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%.     

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.     

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.     

Monoclinic zirconia-supported Fe3O4 for the two-step water-splitting thermochemical cycle at high thermal reduction temperatures of 1400–1600 °C

Two-step thermochemical water-splitting using monoclinic ZrO₂-supported Fe₃O₄ (Fe₃O₄/m-ZrO₂) for hydrogen production was examined at high thermal reduction temperatures of 1400–1600 °C. After thermal reduction of Fe₃O₄/m-ZrO₂, the reduced sample was quenched in liquid nitrogen, and was subsequently subjected to the water-decomposition step at 1000 °C. Quenching of the solid sample was conducted for analysis of the chemical reactions, such as phase transitions, occurring at high-temperature. The hydrogen productivity of Fe₃O₄ on a m-ZrO₂ support and the conversion of Fe₃O₄ to FeO were significantly enhanced with higher thermal reduction temperatures. The Fe₃O₄-to-FeO conversion reached 60% when the Fe₃O₄/m-ZrO₂ was thermally reduced at 1600 °C. The phase transition of m-ZrO₂ support to tetragonal ZrO₂ (t-ZrO₂) did not occur during the thermal reduction at 1400–1500 °C, but it did proceed slightly at 1600 °C. Fe ions from Fe₃O₄ did not enter the ZrO₂ lattice during high-temperature thermal reduction. Thus, the Fe₃O₄ loaded on a m-ZrO₂ support can continuously contribute as a Fe₃O₄–FeO redox reactant for thermochemical water-splitting at high-temperatures of 1400–1600 °C.     

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 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.     

Characterization of Fe2O3/CeO2 oxygen carriers for chemical looping hydrogen generation

Fe₂O₃ is currently the most proper active metal oxide for chemical looping hydrogen generation (CLHG). However, supports are necessary to improve the reactivity and redox stability. CeO₂ can enhance the oxygen mobility, leading to high redox reactivity and carbon deposition resistance, which can be an excellent alternative support for oxygen carriers. In this paper, Fe₂O₃/CeO₂ oxygen carriers prepared by the co-precipitation method with different Fe₂O₃ loadings were investigated on a batch fluidized bed regarding the hydrogen yield and purity, redox reactivity and stability in CLHG with CO as fuel. The results showed that Fe6Ce4 is the best given comprehensive performance with no CO or CO₂ observed in the obtained hydrogen (detection limit 0.01% in volume). The oxygen mobility property for the reducible support CeO₂ and the physical contact between un-integrated Fe₂O₃ and CeO₂ could improve the reduction of Fe₂O₃. In addition, the formation of the hematite-like solid solution and perovskite-type CeFeO₃ could bring about abundant oxygen vacancies and promote the oxygen mobility, which contributes to the elimination of carbon deposition, counteracts the negative effect of serious sintering and guarantees the reactivity and redox stability of the Fe₂O₃/CeO₂ oxygen carriers. The Fe₂O₃/CeO₂ oxygen carriers were characterized by carbon monoxide temperature-programmed reduction measurement and X-ray diffraction patterns, and Fe6Ce4 was also selected to be characterized by scanning electron microscopy images and energy dispersive X-ray spectrometer analysis.     

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.     

Oxygen-releasing step of ZnFe2O4/(ZnO + Fe3O4)-system in air using concentrated solar energy for solar hydrogen production

The oxygen-releasing step of the ZnFe₂O₄/(ZnO + Fe₃O₄)-system for solar hydrogen production with two-step water splitting using concentrated solar energy was studied under the air-flow condition by irradiation with concentrated Xe lamp beams from a solar simulator. The spinel-type compound of ZnFe₂O₄ (Zn-ferrite) releases O₂ gas under the air-flow condition at 1800 K and then decomposes into Fe₃O₄ () and ZnO with a nearly 100% yield (ZnFe₂O₄ = ZnO + 2/3Fe₃O₄ + 1/6O₂). The ZnO was deposited as the thin layer on the surface of the reaction cell wall. A thermodynamic study showed that the ZnO was produced by the reaction between the O₂ gas in the air and the metal Zn vapor generated from ZnFe₂O₄. With the combined process of the present study on the O₂-releasing step and the previous one on the H₂ generation step (ZnO + 2/3Fe₃O₄ + 1/3H₂O = ZnFe₂O₄ + 1/3H₂) for the ZnFe₂O₄/(ZnO + Fe₃O₄)-system, solar H₂ production was demonstrated by one cycle of the ZnFe₂O₄/(ZnO + Fe₃O₄)-system, where the O₂-releasing step had been carried out in air at 1800 K and the H₂ generation step at 1100 K.     

Synergistic effect of PANI and NiFe2O4 for photocatalytic hydrogen evolution under visible light

As an effective photocatalyst, PANI/NiFe₂O₄ nanocomposite was prepared by in situ polymerization of aniline. The physicochemical properties of the composite were characterized by TEM, XRD, FT-IR spectra, UV–vis spectroscopy, XPS and Photoelectrochemical Measurements. Compared with NiFe₂O₄ and PANI, PANI/NiFe₂O₄ nanocomposite has a better photocatalytic activity, which exhibited the remarkable property of hydrogen production under visible light. The photocatalytic mechanism was also discussed. The heterojunction of PANI and NiFe₂O₄ promoted the separation of photogenerated e^? and h⁺ on the surface of PANI/NiFe₂O₄. Besides, the structure of PANI/NiFe₂O₄ in the polymerization was detected by FT-IR. NiFe₂O₄ was proved that in favor of the formation of nucleate phenazine-like structure in the progress of in situ polymerization. Then the chain structure of conductive PANI was formed, which leading to the promotion of photocatalytic activity.     

Redox cycling of CuFe2O4 supported on ZrO2 and CeO2 for two-step methane reforming/water splitting

CuFe₂O₄ supported on ZrO₂ and CeO₂ for two-step methane reforming was evaluated to determine if it could enhance the reactivity, CO selectivity and thermal stability of CuFe₂O₄. Two-step methane reforming consists of a syngas production step and a water splitting step. CuFe₂O₄ supported on ZrO₂ and CeO₂ was prepared using an aerial oxidation method. Non-isothermal methane reduction was carried out on TGA to compare the reactivity of CuFe₂O₄/ZrO₂ and CuFe₂O₄/CeO₂. In addition, a syngas production step was performed at 900 °C and water splitting was conducted at 800 °C alternatively five times to compare the methane conversion, CO selectivity, cycle ability and hydrogen production by water splitting in a fixed bed reactor. If the 1st syngas production step results are excluded due to over-oxidation, CuFe₂O₄/ZrO₂ and CuFe₂O₄/CeO₂ showed approximately 74.0–82.8% and 60.3–87.5% methane conversion, respectively, and 44.0–47.8% and 65.2–81.5% CO selectivity, respectively. Using CeO₂ and ZrO₂ as supports effectively improved the reactivity and methane conversion compared to CuFe₂O₄. CuFe₂O₄/ZrO₂ showed high methane conversion due to the high phase stability and thermal stability of ZrO₂ but the selectivity was not improved. After 5 successive cycles, the CeFeO₃ phase was found on CuFe₂O₄/CeO₂. Furthermore, methane conversion, CO selectivity and the amounts of hydrogen production of CuFe₂O4/CeO₂ increased with increasing number of cycles. Additional test up to the 11th cycle on CuFe₂O₄/CeO₂ revealed that CeO₂ is a better support that ZnO₂ in terms of the reactivity and CO selectivity.     

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.     

Solar hydrogen generation with H2O/ZnO/MnFe2O4 system

The hydrogen generation reaction in the H₂O/ZnO/MnFe₂O₄ system was studied to clarify the possibility of whether this reaction system can be used for the two-step water splitting to convert concentrated solar heat to chemical energy of H₂. At 1273 K, the mixture of ZnO and MnFe₂O₄ reacted with water to generate H₂ gas in 60% yield. X-ray diffractometry and chemical analysis showed that 48 mol% of Mn^II (divalent manganese ion) in the A-site of MnFe₂O₄ was substituted with Zn^II (divalent zinc ion) and that chemical formula of the solid product was estimated to be Zn_0.58Mn^II_0.42Mn^III_0.39Fe_1.61O₄ (Mn^III: trivalent manganese ion). Its lattice constant was smaller than that of the MnFe₂O₄ (one of the two starting materials). From the chemical composition, the reaction mechanism of the H₂ generation with this system was discussed. Since the Mn ions in the product solid after the H₂ generation reaction are oxidized to Mn³⁺, which can readily release the O²⁻ ions as O₂ gas around 1300 K, the two-step of H₂ generation and O₂ releasing seem to be cyclic.     

Reduction reactivity of CeO2-ZrO2 oxide under high O2 partial pressure in two-step water splitting process

The O₂-releasing reaction under the air with the reactive ceramics of CeO₂-ZrO₂ oxides which can be applied to solar hydrogen production via a two-step water splitting cycle using concentrated solar thermal energy was investigated. CeO₂-ZrO₂ oxides were synthesized by polymerized complex method at different Ce:Zr molar ratio. The solid solubility of ZrO₂ in fluorite structure of CeO₂ was in good agreement with the initial content of Zr ions at the preparation in CeO₂-ZrO₂ oxide. The O₂-releasing reaction in air with CeO₂-ZrO₂ oxides was studied. Different solid solubility (0%, 10%, 20%, 30%) of ZrO₂ in CeO₂ were examined. The amount of O₂ gas evolved in the reaction with Ce_1−xZr_xO₂ (0 ? x ? 0.3) solid solutions was more than that with CeO₂, and the largest yield of 2.9 cm³/g was exhibited at x = 0.2 (Ce_0.8Zr_0.2O₂) for an O₂ release at 1500 °C in air. The reduced cerium ion in Ce_0.8Zr_0.2O₂ was about 11%, which is seven times higher than that with CeO₂. The optical absorption and luminescence spectra of the CeO₂-ZrO₂ oxide obtained before and after the O₂-releasing reaction suggest that the reduction of Ce⁴⁺ with formation of oxygen defect in the air. The enhancement of the O₂-releasing reaction with CeO₂-ZrO₂ oxide is found to be caused by an introduction of Zr⁴⁺, which has smaller ionic radius than Ce³⁺ or Ce⁴⁺, in the fluorite structure.     

Cathodic shift and improved photocurrent performance of cost-effective Fe2O3 photoanodes

We successfully fabricated cost-effective and efficient pulse electrodeposited Fe₂O₃ photoanodes for PEC water splitting application. Surface modifications of Fe₂O₃ by oxygen evolution catalysts like cobalt phosphate (Co–Pi), a monoclinic BiVO₄ or both showed cathodic/anodic shift in photocurrent with significantly improved photo-response.     

In-situ La doped Co3O4 as highly efficient photocatalyst for solar hydrogen generation

Photocatalytic hydrogen production via water splitting using metal oxide semiconductors has attract great interests because of the two electrons on the kinetics. Pristine Co₃O₄ was widely studied as efficient photocatalyst, but prefers to produce oxygen due to its lower band-edge positions with regard to water redox potentials. In this work, high efficient photocatalyst basing on non-noble La doped Co₃O₄ on graphene, i.e., La_xCo_3-xO₄/G, were first reported and prepared by the microwave hydrothermal synthesis. In this newly developed hybrids, La and Co ions were adsorbed on the surface of graphene (G) and subsequently reacted with ammonia to yield the La_xCo_3-xO₄/G nanohybrid by in-situ chemical deposition methods. The activity for hydrogen generation of the nanohybrid exhibits 2 times higher than undoped Co₃O₄/G under visible light irradiation. The H₂ evolution of nanohybrid reaches 6.543 mmol g^?1 h^?1 when the molar ratio of La/Co is 10% in the nanohybrid. Our experimental results indicate the incorporation of La doped in the Co₃O₄ crystal lattice not only forms the lattice defects, resulting in provision for capture trap and the separation of electrons and holes, but also changes the band structure to eventually improve the photocatalytic activity under visible light. Therefore, non-noble La is a promising substitute to prepare highly efficient hydrogen photocatalyst and can be extendedly applied to the other metal oxide semiconductors for solar hydrogen production.     

Thermodynamic study based on the phase diagram of the Na2O–MnO–Fe2O3 system for H2 production in three-step water splitting with Na2CO3/MnFe2O4/Fe2O3

The phase diagram of the Na₂O–MnO–Fe₂O₃ system forms the basis for thermodynamic consideration of H₂ production in water splitting with the Na₂CO₃/MnFe₂O₄/Fe₂O₃ system. Sodium iron manganese oxides Na_0.71(Mn_1−x, Fe_x)O_2+δwere observed in the phase diagram at T=1273 K under P_O2=1.23×10⁻⁵ atm. Confirmation of this phase, especially for x>0.5, suggests the possibility of H₂ generation under this oxygen partial pressure and results in a value of 1:37 for the ratio of P_H2:P_H2O in the H₂ generation step of water splitting. This oxygen partial pressure is realized by the decomposition of carbon dioxide (CO₂=CO+0.5O₂) and it is concluded that the H₂ generation step can proceed under CO₂ atmosphere.     

Photoelectrochemical study on charge transfer properties of nanostructured Fe2O3 modified by g-C3N4

Novel photocatalysts, which consist of two visible light responsive semiconductors including graphite-like carbon nitride (g-C₃N₄) and Fe₂O₃, were successfully synthesized via electrodeposition followed by chemical vapor deposition. The morphology of the g-C₃N₄/Fe₂O₃ can be tuned from regular nanosheets to porous cross-linked nanostructures. Remarkably, the optimum activity of the g-C₃N₄/Fe₂O₃ is almost 70 times higher than that of individual Fe₂O₃ for photoelectrochemical water splitting. The enhancement of photoelectrochemical activity could be assigned to the morphology change of the photocatalysts and the effective separation and transfer of photogenerated electrons and holes originated from the intimately contacted interfaces. The g-C₃N₄/Fe₂O₃ composites could be developed as high performance photocatalysts for water splitting and other optoelectric devices.     

Reduction characteristics of CuFe2O4 and Fe3O4 by methane; CuFe2O4 as an oxidant for two-step thermochemical methane reforming

The reduction characteristics of CuFe₂O₄ and Fe₃O₄ by methane at 600–900 °C were determined in a thermogravimetric analyzer for the purpose of using CuFe₂O₄ as an oxidant of two-step thermochemical methane reforming. It was found that the addition of Cu to Fe₃O₄ largely affected the reduction kinetics and carbon formation in methane reduction. In the case of CuFe₂O₄, the reduction kinetics was found to be faster than that of Fe₃O₄. Furthermore, carbon deposition and carbide formation from methane decomposition were effectively inhibited. In case of Fe₃O₄, Fe metal formed from Fe₃O₄ decomposed methane catalytically, that lead to the formation of graphite and Fe₃C phases. It is deduced that Cu in CuFe₂O₄ enhanced reduction kinetics, decreased reduction temperature and prevented carbide and graphite formation. Additionally, methane conversion and CO selectivity in the syngas production step with CuFe₂O₄ were in the range of 33.5–55.6% and 54.9–59.6%, respectively.     

Photocatalytic hydrogen production with nickel oxide intercalated K4Nb6O17 under visible light irradiation

Nickel oxide nanoclusters were intercalated to layered niobate, K₄Nb₆O₁₇, to improve the photocatalytic hydrogen production for water splitting under visible light irradiation. A K₄Nb₆O₁₇–SSRx (Ni/Nb ratio range of 0.8–5%) series of nickel oxide intercalated layered niobates was prepared by a two-step solid-state reaction and characterized by Extended X-ray Absorption Fine Structure (EXAFS), Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Energy Dispersive Spectrometer (EDS), X-ray Photoelectron Spectroscopy (XPS) and Ultraviolet–visible spectroscopy (UV–vis). The photocatalytic reaction was carried out in a quartz reactor irradiated under a 500-W halogen lamp. The K₄Nb₆O₁₇–SSR0.2 catalyst exhibited a much higher photocatalytic activity for water splitting than unloaded K₄Nb₆O₁₇ catalyst and NiO_y/K₄Nb₆O₁₇ catalyst prepared by the conventional impregnation method. The high catalytic performance was attributed to the well dispersed nickel oxide nanoclusters intercalated into the bulk structure of K₄Nb₆O₁₇ catalyst and the lack of NiO particles on the external particle surface.