Researches On The Cathodic Kinetics And Products Of CO₂ Electrochemical Reduction In Molten Ternary Carbonates

CO₂ capture and utilization(CCU)is considered to be one of the promising ways for reduction of CO₂ emission.CO₂,as a cheap carbon source,can be converted into a variety of carbon-containing products through physical,chemical and biological processes.Recently,molten salt CO₂ capture and electrochemical transformation(MSCC-ET)technology is proposed for high-flux capture and conversion of CO₂,which is becoming a hot issue.In the MSCC-ET process,captured CO₂(in the form of carbonate ions)is converted into carbon materials,CO or syngas via reduction of CO₃^2-with relatively high current density.Many groups have conducted several studies using this technology,focusing mainly on the production of CO₂-derived carbon materials with various nanostructures and their performance in application.Less attention has been paid to the reaction kinetics.However,it is of great significance to fully understand and control the cathodic kinetics of the MSCC-ET process for engineering this technology.Herein,the electrochemical reduction mechanism of CO₂ in molten carbonates and its effects on the cathodic products was explored.The way to accelerate the cathode kinetics was also studied.The main work and results are as follows:(1)Several electrochemical tests such as cycle voltammetry,linear sweep voltammetry and potentiostatic electrolysis were conducted on a Ni working electrode to investigate the cathodic reduction kinetics in molten Li-Na-K ternary carbonate.Sn O₂ was used as the counter electrode and Ag/Ag₂SO₄ was used as the reference electrode.The electrolytic products prepared at selected potentials were also characterized by X-ray diffraction.It was found that the cathode surface was partially covered by insoluble lithium oxide and that the sluggish diffusion of the O^2-ion was the rate-determining step during carbon deposition at 723 K.When the temperature was increased to 923 K,the reaction kinetics were accelerated around 100-fold with a limiting current density of 1.5 A/cm².(2)In the previous studies,constant cell voltage electrolysis was usually used for CO₂electrochemical reduction and the cathodic potential was not constant.According to the results of the kinetics study in the previous chapter,three characteristic potentials(-2.1 V,-2.4 V,and-2.6 V)were selected to prepare carbon materials.The morphologies of the deposited carbon materials were characterized and the capacitive performances of the three carbon materials in 1 M H₂SO₄ were tested.It was found that when the alkali metal precipitated(-2.6 V),carbon nanoparticles(CNPs)with the particle size of 10-20 nm were obtained.The specific surface area was 762 m²/g and the total pore volume was 0.51 cm³/g.The carbon deposited at-2.6 V showed the highest capacitance value.When testing in a two-electrode cell,the specific capacitance was 280 F/g under galvanostatic charge/discharge at 0.2 A/g.Under a high charge/discharge current density of 10 A/g,the specific capacitance could still reach 165 F/g.The carbon deposited-2.1 V contained some microcrystalline graphite structures and the cycle performance was the best among the three carbon materials.After cycling 10,000 times under charge/discharge at 10 A/g,the specific capacitance could still be maintained at about 90%.At the potential of-2.4 V,the carbon particles were serious agglomeration.The specific capacitance was only 208 F/g(two-electrode cell)under charge/discharge at 0.2 A/g.(3)The electrochemical reduction behavior of CO₂ on Ni,Cu,Ti and stainless steel cathodes was investigated in molten ternary carbonates at 723 K.It was found that the?cathode passivation?would ouccr on different cathodes.In the cyclic voltammetry curve,the reduction peaks were all observed at about-2.25 V on different cathodes and the limiting currents were all closed to 11.7 m A/cm².Particularly,the pre-polarized Ti electrode exhibited different properties from other three cathodes in molten ternary carbonate.By pre-polarizing at-2.0 V for 10 min and then electrolyzing at-2.6 V,the current density can reach about 80 m A/cm²,which may be related to the formation of CO.Constant current electrolysis were also conducted on above four cathodes at different current densities with the same amounts of charges.When the current density was lower than the limiting current(10 m A/cm²),the honeycomb-like structures could be found in the carbon deposited on different electrodes.The carbon obtained on Ni cathode showed the most uniform honeycomb-like structure.When the current density was 40 m A/cm²,the carbon particles obtained from different cathodes exhibited a certain agglomeration.Carbon nanoparticles with very small sizes were obtained on different cathodes when the alkali metal was precipitated(80 m A/cm²).(4)In order to eliminate the concentration polarization of Li₂O on the cathode,a hollow gas bubbling(HGB)electrode was designed to investigate the effect of gas convection and chemical reaction on the cathode kinetics.It was found that the in-situ CO₂ bubbles can accelerate the mass transfer of O^2-ions by force convection of the melt and CO₂can rapidly chemically react with O^2-to generate CO₃^2-.In the steady-state polarization curve,the limiting current was increased from 15.3 m A/cm²(no gas bubbling)to 200 m A/cm².During the long-term electrolysis at the current of50 m A/cm²,the cell voltage was also reduced by about 300 m V when CO₂ was bubbled,which meaned a higher energy efficiency.(5)The elimination effect of pulse electrolysis on the cathodic passivation was further investigated.At the current density of 100 m A/cm² and a duty ratio of 1:10,when the pulse period increased from 1 s to 40 s,the Li₂O contained in the cathodic products decreased firstly and then increased.When the pulse period was 10 s,The Li₂O in the product was reduced by more than 80%compared to the constant current electrolysis at the same current density.The carbon material obtained by pulse electrolysis presented a uniform honeycomb-like structure.As the amounts of Li₂O in the product increased,the honeycomb-like structures decreased.When the pulse period was 40 s,there was no honeycomb-like structure in the product owing to the accumulation of large amounts of Li₂O.