Water Splitting Cycle Research Articles

Simulation study on the microscopic characteristics of electrochemical Bunsen reaction in the sulfur–iodine cycle for renewable hydrogen production

The sulfur–iodine (SI) thermochemical water-splitting cycle is a promising hydrogen production process with high thermal efficiency and low pollution. The Bunsen reaction in the cycle using an electrochemical cell (EC) has been proposed and R&D on the cell has been carried out. To fill the knowledge gap in the microscopic characteristics of the electrochemical Bunsen reaction, a numerical study was conducted in this work. A two-dimensional, steady state, laminar and isothermal model of EC with detailed processes of flow, species transfer and electrode reactions was developed, and the simulated results showed good agreement with experimental data. The reaction rates, indicated in the form of species molar flux variation, increase with rising current density due to more electron transfer at a certain time, along with the increase of electric energy consumption from 9.1 W/m2 to 227.9 W/m2. Increasing the temperature inhibits the conversion and generation of species. Although higher flow rate reduces the flow residence time as well as the concentration variation of species, faster flow and higher bulk reactants concentration around the electrode surface lead to the increase of overall reaction rates. The consumption of pumping power also increases from 0.264 W/m2 to 6.661 W/m2 with rising flow rate. The optimal operating conditions of 4–5 A/dm2, 303–313 K and 0.01–0.02 m/s are obtained. The simulation favors a better understanding of the phenomena occurring in the EC and its further optimization.

Application of corrosion-resistant Corning advanced-flow reactors for multiphase Bunsen reaction - Part two: investigation on multiphase reaction

Bunsen reaction ( 2 H 2 O + I 2 + S O 2 → H 2 S O 4 + 2 H I ) is a key step for hydrogen production from either the H2S splitting cycle or the sulfur-iodine (S-I) cycle of water splitting. As pointed out in part one, when engineering this reaction, many challenges such as side reactions and corrosion impede scaling up this process. Using iodine-toluene solution to provide flowing iodine below the melting point of iodine renders the Bunsen reaction to be conducted at ambient temperature such that these challenges can be either overcome or eased. However, using toluene as the iodine solvent makes the Bunsen reaction a multiphase reaction system which includes gas, aqueous, and organic phases. Glass-made Corning® advanced-flowTM reactors (AFRs) can be used for Bunsen reaction because they are good at resisting corrosion, improving mixing efficiency of multiphase fluids, and allowing seamless scaling up. Part one has studied the absorption behavior of SO2 gas in the liquids used for Bunsen reaction (water, toluene and water-toluene mixture). Part two (this work) mainly studies the Bunsen reaction using the Corning® microscale (LF) and milliscale (G1) AFRs. When I2 was dissolved in toluene, the Bunsen reaction was conducted by feeding SO2 gas, water, and I2/toluene solution into the AFRs. SO2 and I2 were used as the limiting reactants in turn, and the effects of operating conditions such as gas and liquid flow rates, water to toluene ratio, and temperature in the range (22-80 oC) on the absorption rates of SO2 and the I2 reaction rate were studied. The results confirm the seamless scaling-up capability of the Corning reactors when the flow rates were increased twenty times from AFR-LF to AFR-G1.

First-principles investigation of BiVO3 for thermochemical water splitting

Thermochemical water splitting is a promising clean method of hydrogen production of high relevance in a society heavily reliant on fossil fuels. Using evolutionary methods and density functional theory, we predict the structure and electronic properties of BiVO3. We build on previous literature to develop a framework to evaluate the thermodynamics of thermochemical water splitting cycles for hydrogen production. We use these results to consider the feasibility of BiVO3 as a catalyst for thermochemical water splitting. We show that for BiVO3, both the thermal reduction and gas splitting reactions are thermodynamically favorable under typical temperature conditions. We predict that thermochemical water splitting cycles employing BiVO3 as a catalyst produce hydrogen yields comparable to those of commonly used catalysts.

Hydrogen production methods efficiency coupled to an advanced high-temperature accelerator driven system

Hydrogen, rather than oil, must be produced in volumes not provided by the currently employed methods. In this work, two high-temperature hydrogen production methods coupled with an advanced nuclear system are presented. A new design of a pebble-bed accelerator nuclear-driven system called TADSEA (Transmutation Advanced Device for Sustainable Energy Applications) was chosen because of the advantages in transmutation and safety. A detailed flowsheet of the high-temperature electrolysis process coupled to TADSEA through a Brayton gas cycle was developed using chemical process simulation software: Aspen HYSYS®. It is obtained 0.1627 kg/s of hydrogen with the model with optimized operating conditions, resulting in an overall process efficiency of 34.51%, a value in the range of results reported by other authors. A conceptual design of a plant using the iodine-sulfur thermochemical water splitting cycle was carried out producing 5.66e-2 kg/s and electric energy in cogeneration. The overall efficiency was calculated performing an energy balance resulting in 22.56%. A brief hydrogen production cost estimation was performed for both methods obtaining 5.96$/kg for the sulfur-iodine (SI) and 4.8 $/kg for the high-temperature electrolysis (HTE) process.

Metal sulfate decomposition using green Pd-based catalysts supported on γAl2O3 and SiC: A common step in sulfur-family thermochemical cycles

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.

A review of recent researches on Bunsen reaction for hydrogen production via S–I water and H2S splitting cycles

Abstract The Bunsen reaction is the center reaction for both the sulfur–iodine water splitting cycle for hydrogen production and the novel hydrogen sulfide splitting cycle for hydrogen and sulfuric acid production from the sulfur-containing gases. This paper reviews the research progress of the Bunsen reaction in recent 10–15 years. Researches were initially focused on the optimization of the operating conditions of the conventional Bunsen reaction requiring excessive water and iodine to improve the products separation efficiency and to avoid the side reactions and iodine vapor deposition. Alternative methods including electrochemical methods, precipitation methods, and non-aqueous solvent methods had their respective advantages, but still faced challenges. In development of the technology of H2S splitting cycle, dissolving iodine in toluene solvent could render the Bunsen reaction to occur with the flowable I2 stream at ambient temperature such that the side reactions and iodine vaporization can be avoided and the corrosion hazard lessened. It also prevented the Bunsen reaction from using excessive iodine and water. The products from the Bunsen reaction including HI, H2SO4, H2O, and toluene could be directly electrolyzed.

Sustainability analysis of different hydrogen production options using hesitant fuzzy AHP

Hydrogen is seen as the key component of energy systems for a sustainable future. In the literature, there has been extensive efforts on making hydrogen energy systems more sustainable. True sustainability of such systems requires hydrogen to be produced in clean, reliable, affordable, and safe manners without harming neither the environment nor the societies. In the literature, there is a lack of studies focusing on a complete technical, environmental, social, and economic evaluation of hydrogen production systems by taking availability and reliability into account. Therefore, the primary aim of this study is to provide a comprehensive review and investigation on sustainability of hydrogen production systems, which could potentially guide researchers, policy makers, different industries, and energy market customers. The selected hydrogen production methods are grid electrolysis (electricity from fossil fuels), wind electrolysis, PV electrolysis, nuclear thermochemical water splitting cycles, solar thermochemical water splitting cycles and photoelectrochemical cells. To deal with the ambiguity and vagueness in the evaluation process, and to overcome the observed hesitancy in decision makers’ preferences, a novel approach, hesitant fuzzy AHP, is used to evaluate sustainability of the selected hydrogen production methods. In the proposed model, five criteria; economic performance (initial cost and running cost), environmental performance (GHG emissions, land use, water discharge quality, and solid waste), social performance (impact on public health, employment and training opportunities, and public acceptance), technical performance (energy and exergy efficiencies, process control, and raw material input), and availability/reliability (dependence on imported resources, predictability, and scalability) are taken into account. The results show that grid electrolysis is expected to hold the key of sustainable hydrogen production in the near future while the technologies of other methods advance and their associated costs decrease.

Thermodynamic development and design of a concentrating solar thermochemical water-splitting process for co-production of hydrogen and electricity

A concentrating solar plant is proposed for a thermochemical water-splitting process with excess heat used for electricity generation in an organic Rankine cycle. The quasi-steady state thermodynamic model consisting of 23 components and 45 states uses adjustable design parameters to optimize hydrogen production and system efficiency. The plant design and associated thermodynamic model demonstrate that cerium oxide is suitable for thermochemical water-splitting cycles involving the co-production of hydrogen and electricity. Design point analyses at 900 W/m2 DNI indicate that a single tower with solar radiation input of 27.74 MW and an aperture area of 9.424 m2 yields 10.96 MW total output comprised of 5.55 MW hydrogen (Gibbs free energy) and 5.41 MW net electricity after subtracting off 22.0% of total power generation for auxiliary loads. Pure hydrogen output amounts to 522 tonne/year at 20.73 GWh/year (HHV) or 17.20 GWh/year (Gibbs free energy) with net electricity generation at 14.52 GWh/year using TMY3 data from Daggett, California, USA. Annual average system efficiency is 38.2% with the constituent hydrogen fraction and electrical fraction being 54.2% and 45.8%, respectively. Sensitivity analyses illustrate that increases in particle loop recuperator effectiveness create an increase in hydrogen production and a decrease in electricity generation. Further, recuperator effectiveness has a measurable effect on hydrogen production, but has limited impact on total system efficiency given that 81.1% of excess heat is recuperated within the system for electricity generation.

Catalytic performance of semi-coke on hydrogen iodide decomposition in sulfur-iodine thermochemical cycle for carbon dioxide-free hydrogen production

Sulfur-iodine thermochemical water splitting cycle is a promising and carbon dioxide-free method for hydrogen production. Among the reactions in this cycle, hydrogen iodide catalytic decomposition is the rate-determining step. In this study, catalytic reactivity test combined with characterizations of nitrogen physisorption, X-Ray diffraction, and Raman spectroscopy provide evidences that hydrofluoric acid modified semi-coke is a promising catalyst candidate for hydrogen iodide decomposition because of its high active and low cost. The raw semi-coke enhanced the hydrogen iodide conversion rate. After modification of hydrofluoric acid, the catalytic activity of semi-coke increased largely. Semi-coke modified by 40 wt% hydrofluoric acid showed better performance than commercial activated carbon catalyst. Combine the characterization results and catalytic reactivity, the graphitic edge carbon atoms in semi-coke are the active sites for hydrogen iodide decomposition. This finding pointed out the direction of carbon material catalyst design for hydrogen iodide decomposition.

The stability of Pt–Ir/C bimetallic catalysts in HI decomposition of the iodine–sulfur hydrogen production process

Decomposition of HI is the key reaction of hydrogen production in the iodine–sulfur thermochemical water splitting cycle, so studies about the catalysts for HI decomposition have drawn increasing attention. In this study, a series of monometallic Pt/C((Pt/C-400, Pt/C-500, Pt/C-600, Pt/C-700 and Pt/C-800), Ir/C(Ir/C-400, Ir/C-500, Ir/C-600, Ir/C-700 and Ir/C-800) and bimetallic Pt–Ir catalysts supported on active carbon (Pt–Ir/C-400, Pt–Ir/C-500, Pt–Ir/C-600, Pt–Ir/C-700 and Pt–Ir/C-800 were prepared by the impregnation-reduction-calcination method. Their catalytic activities were evaluated for HI decomposition in a fixed bed reactor at 400 and 500 °C under atmospheric pressure. Their structures, metal particles size and distribution, and specific surface area were characterized by X-ray diffraction (XRD), Transmission electron microscopy (TEM) and Brunauer-Emmett-Teller (BET) surface area, respectively. The results showed that the bimetallic Pt–Ir catalyst had the excellent stability in terms of the anti-sintering structure and catalytic activity. Therefore, the bimetallic Pt–Ir catalysts are the good candidates to take the place of the traditional monometallic Pt/C catalyst for catalyzing the HI decomposition.

Stability of Molten-Phase Cs–V–O Catalysts for SO3 Decomposition in Solar Thermochemical Water Splitting

Supported molten cesium vanadate catalysts (Cs–V–O/SiO2) showed activities comparable to that of a reference Pt catalyst (1 wt % Pt/TiO2) for SO3 decomposition at moderate temperatures (∼600 °C), which is essential as an O2 evolution reaction in solar thermochemical water splitting cycles. Stability testing of the catalyst over a 1000 h continuous reaction at 600 °C resulted in deactivation by ∼20% of the initial activity. Kinetic analysis of the activity versus time-on-stream indicated that the observed deactivation behavior can be divided into an induction period (≤100 h) and an acceleration period (>100 h). The deactivation is mainly caused by the vaporization loss of active components (Cs and V) from the molten phase. At the earliest stage, most vapor is generated in the upstream section of the catalyst bed and then redeposits therebelow. Upon repeating these vaporization and deposition cycles, Cs and V move gradually downstream. During this induction period, the deactivation is not obvious because th...

Na[sbnd]O[sbnd]H thermochemical water splitting cycle: A new approach in hydrogen production based on sodium cooled fast reactor

Currently, the main energy options employed to maintain essential living standards, such as electricity, hot water and refrigeration, come from fossil fuels whose burning contributes to high environmental impacts like global warming. Then, the development of clean fuels is an important step towards sustainability. Hydrogen (H2) can achieve such goal because its combustion mainly releases water. It can be obtained in different ways, including thermochemical cycles that consist of a sequence of chemical reactions to split water molecules into hydrogen and oxygen through a heat source at specific temperature conditions. Some traditional thermochemical processes available in the literature, like the cycles S-I (sulfur-iodine) and Cu-Cl (copper-chlorine) require temperature limits near to 900 °C and 550 °C, respectively. Additionally, the Mg-Cl (magnesium-chlorine) cycle can operate at temperatures about 450 °C while the U-Eu-Br (uranium-europium-bromium) cycle has its maximum operational temperature of 300 °C. In contrast to Cu-Cl, Mg-Cl and U-Eu-Br processes, which have relatively low and viable temperature ranges, there are thermochemical cycles that demand temperatures higher than 1000 °C. The low temperature requirement of a thermochemical process facilitates hydrogen production because it allows the use of many different heat sources like solar, nuclear and waste heat. In this line of reason, in a past work, it was proposed a new set of chemical reactions able to produce hydrogen, as a thermochemical process, which basic elements are sodium (Na), oxygen (O) and hydrogen (H). This system is named in this work as NaOH cycle and has potential to operate at temperatures about 400–500 °C or even below 400 °C. So, the aim of the paper is to present and evaluate a theoretical hydrogen production plant based on the NaOH cycle considering a Sodium Cooled Fast Reactor (SFR) as the heat source. The system was modeled in the Engineering Equation Solver (EES) software according to mass balances in addition to the first and second laws of thermodynamics. In this way, it was possible, for the first time, to estimate the amount of hydrogen obtained in this process. According to the results, the system can produce 1.321 kg/s of H2, equivalent to 114 ton/day. This is a theoretical maximized value, because some aproximations were considered in the calculations. Additionally, the NaOH system has the potential for improvements through more research because it is in the initial stage of development.

Numerical analysis of the electrochemical Bunsen reaction for hydrogen production from sulfur–iodine cycle

The sulfur–iodine (S-I) water-splitting cycle is one of the most promising hydrogen production methods. The Bunsen reaction in the cycle affects the flowsheet complexity and thermal efficiency, but an electrochemical technique has recently been applied to make the S-I cycle more simplified and energy efficient. However, the performance of the electrochemical Bunsen reaction, especially the electrode reactions inside the electrolytic cell (EC) are not clear at present. In this work, a two-dimensional numerical model of EC was developed. The detailed reaction process was numerically calculated with considering the coupling of mass transfer and electrochemical reactions, and was verified using experimental data. The effects of various operating parameters on the reactions were investigated. The results showed that the increase of current density significantly improves the conversion rates of reactants. A higher temperature is unfavorable for concentrating H2SO4 and HI. Increase in the inlet flow rate reduces the conversion rates of reactants, but the impact declines with further rising flow rate. An optimal operating condition is also proposed. The theoretical simulation study will provide guidance for the improvement of experimental work.

Direct electrolysis of Bunsen reaction product HI/H2SO4/H2O/toluene mixture for hydrogen production: Pt electrode characterization

In chemical cycles to produce hydrogen, the H2S splitting cycle and the sulfur-iodine (SI) water splitting cycle both share the Bunsen reaction and HI decomposition. Therefore, they have to overcome the same challenges in the technology development, one of them being the complex and difficult separations of the mixed hydroiodic acid and sulfuric acid solution after the Bunsen reaction. To avoid the separations, this paper studies the electrolysis of the HI/H2SO4/H2O/toluene mixture, focusing on the electrochemical characterization of the Pt electrode by using linear sweep voltammetry (LSV) and cyclic voltammetry (CV). The results show that hydrogen is identified from the gas generated from the cathode in electrolysis. Iodide oxidation is the main reaction in the anode chamber and no significant side reactions are observed. Iodine deposition on the anode surface increases the resistance to iodide diffusion to the anode. However, it can be mitigated by adding toluene in or applying stirring to the anolyte HI/H2SO4 solution. The Pt cathode and sulfuric acid catholyte also work stably.

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

Abstract 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).

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.

Solar Thermochemical Hydrogen (STCH) Processes

In order for hydrogen to play a key role in the move towards renewable energy, CO2-neutral hydrogen must be generated efficiently, inexpensively, and at a large scale. This article focuses on thermochemical water-splitting cycles, especially when coupled with solar energy. There are hundreds of possible thermochemical cycles, but only a few have been considered commercially viable. Commonalities among thermochemical cycles are a series of reactions that split water at lower temperatures (∼500–1000°C) than thermal dissociation (>2500°C), with other species recycled in the system. Thermochemical cycles can be direct (all chemical steps) or hybrid (i.e., a combination of chemical and electrochemical steps) processes. If concentrated solar energy is used for the thermal dissociation step, all thermochemical cycles can be classed as solar thermochemical hydrogen (STCH) processes. This article discusses two cycles: 1) a direct thermochemical cycle (zinc oxide cycle), and 2) a hybrid cycle (hybrid sulfur cycle). The latter combines a chemical thermal dissociation step with an electrochemical step. As two-step cycles, their appeal is in their relative simplicity. Although probably several years away, the outlook for solar thermochemical hydrogen generation is strong, especially with continued advances in materials, and system design and optimization.

Kinetics and mechanism of solar-thermochemical H2 and CO production by oxidation of reduced CeO2

The combination of high thermal stability and fast reduction/oxidation kinetics has drawn attention to CeO2 for use in solar-driven, two-step thermochemical cycles for water and/or carbon dioxide splitting. In accordance with this renewed interest in CeO2, there is a need to better understand the gas splitting chemistry on the reduced oxide. In this study, we measured the H2 production rates over ceria powder, thermally reduced by laser irradiation, during oxidation by H2O and CO2 gases in a stagnation flow reactor. The reaction kinetics was extracted using a model-based analytical approach to account for the effects of mixing and dispersion in the reactor. We find the rigor of this approach necessary in order to identify the rate controlling mechanism and assign parameters to the kinetic model, and to successfully model the production of H2 and CO production over the entire envelope of the fuel curve. The water splitting reaction, in the range of 750–950 °C and 20–40 vol% H2O, can best be described by a first-order kinetic model with low apparent activation energy (29 kJ/mol); while the carbon dioxide splitting reaction, in the range of 650–875 °C and 10–40 vol% CO2, is a more complex surface-mediated phenomena.

Computational model of a sulfur-iodine thermochemical water splitting system coupled to a VHTR for nuclear hydrogen production

Sulfur-Iodine thermochemical water splitting cycle coupled is one of the most promising methods for hydrogen production using a nuclear reactor as the primary energy source. However, there are not references in the scientific publications of a test facility that allow to evaluate the efficiency of the overall process. A computational model for the evaluation and optimization of the sulfur-iodine cycle coupled to a very high temperature reactor for nuclear hydrogen production was developed using a chemical process simulator Aspen HYSYS®. Some operational and design parameters of the cycle sections can be optimized in order to obtain the maximum hydrogen production and higher efficiency. The optimized sections of the flowsheet are coupled to a very high temperature nuclear system (TADSEA) through a Brayton gas cycle for power cogeneration. It is proposed a closed flowsheet for the sulfur-iodine thermochemical water splitting cycle coupled to an accelerator driven system, considering a Brayton cycle for the energy production. It is obtained an acceptable value of global efficiency for the initial operating condition. Several parametric studies are conducted using the flowsheet proposed to evaluate important operating parameters in the overall process efficiency.

Catalytic SO3 Decomposition Activity and Stability of Pt Supported on Anatase TiO2 for Solar Thermochemical Water-Splitting Cycles.

Pt-loaded anatase TiO2 (Pt/TiO2-A) was found to be a highly active and stable catalyst for SO3 decomposition at moderate temperatures (∼600 °C), which will prove to be the key for solar thermochemical water-splitting processes used to produce H2. The catalytic activity of Pt/TiO2-A was found to be markedly superior to that of a Pt catalyst supported on rutile TiO2 (Pt/TiO2-R), which has been extensively studied at a higher reaction temperature range (≥800 °C); this superior activity was found despite the two being tested with similar surface areas and metal dispersions after the catalytic reactions. The higher activity of Pt on anatase is in accordance with the abundance of metallic Pt (Pt0) found for this catalyst, which favors the dissociative adsorption of SO3 and the fast removal of the products (SO2 and O2) from the surface. Conversely, Pt was easily oxidized to the much less active PtO2 (Pt4+), with the strong interactions between the oxide and rutile TiO2 forming a fully coherent interface that limited the active sites. A long-term stability test of Pt/TiO2-A conducted for 1000 h at 600 °C demonstrated that there was no indication of noticeable deactivation (activity loss ≤ 4%) over the time period; this was because the phase transformation from anatase to rutile was completely prevented. The small amount of deactivation that occurred was due to the sintering of Pt and TiO2 and the loss of Pt under the harsh reaction atmosphere.