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Hydrogen production from water splitting is considered one of the most environmentally friendly processes for replacing fossil fuels. Among the various technologies to produce hydrogen from water splitting, thermochemical cycles using chemical reagents have the advantage of scale up compared to other specific facilities or geological conditions required. According to thermochemical processes using chemical redox reactions, 2-, 3-, 4-step thermochemical water splitting cycles can generate hydrogen more efficiently due to reducing temperatures. Increasing the number of cycles or steps of thermochemical hydrogen production could reduce the required maximum temperature of the facility. In addition, recently developed hybrid thermochemical processes combined with electricity or solar energy have been studied on a large scale because of the reduced cost of hydrogen production. Additionally, hybrid thermochemical water splitting combined with renewable energy can result in not only reducing the cost, but also increasing hydrogen production efficiency in terms of energy. As for a green energy, hydrogen production from water splitting using sustainable and renewable energy is significant to protect biological environment and human health. Additionally, hybrid thermochemical water splitting is conducive to large scale hydrogen production. This paper reviews the multi-step and highly developed hybrid thermochemical technologies to produce hydrogen from water splitting based on recently published literature to understand current research achievements.

Hydrogen is a clean, renewable secondary energy source. The development of hydrogen energy is a common goal pursued by many countries to combat the current global warming trend. This paper provides an overview of various technologies for hydrogen production from renewable and non-renewable resources, including fossil fuel or biomass-based hydrogen production, microbial hydrogen production, electrolysis and thermolysis of water and thermochemical cycles. The current status of development, recent advances and challenges of different hydrogen production technologies are also reviewed. Finally, we compared different hydrogen production methods in terms of cost and life cycle environmental impact assessment. The current mainstream approach is to obtain hydrogen from natural gas and coal, although their environmental impact is significant. Electrolysis and thermochemical cycle methods coupled with new energy sources show considerable potential for development in terms of economics and environmental friendliness.

Biomass and organic solid waste are considered as very potential alternative energy sources in the future, leading to the realization of a clean and CO2-free energy system. Therefore, the effective conversion of biomass and organic solid waste to a secondary energy source is urgently demanded. In addition, hydrogen is considered very promising among the secondary energy sources due to its advantages of cleanliness, wide range of conversion and utilization technologies, high energy efficiency, and high gravimetric energy density. This paper reviews several possible routes and key conversion technologies of biomass and organic solid waste to hydrogen. Recent progress related to biological and thermochemical conversion technologies is described. Thermochemical route includes gasification, pyrolysis, steam reforming, partial oxidation, and thermochemical cycle; while biological route covers fermentation (dark and photo), biophotolysis (direct and indirect), enzymatic, and microbial electrolysis. In addition, several challenges regarding the conversion and utilization of biomass and organic solid waste to hydrogen are also discussed in order to clarify the feasibility of biomass and the organic solid waste-based hydrogen economy.

A novel hybrid combination of the copper–chlorine (Cu–Cl) thermochemical cycle and the Goswami cycle is planned for electrical, cooling, and hydrogen production. In this cycle, geothermal (renewabl...

In this paper, neutronic analysis and hydrogen production potential of a laser inertial fusion driver (LIFE) fueled molten salt thorium was investigated. Neutronic calculations were performed with MCNP code. The thorium molten salt was investigated both in six different ratios (2% ThF4 - 98% Li2BeF4 coolant, 4% ThF4 - 96% Li2BeF4, 6% ThF4 - 94% Li2BeF4, 8% ThF4 - 92% Li2BeF4, 10% ThF4 - 90% Li2BeF4, 12% ThF4 - 88% Li2BeF4) and two different fuel zone thickness (50-100 cm). In addition, Li-6 enrichment (in 15-25-50-75 and 90% vol. ratios) in molten salts were examined effect to neutronic performance. As a result of the neutronic assessments, hydrogen production potential of the laser driver fusion breeder was investigated by using three step copper chlorine (Cu-Cl) cycle that one of the thermochemical water splitting methods. As a result, it was observed that tritium-breeding ratio (TBR) increased with increasing fuel zone thickness and Li-6 enrichment, and it decreased with increasing ThF4 ratio. The energy multiplication value (M) has changed in direct proportion to fuel zone thickness, Li-6 enrichment and ThF4 ratio. Since produced hydrogen mass flow depend on energy multiplication value, the highest hydrogen mass flow rate ((m) over dot(H2)) was observed in option 100 cm fuel zone thickness, 12% ThF4 ratio and 90% ratio Li-6 enrichment. (C) 2021 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

In this work, a novel thermochemical cycle (Boron-based) to produce ammonia is coupled with chemical looping combustion (CLC) process to produce final primary products of ammonia, CO2, water, and electricity. Manganese oxide-based CLC provides high purity N2, water and thermal energy for the carbothermal reduction of liquefied natural gas (LNG) occurring at 1200 °C. Gaseous synthesis gas from the carbothermal reduction is used as a fuel in the CLC's fuel reactor. Ammonia is produced through the hydrolysis of boron nitride (BN) and liquefied at atmospheric pressure. Thermodynamic equilibrium computations are used to predict the conversions of reactions involved in this proposed system. The overall system is then evaluated from energetic and exergetic perspectives to reflect upon the efficiency of reactors and subsystems. The production of approximately 25 metric t/h of NH3 is achieved while power production reaches 232 MW. The exergetic efficiency of the overall system is calculated to be 53.8%. Moreover, life cycle assessments are performed to assess boron oxide environmental impacts and evaluated the exergy-based allocation of greenhouse gases emission to ammonia at 0.772 kg CO2 (eq.)/kg NH3. About 61% reduction in emissions relative to the global average of ammonia synthesis is estimated.

Recently, we reported detailed investigations on a hydrolysis step of Cu–Cl thermochemical cycle (Singh et al. in Int J Energy Res 44:2845–2863, 2020) where we demonstrated CuCl2 hydrolysis using a fixed-bed reactor under optimized conditions. Thermolysis of CuCl2 and oxide formation are associated hindrances in achieving 100% phase-pure hydrolysis product, Cu2OCl2. With an objective to understand the thermolysis and hydrolysis processes at molecular level, both these reactions were investigated independently in the present study using in situ XAS as a probe. The progress of both the reactions was recorded from RT to 400 °C by monitoring temperature-dependent evolution of Cu species using Cu K-edge XAS measurements at BL-09, Indus-2 SRS, RRCAT, Indore, India. XAS results are also corroborated with ex situ analysis by XRD and TG of samples containing various compositions of CuCl2 mixed with boron nitride (pellets employed for XAS). Besides LCF, hydrolysis product Cu2OCl2 yield was further supported by chemical titrations. EXAFS revealed that with increasing temperature, coordination of Cu atoms in reactant CuCl2 progressively decreased during thermolysis. For hydrolysis process, coordination of Cu atoms in Cu–Cu, Cu–O and Cu–Cl linkages approached towards that of Cu2OCl2 and CuO. CuCl2 diminished and transformed into CuCl (88 mass%) at 350 °C during thermolysis and ~ 60 mass% of Cu2OCl2 and 40 mass% of CuO during hydrolysis reaction. Based on in situ investigations in the present study, the most probable reactions taking place during CuCl2 hydrolysis at different temperatures are proposed for S/Cu of 14.6.

La0·3Sr0·7Co0·7Fe0·3O3 (LSCF3773) was chosen as an oxygen carrier material for synthesis gas production and synthesized using ethylene-diamine-tetra-acetic acid (EDTA) citrate-complexing method. LSCF exhibited a pure cubic structure where 110 and 100 plane diffractions were active for CO2 splitting, while 111 was more favored by H2O splitting. Overall oxygen storage capacity (OSC) of LSCF was 4072 μmol/gcat. During the reduction process, regular cations (Co4+, Fe4+), polaron cations (Co3+, Fe3+) and localized cations (Co2+, Fe2+) were achieved when the LSCF was reduced at 500, 700 and 900 °C, respectively. The strength of the active sites depended on reduction temperatures. An increase in oxidation temperature enhanced H2 production at temperature ranging from 500 °C to 700 °C while effected CO production at 900 °C. H2O and CO2 was competitively split during the oxidation step, especially at 700 °C. The activation energy of each reaction was ordered as; CO2 splitting > H2O splitting > CO2 adsorption, supporting the above evidence where H2 and CO production were found to increase when the operating temperature was increased.

Most thermochemical cycles require complex thermal processes at very high temperatures, which restrict the production and the use of hydrogen on a large scale. Recently, thermochemical cycles producing hydrogen at relatively low temperatures have been developed in order to be competitive with other kinds of energies, especially those of fossil origin. The low temperatures required by those cycles allow them to work with heats recovered by thermal, nuclear and solar power plants. In this work, a new thermochemical cycle is proposed. This cycle uses the chemical elements Magnesium-Chlorine (Mg-Cl) to dissociate the water molecule. The configuration consists of three chemical reactions or three physical steps and uses mainly thermal energy to achieve its objectives. The highest temperature of the process is that of the production of hydrochloric acid, HCl, estimated between 350-450℃. A thermodynamic analysis was performed according to the first and second laws by using Engineering Equation Solver (EES) software and the efficiency of the proposed cycle was found to be 12.7%. In order to improve the efficiency of this cycle and make it more competitive, an electro-thermochemical version should be studied.

To achieve carbon-neutral energy vectors, researchers have investigated various sulfur-based thermochemical cycles. The sulfur–iodine cycle has emerged as a cost-effective global process with massi...

Copper–Chlorine cycle has been identified as the most prospective among the low temperature thermochemical cycles for hydrogen production. The cycle consists of two thermal reaction steps, one electrochemical step and a physical separation step. The two thermal reaction steps, hydrolysis and thermolysis are carried out in series for water splitting and oxygen production, respectively. The solid product from hydrolysis step Cu2OCl2 enters the thermolysis step where it undergoes decomposition to CuCl and O2. In the present work, thermolysis experiments were carried out in a laboratory scale horizontal furnace reactor with CuO–CuCl2 equimolar mixture and Cu2OCl2 in the temperature range of 470–575 °C. Experiments in furnace reactor show that, under otherwise same conditions, similar conversions are obtained with Cu2OCl2 as well as with the equimolar mixture of CuO–CuCl2. It was also observed that the conversion increased with an increase in CuCl2 percentage in the reaction mixture. From the experimental data, an attempt has been made to provide insights into the reaction mechanism and kinetics. These results are expected to be useful for the design and scale-up of the thermolysis reactor.

In recent years, oxygen storage materials (OSMs) have been widely used in many fields. It would be particularly important for researchers to design high-oxygen-uptake/release-rate materials. In this study, various synthesis processes were used to successfully synthesize YBaCo4O7+δ and comprehensively investigate their potential applications. Compared with traditional solid-state reaction method and co-precipitation method, the results demonstrated that the utilization of mechanical ball milling treatment on co-precipitated precursors could lead to samples with reversible oxygen uptake/release under an oxidative atmosphere at low temperatures. The resultant materials exhibited fast oxygen absorption/desorption rate that could uptake/release oxygen directly to the equilibrium state within 9 min and 20 min, respectively. The mechanochemically ball-milled sample possessed outstanding oxygen storage performance, which could be attributed to their small particle size, the active outer surface of particles, large specific surface area, and relatively low activation energy. Moreover, the ball-milled sample also exhibited excellent cycling stability during relatively short time spacing. TG results also demonstrated that the ball-milled samples could reversibly uptake/release 2.90 wt.% of excess oxygen (while only 0.70 wt.% for solid-state samples) by adjusting the ambient temperature under pure O2 atmosphere, which would make them promising candidates in various applications. This research demonstrated that mechanical treatment could be an effective strategy to tune the properties and oxygen storage capacity(OSC) performances of YBaCo4O7+δ.

This perspective highlights and evaluates recent key developments in the thermodynamic approach used to analyze trends in acid and base strength variation. According to this approach, acid and base strength ranking can be interpreted by using thermodynamic or thermochemical cycles. Each cycle generally consists of three independent but well-defined steps. The modus operandi described here entails the identification of the dominant step and the rationalization of its free energy/enthalpy/energy change along a selected series in terms of known structural chemical concepts. Developments in this approach are described by focusing on two related series of bases and two series of acids. In the case of the former the protonation of a series of N-heterocyclic amine bases together with their methyl-substituted analogs receives particular attention while in the case of acids, the acidic properties of aqua dications of elements in period 4 and group 2 are probed. It is illustrated how significant progress in computational chemistry and mass spectrometric techniques can be employed to compare ‘inherent’ basicity or acidity in the selected families of compounds by using simple gas-phase energy cycles. Unique, dual functions for both electronegativity (element and orbital) and charge density (for aqua cations) indicators are identified and used to evaluate these cycles. Solvent effects (in aqueous solution) are accommodated by including dehydration and hydration changes in appropriately-extended, three-step free energy cycles. It is further suggested that the dominant step in the extended thermodynamic cycle for monomeric aqua cations is the transfer of M(H2O)n2+ complex hydrates from the gas-phase to bulk water. Charge density of the aqua cations again features prominently in proposed rationalizations. Finally, this article also sheds light on salient relationships that exist between empirically and quantum-chemically estimated enthalpy and entropy changes for the aforementioned transfer process.

Hydrogen is a clean energy carrier that can help mitigate greenhouse gas (GHG) emissions if it is used to replace fossil fuels for power production. One way to produce hydrogen on a large scale is through the use of water splitting thermochemical cycles such as the hybrid copper chlorine (Cu–Cl) cycle. Canadian Nuclear Laboratories Ltd. (CNL) chose to develop the Cu–Cl cycle because the highest temperature required by this cycle is about 530 °C, compatible with the Canadian Super Critical Water Reactor (SCWR) or some small modular reactors (SMR). The on-going effort at CNL is to demonstrate a fully integrated Cu–Cl cycle at laboratory scale with a hydrogen production rate of 50 L/h. Some recent experimental results of the electrolysis step, one of the main steps of the cycle, are discussed in this paper. The anode reaction of CuCl oxidation was investigated using a three-electrode electrochemical cell. Half-cell experiments found that CuCl oxidation did not require noble metals as catalyst. The CuCl oxidation on carbon was found to be a mass-transfer controlled process. Hence the limiting current density increased with increasing turbulence on the electrode surface. Increasing the CuCl concentration and the solution temperature also resulted in higher limiting current densities. A current density of 0.53 A/cm2 was achieved for a 1.0 M CuCl solution at 80 °C. Single cells with electrode areas up to 100 cm2 were used to establish the operating conditions for the electrolysis step. The effects of flow rate, temperature, and current density on the cell voltage were studied. A hydrogen production rate of 50 L/h was successfully achieved at 0.4 A/cm2 in a 2.0 M CuCl solution at 80 °C. The electrolysis step is fully developed for integration in a laboratory-scale demonstration of the Cu–Cl cycle.

Concentrated solar thermal technology is considered a very promising renewable energy technology due to its capability of producing heat and electricity and of its straightforward coupling to thermal storage devices. Conventionally, this approach is mostly used for power generation. When coupled with the right conversion process, it can be also used to produce methanol. Indeed methanol is a good alternative fuel for high compression ratio engines. Its high burning velocity and the large expansion occurring during combustion leads to higher efficiency compared to operation with conventional fuels. This study is focused on the system level modeling of methanol production using hydrogen and carbon monoxide produced with cerium oxide solar thermochemical cycle which is expected to be CO2 free. A techno-economic assessment of the overall process is done for the first time. The thermochemical redox cycle is operated in a solar receiver-reactor with concentrated solar heat to produce hydrogen and carbon monoxide as the main constituents of synthesis gas. Afterwards, the synthesis gas is turned into methanol whereas the methanol production process is CO2 free. The production pathway was modeled and simulations were carried out using process simulation software for MW-scale methanol production plant. The methanol production from synthesis gas utilizes plug-flow reactor. Optimum parameters of reactors are calculated. The solar methanol production plant is designed for the location Almeria, Spain. To assess the plant, economic analysis has been carried out. The results of the simulation show that it is possible to produce 27.81 million liter methanol with a 350 MWth solar tower plant. It is found out that to operate this plant at base case scenario, 880685 m2 of mirror's facets are needed with a solar tower height of 220 m. In this scenario a production cost of 1.14 €/l Methanol is predicted.

Apart from being a major feedstock for chemical production, hydrogen is also a very promising energy carrier for the future energy. Currently hydrogen is predominantly produced via fossil routes, but as green energy sources are gaining a larger role in the energy mix, novel and green production routes are emerging. The most abundant renewable hydrogen sources are water and biomass, which allow several possible processing routes, such as electrolysis, thermochemical cycles and gasification. By introducing heat to the process the required electricity demand can be reduced (high temperature electrolysis) or practically eliminated (thermochemical cycles). Each renewable hydrogen production route has its own strength and weaknesses; the choice of the most suitable method is always dependent on the economical potentials and the location. The aim of this paper is to evaluate the different high temperature, renewable hydrogen production technologies.

In this paper, an atmospheric-pressure distillation system is designed and constructed for partial to separation of hydrochloric acid and water. The system concentrates HCl(aq) between the electrolyzer and hydrolysis processes of the Copper–Chlorine (Cu–Cl) cycle for hydrogen production. The motivation behind this study is to investigate azeotropic separation of HCl(aq), as needed for integration of unit operations in the Cu–Cl cycle. The separation is only partial, as the mixture is unable to cross the azeotrope with only a single pressure. The distillation system consists primarily of one packed distillation column, which employs heating tapes and thermocouples to achieve a desired axial temperature profile. The column can be operated in batch or continuous mode. The distillate is H 2O(l) and the bottoms is HCl(aq) near the azeotropic concentration; feed concentrations are less than azeotrope. Thus, the degree of separation is determined to be independent of the feed concentration. The bottoms concentration varies from experiment to experiment, but does so independently of feed concentration, likely the result of corrosion impurities affecting the calculation of its concentration. It is found that HCl(aq) can be concentrated up to approximately 0.1068 mol/mol from an initial concentration of 0.0191 mol/mol. A simulation of pressure-swing distillation (PSD) is also performed, but due to safety constraints (a column operating at 10 atm must be certified to CSA B51), a single-pressure (single-column) distillation is physically performed. A single-pressure column is beneficial to the Cu–Cl cycle because it partially recycles HCl, which reduces the cost of the cycle, and still provides valuable results for analysis. The maximum HCl concentration achieved experimentally is 0.1068 mol/mol and the maximum HCl concentration determined from simulation is 0.11 mol/mol (the azeotropic concentration). The novelty of this research is that the experimental column built to study HCl partial separation is designed to be simple yet safe to integrate within the Cu–Cl cycle for hydrogen production.

In this paper, a theoretical and experimental study on a novel cobalt-chlorine thermochemical cycle for hydrogen production is presented. The cobalt-chlorine cycle comprises a closed loop of four thermochemical reactions occurring at 700 °C that is a reaction temperature compatible with the present generation of high-temperature gas-cooled reactors. Firstly, a thermodynamic analysis was done for determining whether this cycle is attractive for hydrogen production in terms of both energy and exergy efficiencies. Following, proof-of-principle experiments were carried out at laboratory scale in a batch reactor at temperatures in the range from 550 °C to 950 °C and holding times between 1 h and 72 h. Experimental results complemented by the characterization of condensed compounds deposited on the reactor walls allowed confirm the reaction pathway of thermochemical reactions originally proposed, define the slowest step of the global process, and explain the beneficial effect of increasing the system pressure on the hydrogen yield. Even both performance assessment and proof-of-principle experimental results look like promising more research will be required in the future to confirm these preliminary findings. Finally, a modified version of the cobalt-chlorine cycle is proposed for enhancing the global kinetics, based on the experimental evidence found in the proof-of-principle tests.

HIx processing section of Iodine-Sulphur (IS) thermochemical cycle dictates the overall efficiency of the cycle, which poses extremely corrosive HI–H2O–I2 environment, coupled with a very low equilibrium conversion (~22%) of HI to hydrogen at 450 °C. Here, we report the fabrication, characterization and operation of a 4-tube packed bed catalytic tantalum (Ta) membrane reactor (MR) for enhanced HI decomposition. Gamma coated clay-alumina tubes were used as supports for fabrication of Ta membranes. Clay-alumina base support was fabricated with 92% alumina (~8 μm particle size) and 8% clay (~10 μm particle size), sintered at a temperature of 1400 °C. An intermediate gamma alumina coating was provided with 4% polyvinyl butyral as binder for a 10% solid loading. Composite alumina tubes were coated with thin films of Ta metal of thickness 80% single-pass conversion of HI to hydrogen at 450 °C. The hydrogen throughput of the reactor was ~30 LPH at a 2 bar trans-membrane pressure, with >99.95% purity. This is the first time a muti-tube MR is reported for HIx processing section of IS process.

Thermochemical water splitting cycles are recognized as one of the promising pathways for sustainable hydrogen production. In the present study, Iron-chlorine (Fe–Cl) cycle as one of the chlorine family thermochemical cycles where iron chloride is consumed for hydrogen production from water, is considered for a study. This four-step cycle is modelled by Aspen Plus software package and analyzed for performance investigation of each reaction step and system's components. The parametric studies are also performed to assess the effect of operation conditions such as temperature, pressure and steam to feed ratio on the reaction products and conversion rates. Results indicated that although the effect of pressure is not significant on reaction's production rates, an increase in temperature favors oxygen production in reverse deacon reaction and magnetite production in hydrolysis and lowers hydrogen production in the hydrolysis step. On the other hand, steam to chlorine (Cl2) ratio is directly correlated with hydrochloric acid (HCl) and oxygen production in reverse deacon reaction and hydrogen production in hydrolysis.

Sulfur-based thermochemical cycles, such as the hybrid sulfur-ammonia (HySA) cycle, offer a valuable approach in which hydrogen is produced by exploiting sulfur dioxide (potentially pollutant emissions) through the electrochemical oxidation of aqueous sulfite. In this study, the effect of pH on electrooxidation rate was assessed by comparing different reaction scenarios. Then, a Central Composite Design (CCD) combined with a Response Surface Methodology (RSM) was used to optimize batch electrooxidation of ammonium sulfite at near-neutral pH. Results show that the use of an anion exchange membrane (AEM) greatly improves sulfite electrooxidation rate while pH is effectively stabilized. Furthermore, a second-order model that relates applied potential and sulfite concentration with the normalized half-life of the reaction was obtained and verified experimentally at long-term batch electrooxidations. A good agreement between the model and experimental tests, adequate hydrogen recoveries and low sulfur crossover through the membrane demonstrate practical robustness of this approach.

Solar thermal-driven thermochemical H2O and CO2 splitting offers a carbon-neutral path to produce feedstocks for synthetic fuel production such as hydrogen or synthesis gas. This paper assesses research outcomes for perovskite materials in two-step thermochemical cycles. Experimental, computational and thermodynamic studies are summarized and critically discussed, identifying key attributes for future research. In addition to the critical review, a fast method for the classification of effective thermochemical properties (oxygen vacancy formation enthalpy and entropy) in a wide range of operational temperatures is provided. These properties together with a high-grade of sintering resistance and fast kinetics are the main characteristics required to maximize the solar-to-fuel efficiency of the process. The discovery of optimum material compositions for this application could be effectively achieved by a combination of machine learning, DFT, experimental testing and system modelling, and will require an extensive international research effort. If successful, this could lead to the ultimate development and practical application of thermochemical cycles for fuel production.

Thermochemical water splitting cycles are considered an attractive option to produce H2, a substance with many important applications for society such as ammonia production because they do not require fossil fuels. In contrast, they obtain H2 decomposing water by means of cyclic chemical reactions. There are a wide variety of thermochemical ones under research, including the relatively new Na–O–H (sodium–oxygen–hydrogen) cycle proposed in 2012 by a research group. Despite introducing a new potential thermochemical cycle, those previous researchers do not investigate the thermal performance of it. Such task can be done through an exergy analysis. So, the aim of the paper is to evaluate the exergy performance of the Na–O–H cycle and its chemical reactions. This goal is accomplished by implementing in EES (Engineering Equation Solver) software exergy efficiency ( e 1 ) and exergy destroyed (ED_1) balances for the cycle and their reactions considering specific conditions of pressure (p) and temperature (T). This approach allows understanding how these two variables influence the system performance, which could compromise its actual implementation and economic feasibility if it has low thermal efficiency. According to the results: reaction 1 (hydrogen production step) has mean e 1 = 96 % and ED_1 = 8.5 kJ under 100–300 °C at 0–1 bar; reaction 2 (metal separation step) has ED_2 = 280 kJ and e 2 = 56 % at 450 °C considering vacuum condition; hydrolysis step (reaction 3) has average e 3 = 87 % and ED_3 = 18 kJ at 25–200 °C under 0–1 bar. Then, the Na–O–H cycle has overall e = 82 % and ED = 306 kJ to produce 1 mol of H2. These values of e and ED are theoretical and maximized ones. In practical situations, such amounts probably will reduce in function of unavoidable irreversibility present in actual systems such as pressure drop and heat loss that were neglected in the work to facilitate its development. Finally, it concludes the Na–O–H cycle has relatively high exergy efficiency, making it a potential hydrogen production method. However, its practical implementation in the future must overcome some of its drawbacks, by means of more research, like the low thermal efficiency of reaction 2 when compared to reactions 1 and 3 beyond the low pressure needed to perform such step.

The two-step cycle for solar-thermochemical fuels production and thermochemical energy storage benefits from low oxygen partial pressure in the high-temperature thermal reduction step. To be practical, low oxygen partial pressures must be reached by energetically efficient and economically affordable methods—a challenge currently not met by either mechanical vacuum pumping or by inert gas sweeping. To address this challenge, we have examined a promising, thermally-driven surface adsorption/desorption approach. Providing that appropriately designed materials can be identified as expected, this approach would substantially advance solar-thermochemical fuel production (water and carbon dioxide splitting), thermochemical energy storage, and related technologies.

Using solid oxide membrane, this paper presents the theoretical modeling of the high temperature electrolysis of hydrogen bromide gas for hydrogen production. The electrolysis of hydrogen halides such as hydrogen bromide is an attractive process, which can be coupled to hybrid thermochemical cycles. The high temperature electrolyzer model developed in the present study includes concentration, ohmic, and activation losses. Exergy efficiency, as well as energy efficiency parameters, are used to express the thermodynamic performance of the electrolyzer. Moreover, a detailed parametric study is performed to observe the effects of various parameters such as current density and operating temperature on the overall system behavior. The results show that in order to produce 1 mol of hydrogen, 1.1 V of the applied potential is required, which is approximately 0.8 V less compared to high temperature steam electrolysis under same conditions (current density of 1000 A/m2 and temperature of 1073 K). Furthermore, it is found that with the use of the presented electrolyzer, one can achieve energy and exergy efficiencies of about 56.7% and 53.8%, respectively. The results presented in this study suggest that, by employing the proposed electrolyzer, two-step thermochemical cycle for hydrogen production may become more attractive especially for nuclear- and concentrated solar-to-hydrogen conversion applications.

There is a renewed interest in CeO2 for use in solar-driven, two-step thermochemical cycles for water splitting. However, despite fast reduction/oxidation kinetics and high thermal stability of ceria, the cycle capacity of CeO2 is low due to thermodynamic limitations. In an effort to increase cycle capacity and reduce thermal reduction temperature, we have studied binary zirconium-substituted ceria (ZrxCe1-xO2, x = 0.1, 0.15, 0.25) and ternary praseodymium/gadolinium-doped Zr-ceria (M0.1Zr0.25Ce0.65O2, M = Pr, Gd). We evaluate the oxygen cycle capacity and water splitting performance of crystallographically and morphologically stable powders that are thermally reduced by laser irradiation in a stagnation flow reactor. The addition of zirconium dopant into the ceria lattice improves O2 cycle capacity and H2 production by approximately 30% and 11%, respectively. This improvement is independent of the Zr dopant level, up to 25%, suggesting that above 10% Zr dopant level, Zr might be displaced during the high temperature annealing process. The addition of Pr and Gd to the binary Zr-ceria mixed oxide, on the other hand, is detrimental to H2 production. A kinetic analysis is performed using a model-based analytical approach to account for effects of mixing and dispersion, and to identify the rate controlling mechanism of the water splitting process. We find that the water splitting reaction at 1000 °C and with 30 vol% H2O, for all doped ceria samples, is surface limited and best described by a deceleratory power law model (F-model), similar to undoped CeO2. Additionally, we used density functional theory (DFT) calculations to examine the role of Zr, Pr, and Gd. We find that the addition of Pr and Gd induce non-redox active sites and, therefore, are detrimental to H2 production, in agreement with experimental work. The calculated surface H2 formation step was found to be rate limiting, having activation barriers greater than bulk O diffusion, for all materials. This agrees with and further explains experimental findings.

Dissociation of CO2 to form CO can play a key role in decarbonizing our energy system. We report here a two-step thermochemical cycle using a variety of iron-poor (Fe-poor) ferrites (FeyM1−yOx where y < 2/3) that produce CO with unusually high yield using Fe as the redox active species. Conventional wisdom suggests that increasing the Fe fraction would increase the capacity for CO2 dissociation. Here, we report the opposite result: at partial pressure ratio CO : CO2 = 1 : 100, we demonstrated CO yields of 8.0 ± 1.0 mL-CO per gram from Fe0.35Ni0.65Ox, and 3.7 ± 1.0 mL-CO per gram from Fe0.45Co0.55Ox, at a thermal reduction temperature of 1300 °C; remarkably, these CO2 dissociation capacities are significantly higher than those of state-of-the-art materials such as spinel ferrites (Fe2MO4), (substituted) ceria, and Mn-based perovskite oxides. Optimization of the kinetics of Fe-poor ferrites with a ZrO2 support resulted in higher CO yields per gram of ferrite. The unexpected CO yield vs. Fe ratio trend is consistent with the prediction of calculated ternary phase diagrams, which suggest a swing between spinel and rocksalt phases. These Fe-poor ferrites open new opportunities for tuning the redox properties of oxygen exchange materials.

© 2021 Hydrogen Energy Publications LLCIn the presented paper, energy and exergy analysis is performed for thermochemical hydrogen (H2) production facility based on solar power. Thermal power used in thermochemical cycles and electricity production is obtained from concentrated solar power systems. In order to investigate the effect of thermochemical cycles on hydrogen production, three different cycles which are low temperature Mg–Cl, H2SO4 and UT-3 cycles are compared. Reheat-regenerative Rankine and recompression S–CO2 Brayton power cycles are considered to supply electricity needed in the Mg–Cl and H2SO4 thermochemical cycles. Furthermore, the effects of instant solar radiation and concentration ratio on the system performance are investigated. The integration of S–CO2 Brayton power cycle instead of reheat-regenerative Rankine enhances the system performance. The maximum exergy efficiency which is obtained in the system with Mg–Cl thermochemical and recompression S–CO2 Brayton power cycles is 27%. Although the energy and exergy efficiencies decrease with the increase of the solar radiation, they increase with the increase of the concentration ratio. The highest exergy destruction occurred in the solar energy unit.

The focus of this chapter is on thermochemical water-splitting cycles, with an emphasis on solar energy as the input. However, other sources of thermal and electrical energy (e.g., nuclear) can drive these cycles. Further emphasis will be placed on comparing those with no electrochemical step to those that take advantage of one. In general, water splitting that employes an electrochemical step is less complex but often less efficient. The simplest example is water electrolysis, where water is split directly into oxygen and hydrogen using electricity. However, since there are thermodynamic inefficiencies in generating electricity from thermal energy, thermal water splitting would be attractive from an efficiency standpoint. Unfortunately, thermal dissociation of water occurs at temperatures in excess of 2500°C. Therefore, hundreds of different thermochemical cycles involving chemical species in addition to water are under investigation. These various thermochemical cycles split water at lower temperatures (∼500–1000°C), with the other chemical species recycled in the system. Thermochemical cycles can be divided into two broad categories: (1) direct processes (i.e., all chemical steps); or (2) hybrid processes (i.e., a combination of chemical and electrochemical steps).

International audience; Porous ceria-based architected materials offer high potential for solar fuels production via thermochemical H 2 O and CO 2-splitting cycles. Novel porous morphologies and micro-scale architectures of redox materials are desired to provide suitable thermochemical activities and long-term stability. Considering particle-based solar reactors, porous ceria microspheres are promising because of their excellent flowability and large surface area. In this work, such porous microspheres with perfect spherical shape, high density and interconnected pore network were fabricated by a chemical route involving ion-exchange resins. The method involved the cationic loading of the resin in aqueous medium followed by thermal treatment for oxide formation and porous microstructure stabilization. The utilization of these microspheres (~150-350 µm in size) as redox materials for solar fuel production was investigated in packed-bed solar reactors (directly and indirectly-irradiated). Superior redox performance was obtained for the pure ceria microspheres in comparison with other morphologies (powders and reticulated foams). Low p O2 values thermodynamically favored the reduction extent and associated fuel yield, whereas high p CO2 kinetically promoted the oxidation rate. The highest fuel production rate reached 1.8 mL/min/g with reduction step at 1400°C and low total pressure (~0.1 bar), and oxidation step below 1050°C under pure CO 2. Low pressure during reduction both improved reduction extent (oxygen under-stoichiometry  up to 0.052) and associated fuel production yield (331 µmol/g CO). After 19 redox cycles (~32h under high-flux solar irradiation), the porous microspheres maintained their individual integrity (no agglomeration), spherical shape, and internal porosity, with great potential for stable fuel production capacity in particle-based solar reactors.

Inspired by the promising hydrogen production in the solar thermochemical (STC) cycle based on non-stoichiometric oxides and the operation temperature decreasing effect of methane reduction, a high-fuel-selectivity and CH4-introduced solar thermochemical cycle based on MoO2/Mo is studied. By performing HSC simulations, the energy upgradation and energy conversion potential under isothermal and non-isothermal operating conditions are compared. In the reduction step, MoO2:CH4 = 2 and 1020 K < Tred < 1600 K are found to be most favorable for syngas selectivity and methane conversion. Compared to the STC cycle without CH4, the introduction of methane yields a much higher hydrogen production, especially at the lower temperature range and atmospheric pressure. In the oxidation step, a moderately excessive water is beneficial for energy conversion whether in isothermal or non-isothermal operations, especially at H2O: Mo = 4. In the whole STC cycle, the maximum non-isothermal and isothermal efficiency can reach 0.417 and 0.391 respectively. In addition, the predicted efficiency of the second cycle is also as high as 0.454 at Tred = 1200 K and Toxi = 400 K, indicating that MoO2 could be a new and potential candidate for obtaining solar fuel by methane reduction.

Induced by promising hydrogen production of CeO2-based solar thermochemical cycle and evident temperature decreasing effect of methane reduction, a moderately high-temperature solar thermochemical ceria-methane cycle is investigated thermodynamically. In this paper, isothermal and non-isothermal solar-to-fuel efficiencies (ηsolar-to-fuel) under different temperatures and reactant ratios are compared carefully. The calculated results indicate that the condition of CH4:CeO2 = 0.5 is favorable for oxygen release, fuel selectivity and methane conversion. The introduction of methane could increase the maximum yield of H2, and more solar energy could be converted to chemical energy as the increase of nH2O:nCeO2. nH2O:nCeO2 = 0.5, Tred = 1400 K and Toxi = 750 K are suggested for the maximum non-isothermal ηsolar-to-fuel of 0.35, which is larger than the maximum isothermal ηsolar-to-fuel of 0.24. The result shows that non-isothermal solar thermochemical ceria-methane cycle is more feasible for fuel production.

Hydrogen production thermochemical cycles, based on the recirculation of sulfur-based compounds, are among the best suited processes to produce hydrogen using concentrated solar power. The sulfuric acid decomposition section is common to each sulfur-based cycle and represents one of the fundamental steps. A novel direct solar receiver-reactor concept is conceived, conceptually designed and simulated. A detailed transport phenomena model, including mass, energy and momentum balance expressions as well as suitable decomposition kinetics, is described adopting a finite volume approach. A single unit reactor is simulated with an inlet flow rate of 0.28 kg/s (corresponding to a production of approximately 11 kgH2/h in a Hybrid Sulfur process) and a direct solar irradiation at a constant power of 143 kW/m2. Results, obtained for the high temperature catalytic decomposition of SO3 into SO2 and O2, demonstrate the effectiveness of the proposed concept, operating at pressures of 14 bar. A maximum temperature of 879 °C is achieved in the reactor body, with a corresponding average SO2 mass fraction of 27.8%. The overall pressure drop value is 1.7 bar. The reactor allows the SO3 decomposition into SO2 and O2 to be realized effectively, requiring an external high temperature solar power input of 123.6 kJ/molSO2 (i.e. 123.6 kJ/molH2).

Separation of O2 from air to produce a high-purity stream of N2 is considered via a two-step solar thermochemical cycle based on aluminum-doped strontium ferrite reduction/oxidation reactions. The cycle steps encompass the (1) thermal reduction, driven by concentrated solar irradiation; and (2) re-oxidation with atmospheric air to produce a N2 stream. Samples were synthesized via the sol-gel method, and crystalline structures were confirmed with x-ray powder diffractometry. Thermogravimetry was used to measure the nonstoichiometry at chemical equilibrium for molar B-site Al dopant fractions between 0 and 0.20, temperatures between 673 and 1373 K, and volumetric gas flow compositions between 1 and 90% O2–Ar. The compound energy formalism was applied to thermogravimetric measurements to predict equilibrium nonstoichiometry as a function of dopant fraction, temperature, and O2 partial pressure. The results were used to predict partial reaction enthalpies and entropies. Aluminum doping increased the strontium ferrite oxygen affinity but reduced the range of nonstoichiometries. A strontium ferrite sample underwent 10 thermal reduction-oxidation steps in an upward flow reactor coupled to a high-flux solar simulator to study cyclability. A thermodynamic cycle analysis was performed, and cycle efficiencies were determined relative to an ideal separation process. For a solar reactor at 1073 K, 0.59 mol N2 per mol strontium ferrite could be produced at a purity of nearly 99% with a cycle efficiency of 4.0%.

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 MFeO 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 (H/O) for Co ferrite thermochemical cycles in comparison with those corresponding to Ni ferrite., European Regional Development Fund is gratefully acknowledged by the partial funding of the XRF and XRD equipment employed for this study (Projects FEDER 2004 CIEM05-34-030 and FEDER 2004 CIEM05-34-031).

A pure barium borate of BaB8O11(OH)4·3H2O has been synthesized and characterized by XRD, FT-IR, TGA, and chemical analysis. The molar enthalpies of solution of BaB8O11(OH)4·3H2O in 1 mol dm−3 HCl(aq), and Ba(OH)2·8H2O in the mixture solvent of {2.00 cm3 of 1 mol·dm−3 HCl + H3BO3(aq)} were measured by microcalorimeter at T = 298.15 K, respectively. With the use of previously determined enthalpy of solution of H3BO3 (s) in 1 mol·dm−3 HCl(aq), and the standard molar enthalpies of formation for Ba(OH)2·8H2O(s), H3BO3(s), and H2O(l), the standard molar enthalpy of formation of −(7500.6 ± 6.5) kJ·mol−1 at T = 298.15 K was obtained based on an appropriate thermochemical cycle. Moreover, the molar enthalpy of formation of −6091.7 kJ mol−1 for [B8O11(OH)4]2− was estimated by a group contribution method, which was used to predict the ΔfHmo of a series of alkaline-earth metal borates for MB8O11(OH)4 (M = Ca, Sr, Ba).

Ammonia (NH3) is one of the most valuable chemicals due to its multipurpose utilization in many applications. Beside major application as fertilizer in agriculture, NH3 is a promising hydrogen (H2) carrier because it has a high content of H2 and technically available storage and transportation systems. At present, most of NH3 is synthesized from a series of steam reforming process to the feedstock of H2 and nitrogen (N2) production, followed with the catalytic Haber-Bosch reaction. In this study, an integrated system of N2 production system, NH3 synthesis system and a power generation process is proposed to produce NH3 efficiently. Thermochemical cyclic process of an endothermic reaction of Al2O3 reduction by carbon in N2 atmosphere, is coupled with oxidation/steam-hydrolysis of aluminum nitride (AlN) back to Al2O3, producing NH3 without catalyst and the need of H2 production system. The thermal energy required for reduction reaction is covered by the heat generated during exothermic reaction and combustion process, and the remaining heat is utilized in power generation. Heat circulation optimization is carried out by applying the enhanced process integration, resulting in a highly efficient system. The proposed system is analyzed through an adjustment of the main parameters: oxidation temperature, steam turbine inlet pressure, and temperature, to observe their effect on system performance and efficiency. The proposed system can reach a very high system efficiency of 69.3%. The oxidation reaction temperature is observed to play major role in determining the system performance due to temperature dependent NH3 yield.

An integrated lab-scale copper-chlorine (Cu-Cl) thermochemical cycle for hydrogen production at the University of Ontario Institute of Technology (UOIT) is presented and analyzed in this paper. In a practical operation of the Cu-Cl cycle, besides the main steps of hydrolysis, thermolysis, electrolysis and drying, the oxidized anolyte (consumed anolyte at the electrolyzer cell) needs to be recycled to be concentrated sufficiently for the electro-chemical process. Recycling of the oxidized anolyte through the separation processes is achieved by distillation of anolyte, drying unit, separation cell, pressure swing distillation and CuCl2 concentrator. This study examines the thermodynamic performance of all unit operations in the lab-scale Cu-Cl cycle. A process simulation model with Aspen Plus is used to assess the system by energy and exergy analyses. For the specific system design characteristics, the cycle is capable of producing 100 L/h of hydrogen. From the simulation results, the overall energy and exergy efficiencies of the lab-scale Cu-Cl cycle are determined to be 11.6% and 34.9%, respectively. Furthermore, after the thermolysis and hydrolysis reactors, the quench cell and CuCl2 concentrator have the highest exergy losses with thermal energy transferred through CuCl solidification and water vaporization phase-change processes at relatively high temperature. Additional results of the processes are presented and discussed.

A thermochemical study of [SbCl3(L)] and [BiBr3(L)] complexes, wherein L is 2,2′-bipyridine (bpy) or 1,10-phenanthroline (phen), was performed by calorimetric measurements in solution. The complexes were synthesized and characterized by elemental analysis, FTIR spectroscopy and thermal analysis. From the enthalpies of dissolution of the complexes, salts and ligands in combination with auxiliary thermodynamic data, some thermochemical parameters of the complexes were determined by use of thermochemical cycles. Theoretical study on the complexes was performed using M06-2X/def2-TZVPP method. Donor-acceptor bond energies were obtained taking into account reorganization energies of the fragments. Our findings suggest that both the reorganization energy of the acceptor metal trihalide MX3 (113–136 kJ · mol−1) and the rigidity of the phen govern the dissociation energy of the complexes. The theoretical results indicated that the chemical stabilities of the complexes decreases in order [BiBr3(phen)] > [SbCl3(phen)] > [BiBr3(bpy)] > [SbCl3(bpy)], agreeing with the experimental enthalpies of coordination in solid state.

The enthalpies of formation and isomerization energies of P4Sn molecular cages are not experimentally (or theoretically) well known. We obtain accurate enthalpies of formation and isomerization energies for P4Sn cages (n = 3, 4, 5, 6, and 10) by means of explicitly correlated high-level thermochemical procedures approximating the CCSD(T) and CCSDT(Q) energies at the complete basis set (CBS) limit. The atomization reactions have very significant contribution from post-CCSD(T) correlation effects and, due to the presence of many second-row atoms, the CCSD and (T) correlation energies converge exceedingly slowly with the size of the one-particle basis set. As a result, these cage structures are challenging targets for thermochemical procedures approximating the CCSD(T) energy (e.g., W1-F12 and G4). Our best enthalpies of formation at 298 K (∆fH°298) are obtained from thermochemical cycles in which the P4Sn cages are broken down into P2S2 and S2 fragments for which highly accurate ∆fH°298 values are available from W4 theory. For the smaller P4S3 and P4S4 cages, the reaction energies are calculated at the CCSDT(Q)/CBS level and for the larger P4S5, P4S6, and P4S10 cages, they are obtained at the CCSD(T)/CBS level. Our best ∆fH°298 values are − 94.5 (P4S3), − 108.4 (α-P4S4), − 98.7 (β-P4S4), − 126.2 (α-P4S5), − 126.1 (β-P4S5), − 112.7 (γ-P4S5), − 144.7 (α-P4S6), − 153.9 (β-P4S6), − 134.4 (γ-P4S6), − 136.3 (δ-P4S6), − 118.7 (e-P4S6), and − 215.4 (P4S10) kJ mol−1. Interestingly, we find a linear correlation (R2 = 0.992) between the enthalpies of formation of the most stable isomers of each molecular formula and the number of atoms in the P4Sn cages. We use our best ∆fH°298 values to assess the performance of a number of lower-cost composite ab initio methods. For absolute enthalpies of formation, G4(MP2) and G3(MP2)B3 result in the best overall performance with root-mean-square deviations (RMSDs) of 10.6 and 12.9 kJ mol−1, respectively, whereas G3, G3B3, and CBS-QB3 result in the worst performance with RMSDs of 27.0–38.8 kJ mol−1. In contrast to absolute enthalpies of formation, all of the considered composite procedures give a good-to-excellent performance for the isomerization energies with RMSDs below the 5 kJ mol−1 mark.

Thermochemical water splitting cycles (TWSCs) are processes with the potential for large-scale production of carbon-free hydrogen. Among these, the sulfur-family thermochemical cycles are considered the most promising due to both, the use of readily affordable chemical reagents and the temperature required to thermally decompose oxygenated sulfur compounds, which is achievable by solar means. Indeed, solar heat assisted metal sulfate decomposition is a key step, where catalysis can be employed to reduce decomposition temperature. Here we present a green route to synthesize Ag-Pd and Fe-Pd intermetallic alloy catalysts supported over γ-Al2O3 and Si-C by a microwave-assisted method using glycerol both as a solvent and as a reducing and stabilizing agent. The obtained supported catalysts were physicochemically characterized. Fe-Pd/Al2O3 catalyst exhibited the best performance, abating the zinc sulfate decomposition temperature by ca. 85 °C in comparison with other reported catalysts.

Solar thermochemical cycle is considered a promising strategy for hydrogen production. The reaction process and oxygen carrier (OC) are the key issues for solar hydrogen production. In this study, a solar-driven chemical-looping steam methane reforming (S-CL-SMR) process by using perovskite as the oxygen carrier is proposed. In this process, the heat required by the endothermic reduction is supplied by solar heat, and the heat is converted into the chemical energy of solar fuels. Perovskites La1−yCayNi0.9Cu0.1O3 were synthesized and characterized. Meanwhile, the reactivity was studied via thermogravimetry analysis at 400 °C for reduction, and 900 °C for re-oxidation. The results show that the reactivity of the La1−yCayNi0.9Cu0.1O3 is improved as the calcium substitution increases. In the reduction reactor, the fractional oxidation of La0.1Ca0.9Ni0.9Cu0.1O3 reaches to 0.61, and the methane conversion is about 52% with 60% CO selectivity. In the steam reactor, oxygen vacancies and reduced metal ions (Ni2+ and Cu2+) in doped perovskites can provide more active sites to break chemical bonds of H O. It is found that La0.1Ca0.9Ni0.9Cu0.1O3 has the best reactivity, regenerability and high resistance to carbon deposition among the synthesized perovskites. Our study is expected to provide a promising potential for hydrogen production by mid-temperature solar heat.