Sunlight may be the secret weapon for direct CO2 capture 

Global warming demands immediate and effective actions. One critical aspect of tackling this challenge is the reduction of greenhouse gas emissions, specifically carbon dioxide (CO2). 

Traditional approaches are not sufficient to meet the ambitious climate targets set by the global community. Consequently, there is a growing emphasis on innovative methods to capture CO2 directly from the atmosphere.

Carbon capture technologies 

Current carbon capture technologies are energy-intensive and costly. This limitation highlights the urgent need for alternative, efficient methods. 

A team of researchers at ETH Zurich led by Professor Maria Lukatskaya has developed a novel approach to direct CO2 capture by harnessing the power of sunlight. 

Chemical phenomenon 

In acidic aqueous solutions, CO2 exists in its original state, while in alkaline solutions, it converts to carbonates. This reaction is reversible, depending on the solution’s acidity.

To manipulate this acidity, the ETH team introduced photoacids into the solution. These special molecules respond to light. In the presence of light, they acidify the liquid, and in darkness, they revert, making the liquid more alkaline.

Breakthrough technique 

The process begins by passing air through an alkaline solution containing photoacids in the dark, causing CO2 to react and form carbonates. 

Once a significant amount of carbonates accumulate, the solution is exposed to light, turning acidic and releasing CO2 gas, similar to the effervescence observed in a cola bottle. The released CO2 is then collected in gas tanks. After most CO2 is expelled, the light is turned off, and the cycle restarts.

Overcoming challenges 

Initially, the researchers found that the photoacids were unstable in water. “In the course of our earliest experiments, we realized that the molecules would decompose after one day,” said study lead author Anna de Vries.

To overcome this, the team experimented with a mix of water and an organic solvent, eventually finding an optimum ratio that stabilized the photoacids for nearly a month and allowed the required reversible reaction.

Advantages over traditional methods

The new technique stands out from existing carbon capture processes, which typically involve cyclical temperature-based systems requiring significant energy for heating and cooling. 

By contrast, the ETH Zurich method is more energy-efficient as it eliminates the need for heating, potentially operating solely on sunlight.

“Another interesting aspect of our system is that we can go from alkaline to acidic within seconds and back to alkaline within minutes. That lets us switch between carbon capture and release much more quickly than in a temperature-​driven system,” explained de Vries.

Looking ahead, the researchers aim to enhance the stability of photoacid molecules and optimize the process parameters for potential market application.

The study is published in the journal Chemistry of Materials. 

Image Credit: ETH Zurich

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