Lower Temperature Electrolytic Reduction of CO[sub 2] to O[sub 2] and CO with High-Conductivity Solid Oxide Bilayer Electrolytes

Abstract

Ceramic oxygen generators ͑COGs͒ with a bilayer-electrolyte ͑erbia-stabilized bismuth oxide/samaria-doped ceria͒ architecture were developed to produce pure oxygen from CO 2 at lower temperature ͑below 700°C͒ for potential use in NASA's Mars exploration mission. Major factors that influence oxygen generation include the oxygen-ion conductivity of the solid-oxide electrolyte, applied current, operating temperature, and fuel utilization ͑CO/CO 2 ratio͒. The COG voltage power losses were due to internal resistance and electrode polarization. Higher temperatures resulted in higher oxygen generation rates due to reduced cell resistance. However, lowering the COG operating temperature is very important and the bilayer COGs showed promise for operation below 700°C, thus reducing the required power consumption, expanding ancillary material selection, decreasing fabrication cost, and potentially extending mission time.Basic concept and application of ceramic oxygen generators.-Ceramic oxygen generators ͑COGs͒ based on oxygen ion conductors produce pure O 2 via selective oxygen ion flux at high temperatures. 1,2 This novel technology provides many advantages over traditional cryogenic oxygen generating equipment, in that it is potentially economical and produces exceedingly high purity oxygen with negligible emissions of toxic gases. 3 COGs are classified according to their driving force for oxygen generation. 1,3 One subgroup of COGs uses mixed ionic-electronic conductors ͑MIECs͒, which employ an oxygen partial pressure ͑p O 2 ͒ gradient as the driving force for oxygen separation. The MIEC has substantial electronic conductivity as well as ionic conductivity, which results in an internally short-circuited oxygen-concentration cell. 4 Thus oxygen ions are transported from the high p O 2 side of the membranes to the low p O 2 side. The second subgroup uses an ion conducting membrane, such as yttria-stabilized zirconia ͑YSZ͒. Here, the driving force is an electric potential which is applied across the membrane. Under the influence of an applied electric potential, oxygen ions migrate from the cathode side to the anode side, through the dense membranes. The flux of oxygen ion migration for these membranes is predicted using the Faraday equation 5where J O 2 ͑mol/cm 2 /s͒ is the molar flux of oxygen produced, i applied is the applied current density ͑A/cm 2 ͒, F is 96 500 ͑C/mol͒, and n is the number of electrons associated with the ionization of an oxygen molecule. COG technologies have considerable potential for a wide range of applications in life-support systems, and semiconductor and gasenergy industries. 3 Ceramic oxygen-ion conducting membranes can be used for gas purification ͑separation of oxygen from an inert gas stream containing higher amounts of oxygen͒, for controlling oxygen levels in gas streams, and for producing high-purity oxygen. This technology can also be used in the combustion processes for hydrocarbon fuels ͑HCs͒, in the production of hydrogen by electrolysis of steam, and in the reduction of nitric oxi...

Highly Sensitive/Selective Miniature Potentiometric Carbon Monoxide Gas Sensors with Titania‐Based Sensing Elements

“…More recently, NASA's Human Mars Exploration Mission 2,15,16 requires CO sensors. The Martian atmosphere consists essentially of 97% CO 2 17,18 .…”

A high performance IT-EOG cell based on a solid/molten Bi2O3–B2O3 composite electrolyte

“…In majority of IT-EOGs, the samaria-doped ceria Ce 0.8 Sm 0.2 O 2Àd (SDC), ittria-stabilized bismuth oxide Bi 1.46 Y 0.54 O 3 (YSB), erbia-stabilized bismuth oxide Bi 1.6 Er 0.4 O 3 (ESB), dysprosium-and tungsten-stabilized bismuth oxide Dy 0.08 W 0.04-Bi 0.88 O 1.56 (DWSB), and bilayer SDC/ESB ceramic electrolytes are used. [7][8][9][10][11][12][13] However, the ionic conductivity of these ceramic electrolytes is lower than that of the newly developed solid/ molten Bi 2 O 3 -0.2 wt% B 2 O 3 composite electrolyte (Fig. 1b).…”

Oxygen Generation from Carbon Dioxide for Advanced Life Support

“…However, by using electrolytes known to perform well at lower temperatures the potential weight of the proposed COG-based life support system can be reduced because of reduced insulation and power needs. Two such materials are gadolinium doped ceria (GDC) and erbia stabilized bismuth oxide (ESB) (1). GDC is known to be prone to electronic conductivity at low partial pressures of oxygen, but is thermodynamically stable.…”