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.
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.
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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