| Hydrogen is one of the most promising clean energy by virtue of many prominent advantages,such as wide availability,clean combustion,high energy density and so on,and it is also an important raw material in the food and chemistry industries at the same time.Although the ratio of hydrogen production in water electrolysis is relatively low in the total hydrogen production technologies,hydrogen production in water electrolysis is an irreplaceable method for ultra-pure hydrogen supply and an important component of distributed clean energy system,and it has bright prospect in the future.The time-limited and geography-constrained energy,such as the solar and wind energy,can be effective integrated into the sustainable clean energy system if water electrolysis for hydrogen production is employed as a medium in the system,thus hydrogen energy can be used more widely.Compared with other methods,water electrolysis for ultra-pure hydrogen production is a necessary method for hydrogen supply in some industrial applications,such as large scale integrated circuit manufacturing and float glass production.The high energy consumption is always the bottleneck of restricting the development of water electrolysis for hydrogen production.The energy consumption is about 4.5~5.0 kWh for 1 Nm3 hydrogen product,the efficiency is below 60%,so the work of intensifying water electrolysis and improving its efficiency is important and necessary.In the process of water electrolysis,the management of gas-phase product is always an important work.The effective method of expelling gas-phase product timely concerns the reduction of electric power consumption.Up to now,the gas product is usually expelled by the electrolyte flow driven through the pump.The previous reports showed that the multi-scale convections induced by Lorentz force in electrolyte under magnetic field have the effect of expelling the gas product.However,the mechanism of magnetic influence on the electrolyte mixed with gas product is not clear,and the work of how to intensify the magnetic field effect in order to increase the electrolysis efficiency needs to be done.In order to analyze the impact and mechanism of magnetic field influence on the process of water electrolysis,based on the theoretical analysis,the bubbles behavior at the electrode surface and gas-liquid two-phase flow between electrodes,which is driven by magnetic field,are given specific analysis by the combined method of experiment and numerical simulation.The content includes the main aspects as shown below:1.By the method of numerical simulation,based on the FLUENT,the volume of fluid(VOF)model was used to simulate the process of hydrogen desorption at the electrode surface.The production rate of species of hydrogen was calculated through Faraday law.The bubble growth model was constructed based on the hydrogen supersaturation mass transfer model.It was compiled into the calculation model through the user define function(UDF),and verified through the experiment data.Within the stages of bubble nucleation and growth,various convections exist near the electrode surface,which include the natural convection,induced by the concentration gradient of hydrogen species in electrolyte,and the convection which results from the gas-liquid interface expansion due to bubble evolution.The effect of mass transfer intensification due to bubble evolution was quantized through the calculation of mass transfer coefficient under various convections.2.Based on the analysis of the process of bubble evolution at the electrode surface,the influence of external magnetic field on the bubbles behavior was observed by the experiment method with micro electrode and macro one partially covered with hydrophobic materials respectively.The bubbles evolution was recorded by the high speed photography,their growth curves and the influence of magnetic field on bubble growth rate are given.At the same,the change of supersaturation concentration of hydrogen species around the bubble was analyzed under various current conditions.The partially hydrophobic electrodes were prepared with the method of photolithography and electrochemical deposition,and the characteristics of bubbles growth and release at thus electrodes were observed.The reason of the big bubble random release was analyzed.It has been compared of the change of bubbles behavior growing at micro and macro electrodes surface with magnetic field influence,and the mechanism of micro-MHD convection changing pressure distribution around bubble,which results in the change of bubble behavior,was revealed through numerical simulation.3.A pair of parallel platinum electrodes was used to test the magnetic field influence on the electric potential difference.Based on the FLUENT,the Euler-Euler model was used to analysis the electrolyte flow and the gas product distribution under magnetic field influence between electrodes gap,it was revealed that the physical mechanism of magnetohydrodynamics(MHD)convection affects the electric potential between electrodes through reducing the gas-phase fraction and the electrode surface gas fraction. |
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