Water Splitting Cycle Research Articles

Kinetics of Thermal Reduction Step of Thermochemical Two-step Water Splitting Using CeO2 Particles: MASTER-plot Method for Analyzing Non-isothermal Experiments

Thermochemical two-step water splitting cycle by concentrated solar heat has been studied as an energy conversion of solar energy into hydrogen energy. The thermochemical two-step water splitting cycle using metal oxide redox pair was composed of thermal reduction (T-R) step of metal oxide at higher temperature to release oxygen and, subsequent water decomposition (W-D) step of the reduced metal oxide with steam at lower temperature to produce hydrogen. Recently, cerium oxide (CeO2) is concerned as a redox pair of the thermochemical cycle for highly reactivity and cyclicity at high temperature. In this study, we focused a kinetic analysis of thermochemical two-step water splitting using cerium oxide. Various theoretical reaction models for thermal reduction (T-R) step of CeO2 particles are examined, and the appropriate reaction model for experimental results of thermogravimetric analysis was found by Master plot method. Finally, the reaction rate equation of thermal reduction is estimated.

Quantum Chemical Calculations on the Reaction of Zinc and Water in Gas Phase

The reaction of zinc with water is one of two steps of the Zn/ZnO water-splitting thermochemical cycle and is of interest for the hydrogen production. The reaction of zinc with water was investigated here by ab initio quantum chemical methods, and the results provided fundamental understanding of the zinc oxide particle formation in the gas phase. The geometries and frequencies of all reactants, products, intermediates, and transition states were calculated at the B3LYP/6-311G++(3df, 2p) level. The Zn and water first form an atom–molecule adduct Zn · OH2, and then either form HZnOH by an H-migration process or directly form ZnO + H2 by an H-dissociation process. The higher-level energies, barrier heights of the two paths, and rate constants were calculated. The results showed that H-migration process was dominant in the studied temperature range.

Molten copper hexaoxodivanadate: an efficient catalyst for SO3 decomposition in solar thermochemical water splitting cycles

Molten copper hexaoxodivanadate (CuV2O6) was identified as an active catalyst for SO3 decomposition, which is an oxygen generation step in solar thermochemical water splitting cycles, at moderate temperatures (ca. 600 °C). The SO3 decomposition over CuV2O6 was significantly accelerated when the reaction temperature approached the melting point (ca. 630 °C) compared with solid phases of Cu2V2O7 as well as other compounds in the CuO–V2O5 system with higher melting points (≥780 °C). A possible intermediate CuSO4 species formed by SO3 adsorption onto the Cu oxide site may decompose promptly to evolve SO2 and O2 on contact with the molten catalyst phase. Furthermore, the molten catalyst contained a large fraction of monovalent Cu formed by spontaneous desorption of oxygen. A possible reaction mechanism consisting of the fast dissolution of CuSO4 and Cu2+/Cu+ redox cycles in the melt is proposed.

Role of Oxygen Vacancies in Catalytic SO3 Decomposition over Cu2V2O7 in Solar Thermochemical Water Splitting Cycles

We report the structure–activity relationship of copper pyrovanadate (Cu2V2O7) as an efficient catalyst for SO3 decomposition in solar thermochemical water splitting cycles. Of the α, β, and γ polymorphs of Cu2V2O7, the α-phase, which has a blossite-type structure, was stable under the catalytic reaction conditions. Spontaneous oxygen desorption accompanied by charge compensation through the reduction of Cu2+ to Cu+ produced an oxygen deficiency corresponding to Cu16V16O55 at 600 °C. Density functional theory calculations based on these results showed that oxygen vacancy formation is more favorable on the Cu–O–V bridging sites than on the V–O–V site in the pyrovanadate unit. The oxygen vacancy formation energy of the (100) surface is considerably less than that of bulk Cu16V16O56. The reaction, Cu16V16O55 + SO3 → Cu16V16O56 + SO2, is exothermic, suggesting that oxygen vacancies play a key role in catalytic SO3 decomposition over a Cu2V2O7 catalyst.

Performance investigation of magnesium–chloride hybrid thermochemical cycle for hydrogen production

In this paper, we study the yields of reactants in hydrolysis and chlorination chemical processes of the low temperature Mg–Cl hybrid thermochemical cycle to investigate the requirements of temperature, pressure and product ratios for individual reactors of the cycle. A simulation of both hydrolysis and chlorination processes is conducted using the Aspen Plus software. A Mg–Cl cycle is developed by considering the results obtained from the present simulations. Both energy and exergy efficiencies of Mg–Cl cycle are comparatively evaluated under varying system and environmental parameters, and an efficiency comparison of the cycle with other promising thermochemical water splitting cycles is conducted. The results show that, compared to other cycles, lower pressure, higher temperature and higher steam to magnesium–chloride ratio are required for full conversion of reactants in the hydrolysis step; and hence, lower temperature, higher pressure and higher chlorine to magnesium oxide ratio is required for full conversion in chlorination reactor. The efficiency results show that Mg–Cl cycle can compete with other low temperature thermochemical water splitting cycles and under influence of various internal and external parameters.

S0-State Model of the Oxygen-Evolving Complex of Photosystem II

The S0 → S1 transition of the oxygen-evolving complex (OEC) of photosystem II is one of the least understood steps in the Kok cycle of water splitting. We introduce a quantum mechanics/molecular mechanics (QM/MM) model of the S0 state that is consistent with extended X-ray absorption fine structure spectroscopy and X-ray diffraction data. In conjunction with the QM/MM model of the S1 state, we address the proton-coupled electron-transfer (PCET) process that occurs during the S0 → S1 transition, where oxidation of a Mn center and deprotonation of a μ-oxo bridge lead to a significant rearrangement in the OEC. A hydrogen bonding network, linking the D1-D61 residue to a Mn-bound water molecule, is proposed to facilitate the PCET mechanism.

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.

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 ...

Numerical investigation of thermal disassociation of ZnO for hydrogen production: Parametric study and reactor configuration

SUMMARY The present research is focused on the two-step ZnO/Zn thermochemical water splitting cycle for hydrogen production. In the present paper, the numerical modeling of the first step, which involves endothermic reduction of zinc oxide (ZnO), is carried out in a cylindrical reactor using Computational Fluid Dynamics (CFD). The parametric study shows that the fractional conversion of ZnO increases with an increase in the flow rate of ZnO, while it decreases with an increase in the ZnO particle diameter and carrier gas mass flow rate. Six different reactor configurations are also assessed comprehensively. It is observed that a cylindrical reactor with a tangential inlet at the top plane and a tangential outlet at the bottom plane has higher robustness to the variation of various operating parameters with consistently high ZnO fractional conversion. Copyright © 2013 John Wiley & Sons, Ltd.

Gold peroxide complexes and the conversion of hydroperoxides into gold hydrides by successive oxygen-transfer reactions

Gold catalysts are widely studied in chemical and electrochemical oxidation processes. Computational modelling has suggested the participation of Au-OO-Au, Au-OOH or Au-OH surface species, attached to gold in various oxidation states. However, no structural information was available as isolable gold peroxo and hydroperoxo compounds were unknown. Here we report the syntheses, structures and reactions of a series of gold(III) peroxides, hydroperoxides and alkylperoxides. The Au-O bond energy in peroxides is weaker than in oxides and hydroxides; however, the Au-OH bond is also weaker than Au-H. Consequently Au-OH compounds are capable of oxygen-transfer generating gold hydrides, a key reaction in a water splitting cycle and an example that gold can react in a way that other metals cannot. For the first time it has become possible to establish a direct connection from peroxides to hydrides: Au-OO-Au→Au-OOH→Au-OH→Au-H, via successive oxygen-transfer events.

Efficiency of the sulfur–iodine thermochemical water splitting process for hydrogen production based on ADS (accelerator driven system)

The current hydrogen production is based on fossil fuels; they have a huge contribution to the atmosphere's pollution. Thermochemical water splitting cycles don't present this issue because the required process heat is obtained from nuclear energy and therefore, the environmental impact is smaller than using conventional fuels. Although, solar hydrogen production could be also used for practical applications because it's lower environmental impact. One of the promising approaches to produce large quantities of hydrogen in an efficient way using nuclear energy is the sulfur–iodine (S–I) thermochemical water splitting cycle. The nuclear source proposed in this paper is a pebble bed gas cooled transmutation facility. Pebble bed very high temperature advanced systems have great perspectives to assume the future nuclear energy. Softwares based on CPS (chemical process simulation) can be used to simulate the thermochemical water splitting sulfur-iodine cycle for hydrogen production. In this paper, a model for analyzing the sulfur-iodine process sensibility respect to the thermodynamics parameters: temperature, pressure and mass flow is developed. Efficiency is also calculated and the influence of different parameters on this value. The behavior of the proposed model for different values of initial reactant's flow, is analyzed.

3D numerical investigation of ZnO/Zn hydrolysis for hydrogen production

The current research is focused on the hydrogen production through a two-step ZnO/Zn thermochemical water splitting cycle. In the present paper, numerical modeling of the second step is conducted using Computational Fluid Dynamics (CFD)2, where steam reacts with zinc to produce hydrogen. The parametric study shows that the hydrogen yield is relatively insensitive to the steam/zinc molar ratio and inversely proportional to the argon/steam molar ratio. For large argon to steam molar ratios, hydrogen yield is relatively insensitive to the inlet temperature of zinc and steam, and increases marginally with an increase in the argon inlet temperature. Five different reactor configurations were evaluated comprehensively. Among all configurations, a cylindrical reactor with a tangential inlet for argon and zinc, and a radial inlet for steam (both in the bottom plane of the reactor) and a tangential outlet in the top plane of the reactor produced the highest hydrogen yield of 88%. Copyright © 2013 John Wiley & Sons, Ltd.

Understanding Morphology-Controlled Synthesis of Zinc Nanoparticles and Their Characteristics of Hydrolysis Reaction

Two-step thermochemical water-splitting cycle based on a Zn/ZnO redox pair is considered as a potential route for carbon-free production of hydrogen because the first hydrolysis step of the cycle highly depends on the method of preparation and the resultant particle characteristics, such as size, morphology, surface state, and initial oxide content. Here, employing a conventional evaporation and condensation method, we successfully produce three types of Zn nanoparticles ranging from nanorods, mesoporous nanorods with nanospheres on their surfaces, and fully sintered nanocrystals. The achievement in morphology control is realized simply by changing the injection position of the quenching gas. We found that the resultant hydrolysis kinetics is highly dependent on the morphology and porosity of the Zn nanoparticles. Finally, a series of simple mathematical modeling is made in an effort to understand the formation mechanism of Zn nanoparticles.

Spinel Metal Oxide-Alkali Carbonate-Based, Low-Temperature Thermochemical Cycles for Water Splitting and CO2 Reduction

A manganese oxide-based, thermochemical cycle for water splitting below 1000 °C has recently been reported. The cycle involves the shuttling of Na+ into and out of manganese oxides via the consumption and formation of sodium carbonate, respectively. Here, we explore the combinations of three spinel metal oxides and three alkali carbonates in thermochemical cycles for water splitting and CO_2 reduction. Hydrogen evolution and CO_2 reduction reactions of metal oxides with a given alkali carbonate occur in the following order of decreasing activity: Fe_(3)O_4 > Mn_(3)O_4 > Co_(3)O_4, whereas the reactivity of a given metal oxide with alkali carbonates declines as Li_(2)CO_3 > Na_(2)CO_3 > K_(2)CO_3. While hydrogen evolution and CO_2 reduction reactions occur at a lower temperature on the combinations with the more reactive metal oxide and alkali carbonate, higher thermal reduction temperatures and more difficult alkali ion extractions are observed for the combinations of the more reactive metal oxides and alkali carbonates. Thus, for a thermochemical cycle to be closed at low temperatures, all three reactions of hydrogen evolution (CO_2 reduction), alkali ion extraction, and thermal reduction must proceed within the specified temperature range. Of the systems investigated here, only the Na_(2)CO_3/Mn_(3)O_4 combination satisfies these criteria with a maximum operating temperature (850 °C) below 1000 °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.

HI decomposition over PtNi/C bimetallic catalysts prepared by electroless plating

The catalytic decomposition of hydrogen iodide (HI) has drawn increasing attention because it is the key reaction for hydrogen production in the Iodine–sulfur (IS) thermochemical water splitting cycle, which is considered one of the most promising alternative methods for massive hydrogen production with high efficiency and without CO2 emissions. Because it is very difficult for HI to decompose without the catalysts even at 500 °C, some catalysts have to be used to catalyze this reaction. In this study, four kinds of PtNi bimetallic catalysts supported on activated carbon (PtNi/C) were prepared by electroless plating and their catalytic activities were compared for HI decomposition in a fixed bed reactor at 400 and 500 °C under atmospheric pressure. Their differences in structures, surface areas, and morphology were characterized by XRD, BET and TEM, respectively. The used catalysts were also analyzed by TEM characterization in order to investigate the stability of catalysts. The results showed that the PtNi/C bimetallic catalysts are promising catalysts for HI decomposition because of their high activity and good stability, especially at high reaction temperature.

Sr- and Mn-doped LaAlO3−δ for solar thermochemical H2 and CO production

The increasing global appetite for energy within the transportation sector will inevitably result in the combustion of more fossil fuel. A renewable-derived approach to carbon-neutral synthetic fuels is therefore needed to offset the negative impacts of this trend, which include climate change. In this communication we report the use of nonstoichiometric perovskite oxides in two-step, solar-thermochemical water or carbon dioxide splitting cycles. We find that LaAlO3 doped with Mn and Sr will efficiently split both gases. Moreover the H2 yields are 9× greater, and the CO yields 6× greater, than those produced by the current state-of-the-art material, ceria, when reduced at 1350 °C and re-oxidized at 1000 °C. The temperature at which O2 begins to evolve from the perovskite is fully 300 °C below that of ceria. The materials are also very robust, maintaining their redox activity over at least 80 CO2 splitting cycles. This discovery has profound implications for the development of concentrated solar fuel technologies.

Coupling the nickel-iodine-sulphur cycle with a nuclear reactor

Nuclear hydrogen production is a technically feasible and economically viable option for addressing future energy needs. Several projects have been started on the co-generation of hydrogen and electricity from nuclear energy. In this report, the nickel sulphur iodine (NIS) cycle, a thermochemical water splitting cycle originally developed in ENEA for solar hydrogen production was studied to be coupled with a new generation nuclear reactor for massive hydrogen production.

Phase separation characteristics of HI–I2–H2SO4–H2O mixture at elevated temperatures

The iodine–sulfur (IS) thermochemical water splitting cycle is one of the most promising massive hydrogen production approaches using nuclear or solar energy. Bunsen reaction is a crucial reaction of the process by which sulfuric and hydriodic acids form. The simultaneously phase separation of the Bunsen production products, composed of HI, I2, H2SO4, and H2O are essential for the smooth operation of the process. In this work, the phase separation characteristics of the quaternary mixtures were studied at elevated temperatures of 40–80°C to clarify the phase separation characteristics. The critical conditions of the phase separation and iodine solubility in the mixture, as well as the effects of the composition and temperature on the purity of the two phases, were investigated. Favorable concentration range of each component in the quaternary solutions at various temperatures was obtained and presented in a tetrahedron diagram. The solution separates into two liquid phases spontaneously without iodine precipitation within the range. The amount of I2 should be as high as possible, with no I2 precipitation, to keep impurities at minimum content in the two phases. The results of this study offer technical basis for the design and operation of the IS process.