| Hydrogen is considered as a kind of clean, effective, safe, and sustainable alternative energy, and has received increasing concern. It is desirable to develop a hydrogen energy system, integrated by hydrogen production, storage, transportation, and utilization, and finally realize the transition of global fossil energy economy to hydrogen energy economy. Hydrogen production is the primary work for developing a hydrogen energy system. Water, the abundant resource on the earth and free of pollution, is considered as the best raw material for hydrogen production. Among the various methods, the thermochemical water-splitting cycles for hydrogen production has been closely concerned due to their advantages. Especially, the thermochemical sulfur-iodine (SI) cycle is taken as one of the most promising methods.In the SI cycle, the operation mode and progress of the initial Bunsen reaction (SO2+I2+2H2Oâ†'H2SO4+2HI) affect the following decomposition of HI and H2SO4, and thus dominate the whole SI cycle system. The traditional Bunsen reaction, proposed by General Atomics, requires a large excess of both iodine and water, and the produced HI and H2SO4 spontaneously separate. However, to reduce and even avoid the use of excess iodine and water, an alternative way, the electrochemical Bunsen reaction, which can simplify the SI cycle system, was proposed.Considering the fact that the HI in the real SI cycle system will return to the Bunsen reaction, the kinetic and thermodynamic equilibrium performance of four quaternary Bunsen reaction with the existence of initial HI were studied. An increase in the initial HI concentration or temperature accelerated the reaction kinetic rate, leading to the earlier appearance of liquid-liquid phase separation, and shorter time to achieve thermodynamic equilibrium. Yet the phase separation became difficult if the initial HI content was too high, and thus the initial HI/H2O molar ratio should be controlled in 0～1/18, which could be a proper range in this work. Increasing the initial HI content amplified the impurities in the H2SO4 phase, whereas the increase of temperature improved the liquid-liquid phase equilibrium separation. A hyper-azeotropic HI concentration was obtained with feeding HI, which favored reducing the concentration process for HI, and then optimizating the SI cycle system. The conversion rate of SO2 slightly decreased with increasing the initial HI content or temperature.In conclusion, an initial HI/H2O molar ratio of 0～1/36 at high temperature of 343-358K was comparatively the optimal operating conditions.Fundamental experiments concerning the electrochemical Bunsen reaction were conducted, and the variation law of the acid concentration and cell voltage was explored. During electrolysis, both the concentration of H2SO4 and HI increased while the I2 concentration decreased, the voltage presented a rising trend. An increase in current density increased the gain and loss number of electrons at unit time, and thus elevated the produced amount of H2SO4 and HI, but also increased the voltage. The anode-side graphite electrode could be corroded once the voltage reached over 3V. The electrode reaction rates was increased with rising temperature. Increase in I2/HI molar ratio reduced the overpotential of cathode, and meanwhile increased the solution ohmic resistance. The current efficiency was higher than 90% and even close to 100% at most conditions, indicating the good performance for energy conversion. A lower energy consumption was obtained at lower current density, higher temperature, or higher H2SO4 concentration; whereas the variation of HI concentration or I2/HI molar ratio led to a peak value of consumed energy.The performance of two typical proton exchange membranes, Nafion 117 and 115, in the cell was explored. The proton transport number (t+) was almost greater than 0.9, and even close to 1, except at high temperatures, the proton transport ability of Nafion 115 declined markedly. The variation of electro-osmosis coefficient (β) was different according to the operating conditions. The transport properties of membrane is closely related to concentration of acids, because a high t+ and low β favors obtaining a high concentration of HI; if not, a high concentration of H2SO4 may be produced. According to the measured cross-contamination between two electrolytes, the concentration of HI in anolyte was 2-10 times higher than that of H2SO4 in catholyte. An increase in current density, temperature, or I2/HI molar ratio resulted in a higher molar flux of HI and H2SO4. Increasing H2SO4 concentration promoted the transport of H2SO4, but inhibited HI permeation; whereas increasing HI concentration had a contrary effect. The microscopic characterization of membrane indicated that the membrane after electrolysis became crumpled and some particles deposited on its surface, which increased its BET surface area and decreased its average pore diameter. The membrane could be activated, which favored extending its life-span.The equilibrium potential for the electrochemical Bunsen reaction was experimentally and theoretically studied. First, a theoretical equilibrium potential model was deduced according to the electrochemical theory. Second, the effects of operating parameters on the equilibrium potential were experimentally explored. An increase in temperature, SO2 or I2 concentration reduced the potential, whereas the increase of H2SO4 or HI concentration increased the potential. The theoretical model was verified through the experimental data fitting, and two important parameters, M and Z, respectively represent the non-ideal nature of electrolyte and the concentration distribution inside the membrane, were found independent of solution concentration. Z almost kept unchanged with temperature, while M and temperature showed an exponential relationship, a higher temperature resulted in a lower M. Finally, an empirical equilibrium potential formula was proposed, and it well reproduced the experimental data, which made it possible for estimating the equilibrium potential.The electrode reaction mechanism and kinetic characteristics for the electrochemical Bunsen reaction were studied using the electrochemical workstation. The measurement of electrode reactions using cyclic voltammetry showed that both the anodic SO2 oxidation and the cathodic I2 reduction were irreversible; the diffusion-mass transfer rate was qualitatively analyzed according to the peak current in the cyclic voltammograms. Both electrode reactions were characterized using electrochemical impedance spectroscopy, and the measured Nyquist plots were regressed by equivalent circuit. The equivalent circuit for the cathode reaction was a solution ohmic resistance, in series with a parallel combination of a charge transfer resistor and a constant phase element; while the equivalent circuit for the anode reaction consisted of a parallel combination of a charge transfer resistor and a constant element. Two important kinetic parameters, the exchange current density (jo) and the standard reaction rate constant (k0), were evaluated, and indicated that a high electrode reaction kinetic rate can be obtained with a HI concentration of 8mol/kgH2o, I2/HI molar ratio of 0.5, and H2SO4 concentration of 13mol/kgH2o-The flowsheet design and simulation of the sulfur-iodine cycle using the electrochemical Bunsen reaction were conducted, and the mass balance and energy balance of system were calculated with 1mol/s of hydrogen production. Take interior heat exchange and recovery of waste heat for generating electricity into account, the calculated thermal efficiency reached 48.98%. Compared with the SI cycle using traditional Bunsen reaction, the new SI cycle with the application of the electrochemical Bunsen reaction markedly simplifies the flowsheet, reduces the energy consumption, and increases the thermal efficiency. By means of optimization and improvement, the thermal efficiency and economy can be further improved in the future. |
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