From CO2 to green chemicals—researchers are one step closer

Too much CO₂ in the atmosphere is one of the major culprits of global warming. But imagine if we could convert captured CO₂ into valuable green chemicals—and we can. The conversion is made possible through CO₂ electrolysis, but the process is complex and costly.

Researchers are now one step closer to the development of a technology that can transform captured CO₂ into useful green chemicals such as ethylene and ethanol, which can be used in the production of plastic.

Most of the plastics we currently use are produced from fossil-based chemicals, which are responsible for around 5% of our global CO₂ emissions. CO₂ electrolysis offers a green alternative to fossil-based chemicals while also using captured CO₂ as a resource. This means the technology has great potential to play a role in society’s green transition.

“We have found out why and where in the chemical process of electrolysis of CO/CO₂ the electrolysis device degrades. Our results provide clear guidelines for researchers and the industry on how to extend the durability of devices for CO/CO₂ electrolysis, which will strengthen the commercialization of the technology,” says DTU Professor Brian Seger.

The results have recently been published in the international journal Nature Catalysis under the title “Identifying and alleviating the durability challenges in the membrane-electrode-assembly for high-rate CO electrolysis”.

Electrolysis using CO₂
To understand the importance of the results, we first need to understand the principles of electrolysis. Electrolysis enables you to separate matter into its basic elements or make new chemical compounds. It is a known chemical process that takes place by adding an electric current to an electrolyte, which is a solution or molten compound that conducts electricity.

Here, the positive ions of the electrolyte will be attracted to a cathode, while the negative ions of the electrolyte will be attracted to an anode. In electrolysis of, e.g., water (H₂O), the anode in a water-filled electrolysis vessel will attract oxygen (O₂), while the cathode will attract hydrogen (H₂), splitting the water into its basic components.

The first intermediate product produced from CO₂ electrolysis is CO (carbon monoxide). This is followed by a CO electrolysis, whereby the valuable chemicals ethanol (alcohol, which can be used as fuel) and ethylene (hydrocarbon, which can be used to make the plastic material polyethylene) can be produced.

Durability increased from half a day to 100 hours
CO₂ electrolysis is a complex multi-step process, and several factors can affect the effectiveness of the process. A particular challenge is the breakdown of the anode in the electrolyte solution, which leads to the device failing after about half a day’s use. This makes the process very costly and difficult to scale up for industrial use.

But since it is precisely the durability of the anode that the researchers have increased, there may be good news on the way for companies working with CO₂ electrolysis.

The researchers have shown that one of the biggest causes of anode degradation is the production of acetate on the cathode, where the environment is alkaline. This causes acetic acid to form at the anode, thus lowering the pH. If the material on the anode cannot handle the now low pH of the electrolysis solution, it will degrade, and the electrolysis device will fail after about 12 hours’ use.

By removing the acetate and thereby maintaining the pH of the electrolyte solution, the researchers have found that the durability of the anode can be extended from half a day to more than 100 hours. While the researchers achieved this by manually replacing the electrolysis solution every 12 hours when the pH became too low, a simple filter could solve this when commercialized.

“Our guidelines tell researchers and the industry that they need to monitor the pH value on the anolyte side in order to maintain a pH that does not corrode the anode. This is a simple but crucial point for the companies who are already starting to commercialize the technology”, says DTU Professor Brian Seger.

FACT BOX:
 

CO₂ electrolysis

Electrolysis with CO₂ is a process in which carbon dioxide (CO₂) is decomposed using electricity. This process has the potential to contribute to CO₂ reduction and the production of useful products such as hydrogen (H2) and carbon monoxide (CO) that can be used as fuels or in chemical processes. Here follows a basic explanation of CO₂ electrolysis:

Electrolyte cell: The process requires an electrolyte cell in which CO₂ is present as gas or dissolved in a liquid, such as water (H₂O). The electrolyte cell contains an anode and a cathode, as well as an electrolyte.

Anode and cathode: The anode is the positive electrode, and the cathode is the negative electrode. They are made of materials that can conduct electricity and withstand corrosion. Typically, materials such as platinum or graphite foam are used.

Electric power source: An external power source, such as a battery or power supply, connects to the anode and cathode to create an electrical circuit.

Electrochemical reaction: When an electric current runs through the electrolyte, an electrochemical reaction occurs at the anode and cathode. At the anode, CO₂ molecules are oxidized to carbonate ions (CO₃^2-) or carbonic acid (H₂CO₃), depending on the conditions. At the cathode, water (H₂O) is reduced to hydrogen (H₂) and possibly also to carbon monoxide (CO), depending on the process.