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Developing sustainable fuels that work with our existing infrastructure (fuel distribution and vehicles) will help achieve a long-term sustainable energy future.  The two most widely explored options are hydrocarbons produced from fossil fuel and biomass feedstocks.  However, similar hydrocarbons can be produced using renewable and/or nuclear energy (heat, electricity, and/or light) to dissociate CO₂, water, or both, resulting in energy-rich gas mixtures which are converted to convenient fuels.  This process is made up of three stages:

(1) CO₂ capture—Initially, CO₂ captured from large industrial sources could be utilized.  In the long term, capturing CO₂ from the atmosphere (“air capture”) would enable a closed-loop fuel cycle analogous to a renewable hydrogen fuel cycle.  Our research group has demonstrated the technical feasibility of air capture.  We have continued to develop the technology and economic feasibility is on the horizon

(2) Storage of the renewable/nuclear energy as chemical energy by dissociation of CO₂ and/or H₂O—A promising means of efficient dissociation is by high temperature electrolysis of CO₂ and/or H₂O in solid oxide cells (SOCs) to yield synthesis gas (syngas, an energy-rich gas mixture of CO and H₂).  The present research focuses on this stage and technology.

(3) Fuel synthesis using the dissociation products—Liquid fuel synthesis from syngas, e.g. Fischer-Tropsch synthesis, is a mature technology that has long been used in coal and natural gas to liquids projects. 

During the first year of the PRF grant, we have accomplished a number of goals.  We have characterized the electrolysis performance and durability of two types of state-of-the-art SOCs that were designed as solid oxide fuel cells.  Our ongoing work is on materials development for the negative-electrode at which CO₂/H₂O are dissociated.  A large part of this work has been carried out by our PhD student Christopher Graves, during a visit to Risø National Laboratory for Sustainable Energy in Denmark.  This collaboration, which was introduced in our original Petroleum Research Fund proposal, has been a large success for both Columbia and Risø, and we are planning future work together which takes advantage of our respective strengths.

We built a test rig at Columbia for studying solid oxide cells (and high temperature ceramic materials) with DC and AC electrochemical characterization methods.  We used commercially obtained electrolyte-supported button cells (composed of a Ni-YSZ/GDC negative-electrode, YSZ-based electrolyte, and LSM-YSZ/GDC oxygen electrode; YSZ = yttria-stabilized zirconia, GDC = gadolina-doped ceria, LSM = lanthanum strontium manganite).  By varying the conditions (temperature, gas composition to each electrode) between measurements, we were able to identify some of the rate-limiting processes.  Similar experiments on negative-electrode supported full cells at Risø were then conducted (cell composition: Ni-YSZ / YSZ / LSM-YSZ).  These cells showed a much higher total performance, which was only partly due to the small ohmic resistance of the thinner electrolyte. 

Both types of cells showed higher activity for H₂O electrolysis than CO₂ electrolysis.  The difference was more dramatic for the button cells, which was mostly due to gas concentration effects based on analysis of the impedance spectra.  This clearly demonstrated the sensitivity of measured cell performance to electrode microstructure and the test setup.  With the Risø cells, a more extensive systematic variation of test conditions was conducted in an automated setup, which by careful analysis of the impedance measurements, enabled a break-down of the total internal resistance of the cell into individual contributions from the different components and reactions.  This method was used to conduct a meaningful study of a long-term durability test.  We performed a durability test of co-electrolysis of CO₂ and H₂O for several hundred hours at consecutively higher current densities (and corresponding overpotentials).  For these cells, which are more active for H₂O than CO₂ electrolysis, co-electrolysis can partly rely on the reverse water-gas shift internally in the electrode, which may be advantageous for process simplicity, eliminating the need for a separate reverse WGS reactor to prepare syngas with the H₂/CO ratio needed for catalytic fuel synthesis.  By analyzing the impedance spectra before and after each segment, it was found that at low current density operation (-0.25 A/cm²) degradation at the Ni/YSZ electrode dominated, whereas at higher current densities (-0.5 A/cm² and -1.0 A/cm²), the serial resistance and degradation at the LSM/YSZ electrode began to play a major role.  At higher current density, the durability needs to be improved if high current density operation is needed for an economical synthetic fuel production process.  At low current density the durability was high, maybe sufficiently high for synthetic fuel production.  These results have been submitted for journal publication.

We have also conducted a critical review and assessment of the feasibility of the process, with a focus on the many possible electrochemical, thermochemical, and photochemical pathways that could be used for the dissociation stage.  This includes energy balance and economics estimates based on the results described above.  These estimates include the important effects of the energy source, e.g.  intermittent renewable energy (e.g. excess wind power supply, or solar arrays built in remote, sunny locations) vs constant-supply nuclear power.  This will soon be submitted for journal publication.  Finally, we are also in the process of writing an analysis of the sustainability of all types of transportation energy carriers, both chemical and electrical, including synthetic fuels from CO₂, H₂O, and non-fossil energy. 

This work has identified the negative-electrode as a cell component could be improved.  While the Ni-YSZ cathode shows high performance for CO₂ and H₂O electrolysis, the durability (tolerance to impurities, and structural “redox” stability against repeated oxidation and reduction) needs improvement for an affordable synthetic fuel process.  Our ongoing work focuses on two routes to accomplish this.  We will discuss the details of the methods and results in next year’s annual report. 

In conclusion, we have determined that it is possible to produce clean synthetic fuels using state-of-the-art solid oxide cells.  To make this process affordable on a larger scale—competing against a lower oil cost and using a wider variety of energy sources—we have set out to improve one of the important parts of the process.