Two-step Thermochemical Water Splitting Research Articles

Towards chemical equilibrium in thermochemical water splitting. Part 2: Re-oxidation

Chemical equilibrium represents the highest efficiency achievable by a thermochemical cycle under specific operational conditions. This study delves into two-step thermochemical water splitting cycles, which dissociate water into hydrogen (H2) and oxygen (O2) via sequential thermal reduction and re-oxidation processes. Building on the thermal reduction analysis presented in Part 1, this paper zeroes in on the re-oxidation reaction within the optimal reactor framework – the counter-current reactor. It elucidates the equilibrium pathway, delineating the progression of chemical equilibrium at each incremental stage of re-oxidation. Using ceria (CeO2) as a model redox-active metal oxide, the investigation analyzes the influence of operational parameters on the re-oxidation reaction. Findings reveal the theoretical feasibility of maintaining near constant temperature (±2.5 °C) during re-oxidation, achieving a total extent of reduction of 0.038 and a hydrogen conversion yield of 32%. While near adiabatic operation is also achievable, practicality is constrained by a maximum total extent of reduction of 6.5·10−3. The study also explores the economic implications of thermochemical water splitting, particularly focusing on the steam-to-hydrogen output ratio. It highlights the severe economic hurdles associated with hydrogen yields below approximately 1%. Furthermore, the investigation reveals that the addition of inert gas to the re-oxidation step offers no significant advantages. Concluding the analysis, a comparative assessment exposes negligible differences in outcomes when substituting carbon dioxide (CO2) for water (H2O) as the oxidant in low-temperature scenarios (800 °C). This comprehensive study not only advances the understanding of re-oxidation dynamics in thermochemical water splitting but also informs practical and economic considerations crucial for the advancement of this technology.

Investigation on the activity of Ni doped Ce0.8Zr0.2O2 for solar thermochemical water splitting combined with partial oxidation of methane

The incredible potential of metal oxide-based two-step solar-driven thermochemical water splitting technology lies in its ability to harness limitless solar energy to produce clean and renewable hydrogen fuel. However, achieving high redox kinetics and lowering the reaction temperature remain the primary challenges in hydrogen production. The CeO2–ZrO2 system is a well-known oxygen carrier for thermochemical water splitting on account of its high temperature stability, quick kinetics, and redox cyclability. While recent studies have explored the combination of methane partial oxidation with metal oxide reduction, the reduction effect of cerium-zirconium oxide is relatively low, and reactivity still needs improvement. The nickel-doped system exhibits good redox performance and is regenerative. In this study, different mass percentages nickel-loaded oxygen carriers mNi/Ce0.8Zr0.2O2 (m = 0, 1, 3, 5, 7, 9) were prepared using the impregnation method. Redox properties of oxygen carriers were investigated in a fixed bed reactor and various characterizations were performed. To assess the durability of mNi/Ce0.8Zr0.2O2, 60 long-cycle experiments were also conducted. The results indicate that the lattice oxygen in mNi/Ce0.8Zr0.2O2 exhibits remarkable selectivity for the partial oxidation of CH4 at temperatures ranging from 880 to 925 °C. The best reactivity performance was achieved with a nickel loading ratio of 5 wt%, and after 60 consecutive redox cycles, 5Ni/Ce0.8Zr0.2O2 demonstrates remarkable robustness, maintaining its redox activity.

Physical system behavior and energy flow analysis of a solar driven hydrogen production plant based on two-step thermochemical redox cycles

Two-step thermochemical water splitting using concentrated solar power offers promising potential for green hydrogen production as a part of a future sustainable energy economy. In this study, a system and energy flow analysis of a large scale 250 kW reactor prototype is presented by using a complex 3D-simulation model. Thus, the physical dynamic system behavior and potential efficiency improvements are discussed with respect to plant design and operational strategy. Regarding the plant design, different kind of heat losses as well as the influence of reactor design parameters and material modifications are analyzed. Regarding the operational strategy, crucial control parameters as cycle durations, temperatures, and mass flow rates are varied in a broad range and analyzed with respect to plant efficiency. By comparing commonly used operational strategies, the temperature swing operation turns out to be by far more efficient than any isothermal or near-isothermal operation for the current plant concept.

Techno-economic assessment of green hydrogen production via two-step thermochemical water splitting using microwave

This study evaluates a two-step thermochemical water-splitting method for green hydrogen production and considers the economic feasibility of technically available designs under harsh hydrogen production conditions. As layouts of hydrogen production, two thermochemical water–splitting systems are evaluated in this study. One system is the process via high temperature from solar concentration power systems. The other system uses microwaves for thermochemical water splitting under low temperatures from advanced nuclear power plants. As part of the hydrogen production system, possible solid–solid and fluid–fluid heat recuperators of printed circuit heat exchanger (PCHE) are proposed and evaluated through the effectiveness-number of transfer units (ε-NTU) method and logarithmic mean temperature difference (LMTD) method. The required heat transfer area and volume are calculated according to the operating conditions and considered in the economic assessment of the hydrogen production system. Optimum geometries of the PCHE are proposed considering the cost analysis. The Levelized cost of hydrogen (LCOH) and system efficiency are calculated for the conventional system with solar power and system-using microwave with HTGR. The importance of heat recuperation systems is confirmed in that they account for approximately 10–20% of the cost for both system layouts. To evaluate the technology development level to achieve the ultimate target, LCOH according to various cost factors is evaluated and further research areas essential for commercialization are represented.

The role of entropy in the success of nonstoichiometric oxides for two-step thermochemical water and CO2 splitting

Owing to their high theoretical efficiency, two-step solar thermochemical water and CO2 splitting cycles, using a metal oxide intermediate, provide a promising pathway to store solar energy in a stable chemical bond. To date, the most successful demonstrations have utilized nonstoichiometric oxides, a class of materials capable of continuously transitioning from an oxidized to reduced state via formation of oxygen vacancies. The success of nonstoichiometric oxides is typically attributed to their ability to maintain their crystallographic phase, allowing them to be cycled many times without destabilizing–a tremendous practical advantage. In this work, we utilize a combined empirical and statistical thermodynamics approach to present an alternative explanation of the success of nonstoichiometric oxides from an entropy perspective. We first illuminate the importance of the entropy change of the reduction reaction as a key material parameter. We then develop a methodology for plotting nonstoichiometric oxides on the Ellingham diagram, enabling a direct comparison to stoichiometric materials. This analysis reveals the unique ability of nonstoichiometric oxides to achieve a significant solid-state entropy contribution, which results from the disorder generated via oxygen vacancy formation. To quantify the solid-state entropy, we develop a simple configurational entropy model applicable to any oxygen-deficient nonstoichiometric oxide. We compare model predictions to existing data for two important nonstoichiometric oxides, CeO2−δ and La0.6Ca0.4Mn0.6Al0.4O3−δ, to reveal the main trends in entropy vs δ and material composition and discuss the causes and implications of deviations from the theory.

EFFECT of SYNTHESIS PARAMETERS on the CRYSTAL STRUCTURE of La1-xCaxMnyAl1-y (LCMA)

Hydrogen is an essential substance for green-energy applications. The production of hydrogen-based on renewable energy sources has a critical role in this context. Thermochemical methods based on solar energy are getting attention for hydrogen production in a sustainable manner. It is possible to produce hydrogen without the need of purification via two-step thermochemical water splitting (TWS) method. The thermodynamics and kinetics of redox reactions in active materials used are the important factors for determining the hydrogen production efficiency. The structural stability is another concern in the TWS reactions. The efficiency is strongly influenced by the structural properties of active materials used in these reactions. In this regard, perovskite-oxides draw attention as an active material that can be used in TWS reactions due to their superior structural stability together with their compositional diversity. In this study, it was aimed to investigate the effect of synthesis parameters on the structural properties of La0.4Ca0.6Mn0.6Al0.4O3 (LCMA4664) and La0.2Ca0.8Mn0.8Al0.2O3 (LCMA2882) perovskite-type oxides that offer high hydrogen production efficiency by TWS. It was found that different stoichiometry in LCMA oxide family has an effect on the resulting crystal structure together with the synthesis parameters.

Efficient generation of hydrogen by two-step thermochemical cycles: Successive thermal reduction and water splitting reactions using equal-power microwave irradiation and a high entropy material

Hydrogen production through two-step thermochemical water splitting cycle based on metal oxide has emerged as a promising strategy to store dilute and intermittent solar energy. However, a typical reaction time of the two-step thermochemical water splitting cycle is lengthy, with at least 0.5 h for thermal reduction step and 1 h for water splitting step, and the energy required in the thermal reduction process for hydrogen regeneration is higher than the generated hydrogen energy. In this work, to overcome the problem of an energy efficiency imbalance, we investigated the possibility of the rapid and successive reactions of thermal reduction and water splitting, using short-term irradiation from a low-energy microwave. To this end, a high entropy material, as a poly-metal oxide used to generate hydrogen, was produced by simultaneously introducing four cations onto a SiC foam – (FeMgCoNi)O1.2@SiC. It was found that the oxygen vacancy of the (FeMgCoNi)O1.2@SiC could be significantly increased by short-term microwave irradiation, and hence the thermal reduction process took only 4 min, which is much less than normal. High H2 generation rates were achieved by re-oxidation of the Fe (II) to Fe (III) of the (FeMgCoNi)O1.2@SiC. In addition, the microwave plasma generated by microwave irradiation induced (FeMgCoNi)O1.2@SiC discharge could enhance the water splitting process. The maximum hydrogen yield was 122 mL/g at 700 W, due to the coupling effect of the thermochemical cycle and microwave plasma. In this way, the power consumption of microwave process is only 3% of that of conventional high-temperature heat treatment during thermal reduction process.

CeTi2O6-A Promising Oxide for Solar Thermochemical Hydrogen Production.

A large entropy of reduction is crucial in achieving materials capable of high-efficiency solar thermochemical hydrogen (STCH) production through two-step thermochemical water splitting cycles. We have recently demonstrated that the onsite electronic entropy of reduction attains an extreme value of 4.26 kB at 1500 K in Ce4+ → Ce3+ redox reactions, which explains the high performance and uniqueness of CeO2 as an archetypal STCH material. However, ceria requires high temperatures (T > 1500 °C) to achieve a reasonable reduction extent because of its large reduction enthalpy, which is a major obstacle in practical applications. Therefore, new materials with a large entropy of reduction and lower reduction enthalpy are required. Here, we perform a systematic screening to search for Ce4+-based oxides which possess thermodynamics superior to CeO2 for STCH production. We first search the Inorganic Crystal Structure Database (ICSD) and literature for Ce4+-based oxides and subsequently use density functional theory to compute their reduction enthalpies (i.e., oxygen vacancy formation energies). We find that CeTi2O6 with the brannerite structure is the most promising candidate for STCH because it possesses three essential characteristics of an STCH material: (i) a smaller reduction enthalpy compared to ceria yet large enough to split water, (ii) a high thermal stability, as reported experimentally, and (iii) a large entropy of reduction associated with Ce4+ → Ce3+ redox. Our proposed design strategy suggests that further exploration of Ce4+ oxides for STCH production is warranted.

A rotating fluidized bed reactor for rapid temperature ramping in two-step thermochemical water splitting

Two-step thermochemical water splitting (TWS) is a promising carbon-free/low-carbon technology for producing hydrogen in a mass production scale, in which water is dissociated in the presence of metal oxide-based catalysts via redox cycles driven by thermal energy. While active research is underway to develop high-performance metal oxide catalysts, less attention has been paid to reactor design and relevant system analysis, which are essential for constructing an actual system. The gas and solid flow configuration is one of the key design parameters in reactor design that determines thermodynamic and kinetic characteristics of the entire two-step TWS system. In this study, we propose a rotating fluidized bed reactor design wherein the rotating current flow configuration allows a much larger relative velocity between sweep gas and redox particles compared to conventional flow configurations. The rotating current flow configuration significantly improves the temperature ramp rate of redox particles via enhanced heat transfer between particles and sweep gas. Through thermodynamic and kinetics analysis of the reactor system, we show that the large temperature ramp rate of the proposed reactor results in a considerable improvement in the hydrogen yield per hour. This work adds a new dimension to the reactor design for two-step TWS.

Thermochemical two-step water splitting cycle using perovskite oxides based on LaSrMnO3 redox system for solar H2 production

A thermochemical two-step water-splitting cycle using perovskite oxides based on LaSrMnAlO3 was examined for hydrogen production from water using concentrated solar radiation. Concurrent Sr and Mn substitutions in La1-xSrxMnxAl1-xO3, Sr and Al substitutions in La1-ySryMn1-yAlyO3, and substitution of Al by Cr in La0.7Sr0.3Mn1-zCrzO3 were examined, and their kinetics, oxygen/hydrogen productivity, and repeatability were compared against LaSrMnAlO3. The impact of the chemical compositions of the perovskite oxides was systematically examined at a thermal reduction temperature of 1350 °C and water decomposition temperatures of 1000–1200 °C. From the viewpoints of average amounts of oxygen and hydrogen produced and the H2/O2 ratios, La0.7Sr0.3Mn0.9Cr0.1O3 and La0.7Sr0.3Mn0.8Cr0.2O3 provided the most reproducible productions of oxygen and hydrogen in the thermochemical two-step water-splitting cycle.

Moving Brick Receiver–Reactor: A Solar Thermochemical Reactor and Process Design With a Solid–Solid Heat Exchanger and On-Demand Production of Hydrogen and/or Carbon Monoxide

Three crucial aspects still to be overcome to achieve commercial competitiveness of the solar thermochemical production of hydrogen and carbon monoxide are recuperating the heat from the solid phase, achieving continuous or on-demand production beyond the hours of sunshine, and scaling to commercial plant sizes. To tackle all three aspects, we propose a moving brick receiver–reactor (MBR2) design with a solid–solid heat exchanger. The MBR2 consists of porous bricks that are reversibly mounted on a high temperature transport mechanism, a receiver–reactor where the bricks are reduced by passing through the concentrated solar radiation, a solid–solid heat exchanger under partial vacuum in which the reduced bricks transfer heat to the oxidized bricks, a first storage for the reduced bricks, an oxidation reactor, and a second storage for the oxidized bricks. The bricks may be made of any nonvolatile redox material suitable for a thermochemical two-step (TS) water splitting (WS) or carbon dioxide splitting (CDS) cycle. A first thermodynamic analysis shows that the MBR2 may be able to achieve solar-to-chemical conversion efficiencies of approximately 0.25. Additionally, we identify the desired operating conditions and show that the heat exchanger efficiency has to be higher than the fraction of recombination in order to increase the conversion efficiency.

Iron precipitated onto ceria-zirconia nanoparticle mixtures for the production of hydrogen via two-step thermochemical water splitting

Several novel materials were synthesized by precipitating iron oxide (using the previously optimized 10% Fe loading by weight) onto mixtures of nanoparticle zirconia and ceria to investigate the effects of adding CeO2 to FeOx/ZrO2 materials in the thermochemical water splitting reaction. At water splitting temperatures of 1000 °C (after thermal reduction at 1450 °C), the stability of the CeO2-containing materials was lower than for the FeOx/ZrO2 material, and there was no advantage to adding CeO2 to the FeOx/ZrO2 material. However, when operating at a water splitting (WS) temperature of 1200 °C, the stability increased and the hydrogen production was significantly higher over most materials compared with a water splitting temperature of 1000 °C. At a WS temperature of 1200 °C the FeOx/Zr75Ce25O2 (75% Zr75O2 and 25% CeO2 by weight) and FeOx/Zr50Ce50O2 materials performed slightly better than the FeOx/ZrO2 material, and X-ray photoelectron spectroscopy data revealed that the surface concertation of iron is less important compared with water splitting at 1000 °C. The temperature programmed reduction data indicated that the FeOx-CeO2 interactions were weaker compared with FeOx-ZrO2 interactions, since the FeOx reduction occurred at lower temperatures for the CeO2-containing materials. The weaker interactions can explain why the stability was lower for the materials containing CeO2 (sintering of FeOx was likely more pronounced) The X-ray diffraction data revealed that ZrO2-CeO2 solid solutions formed after activation at 1450 °C and lattice volume calculations indicated that iron did incorporate into the ZrO2-CeO2 matrices. More incorporation was observed after water-splitting at 1200 °C compared with a lower temperature (1000 °C), and likely explains why the materials were more stable during water-splitting at 1200 °C.

Notable hydrogen production on LaxCa1−xCoO3 perovskites via two-step thermochemical water splitting

High-performance thermochemical water splitting catalyst is the key in solar-driven H₂ production for the development of sustainable and clean energy technology. Perovskite oxides have been considered promising redox catalysts for two-step thermochemical H₂O splitting cycles due to their remarkable oxygen exchange capacity at low thermal heating temperatures. This study is the first to investigate perovskite series of La₁₋ₓCa ₓ CoO₃ for two-step thermochemical H₂O splitting cycles. The Ca doping contents in La₁₋ₓCa ₓ CoO₃ perovskites showed a significant effect on the O₂ and H₂ production performances. Increasing the Ca doping content has greatly increased O₂ evolution during the thermal reduction process. However, high Ca dopant content significantly weakened the reaction thermodynamics of the subsequent H₂O splitting and led to lower re-oxidation yields. After tuning the Ca doping level from 0.2 to 0.8, La₀.₆Ca₀.₄CoO₃ was identified as the best trade-off among the tested La₁₋ₓCa ₓ CoO₃ perovskites. The thermal reduction and water splitting temperatures were also systematically investigated to optimize the thermochemical operational conditions. La₀.₆Ca₀.₄CoO₃ showed maximum H₂ production of 587 µmol g⁻¹ when the two-step thermochemical H₂O splitting carried out between 1300 and 900 °C, eighteen times higher than that of CeO₂ under the same experimental condition. More importantly, La₀.₆Ca₀.₄CoO₃ also exhibited fairly good catalytic stability during the thermochemical cycling test and has strong potential for long-term applications.

The use of poly-cation oxides to lower the temperature of two-step thermochemical water splitting

We report the discovery of a new class of oxides – poly-cation oxides (PCOs) – that consist of multiple cations and can thermochemically split water in a two-step cycle to produce hydrogen (H2) and oxygen (O2).

Nickel ferrite supported on calcium-stabilized zirconia for solar hydrogen production by two-step thermochemical water splitting

Solar hydrogen production by thermochemical cycles from water using oxide redox pairs represents a promising technology at medium-long term to storage intermittent solar irradiation. Redox oxides based on nickel ferrite supported on modified zirconia have been prepared and studied for renewable H2 production from water by two-step thermochemical cycles. We report herewith a new material based on nickel ferrite supported on Ca-doped zirconia substrate that presents high efficiency for hydrogen production, which is ascribed to a high active phase dispersion promoted by the inclusion of part of Fe cations in the support lattice and by the formation of calcium zirconate, acting as a physical barrier that hinders sintering.

Giant onsite electronic entropy enhances the performance of ceria for water splitting

Previous studies have shown that a large solid-state entropy of reduction increases the thermodynamic efficiency of metal oxides, such as ceria, for two-step thermochemical water splitting cycles. In this context, the configurational entropy arising from oxygen off-stoichiometry in the oxide, has been the focus of most previous work. Here we report a different source of entropy, the onsite electronic configurational entropy, arising from coupling between orbital and spin angular momenta in lanthanide f orbitals. We find that onsite electronic configurational entropy is sizable in all lanthanides, and reaches a maximum value of ≈4.7 kB per oxygen vacancy for Ce4+/Ce3+ reduction. This unique and large positive entropy source in ceria explains its excellent performance for high-temperature catalytic redox reactions such as water splitting. Our calculations also show that terbium dioxide has a high electronic entropy and thus could also be a potential candidate for solar thermochemical reactions.

La1-xCaxMn1-yAlyO3 perovskites as efficient catalysts for two-step thermochemical water splitting in conjunction with exceptional hydrogen yields

Solar-driven thermochemical water splitting represents one efficient route to the generation of H2 as a clean and renewable fuel. Due to their outstanding catalytic abilities and promising solar fuel production capacities, perovskite-type redox catalysts have attracted significant attention in this regard. In the present study, the perovskite series La1-xCaxMn1-yAlyO3 (x, y = 0.2, 0.4, 0.6, or 0.8) was fabricated using a modified Pechini method and comprehensively investigated to determine the applicability of these materials to solar H2 production via two-step thermochemical water splitting. The thermochemical redox behaviors of these perovskites were optimized by doping at either the A (Ca) or B (Al) sites over a broad range of substitution values, from 0.2 to 0.8. Through this doping, a highly efficient perovskite (La0.6Ca0.4Mn0.6Al0.4O3) was developed, which yielded a remarkable H2 production rate of 429 μmol/g during two-step thermochemical H2O splitting, going between 1400 and 1000 °C. Moreover, the performance of the optimized perovskite was found to be eight times higher than that of the benchmark catalyst CeO2 under the same experimental conditions. Furthermore, these perovskites also showed impressive catalytic stability during two-step thermochemical cycling tests. These newly developed La1-xCaxMn1-yAlyO3 redox catalysts appear to have great potential for future practical applications in thermochemical solar fuel production.

4-2-2 二段階水熱分解セラミックの高活性化に関する研究(4-2 太陽熱発電・水素製造2,Session 4 新エネルギー,研究発表)

4-2-2 二段階水熱分解セラミックの高活性化に関する研究(4-2 太陽熱発電・水素製造2,Session 4 新エネルギー,研究発表)

4-4-5 45kW_ KIER太陽炉によるCeO_2/MPSZ反応デバイスを用いた水分解サイクルの太陽試験(4-4 太陽熱・水素製造,Session 4 新エネルギー)

4-4-5 45kW_ KIER太陽炉によるCeO_2/MPSZ反応デバイスを用いた水分解サイクルの太陽試験(4-4 太陽熱・水素製造,Session 4 新エネルギー)

4-10-4 フェライト内循環流動層反応器の光照射による二段階水熱分解(4-10 太陽熱発電・水素製造2,Session 4 新エネルギー,研究発表)

4-10-4 フェライト内循環流動層反応器の光照射による二段階水熱分解(4-10 太陽熱発電・水素製造2,Session 4 新エネルギー,研究発表)