Co-Electrolysis of CO2 and H2O for the Production of Liquid Fuels

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Diagram of the proposed closed-loop fuel cycle. CO2 is recycled into hydrocarbon fuels in a process based on: capturing CO2 from the atmosphere, high-temperature co-electrolysis of CO2 and H2O in a solid oxide cell to yield syngas (CO/H2 mixture), and catalytic fuel synthesis from the syngas. Source: Lenfest Center. Click to enlarge.

Researchers at Columbia University’s Lenfest Center for Sustainable Energy, in collaboration with Risø National Laboratory for Sustainable Energy, DTU, are investigating the high-temperature co-electrolysis of CO2 and H2O using solid oxide electrolysis cells (SOECs) to produce a syngas for conversion into liquid hydrocarbon fuels.

The Columbia/Risø team currently has a paper in press in the journal Solid State Ionics describing their experimental results on performance and durability of the solid oxide cells. In May, the Lenfest Center and Risø DTU hosted a one-day conference—“Sustainable Fuels from CO2, H2O, and Carbon-Free Energy”—addressing technologies that can be used to recycle CO2 into carbon-neutral liquid hydrocarbon fuels using renewable or nuclear energy. The co-electrolysis process was featured in several of the talks.

A process to produce liquid fuels from CO2 would comprise three basic stages:

  1. CO2 capture
  2. Dissocation of CO2 and/or H2O
  3. using renewable or nuclear energy
  4. Fuel synthesis using the dissociation products.
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Possible pathways for the conversion of CO2 to hydrocarbons and alcohols. Source: Lenfest Center, Christopher Graves. Click to enlarge.

A number of different methods for the dissociation—i.e., the conversion of renewable/nuclear energy to chemical energy—are feasible: thermolysis; a thermochemical cycle; high-temperature electrolysis; low-temperature electrolysis; and photoelectrolysis/photolysis. Lenfest Center and Risø DTU determined that dissociation by electrolytic methods is currently the most feasible.

Of those, they identified high temperature co-electrolysis of CO2 and H2O to produce the syngas (CO/H2 mixture) as a promising method. In one of the papers presented at the May conference, Carl Stoots from the Idaho National Laboratory noted that while it is possible to produce syngas by separately electrolyzing steam and CO2, there are significant advantages to co-electrolysis, including lower cell resistance and the reduced possibility of further reduction of CO to C. However, he noted, co-electrolysis is still not well understood.

The Lenfest/Risø team notes that high temperature electrolysis makes very efficient use of electricity and heat (near-100% electricity-to-syngas efficiency), provides high reaction rates (no need for precious metal catalysts), and the syngas produced can be catalytically converted to hydrocarbons in well-known fuel synthesis reactors (e.g. Fischer-Tropsch). There is no need for a separate reverse water-gas shift reactor to produce syngas, and the waste heat from exothermic fuel synthesis is useful in the process.

An analysis of the system energy balance presented by Christopher Graves at the May conference showed a 70% electricity to hydrocarbon fuel efficiency. Using solar photovoltaic energy at 10-20% efficiency, that would result in an overall 7-14% solar energy to liquid fuel efficiency, he said.

Their analysis of the economics of a co-electrolysis-based synthetic fuel production process, including CO2 air capture (earlier post) and Fischer-Tropsch fuel synthesis, determined that the price of electricity needed to produce competitive synthetic gasoline (at $2/gal wholesale) is $0.02 - $0.03 per kWh.

Dominant costs of the process are the electricity cost and the capital cost of the electrolyzer; this capital cost is significantly increased when operating intermittently (on renewable power sources such as solar and wind).

The core of the process is the solid oxide cell for co-electrolysis. Risø has been developing and testing Ni/YSZ based Solid Oxide Cells. Although high initial performance was observed, long-term durability needs to be improved. Testing showed significant structural degradation at high current densities.

The team’s research involves studying reaction mechanisms at the negative-electrode that limit performance and durability, using simplified-geometry electrodes. They have also recently developed new all-ceramic nano-structured molybdate-based electrode materials that exhibit exceptional electrocatalytic performance and could significantly improve the overall energy use and economics of the CO2-to-fuels system.

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