Oxygen Generation from Carbon Dioxide for Advanced Life Support

The partial electrochemical reduction of CO2 using ceramic oxygen generators (COGs) is well known and has been studied. Conventional COGs use yttria-stabilized zirconia (YSZ) electrolytes and operate at temperatures greater than 700 C (1, 2). Operating at a lower temperature has the advantage of reducing the mass of the ancillary components such as insulation. Moreover, complete reduction of metabolically produced CO2 (into carbon and oxygen) has the potential of reducing oxygen storage weight if the oxygen can be recovered. Recently, the University of Florida developed ceramic oxygen generators employing a bilayer electrolyte of gadolinia-doped ceria and erbia-stabilized bismuth oxide (ESB) for NASA s future exploration of Mars (3). The results showed that oxygen could be reliably produced from CO2 at temperatures as low as 400 C. These results indicate that this technology could be adapted to CO2 removal from a spacesuit and other applications in which CO2 removal is an issue. This strategy for CO2 removal in advanced life support systems employs a catalytic layer combined with a COG so that the CO2 is reduced completely to solid carbon and oxygen. First, to reduce the COG operating temperature, a thin, bilayer electrolyte was employed. Second, to promote full CO2 reduction while avoiding the problem of carbon deposition on the COG cathode, a catalytic carbon deposition layer was designed and the cathode utilized materials shown to be coke resistant. Third, a composite anode was used consisting of bismuth ruthenate (BRO) and ESB that has been shown to have high performance (4). The inset of figure 1 shows the conceptual design of the tubular COG and the rest of the figure shows schematically the test apparatus. Figure 2 shows the microstructure of a COG tube prior to testing. During testing, current is applied across the cell and initially CuO is reduced to copper metal by electrochemical pumping. Then the oxygen source becomes the CO/CO2. This presentation details the results of testing the COG

Concurrent CO2 Control and O2 Generation for Advanced Life Support

The electrochemical reduction of carbon dioxide (CO2) using ceramic oxygen generators (COGs) is well known and widely studied, however, conventional devices using yttria-stabilized zirconia (YSZ) electrolytes operate at temperatures greater than 700 C. Operating at such high temperatures increases system mass compared to lower temperature systems because of increased energy overhead to get the COG up to operating temperature and the need for heavier insulation and/or heat exchangers to reduce the COG oxygen (O2) output temperature for comfortable inhalation. Recently, the University of Florida developed novel ceramic oxygen generators employing a bilayer electrolyte of gadolinia-doped ceria and erbia-stabilized bismuth for NASA's future exploration of Mars. To reduce landed mass and operation expenditures during the mission, in-situ resource utilization was proposed using these COGs to obtain both lifesupporting oxygen and oxidant/propellant fuel, by converting CO2 from the Mars atmosphere. The results showed that oxygen could be reliably produced from CO2 at temperatures as low as 400 C. These results indicate that this technology could be adapted to CO2 removal from a spacesuit and other applications in which CO2 removal was an issue. The strategy proposed for CO2 removal for advanced life support systems employs a catalytic layer combined with a COG so that it is reduced all the way to solid carbon and oxygen. Hence, a three-phased approach was used for the development of a viable low weight COG for CO2 removal. First, to reduce the COG operating temperature a high oxide ion conductivity electrolyte was developed. Second, to promote full CO2 reduction while avoiding the problem of carbon deposition on the COG cathode, novel cathodes and a removable catalytic carbon deposition layer were designed. Third, to improve efficiency, a pre-stage for CO2 absorption was used to concentrate CO2 from the exhalate before sending it to the COG. These subsystems were then integrated into a single CO2 removal system. This paper describes our progress to date on these tasks

Determination of Electrochemical Performance, and Thermo-Mechanicalchemical Stability of SOFCS From Defect Modeling

The objectives of this project were to: provide fundamental relationships between SOFC performance and operating conditions and transient (time dependent) transport properties; extend models to thermo-mechanical stability, thermo-chemical stability, and multilayer structures; incorporate microstructural effects such as grain boundaries and grain-size distribution; experimentally verify models and devise strategies to obtain relevant material constants; and assemble software package for integration into SECA failure analysis models

Preface

Peer reviewed: YesNRC publication: Ye

Bi2Ru2O7 Pyrochlore Electrodes For Bi2O3 Based Electrolyte For IT-SOFC Applications

Bismuth ruthenate pyrochlore (Bi2Ru2O7: BRO) was tested as a cathode in several bismuth oxide-based electrolyte cells. Erbium oxide stabilized bismuth oxide (ESB) (Bi2O3)0.8 (Er2O3)0.2, Bi2V0.9Me0.1O5.35 (BIMEVOX10, Me = Co, Cu, Zn), and Bi2Ru2O7 (BRO) were synthesized via solid state reaction. ESB and BIMEVOX powders were pressed and sintered into dense pellets while BRO was deposited and sintered as porous electrodes onto ESB and BIMEVOX dense pellet surfaces. Morphologies and phases of the materials were characterized by Field Emission Scanning Electron Microscope and x-ray diffraction, respectively. Electrochemical performances of BRO/ESB and BRO/BIMEVOX symmetric cells were tested using Electrochemical Impedance Spectroscopy (EIS) at 200-700 °C in air. High polarization for BIMEVOX cells was observed, while ESB cells showed a promising behaviour. Different electrochemical performances of BRO electrodes, with ESB or BIMEVOX, indicated that the phases formed at electrodes/electrolyte interface controlled the cell polarization process

Enhancement of La0.6Sr0.4Co0.2Fe0.8O3-delta Surface Exchange through Ion Implantation

La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) has been demonstrated to be one of the best performing mixed ionic-electronic conductors (MIEC) for SOFC cathode materials. Surface exchange on LSCF, however, limits oxygen transport and performance. We investigated surface modified LSCF, by Mn ion implantation, for an enhanced surface oxygen exchange rate while maintaining LSCFs oxygen ion conductivity through the bulk. Various implantation energies and ion concentrations were used to create samples with different Mnion depth profiles. The oxygen transport properties, chemical diffusion coefficient (Dchem) and effective chemical surface exchange coefficient (κchem), were characterized by electrical conductivity relaxation (ECR), using DC four-point probe measurements during the oxygen re-equilibration process. The changes of κchem for different Mn doping levels and ion acceleration energies determined with ECR are compared with surface configurations obtained using X-ray photoelectron spectroscopy. The Mn ion implanted LSCF samples show an enhanced κchem, improving the overall oxygen reduction reaction for the LSCF cathode material. Possible factors for the increase in surface exchange for the Mn implanted LSCF samples are discussed. © 2015 The Electrochemical Society. All rights reserved.FALS