Effects of Nafion content in membrane electrode assembly on electrochemical Bunsen reaction in high electrolyte acidity

The electrochemical Bunsen reaction is an alternative way to traditional Bunsen reaction for hydrogen production, which can produce H₂SO₄ and HI in lower iodine and water condition in an electrochemical cell. However, energy consumption cannot be neglected due to its high electrolytic voltage. It is reported that the anode overpotential of electrochemical Bunsen reaction was much higher than other parts of the cell voltage. In this work, membrane electrode assemblies were prepared using a commercial 60 wt% Pt/C catalyst by spray method. The influence of Nafion content on the electrochemical Bunsen reaction was studied at room temperature (298 K) by multiple electrochemical means such as galvanostatic polarization studies, IV measurement and impedance studies. The energy consumed in the electrochemical cell was calculated based on hydrogen production. With the increase of Nafion content (13%–64%), the voltage and energy consumption displayed a V-shape. The experimental results shown that the best performance was achieved with a Nafion content of 37.50% for the voltage was as low as 0.6 V and the corresponding energy consumption was 290.5641 kJ/mol-H₂.     

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.     

Evaluation of Bunsen reaction at elevated temperature and high pressure in continuous co-current reactor in iodine-sulfur thermochemical process

Bunsen reaction is one of the three reaction steps of iodine-sulfur process. In present study, Bunsen reaction is carried out in co-current reactor to identify effect of different operating conditions on concentrations of Bunsen reaction product mixture. Bunsen reaction studies have been done in tubular reactor, which is made of tantalum tube and stainless steel jacket, in 50–80 °C temperature range, 2–6 bar (g) pressure range. Feed flow rates are varied for HIx (mixture of hydroiodic acid, water and iodine) 1.2 l/h - 3 l/h, SO₂ 0.02 g/s – 0.24 g/s and O₂ 0.008 g/s ?0.016 g/s. It has been observed that, increasing SO₂ feed flow rate and pressure results in increased mole fraction of HI in HIx phase and H₂SO₄ in sulfuric acid phase. Increase in temperature increased the mole fraction of HI in HIx phase but decreased the mole fraction of H₂SO₄ in sulfuric acid phase. Increase in feed I₂/H₂O ratio and HIx feed flow rate, decreased the mole fraction of HI in HIx phase. Higher pressure improved the conversion of Bunsen reactants to products.     

Experimental study of two phase separation in the Bunsen section of the sulfur–iodine thermochemical cycle

The Bunsen reaction for the production of hydriodic and sulfuric acids from water, iodine and sulfur dioxide, has been studied with the evaluation of the effect of some operative parameters on product phase behavior. Results show that operative temperature has a minor effect on the phase behavior. In contrast, both iodine and water loads can be adjusted to enhance the downstream operations of the sulfur–iodine thermochemical water-splitting cycle: the effect of iodine and water excess on resulting phases purity, side reaction occurrence and acid concentration was studied and, then, the most favorable operative conditions defined.     

An optimal operating window for the Bunsen process in the I–S thermochemical cycle

We searched for the optimal compromised operating conditions for the Bunsen reaction of IS thermochemical water splitting, considering the key concerns of the IS cycle: the liquid–liquid equilibrium (LLE) phase separation performance, the characteristics of water distribution between the sulfuric acid (SA) phase and the poly-hydroiodic acid (HI_x) phase, side reaction occurrence, and the effect on operating costs. Experimental data available from a literature survey were combined, and common trends were examined through a series of parametric studies. Based on the results, the optimal operating point and allowable operating window for the Bunsen reaction have been proposed: the optimal point is represented by 4 mol of excess iodine and 11 mol of excess water in the stoichiometry at temperature of 330 K, while the allowable window ranges over for the excess iodine, for the excess water, and for the temperature. After the Bunsen reaction and LLE phase separation, 5 mol of the excess water is distributed to the SA phase and to the HI_x phase. Operating within this window makes it possible to avoid the side reaction and iodine solidification, to increase the HI concentration well above the azeotropic point in the HI_x section, and to minimize operating costs arising from excess iodine and water.     

Reaction parameters of the Bunsen reaction under simulated recycling conditions

The iodine–sulfur (IS) thermochemical process is one of the most prospective, efficient, CO₂-free, massive hydrogen production approaches that use nuclear or solar energies. Among the three reactions composed of the IS cycle, the Bunsen reaction is crucial to the smooth operation of the continuous closed cycle. In the Bunsen reaction, sulfuric and hydriodic acids are produced by the reaction of recycled products of the decomposition of these two acids, In this work, a Bunsen reaction under simulated closed-cycle conditions, i.e., reaction between I₂/HI/H₂O solution and SO₂, was investigated. The effects of reaction conditions such as SO₂ flow rate, HI acid concentration, I₂/HI molar ratio, and temperature on the characteristics of the Bunsen products were examined. These characteristics were phase separation, phase volume ratio, and compositions of HI_x and H₂SO₄ phases. The concentration of HI acid and the I₂/HI molar ratio of the initial solution, but not the flow rate of SO₂, were found to affect the phase states and compositions of Bunsen reaction products. The phase states and compositions of the products were predicted or calculated with self-built empirical models. Results well agreed with the experimental results for phase states and HI_x composition. All these findings validated the reliability of the models and offered crucial reference and guidance for the operation of the closed-cycle IS process.     

Experimental and numerical investigation of the acoustic response of multi-slit Bunsen burners

Experimental and numerical techniques to characterize the response of premixed methane-air flames to acoustic waves are discussed and applied to a multi-slit Bunsen burner. The steady flame shape, flame front kinematics and flow field of acoustically exited flames, as well as the flame transfer function and matrix are computed. The numerical results are compared with experiments. The influence of changes in the mean flow velocity, mixture equivalence ratio, slit width and distance between the slits on the transfer function is studied, both numerically and experimentally. Good agreement is found which indicates the suitability of both the experimental and numerical approach and shows the importance of predicting the influence of the flow on the flame and vice versa. On the basis of the results obtained, the role and physical nature of convective flow structures, heat transfer between the flame and burner plate and interaction between adjacent flames are discussed. Suggestions for analytical models of premixed flame-acoustics interaction are formulated.     

An entropy production and efficiency analysis of the Bunsen reaction in the General Atomic sulfur-iodine thermochemical hydrogen production cycle

An entropy production and efficiency analysis of the first reaction in the General Atomic sulfur-iodine thermochemical hydrogen production cycle has been carried out by simulating the reaction including the mixing of reactants and separation of the resulting phases. The reaction:

was simulated at 388 K, which is slightly above the melting point of I₂. Analysis of only this reaction shows that the reaction should be run at 15–25% I₂ reacted and the greatest excess of H₂O which will produce two product phases. Actual operating conditions are however dependent on the total processing scheme. An entropy production and efficiency analysis along with economic factors for the entire process is necessary to obtain these conditions.     

Overvoltage reduction in membrane Bunsen reaction for hydrogen production by using a radiation-grafted cation exchange membrane and porous Au anode

An electrochemical membrane Bunsen reaction using a cation exchange membrane (CEM) is a key to achieving iodine-sulfur (IS) thermochemical water splitting for the mass-production of hydrogen. In this study, we prepared a radiation-grafted CEM with a high ion exchange capacity (IEC) and a highly-porous Au-electroplated anode, and then used them for the membrane Bunsen reaction to reduce cell overvoltage. The high ionic content of our CEM led to low resistivity for proton transport, while the high porosity of the electrode led to a large effective surface area for anodic SO₂ oxidation. The cell overvoltage for the membrane Bunsen reaction was significantly reduced to 0.21 V at 200 mA/cm², one-third of that achieved using a commercial CEM and non-porous anode. From the analysis of the current-voltage characteristics, the grafted CEM was demonstrated to play a dominant role in the overvoltage reduction compared to the porous Au anode.     

Tunably fabricated nanotremella-like Bi2S3/MoS2: An excellent and highly stable electrocatalyst for alkaline hydrogen evolution reaction

Reasonable design of efficient and stable catalysts with low cost and abundant natural reserves is vital for electrocatalytic water splitting. Herein, novel nanotremella-like Bi₂S₃/MoS₂ composites with different mass ratios between Bi₂S₃ and MoS₂ have been successfully prepared through a hydrothermal approach and further applied to hydrogen evolution reaction (HER) in 1.0 M KOH electrolyte for the first time. When the mass ratio of Bi₂S₃ and MoS₂ is 5:5, as-prepared nanotremella-like Bi₂S₃/MoS₂ (marked as BMS-5) manifests favorable HER catalytic activity with overpotential of 124 mV at current density of 10 mA cm⁻² and relatively low Tafel slope of 123 mV dec⁻¹. Moreover, it exhibits an extraordinary durability for uninterrupted hydrogen generation. The enhanced HER performances are ascribed to the synergistic effects between Bi₂S₃ and MoS₂, giving rise to large electrocatalytic active area and fast HER kinetics. The results pave a new path to design and construct excellent Bi₂S₃/MoS₂ nanomaterials for electrocatalytic hydrogen generation.     

硫碘热化学循环水分解制氢流程优化与模拟

选择大型化工流程模拟软件Aspen Plus对硫碘循环制氢系统进行流程优化设计与模拟,计算系统的质量平衡、能量平衡,并对系统热效率进行评估.碘化氢相中HI浓度采用本生反应实验中的过恒沸浓度,避免高能耗电渗析的使用,从而大大提高系统效率.在不考虑废热发电情况下,与文献值56.8％相比,系统产氢热效率高达68.46％.     

Analyzing the effects of reaction temperature on photo-thermo chemical synergetic catalytic water splitting under full-spectrum solar irradiation: An experimental and thermodynamic investigation

Photocatalysts currently in use are only able to utilize very small part of the solar spectrum that arrives at the earth's surface (mainly ultraviolet light). Most of the photons that are not absorbed by the photocatalysts are converted to heat. However, there is no consensus on the effect of reaction temperature on photo-thermo chemical synergetic catalysis, which has been studied herein using experimental investigations combined with thermodynamic analysis. An elaborate photo-thermo chemical reaction test rig was initially designed and set up that can test experimental variable while the other influence factors were kept constant. The effects of ultrasonic and operation temperature on Pt/TiO₂ particle cluster distribution during the photo-thermo chemical synergetic catalytic water splitting process were analyzed by an upright microscope for the first time. The results indicated that the H₂ production rate varies with reaction temperature, and 55 °C is the optimum temperature for the photo-thermo chemical synergetic catalytic water splitting process studied here. A maximum H₂ production rate of 11.934 mmol/(h g) could be achieved using a classical Pt/TiO₂ catalyst, when operating under the optimum reaction conditions.     

Hydrogen production by using manganese ferrite: Evidences and benefits of a multi-step reaction mechanism

Hydrogen production by water splitting with _MnFe2O4/_Na2CO3${\mathrm{MnFe}}_2{\mathrm{O}}_4/{\mathrm{Na}}_2{\mathrm{CO}}_3$ system was studied at 973 K. An intermediate phase, resulting from decarbonatation of _MnFe2O4/_Na2CO3${\mathrm{MnFe}}_2{\mathrm{O}}_4/{\mathrm{Na}}_2{\mathrm{CO}}_3$ mixture in inert atmosphere, proved to be effective in hydrogen reduction from water with stoichiometric yield. The presence of a highly reactive intermediate phase suggests the feasibility of a high efficiency, three-step, thermochemical cycle for hydrogen production. In fact, the possibility of obtaining _CO2${\mathrm{CO}}_2$ separately from the gases mixture dramatically enhances process efficiency.     

Study of the miscibility gap in H2SO4/HI/I2/H2O mixtures produced by the Bunsen reaction – Part I: Preliminary results at 308 K

In the framework of the optimization of the sulfur–iodine thermochemical cycle for massive hydrogen production, investigations were performed in order to characterize the liquid phase (HIx and H₂SO_4(aq) phases) separation of solutions resulting from Bunsen reaction. Quaternary H₂SO₄/HI/I₂/H₂O mixtures were prepared at 308 K with different relative proportions of reactants and the chemical composition of each of the two phases formed was analyzed. An increase in iodine concentration and a decrease in water concentration appeared to improve the liquid–liquid equilibrium phase separation. However, a too low concentration of water also promoted the formation of byproducts. An increase in the [H₂SO₄]/[HI] ratio tended to favor the separation and seemed to lead to a dehydration of the HIx phase.     

Co/Mo2C composites for efficient hydrogen and oxygen evolution reaction

Non-precious transition metal electrocatalysts with high catalytic performance and low cost enable the scalable and sustainable production of hydrogen energy through water splitting. In this work, based on the polymerization of CoMoO₄ nanorods and pyrrole monomer, a heterointerface of carbon-wrapped and Co/Mo₂C composites are obtained by thermal pyrolysis method. Co/Mo₂C composites show considerable performance for both hydrogen and oxygen evolution in alkaline media. In alkaline media, Co/Mo₂C composites show a small overpotential, low Tafel slope, and excellent stability for water splitting. Co/Mo₂C exhibits a small overpotential of 157 mV for hydrogen evolution reaction and 366 mV for oxygen evolution reaction at current density of 10 mA cm⁻², as well as a low Tafel slope of 109.2 mV dec⁻¹ and 59.1 mV dec⁻¹ for hydrogen evolution reaction and oxygen evolution reaction, respectively. Co/Mo₂C composites also exhibit an excellent stability, retaining 94% and 93% of initial current value for hydrogen evolution reaction and oxygen evolution reaction after 45,000 s, respectively. Overall water splitting via two-electrode water indicates Co/Mo₂C can hold 91% of its initial current after 40,000 s in 1 M KOH.     

An experimental investigation of convective mass transfer characterization in two configurations of electrolysers

This work is devoted to the study of hydrodynamics behavior and mass transfer performance in water electrolysis processes, two configurations of containers and electrodes are studied in laboratory experiments under different current densities, the platinum (0.2 mm in diameter) in an acidic environment (36% CH₃COOH) as electrode material, surrounds the sides of the container in horizontal mode. The system is studied using particle image velocimetry (PIV), microscope enhanced visualization. The experimental results show that the velocity distribution in most regions of electrolyser is dominated by two asymmetry bubble buoyancy induced flow patterns. The greater reaction rate of water electrolysis and better mass transfer arise in the smaller space of electrodes. By comparing hydrodynamic behavior in two containers with different current densities, hydrogen production, bubble-driven convection and convective mass transfer increase at higher current densities, however, this increase is not linear, the interaction mechanisms are analyzed on mass transfer, electrochemical reaction and bubble effect. Results facilitate the understanding and the design of the transport phenomena in electrolyser.     

Oxidation reaction kinetics for the steam-iron process in support of hydrogen production

A hydrogen generation research program is focused on solar-driven hydrogen production by means of reactive metal water splitting. In order to dissociate water molecules at significantly reduced thermal energies as well as providing a practical means for efficient hydrogen and oxygen separation, an intermediary reactive material is introduced to realize water splitting in the form of an oxidation reaction. Elemental iron is used as the reactive material in the process commonly referred to as the steam-iron process. In order to exploit the unique characteristics of highly reactive materials and ultimately achieve the potential efficiency gains at the solar reactor scale, a monolithic laboratory-scale reactor has been designed to explore the fundamental kinetic rates during the iron oxidation reaction at temperatures ranging from about 650 to 900 K. Results show hydrogen production rates on the order of 1E-8 g/cm² s. Micro-Raman spectroscopy is used to access information on the exact iron oxide phase produced, and high resolution SEM and electron dispersion spectroscopy (EDS) are used to assess the oxide morphology and further quantify the oxide state, including spatial distributions.     

A review on nickel cobalt sulphide and their hybrids: Earth abundant,pH stable electro-catalyst for hydrogen evolution reaction

The production of hydrogen from electrocatalytic water splitting using a clean energy source has become a sustainable route to overcome the problems related to fossil fuels. Therefore, it is essential to develop a non-precious, stable and highly efficient electrocatalyst. Nickel and cobalt sulphide have gained much attention as hydrogen evolution reaction (HER) catalysts due to their pH stability, low cost and high activity. But the application of these sulphides is limited due to high over-potentials, low surface area and less catalytic active sites available for HER. Furthermore, Nickel and cobalt sulphide can be coupled with different functional components to enhance their catalytic activity. This comprehensive review focuses on the progress made so far to enhance the electrochemical properties of nickel and cobalt sulphides and their composites with various active materials. The comparative survey of their HER activities is made in terms of their electrocatalytic performance parameters.