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.     

Photocatalytic properties of two-dimensional graphene and layered transition-metal dichalcogenides based photocatalyst for photoelectrochemical hydrogen generation: An overview

Hydrogen production through photoelectrochemical (PEC) water-splitting process has drawn significant research attention because it is a promising clean source of energy for improving earth climate in the future. Two-dimensional (2D) graphene and transition metal dichalcogenides (TMDCs), as the core of the system, have become versatile materials for the development of photocatalyst due to their distinct optical, electrical, thermal and mechanical properties. TMDCs have received significant consideration because of low-cost and earth-abundant catalysts that can replace noble metals, such as Pt. Therefore, comprehensive discussions on the structure and properties of 2D graphene and layered TMDCs materials are presented. We also gather and review various fabrication methods for TMDCs-based and graphene-TMDCs-based photocatalysts that can affect the PEC performance and hydrogen evolution. The inherent limitations and several future trends on 2D graphene and layered TMDCs-based photocatalyst for PEC water-splitting application are also discussed.     

Improvement of TiO2 nanotubes for photoelectrochemical water splitting: Review

Photoelectrochemical (PEC) water splitting is an ideal method to produce clean hydrogen. Developing photoelectrodes that fulfill the PEC water-splitting criteria has become the greatest challenge for commercialization of this technology. Titanium dioxide, the first material used for this application, remain appealing due to its one-dimensional nanotube structure. However, the bandgap of TiO₂ nanotubes, ~3.0 eV, is relatively wide, leading to problems such as limited utilization of light energy and easy recombination of the photogenerated products, i.e., electrons and holes. Several approaches have been developed to overcome this problem, including (i) modification of surface morphology to enhance the active catalytic area, (ii) band structure modification to reduce photogenerated charge recombination, and (iii) surface sensitization to improve light absorption ability. This review reports the improvements achieved by all of these approaches for TiO₂ nanotubes, including the basic principles of the photocatalytic water-splitting process and the preparation and polymorphs of TiO₂ nanotubes. This review also discusses combinations of several methods that enable high photocurrent density with fabulous stability.     

A low-temperature electro-thermochemical water-splitting cycle for hydrogen production based on LiFeO2/Fe redox pair

The thermochemical water-splitting cycles have been paid more attention in recent years because they directly convert thermal energy into stored chemical energy as H₂. However, most thermochemical cycles require extremely high temperatures as well as a temperature switch between reduction and oxidation steps, which are the main obstacles for their development. Herein, we introduced an electrochemical reaction into the thermochemical cycle and established a novel two-step water-splitting cycle based on LiFeO₂/Fe redox pair. The two-step water-splitting process involves a cyclic operation of electrochemical reduction and water-splitting steps. The feasibility of the water-splitting cycle for the hydrogen production was thermodynamically and experimentally investigated. A mechanism of hydrogen production based on LiFeO₂/Fe redox pair was developed. Compared with the traditional high-temperature thermochemical cycles, the electrochemical reduction and water-splitting steps of the process can be isothermally operated in the same cell at a relatively low temperature of 500 °C. The main advantages of the cycle are not only easily available heat sources without involvement of the associated engineering and materials issues, but also without any temperature swings. This is a promising method to achieve water splitting for hydrogen production in the future.     

A critical review on integrated system design of solar thermochemical water-splitting cycle for hydrogen production

The development of clean hydrogen production methods is important for large-scale hydrogen production applications. The solar thermochemical water-splitting cycle is a promising method that uses the heat provided by solar collectors for clean, efficient, and large-scale hydrogen production. This review summarizes state-of-the-art concentrated solar thermal, thermal storage, and thermochemical water-splitting cycle technologies that can be used for system integration from the perspective of integrated design. Possible schemes for combining these three technologies are also presented. The key issues of the solar copper-chlorine (Cu–Cl) and sulfur-iodine (S–I) cycles, which are the most-studied cycles, have been summarized from system composition, operation strategy, thermal and economic performance, and multi-scenario applications. Moreover, existing design ideas, schemes, and performances of solar thermochemical water-splitting cycles are summarized. The energy efficiency of the solar thermochemical water-splitting cycle is 15–30%. The costs of the solar Cu–Cl and S–I hydrogen production systems are 1.63–9.47 $/kg H₂ and 5.41–10.40 $/kg H₂, respectively. This work also discusses the future challenges for system integration and offers an essential reference and guidance for building a clean, efficient, and large-scale hydrogen production system.     

Research and development on membrane IS process for hydrogen production using solar heat

Thermochemical hydrogen production has attracted considerable interest as a clean energy solution to address the challenges of climate change and environmental sustainability. The thermochemical water-splitting iodine-sulfur (IS) process uses heat from nuclear or solar power and thus is a promising next-generation thermochemical hydrogen production method that is independent of fossil fuels and can provide energy security. This paper presents the current state of research and development (R&D) of the IS process based on membrane techniques using solar energy at a medium temperature of 600 °C. Membrane design strategies have the most potential for making the IS process using solar energy highly efficient and economical and are illustrated here in detail. Three aspects of membrane design proposed herein for the IS process have led to a considerable improvement of the total thermal efficiency of the process: membrane reactors, membranes, and reaction catalysts. Experimental studies in the applications of these membrane design techniques to the Bunsen reaction, sulfuric acid decomposition, and hydrogen iodide decomposition are discussed.     

Understanding water-splitting thermochemical cycles based on nickel and cobalt ferrites for hydrogen production

Two step water-splitting cycles by using metal ferrites are considered as a clean and sustainable hydrogen production method, when concentrated solar energy is used to drive the thermochemical reactions. This process involves the reduction at very high temperature of the ferrite, followed by the water reoxidation to the original phase at moderate temperature, with the release of hydrogen. In order to decrease the temperature required to decompose the oxide, mixed ferrites of the type MFe₂O₄ with spinel crystal structure have been examined. In this sense, ferrites with the partial substitution of Co and Ni for Fe appear as successful materials in terms of hydrogen production and cyclability. In this work, commercial Ni and synthetic Co ferrites have been subjected to two water splitting cycles. The solid products obtained after thermal reduction and water decomposition reactions have been chemically and structurally characterized by WDXRF, XRD, XPS and SEM techniques, in order to get a deeper understanding of the mechanisms controlling the water splitting process. This knowledge contributes to improve the process involved in thermochemical cycles and to understand the lower efficiencies (H₂/O₂) for Co ferrite thermochemical cycles in comparison with those corresponding to Ni ferrite.     

金属氧化物热化学循环分解水制氢热力学基础及研究进展

介绍了以金属氧化物为介质的热化学循环分解水制氢。与其它循环体系相比,金属氧化物循环仅由两步反应组成,过程简单、不向环境排放有害物质、避免了高温下分离气体的困难。研究发现,以铁酸盐为代表的复合体系有望在较温和的条件下进行反应,如果能与太阳能或者高温核反应堆耦合,则有望成为清洁的、具有经济性的制氢方法。     

An overview of photocells and photoreactors for photoelectrochemical water splitting

Solar hydrogen production from direct photoelectrochemical (PEC) water splitting is the ultimate goal for a sustainable, renewable and clean hydrogen economy. While there are numerous studies on solving the two main photoelectrode (PE) material issues i.e. efficiency and stability, there is no standard photocell or photoreactor used in the study. The main requirement for the photocell or photoreactor is to allow maximum light to reach the PE. This paper presents an overview of the PE configurations and the possible photocell and photoreactor design for hydrogen production by PEC water splitting.     

The production and application of hydrogen in steel industry

Due to the increasingly serious environmental issues and continuous depletion of fossil resources, the steel industry is facing unprecedented pressure to reduce CO₂ emissions and achieve the sustainable energy development. Hydrogen is considered as the most promising clean energy in the 21st century due to the diverse sources, high calorific value, good thermal conductivity and high reaction rate, making hydrogen have great potential to apply in the steel industry. In this review, different hydrogen production technologies which have potential to provide hydrogen or hydrogen-rich gas for the great demand of steel plants are described. The applications of hydrogen in the blast furnace (BF) production process, direct reduction iron (DRI) process and smelting reduction iron process are summarized. Furthermore, the functions of hydrogen or hydrogen-rich gas as fuels are also discussed. In addition, some suggestions and outlooks are provided for future development of steel industry in China.     

Carbon cloth/transition metals-based hybrids with controllable architectures for electrocatalytic hydrogen evolution - A review

Recently, the demand for energy consumption has been increasing exponentially due to the exhaustion of fossil fuels in the environment. This is the foremost technical challenge to the researchers to progress clean and alternative sustainable energy sources. Among various kinds of energy sources, an environment-friendly fuel, hydrogen is recognized as a favorable energy carrier to reduce the necessity on fossil fuels and protect the environment by reducing the discharge of greenhouse and other toxic gases. Thus, effective production and storage of hydrogen through a cost-effective and significant approach are the important factors of sustainable hydrogen production. Electrocatalytic water splitting is a favorable method for the hydrogen evolution reaction (HER), which requires an efficient and strong electrocatalyst to accelerate the kinetics of HER. To date, the well-developed electrocatalysts for HER activity are Pt-group metals, but, these electrocatalysts are inadequate and more expensive. In recent years, significant improvement has been achieved in the development of carbon cloth-based HER electrocatalysts as a replacement to Pt-based catalysts for hydrogen production in acidic medium. In this review, we mainly focused on the recent growth in the establishment of carbon-cloth functionalized transition metal (Fe-, Co-, Ni-, Mo-, and W-) based electrocatalysts towards the enhancement of HER activity. Depending on the results, we believed that the transition metal-based electrocatalysts have been appearing as fascinating and future alternative catalysts due to their morphology improvements, synergistic effects, a significant enhancement in the production of active sites, charge transfer efficiency, and superior HER activity with great durability. In addition, we outline the remarkable challenges and future prospects in this inspiring field.     

Analysis of copper chlorine electrolysis for large-scale hydrogen production

Nuclear hydrogen production is experiencing an unprecedented momentum worldwide, in response to the increasing demand for clean large-scale hydrogen production in line with the outcomes of the UN Conference of the Parties (COP26). A seamless integration of several innovative nuclear designs including Small Modular Reactors with steam Rankine cycle and the cogeneration of Hydrogen using thermochemical water-splitting cycles (e.g., the Cu–Cl cycle) is possible for a complete solution of hydrogen, oxygen, and electric power generation. In this paper, a process and flow sheet for large-scale hydrogen production by CuCl electrolysis at 50 tonnes per day is proposed and analyzed. The scaled-up process and flow sheet is based on lab-scale experience with 50 l/h hydrogen generation. Pressurized Cu–Cl electrolysis and basic electrolysis are reported to support the scaling up parameters, assumptions, and considerations. Based on determined sizing parameters and energy analysis, the Cu–Cl cycle consumes substantially less primary energy (thermal) than water electrolysis, which makes it a serious competitor, despite its obvious higher investment cost in the hardware.     

Hydrogen evolution in enzymatic photoelectrochemical cell using modified seawater electrolytes produced by membrane desalination process

In the near future, potential water shortages are expected to occur all over the world and this problem will have a significant influence on the availability of water for water-splitting processes, such as photocatalysis and electrolysis, as well as for drinking water. For this reason, it has been suggested that seawater could be used as an alternative for the various water industries including hydrogen production. Seawater contains a large amount of dissolved ion components, thus allowing it to be used as an electrolyte in photoelectrochemical (PEC) systems for producing hydrogen. Especially, the concentrate (retentate) stream shows higher salinity than the seawater fed to the membrane desalination process, because purified water (fresh water) is produced as the permeate stream and the waste brine is more concentrated than the original seawater. In this study, we investigated the hydrogen evolution rate in a photoelectrochemical system, including the preparation and characterization of an anodized tubular TiO₂ electrode (ATTE) as both the photoanode and the cathode with the assistance of an immobilized hydrogenase enzyme and an external bias (solar cell), and the use of various qualities of seawater produced by membrane desalination processes as the electrolyte. The results showed that the rate of hydrogen evolution obtained using the nanofiltration (NF) retentate in the PEC system is ca. 105 μmol/cm² h, showing that this is an effective seawater electrolyte for hydrogen production, the optimum amount of enzyme immobilized on the cathode is ca. 3.66 units per geometrical unit area (1 cm×1 cm), and the optimum external external bias supplied by the solar cell is 2.0 V.     

Use of bioethanol for sustainable electrical energy production

Nowadays, the climatic change has addressed the research targets to find renewable energy sources and in order to develop more efficient technologies in a simultaneous way, with the object of promoting a rational use of the energy in the frame of the sustainable development. In this case, an integrated process for sustainable electrical energy production from bioethanol was designed, taking advantage of hydrogen fuel as an energy carrier and fuel cells as efficient and clean devices. The calculated efficiency for this process is better than traditional power cycles, which constitutes a starting point for future developments of this technology.     

Revisiting solar hydrogen production through photovoltaic-electrocatalytic and photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting is regarded as a promising way for solar hydrogen production, while the fast development of photovoltaic-electrolysis (PV-EC) has pushed PEC research into an embarrassed situation. In this paper, a comparison of PEC and PV-EC in terms of efficiency, cost, and stability is conducted and briefly discussed. It is suggested that the PEC should target on high solar-to-hydrogen efficiency based on cheap semiconductors in order to maintain its role in the technological race of sustainable hydrogen production.     

Nanomaterials for photoelectrochemical water splitting – review

Photoelectrochemical (PEC) water splitting using nanomaterials is one of the promising techniques to generate hydrogen in an easier, cheaper and sustainable way. By modifying a photocatalyst with a suitable band width material can improve the overall solar-to-hydrogen (STH) energy conversion efficiency. Nanomaterials can tune their band width by controlling its size and morphology. In many studies, the importance of nanostructured materials, their morphological and crystalline effects in water splitting is highlighted. Charge separation and transportation is the major concern in PEC water splitting. Nanomaterials are having high surface to volume ratio which facilitates charge separation and suppress electron-hole pair recombination. This review focuses on the recent developments in water splitting techniques using PEC based nanomaterials as well as different strategies to improve hydrogen evolution.     

Recent advances in ammonia synthesis technologies: Toward future zero carbon emissions

As a carbon-free molecule, ammonia has gained great global interest in being considered a significant future candidate for the transition toward renewable energy. Numerous applications of ammonia as a fuel have been developed for energy generation, heavy transportation, and clean, distributed energy storage. There is a clear global target to achieve a sustainable economy and carbon neutrality. Therefore, most of the research's efforts are concentrated on generating cost-effective renewable energy on a large scale rather than fossil fuels. However, storage and transportation are still roadblocks for these technologies, for example, hydrogen technologies. Ammonia could be replaced as a viable fuel for a clean and sustainable future of global energy. More efforts from governments and scientists can lead to making ammonia a clean energy vector in most energy applications. In this review, ammonia synthesis was assessed, including conventional Haber–Bosch technology. Current hydrogen technologies as the key parameters for ammonia generation are also evaluated. The role of ammonia as a hydrogen-based fuel and generation roadmap are discussed for future utilization of energy mix. Further, ammonia generation processes are addressed in depth, including blue and green ammonia generation. A survey of ammonia synthesis catalytic materials was conducted and the role of catalyst materials in ammonia generation was compared, which showed that the Ru-based catalyst generated the maximum ammonia after 20 h of starting experiment. An end-use plan for using ammonia as a clean energy fuel in vehicles, marines, gas turbines as well as fuel cells, is briefly discussed to recognize the potential applications of ammonia use. The practical and future end-use vision of energy sources is proposed to achieve great benefits at low carbon emissions and costs. This review can provide prospective knowledge of large-scale aspects and environmental considerations of ammonia. Herein, we conclude that ammonia will become the “clean energy carrier link” that will achieve the global energy and economy sustainability targets.     

Screening of water-splitting thermochemical cycles potentially attractive for hydrogen production by concentrated solar energy

Hydrogen, a promising and clean energy carrier, could potentially replace the use of fossil fuels in the transportation sector. Currently, no environmentally attractive, large-scale, low-cost and high-efficiency hydrogen production process is available for commercialization. Solar-driven water-splitting thermochemical cycles may constitute one of the ultimate options for CO₂-free production of hydrogen. The method is environmentally friendly since it uses only water and solar energy. First, the potentially attractive thermochemical cycles must be identified based on a set of criteria. To reach this goal, a database that contains 280 referenced cycles was established. Then, the selection and evaluation of the promising cycles was performed in the temperature range of 900–2000 °C, suitable to the use of concentrated solar energy. About 30 cycles selected for further investigations are presented in this paper. The principles and basis for a thermodynamic evaluation of the cycles are also given.