The threat of global climate change has brought considerable attention to the need for a reduction in anthropogenic CO2 emissions. The most significant sources for carbon dioxide emissions are electric power plants, accounting for around 40% of U.S emissions. Unlike transportation emissions, which is a collection of millions of small sources, 250 power plants comprise > 50% of the total CO2 emissions in the U.S., making them prime targets for immediate reduction in the CO2 emissions. Chemical sorption is widely viewed as the state-of-the-art technology for scrubbing CO2 from flue gas; however, it has been estimated that adding an monoethanolamine sorption system to a new pulverized coal power plant would increase the cost of electricity by 80% and derate the plant’s net generating capacity by approximately 30% (and some studies show that the energy penalty may be as high as 45%). Therefore, there is an urgent need for new technologies that approach CO2 capture from a fresh perspective.
Our work focuses on electrochemical CO2 capture by leveraging carbonate chemistry in anion exchange membranes (left figure below). Since electrochemical devices are not bound by thermochemical cycles, the thermodynamic energy requirement for our system is 80% less than sorption. The minimum energy requirement for chemical sorption is 11% of the power plant rating based on the heating requirement to produce steam and release CO2 from the amine sorbent, where the minimum energy requirement for our electrochemical separator is 2.1% of the power plant rating based on the Nernst Equation. However, improvements in both catalyst and anion-exchange membrane materials are still needed to drive the performance up and the costs down, and this is exactly what our focuses. Our overall goal for the project and its relationship to the energy demands of a powerplant are shown in the right figure below.
An illustration showing the operating principle of the AEM-based CO2 separation cell is shown in the left-figure below. The exhaust from a coal-fired or natural gas-fired power plant is fed to the cathode where the CO2 and O2 are separated from the incoming flue gas by electrocatalytic reduction to carbonate (CO32-) and bicarbonate (HCO3–). The (bi)carbonate anions are transported across an AEM, whose functional groups play a role in anionic species balance and transport mechanism by modulating the internal pH of the device, to the anode where they are electrolyzed back to O2 and CO2, forming a CO2/O2-rich stream. In the context of a power plant, the O2 would be post-combusted to yield a high purity CO2 stream, which will add some capital cost to the system, but also produce energy.