Carbon capture initiatives (CCI) are technologies which filter out CO2 from exhaust gases before they reach the atmosphere. There is substantial scientific evidence that CO2 released by human activities is causing increases in global average temperature, otherwise known as climate change. For this reason, technology to remove carbon from the atmosphere, called carbon capture, is receiving greater attention from governments and investors around the world.
There are several existing carbon capture methods that use different pressures, temperatures, and membranes to filter out CO2. Other methods involve a chemical reaction known as solvent scrubbing. However, all of these techniques are difficult to scale, require lots of energy, and have little capacity for complicated design shapes, making them hard to build into existing devices. Furthermore, most are designed for large-scale combustion and other industrial fumes that produce gases with CO2 concentrations above 10%.
Researchers Sahag Voskian and T. Alan Hatton at MIT engineered a revolutionary new carbon-capture technique called ‘electro-swing-absorption’ (ESA), which overcomes many of these existing issues. They created a specialized battery that absorbs CO2 from air as it passes through the device. In addition to their study on this device, they plan to commercialize the process through their startup company, Verdox.
The battery uses some newly engineered materials which make up a cathode (negative electrode) and an anode (positive electrode). The researchers built these from tiny particles called carbon nanotubes. They coated the anode with an iron-based chemical called ferrocene and the cathode with a large organic molecule called polyanthraquinone. They sandwiched these together into layers with channels so that gas can flow between them. The researchers analyzed the pressure of a stream of air they passed through the device during charging and discharging to test the capability for absorbing CO2. Experiments were repeated for a variety of CO2 concentrations to mimic exhaust gas and different types of ambient air and determine the effectiveness of filtration.
On charging, they found CO2 molecules bind to the cathode surfaces through a chemical process known as reduction. In reduction, the CO2 molecule gains two extra electrons (“reducing” the charge on the carbon atom) and attaches them to the chain of the polymer polyanthraquinone. On discharging, they reported the reverse. Instead, an oxidation reaction released the bound CO2 molecules from the polymer. This is essential so the device can capture new CO2 on the next charge-discharge cycle. During discharge, the battery could provide some of the original energy required for the charging process. This reduces the energy consumption of the charge/discharge process.
When two of these cells were linked together to perform reduction and oxidation simultaneously, it reduced the wait-time required between cycles. As the oxidation cell discharges, the exhaust stream containing CO2 can be fed to the reduction cell for CO2 absorption. Furthermore, pure CO2 released can be used to supply other industries. Examples include a source for carbonating drinks or for farmers who add CO2 to their greenhouses to maximise crop growth. Alternatively, the CO2 can be stored using compression tanks or by burying the CO2 underground so it does not reach the atmosphere. This prevents the CO2 from ever reaching the atmosphere.
This carbon capturing battery is unique because it can switch between absorbing or releasing CO2. Other carbon capture methods require significant amounts of time, chemical processing, or energy to regenerate before they can absorb CO2 again. The device is highly scalable and can be designed in many shapes and sizes, only requiring a power supply. The researchers found that this technology was quite durable, lasting over 7,000 cycles with less than 30% loss in capacity for absorbing CO2. The device effectively absorbed CO2 at concentrations as low as 0.8%, similar to levels expected for ventilation of confined living spaces such as space modules, submarines, and buildings.
The researchers conducted a preliminary financial analysis for their technology and concluded the device could be economically feasible for industries. Costs ranged from $50–$100 per ton CO2 depending on the feed concentrations of CO2 being fed in and the applications under consideration.
While there is a global drive to reduce fossil fuel combustion, they will continue to be combusted long into the future, whether that be by the chemical, pharmaceutical or transport industry. The 2015 Paris Agreement, therefore, heavily emphasizes the need for CO2-reducing technology. The efficiency in design and scalability of ESA carbon capture brings significant potential to support sustainable economies.