Making Fuel on Mars: Methane Synthesis from Martian-Derived CO2 and H2o Using a Sabatier Electrolyzer Based on Proton-Conducting Ceramics

Abstract

We present our development of dense proton-conducting ceramic membranes to synthesize CH4 and O2 from CO2 and H2O feed streams. The National Aeronautics and Space Administration (NASA) seeks such technology to enable manned missions to Mars. The CO2 and H2O feedstocks are in abundant supply on the Red Planet; solar power can be harnessed to convert these feedstocks via electrochemical reactors into fuels and O2 to sustain human life and enable permanent bases. NASA’s current approach features a two-stage process that starts with H2 synthesis through solar-driven H2O electrolysis using polymer-electrolyte membranes (PEM) at ~ 100 °C. The H2 product is then mixed with CO2 drawn from the Martian atmosphere and fed to a separate reactor held at ~ 400 °C in which a ruthenium catalyst promotes the Sabatier reaction to form methane: CO2 + 4 H2 → CH4 + 2 H2O This two-stage process joining a steam electrolyzer with a Sabatier reactor results in high conversion of CO2 and reasonable rates of CH4 production. However, the PEM-based H2O electrolysis suffers from low efficiency, resulting in an energy-intensive CH4 and O2 production. Our work seeks to combine these two processes into a single Sabatier Electrolyzer reactor based on proton-conducting ceramic membranes. Like PEMs, proton-conducting ceramic membranes transport reasonable rates of H+ ions at lower overall cell voltages due to increased temperature operation and low overpotentials for H2O splitting. The decreased electric power requirements for the Sabatier Electrolyzer are further enabled by autothermal operation where the Ohmic heating maintains the cell at the necessary temperatures for high H+ conduction and effective Sabatier chemistry (~ 400 °C). Integration of the catalyst with the electrolyzer simplifies the system, improves reliability, and provides increased performance within a smaller package. Figure 1 illustrates the Sabatier Electrolyzer concept. Martian derived H2O and CO2 are fed to opposing electrodes of a protonic-ceramic electrolysis cell. Steam is electrolyzed (top) with the product O2 exhausting 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, promoting thermal balance and high efficiency. This novel process intensification of intermediate temperature H2O electrolysis with Sabatier chemistry brings many questions. For example, do the protons crossing the membrane directly react with CO2 at the fuel electrode (as shown in Figure 1), or do they recombine to form H2 that subsequently reacts with CO2 within the nickel-laden fuel electrode to form CH4? What are the optimal operating conditions for integrating the protonic-ceramic electrolyzer with the Sabatier chemistry? What performance and reliability can NASA expect from a protonic-ceramic Sabatier Electrolyzer, in terms of electrical efficiency, methane production per unit mass, and long-term stability of cell voltage and current? CSM researchers have previously demonstrated encouraging results with proton-conducting ceramics for a number of applications, including electric-power generation and fuels synthesis. In this presentation, we will review our efforts to extend development of protonic-ceramic cells to the Sabatier Electrolyzer shown in Figure 1. The cells feature a BaCe0.2Zr0.6Y0.2O3-d (BCZY26) electrolyte, a Ni-BCZY26 fuel electrode support, and a BaCo0.4Fe0.4Zr0.1Y0.1O3-d (BCFZY) steam electrode. Results will be presented for CO2 conversion and CH4 selectivity under electrolysis operation. Figure 1