48th International Conference on Environmental Systems ICES-2018-48 8-12 July 2018, Albuquerque, New Mexico

State of NASA Oxygen Recovery

Zachary W. Greenwood1, Morgan B. Abney2, Brittany R. Brown3, Eric T. Fox4, and Christine Stanley5 NASA, George C. Marshall Space Flight Center, Huntsville, Alabama, 35812, USA

Life support is a critical function of any crewed space vehicle or habitat. One of the key elements of the life support system is the provision of oxygen to the crew. For missions close to Earth, oxygen may be resupplied from the ground, but as we look at exploring further out into the solar system for longer periods of time oxygen recovery from metabolic carbon dioxide (CO2) becomes a priority to minimize resupply requirements and enable feasible mission architectures. For more than half a century NASA has pursued the development of technology to enable oxygen recovery from metabolic CO2. Development work has included Bosch, Sabatier, and CO2 electrolysis systems and more recently plasma reactors, ionic liquids, and other exotic processes. NASA’s historical oxygen recovery work as well as the current state of oxygen recovery work currently going on within the agency is presented and discussed.

Nomenclature IL = Ionic Liquid AR = Atmosphere Revitalization ISS = International Space Station C2H2 = Acetylene MSFC = Marshall Space Flight Center C2H4 = Ethylene OGA = Oxygen Generation Assembly C2H6 = Ethane PPA = Plasma Pyrolysis Assembly C = Carbon PSI = Pounds per Square Inch CM = Crew Member PSIG = Pounds per Square Inch Gauge CDRA = Carbon Dioxide Removal Assembly RGA = Residual Gas Analyzer CFR = Carbon Formation Reactor RTV = Room Temperature Vulcanizing CH4 = Methane RWGS = Reverse Water-Gas Shift CORTS = Carbon Dioxide Reduction Test Stand SBIR = Small Business Innovation Research CRA = Carbon Dioxide Reduction Assembly SCOR = Spacecraft Oxygen Recovery ECLSS = Environmental Control and Life SDU = Sabatier Development Unit Support Systems SI = Sustainable Innovations, Inc. GDC = General Dynamics Corporation SmLPM = Standard milliLiters Per Minute H = Enthalpy UTAS = UTC Aerospace Corporation H2 = Hydrogen W = Watts H2O = Water

1 Aerospace Engineer, Space Systems Dept., NASA-MSFC/ES62. 2 Branch Chief, Space Systems Dept., NASA-MSFC/ES62. 3 Project Lead and Reactor Designer, Space Systems Dept., NASA-MSFC/ES62. 4 Ionic Liquids SME, Materials Branch, NASA-MSFC/EM22 5 Design Engineer, Space Systems Dept., NASA-MSFC/ES62.

I. Introduction MONG the most demanding of the challenges that engineers face is the development of efficient and reliable A human life support systems for space exploration. Reduced atmosphere and lack of breathable oxygen and drinkable water on alternative planets requires technology capable of providing these resources while simultaneously purifying and recycling human waste. Additionally, this technology must be self-sufficient, requiring minimal replacement of essential elements like oxygen and hydrogen. For this purpose, the Environmental Control and Life Support Systems (ECLSS) project is continuously seeking and developing new technologies. One of the key components of ECLSS is Atmosphere Revitalization (AR). AR seeks to maintain a breathable atmosphere during space missions. Thus, it is imperative that trace contaminants and carbon dioxide be removed from circulating air to maintain the health of astronauts. NASA has designed AR systems for multiple missions including Apollo and the International Space Station (ISS). Human life support systems on the International Space Station (ISS) include a number of AR technologies to provide breathable air and a comfortable living environment to the crew. The Trace Contaminant Control System removes harmful volatile organic compounds and other trace contaminants from the circulating air. The Carbon Dioxide Removal Assembly (CDRA) removes metabolic carbon dioxide (CO2) and returns air to the cabin. Humidity is kept at comfortable levels by a number of condensing heat exchangers. The Oxygen Generation Assembly (OGA) electrolyzes water to produce oxygen for the crew and hydrogen (H2) as a byproduct. A Sabatier reaction-based CO2 Reduction Assembly (CRA) was launched to the ISS in 2009 and became fully operational in June 2011. The CRA interfaces with both the OGA and CDRA. Carbon dioxide from the CDRA is compressed and stored in tanks until hydrogen is available from OGA water electrolysis. When the OGA is operational and there is CO2 available, the CRA is activated and produces methane and water via the Sabatier reaction shown in Equation 1.

Sabatier Reaction CO2 + 4H2  CH4 + 2H2O ΔH°rxn = -165 kJ/mol (1)

The water product is condensed out of the product stream, separated, and purified in the Water Processing Assembly before being recycled back to the OGA to be used to produce O2 for the crew. Methane, saturated with water vapor at a dewpoint similar to the temperature of the ISS moderate temperature cooling loop that is used to cool the condensing heat exchanger, is vented to space as a waste product. The loss of H2 in the form of vented CH4 and uncondensed water vapor in the CH4 stream limits the oxygen recovery to approximately 50% from metabolic CO2. A Bosch reactor was strongly considered for Space Station Freedom. A Bosch reactor was appealing because of the potential for high levels of oxygen recovery and because it would be able to meet the no-venting requirement that was initially imposed. However, catalyst handling complexities and the removal of the no-venting requirement with the program change from Space Station Freedom to the ISS meant that a Bosch reactor has not yet flown. See below for a discussion of the constituent Bosch reactions and a discussion of the process. As NASA looks to conduct more ambitious missions further away from Earth, the resupply and/or initial launch mass requirements must be minimized. The mass requirements imposed by only recovering 50% of the oxygen from metabolic CO2 will simply not be sufficient to enable the development of feasible mission architectures, so the total oxygen recovery must be increased. Several technology development paths are currently being pursued to increase oxygen recovery beyond the ISS state-of-the-art. Figure 1 graphically depicts NASA’s intended development plan for CO2 reduction technology. In general, there are three paths being explored in parallel. The first path builds on the existing Sabatier state-of-the-art and adds methane post processing to increase total oxygen recovery. This path is represented by Plasma Pyrolysis Assembly (PPA) technology development and culminates with an ISS flight demonstration in 2021. The second path forward includes two technology development efforts within the Spacecraft Oxygen Recovery (SCOR) project. One of these technologies, developed by Umpqua, builds on Bosch technology. The second of these technologies, developed by Honeywell, is a direct competitor to PPA in that it is methane-post processor that would be integrated with Sabatier technology. The third path explores a Series-Bosch approach and explores various options for carbon formation within the system. The second two paths target an ISS flight demonstration in the 2024 timeframe.

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Also shown on Figure 1 is the status of the ISS Sabatier reactor. The ISS Sabatier began showing reduced performance and was returned to the ground for testing and analysis in early Fiscal Year 2018. After analyzing, understanding, and correcting the mechanisms that led to the Sabatier’s performance degradation the plan is to build a replacement unit and begin operating it on the ISS in March 2021. This will put the new Sabatier on-orbit in time to support a PPA flight demonstration.

Figure 1: NASA CO2 Reduction/Resource Recovery Development Plan

The following sections of this paper start by discussing historical Bosch and Sabatier work. The intent of these sections is to provide reference for anyone that wishes to understand how NASA arrived at its current state of CO2 reduction technology. These sections are followed by discussion of current NASA CO2 reduction work.

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II. Historical Background

A. Bosch

The Bosch process involves catalyzing CO2 and H2 gas to produce solid carbon and water. It was initially understood to be a two-step process including the Reverse Water-Gas-Shift (RWGS) and CO Hydrogenation reactions. In the RWGS reaction, CO2 and H2 react to form carbon monoxide (CO) and water. As traditionally understood, this is followed by hydrogen reduction of CO to form solid carbon and water. The Boudourd reaction, in which CO is converted to CO2 and solid carbon, is also a critical contributor to the Bosch process but was not recognized until later.

RWGS CO2 + H2 ↔ H2O + CO (2) CO Hydrogenation CO + H2 ↔ H2O + C(s) (3) Boudouard 2CO ↔ CO2 + C(s) (4) Bosch Process CO2 + 2H2 ↔ 2H2O + C(s) (5)

This technology has been under consideration throughout the history of NASA. Historical concerns with the reactor include the loss of catalyst due to carbon fouling and the crew intensive requirements for the monitoring and disposal of accumulated carbon waste.

1. Bosch History at NASA

NASA (through Langley Research Center) began development of the Bosch reactor in the mid-1960’s in conjunction with General Dynamics Corporation (GDC).1 The first prototype to be delivered to NASA was completed in 1970 with development and integration continued by the Life Systems Department in 1975.2-13 Although relatively limited, academic research on Bosch, concentrating primarily on catalyst optimization, was also in progress during this time period and continued through the early 1990’s.3-10, 14, 15

2. Historical Bosch Research

Although the Bosch reaction had not been studied prior to NASA’s investigations in the 1960’s, the RWGS reaction had been examined by a number of groups. Kusner reported that the RWGS reaction went to completion over an iron or iron oxide catalyst at 922K.8 Similarly, Barkley et al. reported that the reaction went to completion over a copper-oxide catalyst at 811K.9 Iron carbide studies were also prevalent in the 1940-60’s and would later prove pertinent to Bosch iron catalysts. From these studies, three forms of iron carbide were identified. The first was cementite (Fe3C) which was later shown to be a deactivated form of iron-carbide. This form of carbide also led to unwanted byproducts, predominantly 10 methane, in Bosch reactors. The second was Fe5C2, also called Hagg carbide and was shown to decompose to 11 cementite and carbon at temperatures above 773K. Finally, Fe7C3 was shown to form only at pressures greater than 20 atm.12 In 1966, Armstrong of GDC reported the results of a two-year study aimed at developing a CO2 reduction system capable of 90 consecutive days of operation in a 117.5 m3 spacecraft with a crew of four. For this purpose, Bosch systems from two existing companies (Tapco and General American Transportation Corporation) were evaluated.1 From this information, GDC began internal development of their own Bosch reactor capable of operating as a Sabatier reactor for emergency back-up. The reactor was integrated with an electrolysis unit and CO2 concentration unit and successfully operated per requirements. However, engineers determined the need for improved materials for the high temperature operation and improved fabrication techniques to eliminate leaks in the system. Additionally, significant concerns were noted regarding the sustainability of the Bosch cooperative subsystems (CO2 concentrator and electrolysis unit).

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Following their 1966 report, GDC continued development of the Bosch reactor and reported their findings in a 1970 report.13 During the four-year development period, GDC used steel wool as a catalyst. They found that the optimum feed settings were 40% hydrogen, 30% methane, 20% CO, and 10% CO2 (vol %). After exploring a range of temperatures from 874-977K, it was determined that higher temperatures yielded more CO and less methane. Additionally, it was found that pre-activation of the steel wool significantly decreased the lag time for initial water production. During the development of the Bosch in the 1970’s, a series of dissertations were produced from Robert Reid’s group at the Massachusetts Institute of Technology. Robert Wilson reported a fundamental investigation of the Bosch reaction in his 1971 Master’s thesis.3 His work indicated that, rather than the accepted hydrogen reduction reaction to form carbon, a third reaction, the Boudouard reaction, was the primary source of carbon at 873K and 1 atm over an iron rod catalyst. Additionally, Wilson determined that kinetics played a much larger role than equilibrium considerations in the observed reactions. In 1976, Michael Manning published his dissertation exploring multiple aspects of the Bosch process at 823K and 1 atm over a steel wool catalyst.4 In this work, he examined ratios of recycle gases to determine the resulting products and by-products. He found that recycling high quantities of methane resulted in the cessation of carbon deposition. Feed with a high concentration of hydrogen resulted in higher yields of methane. Feed with high carbon dioxide concentration yielded no reaction except catalytic oxidation. The best carbon deposition was found when the recycle stream had a high concentration of carbon monoxide. Additionally, once deposited, carbon did not facilitate further carbon deposition. Rather, carbon deposition led to increased iron catalyst surface area, thus yielding improved carbon deposition although the mechanism was not fully understood by Manning. Finally, Manning proposed the best way to improve the performance of the Bosch reaction was to split the reactor into two separate reactors. The first reactor would be dedicated to forming carbon monoxide and water in the RWGS while the second reactor would be dedicated to carbon deposition with control parameters optimized for the hydrogen reduction reaction. Following on the heels of Manning, Sacco completed his dissertation in July 1977.5 His work was directed at determining how carbon deposition was affected by iron, iron carbides, and iron oxides as catalysts. He found that wustite (FeO) inhibited carbon formation. Unfortunately, he was unable to draw any conclusions regarding magnetite 14 (Fe3O4) or iron carbides. However, Everett had previously shown that the presence of magnetite inhibited carbon deposition. Another important finding of Sacco’s study was the influence of pre-conditioning parameters on the resulting catalyst. Two methods of pre-conditioning were examined. Both methods involved cleaning by oxidizing with a CO2/H2O mixture. This was followed by reduction with hydrogen at 800K for the first method, and 900K for the second method. The resulting catalysts had very different surface morphologies. For pre-conditioning at 800K, a thin, dense layer was observed on the outer surface of the catalyst. For pre-condition at 900K, a highly porous shell structure was observed surrounding a dense core. This resulted in a 500% increase in the effective surface area of the catalyst. In 1980, Dr. Reid’s group continued Bosch related research in the form of a dissertation by Garmirian. His work focused on nickel and cobalt (wire) catalysts and their respective oxide forms as they related to carbon deposition and necessary recycle ratios. He compared an optimum recycle ratio of 10.5 for iron with ratios of 6.0 (at 825K) and 4.3 (at 800K) for nickel and cobalt, respectively. This data implied that cobalt would be the best catalyst with respect to limiting the necessary recycle ratio for maximum carbon deposition. However, he suggested that both nickel and cobalt, in wire form, be explored in a prototype reactor. The final work from Dr. Reid’s group regarding Bosch was a single paper published in 1982 by Sophonpanich15 in which the required recycle ratio and rates of carbon deposition on unsupported ruthenium (pure ruthenium, 50Ru- 50Fe, 33Ru-67Fe) were explored. He found that pure ruthenium did not have catalytic qualities for the Bosch system. While the ruthenium-iron alloys performed better than iron alone, they were not as effective as the nickel and cobalt catalysts. Lastly, the minimum recycle ratio was determined to be 10.9 for the 50Ru-50Fe as compared to 10.5 for the iron alone. In 1992, Vaidyaraman completed a Georgia Tech master’s thesis in which he studied the kinetics of the Bosch 7 reaction over three catalysts including Ni/SiO2, Fe/γ-Al2O3, and Fe/SiO2 . In this study, multiple parameters were studied to develop a kinetic model including reaction temperature (673-873K), pressure (sub-atmospheric), and inlet gas composition. While Vaidyaraman was able to determine a simple representative kinetic equation for his system, this system did not adequately represent a total Bosch system in that only the RWGS and Boudouard reactions were considered and no recycle was considered in the modeling. Despite this fact, certain points can be taken from the work. First, regardless of inlet concentrations of reactants, 773K was consistently the best temperature for carbon deposition. This is possibly due to the difference in optimum conditions for each of the reactions involved (low 5

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temperature for one, high temperature for the other). Secondly, it was shown that using pre-conditioned iron supported catalysts prevented the formation of fibrous carbon, instead promoting lamellar carbon and allowing for higher concentrations of deposited carbon during reactions. Finally, Vaidyaraman suggested that using an alloy catalyst with separate metals contributing to each reaction individually could potentially improve overall performance of the Bosch reaction.

B. Sabatier

The Sabatier reaction involves the conversion of CO2 and hydrogen to form water and methane. The Sabatier reactor is designed to operate at 788K and at slightly sub-ambient pressure (to protect against hydrogen leaks). CO2 enters the reactor at 322K and operating pressure to produce the desired products over a Hamilton Sundstrand proprietary ruthenium/alumina catalyst.

1. Sabatier History at NASA

Development of the Sabatier reactor began in the late 1950’s by Isomet Corporation.27 Initially dubbed the “Methoxy” reaction, Isomet continued Sabatier development through the mid-1960’s to successfully develop a 3-crew reactor. During this time, engineers proposed that, providing proper insulation, the reaction could potentially be self- sustaining on an alumina supported ruthenium catalyst. This self-sustainability hypothesis would later prove correct and very valuable for resource management. Development was adopted in the late 1960’s by Hamilton Standard with the first pre-prototype completed in the early 1980’s.28 The Sabatier Engineering Development Unit (EDU) was described in 2003 to include a Sabatier reactor, carbon dioxide management system (CDMS) which includes a 29 compressor, valves and sensors, and a CO2 accumulator to ensure consistent flow during OGA operation. This was followed by integrated testing of the EDU with the CDRA ground simulator in 2005 and 2006.37, 38 A Sabatier reaction- based CO2 Reduction Assembly (CRA) was launched to the ISS in 2009 and became fully operational in June 2011.

2. Historical Sabatier Research

There has been a significant amount of research completed since the late 1950’s with regards to the Sabatier reaction. Internal research has been primarily focused on reactor design, reaction optimization and reactor integration27-39 while external research has been primarily focused on catalyst development.40-47 Initial studies by Isomet Corporation from 1959 to 1964 yielded three important facts. First, a Sabatier reactor for the hydrogenation of CO2 was possible. Secondly, the Sabatier reaction was potentially self-sustaining when a reactor was properly insulated. Thirdly, tests with CO indicated that small quantities (<5% of the feed stream) did not adversely affect the conversion efficiency of CO2 to CH4. Additionally, CO was entirely reacted with no trace detectable in the product stream.30,31 In 1971, Hamilton Standard published a final report on their continued development of the Sabatier reactor. This work was the first test to fully evaluate the effect of flowrates, system pressure, H2:CO2 ratio and cooling rates on the efficiency of the reactor. This work included a complete mathematical model of the reaction for steady state and transient conditions.28 In 1974, NASA published a kinetic study of the Sabatier reaction using a catalyst made up of 20% ruthenium on an alumina support.39 Later research on a similar catalyst led to the development of a self-sustaining reactor capable of supporting a crew of 3 in 1980 and a crew of 5 in 1981 with >99% efficiency for ratios of H2:CO2 of 1.8-5.32-34 By 2003, a fully operable Sabatier Engineering Development Unit was completed29 with integration testing completed in 2005-2006.37, 38 To supplement the extensive work completed by NASA and Hamilton Sunstrand, external sources have continued work on the Sabatier catalysts. A variety of catalysts have been explored for this purpose including ruthenium on alumina, titania, silica, anatase and rutile in various percentages. Other catalysts include rhodium on alumina, titania and silica, nickel on magnesium oxide, titania, alumina and silica and ruthenium catalysts with alkali additives on alumina.40-47 The current EDU contains a proprietary ruthenium on alumina catalyst developed by Hamilton Sunstrand.

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III. Current NASA Oxygen Recovery Work

A. Bosch Derived Development

Two of the three paths toward increasing oxygen recovery from metabolic CO2 that NASA is pursuing are based on improvements to the Bosch process. These efforts include Series-Bosch technology development and Umpqua’s Continuous Bosch Reactorwhich are show in Figure 1. Ionic Liquids are also being investigated to improve the Bosch process. Each are discussed below.

1. Series-Bosch Development

To further the development of oxygen recovery systems, NASA awarded Phase I contracts to several entities under the Game Changing Development Program addressing Spacecraft Oxygen Recovery. Two carbon formation reactors (CFR) were delivered to Marshall Space Flight Center (MSFC) at the close of the contracts. Each of the reactors were evaluated in a stand-alone configuration prior to delivery. Once received by MSFC, the reactors were integrated into the Carbon Dioxide Reduction Test Stand (CORTS) separately and integrated performance evaluations were executed in 2017 and 2018. The first one evaluated was a CFR developed by pH Matter, LLC (Columbus, OH). The second one was developed by UMPQUA Research Company (Myrtle Creek, OR) as a single reactor Bosch process, but was integrated in the CORTS as the CFR component of the Series Bosch process. Although, there are two Phase II SCOR contracts currently on-going, this work falls under the Series-Bosch work. The data collected from the evaluations of these reactors along with the data collected for the NASA developed CFR will be used in a down-selection process. A reactor will be selected for further development to compete with the Phase II SCOR developed reactors. The final selection will undergo further development to become a flight-demo unit onboard the ISS. Series-Bosch (S-Bosch) technology has a theoretical maximum recovery of 100% and is one approach to achieve the goal of >90% recovery. A short overview of the process is presented here. The system involves two reactors, the Reverse Water Gas Shift (RWGS) reactor and the CFR and two membranes, the Carbon Dioxide Extraction Assembly (CDEA) membrane (Polaris) and the Hydrogen Extraction Assembly (HEA) membrane (Proteus). Figure 2 shows an illustration of the integrated S-Bosch process. Fresh CO2 enters the system as the sweep stream for the HEA and picks up H2 from the recycle stream while fresh H2 enters as the CDEA sweep stream picking up CO2. The membranes are operated at a pressure differential of ~5 psid to increase permeability. The sweep streams are combined prior to entering the RWGS where the reaction shown by Eq. 2 occurs. The stream exiting the RWGS enters the compressor where it is mixed with the CFR effluent. The mixed stream becomes the process/recycle stream prior to entering the condensing heat exchanger. Water vapor is condensed while the remaining gases flow through the HEA and then the CDEA. The effluent of the CDEA enters the CFR where Figure 2. Illustration of the Series Bosch System. the Carbon Monoxide (CO) Hydrogenation (Eq. 3) and/or the Boudouard (Eq. 3) reactions

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occur. The resulting net reaction is the Bosch process as shown in Eq. 5. An integrated evaluation of the NASA developed reactor was performed in 2017 and the results were discuss previously.21 Those results along with the results for the reactors received from pH Matter and UMPQUA will be considered during the down-select process. The selected reactor will undergo further development. The pH Matter and UMPQUA reactors were each integrated into the MSFC CORTS for testing and evaluation. Testing of the UMPQUA reactor was delayed and results are expected by the end of June. The pH Matter CFR produced carbon in the form of fragile flakes interspersed with active metal catalyst particles as shown in Figure 3. This is typical behavior for metallic catalysts for the Bosch process. While the catalyst is broken down by carbon deposition, the particles remain active as long as contact is made with the gas. This, in effect, increases the life of the catalyst. During this test campaign the reactor was not run long enough to completely fill with carbon or to deactivate the catalyst. This would be the goal of future evaluations. pH Matter performed a series of stand-alone tests that resulted in a carbon formation rate in the range of 4.06 to 6.87 mg C /cm2/hr. This resulted from a steady, constant CFR inlet gas composition and flow rate. In the integrated tests, the CFR inlet gas composition and flow rates were dependent on the reaction products from the CFR mixed with Figure 3: Carbon formed in the the reaction products from the RWGS reactor, which make up the pH Matter CFR. The carbon recycle loop. The integrated test showed a higher carbon production formed as delicate flakes with bits of per area of catalyst. Additional details of this testing may be found in metal catalyst dispersion throughout. Ref. 22.

2. SCOR Phase II

In February 2017, NASA announced the selection of two Phase II Space Craft Oxygen Recovery development efforts. Honeywell Aerospace was selected to advance their technology entitled “Methane Pyrolysis System for High- Yield Soot-Free Recovery of Oxygen from Carbon Dioxide.” UMPQUA Research Company was selected to advance their “Continuous Bosch Reactor” technology. Eight key requirements were levied on the projects:

 Meets Technology Readiness Level 5 brassboard maturity by the end of the project  Achieves at least 75% oxygen recovery from the carbon dioxide produced by 4 crew members at an integrated life support system level.  The new component may accept feed from and deliver conditioned product streams to existing components, such as water electrolysis or Sabatier as part of the system.  Excess oxygen cannot be produced to achieve the minimum targeted 75% recovery from carbon dioxide.  Includes all functionality, including sub-systems and balance of plant necessary to integrate with and demonstrate the required recovery rate with input streams and resources similar to those available on ISS (e.g., Sabatier product gases, vacuum vent, power and cooling).  Includes all software and controls necessary for operation.  Includes a concept of operations for implementation in a future mission scenario, including, but not limited to Martian transit and Martian surface missions.  Built to NASA standards for safety, including, but not limited to, oxygen, hydrogen, and pressure systems.

Both efforts were initiated in September 2017 and will be subject to a Continuation Review approximately halfway through the two year projects. At the completion of the efforts, NASA will review the capability of the technologies and compare them to other O2 Recovery technologies with the goal of down-selecting for a flight demonstration.

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3. Ionic Liquids

Ionic Liquids (ILs) are liquid organic salts and are of particular interest for space applications due to their characteristics such as low flammability, no vapor pressure, and their ability to be modified to be task specific. Several efforts involving ILs in O2 Recovery approaches have been explored including ILs for catalyst recycling, ILs to support conversion of CO2 to methane (not discussed here), and ILs to support conversion of CO2 to carbon monoxide.23

a. MSFC Ionic Liquid Work

Historically, Bosch systems have produced carbon at a volume of 1.2 kg per day, which quickly fouls the catalyst. For long duration missions, this issue is concerning due to the high amount of catalyst resupply that would be required to maintain oxygen for the crew throughout the entire duration of the mission. In order to solve this issue, advanced development of a fully regenerable oxygen recovery system is desirable. Regenerable catalysts can be achieved with the aid of ILs. Previous studies at NASA MSFC have used ILs to extract iron (Fe) and nickel (Ni) from a Campo del Cielo meteorite and electroplate the extracted metals onto a carbon substrate.24 This same approach can be applied to create an IL-based Bosch system which could eliminate the need for resupply for long duration missions. An IL-based Bosch system is a fully regenerable oxygen recovery system that involves the following three steps:

1.) Designer IL is used to extract Fe and/or Ni from Martian regolith. 2.) Designer IL is used to electroplate Fe and/or Ni onto a Copper (Cu) substrate to generate a Fe-Cu catalyst substrate to be used in the Bosch process for O2 recovery. 3.) Designer IL is used to regenerate the Fe and/or Ni from the carbon fouled catalyst. Upon completion, the Fe and/or Ni is suspended in the IL leaving behind pure carbon. The solution is then ready to be reused in step two.

In 2015 and 2016, the feasibility of using ILs to reduce catalyst resupply in a closed-loop Environmental Control Life Support System (ECLSS) Atmosphere Revitalization (AR) system was proven and achieved a TRL 2.5.25 Fe was successfully electroplated onto Cu substrates using a traditional method with three different ILs, and electroplated Fe- Cu substrates were shown to be catalytic from all sources. Also, Fe extraction from a high carbon-content mixture using IL was demonstrated. In 2017, efforts were initiated to scale the proposed technology. A multi-substrate plating apparatus and a bulk regeneration system was designed, constructed, and successfully tested. Based on the data gathered throughout the study, an initial reactor design was generated. The results of that effort are detailed in Ref. 26. Future work will involve further optimization of the bulk regeneration system that mimics the design of the IL Bosch-based reactor design, and enhances the carbon cleaning capabilities.

b. University of Colorado Ionic Liquid Work

The University of Colorado-Boulder, under a NASA Space Technology Research Grant, has pursued development of an O2 recovery technology that uses ILs to convert CO2 to carbon monoxide and oxygen. Benefits and challenges of this approach include room temperature operation, direct O2 production, and a product that can be combined with a variety of other technologies to meet mission needs. Holquist et al have previously reported advantages and challenges of this technology.23

4. Alternative Carbon Uses

The Bosch process is a desirable technology for future long duration life support missions, due to the potential recovery rate of 100% oxygen from metabolic CO2. Although the process has a high recovery rate, it comes with a great challenge. The Bosch process reacts carbon dioxide (CO2) with hydrogen (H2) to produce water (H2O) and elemental carbon (C) in the presence of a catalyst. The carbon produced builds up and fouls the catalyst at a rate of 1 kg per day. Finding useful ways to utilize the carbon produced in the Bosch process would be very beneficial. One area of particular interest is additive manufacturing. 9

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Additive manufacturing onboard ISS provides the ability to manufacture parts on demand allowing for critical replacement parts and tools to be manufactured without the need to wait for them to arrive. ABS filament is currently being used on ISS for additive manufacturing, and although ABS is a relatively strong plastic, when ABS manufactured parts are used as tools there is a great chance of the manufactured parts to fracture. The addition of carbon to the ABS filament is likely to solve this issue. Carbon has long been used in industry as a reinforcement providing structural strength to materials. The addition of Bosch carbon to ABS filament could greatly increase the structural strength of manufactured parts allowing for more durable and reliable parts to be printed, as well as utilizing a waste product of the Bosch reaction. Research is being done at NASA MSFC in conjunction with the University of Alabama to determine how Bosch carbon effects the physical properties of additive manufactured parts. Bosch carbon ABS filament has been generated with 1% and 3% Bosch carbon by combining Bosch carbon with ABS filament through mechanical mixing and extruded into filaments. ABS is known to fracture at higher carbon content, therefore lower carbon to ABS ratios were used in initial Bosch carbon ABS filament generation. Future efforts will determine what the maximum Bosch carbon content in ABS filament is required in order to maintain suitable mechanical properties. Also, other thermoplastics as well as ionic ultra-high performance polymers will be investigated. Additional research will investigate Bosch carbon in thermoplastics. This approach will use 3D printed ABS reusable molds filled with a Room Temperature Vulcanizing (RTV) silicone and Bosch carbon mixture. This approach will allow for more bulk carbon consumption, and the use of RTV silicone will allow for greater physical properties not achievable by ABS such as flexibility and compressibility.

B. Sabatier and Methane Post-Processing

The other primary path toward greater oxygen recovery being pursued by NASA is to develop a methane post- processing architecture to augment the Sabatier process. This work is represented by the Plasma Pyrolysis Assembly (PPA) work in Figure 1 and is discussed below.

Methane Post-Processing

NASA developments relating to Sabatier based technology currently focus on post-processing of Sabatier produced methane in order to increase total oxygen recovery. NASA has been developing the Plasma Pyrolysis 54-60 Assembly (PPA) to fill the role of a methane post-processor. The PPA uses a magnetron to generate an H2/CH4 plasma targeting Sabatier CH4 conversion to hydrogen and acetylene (C2H2) as shown in Eq. 6. Secondary reactions with CH4, as shown in Eqs. 7-9, and reactions with residual water vapor as shown in Eqs. 10-11, also occur in the PPA resulting in an effluent mixture containing H2, unreacted CH4, product C2H2, and trace quantities of H2O, carbon monoxide (CO), ethylene (C2H4), ethane (C2H6), and solid carbon (C).

Targeted PPA Reaction 2CH4 ↔ 3H2 + C2H2 (6) CH4 Conversion to Ethane 2CH4 ↔ H2 + C2H6 (7) CH4 Conversion to Ethylene 2CH4 ↔ 2H2 + C2H4 (8) CH4 Conversion to Solid C CH4 ↔ 2H2 + C(s) (9) CO Production C(s) + H2O ↔ CO + H2 (10) CO Production CH4 + H2O ↔ CO + 3H2 (11)

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When H2 recovered by the PPA is recycled back to the CRA, and the CRA is operated at a H2:CO2 ratio of 4.25, a theoretical O2 recovery of >86% may be realized (assuming a respiratory quotient of 0.92) from metabolic CO2. This further reduces the water resupply requirement to ~0.18 kg/CM-day. Figure 4 shows the current baseline integration architecture for the PPA with the existing ISS AR architecture. This architecture makes use of an electrochemical cell stack produced by Sustainable Innovations, Inc. to carry out the separation of acetylene from hydrogen before the hydrogen is recycled back to the CRA. Testing of the PPA has been conducted in both a stand- alone mode and integrated with the MSFC Sabatier Development Unit and has been reported previously.54-60 A demonstration of the full hydrogen recycling architecture is planned for 2018.

To Vent

CO2 from Flow CRA CH4+ H2 PPA crew Meter

Flow

Controller

H2 Carbon Capture Flow WPA Controller H2O H2

Pressure

Controller H2+C2H2+ H2O+CH4... H2 Accumulator To Vent Tank To Vent

O2 to OGA C2H2+CH4+H2O crew to vent

H2 Sep.

Pressure Controller Figure 4: Proposed PPA system architecture. A flight experiment on the ISS is currently in the planning stages to demonstrate the PPA methane post-processing architecture but an ISS flight experiment presents a number of challenges. Included among these challenges are the necessarily tight physical and operational coupling with critical ECLSS assemblies in the Oxygen Generation System rack, the additional volume of combustible hydrogen necessary for the accumulator, and the perceived risks of flying a high powered microwave induced plasma in a habitable volume. The PPA could be flown and demonstrated by flying a supply of compressed methane and hydrogen; however, this would severely limit its operation time, raise questions about the behavior of the PPA system with the “real” methane mix coming from the Sabatier reactor, and pose its own challenges by necessitating the flight of large volumes of combustible gases. The accumulator volume and required system operating pressures are currently under investigation at NASA MSFC. A test is planned to start later this year to demonstrate and evaluate this architecture. Of primary interest is minimizing the accumulator volume and pressure. Ideally, it will be possible to operate the system entirely at sub ambient (1 atm) pressure. This would mitigate any concerns of combustible gas leakage into the cabin environment since any leakage that occurred would push air into the system instead of pushing combustibles out. There have been concerns expressed about crew exposure to microwave radiation. The PPA makes use of a substantial microwave power system to produce methane/hydrogen plasma. During normal processing at a 4-CM rate the PPA applies ~800 W of microwave power to the plasma. However, because of the use of effectively sealed microwave system components the emitted microwave radiation from the PPA is substantially less than a cell phone during a call. Current technology development to support the PPA methane post-processing architecture focuses on hydrogen separation, carbon capture, and the evaluation of microgravity plasma dynamics. A prototype hydrogen separator was delivered to NASA by Skyre, Inc (formerly Sustainable Innovations) in October 2017. However, during development, several necessary improvements to electrochemical membrane technology (to maximize hydrogen transport and minimize C2H2 hydrogenation) and sealing techniques were recognized as essential before incorporation into a flight system. 11

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During 2018 Skyre will work to make improvements to the electrochemical hydrogen separation technology. The goal of this project is to advance development of the Skyre electrochemical hydrogen separator membrane technology such that integrated operation of the cell stack results in at least 85% hydrogen recovery from a nominal PPA effluent stream at a processing rate of four crew members. Additionally, this effort will seek to improve sealing technology and techniques of the stack in an effort to minimize all undesirable leak paths (e.g. cross-cell leakage, leakage out of the stack, etc.) and total pressure drop across the cell stack. Also during 2018, UMPQUA will work to investigate the effects of microgravity on the PPA plasma. This effort began with a development of computer models of the PPA in both Earth normal gravity and microgravity and will be validated with empirical data gathered on a reduced gravity parabolic flight. With knowledge of the approximate masses of hardware and a basic mass calculation for oxygen or water resupply, it is possible to show, as in Figure 5, a simplified breakeven for various levels of loop closure. Compressed oxygen mass includes the mass of tanks (0.429 kg tank/kg O2). Addition of an OGA (~318kg) and resupply in the form of water instead of gaseous oxygen adds initial mass, but surpasses tanks of oxygen beyond 315 days. Adding Sabatier to the architecture further increases initial mass (318 kg for OGA + ~ 150 kg for Sabatier), but results in a simplified breakeven of ~170 days. Finally, addition of a PPA to the architecture adds initial mass ( ~100kg), improves in breakeven time of Sabatier alone by less than a week, but shows considerable benefit as mission duration increases. When the calculation is extrapolated to 1000-day class missions, the use of the PPA would save nearly 600 kg in launch mass over the OGA + Sabatier system architecture.

Figure 5: Mass break-even for Sabatier-based oxygen recovery. IV. Conclusion Extensive work has been conducted and will continue to be conducted to close the loop on oxygen recovery from metabolic carbon dioxide. But despite extensive and ongoing research, none of the proposed technologies is fully ready to fulfill the task of complete loop closure. Each has the potential, with the appropriate development, to meet this need but due to the many potential missions requiring oxygen loop closure, it is likely that multiple technologies will be adopted. Ultimately, environmental conditions, volume, weight, power restrictions and level of development will decide which technologies take us to the Moon and Mars. 12

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Jr., Hadley, N.M., Dahl, R.W., Abney, M.B., Greenwood, Z., Miller, L., Medlen, A., “Advanced PPA Reactor and Process Development,” AIAA Paper No. 2012-3553-145, presented at the 42nd International Conference on Environmental Systems, San Diego, CA, 2012. 59 Mansell, J. M., Abney, M. B., and Miller, L. A., “Influence of Oxygenated Compounds on Reaction Products in a Microwave Plasma Methane Pyrolysis Assembly for Post-Processing of Sabatier Methane,” AIAA Paper No. 2011-5035, presented at the 41st International Conference on Environmental Systems, Portland, 2011. 60 Atwater, J. E., Wheeler, R. R., Jr., Hadley, N. M., Dahl, R. W., Carrasquillo, R. L., “Development and Testing of a Prototype Microwave Plasma Reactor for Hydrogen Recovery from Sabatier Waste Methane,” 39th International Conference on Environmental Systems, SAE International, SAE 2009-01-2467, Savannah, Georgia, 2009. 14

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