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Posted: Jul 20, 2015

Carbon-negative CO2 conversion using renewable energy

(Nanowerk Spotlight) Putting some of the rising amounts of carbon dioxide in the atmosphere to good use again, researchers are looking for ways to convert atmospheric CO2 emissions into industrially relevant, valuable chemicals and fuels; ideally powered by clean, renewable energy sources to make the whole process 'carbon-negative' or at least 'carbon-neutral', i.e. by using at least – if not more – CO2 than is created in the process.
New work just published in ACS Applied Materials & Interfaces ("Efficient Electrochemical CO2 Conversion Powered by Renewable Energy") demonstrates that current, state-of-the-art renewable energy sources can efficiently power large-scale CO2 conversion systems.
"Our data shows that large-scale CO2 conversion technologies are practical, and we provide some of the first performance metrics needed to move CO2 conversion technologies out of the lab and into the real world," Douglas R. Kauffman, a research scientist at the U.S. Department of Energy's National Energy Technology Laboratory (NETL), tells Nanowerk. "Demonstrating that current renewable energy technology can power CO2 conversion systems is a game changer, and it will hopefully accelerate the development of larger-scale installations."
electrochemical CO2 reactor
(a, b) Photographs of an electrochemical CO2 reactor powered by inexpensive ($10-20 USD) solar panels and a solar-rechargeable battery. (c, d) CO2 → CO selectivity as a function of turnover number when powered with a solar cell or solar-rechargeable battery (where, for example, 2E5 represents 2 105). The panels summarize operating time, average cathode voltage, CO production rate, and CO selectivity. Solar power operation mimics a 12 hour sunny day, and battery-powered operation mimics nighttime hours, low-light conditions, or periods of unavailable renewable electricity. (Reprinted with permission by American Chemical Society) (click on image to enlarge)
The technology developed by the NETL team uses a solar powered reactor and a special form of gold nanoparticles to turn CO2 into valuable chemicals and fuels. It has the potential to help enable affordable, sustainable utilization of fossil energy resources.
"The challenge associated with this project is that CO2 is a very stable molecule, and converting CO2 into anything useful requires a lot of energy," Kauffman points out.
Electrochemical conversion is a leading candidate that uses electrical voltages to efficiently convert CO2 into other products. The drawback to this approach is that it requires electricity, and CO2 emissions from fossil-fuel derived electricity typically produce a net increase in CO2 emission.
In other words, most fossil-fuel powered CO2 conversion processes are currently 'carbon-positive' and do not help mitigate CO2 emissions.
"The most important and exciting aspect of our paper is that we show 'carbon-negative' CO2 conversion," says Kauffman. "We do this by using renewable energy sources to convert CO2 into other chemicals without producing additional CO2 emissions. We used a ligand-protected gold nanocluster that contains exactly 25 gold atoms to convert CO2. This Au25 catalyst is so efficient we were able to power our laboratory-scale CO2 reactor with inexpensive ($10-$20) and off-the-shelf renewable energy sources such as solar panels and solar-rechargeable batteries from retail hobby shops."
This finding grew out of previous work from Kauffman's and other groups working on CO2 conversion. "However, there are very few literature reports assessing the scalability and technical viability of larger-scale, renewably-powered CO2 conversion technologies," he says. "Our paper fills this gap and provides a path to developing industrial-scale CO2 conversion processes."
The also team evaluated long-term CO2 reduction reaction (CO2RR) performance of their device. The charts below show the results of a 6-day experiment where the electrolysis was run for 5-6 hours each day to mimic realistic, on-demand usage that is known to degrade catalysts and carbon electrodes in real-world applications such as fuel cells.
carbon dioxide conversion experiment
Day-to-day (a) product formation rates, (b) cumulative turnover number (TON, mol of CO/(mol of Au25)), and (c) Faradaic efficiency during a 36 h CO2RR experiment. The electrode contained 0.96 µgAu cmgeo-1, it was operated at -1 V vs RHE, and CO2 was bubbled into solution with a flow rate of 50 mL min-1. (Reprinted with permission by American Chemical Society) (click on image to enlarge)
In a broader sense, these results provide some crucial performance metrics that are needed to evaluate potential industrial-scale CO2 conversion systems.
The scientists estimate that state-of-the-art renewable energy sources like solar-cells and wind turbines are sufficient to convert metric tonnes of CO2 per day. Data and performance estimates like these are essential for deploying CO2 conversion technology on an industrially-relevant scale.
Future directions of the team include scalability and materials development to find new CO2 conversion materials. "Our lab is one of several around the world that are working on the CO2 conversion challenge," notes Kauffman. "Continued work in this area will hopefully produce even better CO2 conversion materials with longer lifetimes and higher catalytic activity."
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