Researchers at MIT have developed a new, efficient way to capture carbon that addresses the inherent inefficiencies (earlier post) of incumbent technologies, due to their thermal energy losses, large footprint or degradation of sorbent material.

In an open-access paper in the journal Energy & Environmental Science, they report a solid-state faradaic electro-swing reactive adsorption system comprising an electrochemical cell that exploits the reductive addition of CO₂ to quinones for carbon capture. Their device is compact and flexible, obviates the need for ancillary equipment, and eliminates the parasitic energy losses by using electrochemically activated redox carriers.

The device is essentially a large, specialized battery that absorbs carbon dioxide from the air (or other gas stream) passing over its electrodes as it is being charged up, and then releases the gas as it is being discharged.

The electrochemical cell with a polyanthraquinone–carbon nanotube composite negative electrode captures CO₂ upon charging via the carboxylation of reduced quinones, and releases CO₂ upon discharge. The cell architecture maximizes the surface area exposed to gas, allowing for ease of stacking of the cells in a parallel passage contactor bed.

In operation, the device alternates between charging and discharging, with fresh air or feed gas being blown through the system during the charging cycle, and then the pure, concentrated carbon dioxide being blown out during the discharging.

Schematic of a single electro-swing adsorption electrochemical cell with porous electrodes and electrolyte separators. The outer electrodes, coated with poly-1,4-anthraquinone composite, can capture CO₂ on application of a reducing potential via carboxylation of quinone, and release the CCO₂O2 on reversal of the polarity. The inner polyvinylferrocene-containing electrode serves as an electron source and sink for the quinone reduction and oxidation, respectively. Credit: RSC, Voskian and Hatton (2019)

The MIT team demonstrated the capture of CO₂ both in a sealed chamber and in an adsorption bed from inlet streams of CO₂ concentrations as low as 0.6% (6,000 ppm) and up to 10%, at a constant CO₂ capacity with a faradaic efficiency of >90%, and a work of 40–90 kJ per mole of CO₂ captured, with great durability of electrochemical cells showing <30% loss of capacity after 7000 cycles.

Most methods of removing carbon dioxide from a stream of gas require higher concentrations, such as those found in the flue emissions from fossil fuel-based power plants. A few variations have been developed that can work with the low concentrations found in air, but the new method is significantly less energy-intensive and expensive, the researchers say. Their entire system operates at room temperature and normal air pressure.

The binary nature of the adsorbent’s affinity to carbon dioxide is the greatest advantage of this technology over most other carbon capture or carbon absorbing technologies, said MIT postdoc Sahag Voskian, who developed the work during his PhD.

In other words, the electrode material, by its nature, “has either a high affinity or no affinity whatsoever,” depending on the battery’s state of charging or discharging. Other reactions used for carbon capture require intermediate chemical processing steps or the input of significant energy such as heat, or pressure differences.

This binary affinity allows capture of carbon dioxide from any concentration, including 400 parts per million, and allows its release into any carrier stream, including 100 percent CO₂.

—Sahag Voskian

That is, as any gas flows through the stack of these flat electrochemical cells, during the release step the captured carbon dioxide will be carried along with it. For example, if the desired end-product is pure carbon dioxide to be used in the carbonation of beverages, then a stream of the pure gas can be blown through the plates. The captured gas is then released from the plates and joins the stream.

In some soft-drink bottling plants, fossil fuel is burned to generate the carbon dioxide needed to give the drinks their fizz. Similarly, some farmers burn natural gas to produce carbon dioxide to feed their plants in greenhouses. The new system could eliminate that need for fossil fuels in these applications, and in the process actually be taking the greenhouse gas right out of the air, Voskian says.

Alternatively, the pure carbon dioxide stream could be compressed and injected underground for long-term disposal, or even made into fuel through a series of chemical and electrochemical processes.

In my laboratories, we have been striving to develop new technologies to tackle a range of environmental issues that avoid the need for thermal energy sources, changes in system pressure, or addition of chemicals to complete the separation and release cycles. This carbon dioxide capture technology is a clear demonstration of the power of electrochemical approaches that require only small swings in voltage to drive the separations.

—T. Alan Hatton, the Ralph Landau Professor of Chemical Engineering

In a working plant—for example, in a power plant where exhaust gas is being produced continuously—two sets of such stacks of the electrochemical cells could be set up side by side to operate in parallel, with flue gas being directed first at one set for carbon capture, then diverted to the second set while the first set goes into its discharge cycle.

By alternating back and forth, the system could always be both capturing and discharging the gas. In the lab, the team has proven the system can withstand at least 7,000 charging-discharging cycles, with a 30 percent loss in efficiency over that time. The researchers estimate that they can readily improve that to 20,000 to 50,000 cycles.

The electrodes themselves can be manufactured by standard chemical processing methods. While today this is done in a laboratory setting, it can be adapted so that ultimately they could be made in large quantities through a roll-to-roll manufacturing process similar to a newspaper printing press, Voskian says. He estimates it could be produced for something like tens of dollars per square meter of electrode.

Compared to other existing carbon capture technologies, this system is quite energy efficient, using about one gigajoule of energy per ton of carbon dioxide captured, consistently. Other existing methods have energy consumption which vary between 1 to 10 gigajoules per ton, depending on the inlet carbon dioxide concentration, Voskian says.

The researchers have set up a company called Verdox to commercialize the process, and hope to develop a pilot-scale plant within the next few years.

Resources

• Sahag Voskian and T. Alan Hatton (2019) “Faradaic electro-swing reactive adsorption for CO₂ capture” Energy Environ. Sci. doi: 10.1039/C9EE02412C