Carbon capture sorbent charges like a battery

BY ANTHONY KING | IMAGE FROM THE STUDY

Charged charcoal uses battery-like process to capture carbon dioxide from air at low temperatures.

A new type of material can absorb carbon dioxide from the air using a charged sorbent. This relies on battery-like charging to accumulate ions in the pores of low-cost charcoal (Nature, DOI: 10.1038/s41586-024-07449-2).

The voltage creates positive charges inside the graphite to which hydroxide ions bind. The hydroxide ions in the pores then capture carbon dioxide from air, forming bicarbonate.

The research, carried out in US and UK labs, was inspired by aqueous solutions of negatively charged hydroxide anions reacting with CO₂. Their biggest downside is that the carbonates formed must be heated over 800°C to release the CO₂. For the past five years, the US-UK collaboration studied how to encapsulate the hydroxides within porous solids to reduce energy costs.

The lab of Alex Forse at the University of Cambridge showed that they could be charged into charcoal to make low-cost sorbents. In the US, the lab of Phillip Milner at Cornell University, Ithaca, demonstrated fast reaction with CO₂ and stability towards oxygen in air.

The charged sponges were heated to only 90-100°C for regeneration – temperatures that can be achieved using renewable electricity.

‘This material exhibits exceptional stability compared to state-of-the-art amine-based sorbents,’ says Milner. ‘It is also very inexpensive.’ Amines are an alternative to hydroxide ions: they selectively react with CO₂ to form a range of species and are usually regenerated using heat and vacuum.

Direct air capture, which uses sponge-like materials to snag carbon dioxide, is one potential approach for carbon capture, but is often expensive.

The University of Cambridge is working on commercialising the new sorbent. ‘We could have a technology on hand that is competitive with leading amine-based sorbents and beats aqueous hydroxide solutions,’ says Milner.

‘Its main limitation right now is its relatively low uptake of carbon dioxide from air due to the low density of hydroxide anions that can be charged into the carbon,’ he adds. The collaboration is working to improve uptake.

‘At this point, we have no choice any more but to figure something like direct air capture out, because we have enough momentum that we will overshoot our emission targets,’ says Klaus Lackner, director of the Center for Negative Carbon Emissions at Arizona State University.

‘Most sorbents available are expensive,’ he says. ‘This new sorbent is remarkably cheap, and it can be easily regenerated by simply running an electric current through the graphitic material.’

‘It is an interesting way of lowering the cost of the material,’ Lackner adds. ‘There’s still a lot of R&D to be done before it’s practical, but keep in mind the huge amounts of sorbent you need to capture gigatons of carbon.’ Manufacturing those should greatly reduce costs, he concludes.