Harvesting Energy From Carbon Dioxide Emissions

Energy: Device generates electricity from the entropy created when the greenhouse gas mixes with fresh air

An electrochemical cell could someday generate electricity from carbon dioxide emitted by power plants as the gas wafts into the atmosphere. Researchers demonstrate that the cell harvests energy released by the entropy created when CO2 mixes with fresh air (Environ. Sci. Technol. Lett. 2013, DOI: 10.1021/ez4000059). The device could help power plants increase electricity output without producing additional CO2.


Electricity From CO2            
 A new electrochemical cell generates electricity from carbon dioxide dissolved in water solutions. When dissolved, the gas forms carbonic acid (H2CO3), which then dissociates into H+ and HCO3 ions. These ions adsorb selectively onto one of the two electrodes (left and right), depending on the type of membrane on the electrode (yellow and red). This process generates a current between the electrodes.            Credit: Environ. Sci. Technol. Lett
Bert Hamelers of Wetsus, a research center focused on water treatment technology in Leeuwarden, the Netherlands, and his team developed the new device based on one they created to tap energy released when seawater and freshwater mix. The previous cell consisted of electrodes coated with ion-exchange membranes. As seawater and freshwater flowed through the cell, the membranes absorbed and released sodium and chloride ions, creating a current.

Hamelers realized that the same cell design could harvest the energy released when two gases mix. To do so with CO2, the team first mixed it with a liquid, using either deionized water or a 0.25 M water solution of monoethanolamine (MEA), which is often used to remove CO2 from exhaust gases. In water, the CO2 forms carbonic acid, which then dissociates into H+ and HCO3 ions. These ions act like the sodium and chloride ions in the previous entropy-harvesting device. As the solution passes through the cell, ion-exchange membranes on the cell’s electrodes absorb the ions, H+ on one electrode and HCO3 on the other. This process produces current between the electrodes.

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                       Mixing Gases            
To harvest energy from mixing CO2 and fresh air, researchers first must dissolve the gases in water solutions (CO2, right; air, left). The water then passes by membranes in an electrochemical cell (rectangular block in the middle) in alternating pulses. The cell generates electricity as ions in the solutions adsorb onto and desorb from the electrodes.    Credit: Bert Hamelers/Wetsus        
Then water with dissolved fresh air flushes through the cell. Since this water is mostly ion free, the membranes release the H+ and HCO3 ions into the water, producing current in the opposite direction as before. This now ion-laden water leaves the cell and gets flushed with air. The CO2 gas reforms and is then released. The fluidics system continually repeats this cycle, sending alternating pulses of the dissolved CO2 and dissolved air through the cell.

With the small-scale system the researchers built in their lab, they could harvest 24% of the energy released when they used deionized water and 32% when they used MEA. At its most efficient, the lab setup generates only milliwatts of power. But with a scaled-up system, the researchers calculate that power plants could produce megawatts of power using CO2 emissions. They estimate that flue gases from power plants worldwide contain enough CO2 to generate 850 TWh of energy every year.

But the system has a few obstacles to overcome before it can be used in such large-scale applications, the team and outside experts say. For example, impurities in a power plant’s flue gas, such as sulfur dioxide or nitrogen oxides, could foul the cell’s membranes. The immediate problem is getting CO2 emissions dissolved into a liquid upon exiting the stacks. With current technology, dissolving that much gas in liquid would require more energy than the researchers’ system could generate. So it will take more research to find the optimal process to dissolve CO2 using as little energy as possible, Hamelers says.

Still, the concept is “marvelous,” says Volker Presser of the Leibniz Institute for New Materials in Germany. Now the researchers “need to envision a system that can take up tonnes and tonnes of CO2,” over multiple cycles, he says. With such a system generating extra electricity, Presser says, coal plants could produce energy more efficiently, without emitting more CO2.

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