Comparative study on discharge characteristics of low pressure CO₂ driven by sinusoidal AC voltage: DBD and bare electrode structure

Abstract

The low-pressure atmosphere rich in CO₂ (~95%) on Mars makes the <i>in-situ</i> resource utilization of Martian CO₂ and the improvement of oxidation attract widespread attention. It contributes to constructing the Mars base which will support the deep space exploration. Conversion of CO₂ based on high voltage discharge has the advantages of environmental friendliness, high efficiency and long service life. It has application potential in the <i>in-situ</i> conversion and utilization of Martian CO₂ resources. We simulate the CO₂ atmosphere of Mars where the pressure is fixed at 1 kPa and the temperature is maintained at room temperature. A comparative study is carried out on the discharge characteristics of two typical electrode structures (with/without barrier dielectric) driven by 20 kHz AC voltage. Combined with numerical simulations, the CO₂ discharge characteristics, products and their conversion pathways are analyzed. The results show that the discharge mode changes from single discharge during each half cycle into multi discharge pulses after adding the barrier dielectric. Each discharge pulse of the multi pulses corresponds to a random discharge channel, which is induced by the distorted electric field of accumulated charge on the dielectric surface and the space charge. The accumulated charge on the dielectric surface promotes the primary discharge and inhibits the secondary discharge. Space charge will be conducive to the occurrence of secondary discharge. The main products in discharge process include <inline-formula><tex-math id="Z-20220530120205">\begin{document}${\rm{CO}}^+_2 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20220086_Z-20220530120205.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20220086_Z-20220530120205.png"/></alternatives></inline-formula>, CO, O₂, C, and O. Among the products, CO is produced mainly by the attachment decomposition reaction between energetic electrons and CO₂ at the boundary of cathode falling zone, and the contribution rate of the reaction can reach about 95%. The O₂ is generated mainly by the compound decomposition reaction between electrons and <inline-formula><tex-math id="Z-20220530120347">\begin{document}${\rm{CO}}^+_2 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20220086_Z-20220530120347.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20220086_Z-20220530120347.png"/></alternatives></inline-formula> near the instantaneous anode surface or instantaneous anode side dielectric surface, and the contribution rate of the reaction can reach about 98%. It is further found that the dielectric does not change the generation position nor dominant reaction pathway of the two main products, but will reduce the electron density from 5.6×10¹⁶ m⁻³ to 0.9×10¹⁶ m⁻³ and electron temperature from 17.2 eV to 11.7 eV at the boundary of the cathode falling region, resulting in the reduction of CO production. At the same time, the deposited power is reduced, resulting in insufficient <inline-formula><tex-math id="Z-20220530120420">\begin{document}$ {\rm{CO}}^+_2 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20220086_Z-20220530120420.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20220086_Z-20220530120420.png"/></alternatives></inline-formula> yield near the instantaneous anode surface and instantaneous anode side dielectric surface and further the decrease of O₂ generation.