The Sabatier Electrolyzer: Harnessing Proton-Conducting Ceramics to Upgrade Carbon Dioxide into Methane

Abstract

We present our development of the Sabatier Electrolyzer to synthesize CH4 and O2 from CO2 and H2O feed streams. Our work seeks to integrate steam electrolysis and hydrogen production with carbon dioxide hydrogenation into a single Sabatier Electrolyzer reactor based on proton-conducting ceramic membranes and Sabatier catalysts. When powered by solar- and / or wind-energy, the Sabatier Electrolyzer has the potential to store intermittent renewable electricity in the form of commodity chemicals while simultaneously reducing carbon dioxide emissions, thereby addressing two principle societal concerns in a single device. The Sabatier Electrolyzer concept is illustrated in the figure. H2O and CO2 are fed to steam and fuel electrodes, respectively, of a protonic-ceramic electrolysis cell. An external power source drives H2O splitting at the steam electrode. The product O2 is exhausted from the cell. The product protons are driven across the protonic-ceramic membrane to the fuel electrode, where they react with CO2 to form CH4 and H2O. The combination of processes can match the exothermicity of CO2 hydrogenation with the endothermicity of H2O electrolysis to facilitate thermal balance and high efficiency. Proton-conducting ceramic membranes transport reasonable rates of H+ ions at 400 - 500 ºC with low overpotentials for H2O splitting. This lower operating temperature is reasonably well matched with the CO2-methanation reaction that thermodynamically favors lower-temperatures. The Ni-based catalyst is active for both H2 evolution from the protonic-ceramic material and catalytic methanation of CO/CO2, further promoting the technological combination. Integration of the methanation reactor with the electrolyzer simplifies the system, improves reliability, and provides increased performance within a smaller package than more-prevalent two-stage systems. We have pioneered a larger-area (active area is 5 cm2) Sabatier Electrolyzer cell composed of a BaCe0.4Zr0.4Y0.1Yb0.1O3-δ (BCZYYb4411) electrolyte, a Ni-BCZYYb4411 fuel electrode support, and a BaCo0.4Fe0.4Zr0.1Y0.1O3-δ (BCFZY) steam electrode. The Ni-cermet fuel electrode serves as the CO2-upgrading catalyst. To date, we have demonstrated CO2 conversion and CH4 selectivity at 55% and 63%, respectively, at 450 °C. This result represents the highest reported electrochemical conversion of CO2 and H2O into CH4 in a single device. In this presentation, we will review our efforts in Sabatier Electrolyzer development, targeting the thermodynamically predicted CO2 conversion and CH4 selectivity of 70% and 95%, respectively. Figure 1