Two-step Water Splitting Cycle Research Articles

Enhanced hydrogen production by doping Pr into Ce0.9Hf0.1O2 for thermochemical two-step water-splitting cycle

We synthesized (Ce0.9Hf0.1)1−xPrxO2−δ (x=0, 0.05 and 0.1) using the polymerized complex method. The synthesized samples, as well as the samples after thermochemical two-step water-splitting cycles have a fluorite structure and Pr exists in the solid solutions with both trivalent and tetravalent states, as suggested by powder X-ray Diffraction (XRD) Patterns. The reduction fraction of Ce4+ in redox cycles (oxidation step in air) and two-step water-splitting cycles (oxidation step in steam) indicates that the addition of Pr into Ce–Hf oxide solid solution cannot improve the reduction fraction of Ce4+ during the redox cycles but both the reduction fraction of Ce4+ and H2 yield are significantly enhanced during two-step water-splitting cycles. The chemical composition of 10mol% Pr doped Ce0.9Hf0.1O2 exhibits the highest reactivity for hydrogen production in H2-generation step by yielding an average amount of 5.72mlg−1 hydrogen gas, which is much higher than that evolved by Ce0.9Hf0.1O2 (4.50mlg−1). The enhancement effect of doping Pr on the performance during two-step water-splitting cycles is because of the multivalent properties of Pr, which can: (1) reduce the amount of Ce3+ oxidized by contamination air (contamination air eliminated by partial oxidation of Pr3+ to Pr4+) in H2-generation step; (2) enhance the reaction rate in H2-generation step by improving the ionic conductivity (extrinsic oxygen vacancies created by the substitution of Ce4+ by Pr3+).

Reactivity of Doped Ceria-Based Mixed Oxides for Solar Thermochemical Hydrogen Generation via Two-Step Water-Splitting Cycles

Ceria-type materials were investigated as reactive chemical intermediates, in view of solar thermochemical hydrogen production via two-step water-splitting. Ceria/zirconia mixed oxides and ceria doped with yttrium, lanthanum, praseodymium, or gadolinium were studied using a thermobalance to evaluate their thermal reduction capacity in inert atmosphere and their subsequent reactivity with water steam to generate hydrogen. Ceria/zirconia materials present the highest reduction yields with a noticeable linear increase as a function of the zirconium content (in the range 0–54% Zr), while the gravimetric amount of O2 released during reduction tends to level off for Zr atomic contents above 25%. Temperature-programmed reduction experiments demonstrate that the zirconium insertion favors the bulk reduction. The addition of different dopants (among Y, La, Pr, and Gd) did not affect the global materials reducibility, although it should favor the material thermal stability during repeated cycles. The marked effect ...

Ab initiothermodynamics of intrinsic oxygen vacancies in ceria

Nonstoichiometric ceria(CeO${}_{2\ensuremath{-}\ensuremath{\delta}}$) is a candidate reaction medium to facilitate two-step water splitting cycles and generate hydrogen. Improving upon its thermodynamic suitability through doping requires an understanding of its vacancy thermodynamics. Using density functional theory (DFT) calculations and cluster expansion-based Monte Carlo simulations, we have studied the high-temperature thermodynamics of intrinsic oxygen vacancies in ceria. The DFT$+U$ approach was used to get the ground state energies of various vacancy configurations in ceria, which were subsequently fit to a cluster expansion Hamiltonian to efficiently model the configurational dependence of energy. The effect of lattice vibrations was incorporated through a temperature-dependent cluster expansion. Lattice Monte Carlo simulations using the cluster expansion Hamiltonian were able to detect the miscibility gap in the phase diagram of ceria. The inclusion of vibrational and electronic entropy effects made the agreement with experiments quantitative. The deviation from an ideal solution model was quantified by calculating as a function of nonstoichiometry, (a) the solid state entropy from Monte Carlo simulations, and (b) Warren-Cowley short range order parameters of various pair clusters.

Solar Hydrogen Productivity of Ceria–Scandia Solid Solution Using Two-Step Water-Splitting Cycle

The reactivity of CeO2–Sc2O3 solid solution for solar hydrogen production via two-step water-splitting reaction has been studied in this work. The CeO2–Sc2O3 solid solution was synthesized by polymerized complex method (PCM) with various Sc content between 0 and 20 mol. %. Analysis results from online direct gas mass spectrometry (DGMS) suggest that Ce3 + formed by CeO2–Sc2O3 solid solution in the O2-releasing step could be completely oxidized by H2O to generate hydrogen and return to Ce4 + in the H2-generation step. A Ce0.97Sc0.03O1.985 generates the largest amount of O2 and H2 among present samples, and the reduction and oxidation ratios are about 9.9% (Ce) and 10% (Ce), respectively. An estimated H2-generation reaction rate is about 4 ml g−1min−1 for Ce0.97Sc0.03O1.985. This value is about seven times greater than that of Ce0.89Zr0.11O2. The high reaction rate of Ce0.97Sc0.03O1.985 makes all formed Ce3 + completely oxidized by H2O in 5 min in the H2-generation step. The reasons for high performance are discussed from the views of lattice distortion and the amount of oxygen vacancies formed in the lattice.

Solar hydrogen production using Ce1−xLixO2−δ solid solutions via a thermochemical, two-step water-splitting cycle

The reactivity of Ce1−xLixO2−δ (x=0.025, 0.05, 0.075 and 0.1) solid solutions during the redox and two-step water-splitting cycles has been investigated in this work. Thermogravimetric analysis (TGA), X-ray diffraction (XRD) patterns and field-emission scanning electron microscopy (FE-SEM) indicate that there are two reaction mechanisms in the O2-releasing step and the shift in the reaction mechanisms occurs in the O2-releasing step because of sintering at high temperatures, and a decrease in the concentration of lattice oxygen occurs as the O2-releasing step proceeds. The reaction in the O2-releasing step follows a second-order mechanism over a temperature range of 1000–1170°C and a contracting-area model over a temperature range of 1170–1500°C. According to direct gas mass spectroscopy (DGMS), ceria doped at 5mol% Li exhibits the highest reactivity in the O2-releasing step during both redox cycles in air and two-step water-splitting cycles, whereas ceria doped at 2.5mol% Li yields the highest amount of hydrogen (4.79ml/g) in the H2-generation step during the two-step water-splitting cycles, which is higher than ceria doped with other metals. DGMS and electrochemical impedance spectroscopy (EIS) suggest that the average reaction rate in the H2-generation step is influenced by the concentration of extrinsic oxygen vacancies, and thus, the reactivity in the H2-generation step, to some degree, could be tuned by varying the concentration of extrinsic oxygen vacancies (Li content).

Reactivity of CeO 2-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 CeO 2-based ceramics in the O 2-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 H 2 evolved in the H 2-generation step is dominated by the amount of O 2 (Ce 3+) evolved in the O 2-releasing step. Electrochemical impedance spectroscopy (EIS) investigations show that the higher bulk conductivity of CeO 2-based ceramics at intermediate temperatures could promote reactivity by enhancing the molar ratio of H 2–O 2 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 0.1O 2.

4-2-3 内循環流動層反応器による二段階水熱分解反応の一段階システム化に関する研究(4-2 太陽熱発電・水素製造2,Session4 新エネルギー,研究発表)

4-2-3 内循環流動層反応器による二段階水熱分解反応の一段階システム化に関する研究(4-2 太陽熱発電・水素製造2,Session4 新エネルギー,研究発表)

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

Abstract 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 Ce1−xZrxO2 (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 (Ce0.8Zr0.2O2) for an O2 release at 1500 °C in air. The reduced cerium ion in Ce0.8Zr0.2O2 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.

Solar thermochemical process for hydrogen production via two-step water splitting cycle based on Ce1−xPrxO2−δ redox reaction

Ce1−xPrxO2−δ (x=0, 0.05, 0.1, 0.2 and 0.3), as reactive ceramics synthesized by combustion method for H2 production via two-step water splitting reaction with concentrated solar energy, was investigated in present work. XRD analysis of the samples shows that Pr6O11 underwent a phase transition into Pr2O3and Pr2O3 cannot be oxidized by steam to yield H2 in two-step water splitting reaction. The solid solutions of the samples containing Ce were formed with a single cubic CeO2 fluorite structure after synthesis and even after two-step water splitting reaction. Although Pr cannot be oxidized by steam to generate H2 in two-step water splitting reaction, the promotion effect is present with the introduction of 5mol% and 10mol% Pr into CeO2 on the amounts of O2 and H2, but the volume of H2 evolved is enhanced or not, strongly dependent on the H2-generation temperature. The optimum H2-generation temperature is 1023K.

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.

A Reactive Fe-YSZ Coated Foam Device for Solar Two-Step Water Splitting

A thermochemical two-step water-splitting cycle using a redox system of iron-based oxides or ferrites is one of the promising processes for converting solar energy into clean hydrogen in sunbelt regions. An iron-containing yttrium-stabilized zirconia (YSZ) or Fe-YSZ is a promising working redox material for the two-step water-splitting cycle. Fe2+-YSZ is formed by a high-temperature reaction between YSZ and Fe3O4 supported on the YSZ at 1400°C in an inert atmosphere. Fe2+-YSZ reacts with steam and generates hydrogen at 1000–1100°C to form Fe3+-YSZ that is reactivated by thermal reduction in a separate step at temperatures above 1400°C under an inert atmosphere. In the present study, ceramic foam coated with Fe-YSZ particles is examined as the thermochemical water-splitting device to be used in a solar directly irradiated receiver/reactor system. The Fe-YSZ particles were coated on an Mg-partially stabilized zirconia foam disk, and the foam device was tested during the two-step water-splitting cycle; this was performed alternately at temperatures between 1100°C and 1400°C. The foam device was irradiated by concentrated visible light from a sun simulator at a peak flux density of 925 kW/m2 and an average flux density of 415 kW/m2 (total power input on the surface of the foam was 0.296 kW) in a N2 gas stream; subsequently, it was reacted with steam at 1100°C while heating by an infrared furnace. Hydrogen successfully continued to be produced in the repeated cycles.

New Solar Water-Splitting Reactor With Ferrite Particles in an Internally Circulating Fluidized Bed

The thermal reduction of metal oxides as part of a thermochemical two-step water-splitting cycle requires the development of a high-temperature solar reactor operating at 1000–1500°C. Direct solar energy absorption by metal-oxide particles provides direct efficient heat transfer to the reaction site. This paper describes the experimental results of a windowed small reactor using an internally circulating fluidized bed of reacting metal-oxide particles under direct solar-simulated Xe-beam irradiation. Concentrated Xe-beam irradiation directly heats the internally circulating fluidized bed of metal-oxide particles. NiFe2O4∕m‐ZrO2 (Ni-ferrite on zirconia support) particles are loaded as the working redox material and are thermally reduced by concentrated Xe-beam irradiation. In a separate step, the thermally reduced sample is oxidized back to Ni-ferrite with steam at 1000°C. The conversion efficiency of ferrite reached 44% (±1.0%), which was achieved using the reactor at 1kW of incident Xe lamp power. The effects of preheating temperature and NiFe2O4∕m‐ZrO2 particle size on the performance of the reactor for thermal reduction using an internally circulating fluidized bed were evaluated.

Thermochemical two-step water splitting cycles by monoclinic ZrO 2-supported NiFe 2O 4 and Fe 3O 4 powders and ceramic foam devices

A thermochemical two-step water splitting cycle is examined for NiFe 2O 4 and Fe 3O 4 supported on monoclinic ZrO 2 (NiFe 2O 4/ m-ZrO 2 and Fe 3O 4/ m-ZrO 2) in order to produce hydrogen from water at a high-temperature. The evolution of oxygen and hydrogen by m-ZrO 2-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 NiFe 2O 4/ m-ZrO 2 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-ZrO 2. A hydrogen/oxygen ratio for these redox powder systems exhibited good reproducibility of approximately two throughout the repeated cycles. The foam device loaded NiFe 2O 4/ m-ZrO 2 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.

The oxygen-releasing step in the water splitting cycle by MnFe2O4–Na2CO3 system

Investigation of the feasibility of the thermochemical two-step water splitting cycle based on MnFe2O4/Na2CO3 system is reported. Influence of temperature and carbon dioxide pressure on the oxygen-releasing step was investigated. XRD analysis was applied to obtain phase identification of reacted powders at investigated experimental conditions. Different sodium sub-stoichiometric Na1−δ(Mn1/3Fe2/3)O2−δ/2 compounds were observed and their structure determined by using Rietveld analysis. Selected experimental conditions permitted to define a T/pCO2 phase diagram, showing different solid phases coexistence regions. Experimental conditions that permit complete regeneration of the initial MnFe2O4/Na2CO3 mixture were identified (field I in the reported diagram), demonstrating the possibility of full chemical cyclical operation of the system.

4-4 YSZ担持Fe_3O_4による二段階水分解サイクル((1)太陽熱発電・水素製造1,Session 4 新エネルギー)

4-4 YSZ担持Fe_3O_4による二段階水分解サイクル((1)太陽熱発電・水素製造1,Session 4 新エネルギー)

4-5 m-ZrO_2担持フェライトによる二段階水分解サイクル((2)太陽熱発電・水素製造2,Session 4 新エネルギー)

4-5 m-ZrO_2担持フェライトによる二段階水分解サイクル((2)太陽熱発電・水素製造2,Session 4 新エネルギー)

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

Abstract 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 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 NiFe 2 O 4 / m - ZrO 2 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 NiFe 2 O 4 / m - ZrO 2 sample to remain in powder form without sintering and agglomerating during direct Xe-beam irradiation over 30 min. Approximately 45% of the NiFe 2 O 4 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.

Application of an Internally Circulating Fluidized Bed for Windowed Solar Chemical Reactor with Direct Irradiation of Reacting Particles

Solar thermochemical processes require the development of a high-temperature solar reactor operating at 1000–1500°C, such as solar gasification of coal and the thermal reduction of metal oxides as part of a two-step water splitting cycle. Here, we propose to apply “an internally circulating fluidized bed” for a windowed solar chemical reactor in which reacting particles are directly illuminated. The prototype reactor was constructed in a laboratory scale and demonstrated on CO2 gasification of coal coke using solar-simulated, concentrated visible light from a sun simulator as the energy source. About 12% of the maximum chemical storage efficiency was obtained by the solar-simulated gasification of the coke.

Preparation of CuFe<SUB align=right>2O<SUB align=right>4 and its preliminary application study in two-step thermochemical water-splitting cycle

Thermochemical water splitting cycles based on metal oxides were developed. Copper ferrite (CuFe2O4) with spinel structure was prepared by the co-precipitation method, and the thermo-decomposition conditions to form the oxygen-deficient oxides by calcination were investigated. CuFe2O4 was thermochemically activated to form an oxygen-deficient one, CuFe2O4-8 under nitrogen at elevated temperature below 1000?C. The activated copper ferrite reacted with water to form hydrogen gas at about 800?C. The feasibility of the thermochemical cycle using CuFe2O4 was confirmed.