Three-dimensional electrochemical Bunsen reaction characteristics in the sulfur–iodine water splitting cycle for hydrogen production

Hydrogen production from the sulfur–iodine water splitting cycle integrated with solar or nuclear energy has been proposed as a promising technique. Bunsen reaction is one of the three main steps in the cycle and electrochemical method has been applied to this reaction. In present work, a three-dimensional numerical study of the electrochemical Bunsen reaction was conducted. A three-dimensional, steady state, laminar and isothermal mathematical model of electrolytic cell was developed and verified by experiments. The spatial maldistribution of species concentration was found between electrodes and proton exchange membrane (PEM). The electric power drives most H₂SO₄ and I₂ to the anode and cathode surface, respectively, while the proton attraction contributes to HI enrichment on the surface of PEM. At the high inlet H₂SO₄ concentration of 50 wt%, the transformation of flow channel from single serpentine to single entry & double serpentine with the same inlet flow rate cannot solve the insufficient problem of SO₂. But the increase of the overall inlet flow rate in the double entry & double serpentine flow channel make SO₂ sufficient for anode reaction. Further decreasing the inlet H₂SO₄ concentration to 40 wt% and 30 wt% make the initial SO₂ sufficient for overall reactions. The single serpentine channel gives the highest SO₂ conversion rate, followed by the single entry & double serpentine and double entry & double serpentine flow channels. The single serpentine flow channel at the H₂SO₄ inlet concentration of 40 wt% is found optimal for achieving a high electrochemical Bunsen reaction performance.     

Electrochemical characterization of electrodes in the electrochemical Bunsen reaction of the sulfur–iodine cycle

In the electrochemical Bunsen reaction, SO₂ is oxidized to H₂SO₄ at the anode while I₂ is reduced to HI at the cathode. Both electrodes were electrochemically characterized using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The effects of H₂SO₄ concentration in the anolyte, as well as HI concentration and I₂/HI molar ratio in the catholyte, were explored. The cyclic voltammograms of platinum electrode shift with rising scan rate, indicating the irreversibility of two electrode reactions. The equivalent circuit for the cathode reaction impedance consists of an ohmic resistance of the solution, in series with a parallel combination of a charge transfer resistor and a constant phase element, yet the impedance spectra for the anode reaction can be modeled using a parallel combination of a charge transfer resistor and a constant phase element. The electrode reaction kinetics was also analyzed using the exchange current density (j₀) and the standard reaction rate constant (k⁰). The results indicate that a high electrode reaction rate in the cell can be obtained for a HI concentration of 8 mol/kg_H2O and an I₂/HI molar ratio of 0.5 in the catholyte and a H₂SO₄ concentration of 13 mol/kg_H2O in the anolyte.     

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.     

Electrochemical investigation of the Bunsen reaction in the sulfur–iodine cycle

The Bunsen reaction, as a part of the sulfur–iodine thermochemical cycle, was studied using an electrochemical cell. The effects of current density, operating temperature, H₂SO₄ concentration in the anolyte, HI concentration and I₂/HI molar ratio in the catholyte were explored. Both the H₂SO₄ in anolyte and the HI in catholyte were concentrated during electrolysis. Increasing current density amplified this H₂SO₄ and HI concentration, while the other operating parameters also varied the anolyte and catholyte concentration. The transport properties of the cation exchange membrane were examined. The electrode current efficiency remained close to 100% for most runs except those at high current density. Both the average cell voltage and the heat equivalent of electric energy were determined at different conditions.     

Influence of the initial HI on the multiphase Bunsen reaction in the sulfur–iodine thermochemical cycle

In the sulfur–iodine cycle flowsheet, HI may exist in the feeds of Bunsen reaction. The effects of the initial HI and the operating temperature on the kinetic process and thermodynamic equilibrium of the multiphase Bunsen reaction were investigated. Increasing initial HI concentration (HI/H₂O = 0–1/18) or temperature (303 K–358 K) amplified the reaction kinetic rate, and led to the earlier appearance of liquid–liquid separation and less time to reach the thermodynamic equilibrium. But the separation became difficult for further increase of the initial HI content. The liquid–liquid equilibrium (LLE) phase separation was enhanced with rising temperature. An increase in the initial HI content slightly weakened the LLE phase separation at a lower temperature, while at 345 K and 358 K, the LLE phase separation characteristics showed little variation in the HI/H₂O molar ratio range of 0–1/18. A hyper-azeotropic HI concentration in the HIx phase was obtained with feeding HI. The conversion of SO₂ lowered as the initial HI content and the temperature increased.     

Survey of Bunsen reaction routes to improve the sulfur–iodine thermochemical water-splitting cycle

A large excess of water and iodine is typically employed in the Bunsen reaction step of the sulfur–iodine thermochemical cycle in order to induce liquid–liquid phase separation of the two acid products. This paper presents an overview of some alternative routes for carrying out the Bunsen reaction. The use of a reaction solvent other than water is first discussed, and experimental results obtained with tributylphosphate are presented. Another approach is separation of the product acids by selective precipitation of insoluble salts, and the addition of lead sulfate as the precipitating agent is discussed in detail. Finally, the electrochemical Bunsen reaction route is investigated. All of these methods have the potential to reduce the iodine and/or water requirement of the sulfur–iodine cycle.     

Bunsen section thermodynamic model for hydrogen production by the sulfur–iodine cycle

A model for the Bunsen section of the Sulfur–Iodine thermo-chemical cycle is proposed, where sulfur dioxide reacts with excess water and iodine to produce two demixing liquid aqueous phases (H₂SO₄ rich and HI rich) in equilibrium. Considering the mild temperature and pressure conditions, the UNIQUAC activity coefficient model combined with Engels' solvation model is used. The complete model is discussed, with HI solvation by water and by iodine as well as H₂SO₄ solvation by water, leading to a very high complexity with almost hundred parameters to be estimated from experimental data. Taking into account the water excess, a successful reduced model with only 15 parameters is proposed after defining new apparent species. Acids total dissociation and total H⁺ solvation by water are the main assumptions. Results show a good agreement with published experimental data between 25 °C and 120 °C.     

Development of a flowsheet for iodine–sulfur thermo-chemical cycle based on optimized Bunsen reaction

Based on the Bunsen reaction process whose operating conditions are optimized to yield an over-azeotropic HI liquid solution, we devised a new flowsheet of iodine–sulfur thermo-chemical cycle. A highly enriched hydrogen-iodide gas can be generated through a series of processes of liquid–liquid separation of product mixture from Bunsen reaction and flash of over-azeotropic HI solution. Operating temperature and pressure for HI enrichment need not to be increased as high as those for existing flowsheets; as a result, the operating conditions become less corrosive. Chance of pipe clogging due to iodine solidification is low because there is no process where iodine is concentrated that high. Enrichment of HI through spontaneous liquid–liquid separation and simple flash processes avoiding complicated separate process is considered to be an additional benefit. Analysis of overall and component material balances showed that excess amount of feed to each process to get a desired output depends on the efficiency of flash and decomposition processes. Compared to previous ones, the proposed flowsheet requires more recirculation flows throughout the whole cycle mainly because only a portion of HI content exceeding the azeotrope is allowed to evaporate in the flash without employing a separate HI enrichment process. Thermal efficiency of the proposed flowsheet was evaluated, together with a series of parametric analyses for the sensitivity to key operating parameters and component performances. It was observed that the thermal efficiency can be raised above 60% at optimal condition.     

Improved solvation routes for the Bunsen reaction in the sulphur iodine thermochemical cycle: Part III–Bunsen reaction in molecular solvents

Operating the Bunsen reaction in a solvent has the potential to increase SI cycle efficiency and decrease operating costs. Analysing the solvent–acid mixtures produced is complicated as additional acid is formed when SO₂ comes into contact with water. Tri-n-butyl phosphate (TBP) is suitable for HI extraction; however, it is susceptible to acid catalysed dealkylation, resulting in solvent decomposition and the production of butyl iodide. Cyanex^® 923 is found to be superior to TBP in the Bunsen reaction due to its high affinity for HI. Strong orange complexes between HI, SO₂ and the phosphoryl group in Cyanex^® 923 are formed, giving good product separation, however severely hampering HI recovery. Washing the organic phase with water resulted primarily in the removal of H₂SO₄. HI could then be recovered either thermally or with a second wash step.     

Kinetic and thermodynamic studies of the Bunsen reaction in the sulfur–iodine thermochemical process

This work presents the kinetic and thermodynamic studies of the Bunsen reaction in the sulfur–iodine thermochemical cycle for hydrogen production by water splitting. A series of experimental runs have been carried out by feeding the gas mixture SO₂/N₂ in an I₂/H₂O medium in the temperature range of 336–358 K. The effects of the various operating parameters on the SO₂ conversion ratio have been evaluated. The results showed that the efficiency of SO₂ conversion into H₂SO₄ increased with the amount of I₂ or H₂O increase. The increasing reaction temperature impeded SO₂ conversion into H₂SO₄. A kinetic model has been developed to fit to the experimental data obtained in a semi-batch reactor. A good fitting can be observed for each experiment, which discloses the overall kinetic mechanism of the complex Bunsen reaction. The apparent activation energies were found to be 23.513 kJ mol⁻¹ and 9.212 kJ mol⁻¹ for the sequential reactions  and , respectively.     

Experimental investigation of reaction kinetics of gas–liquid–solid Bunsen reaction in iodine–sulfur process

Iodine–sulfur (IS) cycle is the most promising thermochemical water-splitting process for nuclear hydrogen production. The Bunsen reaction, which produces sulfuric and hydriodic acid for the two decomposition reactions, plays a crucial role for the continuous stable operation of the IS cycle. Insufficient kinetics studies on Bunsen reaction, particularly under the gas–liquid–solid heterogeneous conditions, have caused difficulties for the design of Bunsen reactor, as well as the optimization and improvement of the efficiency of the process. In this work, the reaction kinetics of gas–liquid–solid Bunsen reaction denoted in the pressure drop of SO₂ was experimentally investigated, and the effluences of the main factors, including the initial SO₂ pressure, molar ratio of I₂ to H₂O, temperature, and stirring rate, were studied. In addition, a kinetics model for simulating the heterogeneous reaction was proposed and verified by the experimental data obtained under the three-phase Bunsen reaction conditions.     

Apparent kinetics of the Bunsen reaction in I2/HI solution for the iodine–sulfur hydrogen production process

Massive hydrogen production featuring high efficiency, CO₂ free, and cost effectiveness is a crucial challenge for the hydrogen economy. Nuclear hydrogen production through thermochemical iodine–sulfur (IS) process is a potential candidate for this purpose. Chemical reaction kinetics data are indispensable for developing a high-performance reactor as well as the scaling up of the process. The apparent kinetics of the reaction under simulated recycling conditions of IS closed cycle operation was studied by initial rate method. The effects of key parameters, including agitation speed, SO₂ partial pressure, I₂ concentration, and reaction temperature, on reaction rate, were systematically investigated by measuring the variation in SO₂ pressure with reaction time. Initial rate analysis method indicated that the Bunsen reaction rates were 0.23 ± 0.01 and 0.77 ± 0.01 order with respect to SO₂ pressure and I₂ concentration. The apparent activation energy was 5.86 ± 0.21 kJ/mol. Based on these results, an exponential rate expression of the Bunsen reaction was established. In addition, a simplified method for calculation of kinetics parameters was proposed and compared with conventional techniques. Experimental results provide theoretical basis for design and development of Bunsen reactors and for elucidating the reaction process.     

Membrane electrolysis of Bunsen reaction in the iodine–sulphur process for hydrogen production

In traditional IS process for production of hydrogen by water decomposition, the Bunsen reaction (SO₂ + I₂ + 2H₂O → H₂SO₄ + 2HI) was carried out by direct contact of SO₂ with aqueous solution of I₂ where a large excess of I₂ (8 mol) and H₂O (16 mol) were required. Excess amounts of these chemicals severely affected the overall thermal efficiency of the process and new ways including membrane electrolysis was reported in literature for carrying out Bunsen reaction where the amount of excess chemicals can be greatly reduced. We have carried out Bunsen reaction in a two-compartment membrane electrolysis cell containing graphite electrodes and Nafion 117 membrane as a separator between the two-compartments. Electrolysis was carried out at room temperature with continuous recirculation of anolyte and catholyte. Electrolysis was done in constant-current mode with current density in the range of 1.6 A/dm² to 4.8 A/dm². Initial concentrations of H₂SO₄ and HI were about 10 and 5 N, respectively and I₂/HI molar ratio in the catholyte was varied in the range of 0.25–1.5. Current efficiency was found to be close to 100% indicating absence of any side reaction at the electrodes. Cell voltage was found to vary linearly with current densities up to 80 A/dm² and for I₂/HI molar ratio in the range of 0.25–1.5 the cell voltage was found to be lowest for the value of 0.5.     

Process optimization and safety assessment on a pilot-scale Bunsen process in sulfur–iodine cycle

This study investigates Bunsen reaction in the sulfur-iodine (SI) cycle for optimal conditions and specification of equipment in terms of the maximum HI yield and the least impurities in HIx (mixture of HI, I₂ and H₂O), the reaction safety, and dispersion of SO₂ gas and HI_X solution for leakage accident. The pilot-scale Bunsen process was simulated and validated. The optimization of the Bunsen reactor, 3-phase separator, and HI_X purifier have been investigated in order to parameterize the operating conditions and equipment specification for three cases: (1) Maximize the HI yield for the final product (2) Minimize the H₂SO₄ impurities (3) Multi-objective case of both maximum HI production and minimum impurities. The gas reactivity safety was investigated on HI, H₂SO₄, I₂, SO₂, H₂O, and O₂. Also, the SO₂ gas dispersion distance for 30 ppm, 0.75 ppm, and 0.2 ppm and HI dispersion distance for 120 ppm, 25 ppm, and 1 ppm was investigated for targeted unit operators at each optimization scenario. The deviation between pilot-scale experiment and simulation case falls within 1–3% for Bunsen reactor, 6~8% for 3-phase separator, and 2~4% for HI_X purifier. The maximized HI production was increased by 17% for the maximum HI yield case from the designed case. The size and temperature of the Bunsen reactor was increased to enhance the reaction. However, the HI_X purifier size was reduced since reverse Bunsen reaction causes loss in HI product. The H₂SO₄ impurities in the minimize H₂SO₄ impurities case were reduced by 71% from the designed case. The size of the Bunsen reactor remained the same as design case, but the HI_X purifier size was increased to enhance the reverse Bunsen reaction. For multi-objective case, the HI productivity was increased by 16% and the H₂SO₄ impurities were reduced by 67% simultaneously. According Chemical Reactivity Worksheet (CRW) result, O₂ should therefore not be stored with any components except iodine. For SO₂ and HI_X dispersion assessment, the maximum HI yield case reveals the maximum dispersion of SO₂ gas and HI_X solution from the Bunsen reactor. The dispersion from 3-phase separator was almost the same for all the cases. For HI_X purifier, the minimum H₂SO₄ case exhibited the longest distance of SO₂ gas and HI solution dispersion. At 3 bar and 140 °C, the maximum SO₂ and HI_X dispersion distance were occurred.     

Improved solvation routes for the Bunsen reaction in the sulphur iodine thermochemical cycle: Part I – Ionic liquids

The Bunsen reaction is central to the Sulphur Iodine cycle, however large excesses of both water and iodine are currently employed to enable phase separation of the two acids produced. This causes separation issues later in the cycle and induces a large thermal burden for water evaporation. The use of solvents in the reaction has the potential to reduce these large excesses, thereby increasing the cycle efficiency. This paper investigates ionic liquids as solvents for the Bunsen reaction. Several potential ionic liquids are identified based on their anion properties. The extraction of HI into the ionic liquid is then investigated experimentally. [FAP]⁻ ionic liquids were examined but their extreme hydrophobicity prevented water being taken up into the organic phase, severely retarding the extraction of acid by the solvent. Results for the [TMPP]⁻ ionic liquid showed discrepancies in the component balance and it is thought that the solvent may be susceptible to hydrolysis. The extraction of acid by the [Tf₂N]⁻ ionic liquids was more promising, the amount of acid extracted being of the order of 20%. However, the amount of protons and iodide ions extracted by the solvents were not equivalent and evidence is presented demonstrating the presence of an ion exchange mechanism. None of the ionic liquids tested are therefore suitable for use in the Bunsen reaction, however the properties of an ionic liquid can be tailored by the choice of anion and cation. Further investigation of ionic liquids is necessary before they can be conclusively ruled out.     

Continuous Bunsen reaction and simultaneous separation using a counter-current flow reactor for the sulfur–iodine hydrogen production process

The sulfur–iodine (SI) process, which consists of three chemical reactions of the Bunsen reaction, a H₂SO₄ decomposition and a HI decomposition, is an important potential method for hydrogen production among thermochemical water splitting methods. For steady-state operation of the SI process, it is very important to provide information on the composition of each phase that passes from the Bunsen reaction section to the following H₂SO₄ and HI decomposition sections. In this study, the Bunsen reaction was carried out using a counter-current flow reactor, the Bunsen reaction and product separation steps were shown capable of being performed simultaneously, and the composition variation of each phase discharged at the top and bottom of reactor was investigated. The process variables were the SO₂ feed rate, temperature, I₂/H₂O molar ratio. As a result of constant reactant feed and continuous product discharge operation, it was found that the composition remained constant after 120 min of reaction time, indicating steady-state operation. The phase separation characteristics of the Bunsen reaction were minimally affected by the SO₂ feed rate. As the amount of I₂ introduced increased with increasing temperature, the volume of the H₂SO₄ phase discharged from the upper phase was unchanged, while that of the HI_x phase discharged from the lower phase increased proportionally. The average molar composition of the H₂SO₄ phase (H₂SO₄/H₂O/HI) obtained at a typical operation condition (353 K, I₂/H₂O molar ratio of 0.406) was 1/5.30–5.39/0.02–0.04, and the composition of the HI_x phase (HI/I₂/H₂O/H₂SO₄) was 1/2.81–3.09/5.67–6.40/0.04–0.06. These results could be used for the design and operation of H₂SO₄ and HI decomposition sections of the SI process.     

Global warming potential of the sulfur–iodine process using life cycle assessment methodology

A life cycle assessment (LCA) of one proposed method of hydrogen production – thermochemical water-splitting using the sulfur–iodine cycle couple with a very high-temperature nuclear reactor – is presented in this paper. Thermochemical water-splitting theoretically offers a higher overall efficiency than high-temperature electrolysis of water because heat from the nuclear reactor is provided directly to the hydrogen generation process, instead of using the intermediate step of generating electricity. The primary heat source for the S–I cycle is an advanced nuclear reactor operating at temperatures corresponding to those required by the sulfur–iodine process. This LCA examines the environmental impact of the combined advanced nuclear and hydrogen generation plants and focuses on quantifying the emissions of carbon dioxide per kilogram of hydrogen produced. The results are presented in terms of global warming potential (GWP). The GWP of the system is 2500 g carbon dioxide-equivalent (CO₂-eq) per kilogram of hydrogen produced. The GWP of this process is approximately one-sixth of that for hydrogen production by steam reforming of natural gas, and is comparable to producing hydrogen from wind- or hydro-electric conventional electrolysis.     

Improved solvation routes for the Bunsen reaction in the sulphur iodine thermochemical cycle: Part II – Molecular solvent properties

Operating the Bunsen reaction in the presence of a molecular solvent has the potential to decrease the large excesses of water and iodine currently required and therefore to increase the efficiency of the sulphur iodine cycle. Tri-n-butyl phosphate (TBP) has been shown to extract HI into the solvent phase, however the TBP breaks down when treated thermally to recover the HI. Alternative solvents which also show a high extraction of HI, but that are stable in the presence of HI at high temperature, are needed. This paper investigates dibutylbutylphosphonate (DBBP) and Cyanex^® 923 as alternatives to TBP. Both DBBP and Cyanex^® 923 were shown to perform as well as or better than TBP against metrics including: HI extraction, H₂SO₄ extraction, SO₂ solubility and I₂ solubility. Using TGA, thermal recovery of the HI was not successful from DBBP, as the solvent broke down in a similar way to TBP. Small amounts of HI were recovered through distillation of Cyanex^® 923, however the yield was low. Partial success was achieved by backwashing the HI loaded Cyanex^® 923 with water. Further work is needed to assess the performance of the actual Bunsen reaction in each of these solvents and to find ways of recovering the HI from the solvent.     

Hydrogen iodide decomposition over nickel–ceria catalysts for hydrogen production in the sulfur–iodine cycle

In this work, pure CeO₂ and three nickel–ceria catalysts prepared by different methods have been tested to evaluate their effect on hydrogen iodide (HI) decomposition in the sulfur–iodine (SI or IS) cycle at various temperatures. BET, XRD, HRTEM and TPR were performed for catalysts characterization. Indeed, the pure CeO₂ also strongly enhance the decomposition of HI to H₂ by comparison with blank yield. Nickel–ceria catalysts show better catalytic activity, especially Ni-doping-G sample. It is found that, through the sol-gel method, the Ni²⁺ ions have dissolved into the ceria lattice instead of the Ce⁴⁺ ions during the synthesis process of Ni-doping-G sample. Oxygen vacancies are formed because of the charge imbalance and lattice distortion in CeO₂. The presence of Ni during the CeO₂ synthesis process of Ni-doping-G also causes smaller average particle size, larger surface area, better thermal stability and better Ni dispersion than the Ni-loading samples. These provide nickel–ceria catalyst with a potential to be used in the SI cycle for HI decomposition.