Electrochemical Synthesis Of Ammonia In Molten Salt

Artificial nitrogen fixation is one of the most important industrial processes for human beings.However,the current Haber-Bosch process based on fossil fuel suffers from unsustainability and a low N₂ to NH₃ conversion.Electrochemical ammonia synthesis is expected to achieve green,low-carbon and sustainable development considering electricity could be heavily generated by renewable energy in the future.Unfortunately,past efforts to directly reduce N₂ on an electrode under ambient conditions encountered industrially unacceptable low reaction rates(<10^-10mol·cm^-2s^-1),noticing the target is a NH₃ synthesis rate of 0.4?1×10^-6 mol·cm^-2s^-1(or a electrolysis current of 0.25?0.5 A cm^-2 with a current efficiency higher than 50%).It was reported that electrosynthesis of NH₃ from N₂ and H₂ with molten chlorides electrolytes at elevated temperatures could be quicker(ca.10^-8 mol·cm^-2s^-1)but the efficiency,energy consumption,the rate of ammonia synthsis cannot meet the requirements of industrialization.Here this paper proposes a magnesium chloride cycle(MCC)process for the synthesis of ammonia with N₂ and H₂O under atmospheric pressure.Compared with the current Haber Bosch process(HBP),MCC has the characteristics of zero CO₂ emission,coulomb efficiency higher than 92%and energy consumption of 14.1 k Wh kg^-1-NH₃,which fills the gap of electrochemical industrialization ammonia synthesis.In addition,this paper explores the possibility of using under potential deposition to reduce the energy consumption of ammonia synthesis by MCC,and explores the ammonia synthesis by Li H inner cycle(LIC).The specific work and main research results are as follows:1.The MCC ammonia synthesis process:Magnesium and chlorine are produced by electrolysis of magnesium chloride,and magnesium is used for nitrogen fixation to produce magnesium nitride.At the same time,chlorine is reacted with water to produce hydrogen chloride.The research shows that when magnesium nitride is directly hydrogenated by hydrogen chloride,because of the high exothermic reaction,the product is sintered,and the nitrification rate is difficult to exceed 90%.In addition,because the amount of hydrogen chloride is difficult to control,the product is easy to produce ammonium chloride instead of hydrogen chloride Ammonia in the form of nitrogen.When hydrogen chloride is captured by circulating ammonia to produce ammonium chloride,and magnesium nitride is hydrogenated by ammonium chloride,pure ammonia gas and anhydrous magnesium chloride which can be directly recycled can be produced.Compared with the current Haber Bosch process(HBP)ammonia synthesis,MCC ammonia synthesis has the characteristics of zero CO₂ emission,higher than 92%coulomb efficiency,and only 14.1 kwh kg^-1-NH₃ ammonia synthesis energy consumption,which is expected to realize industrial electrochemical ammonia synthesis.2.The energy consumption of ammonia synthesis by MCC process depends on the electrolytic magnesium chloride.The method of under potential deposition is tried to reduce the precipitation potential of magnesium.The results show that the reduction of cell voltage is less than 10%by using under potential alloying.However,the structure of alloying blocks the reaction between magnesium and nitrogen,which reduces the nitriding rate of magnesium from 100%to 10%.In addition,under potential alloying only occurs on the surface,and magnesium production is less,so it is difficult to reduce the energy consumption of MCC ammonia synthesis by under potential deposition.3.The Lithium inner cycle(LIC)process:lithium metal is prepared by electrolysis of lithium hydride in the Li Cl-KCl molten salt,lithium is nitrogen fixed to produce lithium nitride,lithium nitride is hydrogenated to produce lithium hydride and ammonia,and then lithium hydride is electrolyzed to release ammonia and recover lithium,so as to construct lithium inner cycle(LIC)for ammonia synthesis.The results show that:the electrolysis of lithium hydride and the nitridation of lithium are relatively easy processes,and their conversion efficiency can reach 100%.The difficulty of ammonia synthesis by LIC is that ammonia is difficult to release due to the strong adsorption of Li H in the hydrogenated product.When lithium nitride is finished to produce 2Li H and Li NH₂,the electrolysis of Li H can release ammonia by pyrolysis of Li NH₂.The current efficiency of ammonia synthesis by LIC process is74.7%,and the ammonia production rate is 0.9×10^-7 mol/s.While the energy consumption of ammonia synthesis is about 5.82 k Wh/kg-NH₃.