Twenty years ago, Bockris and co-workers proposed that high overpotentials are needed to convert CO₂ because the first step in CO₂ conversion is the formation of a “CO₂⁻” intermediate. In this context the term “CO₂⁻” is not necessarily a bare CO₂⁻ anion. Instead it is whatever species forms when an electron is added to CO₂. The equilibrium potential for “(CO₂)⁻” formation is very negative in water and in most common solvents. Consequently, it is necessary to run the cathode very negative (i.e., at a high overpotential) for the reaction to occur. This is very energy inefficient.

The objective of the work described here was to develop a co-catalyst that would lower the potential for formation of the “CO2⁻” intermediate, which then subsequently reacts with H⁺ on the silver cathode to produce CO. If Bockris’ proposal is correct, the overpotential for CO₂ conversion into useful products should decrease upon lowering the free energy of formation of the “CO₂⁻”...Although there would still be a barrier to form the final products of the reaction, the overall barrier to reaction would be reduced.

—Rosen et al.

The researchers used an electrochemical cell as a flow reactor, separating the gaseous CO₂ input and oxygen output from the liquid electrolyte catalyst with gas-diffusion electrodes. The cell design allowed the researchers to fine-tune the composition of the electrolyte stream to improve reaction kinetics, including adding ionic liquids as a co-catalyst.

A weakness of the current system is that the observed rates are lower than what is needed for a commercial process, the team noted in their paper. Commercial electrochemical processes typically run at a turnover rate of about 1-10 per second; by contrast, the new system has a rate of 1 per second or less.

Further development of the reactor configuration and exact operating conditions, e.g., to overcome some mass transport issues, is expected to increase the turnover number. Indeed a rate of 60 turnovers per second is observed with a rotating disk electrode at a cathode potential equivalent to that observed when the cell potential is about 2 V.

Also, scale-up needs to be done. Presently our cathode only has an electrochemical surface area of 6 cm² compared to in the order of 10⁹ cm² in a commercial electrochemical cell for the chlor-alkalai process. At 2 V our cell only produces about a micromol/min of CO, while commercial processes require thousands of moles per minute per cell.

—Rosen et al.

Dioxide Materials. Dioxide Materials, the university Research Park, was founded by retired chemical engineering professor Richard Masel. The company has technology in two areas:

• Processes that convert the carbon dioxide produced by homes and businesses back to transportation fuels, further lowering the carbon footprint, and creating a viable source of renewable fuels; and

• Advanced controls that allow a building's heating, ventilation and air conditioning (HVAC) systems to use less energy, saving money and lowering the building’s carbon footprint.

The company’s patent-pending Dual-Electrocat (DuElCat) process converts a mixture of carbon dioxide and water back to synthesis gas for subsequent processing into synthetic fuels.

Resources

• Brian A. Rosen, Amin Salehi-Khojin, Michael R. Thorson, W. Zhu, Devin T. Whipple, Paul J. A. Kenis, and Richard I. Masel (2011) Ionic Liquid–Mediated Selective Conversion of CO₂ to CO at Low Overpotentials. ScienceDOI: 10.1126/science.1209786