Recent advances in electrocatalysts for seawater splitting in hydrogen evolution reaction

Electrocatalytic water splitting is an important method to produce green and renewable hydrogen (H₂). One of the hindrances for wide applications of electrocatalysis in H₂ production is the lack of freshwater resources. Comparatively, seawater splitting has become an effective approach for large-scale H₂ production due to its abundant reserves. However, the increased complexity of seawater content emerged more problems in electrocatalytic seawater splitting. Recently, various strategies have been reported on improving the performance of electrocatalysts applied in seawater. Herein, this review firstly analyzed the mechanisms and challenges of electrocatalytic seawater splitting to evolve H₂, and summarized the recent progress on H₂ production in electrocatalytic seawater splitting. Furthermore, suggestions for future work have been provided for guidance.     

Carbon fiber supported Co/Co3O4-embedded N-doped carbon microparticles for efficient overall seawater splitting and oxygen reduction

Due to low cost and abundance, direct seawater splitting to produce hydrogen is an encouraging strategy to alleviate the consumption of fossil energy, while also avoiding freshwater stress. However, when natural seawater is utilized as the electrolyte, the catalysts on the cathode and anode of water splitting should not only have high activity to promote energy efficiency, but also have good stability and durability to resist the corrosion of chloride ions. Herein, a hierarchical carbon-based catalyst for hydrogen and oxygen evolution, ZIF-67/CF-1, was prepared by annealing a composite of ZIF-67 and carbon fiber (CF). It exhibits good electrocatalytic activity and stability for overall splitting in natural seawater and neutral PBS solution. Impressively, when an electrolyzer consisting of ZIF-67/CF-1||ZIF-67/CF-1 is applied to overall seawater splitting, the current density of 10 mA/cm² is achieved with a drive voltage of 2.46 V, which is only 0.28 V higher than precious metal-based electrolyzer (Pt/C||IrO₂). Meanwhile, ZIF-67/CF-1 shows outstanding catalytic ability for oxygen reduction (E_1/2 = 0.84 V, Tafel slope = 66.9 mV/dec), also demonstrating its application potential in rechargeable batteries and fuel cells.     

Growth and hydrogen production characteristics of Caldicellulosiruptor saccharolyticus on chemically defined minimal media

Caldicellulosiruptor saccharolyticus is an extreme thermophilic bacterium recognized for its saccharolytic ability and superior ability to produce high yields of hydrogen. However, most studies have been made using yeast extract (YE) as a rich but expensive nutrient source. For the first time, we show that C. saccharolyticus is able to grow on defined minimal media, including essential vitamins, provided that CO₂ was allowed to accumulate sufficiently in the culture broth to activate growth. Growth and hydrogen production performance on minimal media was analyzed in both batch and continuous mode. Absence of YE resulted in similar or higher hydrogen yields and specific hydrogen productivities but lower volumetric hydrogen productivities than with YE. The results also indicate that YE is used as a carbon- and energy source thus affecting metabolic flux calculations. This study clarified that YE is not essential making C. saccharolyticus more attractive for fundamental studies on its metabolism and future industrial exploitation.     

Effect of chlorine-repulsive molecular fragments on photocatalytic seawater splitting performance of BiVO4 for oxygen evolution

Photocatalysis to produce clean energy by splitting seawater have great practical importance for dealing with the energy crisis. However, seawater contains many Cl^? ions whose oxidation competes with the oxygen evolution reaction. In this work, we present a photocatalyst modified with S-containing molecular fragments on the surface to improve its efficiency in the oxygen evolution reaction in seawater splitting. We found that the oxygen evolution performance of BiVO₄ modified with S-containing molecular fragments was 1.7 times higher than the unmodified material. Based on this finding, the modification didn't affect the light absorption and charge separation efficiency of BiVO₄, but lead to a marked (83.6%) decrease of the effective chlorine concentration in the reactor after photocatalytic reaction. The results indicate that the surface modification with S-containing molecular fragments is an effective method to repel chlorine. This work provides a useful reference to improve the efficiency of photocatalysts in overall seawater splitting.     

A current perspective for photocatalysis towards the hydrogen production from biomass-derived organic substances and water

Recently, an increasing interest has been devoted to produce chemical energy – hydrogen (H₂) by converting sustainable sunlight energy via water splitting and reforming of renewable biomass-derived organic substances. These photocatalytic processes are very promising, sustainable, economic, and environment-friendly. Herein, this article gives a concise overview of photocatalysis to produce H₂ as solar fuel via two approaches: water splitting and reforming of biomass-derived organic substances. For the first approach – photocatalytic water splitting, there are two reaction types have been used, including photoelectrochemical (PEC) and photochemical (PC) cell reactions. For the second approach, biomass-derived oxygenated substrates could undergo selective photocatalytic reforming under renewable solar irradiation. Significant efforts to date have been made for photocatalysts design at the molecular level that can efficiently utilize solar energy and optimize the reaction conditions, including light irradiation, type of sacrificial reagents. Critical challenges, prospects, and the requirement to give more attention to photocatalysis for producing H₂ are also highlighted.     

Photoelectrochemical hydrogen production with concentrated natural seawater produced by membrane process

Water shortages are anticipated to occur all over the world and are likely to have a significant effect on the availability of water for processes such as photocatalysis and electrolysis, as well as for drinking and industrial water. To overcome this problem, it has been suggested that seawater could be used as an alternative resource for the various water industries, such as hydrogen production, industrial and drinking water. Seawater contains a large amount of dissolved ion components, thus allowing it to be utilized as an electrolyte in photoelectrochemical system for producing hydrogen. Especially, the concentrated shows higher salinity (total dissolved solids, TDS) than the general seawater fed to the membrane process, because the permeate has a lower salinity and the retentate is more concentrated than the original seawater. For these reasons, the hydrogen evolution rate was investigated in a photoelectrochemical system, including anodized tubular TiO₂ and platinum as the photoanode and cathode, an external bias (solar cell) and the use of various types of seawater prepared by the nanofiltration membrane process as the electrolyte in the photoelectrochemical system.The results showed that the rate of hydrogen evolution obtained using the relatively tight nanofiltration membrane, NF90, operated at 20 MPa in the photoelectrochemical system is ca. 270 μmol/cm² h, showing that the retentate with a higher TDS than the general TDS of seawater acts as a more effective seawater electrolyte for hydrogen production.     

Review on hydrogen production photocatalytically using carbon quantum dots: Future fuel

They are sometimes identified as zero-dimensional (0D) nanoparticles. These particles have gained much attention in water splitting into hydrogen and oxygen through photocatalytic conversion. CQDs act as semiconductor few nm sizes, due to very small size; their optical and electronic properties differ from larger particles. CQDs particle has high stability, mild toxicity besides conductivity. These particles are environmentally friendly due to low toxicity and also have excellent luminescence. Therefore they can be utilized as a potential source for the splitting of water photocatalytically. The parting of water into H₂ and O₂ will enable us to produce or collect hydrogen to be used as a future fuel. The review summarizes the efforts made by various researchers in the field of utilizing carbon quantum dabs for water splitting which may be further followed by future researchers for commercial-scale hydrogen production. Thus, the study concludes the methods for the production of CQDs and their utilization under sunlight by catalytically hydrogen gas production from water.     

Thermochemical two-step water splitting cycles by monoclinic ZrO2-supported NiFe2O4 and Fe3O4 powders and ceramic foam devices

A thermochemical two-step water splitting cycle is examined for NiFe₂O₄ and Fe₃O₄ supported on monoclinic ZrO₂ (NiFe₂O₄/m-ZrO₂ and Fe₃O₄/m-ZrO₂) in order to produce hydrogen from water at a high-temperature. The evolution of oxygen and hydrogen by m-ZrO₂-supported ferrite powders was studied, and reproducible and stoichiometric oxygen/hydrogen productions were demonstrated through a repeatable two-step reaction. Subsequently, a ceramic foam device coated with NiFe₂O₄/m-ZrO₂ powder was made and examined as a water splitting device by the direct irradiation of concentrated Xe-light in order to simulate solar radiation. The reaction mechanism of the two-step water splitting cycle is associated with the redox transition of ferrite/wustite on the surface of m-ZrO₂. A hydrogen/oxygen ratio for these redox powder systems exhibited good reproducibility of approximately two throughout the repeated cycles. The foam device loaded NiFe₂O₄/m-ZrO₂ powder was also successful with respect to hydrogen production through 10 repeated cycles. A ferrite conversion of 24-76% was obtained over an irradiation period of 30 min.     

Hollow heterostructure CoS/CdS photocatalysts with enhanced charge transfer for photocatalytic hydrogen production from seawater

In recent years, there have been many studies on photocatalytic water splitting, but there are still few high-efficiency photocatalysts for photocatalytic seawater splitting. In this study, a series of hollow Co sulphide-supported CdS catalyst (H–CoS/CdS) composite photocatalysts were prepared by loading CdS onto the surface of H–CoS, which can be used for efficient H₂ production in pure water and simulated seawater. The heterojunction H–CoS/CdS exhibited H₂ production of 572.4 μmol g^?1 (4 h) from simulated seawater, which is 97.7 and 2.96 times those of H–CoS and CdS, respectively. The h-CoS cocatalyst extended the light absorption range of CdS, improved the chemical stability, and significantly enhances the charge separation efficiency. This study provides guidance for the reasonable design of a photocatalytic seawater-based H₂ production catalyst with high efficiency and low cost.     

Cost effective and facile low temperature hydrothermal fabrication of Cu2S thin films for hydrogen evolution reaction in seawater splitting

Electrolysis of seawater gets an attention to produce hydrogen for renewable energy technology. It significantly reduces the use of fresh water instead of seawater. Development of low temperature fabrication of electrocatalyst can explore seawater splitting by avoiding chloride reduction during the hydrogen production. In the present work, we fabricated low temperature hydrothermal growth of Cu₂S electrocatalyst on Ni foam at constant temperature of 80 °C at different growth times of 1–3 h. The prepared Cu₂S electrocatalyst grown for 1 h exhibited low overpotentials of 76 and 118 mV at 10 mA/cm² (289 and 358 mV overpotentials at 100 mA/cm²) in 1 M KOH deionized water and seawater, respectively for hydrogen evolution reaction (HER). The Tafel plot of Cu₂S catalyst grown for 1 h showed lesser Tafel slope value of 128 mVdec^?1 than that of other growth times 2 h (136 mVdec^?1) and 3 h (142 mV dec^?1) indicating elevated electrocatalytic behaviour of Cu₂S grown for 1 h. Electrochemical impedance spectroscopy (EIS) showed charge transfer resistance of 12.8Ω, 19.6 Ω and 25.7Ω, for Cu₂S grown for 1, 2 and 3 h, respectively, this lower charge transfer resistance indicated higher charge transfer properties. The Cu₂S electrocatalyst grown for 1 h sustained retention of 80% after 12 h continuous stability test. Therefore, the cost-effective and low temperature fabrication of Cu₂S electrocatalyst proceeds for development of largescale seawater splitting for hydrogen production.     

A review on integrated thermochemical hydrogen production from water

Hydrogen production from water splitting is considered one of the most environmentally friendly processes for replacing fossil fuels. Among the various technologies to produce hydrogen from water splitting, thermochemical cycles using chemical reagents have the advantage of scale up compared to other specific facilities or geological conditions required. According to thermochemical processes using chemical redox reactions, 2-, 3-, 4-step thermochemical water splitting cycles can generate hydrogen more efficiently due to reducing temperatures. Increasing the number of cycles or steps of thermochemical hydrogen production could reduce the required maximum temperature of the facility. In addition, recently developed hybrid thermochemical processes combined with electricity or solar energy have been studied on a large scale because of the reduced cost of hydrogen production. Additionally, hybrid thermochemical water splitting combined with renewable energy can result in not only reducing the cost, but also increasing hydrogen production efficiency in terms of energy. As for a green energy, hydrogen production from water splitting using sustainable and renewable energy is significant to protect biological environment and human health. Additionally, hybrid thermochemical water splitting is conducive to large scale hydrogen production. This paper reviews the multi-step and highly developed hybrid thermochemical technologies to produce hydrogen from water splitting based on recently published literature to understand current research achievements.     

Charge-compensated codoped pseudohexagonal zinc selenide nanosheets towards enhanced visible-light-driven photocatalytic water splitting for hydrogen production

Two-dimensional (2D) pseudohexagonal ZnSe (h-ZnSe) nanosheet has been sythesized and demonstrated to be a stable, cut-price and high-efficiency photocatalyst under the irradiation of ultraviolet light. Herein, we use hybrid density functional theory to rationally design the charge-compensated codoped h-ZnSe sheets, which are simulated by substituting Sb for Se atom, and Sc or Y for Zn atom, for visible-light-driven photocatalytic water splitting to produce hydrogen. It is demonstrated that the Sc–Sb and Y–Sb codoped sheets are energetically favorable compared with Sc, Y, Sb-monodoped sheets because dopants have strong Coulombic interactions with other atoms, and moreover, have the effectively reduced bandgap, the larger absorption region of visible light, and appropriate band edges positions with respect to the water redox level. Meanwhile, compared to monodoped sheets, the codoped sheets do not introduce charge-imbalance defects or unoccupied impurity states which promote the electron-hole recombination. Furthermore, the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are easier driven by photogenerated carriers due to the largely reduced overpotentials and ample active sites on the codoped sheets. Thus, the Sc–Sb and Y–Sb codoped h-ZnSe sheets can be promising photocatalysts for visible-light-driven water decomposition to generate hydrogen.     

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.     

Exergy analysis of hydrogen production from different sulfur-containing compounds based on H2S splitting cycle

There is a tremendous demand for hydrogen production worldwide but the current H₂ production routes from natural gas and other carbon fuels lead to large greenhouse gas emissions. Intentionally coupled with nuclear power, the sulfur–iodine (S–I) thermochemical water splitting cycle is one of the most widely studied cycles for the large-scale hydrogen production that has environmental benignity. Based on the inspiration of the S–I cycle, a novel chemical cycle called hydrogen sulfide splitting cycle has been proposed for hydrogen production. In addition to the SO₂ production from the reaction of H₂S and sulfuric acid, SO₂ can be produced from the burning (direct oxidation) of hydrogen sulfide or elemental sulfur. And it can also be provided by SO₂ capture from flue gas or other SO₂-containing waste gases. This paper performs exergy analysis on the various SO₂ provisions to the Bunsen reaction that make different routes for hydrogen production from waste sulfur-containing compounds as feedstock. It has been found that the route including SO₂ from direct H₂S oxidation potentially makes the best energy-efficient process of H₂ production. The heat that is generated from H₂S oxidation can be recovered and used to support the energy requirements for other steps of the cycle, making the entire hydrogen production cycle more energy-efficient.     

Recent progresses in porphyrin assisted hydrogen evolution

The indiscriminate exploitation of fossil fuels over a period of two centuries has eventually led us to a juncture where search for alternate energy sources and sustainable development has become inevitable. Solar energy remains the most reliable renewable energy source, efficient harnessing of which can serve to meet the future energy demands. Photo-assisted water splitting to generate hydrogen, a potential clean fuel has been the focus of current research in this field. Design and development of suitable materials for efficient solar energy conversion remains the major challenge to be tackled in this aspect. A cost-effective technology for conversion of solar energy is still a distant dream. The present paper attempts a general overview of the basic principles of water splitting with special focus on porphyrin-based systems as promising water splitting systems.     

Kinetic investigations of the hydrogen production step of a thermochemical cycle using mixed iron oxides coated on ceramic substrates

A two‐step thermochemical cycle for solar hydrogen production using mixed iron oxides as the metal oxide redox system has been investigated. The ferrite is coated on a honeycomb structure, which serves as the absorber for solar irradiation and provides the surface for the chemical reaction. Coated honeycomb structures have already been tested in a solar receiver reactor in the solar furnace of DLR in Cologne with respect to their water splitting capability and their long‐term stability. The concept of this new reactor design has proven feasible and constant hydrogen production during repeated cycles has been shown. For a further optimization of the process and in order to gain reliable performance predictions more information about the process especially concerning the kinetics of the oxidation and the reduction step are essential. To examine the hydrogen production during the water splitting step a test rig has been built up on a laboratory scale. In this test rig small coated honeycombs are heated by an electric furnace. The honeycomb is placed inside a tube reactor and can be flushed with water vapour or with an inert gas. A homogeneous temperature within the sample is reached and testing conditions are reproducible. Through analysis of the product gas the hydrogen production is monitored and a reaction rate describing the hydrogen production rate per gram ferrite can be formulated. Using this test set‐up, SiC honeycombs coated with zinc ferrite have been tested. The influences of the temperature and the water concentration on the hydrogen production during the water splitting step have been investigated. An analysis of the ferrite conversion was performed using the Shrinking Core Model. A mathematical approach for the peak reaction rate at the beginning of the water splitting step was formulated and the activation energy was calculated from the experimental data. An activation energy of 110 kJ mol⁻¹ was found. Copyright © 2009 John Wiley & Sons, Ltd.     

Bi shell-BiOI core microspheres modified TiO2 nanotube arrays photoanode: Improved effect of Bi shell on photoelectrochemical hydrogen evolution in seawater

Photoelectrochemical (PEC) seawater splitting can relive the shortage of purified water feedstocks. However, the corrosion from seawater on the photoelectrode becomes an uncertain influence factor for PEC performance. Developing an efficient and stable photoelectrode is a challenge. Herein, we present a Bi–BiOI shell-core microspheres modified TiO₂ nanotube arrays (TNA) photoanode prepared via solvothermal method, affording superior PEC hydrogen evolution activity in simulated seawater under AM1.5G light, which is 3.8 and 7.6 times than those of BiOI/TNA and TNA, respectively. Solar-to-hydrogen conversion efficiency of Bi–BiOI/TNA reaches to 2.21% with Faradaic efficiency up to 85.7%. Based on the optical and PEC measurements, it is verified that surface plasmon resonance effect of metallic Bi promotes transfer and separation of photogenerated charge and enhances visible-light absorption, thus benefiting higher PEC performance. Especially, Bi shell efficiently hinders the corrosion of BiOI by seawater. Our work provides a novel paradigm of photoanode for efficient and stable PEC seawater splitting.     

Spherical Co/Co9S8 as electrocatalyst for hydrogen production from alkaline solution and alkaline seawater

Cobalt-based sulfide catalysts are considered as potential materials for electrocatalytic hydrogen production from seawater. Here, we have successfully prepared a Co/Co₉S₈ electrocatalyst with hollow spherical structure. As-prepared material exhibited excellent electrocatalytic activity in hydrogen evolution reaction (HER) in alkaline seawater. The overpotentials for Co/Co₉S₈ in alkaline seawater were measured as low as 136.2 mV, when reached a current density of 10 mA cm^{− 2}. It also had good stability and could be maintained for 24 h in 1.0 M KOH and alkaline seawater. The results of SEM and TEM confirmed that the catalyst had excellent reaction structure. Due to the hollow structure, Co/Co₉S₈ showed remarkable catalytic performance for HER. The construction method of Co/Co₉S₈ hollow structure is an effective strategy to improve the performance of HER for seawater splitting.     

An evaluation of the potential of the use of wasted hydroelectric capacity to produce hydrogen to be used in fuel cells in order to decrease CO2 emissions in Brazil

The use of water wasted in hydroelectric plants as normalization dam excess, which constitute a hydrodynamic potential useful to generate electric energy which can be subsequently used to produce hydrogen and its subsequent consumption in fuel cells, has been considered as an alternative for hydraulic energy-rich countries like Brazil. The case is examined in which all the water wasted in the hydroelectric plants, spilled by dam gates to maintain acceptable water levels, from the 101 largest Brazilian hydroelectric plants was used to produce hydrogen. During the year of 2008, the electric energy produced from this utilisation would have been equivalent to 106.2 TWh, an amount that corresponds to an increase of ca. 30% of the total electric energy produced in the country. Furthermore, if this amount of hydrogen was used in the replacement of internal combustion vehicles by fuel cells, this would have prevented the production of 2,000,000 tons of CO₂ emissions per day. The economic balance (cost of electricity produced using the wasted water minus cost of gasoline consumed) indicates a savings of ca. 200 million US$. This plan would also significantly decrease production and release of greenhouse gases.     

Oxygen permeation and coal-gas-assisted hydrogen production using oxygen transport membranes

A tubular oxygen transport membrane (OTM) was developed to produce hydrogen via water splitting using fossil sources. In this study, two OTM materials, La_0.7Sr_0.3Cu_0.2Fe_0.8O_3−δ (LSCF) and BaFe_0.9Zr_0.1O_3−δ (BFZ), were prepared by a conventional solid-state technique. In tests with an LSCF thin-film tube (thickness ≈30 μm) as an OTM, hydrogen was produced by flowing simulated product streams from coal gasification on one side of the OTM and steam on the other side. In this method, the coal gas on the oxygen-permeate side drives the removal of oxygen from the other hydrogen-generation side of the OTM, where hydrogen and oxygen are produced by water splitting. With CO (99.5% purity) flowing on the oxygen-permeate side, the hydrogen production rate of the LSCF tube was measured to be ≈19.6 cm³/min at 900 °C, indicating that hydrogen can be produced at a significant rate by using product streams from coal gasification. Concentration polarization effects were found to lower the hydrogen production rate of the LSCF thin-film tube at high temperatures. This process also yields a CO₂-rich product stream that is ready for sequestration. The other candidate OTM material, BFZ, was tested by measuring its oxygen-permeation flux, DC conductivity, and hydrogen production, and by evaluating its microstructure. The dependences of the hydrogen production rate of BFZ disks (thickness, ≈1.6 mm) on water partial pressure and temperature were determined while flowing 80% CO₂/He over a graphite rod on the oxygen-permeate side and humidified N₂ on the hydrogen-generation side. Preliminary results indicate that BFZ is a promising OTM material.