Electrochemical Feature And The Environmental Application Of CO₂ Conversion Carbon Materials

Molten Salt CO₂ Capture and Electrochemical Transformation process?MSCC-ET?is a potential and efficient CO₂ capture and utilization way without any catalyst.This process can convert CO₂ into value-added carbon materials and oxygen,realizaing the reduction of CO₂ emmition and providing a new kind of functional carbon materials at the same time.For the industrialization of this process and the large-scale resourceful usage of CO₂,study on the properity and its application of electrolytic carbons?ECs?prepared through MSCC-ET process is necessary.Hence,we systematically investigated the electrochemical feature of ECs obtained in molten Li2CO3-Na2CO3-K2CO3 under different temperature and electrolysis voltage using carbon powder microelectrode based on the understanding of carbon morphology and structure.Carbon materials with both adsorption and catalytic activity have been widely used in the advanced oxidation system of persulfate.As a new type of nano-carbon,ECs has both excellent adsorption and energy storage properties.The catalytic performance of ECs in activation persulfate for 2,4-DCP degradation was evaluated and the improvement of the catalyst ability by sulfur modification through the novel molten salt redox-sulfidation has also been investigated.In order to improving the organic degradaion efficiency of common zero valent iron in persulfate system,we also prepared the iron carbon composites based on ECs.The catalytic properties of the modified ECs or EC-iron composite materials were investigated and the mechanism of the degradation was also discussed.The main conclusions of this work are as follows:?1?We systematically investigated the electrochemical properties of the ECs using carbon powder microelectrode cyclic voltammetry in 10 mmol/L Na2SO4 solution.It was found that the electrochemical behavior of the EC obtained at 450 ? under cell voltage of 4.5 V is quite different from other carbon materials,i.e.,multi-wall carbon nanotubes,graphene,graphite and acetylene black.In addition to a much larger charging-discharging capacity,an unusual hysteresis of charge/discharge current response of ECs in the negative potential region was observed.The phenomenon was eliminated by annealing the material in argon at 550 ?,demonstrating the unique electrochemical behavior is closely related to the oxygen functional groups on the carbon surface.Furthermore,the pH and different ions of the solution did not affect the CVs and the hysteresis was weakened in certain degree at slower potential sweep speed or in a higher concentration electrolyte.The results supports that the hysteresis is more likely related to the specific adsorption of cations,causing by the special surface properties of the electrolytic carbon.It was found that the CVs of ECs prepared at different cell voltage were similar but the adsorption capacity decreased for the ECs prepared at higher temperature?550 and 650 ??,it's because that the specific surface area and oxygen concentration of ECs are different with the changing of electrolysis temperature and cell voltage.?2?The catalytic activity in persulfate system of EC prepared at 450 ? under cell voltage of 4.5 V in molten Li2CO3-Na2CO3-K2CO3 was evaluated.In order to improve the activation performance of EC,we introduced the sulfur modification through the novel molten salt redox-sulfidation.It is found that the oxygen content of EC decreases after molten salt modifacation,and the introduction of a certain amount of defective sites give higher catalytic activity for EC.The sulfur doped EC?S-EC?has better activation properties for persulfate.The removal rate of 2,4-DCP is 96.3%,with 50 mg/L S-EC and the molar ratio of persulfate to 2,4-DCP is 5/1.The system also possesses high degradation rate in a wider pH range.The results of radical detection and quenching test confirmed that the degradtion is a non-radical process,and the main oxidized active species is the surface adsorbed radical.The active sits of S-EC limits the production of active species in the degradation process,therefore,the increase of S-EC dosage could obviously improve the degradation efficiency of the system,and the effect of the amount of persulfate on the degradation ability of the system was not obvious.In the process of degradation,the decreased specific surface area and surface oxidization of S-EC lead to the significantly decreasing of the activity.We confirmed that the catalytic ability of S-EC could be recovered by reactivation of the molten salt.?3?CO₂ converted carbon materials and zero valent iron composites were prepared by carbon thermal reduction method.The composites were superior to zero valent iron and Fe?II?system in the degradation of 2,4-DCP by combining the advantages of iron carbon micro-electrolysis and persulfate oxidation.The degradation rate of 2,4-DCP after 60 min was 97.8%when the dosage of Fe/EC and potassium persulfate was 50 mg/L and 1 mmol/L,respectively.The removal rate of 2,4-DCP could be kept above 90%in a wider pH range.The main radicals in the degradation process are SO4·-,·OH and O₂^·-.