Plasma-assisted CO2 conversion is a novel technology being developed to address climate change by removing greenhouse gases from the atmosphere and fulfilling the demand for carbon-based products, and as a method of in-situ resource utilization (ISRU) on Mars to reduce launch costs and increase mission independence and efficiency. Because carbon dioxide comprises 96% of the Martian atmosphere, plasma reactors are a promising route to generating oxygen for rocket propellant and respiration, enabling human exploration of Mars. You can read about this proof-of-concept reactor idea here.

Although at a lower stage of maturity compared to other chemical conversion technologies, plasma-based systems present many advantages. Plasma-assisted technologies exploit electron excitation chemistry, rather than thermal activation or surface reactions. There are chemical pathways that can be leveraged to break stable molecular bonds, like CO2, through selective excitation of electronic and vibrational modes, as well as vibrational energy exchanges. This enables lower-temperature reactors that can also be switched on and off on-demand, without any significant startup time associated with thermal processes, enabling the flexibility to use with intermittent power. These technologies are also easily scalable, making it possible to have both portable systems for astronauts and large-scale industrial systems for colonies. Despite the breadth of plasma-assisted CO2 conversion studies, there are limited studies providing parametric sweeps of operational parameters to compare performance and to begin to optimize the design process. These comparisons are necessary to go beyond fundamental plasma studies and engineer a competitive technological candidate for Martian ISRU.

What is the goal of the thesis?

Model and examine the performance of plasma reactors with a time-varying electrical waveform that represents Nanosecond Repetitively Pulsed Discharges (NRPD), and parametrically explore operational parameters under both Earth and Martian conditions. A comprehensive range of temperature, pressure, and electrical settings will be tested to map conversion fraction and efficiency, which can provide valuable insights for the design of future reactors.

How are you modeling this?

A combination of a 0-dimensional chemistry/Boltzmann solver and a 1-dimensional fluid model is used to understand the competing effects of transport and kinetics in physical plasma discharges (splitting phase). A user-defined reduced electric field waveform, an appropriate set of chemical reactions, and some initial operating conditions are the model inputs. Global parameters such as the dissociation fraction of CO2 and energy efficiency, in addition to species density evolution, are calculated as output. To understand the coupling of operating conditions and chemical heating of the plasma, particular emphasis is placed on the energy balance and heat transport within the reactor model. A very detailed chemical kinetic model can be found here.

What is needed to actualize this reactor?

Physically, the reactor needs two stages: a plasma source, for the splitting phase, and a separation stage, to produce a pure stream of oxygen. Future work aims to utilize the 1-D model to predict the temperature, yield, and energy efficiency of an experimental plasma reactor utilizing NRPD, under construction in our laboratory. Optical Emission Spectroscopy and gas chromatography will be used to measure plasma temperature and dissociation fraction, respectively, and energy deposition will also be measured, from the electrical waveforms, to calculate energy efficiency. Additionally, there are potential synergies between the two reactor phases that can be explored through a combination of modeling and experiment in future work. With an accurate understanding of the plasma discharges under various conditions, this work paves the way for designing a future reactor that couples the splitting and separation stages, exploiting synergies, into a proof-of-concept oxygen generation technology for Mars.

L. McKinney acknowledges support through the National Science Foundation Graduate Research Fellowship program, division of Plasma Physics. This work was also funded by the MIT-Portugal program (Project IMPACT), the Portuguese FCT (Fundação para a Ciência e a Tecnologia) under projects UIDB/50010/2020, UIDP/50010/2020, EXPL/FIS-PLA/0076/2021, and MIT-EXPL/ACC/0031/2021.