The method uses a process similar to charging a battery to charge activated charcoal, which is often used in household water filters. By charging the charcoal sponge with ions that bond with CO₂, the researchers found the charged material could successfully capture CO₂ directly from the air. They also found that the charged charcoal sponge is potentially more energy-efficient than current carbon capture approaches, since it requires much lower temperatures to remove the captured CO₂ for storage. The results are reported in the journal Nature.

‘Capturing carbon emissions from the atmosphere is a last resort, but given the scale of the climate emergency, it’s something we need to investigate,’ says Dr Alexander Forse from the Yusuf Hamied Department of Chemistry, who led the research. Direct air capture (DAC), which uses sponge-like materials to remove CO₂ from the atmosphere, is one potential approach for carbon capture, but current approaches are expensive, require high temperatures and the use of natural gas, and lack stability, say the researchers. ‘We wanted to see if activated charcoal might be an option, since it’s cheap, stable and made at scale,’ explains Forse.

Activated charcoal is used in many purification applications, such as water filters, but normally it can’t capture and hold CO₂ from the air. Inside a battery, charged ions are inserted into one of the battery’s electrodes. Forse and his colleagues hypothesised that the same thing could be achieved in an activated charcoal cloth functioning as an electrode. They imagined that ‘charging’ the inexpensive cloth with hydroxides, which have an electrical charge, would make it suitable for carbon capture, since the hydroxide ions forming in the tiny pores of the charcoal form reversible bonds with CO₂.

And so it proved. Tests of the charged charcoal sponge showed that it could successfully capture CO₂ directly from the air, thanks to the bonding mechanism of the hydroxides.

According to Forse, the rates of CO₂ capture are already comparable to existing materials. However, ‘what’s even more promising is this method could be far less energy-intensive, since we don’t require high temperatures to collect the CO₂ and regenerate the charcoal sponge’, he adds.

To collect the CO₂ from the charcoal, the material is heated to reverse the hydroxide-CO₂ bonds. Most materials currently used for direct air capture of CO₂ need to be heated to temperatures as high as 900°C, often using natural gas. However, the charged charcoal sponges developed by the Cambridge team only require heating to 90–100°C, temperatures that can be achieved using renewable electricity. In fact, they are warmed by resistive heating, which essentially heats them from the inside out, making the process faster and less energy-intensive.

The materials do, however, have limitations. ‘We are working now to increase the quantity of CO₂ that can be captured, and in particular under humid conditions, where performance decreases,’ notes Forse.