NASA has a clear need to develop new technology in support of its future goals, including missions beyond low-Earth orbit, the possible development of lunar outposts, and the eventual exploration of Mars. As these missions develop, it is anticipated that crew members will spend extended time outside the spacecraft and established habitats, requiring new, robust, lightweight life support systems for extravehicular activities (EVAs). One area that is critical to life support systems is the control of CO2, and new spacesuits must be able to accommodate longer EVAs without increasing the size or weight of the current portable life support system (PLSS).
The rate of CO2 generation during EVA is dependent on the level of crew activity and ranges from 39 grams CO2 per hour at rest to a maximum of about 393 grams per hour, while the average is about 93 grams per hour. Therefore, the CO2 control system must be sized to handle at least average production rates for the duration of the EVA.
One approach to developing new methods for CO2 removal in EVA applications is the use of rapidly regenerable sorbent systems, which are used alternately to remove CO2 and then taken offline for regeneration. This strategy offers the benefit of smaller adsorption beds and lifetimes limited only by the number of times the sorbent can be loaded and regenerated. However, the tradeoff is the increased complexity, size, weight, power consumption, and potential failures associated with the hardware required to carry out the regeneration.
Another approach is to use a membrane to separate CO2 from O2, but conventional gas separation membranes do not have adequate selectivity for CO2. However, supported liquid membranes (SLMs), where a reactive liquid is incorporated or immobilized in the pores of a microporous membrane, have the potential to perform well in this application because they are a continuous system in which the chemistry of the liquid can be tailored to selectively absorb CO2 on the crew side of the SLM, and continuously desorb CO2 to the vacuum of space on the low-pressure side of the SLM.
In this work, ionic liquids were used to develop new SLM sorbents that have good reversible CO2 absorption capacity, low viscosity, and effectively zero vapor pressure. Ionic liquids are a relatively new type of organic chemicals that consist of positive and negative ions and are liquids at room temperature. Ionic liquid cations were functionalized with primary, secondary, or tertiary amine groups, which are known to absorb CO2. The compounds were then ion exchanged with various anions to reduce viscosity and O2 permeance. The structures of the compounds prepared were confirmed with NMR spectroscopy, and their total and reversible CO2 absorption capacities were characterized. To prepare the SLM, the liquids were immobilized in a thin, layered, microporous membrane designed to contain the liquid in the structure against differential pressures of up to one atmosphere. With the primary amine-functionalized ionic liquid, the maximum CO2 permeation rate obtained was 1.1 × 10–4 scc/(cm2 s cm Hg), which is high enough to allow the SLM to meet size requirements for the PLSS. Moreover, the selectivity for CO2 over O2 was 1450, which is sufficient to prevent excessive O2 losses. Finally, the water vapor permeance was found to be 1.5 × 10–3 scc/(cm2 s cm Hg), which is high enough to control relative humidity in the suit.
Thus, this work has demonstrated the feasibility of using an SLM containing a functionalized ionic liquid to control CO2 in EVA. Both the permeation rate and selectivity for CO2 over O2 meet initial requirements. Moreover, the extremely low vapor pressure of the sorbent makes SLMs ideal candidates for use in EVA applications because they overcome the instability that plagued early versions of SLMs that relied on aqueous liquid sorbents.
This work was done by David Wickham, Kevin Gleason, and Jeffrey Engel of Reaction Systems LLC, and Scott Cowley of Catalyst Consultants for Johnson Space Center. For further information, contact the JSC Technology Transfer Office at (281) 483-3809. MSC-25591-1