Supercapacitor boost for carbon capture
Researchers in Cambridge have developed a low-cost supercapacitor that can selectively capture carbon dioxide gas while it charges.
The supercapacitor device, which is similar to a rechargeable battery, is the size of a two-pence coin, and is made in part from sustainable materials including coconut shells and seawater.
“The trade-off is that supercapacitors can’t store as much charge as batteries, but for something like carbon capture we would prioritise durability,” said researcher Grace Mapstone from the Department of Chemistry (above, right). “The best part is that the materials used to make supercapacitors are cheap and abundant. The electrodes are made of carbon, which comes from waste coconut shells.
“We want to use materials that are inert, that don’t harm environments, and that we need to dispose of less frequently. For example, the CO2 dissolves into a water-based electrolyte which is basically seawater,” she said.
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When the electrodes become charged, the negative plate draws in the CO2 gas, while ignoring other emissions, such as oxygen, nitrogen and water, which don’t contribute to climate change. Using this method, the supercapacitor both captures carbon and stores energy.
This would use renewable energy in a carbon capture system, with the CO2 released in a controlled way when the supercapacitor discharges and collected to be reused or disposed of responsibly.
The symmetric supercapacitor is built with the electrolyte sandwiched between a bottom current collector and a titanium mesh contacting a top current collector, while the volume above the mesh was filled with pure CO2 gas. A potentiostat attached to the two current collectors applied a potential across the electrodes to charge the supercapacitor.
Monitoring the gas reservoir with a PX309-030A5V pressure transducer from Omega determined the takeup of CO2. Titanium was used for the current collectors and mesh to minimize corrosion. Chloride ions are particularly corrosive, so stainless steel is unsuitable for material in contact with the NaCl electrolyte.
A constant current (30 mA / g of the gas-exposed electrode) was applied until a target cell potential between the electrodes was reached. This potential was then held for 30 minutes (to give more time to equilibrate), then a 30 mA/g constant current applied to return the system to the initial potential, and a final 30 minute potential holding step.
Testing out various designs of the cell with the switching technique saw adsorption of 123mmol/kg of CO2 for an energy input of 621 kJ/mol
“We found that that by slowly alternating the current between the plates we can capture double the amount of CO2 than before,” said Dr Alexander Forse who led the research.
“The charging-discharging process of our supercapacitor potentially uses less energy than the amine heating process used in industry now,” said Forse. “Our next questions will involve investigating the precise mechanisms of CO2 capture and improving them. Then it will be a question of scaling up.”
The results are reported in the journal Nanoscale.
Dr Israel Temprano (above, left) developed a gas analysis technique for the device. This help narrow down the precise mechanism at play inside the supercapacitor when CO2 is absorbed and released. Understanding these mechanisms, the possible losses, and the routes of degradation are all essential before the supercapacitor can be scaled up.
“This field of research is very new so the precise mechanism working inside the supercapacitor still isn’t known,” said Temprano.
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