49th International Conference on Environmental Systems ICES-2019-379 7-11 July 2019, Boston, Massachusetts

Solid State Electrochemical Oxygen Separation and Compression Michael Reisert1, Ashish Aphale2, Boxun Hu3, Su Jeong Heo4, Junsung Hong5, and Prabhakar Singh6 Department of Materials Science and Engineering, University of Connecticut, Storrs, CT 06269

Dale Taylor7 American Oxygen, Salt Lake City, UT

and

John Graf8 NASA Johnson Space Center, Houston, TX

Ceramic solid state electrochemical oxygen separation and compression systems offer the ease of producing high purity and high pressure oxygen from a variety of gaseous streams representative of ambient and constrained systems exposure conditions (terrestrial and space). The electrochemical cells utilize exclusive oxygen ion conducting membranes (fluorites, doped) and operate in 550-850 °C temperature range. Advanced perovskites synthesized from non-noble and non-strategic materials serve as electrodes for both oxygen reduction and evolution. A number of electrochemical cells, connected in series using dense electronically conducting perovskite interconnect form the basis of “cell stack” for increased oxygen production. Thermochemical-electrochemical principles for oxygen separation and compression will be discussed. Materials for the construction of cells and stack along with fabrication techniques will be examined and basis for the selection will be described. Approaches for the electrochemical performance improvement will be discussed.

Nomenclature

VN = Nernst voltage LSCF = lanthanum strontium cobalt ferrite R = universal gas constant LSM = lanthanum strontium manganite F = Faraday constant LCM = lanthanum calcium manganite PO2 = partial pressure of oxygen SOEC = solid oxide electrolysis cell I = current SOFC = solid oxide fuel cell jO2 = flux of oxygen CTE = coefficient of thermal expansion QO2 = flow rate of oxygen FEA = finite element analysis OTM = oxygen transport membrane ppm = parts per million MIEC = mixed ionic-electronic conductor ppb = parts per billion YSZ = yttria-stabilized zirconia EVA = extravehicular activity CSO = cerium samarium oxide

1 Graduate Student, Materials Science and Engineering, University of Connecticut, 44 Weaver Rd., Storrs, CT 06269. 2 Postdoctoral Researcher, Materials Science and Engineering, University of Connecticut, 44 Weaver Rd., Storrs, CT 06269. 3 Assistant Professor, Materials Science and Engineering, University of Connecticut, 44 Weaver Rd., Storrs, CT 06269. 4 Postdoctoral Researcher, Materials Science and Engineering, University of Connecticut, 44 Weaver Rd., Storrs, CT 06269. 5 Graduate Student, Materials Science and Engineering, University of Connecticut, 44 Weaver Rd., Storrs, CT 06269. 6 UTC Endowed Chair Professor, Materials Science and Engineering, University of Connecticut, 44 Weaver Rd., Storrs, CT 06269. 7 CEO, American Oxygen, LLC., 4054 S 685 E UNIT F, Salt Lake City, UT 84107. 8 Engineer, NASA Johnson Space Center, 2101 E NASA Pkwy, Houston, TX 77058.

Copyright © 2019 University of Connecticut I. Introduction lectrochemical oxygen compression can provide high-purity oxygen for life support systems in both terrestrial E settings and in human spaceflight.1 Currently, oxygen is mechanically pumped to be distributed into portable oxygen tanks for use in medical applications or as a means of supplying oxygen to the confines of space stations or space suits.1 Mechanical pumping requires a great deal of energy and oftentimes cannot be feasible for direct use within structures in space. Tanks which have already been filled on earth are subsequently transported to the space station, which drastically increases payload weights and limits transport of other necessary supplies. Furthermore, these tanks must be presurized to 300 bar for safe handling and proper confinement. It is imperative to determine a means of oxygen compression within the confines of the space station which can pressurize atmospheric-level oxygen up to 300 bar while utilizing the oxygen output of current fuel cell and electrolysis cell systems onboard the station.1 Separation of oxygen from air or other oxygen-containing gases and subsequent oxygen compression are achieved by using conductive ceramic components which foster exclusively oxygen-ionic transport. This transport can be preferentially driven by an applied bias across the selectively-ionic conducting electrolyte.2 Confining or housing the produced oxygen results in a single-step means of pressurization, eliminating the need for mechanical pumping. The electrochemical principles enabling this are the foundation of other systems such as fuel cells and electrolysis cells.2,3 These systems are well-understood and have been implemented in small and large-scale industrial settings. However, these systems often operate under reducing or mixed reducing/oxidizing gas atomspheres with ppm-ppb contaminant levels, which has historically lead to certain material selection issues due to electrode poisoning.3–5 The oxygen compressor of question would operate in a pure or relatively pure oxygen atmosphere, which influences material selection and avoids some contaminant issues which will be later discussed. This all-oxygen atmosphere and past demonstration of material functioning in electrochemical systems highlights the feasibility of an all solid-state and single-step electrochemical oxygen separator/compressor. In this paper, the different ceramic components of the specified oxygen separator/compressor will be discussed from a materials standpoint. This discussion will emphasize why these materials have been chosen for pure oxygen separation and compression. Furthermore, fabrication methods for system production will be discussed. This will culminate in a discussion of the current trends in this technology and the means by which this particular system compares and may be improved.

II. Oxygen Transport Membrane Operating Principles The electrochemical oxygen separator/compressor operates as an oxygen/ionic transport membrane (OTM/ITM). These systems can be either pure oxygen conducting or a mixed ionic/electronic conductor (MIEC).2 Pure oxygen conduction is achieved using an electrolyte which only allows transport of oxygen ions. This is used in conjunction with an anode and a cathode, where an applied electric potential allows oxygen within the feed gas to ionize at the cathode and move through the electrolyte towards the anode.2,3 The application of a bias makes this OTM electrically driven or passive. A MIEC transports oxygen ions and electrons Figure 1. Schematic of an electrically-driven oxygen transport simultaneously due to an oxygen partial membrane pressure differential across the singular ceramic membrane. This membrane is advantageous for systems which lack an external power source.2 However, for high-purity oxygen and control of oxygen volume produced via external voltage control, the use of a passive OTM is most efficient.2,6 This variation of OTM is pictured in Figure 1. With the constant application of bias, constant feed of gas at the cathode, and a means of housing the pure oxygen at the anode, the production oxygen can be concentrated at high pressure. This pressurized oxygen is needed for proper containment and controlled flow in life-support systems and can feed oxygen to cabin air supplies within confined areas. This

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device provides a means of both separating and compressing oxygen from various gas streams, however in the particular setting with a pure oxygen inlet gas, only compression is required and will be subsequently discussed. The partial pressure of the production oxygen at the anode is related to the applied bias via the Nernst equation in Eq. (1).7

RT pO2(anode) VN = ln⁡( ) (1) 4F pO2(cathode)

By applying an electrical bias at or above the Nernst voltage (VN), low pressure air can be fed at the cathode and electrically driven toward the anode, where it can then be housed within a pressure vessel. This means of oxygen separation and compression is robust and efficient, delivering compressed, extremely pure oxygen in a single step. A device designed by Meixner et al., which provided the basis for this design, produced 99.99% purity oxygen by the principle outlined in Eq. (1).7 A source of oxygen can be fed into the cathode region of the OTM, where oxygen is reduced by the electric potential via the following reaction in Eq. (2):

− 2− O2 + 4e = 2O (2)

Oxygen ions can now travel through the electrolyte via oxygen vacancy diffusion, being driven by the electric potential. Ions reach the anode and lose the excess electrons to again form diatomic molecular oxygen via the reaction in Eq. (3).

2− − 2O = O2 + 4e (3)

These reactions are nearly identical, however they differ in the oxygen species that is produced: whether that be ionized oxygen or diatomic oxygen gas. The flux of oxygen through the electrolyte is determined using a general form of the Wagner equation shown in Eq. (4).8

′′ RT lnP j = − ∫ O2 σ dlnP (4) O2 16F2L lnP′ i O2 O2

2 In Eq. (4), 푗푂2 is the oxygen flux in mol/m s, R is the universal gas constant, T is temperature, F is the Faraday constant, ′′ ′ L is the electrolyte thickness, 푃푂2 and 푃푂2 are the partial pressures of oxygen at the outlet and the feed side, respectively, and 휎푖 is ionic conductivity. The electrolyte thickness directly correlates with oxygen flux and dictates the electrolyte material’s ability to conduct oxygen ions. The above equation correctly assumes that bulk diffusion and ionic conductivity dominate in the electrolyte. The flow rate of production oxygen is dependent on the electrical current generated from the applied voltage. This relationship is demonstrated in Eq. (5).7

I Q = N (5) O2 4F cells

In Eq. (5), 푄푂2 is the flow rate in mol/s, I is the electrical current,. F is the Faraday constant, and 푁푐푒푙푙푠 is the number of cells within the stack. Controlling feed gas flow rate determines the pressure drop which occurs during cell stack operation. This needs to be properly controlled for determining mechanical stability of the cell stack, as well as for proper feed gas heating before it enters the stack.6

III. Materials for Oxygen Separation/Compression

A. Electrolyte For efficient function of an electrically-driven oxygen transport membrane, the selected electrolyte must only allow transport of oxygen ions. Certain ionic-conducting ceramics have been utilized in solid oxide fuel cell systems as electrolytes. These include doped zirconias and cerias, which exhibit high oxygen ionic conductivities at different

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temperature ranges.3,9,10 Zirconia exhibits three temperature-dependent polymorphs. At room temperature up to 1100 °C, the monoclinic fluorite structure is stable. The cubic polymorph is preferred for its thermal and mechanical properties, however it does not form until ~2400 °C because Zr4+ is too small of a cation to stabilize this structure at lower temperatures. However, when doped with a larger cation such as yttrium, Y3+ substitutes for Zr4+ and stabilizes the cubic lattice at room temperature up to its usual transition temperature. In this substitution, which Figure 2. Schematic of YSZ Crystal Structure involves cations of different valencies, oxygen vacancies develop in order to conserve charge neutrality. This is demonstrated in the defect reaction in Kroger- Vink notation shown in Eq. (6).3

′ 푥 ∙∙ 푌2푂3 → 2푌푍푟 + 3푂푂 + 푉푂 (6)

The introduction of oxygen vacancies enables oxygen ion transport through the lattice and, with an applied bias, drives oxygen ions preferentially from one side of the electrolyte to the other. The introduction of oxygen vacancies into the yttria-stabilized zirconia lattice is shown schematically in Figure 2.

Ceria-based electrolytes are also fluorite-type ionic conductors. These electrolytes have higher theoretical ionic conductivities than zirconia-based electrolytes. This high ionic conductivity is achieved at sufficiently high operating temperatures (>600 °C).6 In reducing atmospheres, however, ceria-based electrolytes become predominantly electronic conductors and can lose mechanical integrity. This is caused by the dual valence state of cerium. Cerium cations can exist in either a 3+ or a 4+ state, with the 4+ state occurring at lower temperatures and in oxidizing atmospheres. As temperatures increase to over 600 °C in a predominantly reducing atmosphere, Ce4+ is reduced to Ce3+ and a subsequent decrease in oxygen vacancies, converting the ionic conductor into an electronic one.11 This reduction is shown in Kroger-Vink notation in Eq. (7).11

1 2퐶푒푥 + 푂푥 → 푂 + 푉∙∙ + 2퐶푒′ 퐶푒 푂 2 2 푂 퐶푒 (7)

The reduction from Ce4+ to Ce3+ is denoted at the end of the equation, where a formal negative charge dictates the change Figure 3. Total conductivity of CSO with respect to in valence at a Ce site.11 This subsequently releases a free partial pressure of oxygen at different operating electron, which promotes the electron conductivity increase. temperatures12 With this reduction also comes a volume expansion, which causes cracking and material breakdown.10,11 This can be combatted with selective doping. Similar to the stabilization of zirconia with yttria dopant, ceria can be doped to induce oxygen vacancies based on nonstoichiometric substitution. This develops oxygen vacancies without any valence state change and electronic conductivity increase. Furthermore, at relatively high oxygen partial pressure, the 4+ state is maintained even at elevated temperatures above 800 °C.6 Ceria-based electrolytes are often doped with samarium or gadolinium to achieve substantial oxygen vacancies within the lattice and enable oxygen ion conduction. Samaria-doped ceria (CSO) exhibits ionic conductivity one order of magnitude higher than YSZ at 800 °C. This electrolyte choice is superior due to the high PO2 atmosphere which will be compressed. The absence of reducing atmosphere ensures proper cerium valence is maintained and ionic conductivity is maximized. The high ionic conductivity is achieved at higher temperatures, as exhibited in Figure 3. At high oxygen partial pressures, the total conductivity of CSO is dominated by ionic conductivity.12 This ionic conductivity reaches ~0.08 S/cm at 800 °C and nominal oxygen partial pressure (in air).12

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B. Electrodes The electrodes of the oxygen separation/compression system must be porous mixed ionic/electronic conductors (MIEC) which facilitate easy reduction/oxidation of diffusing oxygen.13–16 For systems which utilize compression of a pure oxygen feed gas, the cathode and anode materials would be the same as their function only differs in whether oxygen is being reduced or oxidized. If the system is to use a different feed gas, such as air, the cathode must be stable in this atmosphere and preferentially reduce only oxygen within the mixed gas. For the former, which describes the system of discussion, potential electrode materials include lanthanum-based perovskites. Lanthanum strontium manganite (LSM) has been typically used as a cathode material in many solid oxide fuel/electrolysis cell systems. However, a sufficient conductivity is only achieved at high temperatures exceeding 800 °C. This conductivity is exclusively electronic, which limits the oxygen reduction reaction (ORR) to triple-phase boundary area (TPB) where LSM, electrolyte, and oxidizing gas all meet.17 Another lanthanum-based perovskite which shows more promise is lanthanum strontium cobalt ferrite (LSCF). LSCF exhibits both good ionic and electronic conductivity (1x10-2 and 102 S/cm, respectively) at the predicted operating temperature of 800 °C.17 This mixed conductivity expands active ORR area past the TPB and allows reduction at other active interfacial sites. The cobaltite-based perovskites also exhibit both higher oxygen self-diffusion coefficients and lower activation energies when compared to manganite-based perovskites.17 More importantly, it shows good compatability with ceria-based electrolytes, specifically CSO.6 The LSCF-CSO electrode/electrolyte provides another benefit as it avoids electrode delamination issues which occur when other lanthanum-based electrodes are paired with a YSZ electrolyte.18 For efficient function, the electrode and electrolyte must maintain a strong interface and good adherence to promote active TPB area. The mechanism of electrode delamination is exhibited in Figure 4, where the application of a bias during electrolysis/oxygen transport Figure 4. Schematic of chemical and morphological causes the formation of a secondary phase. This secondary changes at SOEC anode-electrolyte interface: a) phase leads to increased porosity and ultimately electrode as-sintered LSM/YSZ interface, b) applied 18 delamination at the electrode/electrolyte interface.18 With electrical bias, c) bias continued, d) bias removed the LSCF-CSO electrode/electrolyte combination, the secondary phase is avoided and strong contact at the interface is maintained. 3,19 In typical SOFC/SOEC cathode airstreams, sulfur impurities can exist as SO2. The presence of sulfur impurities 3 poisons the LSCF cathode by reacting with strontium and forming a secondary SrSO4 phase. Rather than causing delamination, this secondary phase forms on the cathode surface and blocks bulk oxygen diffusion. While this is problematic in SOFC/SOEC systems, LSCF electrode poisoning may be alleviated in the oxygen compressor setting. This again is due to the presence of pure or nearly pure oxygen on both sides of the cell, a result of the feed stream being directly transported from the outlet of existing SOFC/SOEC systems which will be utilized onboard the space station.

C. Interconnects Interconnects are structural supports for the cell stack and provide electrical contacts to cell electrodes. In fuel/electrolysis cells, these components are required for separation of the cathodic and anodic gases and therefore require chemical stability in both atmosphere types. Typical ceramic materials used in these systems are lanthanum- based perovskites, specifically lanthanum chromite. Lanthanum chromite is used for its good electrical conductivity and chemical stability in both reducing and oxidizing atmospheres, however it is difficult to process and can lose chromium via evaporation in oxygen-rich atmospheres.7,20,21 The evaporated chromium can be deposited as a solid at active boundary areas at electrode/electrolyte interfaces, greatly reducing cell efficiency by preventing oxygen reduction/oxidation.4,20 5 International Conference on Environmental Systems

In a pure oxygen separator/compressor, interconnect materials would only require stability in a high PO2 atmosphere, hence they would not be subject to a reducing atmosphere. In this atmosphere, they must also maintain a high degree of electrical conductivity and good mechanical properties for efficient function. This makes other lanthanum-based perovskites more favorable to use as interconnects. Lanthanum manganites fulfill the conductivity Table 1. Comparison of mechanical/electrical properties of lanthanum-based interconnect materials7 Nominal Composition Deformation Elastic modulus Fracture Strength Electronic parameter (cm) (GPa) (MPa) Conductivity (S/cm) La0.50Sr0.50Mn1..00Co0.04O3-δ 0.008 35 56.9 289 La0.40Ca0.60Mn1.02O3-δ 0 128 145.5 313

and stability requirements in high PO2 atmospheres. These are often doped with strontium to enhance conductivity, however the lanthanum strontium manganite (LSM) interconnect has been shown to plastically deform when subjected to moderate stresses and ultimately lose mechanical integrity.7 Since the cell would be subject to a large pressure differential when compressing oxygen, the mechanical integrity of structural interconnects is imperative. Another variety of lanthanum manganite, one doped with calcium, shows more promise. Lanthanum calcium manganite (LCM) shows superior mechanical properties compared to LSM-type interconnects while still maintaining high electrical conductivity, as shown in Table 1.7 The combination of high conductivity, mechanical strength, and non-reactive components in a highly oxidizing atmosphere makes LCM a good candidate material for cell stack interconnects.

D. Pressure Vessel/Housing In order to safely and sufficiently pressurize the production oxygen, a high-strength pressure vessel is required. The pressure vessel must withstand the elevated operating temperature of ~800 °C while also properly confining the highly-pressurized oxygen. It is expected to maintain a gas at a pressure of 300 bar, or 3E+7 Pa; this immense pressure is required for filling extravehicular activity (EVA) oxygen tanks. The pressure requirement makes metal alloys the most ideal for housing the separator/compressor due to their manufacturability, durability, and high strengths/toughness. However, the combination of elevated temperature and high PO2 may lead to rapid metal loss and a subsequent decrease in mechanical properties via metal oxidation. To combat this, the material of choice must be corrosion resistant in a heavily oxidizing atmosphere and must maintain mechanical integrity under immense pressure differentials.

IV. Fabrication Method Oxygen transport membranes, much like other ceramic-based electrochemical systems, can come in a variety of shapes to develop high electrochemically-active area and promote efficient function.6,22,23 Most often, these systems come in either planar or tubular forms, both design types having certain advantages and disadvantages. Planar configurations offer high efficiencies due to large active cell areas, with multiple cells arranged into relatively compact stacks.6 However, this configuration requires sufficient sealing between cells within the stack, which are oftentimes the points of failure within a system.24 Tubular designs can be easily manufactured via extrusion techniques. Converesly to planar designs, these configurations require little sealing. They are less efficient, however, with less active cell area and higher cell resistance. For this reason, they are often used at more elevated temperatures above 800 °C. For the oxygen separator/compressor of question, the planar configuration is most appropriate.6,7 The means by which this design is efficiently fabricated, a process known as tape casting, will now be discussed.

A. Tape Casting Tape casting is a ceramic processing technique used mass produce and easily shape ceramic components which can be fired to a target sintered density.6,25 This technique has been used for ceramic components in other electrochemical systems, such as solid oxide fuel cells. During tape casting, a ceramic slurry is created and a layer of tuneable thickness is evenly spread on a flat surface using a doctor blade.25 This leaves a uniform dispersion of ceramic slurry which can be as thin as one micrometer. The resistance of ceramic electrolytes is inversely proportional to the thickness of the electrolyte. Thickness control makes tape casting an ideal manufacturing process for ceramic components in electrochemical systems. Using sufficient additives within the slurry, such as solvents, surfactants, binders, and plasticizers, the ceramic green body tape that is casted and dried is flexible and easily cut to desirable shapes.25 For ceramics used in the oxygen compressor, specific microchannels for inlet oxygen and stack cell geometries can be easily machined from this flexible green body tape. This is to be combined with a laminate-object- 6 International Conference on Environmental Systems

manufacturing approach to combine ceramic components and create full electrochemical stacks.6 This involves continuously pressing tape layers in the desired interconnect-electrode-electrolyte-electrode sequence to achieve a cell which can be further processed via cosintering.

B. Cosintering Cosintering is a method of sintering two ceramic components in a single firing process, creating a ceramic cell with strong, continuous interfaces between different components. After the tapes of different components are properly laminated, the binder additive is slowly exolved at elevated temperature.6 After this, the components are cosintered to achieve a dense electrolyte, dense interconnects, and porous electrodes within the cell configuration. However, this process is oftentimes difficult as different components have different sintering characteristics. Furthermore, thermal mismatching can cause delamination of individual cell components due to mismatched coefficients of thermal expansion (CTE), particularly between the electrodes and Table 2. Comparison of select thermal/mechanical the other cell components. Table 2 reveals the differences in properties of different cell components6 CTE values for various cell components. The electrolyte and LCM interconnect can be well-matched, however the mismatch with the electrodes is challenging for cosintering. Hutchings et al. carried out a finite element analysis (FEA) of the cell configuration after cosintering and found that the electrodes undergo a high biaxial stress state when cooling from sintering temperature to room temperature.6 The FEA study revealed this stress would most likely lead to either catastrophic failure via electrode fracture or microcracking of the electrode. It was determined that microcracking may be allowable for the oxygen compressor to function, however this design allowance may need more understanding at more elevated pressure differentials.6 Developing composite electrode/electrolyte designs may be one method to reduce any expansion coefficient disparities.17

C. Stack and System Assembly To sufficiently generate and pressurize the pure product oxygen for storage, multiple electrode-electrolyte- electrode cells are needed within the pressure vessel. The planar cells are connected in series by interconnects, which are subsequently joined using sealants. Glass-based rigidly bonded sealants are often used in electrochemical systems to maintain hermetic sealing of areas between cell components during operation.6,24 For planar cell configurations, sealing is required at the edges of each cell components and between each cells used within the overall stack.24 These seal types can be either compressive, compliant, or rigidly bonded seals. For a high-pressure system, the glass-based rigidly bonded seals are most favorable. This is due to the nature of the bond and durability of the sealant. Compressive sealing utilize pliable seals which require uniform loading and are often not efficient in preventing leaking with nominal gas flow rates.24 This type of seal would most likely fail under high gas pressures. Compliant sealing requires the use of nobel metals to form malleable sealing at high temperatures.24 These nobel metals are expensive and can easily oxidize, especially when met with the high oxygen partial pressures that the cell stack would face. Conversely, rigidly bonded seals provide strong chemically-bonded sealing which is required for a high-pressure system.24 They are cost-effective and the glass composition can be tailored to match thermal properties of the other cell components. When a cell stack is sufficiently sealed, it can be integrated with other oxygen compressor components to Figure 5. Schematic of oxygen compressor efficiently function and yield a maximum pressurized integrated with other necessary system oxygen output per cell design. Figure 5 shows a schematic components of the full integration of components within the pressure 7 International Conference on Environmental Systems

vessel. Additional system components include a direct current heater, which must heat incoming oxygen and the cell to an efficient operating temperature of 800 °C, and a ceramic heat exchanger. The heat exchanger is required to cool the non-permeate outlet gas after it leaves the cell stack and initially heat the feed stream oxygen.7

V. Conclusion – System Outlook A solid-state oxygen electrochemical compressor design has been discussed. The overall design and the materials which enable pure oxygen compression in a single-step manner were chosen based on previous materials and systems research.6,7,26,27 This has led to early design development based on the most optimal material choices for oxygen compression in a high PO2 atmosphere. Earlier discussion of system fabrication mentioned a system developed by Hutchings et al. This was a planar, co-sintered ceramic cell designed for oxygen separation and compression from which the design discussed in this paper was derived. The two designs center on a dense, thin electrolyte co-sintered with a porous electrode via a tape casting method.6 The device created by Hutchings et al. was proven to both separate oxygen from mixed gas and compress it to a pressure of 2 MPa, however the claim was that the cell design is rated at 14.3 MPa of inner pressure.6 Another system design, developed by Spirin et al., proved to separate oxygen from air, however compression was not tested. The oxygen “generator” utilized co-sintered YSZ/LSM electrolyte/electrode and functioned at a higher operating temperature of ~800 °C.23 A drawback to these materials is the higher temperature requirement for sufficient ionic transport. Devices using ceria-based electrolytes, such as samaria-doped ceria (CSO/SDC), have better ionic conductivities at 800 °C and below (approx. 0.1 S/cm).11,12,28 Hutchings et al. operated at a lower temperature of ~750 °C. However, the tubular cell design used eliminated a great deal of sealing and may be a promising design concept. Joshi et al. thoroughly describes the state-of-the-art materials used for oxygen separation.29 The review encompasses the material choices for oxygen separation and compression based on oxygen conductivity and the ability to create a functioning cell stack. As resistance is inversely proportional to electrolyte thickness, a thin electrolyte is needed for optimal oxygen conductivity. Electrolyte materials are either zirconia or ceria-based, with transition metal or lanthanide dopants added to create oxygen vacancies for oxygen transportation. In this design, a samaria-doped ceria (CSO) electrolyte is being considered for high ionic conductivity at an operating temperature of ~750 °C. This material choice is also optimal as CSO electrolytes exhibit higher ionic conductivities than zirconia-based counterparts. They can be reduced in certain atmospheres and high temperatures, however the relatively low operating temperature and absence of reducing atmosphere should allow for a stable electrolyte with high performance. Electrode materials are often lanthanum-based perovskites with good mixed electronic-ionic conductivity. Certain doped lanthanum-based perovskites, specifically lanthanum strontium cobaltites, maintain high mixed conductivities and stability at the target operating temperature and are a desirable electrode material choice.17,30 Specifically for planar cells, hermetic sealing by means of glass or ceramic sealants is required. Mahoptra et al. describes the various requirements of glass sealants used in such electrochemical systems. These requirements include: matching CTE with stack components and stability under thermal cycling and for long operating times, limited reactivity with cell components, electrically insulating properties, and mechanical integrity under certain pressures.24 Further understanding of various additives in glass sealants is required to optimize seal composition for the high-pressure system. The gaseous species used in fuel cells of similar design hinder operation by poisoning electrodes. These can be either impurities in the air/fuel stream or vaporous species evolved from interconnects used at high temperatures in the presence of water vapor. The planned system will see pure or relatively pure oxygen from other electrochemical systems, which may increase cell lifetime and durability. This leaves the largest hurdles in high-pressure behavior, seeing as oxygen separation is well documented and compression tested up to 2 MPa, however material and system behavior at a pressure of 300 bar (30 MPa) is yet to be understood.6 Material properties and functionalities are also unknown at this high of oxygen partial pressure and will require investigation to ensure a safe and efficient oxygen compressor design is developed.

Acknowledgments The authors acknowledge the National Aeronautics and Space Administration (NASA) and American Oxygen LLC. for collaboration and technical discussion.

References 1Gilkey, K. M., Olson, S. L., “Evaluation of the Oxygen Concentrator Prototypes : Pressure Swing Adsorption Prototype and Electrochemical Prototype,” NASA Tech. Memo. 2015. 8 International Conference on Environmental Systems

2Gupta, S., Mahapatra, M., Singh, P., “Lanthanum Chromite Based Perovskites for Oxygen Transport Membrane,” Materials Science and Engineering: R: Reports, Vol. 90, April 2015, pp. 1-36. 3Reisert, M., Aphale, A., Singh, P., “Solid Oxide Electrochemical Systems: Material Degradation Processes and Novel Mitigation Approaches,” Materials (Basel), Vol. 11, No. 11, 2018, pp. 2169. 4Chen, K., Jiang, S. P., “Review—Materials Degradation of Solid Oxide Electrolysis Cells,” J. Electrochem. Soc., Vol. 163, No. 11, 2016, pp. F3070–F3083. 5Aphale, A., Liang, C., Hu, B., Singh, P., “Cathode Degradation from Airborne Contaminants in Solid Oxide Fuel Cells - A Review,” In Solid Oxide Fuel Cell Lifetime and Reliability, Critical Challenges in Fuel Cells, 2017, pp 102–114. 6Hutchings, K. N., Bai, J., Cutler, R. A., Wilson, M. A., Taylor, D. M., “Electrochemical Oxygen Separation and Compression Using Planar, Cosintered Ceramics,” Solid State Ionics, Vol. 179, No. 11-12, 2008, pp. 442-450. 7Meixner, D. L., Brengel, D. D., Henderson, B. T., Abrardo, J. M., Wilson, M. A., Taylor, D. M., Cutler, R. A., “Electrochemical Oxygen Separation Using Solid Electrolyte Ion Transport Membranes,” J. Electrochem. Soc., Vol. 149, No. 9, 2002, pp. D132. 8Joo, J. H., Yun, K. S., Kim, J. H., Lee, Y., Yoo, C. Y., Yu, J. H., “Substantial Oxygen Flux in Dual-Phase Membrane of Ceria and Pure Electronic Conductor by Tailoring the Surface,” ACS Appl. Mater. Interfaces, Vol. 7, No. 27, 2015, pp. 14699–14707. 9Mahato, N., Banerjee, A., Gupta, A., Omar, S., Balani, K., “Progress in Material Selection for Solid Oxide Fuel Cell Technology: A Review,” Prog. Mater. Sci., Vol. 72, 2015, pp. 141-337. 10Badwal, S. P. S., Ciacchi, F. T., “Oxygen-Ion Conducting Electrolyte Materials for Solid Oxide Fuel Cells,” Ionics (Kiel)., Vol. 6, No. 1-2, 2000, pp. 1-21. 11Arunkumar, P., Meena, M., Babu, K. S., “A Review on Cerium Oxide-Based Electrolytes for ITSOFC,” Nanomater. Energy, Vol. 1, No. 5, 2012, pp. 288-305. 12Gödickemeier, M., “Engineering of Solid Oxide Fuel Cells with Ceria-Based Electrolytes,” J. Electrochem. Soc., Vol. 145, No. 2, 1998, pp. 414. 13Hu, B., Krishnan, S., Liang, C., Heo, S. J., Aphale, A. N., Ramprasad, R., Singh, P., “Experimental and Thermodynamic Evaluation of La1−xSrxMnO3±Δ and La1−xSrxCo1−yFeyO3−Δ cathodes in Cr-Containing Humidified Air,” Int. J. Hydrogen Energy, Vol. 42, No. 15, 2017, pp. 10208–10216. 14Stambouli, A. B., Traversa, E., “Solid Oxide Fuel Cells (SOFCs): A Review of an Environmentally Clean and Efficient Source of Energy,” Renew. Sustain. Energy Rev., Vol. 6, No. 5, 2002, pp. 433-455. 15Liang, C., Hu, B., Aphale, A., Venkataraman, M., Mahapatra, M. K., Singh, P., “Mitigation of Chromium Assisted Degradation of LSM Cathode in SOFC,” ECS Trans., Vol. 75, No. 28, 2017, pp. 57-64. 16Ormerod, R. M., “Solid Oxide Fuel Cells,” Chem. Soc. Rev., Vol. 32, No. 1, 2003, pp. 17-28. 17Jiang, S. P., “Development of Lanthanum Strontium Cobalt Ferrite Perovskite Electrodes of Solid Oxide Fuel Cells - A Review,” Int. J. Hydrogen Energy, 2019. 18Keane, M., Mahapatra, M. K., Verma, A., Singh, P., “LSM-YSZ Interactions and Anode Delamination in Solid Oxide Electrolysis Cells,” Int. J. Hydrogen Energy, Vol. 37, No. 22, 2012, pp. 16776–16785. 19Xiong, Y., Yamaji, K., Horita, T., Yokokawa, H., Akikusa, J., Eto, H., Inagaki, T., “Sulfur Poisoning of SOFC Cathodes,” J. Electrochem. Soc., Vol. 156, No. 5, 2009, pp. B588. 20Jiang, S. P., Zhang, J. P., Apateanu, L., Foger, K., “Deposition of Chromium Species at Sr-Doped LaMnO3 Electrodes in Solid Oxide Fuel Cells. I. Mechanism and Kinetics,” J. Electrochem. Soc., Vol. 147, No. 11, 2002, pp. 4013. 21Badwal, S. P. S., Deller, R., Foger, K., Ramprakash, Y., Zhang, J. P., “Interaction between Chromia Forming Alloy Interconnects and Air Electrode of Solid Oxide Fuel Cells,” Solid State Ionics, Vol. 99, No. 3, 1997, pp. 297-310. 22Ovtar, S., Gurauskis, J., Bjørnetun Haugen, A., Chatzichristodoulou, C., Kaiser, A., Hendriksen, P. V., “Oxygen Transport Properties of Tubular Ce0.9Gd0.1O1.95-La0.6Sr0.4FeO3−d Composite Asymmetric Oxygen Permeation Membranes Supported on Magnesium Oxide,” J. Memb. Sci., Vol. 523, Sept. 2016, pp. 576-587. 23Spirin, A. V., Nikonov, A. V., Lipilin, A. S., Paranin, S. N., Ivanov, V. V., Khrustov, V. R., Valentsev, A. V., Krutikov, V. I., “Electrochemical Cell with Solid Oxide Electrolyte and Oxygen Pump Thereof,” Russ. J. Electrochem., Vol. 47, No. 5, 2011, pp. 569-578. 24Mahapatra, M. K., Lu, K., “Glass-Based Seals for Solid Oxide Fuel and Electrolyzer Cells - A Review,” Mater. Sci. Eng. R Reports, Vol. 67, No. 5-6, 2010, pp. 65-85. 25Jabbari, M., Bulatova, R., Tok, A. I. Y., Bahl, C. R. H., Mitsoulis, E., Hattel, J. H., “Ceramic Tape Casting: A Review of Current Methods and Trends with Emphasis on Rheological Behaviour and Flow Analysis,” Mater. Sci. Eng. B Solid-State Mater. Adv. Technol., Vol. 212, 2016, pp. 39-61. 26Taylor, D. M., Joshi, A. V., Ceramatec, Inc., Salt Lake City, UT, “Ceramic Solid Electrolyte-Based Electrochemical Oxygen Concentrator Cell,” Patent No. 5,378,345, filed Jan. 3, 1995. 27Setlock, J. A., Green, R. D., Farmer, S., The United States of America as represented by the Administrator of the National Aeronautics and Space Administration, Washington, DC, “Generation of High Pressure Oxygen via Electrochemical Pumping in a Multi-Stage Electrolysis Stack,” Patent No. 9,776,130 B1, filed Oct. 3, 2017. 28Jung, G.-B., Huang, T.-J., Chang, C.-L., “Effect of Temperature and Dopant Concentration on the Conductivity of Samaria- Doped Ceria Electrolyte,” J. Solid State Electrochem. 2002, 6 (4), 225–230. Vol. 47, No. 5, 2011, pp. 569-578. 29Joshi, A. V., Steppan, J. J., Taylor, D. M., Elangovan, S., “Solid Electrolyte Materials, Devices, and Applications,” J. Electroceramics, Vol. 13, No. 1-3, 2004, pp. 619-625. 9 International Conference on Environmental Systems

30 Stevenson, J. W., “Electrochemical Properties of Mixed Conducting Perovskites La1−xMxCo1−yFeyO3−δ (M = Sr, Ba, Ca),” J. Electrochem. Soc., Vol. 143, No. 9, 1996, pp. 2722.

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