Petroleum fuels are not sustainable due to their ultimately limited supply, greenhouse gas emissions, and concentrated geographic availability.  Starting from de-carbonized energy sources, it is possible to replace petroleum with sustainable energy carriers that store energy compactly and conveniently.  The major options for transportation are biofuels, batteries, ultracapacitors, hydrogen, and synthetic hydrocarbon fuels produced from carbon dioxide and water (hereafter referred to as “synfuels”).  To set the context for this research, the life-cycle sustainability and practicality of each energy carrier have been examined.  Synfuels were shown to have the practicality of biofuels—high energy density and ability to “plug in” to the existing infrastructure and vehicles—and the sustainability of hydrogen and batteries—significantly lower land area requirements and environmental impacts than biofuels.  Although plug-in hybrid electric vehicles will somewhat reduce the demand for liquid fuels, it is likely that liquid hydrocarbons will continue to be significant.

Figure 1. Synthetic carbonaceous fuel cycles.  a) once-through recycling, resulting in an emissions reduction of approximately ½, b) continuous closed-loop carbon recycling with air capture of carbon dioxide, resulting in approximately zero net emissions.  These approximations neglect life-cycle emissions of non-fossil energy generation, CO₂ capture, equipment production, etc.

At first, synfuels would likely utilize CO₂ from large industrial sources.  In the long term it may be possible to capture CO₂ directly from the air, forming a closed-loop hydrocarbon fuel cycle.  Figure 1 illustrates this.  The CO₂ and/or H₂O must then undergo dissociation using non-fossil energy.  After examining the many dissociation routes using thermal, electrical, or photonic energy, we expect high temperature solid oxide electrolysis cells (SOECs) to be promising.  SOECs operate at significantly higher efficiencies than today’s commercial alkaline water electrolyzers without the need for expensive catalysts, and because their ceramic electrolyte conducts oxide anions, they may be used to electrolyze CO₂ as well.  Thermodynamically, CO₂ electrolysis to CO has a lower minimum electricity demand than water electrolysis above ~800°C, as more inexpensive heat makes up the remaining reaction enthalpy.  In this temperature range, the equilibrium of the water-gas shift (WGS) reaction can go either way.  The goal is to produce syngas (a mixture of CO and H₂), which is then converted to liquid hydrocarbons via well-known Fischer-Tropsch synthesis.

Figure 2. Possible synfuel production pathways.  Enthalpies given for 298K.

This research builds fundamental understanding of high temperature electrolysis of CO₂ and CO₂/H₂O mixtures to form the basis for a new, more efficient pathway to produce synthetic hydrocarbon fuels.  Experiments entail characterizing dominant cell reactions (CO₂ electrolysis, H₂O electrolysis, and forward/reverse WGS; see Figures 2 and 3), at various feedstock compositions (CO₂/H₂O/CO/H₂), temperatures, and cathode materials in our test chamber.  After electrolyzing with prefabricated baseline cells designed as H₂ solid oxide fuel cells, fabrication of new cathodes will aim to discover materials that have higher electrocatalytic activity for splitting CO₂ than H₂O.  Material choices for the ceramic-metal composite cathode will draw from literature including studies of cathodes in low-temperature electrolysis of aqueous CO₂.  High-temperature CO₂ electrolysis has not yet been explored outside of limited research related to oxygen production for undersea or space exploration.  It is worth noting that $0.03/kWh electricity is equivalent to $1/gallon gasoline in terms of raw energy content (not including conversion inefficiencies and capital).  With a high cost of oil and inexpensive non-fossil energy sources, it will be possible to produce clean synthetic fuels at a lower cost than equally clean fossil fuels.

Figure 3. Solid oxide electrolysis of CO₂ (left) and co-electrolysis of CO₂ and H₂O (right).

Researchers

Christopher Graves, Ph.D. Candidate, Earth and Environmental Engineering, crg2109@columbia.edu
Klaus Lackner, Ewing-Worzel Professor of Geophysics, klaus.lackner@columbia.edu
Alan West, Professor of Chemical Engineering, acw17@columbia.edu
Paul Duby, Professor of Earth and Environmental Engineering, pfd1@columbia.edu