employed for the development of catalysts. The catalysts were deeply reviewed and were classified into four types: (a) perovskite compounds, (b) metal oxides (sulfides and nitrides), (c) Bi‐ and In‐based materials, and (d) multicomponent catalysts. Furthermore, the challenges that remain with the process and catalysts and the potential advances were discussed as an outlook for future research.     

Hydrogen from solar energy,a clean energy carrier from a sustainable source of energy

Solar energy is going to play a crucial role in the future energy scenario of the world that conducts interests to solar-to-hydrogen as a means of achieving a clean energy carrier. Hydrogen is a sustainable energy carrier, capable of substituting fossil fuels and decreasing carbon dioxide (CO₂) emission to save the world from global warming. Hydrogen production from ubiquitous sustainable solar energy and an abundantly available water is an environmentally friendly solution for globally increasing energy demands and ensures long-term energy security. Among various solar hydrogen production routes, this study concentrates on solar thermolysis, solar thermal hydrogen via electrolysis, thermochemical water splitting, fossil fuels decarbonization, and photovoltaic-based hydrogen production with special focus on the concentrated photovoltaic (CPV) system. Energy management and thermodynamic analysis of CPV-based hydrogen production as the near-term sustainable option are developed. The capability of three electrolysis systems including alkaline water electrolysis (AWE), polymer electrolyte membrane electrolysis, and solid oxide electrolysis for coupling to solar systems for H₂ production is discussed. Since the cost of solar hydrogen has a very large range because of the various employed technologies, the challenges, pros and cons of the different methods, and the commercialization processes are also noticed. Among three electrolysis technologies considered for postulated solar hydrogen economy, AWE is found the most mature to integrate with the CPV system. Although substantial progresses have been made in solar hydrogen production technologies, the review indicates that these systems require further maturation to emulate the produced grid-based hydrogen.     

Development of solar thermal systems in China

China has an abundant solar energy resource. Solar thermal conversion systems have been studied for 25 yr, and solar thermal industry has developed rapidly for 10 yr. There are various solar thermal systems, with an area of around 10 million m² in 2002. These systems mainly provide domestic hot water, but some other applications are under extensive study and development as well. The purpose of this paper is to present the developments that have taken place and that are under way.     

On the potential of solar energy conversion into hydrogen and/or other fuels

Solar energy, when used together with water for the production of hydrogen, forms an inexhaustible source of transportable primary energy. Hydrogen is also a potential means of storing solar energy. In this paper the thermodynamic and energetic conditions for the splitting of water are established. The different water decomposition techniques are discussed.Electrolysis. Electrolysis is a proven and convenient way of producing hydrogen. If the very high temperature electrolysis (80–1000°C) development is successful, heat-assisted electrolysis with electric efficiencies of 100% and more looks attractive in connection with thermo-mechanical helio-electricity conversion.Thermal conversion. Highest temperature (≈ 3000°C) direct decomposition (thermolysis) is thermodynamically interesting, but is, for the time being, technologically not feasible. Use of thermochemical cycles is mainly a question of economics and of adaptation to the high temperatures, attainable with solar concentrating devices.Quantum conversion. The thermodynamic potential of light makes quantum conversion highly attractive, requiring much basic research, though.Bioconversion. Biosystems are already operating in nature but with low and lowest efficiencies. With successful R & D to increase efficiencies, bio-energy systems seem to become a convenient way of fuel production.Economics are considered when it seems reasonable to do so, otherwise educated guesses are made as to the economics of the different decomposition techniques and their implications for the possible large-scale hydrogen production by solar energy.Some considerations are made on the influence of large-scale solar power plants on the climate.     

Hetero-nanostructured metal oxide-based hybrid photocatalysts for enhanced photoelectrochemical water splitting – A review

This review is mainly focused on nanostructured metal oxide-based efficient photocatalysts for photoelectrochemical (PEC) water splitting applications. Owing to their distinctive physical and chemical properties, metal-oxide nanostructures have attracted a wide research interest for solar power-stimulated water splitting applications. Hydrogen generation by solar energy-assisted water splitting is a clean and eco-friendly route that can solve the energy crisis and play a significant role in efforts to save the environment. In this review, synthesis strategies, control of morphology, band-gap properties, and photocatalytic application of solar water splitting using hierarchical hetero-nanostructured metal oxide-based photocatalysts, such as titanium dioxide (TiO₂), zinc oxide (ZnO), and tungsten/wolfram trioxide (WO₃), are discussed.     

Hydrogen production through solar energy water electrolysis

Various methods of making hydrogen from water have been proposed, but at the present time the only practical way to make hydrogen from water without fossil fuel is electrolysis. The development of a new, advanced, water electrolyser has become necessary for use in hydrogen energy systems and in electricity storage systems. All the new possible electrolysis processes, suitable for large-scale plants, are being analysed, in view of their combination with solar electricity source. A study of system interactions between large-scale photovoltaic plants, for electrical energy supply, and water electrolysis, is carried out. The subsystems examined include power conditioning, control and loads, as they are going to operate. Water electrolysis systems have no doubt been improved considerably and are expected to become the principal means to produce a large amount of hydrogen in the coming hydrogen economy age. Thus, the present paper treats the subject of hydrogen energy production from direct solar energy conversion facilities located on the earth's oceans and lakes. Electrolysis interface is shown to be conveniently adapted to direct solar energy conversion, depending on technical and economical feasibility aspects as they emerge from the research phases. The intrinsic requirement for relatively immense solar collection areas for large-scale central conversion facilities, with widely variable electricity charges, is given. The operation of electrolysis and photovoltaic array combination is verified at different insolation levels. Solar cell arrays and electrolysers are giving the expected results during continuously variable solar energy inputs. Future markets will turn more and more towards larger scale systems powering significantly bigger loads, ranging from hundreds of kW to several MW in size. Detailed design and close attention to subsystem engineering in the development of high performance, high efficiency photovoltaic power plants, are carried out. An overall design of a 50 MWp photovoltaic central station for electricity and hydrogen co-generation is finally discussed.     

Solar spectrum management for effective hydrogen production by hybrid thermo-photovoltaic water electrolysis

Solar Hydrogen is one of the potential key technologies to fuel human's progress. Optimizing the utilization of sunlight to produce Hydrogen using a hybrid thermo-electrolysis system is useful to promote such technology to broad deployment. Theoretically, it was found that a proper sunlight utilization management by an optimized spectral splitting of the solar spectrum between heating water to produce steam on the one hand and producing electricity via photovoltaic cell to energize the steam electrolysis on the other hand leads to an efficient sunlight to Hydrogen conversion. We report in this theoretical work that 82% sunlight to Hydrogen conversion efficiency can be accomplished from the proposed optimized hybrid thermo-photovoltaic system that employs a 90% efficient solar-thermal convertor. Additionally, it was found that for the proposed optimized hybrid system a quadratic enhancement for both the photovoltaic conversion efficiency and the net solar to hydrogen conversion efficiency can be obtained from employing more efficient solar to thermal convertor. Unlike the previous works, which have handled the optimal photon management in the hybrid thermo-photovoltaic system, our proposed optimization method accounts thoroughly the major losses in the photovoltaic conversion like the thermalization process and the limiting fill factor of the PV cell. Therefore, the methodology and the results of this work are more realistic and could be a useful recipe for an optimal sunlight spectrum management for an effective solar-hydrogen production, which could thrive as a reliable carbon-free-source of energy.     

Photochemical production of hydrogen and oxygen from water: A review and state of the art

Photochemical hydrogen production is potentially one of the most fascinating ways for solar energy conversion and storage. Since 1977, several homogeneous, quasi-homogeneous or microheterogeneous model systems of hydrogen or oxygen generation from water, by visible-light irradiation, have been proposed and are briefly reviewed. These half photosystems are based on different approaches: (i) multimolecular systems, (ii) systems involving a supramolecular structure of polyad type, and (iii) systems incorporated in organized and constrained or confined media. A survey of the different attempts for complete water splitting into hydrogen and oxygen is also made.     

Graphitic-polytriaminopyrimidine (g-PTAP): A novel bifunctional catalyst for photoelectrochemical water splitting

In this study, an electrode of g-PTAP, a novel bifunctional catalyst for photoelectrochemical was fabricated and utilized for water splitting. The graphitic-poly (2,4,6-triaminopyrimidine (g-PTAP) was synthesized by the thermal vapor condensation polymerization (TVCP) method on FTO glass. The structure, morphology, and optical characteristics of the resultant g-PTAP were analyzed using analytical techniques such as FT-IR, Raman, XRD, XPS, CHNS, FE-SEM, EDS, and DRS. The synthesized g-PTAP was graphitic with sheet-like morphology and revealed maximum light absorbance capacity in the visible range. The DFT calculation showed an appropriate HOMO-LUMO band position for overall water splitting which was verified experimentally for H₂ and O₂ generation at photocathode and photoanode, respectively. Moreover, the g-PTAP sample exhibited good photo-stability as a photocathode as compared to a photoanode. This work can provide a pathway for fabricating highly efficient semiconductor photocatalyst for overall water splitting and solar energy such as conversion.