Solar hydrogen production by two-step thermochemical cycles: Evaluation of the activity of commercial ferrites

In this work, we report on the evaluation of the activity of commercially available ferrites with different compositions, NiFe₂O₄, Ni_0.5Zn_0.5Fe₂O₄, ZnFe₂O₄, Cu_0.5Zn_0.5Fe₂O₄ and CuFe₂O₄, for hydrogen production by two-step thermochemical cycles, as a preliminary study for solar energy driven water splitting processes. The samples were acquired from Sigma–Aldrich, and are mainly composed of a spinel crystalline phase. The net hydrogen production after the first reduction–oxidation cycle decreases in the order NiFe₂O₄ > Ni_0.5Zn_0.5Fe₂O₄ > ZnFe₂O₄ > Cu_0.5Zn_0.5Fe₂O₄ > CuFe₂O₄, and so does the H₂/O₂ molar ratio, which is regarded as an indicator of potential cyclability. Considering these results, the nickel ferrite has been selected for longer term studies of thermochemical cycles. The results of four cycles with this ferrite show that the H₂/O₂ molar ratio of every two steps increases with the number of cycles, being the total amount stoichiometric regarding the water splitting reaction. The possible use of this nickel ferrite as a standard material for the comparison of results is proposed.     

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%).     

Comparative study of the activity of nickel ferrites for solar hydrogen production by two-step thermochemical cycles

In this work, we compare the activity of unsupported and monoclinic zirconia – supported nickel ferrites, calcined at two different temperatures, for solar hydrogen production by two-step water-splitting thermochemical cycles at low thermal reduction temperature. Commercial nickel ferrite, both as-received and calcined in the laboratory, as well as laboratory made supported NiFe₂O₄, are employed for this purpose. The samples leading to higher hydrogen yields, averaged over three cycles, are those calcined at 700 °C in each group (supported and unsupported) of materials. The comparison of the two groups shows that higher chemical yields are obtained with the supported ferrites due to better utilisation of the active material. Therefore, the highest activity is obtained with ZrO₂-supported NiFe₂O₄ calcined at 700 °C.     

Solar thermochemical ZnO/ZnSO4 water splitting cycle for hydrogen production

In this paper, solar reactor efficiency analysis of the solar thermochemical two-step zinc oxide–zinc sulfate (ZnO–ZnSO₄) water splitting cycle. In step-1, the ZnSO₄ is thermally decomposed into ZnO, SO₂, and O₂ using solar energy input. In step-2, the ZnO is re-oxidized into ZnSO₄ via water splitting reaction producing H₂. The ZnSO₄ is recycled back to the solar reactor and hence can be re-used in multiple cycles. The equilibrium compositions associated with the thermal reduction and water-splitting steps are identified by performing HSC simulations. The effect of Ar towards decreasing the required thermal reduction temperature is also explored. The total solar energy input and the re-radiation losses from the ZnO–ZnSO₄ water splitting cycle are estimated. Likewise, the amount of heat energy released by different coolers and water splitting reactor is also determined. Thermodynamic calculations indicate that the cycle (η_cycle) and solar-to-fuel energy conversion efficiency (η_{solar-to-fuel}) of the ZnO–ZnSO₄ water splitting cycle are equal to 40.6% and 48.9% (without heat recuperation). These efficiency values are higher than previously investigated thermochemical water splitting cycles and can be increased further by employing heat recuperation.     

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.     

Two-step thermochemical electrolysis: An approach for green hydrogen production

Electrolysis and thermochemical water splitting are approaches to produce green hydrogen that use either an electrical potential (electrolysis) or a chemical potential (thermochemical water splitting) to split water. Electrolysis is technologically mature when applied at low temperatures, but it requires large quantities of electrical energy. In contrast to electrolysis, thermochemical water splitting uses thermal energy, as thermal energy can typically be supplied at a lower unit cost than electrical energy using concentrating solar power. Thermochemical water splitting, however, typically suffers from high thermal losses at the extremely high process temperatures required, substantially increasing the total energy required. We show how, by combining electrical and chemical potentials, a novel and cost-efficient water splitting process can be envisioned that overcomes some of the challenges faced by conventional electrolysis and thermochemical water splitting. It uses a mixed ionic and electronic conducting perovskite with temperature-dependent oxygen non-stoichiometry as an anode in an electrolyzer. If solar energy is used as the primary source of all energy required in the process, the cost of the energy required to produce hydrogen could be lower than in high-temperature electrolysis by up to 7%.     

Enhanced photocatalytic hydrogen production over In-rich (Ag–In–Zn)S particles

The ratio of ZnS to AgInS₂ is usually adjusted to tune the band gaps of this quaternary (Ag–In–Zn)S semiconductor to increase photocatalytic activity. In this study, the [Zn]/[Ag] ratio was kept constant. The hydrogen production rate was enhanced by increasing the content of indium sulfide. Compared to the steady H₂ evolution rate obtained with equal moles of indium and silver ([In]/[Ag] = 1, 0.64 L/m² h), that obtained with In-rich photocatalyst ([In]/[Ag] = 2, 3.75 L/m² h) is over 5.86 times higher. The number of nanostep structures, on which the Pt cocatalysts were loaded by photodeposition, increased with the content of indium. The indium-rich samples did not induce phase separation between Ag_xIn_xZn_yS_2x+y and AgIn₅S₈, instead forming a single-phase solid solution. Although the photocatalytic activity decreased slightly for bare In-rich photocatalysts, Pt loading played a critical role in the hydrogen production rate. This study demonstrates the significant effect of In₂S₃ on this unique (Ag–In–Zn)S photocatalyst.     

Estimation of electrolytic hydrogen production potential in Venezuela from renewable energies

An initial estimation of the potential for hydrogen (H₂) production in Venezuela is made, obtained by water electrolysis using electricity from renewable sources, taking advantage of the great potential of the country for solar, wind and mini hydro energies. For the first two, its potential maps is obtained from insolation and wind speed maps, respectively, prepared from satellite data, and for mini-hydro, the potential is obtained from documentary information. To calculate the amount of H₂ to produce is used the Higher Heating Value, considering the electrolytic system overall efficiency of 75%, including power requirements of the electrolyzer, auxiliary equipment, and system losses. In addition, in the calculation of usable renewable potential are excluded land areas under special administration, marine, lake and urban areas, and other limitations are considered concerning energy conversion efficiencies and useful areas available for the location of the different renewable technologies.     

Photon-absorption-based explanation of ultrasonic-assisted solar photochemical splitting of water to improve hydrogen production

The ultrasonic-assisted solar photochemical splitting of water had been explored in recent years to enhance hydrogen production efficiency. In this study, a photon-absorption-based study was conducted to investigate the mechanism of the ultrasonic-assisted solar photochemical splitting of water. An elaborate test bench for temperature-controlled, ultrasonic-assisted solar photochemical water splitting was designed, set up, and tested. A comparison of the hydrogen production between the ultrasonic-assisted and conventional solar photochemical splitting of water was carried out. The effective nanoparticle size before and after ultrasonic vibration, as well as after solar photocatalysis, was analyzed. Furthermore, the spectral absorptivity of the nanofluids before and after ultrasonic vibration, as well as after solar photocatalysis, was investigated by both experimental and numerical methods. The investigation indicated that the improved particle dispersion in the solution prepared by ultrasonication allowed the absorbance of more incoming sunlight. The amount of hydrogen produced by the ultrasonic-assisted hydrogen production was 3.45 times that of conventional solar photochemical splitting of water without pre-ultrasonicated. Besides, an effective spectral absorptivity coefficient was proposed as a modified measure of spectral absorptivity. In addition, the optimal particle diameter was optimized using the Monte Carlo ray tracing method to identify the best light absorption performance.     

Likely near-term solar-thermal water splitting technologies

Thermodynamic and materials considerations were made for some two- and three-step thermochemical cycles to split water using solar-thermal processing. The direct thermolysis of water to produce H₂ using solar-thermal processing is unlikely in the near term due to ultra-high-temperature requirements exceeding 3000 K and the need to separate H₂ from O₂ at these temperatures. However, several lower temperature (<2500 K) thermochemical cycles including ZnO/Zn, Mn₂O₃/MnO, substituted iron oxide, and the sulfur–iodine route (S–I) provide an opportunity for high-temperature solar-thermal development. Although zirconia-based materials are well suited for metal oxide routes in terms of chemical compatibility at these temperatures, thermal shock issues are a major concern for solar-thermal applications. Hence, efforts need to be directed towards methods for designing reactors to eliminate thermal shock (ZrO₂ based) or that use graphite (very compatible in terms of temperature and thermal shock) with designs that prevent contact of chemical species with graphite materials at high temperatures. Fluid-wall reactor configurations where inert gases provide a blanket to protect the graphite wall appear promising in this regard, but their use will impact process efficiency. For the case of S–I up to 1800 K, silicon carbide appears to be a suitable material for the high-temperature H₂SO₄ dissociation. There is a need for a significant amount of work to be done in the area of high-temperature solar-thermal reactor engineering to develop thermochemical water splitting processes.     

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.     

Experimental study of effect of anolyte concentration and electrical potential on electrolyzer performance in thermochemical hydrogen production using the Cu-Cl cycle

An important process in the copper-chlorine water splitting cycle for hydrogen production is electrolysis which occurs after a series of cycle steps that produce the constituents for the anolyte of the electrochemical cell. In this investigation, an anolyte mixture of HCl/CuCl/H₂O of varying concentrations is circulated through the electrolyzer to assist in optimizing its performance. It is observed that the concentration and temperature of the anolyte directly affect the process. The efficiency of the electrolyzer is adversely affected, after running a series of experiments, due to copper deposition on the membrane. An important implication of the results is that, to determine the optimal electrolyzer performance, one needs to vary the flow rate and the concentration of anolyte, for a given constant voltage source. In addition, this work demonstrates that aqueous CuCl₂ can be recovered from the waste solution exiting the electrolyzer and recycled to the hydrolysis reactor.     

Large-vscale hydrogen production and storage technologies: Current status and future directions

Over the past years, hydrogen has been identified as the most promising carrier of clean energy. In a world that aims to replace fossil fuels to mitigate greenhouse emissions and address other environmental concerns, hydrogen generation technologies have become a main player in the energy mix. Since hydrogen is the main working medium in fuel cells and hydrogen-based energy storage systems, integrating these systems with other renewable energy systems is becoming very feasible. For example, the coupling of wind or solar systems hydrogen fuel cells as secondary energy sources is proven to enhance grid stability and secure the reliable energy supply for all times. The current demand for clean energy is unprecedented, and it seems that hydrogen can meet such demand only when produced and stored in large quantities. This paper presents an overview of the main hydrogen production and storage technologies, along with their challenges. They are presented to help identify technologies that have sufficient potential for large-scale energy applications that rely on hydrogen. Producing hydrogen from water and fossil fuels and storing it in underground formations are the best large-scale production and storage technologies. However, the local conditions of a specific region play a key role in determining the most suited production and storage methods, and there might be a need to combine multiple strategies together to allow a significant large-scale production and storage of hydrogen.     

Operational strategy of a two-step thermochemical process for solar hydrogen production

A two-step thermochemical cycle process for solar hydrogen production from water has been developed using ferrite-based redox systems at moderate temperatures. The cycle offers promising properties concerning thermodynamics and efficiency and produces pure hydrogen without need for product separation.     

Life cycle analysis of processes for hydrogen production

Hydrogen is considered to be an ideal energy carrier in the foreseeable future and can play a very important role in the energy system. A variety of technologies can be used to produce hydrogen. One of the most remarkable methods for large-scale hydrogen production is thermo-chemical water decomposition using heat energy from nuclear, solar and other sources. Detailed simulations of the two most promising water splitting thermo-chemical cycles (the Westinghouse cycle and the Sulphur-Iodine cycle) were performed in Aspen Plus code and obtained results were used for life cycle analysis. They were compared with two different processes for hydrogen production (coal gasification and coal pyrolysis). Some of the results obtained from LCA are also reported in the paper.