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Fossil fuels and biomass are the most common feedstocks for the production of hydrocarbon fuels. However, using renewable or nuclear energy, carbon dioxide and water can be recycled into sustainable liquid hydrocarbon fuels in non-biological processes. Such a CO₂ recycling process can produce gasoline, diesel, or other hydrocarbons or alcohols which can directly substitute into the existing infrastructure and vehicles. Capture of CO₂ from the atmosphere (a technology pioneered in our research group) would enable a closed-loop carbon-neutral fuel cycle. When powered by sunlight, the fuel cycle would be similar to that of biofuels. However, since the fuels are produced in a non-biological process, they would not share the disadvantages of biofuels in terms of land use, resource use, interference with food supplies, and other impacts to the environment and biosphere.

The purpose of this project was to develop critical components of a system that recycles CO₂ into liquid hydrocarbon fuels. A process to produce such fuels has three stages: (1) CO₂ capture, (2) storage of the renewable/nuclear energy as chemical energy by dissociation of CO₂ and/or H₂O, and (3) fuel synthesis using the dissociation products. We have examined the concept at several scales, beginning with a broad scope analysis of the system and ultimately studying electrolysis of CO₂ and H₂O in high temperature solid oxide cells as the heart of the energy conversion via a number of micro- and nano-scale experimental studies. Following is a summary of our accomplishments during the second year of the grant.

1. We completed (and have now published) a critical review of the many possible pathways to recycle CO₂ into fuels, based on different dissociation methods including thermolysis, thermochemical cycles, electrolysis, and photoelectrolysis of CO₂ and/or H₂O. We identified a process based on high temperature co-electrolysis of CO₂ and H₂O to produce syngas (CO/H₂ mixture) as a promising method. Using experimental results for cell performance and durability that we obtained during the first year of the grant (now published), we analyzed the energy balance and economics of an electrolysis-based synthetic fuel production process, including CO₂ air capture and Fischer-Tropsch fuel synthesis. We determined that the system can feasibly operate at 70% electricity-to-liquid fuel efficiency (higher heating value basis) and conducted a parametric analysis of the cost of fuel production versus electricity price. The dominant costs of the process are the electricity cost and the capital cost of the electrolyzer, and the capital cost is significantly increased when operating intermittently (on renewable power sources such as solar and wind). Low cell internal resistance, low degradation, and low manufacturing cost each contribute to a low electrolyzer capital cost. Our subsequent research efforts addressed these avenues to affordability.

2. Our experimental results obtained during the first year of the project (using state-of-the-art cells produced by Risø National Laboratory for Sustainable Energy, DTU, Denmark) indicated that the Ni-YSZ negative-electrode is the major site of degradation (YSZ = yttria-stabilized zirconia). To better understand the reaction mechanisms at the negative-electrode that limit performance and durability, we studied different metal electrodes including nickel using a simplified point-contact electrode geometry with a well-defined three-phase boundary (TPB; the electrode/electrolyte/gas interface where the electrochemical reactions take place). The simple geometry is useful for isolating the electrochemical properties without the effects of the complex microstructure of technological porous electrodes. We found that the same reaction mechanisms are not shared by the different metals, contrary to some recent studies. We also found evidence that supports the explanation that impurities segregated to the TPB play a major role and are largely responsible for inconsistencies in the electrode kinetics literature. Our analysis suggested possible reaction mechanisms for H₂O/CO₂ reduction as well as H₂/CO oxidation.

3. We also completed a study using novel molybdate ceramic materials as possible alternative negative-electrode materials. We characterized the phase stability, microstructure, electrical conductivity, and electrochemical activity for H₂O/CO₂ reduction and H₂/CO oxidation for each material. Unique phenomena were observed for some of the materials – they decomposed into multiple, beneficial phases which formed a nanostructured surface upon exposure to operating conditions (in certain reducing atmospheres). The new phases and surface features enhanced the electronic conductivity and electrocatalytic activity. Performing controlled decomposition of multiple desirable phases and a desirable microstructure (which can take place in situ) using these materials is a new way to produce potentially high-performance electrodes for solid oxide cells. By modifying the composition of the molybdate, it was possible to prevent decomposition. The best performing materials were found to provide very high performance electrodes. Many of the molybdate materials exhibited much higher performance for cathodic (electrolysis) polarization than anodic (fuel cell) polarization, which makes them especially interesting for electrolysis electrodes.

4. We also found another ceramic material, Gd-doped ceria in nanoparticle form, to be an excellent electrocatalyst for CO₂ electrolysis and CO oxidation (moreso than for H₂O/H₂ for which it is known to be good). This remarkable finding – a material that exhibits higher performance for CO₂ electrolysis than for H₂O electrolysis – fulfills one of the original goals of our grant proposal.

5. Near the end of the second year of the grant, our PhD student Chris Graves successfully completed his dissertation on this topic. Another related, positive outcome of this PRF project was a conference entitled “Sustainable Fuels from CO₂, H₂O and Carbon-Free Energy” that our research center at Columbia University, the Lenfest Center for Sustainable Energy, held in New York in collaboration with Risø National Lab. We invited speakers from universities and research labs from around the U.S. and from Denmark to present their state-of-the-art research on a wide range of topics relevant to the CO₂ recycling process, including CO₂ capture technology, photovoltaics, and splitting CO₂/H₂O by low- and high-temperature electrolysis, solar-driven thermal processes, and photochemical conversion.

While this work has investigated several important aspects of recycling CO₂ to fuels, there is of course much more work to be done. Our findings point toward a number of opportunities for exciting research. In the future we intend to pursue aspects of the work completed during this project.