| Hydrogen will have the most important role in the new energy technology revolution in future.There is no doubt that hydrogen economy is the necessary process in the development progress of human society.The large scale,low ce(?)and sustainable hydrogen production technology is the foundation of hydrogen utilization.(?)fur-Iodine(SI)thermochemical water-splitting cycles for hydrogen production,composed by three simple chemical reactions,has lots of distinct advantages and is well known as one of the most promising hydrogen generation methods in the field of thermochemical water-splitting cles.Because of the three main reactions in the SI(?)cycle,it is usually divided into three subsections,including Bunsen subsection,HI decompedition subsection and H2SO4 subsection.HI decomposition subsection mainly consists of H(?)lution purification,pre-concentration,distillation and decomposition,which is the key proc of SI cycle and has a great influence on the total thermal efficiency.So,the research focus(?)ctoral dissertation is HI separation and the new electrochemical HI decomposition reaction.The HIx solution purification is simulated by the mercial software Aspen Plus,based on the Gibbs principle of minimum free energy.The effect(?)water concentration in HIx solution on the purification was evaluated.The increase of water c(?)sed a higher temperature of H2SO4 total conversion and increased the proportion of reverse B(?)en reaction while reduced that of sulfur formation reaction.What mainly distribute in the vapc(?)hase product of purification reactor are H2O,HI,and I2.In contrast,the distribution of SO2 a(?)H2S in vapor phase is very little.When the feed of purification reactor is hyper-azeotropic so(?)on,the gas mixture of HI,SO2 and H2S cannot be separated totally by atmosphere temperature idensation.Considering the fact that the excess of iodine only has positive effects on Bunse(?)action and the separation of two liquid phase acids while has some negative influences in HI(?)composition subsectione some iodine in HIx solution would be recovered by crystallization af(?)cooling down the solution temperature.In order to assess the utilization of iodine precipitatc(?)n HI separation subsection,the energy demand and required input flow rate of three different flowsheets were calculated by Aspen Plus and Office Excel to find the most suitable folwsheet.In order to reduce the energy duty and the operation time of EED,the effects of operation temperature and current density on asymmetric EED bedded in the laboratory SI system were evaluated.Increase in operation temperature reduced the apparent transport number of proton and improved the electroosmosis coefficient,which had some negative effects on HI concentration.The rising of current density could accelerate the HI concentration rate of EED.Current density shared a similar effect with operation temperature on apparent proton transport number and electroosmosis coefficient.However,they had different impacts on the cell voltage.A series of experiments were conducted to explore the distillation characteristics of homemade packed distillation column for HI-I2-H2O ternary solution.Whether the feed is in hyper-azeotropic or hypo-azeotropic region,the distillation column could separate HI and I2 in overhead product by and large.For the separation of HI and H2O,if the feed is hypo-azeotropic HIx solution,HI concentration in overhead product is less than that in the feed.However,when the feed is hyper-azeotropic region,high rich HI vapor could be obtained in overhead product.Limited by the thermodynamic equilibrium decomposition rate,the traditional HI decomposition ratio is only 20%at 450?,which is one third of H2SO4 decomposition rate.Due to the different decomposition rate of two acids and the azeotropy property of HIx solution,there are several inner cycle in the whole SI process,which will limit the improvement of total thermal efficiency and result in large material cycling in vain.To release the limit of equilibrium decomposition rate,the electrochemical HI decomposition were put forward.An electrochemical cell was employed for hydriodic acid decomposition under the presence of iodine.Several commercial proton-exchange membranes(PEMs),namely,Nafion 117 and Nafion 115,were used as separators for the electrochemical cell.Several parameters,such as operating temperature,HI molarity,and I2 molarity in anolyte,which affected current efficiency,iodine permeance,and electric resistance of test cell,were investigated.High operating temperature and I2 molarity in anolyte enhanced the permeability of iodine,which had several negative influences on electrochemical cell performance.Although current efficiency was negatively affected by increasing temperature and I2 molarity,it still remained above 0.85 in the range of 30 ?-75 ?.A series of fundamental studies were performed to investigate the electrolytic cell voltage for HI decomposition.The effects of HI and I2 molality in anolyte,operation temperature and current density on the electrolysis potential were were analyzed to determine the preferable electrolysis conditions.The increase of iodine molality in anolyte rises the electrolytic cell voltage,while the increasing hydrogen iodide molality in anolyte could be able to reduce the electrolytic voltage.In order to identify and measure the contributions of electrolytic cell potential,an in-situ under-limiting current chronopotentiometry method was employed for HI electrolysis.Ohmic potential and mass and charge transfer potential is the major part of electrolytic cell potential,and the proportion is 35%and 57%respectively.The open circuit potential for the electrochemical HI decomposition reaction was experimentally and theoretically studied.A theoretical open circuit potential model was deduced based on the electrochemical knowledge.The parameters of model equation about open circuit potential were experimentally explored.The theoretical equation was verified through the experimental data fitting.For reducing the cell voltage and improving the hydrogen production efficiency of SI cycle system,two home-made membrane electrode assemblies were studied.The difference between the two MEAs was whether the anode catalyst layer contained catalytic Ru/C or not.The effect of utilization of two MEAs on the cellvoltage and current efficiency of the electrolytic cell was studied.Finally,based on the experimental results,the energy consumption for per mole hydrogen produced in the HI electrolytic cell was calculated,and then the thermal efficiency of SI cycle system with electrochemical HI decomposition was calculated.In the future,the structure of the electrolytic cell can be further optimized,better performance of membrane electrode materials will be developed,and the optimization design of the sulfuric acid concentration and decomposotion process can be expected.The system cycle thermal efficiency is expected to be continuously improved. |
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