Iodine speciation studies on Bunsen reaction of S–I cycle using spectroscopic techniques

Bunsen reaction is an important step of sulfur–iodine cycle for hydrogen production from thermochemical splitting of water. Polyiodide species generated during the separation process need to be identified for complete understanding of the mechanism involved. Speciation of these polyiodide species formed during Bunsen reaction can lead to better understanding of kinetics of the process. HIx species formed have been analyzed using UV–visible and Raman spectroscopic techniques. Peak corresponding to HI₃ species have been ascertained and their conversion to higher HI₅, HI₇ …… species has been observed.     

Effect of asymmetric variation of operating parameters on EED cell for HI concentration in I–S cycle for hydrogen production

EED process for HI concentration was studied for the effect of individual operating parameters such as I₂/HI ratio, concentration of HI(xHI_/H₂O)$\text{HI}\left(x_{\text{HI}/{\text{H}}_2\text{O}}\right)$, temperature and pressure. Studies were conducted in an asymmetric system where the effects of operating parameters were varied for anolyte and the catholyte separately. Open circuit voltage (OCV) was found to be a contributor toward the net potential drop across the EED cell. Ohmic resistance was found to decrease with increase in I₂/HI ratio in catholyte and was found to increase with increase in I₂/HI ratio in anolyte. Increase in xHI_/H₂O$x_{\text{HI}/{\text{H}}_2\text{O}}$ decreased the resistance for anolyte section whereas caused an increase in resistance for catholyte section. Increase in temperature reduced the voltage drop and the resistance across the EED cell. A non-zero differential pressure between the two compartments of the cell increased the resistance across the cell without affecting the OCV value. Electrode potential studies at the graphite electrodes showed an increase in the electro potential with increase in the iodine concentration and decrease with the increase in the HI concentration. Energy required for concentrating acid increased linearly with current density favoring operation at low current densities. Energy consumed in overcoming OCV contributed substantial fraction of the total energy consumed in EED process at lower current densities.     

Effect of orthohydrogen–parahydrogen composition on performance of a proton exchange membrane fuel cell

The effect of orthohydrogen–parahydrogen concentration on the performance of a proton exchange membrane fuel cell is calculated and experimentally investigated. Gibbs free energy and reversible cell potential calculations predict that parahydrogen at room temperature produces a voltage 20 mV/cell higher than normal hydrogen and a 1.6% increase in efficiency over normal hydrogen. Experimental data based on a 1 kW proton exchange membrane fuel cell rapidly switched between normal and parahydrogen did not show a statistically significant difference in performance. Variations due to stack humidity and anode purging are found to dominate fuel cell output. The experimental results confirm that, as anticipated, parahydrogen concentration has a negligible impact on fuel cell performance for the majority of practical applications.     

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.     

Energy consumption in iodine section of I–S: Effect of operating parameters of electro-electrodialysis on overall efficiency of cycle

Reducing heat demand for increasing concentration of HI in the HI_x solution of the iodine circuit of the Iodine–Sulphur cycle is considered the most effective way of increasing efficiency of the cycle. Electro-electrodialysis has emerged as an energy efficient way of increasing the HI_x concentration above azeotropic value. Simulation of the iodine circuit consisting of an EED, a flash and a decomposer was carried out in Aspen Plus™ simulation platform to study the effect of EED current density and outlet HI concentration on the efficiency of the cycle. Efficiency reduced strongly with increase in current density. For EED current density of 5 A/dm², maximum efficiency was ∼35.9% and the optimal range of EED catholyte's exit HI concentration, iodine-free, mole fraction was 0.19–0.21. Simulation results showed that reducing EED resistance was most effective, among all EED parameters, in increasing the cycle's thermal efficiency and if the EED resistance is completely eliminated the thermal efficiency value would increase to 39.4%.     

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.     

Effect of cell compression on the performance of a non–hot-pressed MEA for PEMFC

In this study, the effect of cell compression on the performance of a non–hot-pressed membrane electrode assembly (MEA) for a polymer electrolyte membrane fuel cell (PEMFC) is presented. The MEA is made without hot pressing, by carefully placing the gas diffusion electrodes (GDEs) and a membrane in a fuel cell fixture. Cell performance is assessed at five different compression ratios between 3.6% and 47.8%. It has been shown that ohmic resistance of the cell, mass transport resistance of reactants, charge transfer resistance at electrode, and overall cell performance are strongly dependent on the cell compression. On increasing the cell compression gradually, cell performance improves initially, reaches the best, and then deteriorates. The cell performance is assessed at fully humidified condition and at dry condition. Optimum cell performances are obtained at compression ratios of 14.2% and 25.7% for 100% relative humidity (RH) and 50% RH, respectively. It is also found that the cell with proper compression and at fully humidified conditions can deliver similar performance to a conventional hot-pressed MEA. Finally, it is shown that after the tests, GDEs can be peeled out, and the membrane inspection can be done as a postexperimental analysis.     

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.     

Equilibrium potential for the electrochemical Bunsen reaction in the sulfur–iodine cycle

The electrochemical Bunsen reaction was carried out in an electrochemical cell, where the anodic and cathodic compartments were separated by a Nafion 117 membrane. The equilibrium potential of the cell was experimentally measured and theoretically modeled. The effect of electrolyte concentration and temperature was explored. An increase in SO₂ or I₂ concentration reduced the equilibrium potential, whereas increasing H₂SO₄ or HI concentration had a contrary effect. The cell equilibrium potential decreased with increasing temperature. The derived theoretical equilibrium potential model was verified by the experimental data. The regression parameters M and Z in the model were independent of electrolyte concentration, but M decreased and Z kept constant with increasing temperature. An empirical equilibrium potential formula was proposed based on the theoretical and experimental results. The good reproducibility of this formula for measured data indicated its feasibility to estimate the equilibrium potential and also its guidance for optimizing the electrochemical Bunsen reaction.     

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.     

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.     

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.     

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.     

Development of Pt-Carbon catalysts using MCM-41 template for HI decomposition reaction in S–I thermochemical cycle

Different Pt-Carbon catalysts have been synthesized by hard templating route and have been employed for production of hydrogen from liquid phase HI decomposition at 160 °C temperature. The physical properties and catalytic activities of these catalysts are compared with that of the platinum on activated carbon catalysts. These catalysts have been characterised by X-Ray diffraction, Raman, SEM and BET surface area. Eluant analysis has been carried out using ICP-OES for evaluation of the extent of noble metal leaching under the catalytic activity test conditions. From the present study we have concluded that MCM-41 based Pt/carbon has higher catalytic activity and stability than other Pt/carbon catalysts.     

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.     

Effect of anode conditions on the performance of direct borohydride–hydrogen peroxide fuel cell system

Direct borohydride–hydrogen peroxide fuel cells (DBHPFCs) are attractive power sources for space applications. Although the cathode conditions are known to affect the system performance, the effect of the anode conditions is rarely investigated. Thus, in this study, a DBHPFC system was tested under various anode conditions, such as electrocatalyst, fuel concentration, and stabilizer concentration, to investigate their effects on the system performance. A virtual DBHPFC system was analyzed based on the experimental data obtained from fuel cell tests. The anode electrocatalyst had a considerable effect on the mass and electrochemical reaction rate of the fuel cell system, but had minimal effect on the decomposition reaction rate. The NaBH₄ concentration greatly influenced the mass and decomposition reaction rate of the fuel cell system; however, it had minimal impact on the electrochemical reaction rate. The NaOH concentration affected the electrochemical reaction rate, decomposition reaction rate, and mass of the fuel cell system. Therefore, the significant effects of the anode conditions on the electrochemical reaction rate, decomposition reaction rate, and mass of the fuel cell system prompt the need for their careful selection through fuel cell tests and system analysis.     

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.     

Effect of carbon content on Ni–Fe–C electrodes for hydrogen evolution reaction in seawater

Hydrogen evolution reaction (HER) on the Ni–Fe–C electrodes electrodeposited at current density ranging from 100 to 300 A/m², as well as their electrochemical properties in 3.5% NaCl solution at 90 °C and pH = 12, had been investigated by polarization measurements, EIS technique. It was shown that the carbon content and grain size of Ni–Fe–C coatings are affected by current density. In addition, the hydrogen evolution overpotential of Ni–Fe–C electrodes was related with carbon content and grain size. The Ni–Fe–C electrodes with optimum catalytic activity for the HER were found to contain the maximum carbon content 1.59% and the minimum grain size 3.4 nm. The results of a comparative analysis between carbon content and intrinsic activity are that carbon content plays an important role in intrinsic activity of Ni–Fe–C electrodes.     

Effect of swirling vanes on performance of steam–water jet injector

The effect of water swirling vanes and steam swirling vanes on the steam–water jet injector with central water nozzle was experimentally investigated. The performance of the steam–water jet injector with water/steam swirling vanes was compared with that without swirling vanes. The exergy analysis was also carried out. According to the experimental results and analysis, the water swirling vanes could effectively enhance the performance of the jet injector, but the steam swirling vanes weakened the performance of the jet injector. When the water swirling vanes were equipped, the maximum amplitudes of increase in the entrainment ratio and exergy efficiency were 172% and 93%, respectively, and the highest inlet water temperature increased from 70 °C to 80 °C. The performance of steam–water jet injector with water swirling vanes was also investigated. The results showed that the entrainment ratio decreased with the increase of the inlet water pressure and temperature, and increased with the increase of inlet steam pressure. The resistance coefficients increased with the increase of the inlet water pressure and temperature, and decreased with the increase of inlet steam pressure.     

The affects of membrane on the cell performance when using alkaline borohydride–hydrazine solutions as the fuel

The cell performance and the polarization behavior of the fuel cell using alkaline NaBH₄–N₂H₄ solutions as the fuel were investigated. It was found that the use of different membrane: anion exchange membrane (AEM) or cation exchange membrane (CEM) would influence the cell performance and cathode polarization behavior. The direct borohydride fuel cell (DBFC) using CEM gave a higher power density than that using AEM, but the direct hydrazine fuel cell (DHFC) using CEM gave a lower power density compared with the DHFC using AEM. In the DBFCs using CEM, N₂H₄ addition in alkaline NaBH₄ solution improved the cell performance but it did not make any difference when adding more N₂H₄. On the other hand, in the DBFCs using AEM, cell performance was improved with increasing the amount of N₂H₄ in the anolyte.