Researchers use ionic liquid electrolyte for a more efficient electroreduction of CO2 to CO; potential for synthetic fuel pathways
07 October 2011
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| 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.
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. Science DOI: 10.1126/science.1209786
The production of liquid fuels by renewables is unlikely ever to be competitive with doing the same job using nuclear power.
The reason is simple. Intermittency means that using renewables you can only run the plant and amortise the capital of the equipment part of the time, perhaps 18% or so in the case of solar.
Of course to some extent you could store it to cover intermittency, but that costs money.
So unless you have either very cheap equipment to produce the liquid or very cheap storage, neither of which is in prospect, running a nuclear plant 24/7 always beats it by far.
Posted by: Davemart | 07 October 2011 at 02:32 AM
Davemart,
It depends. WHen the capital cost of the factory is low compared to the variable energy costs, then it becomes attractive to start the plant whenever an excess of renewable energy is produced and wholesale electricity prices are low.
And nuclear power is very expensive. Low cost estimates are always based on existing or Chines power plants. New nuclear power in the west is uncompetitive. No investor wants to touch it without 100% government guarantees.
Posted by: Arne | 07 October 2011 at 07:36 AM
Davemart is correct as long as you are operating under a rational government and energy licensing system. In the US and EU that is not the case, presently.
Anne may ultimately be correct once the technology is developed and scaled. Remember that Oxford Catalysts has perfected a "shoebox sized" microchannel F-T reactor that can produce up to 6 barrels per day of high quality diesel from flared or stranded natural gas.
It is difficult for a CO2 to fuels approach to compete with ultra-cheap, ultra-abundant natural gas -- whether powered by nuclear or more intermittent and unreliable wind or solar.
Posted by: Alfin2500.blogspot.com | 07 October 2011 at 09:09 AM
Anne, the notion that nuclear is expensive is not true, even with the best efforts of opponents to increase cost by malicious regulation and exaggeration of the safety issues, and that is true even in the West.
Taking the figures from the nuclear constructions in Finland and France, which were the first of a kind and which they made a mess of, nuclear still comes out as highly competitive with gas and coal and many times cheaper than off-shore wind, let alone solar.
Costs are similar to on-shore wind, but need none of the feed in and backup obligations which disguise the true cost of renewables.
This is true whilst the plant is being amortised, over the first 20 years or so.
After that for the remainder of the 60 year lifespan costs are around 2-3 cents kwh, cheaper than anything save hydro where available.
Posted by: Davemart | 07 October 2011 at 10:37 AM
The heck with renewable fuels on Earth. If this can be gotten up to an acceptable performance level it can be used in spacecraft air regenerators or make fuels and chemical feedstocks from the atmospheres of Mars and Venus.
Posted by: Engineer-Poet | 10 October 2011 at 07:28 PM