Light-sensitive molecules called photoacids could make the process of removing carbon dioxide from the atmosphere more energy efficient. Researchers are now devising ways to make photoacids more practical to use.

This could be especially valuable for direct air capture (DAC) systems, which blow air over carbon-capturing materials called sorbents. Existing systems require a large amount of energy to separate pure CO2 from sorbents in order to store or use it elsewhere. This poses a major barrier to using DAC to remove billions of tonnes of CO2 from the atmosphere each year. “That step is hitting a wall,” says at ETH Zurich in Switzerland. “Every single direct air capture company is struggling and trying to make the most efficient process.”

Adding photoacids to the sorbents could help. When illuminated, each photoacid molecule changes shape and releases a proton, making the solution more acidic. This “pH swing” frees CO2 from the sorbent-photoacid mixture. When the lights go out again, the photoacids – and the solution’s pH – revert, allowing the sorbent to absorb CO2 once more. Then the cycle can be repeated.

Usually heat or pressure is used to release the CO2, but using sunlight or lamps could slash the energy needed for this step, says de Vries, who aims to halve DAC’s energy requirements. However, photoacids tend to be unstable and not very soluble in water, which limits the efficiency with which they can release CO2.

De Vries and her colleagues added various solvents to a photoacid solution and that makes photoacids more soluble and extends their lifespan from just a few hours to nearly a month.

In another approach, at Oak Ridge National Laboratory in Tennessee and her colleagues a different photoacid that can maintain its response to light longer and produce more acid, enabling it to release CO2 from a solution more efficiently.

at Newcastle University in the UK says these are “elegant and innovative” solutions. However, he says a scaled-up system could face challenges – for example, losing solvent to evaporation in the air.

While these researchers focused on capturing CO2 from the atmosphere, the first larger-scale test for photoacids may happen in water. A start-up called Banyu Carbon in Washington state is using photoacids to separate CO2 from seawater, with plans to install a system that can remove one tonne of CO2 per year in 2024.

In this system, photoacids are exposed to light and the resulting acidity is temporarily transferred to seawater, causing the water to release CO2 absorbed from the atmosphere. , the company’s co-founder, says this cuts the energy needed to separate out CO2 and eliminates the need to power fans.

A mixture of cement and charcoal powder could enable houses to store a full day’s worth of energy in their concrete foundations. This new way of creating a supercapacitor – an alternative to batteries that can discharge energy much faster – could be incorporated into the foundations of both buildings and wind turbines. When paired with renewable energy sources, it could also someday let concrete road foundations wirelessly recharge electric vehicles as they drive along.

“The materials are available for everyone all over the place, all over the world,” says at the Massachusetts Institute of Technology (MIT). “Which means we don’t have the same restriction as with batteries.”

Ulm and his colleagues showed how cement and carbon black – a very fine version of charcoal – can mix with water to create a hardened block containing many branching, wire-like structures filled with the carbon. When the concrete is soaked in a common electrolyte solution such as potassium chloride, the charged particles from the electrolyte settle on the carbon-wire structures to provide energy-storing potential.

They then turned two thin slabs of the material just 1 centimetre wide and 1 millimetre thick into a supercapacitor by separating them with a thin insulating layer such as paper. Connecting three of these supercapacitors produced the equivalent of a 3-volt battery capable of lighting up a small LED.

The researchers say that the next step might be to create 12-volt supercapacitors that can also be connected to provide more charging power for larger devices. They calculated that a concrete block equivalent to a cube 3.5 metres on each side could store 10 kilowatt-hours of energy. That is about a third of the and about 1.25 times the .

The material maintained its charging and discharging capabilities beyond 10,000 cycles, which means, in theory, that it could provide energy storage for a solar-powered home for more than 27 years.

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One engineering complication is that traditional concrete slabs would need to be replaced by the equivalent of “concrete plywood” made with the supercapacitors, says at Drexel University in Pennsylvania. He suggested that keeping the supercapacitor wetted with the conductive salt solution for the lifetime of the building or road would be challenging.

Still, the MIT team expressed optimism about how many people worldwide could start experimenting with this relatively simple blueprint for a low-cost supercapacitor.

“The fundamental aspect of this technology is it’s two historical, ancient materials that come together, that we have known for millennia”, says at MIT.