On the kinetics of the thermal decomposition of sulfates related with hydrogen water splitting cycles

The purpose of this paper is to present a set of kinetic results on the decomposition of MgSO₄, NiSO₄, ZnSO₄ and to discuss these in relation to the theoretical models. Isothermal and non-isothermal thermogravimetric studies as functions of temperature, gas flowrate, shape and solid additives have been performed. The gas effluent analysis in the three cases show that the products are SO₂ and O₂. For NiSO₄ and ZnSO₄ the contracting sphere model, chemically controlled, gives a good representation of the results. In the case of MgSO₄ a diffusion shrinking-core model had been derived. The reaction is controlled by the mass transfer in the product layer.     

Thermochemical or hybrid cycles of hydrogen production—technico-economical comparison with water electrolysis

The creation, by means of a computer, of numerous thermochemical cycles allowed a general evaluation of the energy and economic outputs of this process. In the energy field, the thermochemical process is theoretically equivalent to the electrolytic line but its irreversibilities are heavier. The large amount of raw materials to be produced in each reaction of a thermochemical cycle will require too high investment costs to make this process economically competitive. Addition of electric energy in a hybrid cycle improves slightly the efficiency and decreases the amount of materials to be handled; therefore lower investment costs are needed. However, these characteristics remain insufficient to compete with direct electrolysis.Direct electrolysis of water seems to be the best way for a mass production of hydrogen by water decomposition.     

Improved kinetic model for water splitting thermochemical cycles using Nickel Ferrite

In a previous work of the authors (AIChE Journal 2013; 59(4): 1213-1225) on the characterization of the performance of redox material compositions during two-step thermochemical splitting of water, it was observed that fitting of the obtained hydrogen and oxygen concentration profiles with a reaction model based on simple first order reaction rates could describe adequately only the first part of the evolution curves. This suggested that more complicated reaction models taking into account the structure of the redox material are needed to describe the whole extent of the experimental data. Based on the above, a minimum set of experiments for water splitting thermochemical cycles over a Nickel-ferrite was deigned and performed involving an increased duration of the reaction steps. A new extended model was derived for the water splitting and thermal reduction reactions, which considers two oxygen storage regions of the redox material communicating to each other by a solid state diffusion mechanism. The inclusion of two state variables instead of one has a significant effect on the reaction dynamics and renders the model capable to explain the dynamics of the convergence of the thermochemical cycles to a periodic steady state, observed experimentally in the previous work.     

Thermochemical H2 production via solar driven hybrid SrO/SrSO4 water splitting cycle

This article reports the thermodynamic efficiency analysis of the strontium oxide – strontium sulfate (SrO-SrS) water splitting cycle by applying the principles of the second law of thermodynamics and by utilizing the commercially available HSC Chemistry software. Initially, the thermodynamic equilibrium compositions allied with a) the thermal reduction of SrSO₄, b) H₂ production via water splitting reaction (through SrO re-oxidation) are recognized. Moreover, the temperatures desirable for performing the thermal reduction and the water splitting steps are determined. The consequence of the molar flow rate of Ar on the thermal reduction of SrSO₄ is also examined in detail. The effect of the thermal reduction and water splitting temperatures on the total solar energy input mandatory to run the cycle, re-radiation shortfalls from the cycle, heat energy emitted by the coolers and the water splitting reactor, and the cycle and the solar-to-fuel energy conversion efficiency (with heat recuperation) is scrutinized in detail. The attained outcomes specify that the cycle and the solar-to-fuel energy conversion efficiency up to 18.9 and 22.8% can be accomplished if the thermal reduction and the water splitting steps are conducted at 2380 and 1400 K (with 30% heat recuperation).     

Analysis of solar chemical processes for hydrogen production from water splitting thermochemical cycles

This paper presents a process analysis of ZnO/Zn, Fe₃O₄/FeO and Fe₂O₃/Fe₃O₄ thermochemical cycles as potential high efficiency, large scale and environmentally attractive routes to produce hydrogen by concentrated solar energy. Mass and energy balances allowed estimation of the efficiency of solar thermal energy to hydrogen conversion for current process data, accounting for chemical conversion limitations. Then, the process was optimized by taking into account possible improvements in chemical conversion and heat recoveries. Coupling of the thermochemical process with a solar tower plant providing concentrated solar energy was considered to scale up the system. An economic assessment gave a hydrogen production cost of 7.98$ kg⁻¹ and 14.75$ kg⁻¹ of H₂ for, respectively a 55 MW_th and 11 MW_th solar tower plant operating 40 years.     

Hydrogen production via solar-aided water splitting thermochemical cycles: Combustion synthesis and preliminary evaluation of spinel redox-pair materials

Redox-pair-based thermochemical cycles are considered as a very promising option for the production of hydrogen via renewable energy sources like concentrated solar energy and raw materials like water. This work concerns the synthesis of various spinel materials of the iron and aluminum families via combustion reactions in the solid and in the liquid-phase and the testing of their suitability as redox-pair materials for hydrogen production by water splitting via thermochemical cycles. The effects of reactants' stoichiometry (fuel/oxidizer) on the combustion synthesis reaction characteristics and on the products' phase composition and properties were studied. By fine-tuning the synthesis parameters, a wide variety of single-phase, pure and well crystallized spinels could be controllably synthesized. Post-synthesis, high-temperature calcination studies under air and nitrogen at the temperature levels encountered during solar-aided thermochemical cyclic operation have eliminated several material families due to phase composition instabilities and identified among the various compositions synthesized NiFe₂O₄ and CoFe₂O₄ as the two most suitable for cyclic water splitting – thermal reduction operation. First such thermochemical cyclic tests between 800 and 1400 °C with NiFe₂O₄ and CoFe₂O₄ in powder form in a fixed bed laboratory reactor have demonstrated capability for cyclic operation and alternate hydrogen/oxygen production at the respective cycle steps for both materials. Under the particular testing conditions the two materials exhibited hydrogen/oxygen yields of the same magnitude and similar temperatures of oxygen release during thermal reduction.     

A comparative thermodynamic analysis of samarium and erbium oxide based solar thermochemical water splitting cycles

This paper reports a thermodynamic comparison between the samarium and erbium oxide based solar thermochemical water splitting cycles. These cycles are a two-step process in which the metal oxide is first thermally reduced into the pure metal, and the produced metal can be used to split water to produce H₂. The metal oxides can be reused for multiple cycles without consumption. The effect of water splitting temperature on various thermodynamic parameters which are essential to design the solar reactor system for the production of H₂ via water splitting reaction using the samarium and erbium oxides is studied in detail. The total amount of solar energy needed for the thermal reduction of samarium and erbium oxides is estimated. The amount of heat energy released by the water splitting reactor is calculated. Also, the cycle and solar-to-fuel energy conversion efficiency for both cycles are determined by employing heat recuperation. Obtained results indicate that the efficiencies associated with these cycles are comparable to the previously studies thermochemical cycles. It is observed that higher water splitting temperature favors towards higher efficiencies. At constant thermal reduction temperature = 2280 K, by employing 50% heat recuperation, the solar-to-fuel energy conversion efficiency for the samarium cycle (30.98%) is observed to be higher than erbium cycle (28.19%).     

Hydrogen production by splitting water on solid acid materials by thermal dissociation

Conventionally, there have been three basic ways of research on H₂ production from H₂O-splitting with solar energy: photo-catalytic, photo-electrochemical and thermochemical. Among them the thermal dissociation of H₂O has been considered the most efficient, because it is a single step energy conversion process and gives much higher conversion efficiency than those resulted from other methods. However, the major stumbling block of thermal dissociation of H₂O has been the requirement of a high dissociation temperature which causes problems both with materials for the reactor and with energy conversion efficiency for the process. In this study, we show that the dissociation temperature can be drastically lowered when H₂O is thermally dissociated on solid acid materials. A probable mechanism of the thermal H₂O-splitting on solid acid materials is also presented, based on some experimental results of this study and reports in the literature.     

Thermochemical production of hydrogen from hydrogen sulfide with iodine thermochemical cycles

With the goal of eventually developing a replacement for the Claus process that also produces $H_2$, we have explored the possibility of decomposing hydrogen sulfide through a thermochemical cycle involving iodine. The thermochemical cycle under investigation leverages differences in temperature and reaction conditions to accomplish the unfavorable hydrogen sulfide decomposition to $H_2$ and elemental sulfur over two reaction steps, creating and then decomposing hydroiodic acid. This proposed process is similar to ideas put forth in the 1980s and 1990s by Kalina, Chakma, and Oosawa, but makes use of thermochemical hydrogen iodide decomposition methods and catalysts rather than electrochemical or photoelectrochemical methods.Process models describing a potential implementation of this thermochemical cycle were created. Motivated by the process model results, experimentation showed the possibility of using alternative solvents to dramatically decrease the energy requirements for the process. Further process modeling incorporated these alternative solvents and suggests that this theoretical hydrogen sulfide processing unit has favorable economic and environmental properties.     

Feasibility studies of chemical reactions for thermochemical water splitting cycles of the iron-chlorine,iron-sulfur and manganese-sulfur families

The number of chemical reactions in water splitting cycles of the iron-chlorine, iron-sulfur and manganese-sulfur families is quite high from a thermodynamic point of view. Applying chemical engineering criteria to the reactions allows the cancelling of unfavorable steps. The remaining reactions can be systemized to typical groups. Experimental results of these groups of reactions are presented, depending on temperature, ratio of reactants and reaction performance. The results are discussed with respect to the performance of the chemical reactions in water splitting cycles.     

Thermochemical two-step water splitting cycles by monoclinic ZrO2-supported NiFe2O4 and Fe3O4 powders and ceramic foam devices

A thermochemical two-step water splitting cycle is examined for NiFe₂O₄ and Fe₃O₄ supported on monoclinic ZrO₂ (NiFe₂O₄/m-ZrO₂ and Fe₃O₄/m-ZrO₂) in order to produce hydrogen from water at a high-temperature. The evolution of oxygen and hydrogen by m-ZrO₂-supported ferrite powders was studied, and reproducible and stoichiometric oxygen/hydrogen productions were demonstrated through a repeatable two-step reaction. Subsequently, a ceramic foam device coated with NiFe₂O₄/m-ZrO₂ powder was made and examined as a water splitting device by the direct irradiation of concentrated Xe-light in order to simulate solar radiation. The reaction mechanism of the two-step water splitting cycle is associated with the redox transition of ferrite/wustite on the surface of m-ZrO₂. A hydrogen/oxygen ratio for these redox powder systems exhibited good reproducibility of approximately two throughout the repeated cycles. The foam device loaded NiFe₂O₄/m-ZrO₂ powder was also successful with respect to hydrogen production through 10 repeated cycles. A ferrite conversion of 24-76% was obtained over an irradiation period of 30 min.     

Direct thermomagnetic splitting of water

The application of a magnetic field to water tends to cause its decomposition into hydrogen and oxygen. Based upon the thermomagnetochemistry of the phenomenon, a process is suggested for carrying out the reaction and separating the product hydrogen and oxygen. The process would have nearly Carnot efficiency, although the requisite magnetic field (～ 10⁴ tesla) is not at present attainable.     

On the splitting of water

Future energy needs and requirements in manufacturing processes (like fertilizers, synfuels, etc.) makes hydrogen an important chemical commodity. It is projected that hydrogen required for various processes may reach 1.8 × 10⁹ MBTU by the year 2000. This increases the importance of producing hydrogen especially from a cheap raw material like water. A survey of the different approaches for splitting water (electrolysis, plasmolysis, magnetolysis, magmalysis, photolysis, photoelectrochemical methods, radiolysis, etc.) is made and discussed in detail in this review.     

Cadmium sulfide Co-catalyst reveals the crystallinity impact of nickel oxide photocathode in photoelectrochemical water splitting

Nickel oxide (NiO) with p-type semiconducting behaviour was prepared via a direct anodisation of nickel (Ni) foam followed by calcination treatment. This method offers a direct photoelectrode synthesis without the intermediate step using a pre-synthesised NiO powder. NiO photocathodes with modulated crystallinity were prepared under elevated calcination temperatures. The beneficial effect of having higher crystallinity in generating higher cathodic photocurrent became obvious in the aid of cadmium sulfide (CdS) deposition. It was found that CdS can promote the excited charge transportation of NiO towards water reduction, thus revealing the effect of NiO crystallinity modulation. The role of CdS as co-catalyst rather than a photosensitiser can be useful in the future design of photoelectrodes.     

Thermochemical two-step water splitting by internally circulating fluidized bed of NiFe2O4 particles: Successive reaction of thermal-reduction and water-decomposition steps

Thermochemical two-step water splitting using a redox system of iron-based oxides or ferrites is a promising process for producing hydrogen without CO₂ emission by the use of high-temperature solar heat as an energy source and water as a chemical source. In this study, thermochemical hydrogen production by two-step water splitting was demonstrated on a laboratory scale by using a single reactor of an internally circulating fluidized bed. This involved the successive reactions of thermal-reduction (T-R) and water-decomposition (W-D). The internally circulating fluidized bed was exposed to simulated solar light from Xe lamps with an input power of 2.4-2.6 kW_th for the T-R step and 1.6-1.7 kW_th for the subsequent W-D step. The feed gas was switched from an inert gas (N₂) in the T-R step to a gas mixture of N₂ and steam in the W-D step. NiFe₂O₄/m-ZrO₂ and unsupported NiFe₂O₄ particles were tested as a fluidized bed of reacting particles, and the production rate and productivity of hydrogen and the reactivity of reacting particles were examined.     

MOFs in photoelectrochemical water splitting: New horizons and challenges

Growing energy consumption with the augmentation in universal population to more than nine billion by 2050 and exhausting fossil fuel reserves necessitates a harsh revolution from non-renewable energy reservoirs to renewable energy reservoirs with zero carbon emission. In the present scenario, solar energy prompted photoelectrochemical (PEC) water splitting or “Artificial Photosynthesis” via light gripping semiconductor material, originates out as the most promising methodology in accomplishing the global energy crisis. Recent studies have amply demonstrated the potential of metal-organic frameworks (MOF) towards PEC applications. They are porous crystalline coordination polymers assembled through an appropriate choice of metal ions and multidentate organic ligands. Owing to their structural regularity and synthetic tunability, MOFs integration with PEC is considered in terms of enhancing and broadening light absorption, providing active sites and directing charge transfer dynamics. Here, we have explored MOFs role in PEC and classified them into different categories such as photosensitizers, co-catalysts, counter electrode, template and also for imparting additional stability to the electrode system. MOFs mediated PEC water splitting is promising but is still rare and in its infancy. Therefore, it is pertinent and timely to take stock of the advancements made and develop insight on the use of MOFs, as an emerging solution for the problems encountered in PEC. This review covers the basics of MOF & mainly describes various case studies done during last 10 years and providing adequate impetus to researchers for critically assessing the recent advances and challenges that are faced by scientists and researchers at large.     

Graphene-based electrocatalysts: Hydrogen evolution reactions and overall water splitting

Recently, carbon-based materials (e.g., graphene, carbon nanotubes, carbon quantum dots) have been used as electrocatalysts to catalyze the reactions such as hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). Among them, graphene has attracted attention as an electrocatalyst, and its electrocatalytic performances have been improved by doping with metals and non-metals, surface and defect engineering, and hybrid development. In this perspective, the present paper reviewed the recent advances (2018 onwards) on the progress of graphene-based electrocatalysts for HER and overall water splitting (OWS). It is emphasizing strategies for optimizing electrocatalytic properties followed by challenges and future outlook. This review will provide the essential ideas and strategies that can help design graphene-based electrocatalysts of high performance that can be implemented for sustainable energy application.     

Research on PEMFC resistance relaxation characteristics and degradation under thermal cycles with different residual water locations

The storage problem at low temperatures is one of the technical barriers to commercialization in the future for the polymer electrolyte membrane fuel cell (PEMFC). In this study, the resistance relaxation characteristic under different pretreatment methods before low-temperature storage of PEMFC is analyzed. The effect of residual water in different PEMFC locations on its storage performance after thermal cycles are investigated. The evolution curves of resistance after the purging process are different under equilibrium, cold, and hot purge and the percentage drop of cell resistance at relaxation stage under three methods are 14.5%, 66.3%, and 73.1%. It is found that the most likely reason for relaxation is membrane water structure reorganization. The voltage of the cell with no purge and purged to the end of the first stage becomes smaller at low current density after 20 freeze/thaw cycles. The charge transfer resistances of the cell with no purge, purged to the end of the first stage, and purged to the end of the second stage increase by 8.2%, 15.6%, and 7.4%, respectively. And the cell purged to the end of the first stage has the largest decay rate of electrochemical surface area. The result implies that the performance degradation after freeze/thaw cycles is associated with the water in the ionomer of catalyst layers, which freezes between ionomers and Pt particles at low temperatures.