To minimize the effects of climate change, in addition to ceasing the burning of fossil fuels, we must remove significant amounts of the CO2 already pumped into the atmosphere.
One solution is direct air capture devices, which use materials that attract and bind CO2 molecules from the atmosphere. For this solution to be economically viable though, the trapped CO2 must be released so it can be properly stored or else reused. The problem is many direct air capture methods currently require extremely high temperatures to release the captured CO2.
To reduce this energy demand, a team from ETH Zürich have developed a system that captures carbon and releases it using just light. “We have to work collectively to ensure a viable and energy efficient carbon capture solution appears soon,” said Maria Lukatskaya, professor at ETH and co-author of the study. “It is clear that existing technologies are still too energy intensive.”
One molecule, two possibilities
The new technology uses light to release the captured carbon, made possible using a unique molecule called a photoacid.
“Photoacids are organic molecules that change their acidity, depending on being exposed to light or darkness,” explained Anna de Vries, a doctoral student at ETH Zürich and lead author of the paper describing how the team used photoacids to change the pH of a solution which, depending on whether it is acidic or basic, will either capture or release carbon.
A basic, alkaline solution can capture CO2 because at a high pH, CO2 dissolves and forms ionic molecules known as bicarbonates. In a low pH, acidic solution, bicarbonate is reverted to CO2, which can be captured and responsibly stored — which is where the photoacids come in. When a light—even a normal ceiling light will do—is shone on photoacids, they react by releasing a hydrogen proton. The more protons in a solution, the more acidic it becomes.
Using the photoacids, the team made a basic solution to capture CO2 in the dark and then release it when a light is switched on and the photoacids lower the pH. Amazingly the pH shift happens in minutes and is completely reversible; switch off the light and the pH goes back up to basic, meaning the solution can again attract CO2.
Finding the right mix
However, there was still one problem to overcome: the stability of the photoacids in solution. “It was already known that in water, [photoacids] hydrolyze,” said de Vries. “They degrade quite fast.”
In her original experiments, the photoacids only lasted about 24 hours. To extend their lifespan, de Vries turned to an aprotic solvent used commonly in chemical labs called dimethylsulfoxide or DMSO. “Chemists often think of DMSO as the solvent that dissolves everything,” said de Vries.
Photoacids were already shown to be soluble in DMSO and it did increase their stability in water, but as an aprotic solvent, DMSO by nature does not readily give up protons. According to de Vries, this trait diminishes the reversibility of the pH change because the presence of DMSO lowers the chances that a released proton will come back when the lights are switched off.
Determining an optimal ratio of water and DMSO for the system required careful experimentation. To better understand how the solvent and photoacids were interacting, the team turned to the expertise of computational chemists at Sorbonne University who were able to calculate where the DMSO would likely be around the photoacid.
Thankfully, it naturally protects the photoacid at the bond that is prone to degradation. With this information, the team could determine an optimum amount of DMSO to provide protection and still allow for reversibility.
Carbon capture powered by the sun
The lifespan of the photoacids is now up to three weeks. In another proof-of-concept experiment, de Vries brought the system to the roof and allowed it to function using only the change from daylight to night. “It worked perfectly,” she said, “and we’re in Switzerland, not near the equator or anything.”
Eventually, this is how the group envisions the technology will be applied, using natural energy from the Sun. Before it reaches building tops around the world though, the team must continue to tweak the chemistry and optimize the solution to make it an economically viable option.
For de Vries and the Lukatskaya team at ETH, the goal is not to replace already existing technology but rather add another potential solution to the mix that utilizes natural energy. Like renewable power generation, no one carbon capture method can meet demand.
“It’s never like that in this field because we need all the different options,” said de Vries. “Every technology has advantages and disadvantages.”
Reference: Maria R. Lukatskaya, et al., Solvation-Tuned Photoacid as a Stable Light-Driven pH Switch for CO2 Capture and Release, Chemistry Materials (2024). DOI: 10.1021/acs.chemmater.3c02435
Feature image credit: Efe Yağız Soysal on Unsplash