Turning carbon dioxide into solid carbon through electrochemical conversion is one of the ways to reduce carbon dioxide content. Present study adopted the reverse reaction of direct carbon fuel cell. In high temperature molten carbonate, carbon dioxide produces high value-added carbon through electrochemical reduction. The electrochemical conversion rate of the electrode can be obtained from electrolysis in two-electrode cell. The conversion current of oxidation reactions at the anode and reduction reactions at cathode was measured by cyclic voltammetry in three electrode cell. The carbon content and crystal structure deposited on the reduction electrode were further analyzed by thermogravimetric analysis (TGA) and X-ray diffraction analysis (XRD). The difference between the supplied amount of CO2 and the exhaust amount of CO2 measured was the reduction amount of CO2. A suitable electrode material was selected by comparing the reactivity of the electrode materials with the results of CO2 reduction and the amount of carbon produced. The electrolyte was prepared by mixing Li2CO3 and K2CO3 in the mole ratio of 62:38. This study adopted Pt, Ag, Ni, and Ir as a reduction electrode. Electrochemical conversion conditions was under the 600 ℃ with the cell voltage of 4.0 V.

Figure 1 shows the experimental results measured according to the cyclic voltammetry. The conversion current value of Ir electrode is the lowest, and the current of the Ir electrode decreases monotonously with time. Pt and Ir had similar properties as oxide electrode in the molten carbonate electrolyte. However, the experimental results showed that the current value of Ir is less than Pt. Material Ir is not suitable for reduction electrode compared with other materials. Ag electrode has the highest conversion current. Current flows are larger than other materials. It can be seen that the Ag electrode had a high reactivity. It is because of the high reactivity of the Ag electrode that a large amount of carbon will be deposited thereon. The Ag electrode dissolves the ionized Ag, and the Ag+ undergoes a redox reaction with the anion in the electrolyte and is reduced to silver on the reduction electrode. Therefore, when using a Ag electrode, the current will gradually increase due to the change in the effective area of the reduction electrode and the corrosion current. The Pt electrode and the Ag electrode have similar reactivity at the start of the conversion, but the current gradually decreases after 200 min. The difference in conversion time between the Pt electrode and the Ag electrode is due to corrosion of the electrode in the electrolyte. The Pt electrode is continuously etched in the molten carbonate to form Li2PtO3 on the surface of the Pt electrode. The effective reaction area is gradually reduced and the resistance is increased. After a period of increase, the conversion current of the Pt electrode continues to decrease. The Ni electrode has a lower conversion current, and the Ni electrode has a lower reactivity compared with Ag and Pt. The current continues to increase with conversion time. The results of thermogravimetric and thermal differential analysis showed that the crystalline structure of carbon deposited by the four materials is different from that of graphite particles. The carbon crystal structure formed by each of the four materials is similar. Ag was found in the carbon deposited on the Ag electrode in the XRD analysis, and low carbon crystallization deposited on the electrode. Although the Ag electrode has the strongest CO2 reduction performance and the highest carbon production capacity, its chemical durability is inferior to other electrode materials. Therefore, when considering the reactivity and durability of the four electrodes, the Pt electrode seems to be the most suitable oxidation electrode and reduction electrode.

Key words: Electrochemical Conversion，Reduction electrode，Electrode Material，Conversion current