CO2 as a Sustainable Natural Resource

Researchers at TU Berlin have unveiled a combination of two electrolysis cells that can use electricity to convert carbon dioxide and water directly into basic chemicals for the chemical industry. In the first electrolysis, carbon monoxide is produced from carbon dioxide, which then combines with water in the second electrolysis cell to form hydrocarbons. The procedure uses a nickel-nitrogen-doped carbon electrode which only requires a metal content of less than one percent, making it less expensive than the usual metal catalysts. In addition, the researchers have developed a diagnostic system that monitors the condition of the tandem electrolyzer during operation, thus increasing the service life and a gaining a better understanding of the chemical processes in the cells.

Extracting carbon dioxide from the air or directly from exhaust gases and then converting it into valuable chemicals using electricity from renewable energy sources seems like an ideal way to combat the climate crisis. The key to this solution is electrolysis. Using water (H₂O) and electricity, electrolysis can reduce carbon dioxide (CO₂) to virtually pure carbon monoxide (CO), while simultaneously producing oxygen (O₂). Carbon monoxide and additional water can then react in a second step to form useful hydrocarbons such as ethylene, which consist of longer chains of carbon and hydrogen atoms.

Better yield and energy efficiency with tandem electrolysis cells

“When electrolyzing water and CO₂ to produce CO, and some hydrogen as a by-product, the proportion of CO in the total substance converted is much higher than in alternative thermal processes that use green hydrogen and CO₂ to produce CO and water at temperatures of around 800 degrees,” says Professor Dr. Peter Strasser, head of the Electrochemical Catalysis, Energy and Materials Sciences Group at TU Berlin, describing the advantages of the method. Several competing processes exist for the second step, the production of basic chemicals for the chemical industry from CO and water. “When performed electrochemically, this step has mostly been conducted in the same electrolysis cell in which the CO is produced. However, to ensure that all the necessary processes can take place in sequence, compromises must be made with the catalyst and the chemical conditions, which leads to less than ideal results. Our solution is to use two cells in tandem, one of which supplies CO, which is then converted directly into valuable hydrocarbons in the second cell.”

Depending on the catalyst formulation, the second step produces reactive hydrocarbons such as ethylene, propylene, and some acetylene, but also liquid compounds such as methanol, ethanol, and propanol or acetate.

A new carbon electrode means less metal is required

Peter Strasser and his team are also breaking new ground with the electrode that converts CO₂ into CO. Normally, silver is used as a material to catalyze the chemical reactions.

“Silver is not only rare and expensive, but it can also corrode and form films on the surface that impair its function as a catalyst,” says PhD candidate Sven Brückner, first author of the publication. “This is why we use a carbon electrode with nickel and nitrogen atoms incorporated in some places as catalysts.”

Because the carbon used is porous and therefore has a large surface area, and the atoms incorporated using a special process are only located on the surface, the proportion of nickel required can be kept extremely low – less than one percent of the total weight.

“Electrodes like these have only been around for a few years, and always have to be optimized for each respective application, which is why their service life and the maximum achievable catalyst performance are still being researched,” says Brückner.

However, because the electrode is produced at higher temperatures than the operating temperature and other chemical conditions are milder during operation than during production, he is optimistic of a successful outcome in the near future. The team hopes to significantly extend the current stability to several hundred hours.

Diagnostic tool for optimized research and longer service life

Other operating parameters of the tandem cell also need to be optimized.

“The ratio of carbon monoxide and hydrogen as a by-product in the first cell depends on the pH value of the electrolyte,” explains Sven Brückner. Because the process is intended to deliver the largest possible amount of CO as a building block for hydrocarbons, CO₂ electrolysis is carried out under alkaline conditions, which favor the formation of carbon monoxide. “The problem is that this leads to the OH ions in the alkaline electrolyte reacting with the CO₂ to form carbonate, which can accumulate and thus damage the cell.”

According to Brückner, it is therefore important to find the best possible compromise between the various electrolysis parameters.

To do this, the research team has developed a special coefficient that makes it easier to determine the optimum process conditions. Expressed as a mathematical formula, this coefficient contains the production rates of CO and hydrogen as well as the amount of CO₂ still present at the outlet of the first cell.

“We measure the concentrations of these substances in a gas chromatograph. We also determine the concentration of nitrogen gas, which we add in a defined quantity,” says Brückner.

This “calibration” can be used to determine the total flow rate through the cell.

The fact that these values can be recorded throughout the whole time the cell is in operation means that any effects arising from changes to the operating parameters can be determined immediately. At the same time, the measured values also serve as an early warning system for when the cell is at risk of failure and countermeasures are required.

CO₂ as a raw material

“The ultimate aim of this research is to establish a circular, sustainable carbon economy,” explains Peter Strasser. “One major advantage of our process is that we do not need any additional hydrogen for the direct electrolysis of CO₂ into useful hydrocarbons using electricity and water.” This is different with conventional sustainable processes, which either use hydrogen to convert CO₂ into CO or to produce hydrocarbons from CO, which requires the production, storage, and, in some cases, transportation of additional energy. “Base chemicals that use CO₂ from the air instead of petroleum as a carbon source can actually reduce the carbon dioxide content of the atmosphere under certain conditions. Green e-fuels made from CO₂ at least have a balanced carbon footprint after combustion,” concludes Strasser.

Further information

• Publication in the Nature Chemical Engineering journal

Source

TU Berlin, press release, 2024-05-13.

Supplier

Technische Universität Berlin (TU Berlin)

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