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.     

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.     

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.     

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.     

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.     

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.     

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 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.     

Reactivity of CeO2-based ceramics for solar hydrogen production via a two-step water-splitting cycle with concentrated solar energy

Ce_0.9M_0.1O_2−δ ceramics (M = Mg, Ca, Sr, Sc, Y, Dy, Zr and Hf) were synthesized by a polymerized complex method. X-ray powder diffraction (XRD) patterns indicate that solid solutions with a fluorite structure were formed after the synthesis, and this structure was retained after redox cycles. An analysis of the redox cycles using a direct gas mass spectrometer (DGMS) suggests that the reactivity of CeO₂-based ceramics in the O₂-releasing step could be enhanced by doping the ceramics with cations with a higher valence and a smaller effective ionic radius. The investigation of two-step water-splitting cycles indicates that the amount of H₂ evolved in the H₂-generation step is dominated by the amount of O₂ (Ce³⁺) evolved in the O₂-releasing step. Electrochemical impedance spectroscopy (EIS) investigations show that the higher bulk conductivity of CeO₂-based ceramics at intermediate temperatures could promote reactivity by enhancing the molar ratio of H₂–O₂ that is evolved during the two-step water-splitting cycles. The highest reactivity, both in the redox and in the two-step water-splitting cycles, is exhibited by Ce_0.9Hf_0.1O₂.     

Reaction characteristics of two-step methane reforming over a Cu-ferrite/Ce–ZrO2 medium

Methane was reformed over a Cu-ferrite/ZrO₂ medium in a two-step process, consisting of a syn-gas production step and a water-splitting step. In the syn-gas production step, increase in Cu content in the Cu_xFe_3−xO₄/ZrO₂ medium suppressed carbon deposition and enhanced the reaction rate for stoichiometries of x ≤ 0.7. In the water-splitting step, the addition of Cu promoted the gasification of the deposited carbon. Furthermore, the addition of Ce as a binder in the Cu_0.7Fe_2.3O₄/Ce–ZrO₂ medium also improved reactivity in the syn-gas production step and yielded the highest reactivity when the molar ratio of Ce/Zr was 3/1. As a result of the co-addition of Cu and Ce, the Cu_0.7Fe_2.3O₄/Ce–ZrO₂ medium showed high durability, with a constant evolution of the synthesis gas and hydrogen in ten repeated cycles. It is thus expected that the Cu-ferrite/Ce–ZrO₂ medium is favourable for two-step methane reforming.     

Binary nickel ferrite oxide (NiFe2O4) nanoparticles coated on reduced graphene oxide as stable and high-performance asymmetric supercapacitor electrode material

Transition metal oxides are nowadays one of the most important materials in the manufacture of capacitive electrodes. The most important problems with these materials for applied energy storage devices are low specific energy and poor electrical conductivity. In this research nickel ferrite nanoparticles (NiFe₂O₄) and also hybrid of NiFe₂O₄/rGO are synthesized by hydrothermal method and characterized by XRD, Raman, and XPS analysis. The amount of porosity and specific surface area is studied by BET analysis as and surface morphology is studied by SEM and TEM. To investigate the effect of adding rGO to NiFe₂O₄ nanoparticles, from a hybrid electrode superconducting electrochemical tests are performed, including CV, EIS, and charge-discharge. This electrode with a capacitance of 584.63 F/g and capacitance retention of 91% after 2000 consecutive cycles can be a tempting option for supercapacitor applications.     

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.     

Two-step water-splitting at 1273–1623 K using yttria-stabilized zirconia-iron oxide solid solution via co-precipitation and solid-state reaction

Reactive ceramics are investigated for potential use in a rotary-type solar reactor. The two-step water-splitting process, which consists of O₂-releasing (MO_oxidized=MO_reduced+1/2O₂) and H₂-generation (MO_reduced+H₂O(g)=MO_oxidized+H₂) reactions with yttria-stabilized zirconia (YSZ)-iron oxide solid solutions prepared by co-precipitation and solid-state reaction, is examined at temperatures of 1623 K for O₂ release and 1273 K for H₂ generation. The YSZ-iron oxide solid solutions with a single phase are obtained at mole ratios lower than 15% and 20% of iron ions to total cations (Fe³⁺, Zr⁴⁺, Y³⁺) by co-precipitation and the solid-state reaction, respectively. The two-step water-splitting process using YSZ-iron oxide solid solutions prepared by both preparation methods are repeated successfully. The amount of O₂ gas evolved per weight of the sample (ml/g) is observed to increase with the iron content of the YSZ-iron oxide solid solution because of the high reactivity of iron ions in the solid solution. The maximum amounts of H₂ and O₂ gases evolved in the two-step water-splitting process with the YSZ-iron oxide solid solution were 0.89 and 1.2 ml/g, respectively.     

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.     

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.     

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.     

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.     

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.     

Visible light-driven hydrogen evolution by using mesoporous carbon nitride-metal ferrite (MFe2O4/mpg-CN; M: Mn,Fe, Co and Ni) nanocomposites as catalysts

Four different earth-abundant ferrite nanoparticles (MFe₂O₄, M: Mn, Fe, Co, Ni) with spinel structure were synthesized by using the surfactant-assisted high temperature thermal decomposition methods and then assembled on mesoporous graphitic carbon nitride (mpg-CN) to study their comparative catalysis for the photocatalytic hydrogen evolution reaction (HER) in the presence of Eosin-Y (EY) as a visible-light sensitizer. The yielded monodisperse ferrite nanoparticles and the MFe₂O₄/mpg-CN nanocomposites were characterized by using advanced analytical techniques including TEM, XPS, XRD, ICP-MS, and UV–Vis DRS. All the tested MFe₂O₄/mpg-CN nanocomposites provided the better catalytic performance than that of pristine mpg-CN in the photocatalytic HER and their photocatalytic HER rates are in the order of NiFe₂O₄/mpg-CN > CoFe₂O₄/mpg-CN > MnFe₂O₄/mpg-CN > Fe₃O₄/mpg-CN > mpg-CN. Among the tested MFe₂O₄/mpg-CN nanocomposites, NiFe₂O₄/mpg-CN nanocomposite provided the highest hydrogen generation of 14.56 mmol g⁻¹, which is 6.75 times greater than that of pristine mpg-CN and, using EY as a visible light sensitizer and triethanolamine (TEOA) as a sacrificial reagent. According to the optical properties and energy band positions of the nanocomposites, a plausible mechanism for the NiFe₂O₄/mpg-CN catalyzed HER is proposed to give insights on the highest activity of NiFe₂O₄/mpg-CN nanocomposites among others.     

Study of NiFe2O4 nanoparticles optical properties by a six-flux radiation model towards the photocatalytic hydrogen production

At present, synthesis of visible-light active materials for hydrogen production through water splitting has become one of the most important challenges in photocatalysis. Of equal importance, photocatalyst optical properties are essential information in order to determine reaction kinetics involved in water splitting. Transition metals spinel ferrites MFe₂O₄ exhibit remarkable activity under visible light for this reaction. However, their optical properties have not been fully determined in photocatalytic kinetic studies. In this research, synthesized and commercial NiFe₂O₄ nanoparticles were compared. NiFe₂O₄ was synthesized by the Pechini method and both materials were studied: structural, textural and optical characterization was performed through XRD, TEM, TGA, BET surface area and UV/Vis spectroscopy and photocatalytic evaluation was performed for hydrogen production. NiFe₂O₄ optical properties were determined by UV/Vis spectroscopy and using a simplified theoretical model for the Radiative Transfer Equation (RTE) called Six-Flux Model (SFM). All performed characterizations and obtained coefficients for the ferrites were compared finding differences between absorption and scattering coefficients, which were attributed to the porosity of the synthesized ferrite.