MIT had made 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. The reported device is compact and flexible, obviates the need for ancillary equipment, and eliminates the parasitic energy losses by using electrochemically activated redox carriers. An 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.
Above is a diagram of the new system, air entering from top right passes to one of two chambers (the gray rectangular structures) containing battery electrodes that attract the carbon dioxide. Then the airflow is switched to the other chamber, while the accumulated carbon dioxide in the first chamber is flushed into a separate storage tank (at right). These alternating flows allow for continuous operation of the two-step process.
Image courtesy of the researchers
An initial techno-economic analysis shows that the MIT solid state carbon capture systems can be economically feasible with costs ranging from $50–$100 per tonne CO2 depending on the feed concentrations and applications under consideration. This would theoretically be two to four times cheaper than other industrial carbon capture systems.
The lowest cost carbon capture systems are still trees, kelp and other biological methods. Biological carbon capture tends to price out at less than $1 per tonne of CO2.
This new way of removing carbon dioxide from a stream of air could provide a significant tool in the battle against climate change. The new system can work on the gas at virtually any concentration level, even down to the roughly 400 parts per million currently found in the atmosphere. 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.
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.
The cell is made of two cathode electrode substrates coated with a CO2-binding quinone–carbon nanotube (Q–CNT) composite sandwiching an anode electrode substrate coated with a ferrocene–CNT (Fc–CNT) composite, with separator membranes between the electrodes. This cell architecture is employed to maximize the CO2-binding surface area of the cell exposed to gas flow in a parallel passage adsorbent contactor design where stacks of these cells form parallel gas channels. The Fc–CNT electrode serves as an electron source and sink for the reduction and oxidation, respectively, of the Q–CNT electrodes to regulate the uptake and release of the CO2. Wetting of porous non-woven carbon fiber mats used as the electrode substrates by a room temperature ionic liquid (RTIL) electrolyte enables effective ionic currents to pass through the electrolyte on activation and deactivation of the electrodes, and permits the diffusion of CO2 into the electrolyte-wetted cathodes during capture.
MIT researchers 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% (6000 ppm) and up to 10%, at a constant CO2 capacity with a faradaic efficiency of over 90%, and a work of 40–90 kJ per mole of CO2 captured, with great durability of electrochemical cells showing less than 30% loss of capacity after 7000 cylces.