Thermochemical two-step water-splitting for hydrogen production using Fe-YSZ particles and a ceramic foam device

Fe₃O₄ supported on cubic yttria-stabilized zirconia (Fe₃O₄/c-YSZ) is proposed as a promising redox material for the production of hydrogen from water via a thermochemical two-step water-splitting cycle. In this study, the evolution of oxygen and hydrogen during the cyclic reaction was examined using Fe₃O₄/c-YSZ particles in order to demonstrate reproducible and stoichometric oxygen/hydrogen production through a repeatable two-step reaction. Subsequently, a ceramic foam device coated with Fe₃O₄ and c-YSZ particles was prepared and examined as a thermochemical water-splitting device in a directly irradiated receiver/reactor hydrogen production system. The Fe₃O₄/c-YSZ system formed a Fe-containing YSZ (Fe-YSZ) by high-temperature reaction between Fe₃O₄ and the c-YSZ support at 1400 °C in an inert atmosphere. The reaction mechanism of the two-step water-splitting cycle is associated with the redox transition of Fe²⁺–Fe³⁺ ions in the c-YSZ lattice. The Fe-YSZ particles exhibit good reproducibility for reaction with a hydrogen/oxygen ratio of approximately 2.0 throughout repeated cycles. The foam device coated with Fe-YSZ particles was also successful for continual hydrogen production through 32 repeated cycles. A 20–27% ferrite conversion was obtained using 10.5 wt% Fe₃O₄ loading over an irradiation period of 60 min.     

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.     

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.     

Two-step water splitting thermochemical cycle based on iron oxide redox pair for solar hydrogen production

This study deals with solar hydrogen production from the two-step iron oxide thermochemical cycle (Fe₃O₄/FeO). This cycle involves the endothermic solar-driven reduction of the metal oxide (magnetite) at high temperature followed by the exothermic steam hydrolysis of the reduced metal oxide (wustite) for hydrogen generation. Thermodynamic and experimental investigations have been performed to quantify the performances of this cycle for hydrogen production. High-temperature decomposition reaction (metal oxide reduction) was performed in a solar reactor set at the focus of a laboratory-scale solar furnace. The operating conditions for obtaining the complete reduction of magnetite into wustite were defined. An inert atmosphere is required to prevent re-oxidation of Fe(II) oxide during quenching. The water-splitting reaction with iron(II) oxide producing hydrogen was studied to determine the chemical kinetics, and the influence of temperature and particles size on the chemical conversion. A conversion of 83% was obtained for the hydrolysis reaction of non-stoichiometric solar wustite Fe_(1−y)O at 575 °C.     

Comparative study of activity of cerium oxide at thermal reduction temperatures of 1300–1550 °C for solar thermochemical two-step water-splitting cycle

Thermochemical two-step water-splitting using CeO₂ (cerium oxide) particles was studied to examine oxygen and hydrogen productivity and repeatability at thermal reduction (T-R) temperatures of 1300–1550 °C and water decomposition (W-D) temperatures of 400–1000 °C for the production of hydrogen from water using concentrated solar radiation as the energy source. The temperature dependency of oxygen and hydrogen productivity and the cyclic repeatability of CeO₂ are reported in this paper. The characteristic features of CeO₂ particles in the thermochemical two-step water-splitting cycle are compared with the well-known highly active reactive mediums of zirconia-supported Ni-ferrites (NiFe₂O₄/m-ZrO₂ and NiFe₂O₄/c-YSZ) and unsupported NiFe₂O₄.     

Heliostat field layout optimization for high-temperature solar thermochemical processing

The layout of the heliostat field of solar tower systems is optimized for maximum annual solar-to-chemical energy conversion efficiency in high-temperature thermochemical processes for solar fuels production. The optimization algorithm is based on the performance function that includes heliostat characteristics, secondary optics, and chemical receiver-reactor characteristics at representative time steps for evaluating the annual fuel production rates. Two exemplary applications for solar fuels production are selected: the thermal reduction of zinc oxide as part of a two-step water-splitting cycle for hydrogen production, and the coal gasification for syngas production.     

Low-temperature water-splitting by sodium redox reaction

Thermochemical water-splitting by sodium redox reactions was investigated from material science point of view as a future hydrogen production method. The reaction system consists of three separate reactions, which are hydrogen generation by NaOH-Na reaction, metal separation by thermolysis of Na₂O, and oxygen generation by hydrolysis of Na₂O₂. Although the current techniques of thermochemical water-splitting required a temperature higher than 800 °C for whole reaction cycle, the sodium system was able to be operated below only 400 °C by using nonequilibrium techniques to control the entropy of the chemical reactions. Therefore, this system should be recognized as a potential water-splitting technique that can widely utilize any heat sources in contrast to the conventional methods.     

Solar thermochemical ZnO/ZnSO4 water splitting cycle for hydrogen production

In this paper, solar reactor efficiency analysis of the solar thermochemical two-step zinc oxide–zinc sulfate (ZnO–ZnSO₄) water splitting cycle. In step-1, the ZnSO₄ is thermally decomposed into ZnO, SO₂, and O₂ using solar energy input. In step-2, the ZnO is re-oxidized into ZnSO₄ via water splitting reaction producing H₂. The ZnSO₄ is recycled back to the solar reactor and hence can be re-used in multiple cycles. The equilibrium compositions associated with the thermal reduction and water-splitting steps are identified by performing HSC simulations. The effect of Ar towards decreasing the required thermal reduction temperature is also explored. The total solar energy input and the re-radiation losses from the ZnO–ZnSO₄ water splitting cycle are estimated. Likewise, the amount of heat energy released by different coolers and water splitting reactor is also determined. Thermodynamic calculations indicate that the cycle (η_cycle) and solar-to-fuel energy conversion efficiency (η_{solar-to-fuel}) of the ZnO–ZnSO₄ water splitting cycle are equal to 40.6% and 48.9% (without heat recuperation). These efficiency values are higher than previously investigated thermochemical water splitting cycles and can be increased further by employing heat recuperation.     

Features of solar thermochemical redox cycles for hydrogen production from water as a function of reactants’ main characteristics

Promising applications of concentrated solar energy are thermochemical cycles based on metal-metal oxide redox reactions for hydrogen production. These cycles usually consist of two steps: metal hydrolysis followed by solar reduction or thermal decomposition of the metal oxide. Thermodynamic analysis sustained by experimental results obtained for different reactants such as boron, zinc, tin and cadmium indicates that the cycle efficiency essentially depends on molar weight, valence, vapor pressure of reduced metals at reaction temperature, and strength of metal-oxygen (Me-O) bonds. Metals with lower molecular weight-to-valence ratio and stronger Me-O bonds demonstrate higher hydrogen productivity, better conversion and larger amount of heat released during the hydrolysis reaction. However, they require higher temperatures or multiple steps for the reduction of their oxides.This paper compares previously published results about these two-step processes complemented by more recent ones and describes the main aspects of selecting solid reactants to enable effective, reliable and safe operation of both the hydrolysis and the reduction steps.     

Comparative study of the activity of nickel ferrites for solar hydrogen production by two-step thermochemical cycles

In this work, we compare the activity of unsupported and monoclinic zirconia – supported nickel ferrites, calcined at two different temperatures, for solar hydrogen production by two-step water-splitting thermochemical cycles at low thermal reduction temperature. Commercial nickel ferrite, both as-received and calcined in the laboratory, as well as laboratory made supported NiFe₂O₄, are employed for this purpose. The samples leading to higher hydrogen yields, averaged over three cycles, are those calcined at 700 °C in each group (supported and unsupported) of materials. The comparison of the two groups shows that higher chemical yields are obtained with the supported ferrites due to better utilisation of the active material. Therefore, the highest activity is obtained with ZrO₂-supported NiFe₂O₄ calcined at 700 °C.     

Uranium thermochemical cycle: hydrogen production demonstration

In hydrogen production industry, thermochemical cycle technology for converting thermal energy into chemical storage energy of hydrogen owns absolute advantages. Compared with other thermochemical cycles, thermochemical cycle technology based on uranium (UTC) is safer and more efficient. This technology consists of three steps, where only the hydrogen production step is unique. In this paper, the verification has been done for this step. Solid products were characterized by XRD and Raman spectroscopy, which were confirmed to be α-Na₂U₂O₇. Gas chromatographic analyses were performed for gas samples, in which hydrogen output was obtained using an internal standard method.     

Thermochemical hydrogen production using manganese cobalt spinels as redox materials

Mn and Co spinels (Mn_3-xCo_xO₄) were thermally reduced and subsequently oxidized showing successful hydrogen production from water splitting by a three-step thermochemical cycle involving sodium hydroxide (NaOH). The spinel materials overcome the main limitation of the Mn₂O₃/MnO redox cycle, reducing the required temperatures from 1300–1400 °C to 850–1050 °C. Additionally, the reduction process takes place through a single step reaction, avoiding the formation of intermediate species that makes much more complex the chemistry of the Mn₂O₃ redox cycle. On the other hand, the subsequent reaction with NaOH allows a hydrogen production of 52.5 cm³ STP/g_material·cycle, which is comparable to the obtained with other spinel-oxide cycles at similar temperature. The cyclability and stability of the hydrogen production with these materials after operation of several cycles have been assessed in a high temperature tubular furnace.     

Catalytic investigation of ceria-zirconia solid solutions for solar hydrogen production

This study addresses the solar thermochemical production of hydrogen from water-splitting cycles using ceria-zirconia solid solutions prepared via soft chemistry methods. The effect of zirconium doping on the catalytic activity of ceria for hydrogen production was studied using thermogravimetric analysis. The influence of the zirconium content between 10% and 50% on the redox properties of the Ce_1−δZr_δO₂ material was investigated. The higher the amount of zirconium, the higher the reduction yields. The reduction yield at 1400 °C in inert atmosphere was 9% for 10% Zr, 16% for 25% Zr, and 28% for 50% Zr. However, increasing the Zr content did not automatically lead to the highest amount of hydrogen produced during cycling. Indeed, the powder with 25% Zr produced 334 and 298 μmol H₂/g at 1050 °C during the first and the second cycle, respectively. In contrast, the powder with 50% Zr yielded 468 and 266 μmol H₂/g during the two successive cycles. Moderate Zr contents thus favored H₂ production during repeated cycles without any significant reactivity losses. A kinetic study of the reduction and the hydrolysis steps was proposed. The activation energies for the thermal reduction and the hydrolysis of Ce_0.75Zr_0.25O₂ were 221 kJ/mol and 51 kJ/mol, respectively. Finally, the use of a template molecule during synthesis was considered, which improved the reduction yield markedly (up to 52%) but strong sintering phenomena limited the hydrogen production and the material cyclability.     

Hydrogen production via solar-aided water splitting thermochemical cycles: Combustion synthesis and preliminary evaluation of spinel redox-pair materials

Redox-pair-based thermochemical cycles are considered as a very promising option for the production of hydrogen via renewable energy sources like concentrated solar energy and raw materials like water. This work concerns the synthesis of various spinel materials of the iron and aluminum families via combustion reactions in the solid and in the liquid-phase and the testing of their suitability as redox-pair materials for hydrogen production by water splitting via thermochemical cycles. The effects of reactants' stoichiometry (fuel/oxidizer) on the combustion synthesis reaction characteristics and on the products' phase composition and properties were studied. By fine-tuning the synthesis parameters, a wide variety of single-phase, pure and well crystallized spinels could be controllably synthesized. Post-synthesis, high-temperature calcination studies under air and nitrogen at the temperature levels encountered during solar-aided thermochemical cyclic operation have eliminated several material families due to phase composition instabilities and identified among the various compositions synthesized NiFe₂O₄ and CoFe₂O₄ as the two most suitable for cyclic water splitting – thermal reduction operation. First such thermochemical cyclic tests between 800 and 1400 °C with NiFe₂O₄ and CoFe₂O₄ in powder form in a fixed bed laboratory reactor have demonstrated capability for cyclic operation and alternate hydrogen/oxygen production at the respective cycle steps for both materials. Under the particular testing conditions the two materials exhibited hydrogen/oxygen yields of the same magnitude and similar temperatures of oxygen release during thermal reduction.     

Experimental evaluation and energy analysis of a two-step water splitting thermochemical cycle for solar hydrogen production based on La0.8Sr0.2CoO3-δ perovskite

A study of the hydrogen production by thermochemical water splitting with a commercial perovskite La_0.8Sr_0.2CoO_3-δ(denoted as LSC) under different temperature conditions is presented. The experiments revealed that high operational temperatures for the thermal reduction step (>1000 °C) implied a decrease in the hydrogen production with each consecutive cycle due to the formation of segregated phases of Co₃O₄. On the other hand, the experiments at lower thermal reduction operational temperatures indicated that the material had a stable behaviour with a hydrogen production of 15.8 cm³ STP/g_material·cycle during 20 consecutive cycles at 1000 °C, being negligible at 800 °C. This results comparable or even higher than the maximum values reported in literature for other perovskites (9.80–10.50 STP/g_material·cycle), but at considerable lower temperatures in the reduction step of the thermochemical cycle for the water splitting (1000 vs 1300–1400 °C). The LSC keeps the perovskite type structure after each thermochemical cycle, ensuring a stable and constant H₂ production. An energy and exergy evaluation of the cycle led to values of solar to fuel efficiency and exergy efficiency of 0.67 and 0.36 (as a percentage of 1), respectively, which are higher than those reported for other metal oxides redox pairs commonly found in the literature, being the reduction temperature remarkably lower. These facts point out to the LSC perovskite as a promising material for full-scale applications of solar hydrogen production with good cyclability and compatible with current concentrating solar power technology.     

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.     

A decade of ceria based solar thermochemical H2O/CO2 splitting cycle

Since 2006, ceria is used as a redox reactive material for production of H₂, CO, and syngas via a two-step solar driven thermochemical H₂O/CO₂ splitting cycle. Different forms of phase pure ceria were studied over a wide range of temperatures and oxygen partial pressures. To increase the redox reactivity and long-term stability, the effects of incorporation of different dopants in to the ceria fluorite structure (in varying proportions) were studied in detail. A variety of solar reactors, loaded with ceria based ceramics, were designed and developed to investigate the performance of these materials towards thermal reduction and H₂O/CO₂ splitting reactions. The thermodynamics and reaction kinetics of ceria based solar thermochemical H₂O/CO₂ splitting cycles were also explored heavily. This paper presents a detailed chronological insight into the development of ceria-based oxides as reactive materials for solar fuel production via thermochemical redox H₂O/CO₂ splitting cycles.     

Water-splitting mechanism analysis of Sr/Ca doped LaFeO3 towards commercial efficiency of solar thermochemical H2 production

Solar thermochemical (STC) technology utilizes the entire spectrum of solar energy to decompose water to produce hydrogen. This technology reduces carbonic fuels, nearly only producing hydrogen rather than hydrogen-oxygen mixture. However, low water-splitting activity of redox materials restricts improvement of water-hydrogen conversion ratio and fuel production efficiency. Recently, a kind of perovskite LaFeO₃ attracts attention, because of the good performance in photocatalysis hydrogen production. Nevertheless, how LaFeO₃ system works in STC water-splitting cycle is rarely studied. In this paper, the first principle method at density functional theory level is adopted to reveal the hydrogen production mechanism of perovskite LaFeO₃ doped with 25% Sr/Ca at A site. Hydrogen migration on material surface determines hydrogen generation rate. The activation energy of 25%-Ca-doped LaFeO₃ is relatively lower 150.09 kJ/mol. In addition, fuel production efficiency has been calculated. When water to hydrogen conversion ratio is 100%, solar-to-fuel efficiency can reach maximum 0.472. The effect of water-splitting kinetics on hydrogen production is also discussed. The results indicate that when T_red = T_oxi = T = 1200K and water to hydrogen conversion ratio is 10%, the dynamic efficiency of La_0.75Ca_0.25FeO₃ can reach 20%. This research can provide index for improving the hydrogen production performance of STC technology.     

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%).