Production Of Carbon And Oxygen From CO₂ By Electrolysis In Halide-Lithium Carbonate Molten Salts

With the rapid development of modern industry,the demands for energy are increasing.The use of fossil fuels,such as coal,oil and natural gas has provided a great amount of energy to the development of modern industry and society,at the same time,it also emits a lot of carbon dioxide.Carbon dioxide is the primary source of greenhouse gases,a large amount of carbon dioxide emission into the atmosphere not only seriously damage the ecological environment of the earth but also affects the survival environment of human.However,carbon dioxide is also a kind of rich carbon resource.To research carbon dioxide conversion and resource utilization technology,decompose carbon dioxide to carbon,can not only realize the positive cycle of carbon dioxide emissions but also reduce environmental damage.In addition,the main component of Mars atmosphere is carbon dioxide,but oxygen is vital for the future of human exploration of Mars,doing activities on Mars and even living on Mars.Developing the key technology of obtaining oxygen from carbon dioxide,using abundant carbon dioxide in the atmosphere of Mars as primary resource to prepare oxygen and carbon by in situ electrolysis can provide oxygen for human to breath and for rocket to launch when human exploring mars and establishing space stations on Mars.Meanwhile,carbon can be used as reducing agent which reduces metallic oxide in Mars to metal.Therefore,the research of decomposing carbon dioxide into carbon and oxygen has great significance.Aiming at reducing carbon emissions and developing the technology of in situ produced oxygen on Mars,this research work is carried out on producing carbon and oxygen from carbon dioxide by electrolysis in halide-lithium carbonate molten salts.The electrochemical behavior of carbonate ions at Ni electrode in 48.55 mass%LiF-50.25 mass%NaF-1.20 mass%Li₂CO₃ molten salts under CO₂ atmosphere was investigated by using cyclic voltammetry,square wave voltammetry and chronopotentiometry.Electrochemical measurements were performed using a three-electrode system consisting of nickel,platinum and graphite as the working electrode,reference electrode and counter electrode,respectively.The transfer electron number of the electrode reaction was studied by square wave voltammetry.The effects of scanning speed and temperature on the electrochemical behavior of carbonate ions were investigated by cyclic voltammetry.The anodic products of potentiostatic electrodeposition were characterized by XRD and SEM.The results show that the electrochemical reduction of carbonate ion to carbon at Ni electrode is a simple four-electron transfer that occurs in a one-step process,i.e.CO₃^2-+ 4e^-= C + 3O^2-,the reduction of carbonate ion at a Ni-wire working electrode is an irreversible process with diffusion controlled mass transfer and the transfer coefficient a was calculated as 0.21.The diffusion coefficient of carbonate ion in 48.55 mass%LiF-50.25 mass%NaF-1.20 mass%Li₂CO₃ molten salts at 1023 K is 5.31×10-5 cm2/s.The relationship between diffusion coefficient and temperature is InD =-5.60-4308.50/T,with the activation energy of 35.80 kJ/mol.Aiming at electrochemical conversion of CO₂ to carbon,we have calculated the theoretical decomposition voltage of alkali metal carbonate and 24.47 mass%LiF-75.53 mass%Li₂CO₃ melts was used as the molten salt medium to electrochemical-decomposition CO₂.Carbon films deposited on nickel and molybdenum metal electrodes were studied.The morphology of carbon films deposited under different potential and the binding form between carbon coating and metal base were observed by SEM.The influences of cathodic potential on the surface morphology of the deposited carbon films were investigated.The nucleation mechanism of carbon was studied by using chronoamperometry.The property of carbon films was analyzed by XRD and Raman.The current efficiency and electric energy consumption of electrochemical conversion of CO₂ to carbon was calculated.The results show that the combination form is jagged combined between the carburized layer and nickel substrate.The results of the morphology of carbon film deposited on Ni cathode at different cathodic deposition potential show that the surface morphology of carbon film prepared at-1.1 V（vs.Pt）and-1.3 V（vs.Pt）has a spherical particles appearance,while the surface of carbon film prepared at-1.5 V（vs.Pt）appears a whisker structure.Chronoamperometry measurements show that the electrodeposition of carbon involves three-dimensional（3D）instantaneous nucleation under diffusion-controlled growth at Mo electrode.Raman spectroscopic analysis confirms that the graphite carbon was obtained at Mo electrode deposited at-0.75 V（vs.Pt）and ID/IG peak ratio was 1.48.The results of current efficiency and electrical energy consumption of carbon deposited at different potentials on Mo electrode showed that the maximum current efficiency is 78.20%and the lowest power consumption is 21.70 kWh/kg-C at the potential of-0.9 V（vs.Pt）.We have analyzed the electrochemical oxidation reaction of Fe and Ni in iron-nickel alloy anode and calculated the electrode potential of corresponding reaction by thermodynamic calculations.Electrochemical test was carried out on iron-nickel alloy anode with different compositions in 24.47 mass%LiF-75.53 mass%Li₂CO₃ molten salt.The results show that Fe was oxidized preference to Ni in the iron-nickel alloy anode and the oxidation process of Fe is Fe→re2+→Fe3+ The protective film of dense iron oxide was formed on the surface of the alloy anode.Moreover the anodic behavior of Pt electrode in LiF-KF-Li₂CO₃ melts was investigated,and the mechanism of oxygen evolution on Pt electrode was proposed.The rate determining step of oxygen evolution reaction was carried out by theoretical analysis and measuring polarization curves.The results show that Pt electrode was oxidized with fluorine ion to form PtF4 in the 31.73 mass%LiF-68.27 mass%KF molten salt at 813 K at 1.8 V（vs.Pt）,e.g.Pt + 4F^--4e^-= PtF4.At 1.1 V（vs.Pt）,carbonate ions in 31.57 mass%LiF-67.93 mass%KF-0.5 mass%Li₂CO₃ molten salt at 813 K are oxidized at Pt electrode.The results of theoretical analysis and comparison of experimental data show that the measured value of Tafel slope at low potential（E<0.38 V vs.Pt）was close to 0.108 which is the predicted value of the model,the rate determining step of oxygen evolution reaction is step 3 e.g.sO^-（?）-sO+e^-,)while,the measured value of Tafel slope at high potential（E>0.68 V vs.Pt）was close to 0.323 which is the predicted value of the model,The rate determining step of oxygen evolution reaction is step 2 e.g.sCO₃^2-（?）sO-+CO₂+e^-.Electrochemical impedance spectroscopy results show that the electrolyte resistance is about 8.6 Ω in the 31.57 mass%LiF-67.93 mass%KF-0.5 mass%Li₂CO₃ melts at 813 K and the electrolyte resistance decreases with increasing temperature.The charge transfer resistance is 33.71 Ω cm2 at 1.20 V（vs.Pt）and 813 K,the charge transfer resistance decreases with increasing potential,raising the temperature is beneficial to decrease the charge transfer resistance at the same potential.The see-through cell experiments results show that bubbles can be observed in the anode compartment and the formation of bubbles is accelerated with increasing anodic potential.The black deposit was observed in the cathode compartment,so we can confirm that carbon is produced at cathode.CO₂ chemically dissolves into the LiCl-Li₂O melt by reacting with Li₂O,changing the electrolyte LiCl-Li₂O-CO₂ into LiCl-Li₂CO₃.Using titanium,platinum wire and platinum foil as the working electrode,reference electrode and counter electrode,respectively,the cathodic process of Ti electrode in 95.0 mass%LiCl-5.0 mass%Li₂O molten salts under CO₂ atmosphere was investigated by using cyclic voltammetry.The results show that the onset potential of cathodic reduction at Ti electrode in 95.0 mass%LiCl-5.0 mass%Li₂O molten salt under CO₂ atmosphere at 903 K is-1.0 V（vs.Pt）,that is corresponding to the reduction of carbonate ion generated from Li₂O absorption CO₂.Carbon can be obtained when using Ti cathode electrolysis in 95.0 mass%LiCl-5.0 mass%Li20 melts under CO₂ atmosphere at-1.3 V（vs.Pt）and-1.5 V（vs.Pt）.The anodic gas products in electrolysis process were collected and detected by Gas chromatographic method.The compositions of anodic gas obtained at different anode current density were investigated.The results show that oxygen can be obtained at the Pt electrode.Electrolysis of the 95.0 mass%LiCl-5.0 mass%Li₂O molten salt under a CO₂ atmosphere using Ti cathode and Pt anode,The CO₂/O₂ ratio of anodic gas increases with increasing current density.The electrode reaction occurred at the anode of Pt is CO₃^2--2e^-= CO₂ + 0.5O₂.