Researchers use ionic liquid electrolyte for a more efficient electroreduction of CO2 to CO; potential for synthetic fuel pathways
|A plot of the Faradaic efficiency of the process to form the desired CO and the undesired hydrogen, and the turnover rate as a function of the applied cell potential. Credit: Science, Rosen et al. Click to enlarge.|
The electroreduction of carbon dioxide (CO2) to CO—a key component of artificial photosynthesis—has largely been stymied by the impractically high overpotentials necessary to drive the process. Now, University of Illinois chemical and biomolecular engineering professor Paul Kenis and his research group and researchers at startup Dioxide Materials report on the development of an electrocatalytic system that reduces CO2 to carbon monoxide at overpotentials below 0.2 V.
The system relies on an ionic liquid electrolyte to lower the energy of the (CO2)–intermediate and thereby lower the initial reduction barrier. A silver cathode then catalyzes formation of the final products.
Formation of gaseous CO is first observed at an applied voltage of 1.5 V, just slightly above the minimum (i.e., equilibrium) voltage of 1.33 V. The system continued producing CO for at least 7 hours at Faradaic efficiencies over 96%, according to their paper published in the journal Science.
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.—Rosen et al.
In artificial photosynthesis, an electrochemical cell uses energy from a solar collector or a wind turbine to convert CO2 to simple carbon fuels such as formic acid or methanol, which can be further refined to make ethanol and other fuels. However, the first step of turning carbon dioxide into carbon monoxide, has been too energy intensive to allow the process to advance towards commercialization. It requires so much electricity to drive this first reaction that more energy is used to produce the fuel than can be stored in the fuel.
The ionic liquid electrolyte stabilizes the intermediates in the reaction so that less electricity is needed to complete the conversion.
The energy efficiency or the process is 87% at low voltage (1.5V), and drops as the voltage increases, since there is energy loss due to resistive losses in the membrane and solutions.
The one weakness of the system at present is that our observed rates are lower than what is needed for a commercial process. Typically, commercial electrochemical processes run at a turnover rate of about 1-10 per second, in contrast with the rate of 1 per second or less we observe here. 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.
Graduate students Brian Rosen, Michael Thorson, Wei Zhu and Devin Whipple and postdoctoral researcher Amin Salehi-Khojin were co-authors of the paper. The US Department of Energy supported this work.
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. Science DOI: 10.1126/science.1209786