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Life emerged in a geochemical context, possibly in the midst of mineral substrates. However, it is not known to what extent minerals and dissolved inorganic ions could have facilitated the evolutio...

Primary batteries have been used in past deep space exploration missions, providing power for periods of several hours to several days of mission operations [1,2]. Future missions planned by the National Aeronautics and Space Administration (NASA) will require more advanced primary batteries to provide power to missions for up to 60 days [3]. The Li/CFx primary battery chemistry has a high specific energy, low self-discharge rate, and a relatively wide operating temperature range which makes it a suitable power source for extended mission durations in deep space, particularly for applications involving low to moderate discharge rates [4]. Li/CFx batteries have not been used in any NASA missions to date, and are now being developed for the Europa Lander mission concept [5]. An extensive evaluation of EaglePicher’s D-sized Li/CFx cells is underway, to benchmark the calendar life performance, radiation tolerance, and performance under different temperatures and currents. One of the unique characteristics of the Li/CFx chemistry is the extremely flat voltage plateau during cell discharge (Figure 1) [6, 7]. As the cell approaches end of life, its voltage quickly drops prior to reaching the 1.5V recommended discharge limit. While the cell’s stable voltage is a key component to the high specific energy of this chemistry, it also makes it challenging to accurately predict the battery’s remaining capacity. Premature battery depletion is a risk to mission success, and improved methods for determining depth-of-discharge (DOD) in this unique cell chemistry are of great interest. A pulse-discharge test method is developed and implemented to investigate the change in cell direct current internal resistance (DCIR) during discharge. Varied responses based on the discharge rate are observed and will be discussed. The resulting data are analyzed for possible correlation with DOD, along with changes in the cell voltage over various segments of the discharge curve. This talk discusses the test method and results achieved using the pulse-discharge test method. References: M. Hofland, E.J. Stofel, R.K. Taenaka, Aerospace and Electronic Systems Magazine IEEE, 11, 14 (1996). P. Dagarin, R.K. Taenaka, E.J. Stofel, Proc. of 31st Energy Conversion Engineering Conference 1996, 1, 427 (1996). P. Hand et al.2022 Planet. Sci. J. 3 22. Frederick C. Krause et al.2018 Electrochem. Soc. 165 A2312. Crum, R. et al 2021 Advanced Technology Developments for Europa Lander and other In-Situ Ocean World Missions. Bulletin of the AAS,53(4). Watanabe, N., & Fukuda, M. (1970). S. Patent No. 3,536,532. Washington, DC: U.S. Patent and Trademark Office. Watanabe, N., Endo, M., Ueno, K., Solid State Ionics, Vol. 1, Issue 5-6 (1980). The work described here was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration (NASA) and supported by the Europa Lander Pre-Project. Figure 1

Understanding the factors that influence ion-solvent properties for the fluoride ion in organic solvents is key to the development of useful liquid electrolytes for fluoride-ion batteries. Using both experimental and computational methods, we examined a range of chemical and electrochemical properties for a set of organic solvents in combination with dry N,N,N-trimethylneopentylammonium fluoride (Np₁F) salt. Results showed that solvent electronic structure strongly influences Np₁F dissolution, and the pK_a of solvent protons provides a good guide to potential F⁻ reactivity. We found a number of organic solvents capable of dissolving Np₁F while providing chemically-stable F⁻ in solution and characterized three of them in detail: propionitrile (PN), 2,6-difluoropyridine (2,6-DFP), and bis(2,2,2-trifluoroethyl) ether (BTFE). Arrhenius analysis for Np₁F/PN, Np₁F/DFP, and Np₁F/BTFE electrolytes suggests that DFP facilitates the highest F⁻ ion mobility of the three neat solvents. Electrolyte mixtures of BTFE and amide co-solvents exhibit higher ionic conductivity than the neat solvents. This improved ionic conductivity is attributed to the ability of BTFE:co-solvent mixtures to partition between Np₁⁺ and F⁻ ion-aggregates, promoting better ion dissociation.

Future landers to the moons of the outer planets could be powered by higher specific energy primary batteries. Batteries based on lithium carbon monofluoride (Li/CFX) provide ~50% higher specific energy than heritage cells (Li/SO2 or Li/SOCl2) in relevant conditions. Radiation tolerance is a major concern due to the high radiation environment surrounding Jupiter and its moons. Gamma radiation exposure may also become the sterilization (a critical step for any lander to planetary bodies where life may survive) technique of choice because the alternative thermal technique (prolonged exposure to high temperature) results in significant capacity loss for Li/CFX cells. Several D-sized Li/CFX and Li/CFX-MnO2 cells have been exposed to gamma radiation from a60Co source. The energy and capacity of the cells decreased by less than 2% following radiation and trends regarding the open circuit voltage and impedance spectroscopy are reported. To better understand the effects of radiation, samples of individual materials have been exposed to radiation as well as three-electrode cylindrical Li/CFx cells. These component-level tests, along with the experimental cell tests, reveal that the electrolyte/electrode interaction in a full cell is the likely cause of increased impedance and decreased energy/capacity, following exposure to ionizing radiation.

Gradients generated in hydrothermal systems provide a significant source of free energy for chemosynthetic life and may play a role in present-day habitability on ocean worlds. Electron/proton/ion gradients, particularly in the context of hydrothermal chimney structures, may also be relevant to the origins of life on Earth. Hydrothermal vents are similar in some ways to typical fuel cell devices: redox/pH gradients between seawater and hydrothermal fluid are analogous to the fuel cell oxidant and fuel reservoirs; the porous chimney wall is analogous to a separator or ion-exchange membrane and is also a conductive path for electrons; and the hydrothermal minerals are analogous to electrode catalysts. The modular and scalable characteristics of fuel cell systems make for a convenient planetary geology test bed in which geologically relevant components may be assembled and investigated in a controlled simulation environment. We have performed fuel cell experiments and electrochemical studies to better understand the catalytic potential of seafloor minerals and vent chimneys, using samples from a black smoker vent chimney as an initial demonstration. In a fuel cell with Na+-conducting Nafion® membranes and liquid fuel/oxidant reservoirs (simulating the vent environment), the black smoker mineral catalyst in the membrane electrode assembly was effective in reducing O2 and oxidizing sulfide. In a H2/O2 polymer electrolyte membrane (PEM) fuel cell with H+-conducting Nafion membranes, the black smoker catalyst was effective in reducing O2 but not in oxidizing H2. These fuel cell experiments accurately simulated the redox reactions that could occur in a geological setting with this particular catalyst, and also tested whether the minerals are sufficiently active to replace a commercial fuel cell catalyst. Similar experiments with other geocatalysts could be utilized to test which redox reactions could be driven in other hydrothermal systems, including hypothesized vent systems on other worlds.

Working toward fluoride batteries Owing to the low atomic weight of fluorine, rechargeable fluoride-based batteries could offer very high energy density. However, current batteries need to operate at high temperatures that are required for the molten salt electrolytes. Davis et al. push toward batteries that can operate at room temperature, through two advances. One is the development of a room-temperature liquid electrolyte based on a stable tetraalkylammonium salt–fluorinated ether combination. The second is a copper–lanthanum trifluoride core-shell cathode material that demonstrates reversible partial fluorination and defluorination reactions. Science , this issue p. 1144

A novel variable altitude Venus interior probe is currently under development that uses in-situ power and propulsion to lower a balloon down into Venus’s atmosphere, approximately 20 km from the surface, and then bring it back up above the clouds and hover at approximately 60 km from the surface. One of the biggest challenges is finding an energy storage solution that can withstand Venus’s harsh atmosphere. At 20 km the probe will experience temperatures close to 325°C, pressures around 20 atm, and an atmosphere composed of CO2, with trace amounts of sulfur containing compounds. Conventional energy storage technologies, like batteries, are not an option under these conditions. The only technology that can survive under these conditions are solid oxide fuel cells (SOFCs), which operate in a temperature range between 600 – 1000 °C. Fuel cells are devices that convert chemical energy, in this case from hydrogen and oxygen, into electricity with the only byproducts being heat and water. Solid oxide fuel cells can be run reversibly as an electrolyzer, taking the byproduct water and splitting it into hydrogen and oxygen that can then be used to run the fuel cell. A regenerative fuel cell system may therefore be built around a single SOFC stack. The goal is to run the SOFC to power the probe at 20 km and then reverse the SOFC to an electrolyzer when the probe is back above the clouds and use solar power to produce hydrogen and oxygen. The data presented in this work shows the successful operation of a regenerative SOFC. Figure 1A shows a SOFC that was run in H2/O2 as an electrolyzer. Figure 1B shows a SOFC run as a fuel cell in different conditions including H2/O2 (red line) and H2/Air at different flow rates (blue lines). The goal of this work is to pinpoint the exact conditions a SOFC would need to operate at in order to successfully store energy for an interior probe in the Venus atmosphere. This work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration (NASA) and NASA’s Innovative Concepts (NIAC) Program: “Venus Interior Probe Using In-situ Power and Propulsion (VIP-INSPR). Figure 1

In-situ exploration of Venus is challenging due to its severe environment, which is benign (25oC) at an altitude of 55 km, but rapidly becoming more hostile at lower altitudes. The temperature increases at ~7oC/km to ~465°C, with the pressure reaching 90 bars at the surface.1 These challenging conditions have limited in-situ exploration missions to high altitude balloons at 55 km (above the clouds) that lasted for 48 h, or even shorter duration surface missions that survived for two hours.2,3 The high-altitude (55-65km) balloon missions are stymied by the opaqueness of the Venusian clouds, which underlines the need for more long-duration in-situ missions for a better understanding of the Venus atmosphere across the cloud layers and below, as recommended by the Venus science community, Venus Exploration Analysis Group (VEXAG).4 Long-duration variable-altitude balloons (VABs) extending below the clouds have gained particular interest. Durable VABs would allow i) long-term measurements across Venus clouds, ii) determination of chemical species and isotopes underneath the clouds, iii) transport to different longitudes on the planet and measure atmospheric flow patterns, especially with the altitude control, iv) probing the interior structure through close-range imaging, and v) investigation of the seismic activity from acoustic measurements at various altitudes. For these missions, conventional power technologies are inadequate. For example, the performance of photovoltaics (PV) is hampered by the decreasing solar flux deeper in the clouds, the selective loss of short wavelength radiation, and the performance loss from the high temperatures.5 An energy storage system tolerant to high temperatures is needed to compensate for the reduced power generation of PVs at low altitudes, and to support nighttime operations for the VABs. In this paper, we will describe a novel ‘Venus Interior Probe using in-situ Power and Propulsion (VIP-INSPR) architecture we have been developing under NASA-NIAC (Novel Innovative and Advanced Concepts) program for sustained Venus atmospheric exploration. The probe concept utilizes: i) PV as a power source to the probe at high altitudes, and to electrolyze water carried from ground using regenerative solid oxide fuel/ electrolysis cell (SOEC), ii) Solid oxide fuel cell (SOFC)6 to provide power at low altitudes, iii) hydrogen storage bed for on-demand storage or release of hydrogen,7 iv) and a balloon filled with hydrogen and with hydrogen buoyancy-based altitude control system. Both H2 and O2 would be regenerated through electrolysis of the water produced in the fuel cell (a closed–system) at high altitudes. Acknowledgments: This work presented here was carried out at the Jet Propulsion Laboratory, California Institute of Technology under a contract with National Aeronautics and Space Administration and supported NASA-NIAC. References: 1) T. Basilevsky, J. W. Head, "The surface of Venus". Rep. Prog. Phys. 66, 1699 (2003); 2) R. Z. Sagdeev, et al., The VEGA Venus balloon experiment, Science, 231, 1407, 1986; 3) M. Wade, "Venera 1VA". Encyclopedia Astronautica. Retrieved 28 July 2010; 4) “Aerial Platforms For the Scientific Exploration of Venus”, The Venus Aerial Platforms Study Team Summary Report, August 2018; 5) G. A. Landis and T. Vo, "Photovoltaic Performance in the Venus Environment," 34th IEEE Photovoltaic Specialists Conference, Philadelphia PA, June 7-12, 2009; 6) A. B. Stambouli and E. Traversa, Renewable and Sustainable Energy Reviews, 6 (2002) 433-455, 7) G. Sandrock, S. Suda and L. Schlapbach, “Applications,” in Hydrogen in Intermetallic Compounds II, Topics in Appl. Phys. V. 67, Springer-Verlag 1992, ISBN 3-540-54668-5.

To meet the future demands of space exploration will require primary batteries offering higher specific energy, relative to heritage space rated cells based on Li/SO2, Li/SOCl2 and Li/MnO2 chemistries. Cells based on the Li/CFx chemistry are currently under development to support operations in challenging extreme environments. One such proposed application of advanced primary cells is for a proposed Europa Lander, which would analyze the icy surface of this Jovian moon, evaluate its potential for habitability, and investigate for signs of bio-signatures. To address this unique mission requires a detailed understanding of the effects of wide temperatures and radiation on cell electrical and thermal performance, self-discharge characteristics and reliability. In particular, results from screening of cell components will be reviewed, including electrodes and electrolytes experiencing high doses of ionizing radiation. The work described here was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration (NASA) and supported by the Europa Lander Pre-Project.

Interest in commercial off-the-shelf (COTS) cylindrical lithium-ion cells for NASA missions has increased in recent years due to their increasing performance and demonstrated reliability. There is particular interest in cells in the 18650 format, due to their wide availability from well-established vendors. Multiple commercial products are available, offering greater than 250 Wh∙kg-1 and/or 3 kW∙kg-1 at the cell level. High specific energy has made small cells competitive with heritage large-format prismatic lithium-ion cells for spacecraft power systems; high power capability could offer an alternative to thermal batteries in some applications. The descent stage of the Europa Lander mission concept was baselined with a combination of a high specific energy lithium primary battery and thermal batteries to provide the required energy content and high-power pulse capability during deorbiting and landing. High specific energy and power lithium-ion cells could fulfill the functions of both of these, while also allowing for cell pre-screening and testing, unlike primary or thermal batteries. In support of this concept and future NASA missions, we have performed extensive evaluation of the components and performance characteristics of a number of COTS 18650 Li-ion cells, including destructive physical analysis, cycle life, rate capability, and ionizing radiation tolerance. ACKNOWLEDGEMENT The work described herein was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. Support from the Europa Lander Mission concept and the NASA Engineering Safety Commission is gratefully acknowledged. Pre-Decisional Information – For Planning and Discussion Purposes Only.

There is considerable international interest in the origin of life, both on Earth and icy worlds such as Europa, Enceladus, Ceres, and Titan. These ocean worlds may contain hydrothermal vents at the interfaces between their salty oceans and rocky cores and have been prioritized by NASA in the search for evidence of life in our solar system.1 Hydrothermal vents on Earth have been shown to produce voltage and current in the field2,3 and may be related to the origin of life.4 Electrochemistry certainly plays a role here, with hot reductants flowing from the crust and interacting with oceans through porous, partially conductive minerals. Understanding the natural systems that exist on Earth and may have contributed to the emergence of life, and determining similarities to hydrothermal vents on ocean worlds, is a critical step in exploring these unknown environments. We have attempted to model these systems using both cyclic voltammetry and fuel cell experiments,5 with results presented here. While there is a plethora of potential redox couples which could be exploited in nature at hydrothermal vents, one of the most active reductants is H2S, which is produced at many black smoker chimneys and reacts with various metals to generate compounds such as FeS and FeS2, both of which are known electroactive materials. Studies on these materials indicate that they may produce energy which could be directly used by simple organisms on the outside of the chimneys and in turn feed larger organisms. This chain of energy utilization exists without solar irradiation, thus giving credence to the hypothesis that life may exist at the bottom of the ocean on the icy moons around Jupiter and Saturn.6 ACKNOWLEDGEMENT The work described here was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration (NASA) supported by JPL’s Research and Technology Development program. References: A. R. Hendrix et al., Astrobiology, 19 (2019). R. Nakamura et al., Angew. Chemie Int. Ed., 49, 7692–7694 (2010). M. Yamamoto et al., Angew. Chemie Int. Ed., 56, 5944 (2017). G. Macleod, C. McKeown, A. J. Hall, and M. J. Russell, Orig. life Evol. Biosph., 24, 19–41 (1994). L. M. Barge, F. C. Krause, J.-P. Jones, K. Billings, and P. Sobron, Astrobiology, 18, 1147–1158 (2018). K. P. Hand, R. W. Carlson, and C. F. Chyba, Astrobiology, 7, 1006–1022 (2007). Figure 1

Lithium-ion battery technology is now pervasive in all areas and at all scales of energy storage, including consumer electronics, automotive applications and grid level storage. Traditionally limited to operation under terrestrial conditions, these limits of lithium-ion technology are being expanded to encompass operation under more extreme conditions, including aerospace applications. These scenarios require the consideration of new cell components, including electrolytes, cell designs and separators. This talk will focus on the development of new separator materials, targeted for operation in high radiation environments, often encountered in space missions. Conventional polyethyelene/polypropylene based separators have been evaluated for their electrical, chemical and mechanical performance. High doses of radiation have been shown to compromise the mechanical performance of these conventional separators. New aramid based separators are under development to address these limitations, and results from this development and testing will be reported. It is anticipated these separator materials may find application in lithium-ion battery cells used in other types of extreme environments. The work described here was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration (NASA) and supported by the Europa Lander Pre-Project.

Hydrogen-oxygen fuel cell technology is a critical component for crewed space exploration mission beyond low earth orbit. While lowering platinum loading is of paramount importance for automotive fuel cells, it is not a significant constraint for MEAs used for space applications. As such, support-less platinum black catalysts is a viable choice. Historically, surfactant-stabilized polytetrafluoroethylene (PTFE) emulsion is used as the binder for platinum black electrode. High temperature treatment steps needed for the PTFE emulsion prevents the direct deposition of the electrocatalysts onto the Nafion® membrane. In this study, two alternative materials have been used to replace PTFE emulsion. These include surfactant-free PTFE fine particles and functionalized carbon nanotube with Nafion®. An ultrasonic spray deposition technique was applied to directly deposit support-less Pt black catalyst ink onto the Nafion® membrane. The fabrication process was optimized to produce high performance membrane electrode assemblies that approaching NASA performance target.

The performances of ruthenium oxide-based oxygen evolution catalysts fabricated by various techniques have been studied. A thermal processing technique has been identified that can produce viable iridium and lead-doped ruthenium oxide catalyst. The best oxygen evolution performance was obtained with an iridium-doped ruthenium oxide catalyst with nine molar percent iridium. The cell voltage of the Ir(9)-d-RuO2 catalyst at an applied current density of 200 mA/cm2 was 1.45 volts at 70 oC. The oxygen evolution Tafel slope for doped ruthenium oxide catalysts are in the range of 40 to 55 mV/decade, a minimum of 10 mV/decade lower than what was found for ruthenium oxide.

Primary batteries have played a critical role in past deep space planetary exploration missions, providing power to spacecraft such as the Galileo and Huygens Probes [1,2]. Most of these missions were of relatively short duration (Huygens lasted 2.5 hours), and relied on primary battery power alone to execute mission profiles. Future missions planned by the National Aeronautics and Space Administration (NASA) to the outer planets (Jupiter, Saturn) and their moons (Europa, Enceladus, Titan) will require more advanced primary batteries to provide bus power for days or even weeks. These batteries may be stored for several years during an initial cruise period, followed by operation in temperature extremes coupled with exposure to high radiation doses. The requirement for primary batteries with very high specific energy, low self-discharge/long calendar life performance, wide temperature operation and radiation tolerance has prompted the evaluation by NASA of commercially available options, as well as the development of new components and cell designs. This includes higher specific energy battery chemistries such as Li/CFx and Li/CFx-MnO2, which provide for a higher capacity per unit mass at moderate rates, relative to Li/SO2 and Li/SOCl2 chemistries used in previous missions. Current efforts to evaluate the effects of radiation and storage on these primary battery chemistries will be discussed in relation to future NASA needs. ACKNOWLEDGEMENT The work described here was carried out at the Jet Propulsion Laboratory, California Institute of Technology under contract with the National Aeronautics and Space Administration (NASA). L.M. Hofland, E.J. Stofel, R.K. Taenaka, Aerospace and Electronic Systems Magazine IEEE, 11, 14 (1996). B.P. Dagarin, R.K. Taenaka, E.J. Stofel, Proc. of 31st Energy Conversion Engineering Conference 1996, 1, 427 (1996).

As NASA missions become increasingly demanding of robust, lightweight, and compact power sources, there is a need to develop long-life and high-energy battery systems. Lithium-ion batteries have been developed at JPL over the last two decades and have been successfully integrated into a number of NASA missions [1] in conjunction with solar arrays and radioisotope thermoelectric generator (RTG) power sources. In the case of short-term missions in extremely harsh conditions (i.e., thirty days or less and/or at temperatures reaching -40 °C) without an active thermal management system, lithium primary cells may be considered for their high specific energy, especially in environments where recharging secondary cells would be challenging [2]. The icy moons of the Jovian system, for example, offer little sunlight for photovoltaic power and Jupiter’s intense magnetosphere leads to constant potentially damaging radiation. Many types of commercial lithium primary cells are available, reportedly offering high specific energies of ~500 Wh kg-1, with some designed specifically for low temperature operation. These commercial cells must be validated for space missions with extensive testing to demonstrate their performance, safety, reliability, and suitability for space environments. Radiation tolerance is also an important consideration, both as a method of sterilization for planetary protection and for survival in high radiation environments. In the present study, we have evaluated a number of commercial lithium primary cells (ranging from AA to DD+ sizes) of varying chemistries (with CFx, MnO2, SOCl2, SO2, and FeS2 cathodes) and manufacturers for possible future use in NASA’s deep space missions. We have assessed various performance characteristics of these cells, including low temperature energy, rate capability, and tolerance to radiation exposure, which will be reported here. Acknowledgement The work described herein was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration (NASA) and supported by the NASA Space Technology Mission Directorate Game Changing Development Program. References [1] Ratnakumar, B. V., Smart, M. C., Huang, C. K., Perrone, D., Surampudi, S., & Greenbaum, S. G. (2000). Lithium ion batteries for Mars exploration missions.Electrochimica Acta, 45(8-9), 1513–1517. doi:10.1016/S0013-4686(99)00367-9 [2] J.F. Whitacre, W.C. West, M.C. Smart, R. Yazami, G.K. Surya Prakash, A. Hamwi, et al., Enhanced Low-Temperature Performance of Li – CF x Batteries, Electrochem. Solid-State Lett. . 10 (2007) A166–A170.doi:10.1149/1.2735823

Supercapacitors combine efficient energy storage and delivery of power under high pulse current conditions, while retaining a high capacitance and low equivalent series resistance (ESR) during extended cycling. A high specific power is achieved via charge storage at a high surface area electrode/electrolyte interface, supported by a very low cell-level ESR. Another advantage of this type of electrochemical storage is the relatively wide operating range of the technology. This is due to the nature of the charge storage mechanism, which is dominated by non-Faradaic processes. Concerns over solid electrolyte interphase stability and ionic electrode diffusivity at wide temperature limits (associated with lithium ion batteries) are not present. Commercially available supercapacitors are typically limited in operation to ~-40 C to +65ºC. These limits are fixed by the liquid range of the non-aqueous acetonitrile solvent used in most supercapacitor electrolytes. Propylene carbonate possesses a wider liquid range, but limits low temperature power delivery due to its higher viscosity. Wider temperature operation is of interest for numerous applications, such as energy storage for remote distributed sensing and electric vehicle power trains. Previous work in wide temperature supercapacitors has demonstrated that operation to 1-4Primary concerns include preventing precipitation of the salt onto the high surface area carbon electrodes and maintaining a sufficiently low ESR. This is achieved by designing high dielectric/low viscosity solvent blends. Higher temperature operation is limited in large part by the boiling point of traditional non-aqueous solvents, as well as increased decomposition rates. 5-6Supercapacitors for elevated temperature operation have been designed and tested to +160ºC, as part of ongoing efforts to develop wider temperature energy storage technologies. To enable operation at these temperatures, ionic liquids have been utilized as the electrolyte. New electrode formulations, designed for long duration elevated temperature exposure, have also been designed and tested. Initial test data over several thousand cycles will be presented. ACKNOWLEDGEMENT The work described here was carried out at the Jet Propulsion Laboratory, California Institute of Technology under contract with the National Aeronautics and Space Administration (NASA). M.S. Ding, K. Xu, J.P. Zheng and T.R. Jow, J. Power Sources, 138, 340 (2004). Y. Korneblitt, A. Kajdos, W.C. West, M.C. Smart, E.J. Brandon, A. Kvit, J. Jagiello, G. Yushin, Adv. Ener. Mater. 22, 1655 (2012). W.C. West, M.C. Smart, E.J. Brandon, L.D. Whitcanack and G.A. Plett, J. Electrochem. Soc., 155, 716 (2008). E.J. Brandon, W.C. West, M.C. Smart, L.D. Whitcanack and G.A. Plett, J. Power Sources, 170, 225 (2007). T. Sato, G. Masuda and K. Takagi, Electrochim. Acta., 49, 3603 (2004). R.S. Borges, A.L.M. Reddy, M.-T. F. Rodrigues, H. Gullapalli, K. Balakrishnan, G.G. Silva and P.N. Ajayan, Sci. Rep., 3, 2572 (2013).

Introduction The National Aeronautics and Space Administration (NASA) has led a thrust for the development of advanced regenerative fuel cell systems (RFCs) to be used as energy storage for space-based robotics, mobility systems, and human habitats. The goal of the program has been to develop an RFC system capable of a round trip efficiency of 64%. The focus at the Jet Propulsion Laboratory (JPL) in RFC research has been in developing proton exchange membrane electrode assemblies (MEAs) for both the fuel cell and the electrolyzer subsystems [1-3]. In a RFC system, energy storage is achieved via the electrolysis of water to hydrogen and oxygen gases during the charge phase. Consumption of the hydrogen and oxygen gases then occurs during the discharge phase, with the subsequent generation of water. For space applications, the energy for the electrolysis of water will be supplied via solar or nuclear power. The power delivered during the discharge of the RFC system can be used by robots, mobility systems, and human habitats operating on the moon, near-Earth asteroids or Mars. Such a system could also be used for load balancing in both space and terrestrial applications. This paper will discuss fuel cell MEAs that have been developed for a future NASA regenerative fuel cell system. RFC system trades and NASA mission concepts that will feature RFC-based energy storage systems will also be discussed. Results and Discussion The current-voltage polarization of two MEAs are shown in Figure 1. To achieve high voltage efficiencies, the MEAs will be expected to operate at current densities in the range of 200 to 600 mA/cm2. The MEA will operate with reactants at a balance pressure of 30 PSIG at 70 oC. The NASA-JPL developed MEA is designed to operate in a non-flow-through stack. The MEA current-voltage polarization reported are for MEAs that operate with a reactant feed rate of approximately 3x stoic. As shown in Figure 1, the polarization for the NASA-JPL MEA is reported to be 0.92, 0.86 and 0.81 Volts at an applied current density of 200, 600 and 1000 mA/cm2, respectively. The polarization of the NASAJPL MEA is approximately 80 mV greater than the commercially available MEA at a current density of 200 mA/cm2. The voltage efficiency for the JPL-NASA MEA at 200 and 600 mA/cm2 is approximately 75 and 70%, respectively. To achieve the desired RFC system efficiency, the fuel cell MEA will need to operate at approximately 73% voltage efficiency. It is envisioned that MEA operation at current densities above 1 A/cm2 would be for short periods (< 1 hour) for NASA applications. The short-term durability studies for the NASAJPL MEA is shown as Figure 2. The MEA operated at an applied current density of 200 mA/cm2, 70 oC, and 30 PSIG balanced reactants during the durability studies. The initial voltage for the MEA is reported to be greater than 0.92 Volts, the voltage drops to 0.91 Volts after 13 hours of operation. The initial voltage is recoverable after current cycling as shown at 243 hours of operation. The cell degradation is reported to be approximately 7 μVolts after more than 1000 hours of operation. The degradation is, in part, attributed to test hardware. The minimum MEA voltage efficiency is approximately 73% for the duration of the test. Future work will investigate the integration of the NASA-JPL MEA in NASA RFC hardware. Acknowledgements The work presented here was carried out at the Jet Propulsion Laboratory, California Institute of Technology for the National Aeronautics and Space Administration. References [1] S. R. Narayanan, A. Kindler, A. Kisor, T. Valdez, R. J. Roy, C. Eldridge, B. Murach, M. Hoberecht, J. Graf, J. Electrochemical Society, 158 (11), B1348-B1357 (2011) [2] S. R. Narayanan, T. I. Valdez, and S. Firdosy, “Analysis of the Performance of Nafion-Based Hydrogen- Oxygen Fuel Cells,” J. Electrochemical Society, 156(1), B152-159 (2009) [3] T. I. Valdez, J. Sakamoto, K. Billings, S. A. Firdosy, F. Mansfeld, and S. R. Narayanan, Meet. Abstr, Electrochem Soc. 801 360 (2008) Figure 1

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