Carbon Dioxide Electrolysis

Prototype MC Wafer Cell

Figure 2. Prototype MC Wafer Cell (The MC wafer consists of a thin-film platinum electrode on both surfaces of a ceramic substrate or two substrates mounted on top of each other, while the bulk ceramic is impregnated with carbonate salt. The wafer is mounted in a cell. At operating temperature, a carbonate melt forms. A liquid seal isolates the internal cell chamber from the external environment. CO2 electrolysis to oxygen was demonstrated, but work is still required for obtaining a reliable liquid seal.)

Oxygen production via the direct electrochemical reduction of carbon dioxide (CO₂) is being investigated. This technology has direct applications for ongoing space missions (e.g., the International Space Station [ISS], Shuttle) and for Human Exploration and Development of Space (HEDS) missions as a means to remove CO₂ generated from human respiration in space cabins during long-term missions. No room-temperature system has been identified that can sustain CO₂ electrolysis, although one possible approach (i.e., the ionic liquids) is being investigated. At least two high-temperature systems were shown to electrochemically convert CO₂ into oxygen; these are the solid oxide conductors and the molten carbonate (MC) cells. The MC cell has several advantages over solid oxide conductors. The main advantage is that it operates at considerably lower temperature than the solid oxides (550 degrees Celsius [°C] versus 700 to 900 °C). In addition, the MC system is a mature technology in the fuel cell industry and exhibits both high throughput and long lifetime. Accordingly, many of the technical problems associated with high-temperature operation were addressed and solved for the MC fuel cell. The electrochemical process can be envisioned to proceed via either an oxide ion or a carbonate ion intermediate:		The excellent correlation between the observed oxygen and carbon dioxide levels in the anode gases confirms the viability of using the molten carbonate system to generate oxygen via the direct electroreduction of CO₂. Bulk reservoirs of molten electrolytes are not amenable for deployment in space applications. Instead, wafer-electrolyte configurations, similar to those developed by the fuel cell industry, are being developed. Electrodes on porous ceramic wafers saturated with the carbonate electrolyte are being developed for the molten carbonate electrolysis cell. Figure 2 shows a prototype design for the wafer cell. Tests demonstrated that oxygen can be generated via the direct CO₂ electrolysis using this cell configuration. Improvements in the robustness of the cell design and integrity of the seals remain to be accomplished.

It should be noted that in the presence of carbon dioxide, oxide ions would be in equilibrium with carbonate ions the main mobile anion in the molten carbonate cell. Accordingly, CO₂ reduction in molten carbonate could proceed via a carbonate intermediate, although mechanistically the actual electrochemical reaction could be via either the oxide ion or carbonate ion. This general reaction scheme was demonstrated experimentally. Figure 1 illustrates the production of oxygen during CO₂ electrolysis using two platinum electrodes immersed in a bulk reservoir of molten carbonate at an operating temperature of 550 °C. The anode was partially surrounded by an inert tube to facilitate collection of the vapor products. The anode vapors were transferred to a gas chromatograph using a helium sweep gas and analyzed for oxygen and carbon dioxide. The observed oxygen concentration as measured by the gas chromatograph correlated nicely to the oxygen concentration as predicted by Faraday’s law. The reaction scheme also predicts that the anode gas should consist of two parts of CO₂ for each part of oxygen. This was experimentally confirmed.		Key accomplishments: Developed a test system that allowed the evaluation of CO₂ electrolysis in MC with the capability of collecting the anode gases for independent analyses. This demonstrated a high Faradaic efficiency for the generation of oxygen via the electrochemical reduction of CO₂ in a bulk electrolyte at 550 °C. Identified ceramic substrates for developing a wafer electrode/electrolyte structure and made prototype systems that confirm similar electrochemical behavior for the wafer cell as the bulk electrolyte system. Contact: Dr. D.E. Lueck (Dale Lueck-1@ksc.nasa.gov), YA-C3, (321) 867-8764 Participating Organization: Dynacs Inc. (Dr. W.J. Buttner and J.M. Surma)
Figure 1. Oxygen Production Via CO2 Electrolysis on a Platinum Anode and Cathode in a Bulk Reservoir of Molten Carbonate at 550 °C. (The anode vapors were collected in a helium sweep gas and analyzed by a gas chromatograph (GC). The oxygen concentration as measured by the GC was nearly idential to the oxygen concentration as predicted by Faraday’s law. Direct reduction of CO2 predicts that the composition of the anode gases consists of two parts of CO2 for each part oxygen. GC analyses confirm this level of CO2 . Results are shown for 7 days of continuous operation; no significant loss in response was observed during this time.)		Figure 2. Prototype MC Wafer Cell (The MC wafer consists of a thin-film platinum electrode on both surfaces of a ceramic substrate or two substrates mounted on top of each other, while the bulk ceramic is impregnated with carbonate salt. The wafer is mounted in a cell. At operating temperature, a carbonate melt forms. A liquid seal isolates the internal cell chamber from the external environment. CO2 electrolysis to oxygen was demonstrated, but work is still required for obtaining a reliable liquid seal.)