Ferrite/zirconia-coated foam device prepared by spin coating for solar demonstration of thermochemical water-splitting

A Ferrite/zirconia foam device in which reticulated ceramic foam was coated with zirconia-supported Fe₃O₄ or NiFe₂O₄ as a reactive material was prepared by a spin-coating method. The spin-coating method can shorten the preparation period and reduce the coating process as compared to the previous wash-coating method. The foam devices were examined for hydrogen productivity and cyclic reactivity in thermochemical two-step water-splitting. The reactivity of these foam devices were studied for the thermal reduction of ferrite on a laboratory scale using a sun simulator to simulate concentrated solar radiation, while the thermally reduced foam devices were reacted with steam in another quartz reactor under homogeneous heating in an infrared furnace. The most reactive foam device, NiFe₂O₄/m-ZrO₂/MPSZ, was tested for successive two-step water-splitting in a windowed single reactor using solar-simulated Xe-beam irradiation with a power input of 0.4-0.7 kW_th. The production of hydrogen continued successfully in the 20 cycles that were demonstrated using the NiFe₂O₄/m-ZrO₂/MPSZ foam device. The NiFe₂O₄/m-ZrO₂/MPSZ foam device produced hydrogen at a rate of 1.1-4.6 cm³ per gram of device through 20 cycles and reached a maximum ferrite conversion of 60%.     

Oxygen-releasing step of nickel ferrite based on Rietveld analysis for thermochemical two-step water-splitting

We investigated the thermal reduction (T-R) of NiFe₂O₄, either supported by m-ZrO₂ or unsupported, as the oxygen-releasing step of a solar thermochemical water splitting cycle based on a ferrite/wustite redox system, by performing the Rietveld analysis using powder X-ray diffraction. The solid materials obtained after the T-R step at 1300–1400 °C were subjected to Rietveld analysis. The amounts and chemical compositions of the wustite phase produced by the T-R step and the remaining ferrite phase were identified quantitatively. Chemical reaction formulas for the different T-R temperatures were determined from the results. Consistency for the chemical reactions of the thermal reduction was discussed and evaluated comparing the O₂ amounts predicted by the chemical reaction formulas and measured experimentally by mass spectrometry.     

Thermochemical hydrogen production from a two-step solar-driven water-splitting cycle based on cerium oxides

A new thermochemical cycle for H₂ production based on CeO₂/Ce₂O₃ oxides has been successfully demonstrated. It consists of two chemical steps: (1) reduction, 2CeO₂ → Ce₂O₃ + 0.5O₂; (2) hydrolysis, Ce₂O₃ + H₂O → 2CeO₂ + H₂. The thermal reduction of Ce(IV) to Ce(III) (endothermic step) is performed in a solar reactor featuring a controlled inert atmosphere. The feasibility of this first step has been demonstrated and the operating conditions have been defined (T = 2000 °C, P = 100–200 mbar). The hydrogen generation step (water-splitting with Ce(III) oxide) is studied in a fixed bed reactor and the reaction is complete with a fast kinetic in the studied temperature range 400–600 °C. The recovered Ce(IV) oxide is then recycled in first step. In this process, water is the only material input and heat is the only energy input. The only outputs are hydrogen and oxygen, and these two gases are obtained in different steps avoiding a high temperature energy consuming gas-phase separation. Furthermore, pure hydrogen is produced (it is not contaminated by carbon products like CO, CO₂), thus it can be used directly in fuel cells. The results have shown that the cerium oxide two-step thermochemical cycle is a promising process for hydrogen production.     

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.     

Thermochemical hydrogen production by a redox system of ZrO2-supported Co(II)-ferrite

The thermochemical two-step water splitting was examined on ZrO₂-supported Co(II)-ferrites below 1400 °C, for purpose of converting solar high-temperature heat to clean hydrogen energy as storage and transport of solar energy. The ferrite on the ZrO₂-support was thermally decomposed to the reduced phase of wustite at 1400 °C under an inert atmosphere. The reduced phase was reoxidized with steam on the ZrO₂-support to generate hydrogen below 1000 °C in a separate step. The ZrO₂-supporting alleviated the high-temperature sintering of iron oxide. As the results, the ZrO₂-supported ferrite realized a greater reactivity and a better repeatability of the cyclic water splitting than the conventional unsupported ferrites. The Co_xFe_3−xO₄/ZrO₂ with the x value of around 0.4–0.7 was found to be the promising working material for the two-step water splitting when thermally reduced at 1400 °C under an inert atmosphere.     

Solar hydrogen production by two-step thermochemical cycles: Evaluation of the activity of commercial ferrites

In this work, we report on the evaluation of the activity of commercially available ferrites with different compositions, NiFe₂O₄, Ni_0.5Zn_0.5Fe₂O₄, ZnFe₂O₄, Cu_0.5Zn_0.5Fe₂O₄ and CuFe₂O₄, for hydrogen production by two-step thermochemical cycles, as a preliminary study for solar energy driven water splitting processes. The samples were acquired from Sigma–Aldrich, and are mainly composed of a spinel crystalline phase. The net hydrogen production after the first reduction–oxidation cycle decreases in the order NiFe₂O₄ > Ni_0.5Zn_0.5Fe₂O₄ > ZnFe₂O₄ > Cu_0.5Zn_0.5Fe₂O₄ > CuFe₂O₄, and so does the H₂/O₂ molar ratio, which is regarded as an indicator of potential cyclability. Considering these results, the nickel ferrite has been selected for longer term studies of thermochemical cycles. The results of four cycles with this ferrite show that the H₂/O₂ molar ratio of every two steps increases with the number of cycles, being the total amount stoichiometric regarding the water splitting reaction. The possible use of this nickel ferrite as a standard material for the comparison of results is proposed.     

Thermochemical two-step water splitting by ZrO2-supported NixFe3−xO4 for solar hydrogen production

A thermochemical two-step water-splitting cycle using a redox metal oxide was examined for Ni(II) ferrites or Ni_xFe_3−xO₄ (0  x  1) for the purpose of converting solar high-temperature heat to hydrogen. The Ni(II) ferrite was decomposed to Ni-doped wustite (Ni_yFe_1−yO) at 1400 °C under an inert atmosphere in the first thermal-reduction step of the cycle; it was then reoxidized with steam to generate hydrogen at 1000 °C in the second water-decomposition step. Although nondoped Fe₃O₄ powders formed a nonporous, dense mass of iron oxide by the fusion of FeO and its subsequent solidification after the thermal-reduction step, Ni(II)–ferrite powders were converted into a porous, soft mass after the step. This was probably because Ni doping in the FeO phase raised the melting point of wustite above 1400 °C. Supporting the Ni(II) ferrites on m-ZrO₂ (monoclinic zirconia) alleviated the high-temperature sintering of iron oxide; as a result, the supported ferrites exhibited greater reactivity and assisted the repeatability of the cyclic water splitting process as compared to the unsupported ferrites. The reactivity increased with the doping value x, and was maximum at x = 1.0 in the Ni_xFe_3−xO₄/m-ZrO₂ system.     

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.     

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.     

Thermochemical production of hydrogen from hydrogen sulfide with iodine thermochemical cycles

With the goal of eventually developing a replacement for the Claus process that also produces $H_2$, we have explored the possibility of decomposing hydrogen sulfide through a thermochemical cycle involving iodine. The thermochemical cycle under investigation leverages differences in temperature and reaction conditions to accomplish the unfavorable hydrogen sulfide decomposition to $H_2$ and elemental sulfur over two reaction steps, creating and then decomposing hydroiodic acid. This proposed process is similar to ideas put forth in the 1980s and 1990s by Kalina, Chakma, and Oosawa, but makes use of thermochemical hydrogen iodide decomposition methods and catalysts rather than electrochemical or photoelectrochemical methods.Process models describing a potential implementation of this thermochemical cycle were created. Motivated by the process model results, experimentation showed the possibility of using alternative solvents to dramatically decrease the energy requirements for the process. Further process modeling incorporated these alternative solvents and suggests that this theoretical hydrogen sulfide processing unit has favorable economic and environmental properties.     

A comprehensive comparative energy and exergy analysis in solar based hydrogen production systems

In the presented paper, energy and exergy analysis is performed for thermochemical hydrogen (H₂) production facility based on solar power. Thermal power used in thermochemical cycles and electricity production is obtained from concentrated solar power systems. In order to investigate the effect of thermochemical cycles on hydrogen production, three different cycles which are low temperature Mg–Cl, H₂SO₄ and UT-3 cycles are compared. Reheat-regenerative Rankine and recompression S–CO₂ Brayton power cycles are considered to supply electricity needed in the Mg–Cl and H₂SO₄ thermochemical cycles. Furthermore, the effects of instant solar radiation and concentration ratio on the system performance are investigated. The integration of S–CO₂ Brayton power cycle instead of reheat-regenerative Rankine enhances the system performance. The maximum exergy efficiency which is obtained in the system with Mg–Cl thermochemical and recompression S–CO₂ Brayton power cycles is 27%. Although the energy and exergy efficiencies decrease with the increase of the solar radiation, they increase with the increase of the concentration ratio. The highest exergy destruction occurred in the solar energy unit.     

Thermodynamic viability of a new three step high temperature Cu-Cl cycle for hydrogen production

A new three step high temperature Cu-Cl thermochemical cycle for hydrogen production is presented. The performance of the proposed cycle is investigated through energy and exergy approaches. Furthermore, the effects of various parameters, such as the temperatures of the steps of the cycle and power plant efficiency, on various energy and exergy efficiencies are assessed with parametric studies. The results show that the exergy and energy efficiencies of the proposed cycle are 68.3% and 32.0%, respectively. In addition, the exergy analysis results reveal that the hydrogen production step has the maximum specific exergy destruction with a value of 150.9 kJ/mol. The results suggest that proposed cycle may provide enhanced options for high temperature thermochemical cycles by improving thermal management without causing a sudden temperature jump/fall between the hydrogen production step and other steps.     

Preparation of a microalgal photoanode for hydrogen production by photo-bioelectrochemical water-splitting

In this study, a microalga Tetraselmis subcordiformis (synonym: Platymonas subcordiformis)-based photoanode was prepared by a novel method developed in our lab. The optimal photocurrent density of microalgae photoanode, 37 μA/cm², was achieved under illumination of 145 μmol s⁻¹ m⁻² at anode potential of 0.5 V vs Ag|AgCl|sat. KCl, immobilized cell density of 2.08 × 10⁶/cm² and BQ concentration of 300 μmol/L. The results of measurements showed that oxygen evolution peak, hydrogen evolution peak and photocurrent response were all synchronous to light impulse in a three-electrode system. It revealed that there occurred a process of photo-bioelectrochemical water-splitting. Hydrogen can be produced by the method. The investigation for whole photo-bioelectrochemical process also indicated that the electrons for hydrogen evolution had two sources, microalgal metabolic process in dark condition and photosynthetic water oxidation. The photo-hydrogen evolution was twice more than hydrogen evolution in dark condition.     

Operational strategy of a two-step thermochemical process for solar hydrogen production

A two-step thermochemical cycle process for solar hydrogen production from water has been developed using ferrite-based redox systems at moderate temperatures. The cycle offers promising properties concerning thermodynamics and efficiency and produces pure hydrogen without need for product separation.     

High temperature electrolysis of hydrogen bromide gas for hydrogen production using solid oxide membrane electrolyzer

Using solid oxide membrane, this paper presents the theoretical modeling of the high temperature electrolysis of hydrogen bromide gas for hydrogen production. The electrolysis of hydrogen halides such as hydrogen bromide is an attractive process, which can be coupled to hybrid thermochemical cycles. The high temperature electrolyzer model developed in the present study includes concentration, ohmic, and activation losses. Exergy efficiency, as well as energy efficiency parameters, are used to express the thermodynamic performance of the electrolyzer. Moreover, a detailed parametric study is performed to observe the effects of various parameters such as current density and operating temperature on the overall system behavior. The results show that in order to produce 1 mol of hydrogen, 1.1 V of the applied potential is required, which is approximately 0.8 V less compared to high temperature steam electrolysis under same conditions (current density of 1000 A/m² and temperature of 1073 K). Furthermore, it is found that with the use of the presented electrolyzer, one can achieve energy and exergy efficiencies of about 56.7% and 53.8%, respectively. The results presented in this study suggest that, by employing the proposed electrolyzer, two-step thermochemical cycle for hydrogen production may become more attractive especially for nuclear- and concentrated solar-to-hydrogen conversion applications.     

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.     

Thermochemical two-step water-splitting reactor with internally circulating fluidized bed for thermal reduction of ferrite particles

A thermochemical two-step water-splitting cycle using a redox system of iron-based oxides or ferrites was examined on hydrogen productivity and reactivity of ferrite in order to convert solar energy into hydrogen in sunbelt regions. In the present paper, a new concept is proposed for a windowed thermochemical water-splitting reactor, using an internally circulating fluidized bed of NiFe₂O₄/m-ZrO₂ particles, and thermal reduction of the bed is demonstrated on a laboratory scale by using a solar-simulating Xe-beam irradiation. The concept is that concentrated solar radiation passes through the transparent window and directly heats the internally circulating fluidized bed. The fluidized bed reactor enabled the NiFe₂O₄/m-ZrO₂ sample to remain in powder form without sintering and agglomerating during direct Xe-beam irradiation over 30 min. Approximately 45% of the NiFe₂O₄ was converted to the reduced phase by the solar-simulated high-flux beam, and was then completely reoxidized with steam at 1000 °C to generate hydrogen.     

Thermochemical two-step water splitting by internally circulating fluidized bed of NiFe2O4 particles: Successive reaction of thermal-reduction and water-decomposition steps

Thermochemical two-step water splitting using a redox system of iron-based oxides or ferrites is a promising process for producing hydrogen without CO₂ emission by the use of high-temperature solar heat as an energy source and water as a chemical source. In this study, thermochemical hydrogen production by two-step water splitting was demonstrated on a laboratory scale by using a single reactor of an internally circulating fluidized bed. This involved the successive reactions of thermal-reduction (T-R) and water-decomposition (W-D). The internally circulating fluidized bed was exposed to simulated solar light from Xe lamps with an input power of 2.4-2.6 kW_th for the T-R step and 1.6-1.7 kW_th for the subsequent W-D step. The feed gas was switched from an inert gas (N₂) in the T-R step to a gas mixture of N₂ and steam in the W-D step. NiFe₂O₄/m-ZrO₂ and unsupported NiFe₂O₄ particles were tested as a fluidized bed of reacting particles, and the production rate and productivity of hydrogen and the reactivity of reacting particles were examined.     

Hydrogen production by two-step thermochemical cycles based on commercial nickel ferrite: Kinetic and structural study

One of the promising routes for hydrogen production consists of the dissociation of the water molecule through two-step thermochemical cycles based on iron oxides, i.e., ferrites. In a previous work, our group evaluated the activity of five commercial ferrites for this purpose, being Ni ferrite the one that exhibits the highest hydrogen production. In this work, the results obtained after a more exhaustive study of the thermochemical cycle based on a commercially available Ni ferrite are presented. Structural characterization of NiFe₂O₄ after each step of the thermochemical cycle is shown, as well as oxygen and hydrogen production over several cycles. In addition, kinetic parameters of this cycle are also studied, fitting the experimental data obtained under isothermal conditions to the most appropriate model among those ascribed to gas–solid non-catalytic multistep reaction systems.     

Performance assessment study of photo-electro-chemical water-splitting reactor designs for hydrogen production

Solar water splitting is considered a greatly promising technique for producing clean hydrogen fuel. However, limited studies have paid attention to the designs of photo-electrochemical (PEC) reactors. In this regard, two different designs of PEC reactor are proposed and studied numerically in the present paper. The effects of important design parameters on the system performance are also investigated. The PEC governing equations of transport phenomena related to water splitting reactor are developed and numerically solved. According to the current results, the rate of the hydrogen volume production and the solar - to - hydrogen conversion efficiency increase as an applied solar incident flux increases for both proposed designs. The solar - to - hydrogen conversion efficiencies are calculated to be 12.65% for design 1 and 12.48% for design 2. The hydrogen volume production rate is performed to achieve 78.3 L/m² h by design 1, and 74.8 L/m² h by design 2.