CO2 recycling: What is the role of the electrolyte?

The EDX measurement shows that at higher concentrations of dissolved potassium compounds in the electrolyte, potassium crystals are deposited on the cathode (right upper corner).

The EDX measurement shows that at higher concentrations of dissolved potassium compounds in the electrolyte, potassium crystals are deposited on the cathode (right upper corner). © HZB

SEM-image of  the copper cathode at low potassium concentration (left) and at higher potassium concentration (right) in the electrolyte.

SEM-image of  the copper cathode at low potassium concentration (left) and at higher potassium concentration (right) in the electrolyte. © HZB

The architecture of the "zero-gap" electrolysis cell.

The architecture of the "zero-gap" electrolysis cell. © HZB

The greenhouse gas carbon dioxide can be converted into useful hydrocarbons by electrolysis. The design of the electrolysis cell is crucial in this process. The so-called zero-gap cell is particularly suitable for industrial processes. But there are still problems: The cathodes clog up quickly. At the HZB, Matthew Mayer and his team has now investigated what causes this and how this undesirable process can be prevented.

The combustion of oil, coal or natural gas produces carbon dioxide, or CO2. This famous greenhouse gas is a major driver of global warming, but it is also a raw material. It is technically possible to convert CO2 into useful carbon compounds, a process which requires energy, water, suitable electrodes and special catalysts. CO2 can be electrochemically converted to carbon monoxide, formate or methane, but also to ethylene, propanol, acetate and ethanol. However, industrial processes must be designed to be highly selective and extremely efficient to produce only the desired products and not a mixture of products.

Converting CO2 back into fuel

"By electrolytically reducing CO2 to useful hydrocarbons, we can produce new fuels without using fossil resources. We thus are putting the CO2 back into the cycle, just like recycling," explains Dr Matthew Mayer, leader of the Helmholtz Young Investigator Group “Electrochemical Conversion” at HZB. The electrical energy for the electrolysis can be provided by renewable energy from wind or solar, making the process sustainable.

The zero-gap cell: a sandwich of many layers

From school, we know electrolysis can be done in a simple beaker of water; a further development of this is the H-cell, which is shaped like the letter H. However, such cells are not suitable for industrial use. Instead, industrial electrolysers are designed with a sandwich architecture consisting of several layers: On the right and left are the electrodes that conduct the current and are coated with catalysts, a copper-based gas diffusion layer that lets in the CO2 gas, and a separation membrane. The electrolyte (here supplied at the anode and called anolyte) consists of dissolved potassium compounds and allows ions to move between the electrodes. The membrane is designed to allow negatively charged ions to pass through and to block positively charged potassium ions.

The problem: potassium crystals

Nevertheless, potassium ions from the electrolyte pass through the membrane and form tiny crystals at the cathode clogging the pores. "This shouldn't happen," says Flora Haun, a PhD student in Matthew Mayer's team. Using scanning electron microscopy and other imaging techniques, the scientists were able to study the process of crystal formation at the cathode in detail. "With energy-dispersive X-ray analysis, we were able to locate the individual elements and show exactly where potassium crystals were forming," Flora Haun explains.

The more potassium the electrolyte contains, the more the cathode becomes clogged, the investigations showed. But there is no simple way to solve the problem: reducing the potassium concentration is good on the one hand, but bad on the other, since the reaction equilibrium also shifts: instead of the desired ethylene, carbon monoxide is produced.

The electrolyte is the key

"The most important observation is that cations can still penetrate the anion exchange membrane, but to an extent that depends on the concentration of the electrolyte. And that with the concentration of the electrolyte we simultaneously regulate which products are formed from the CO2," says Dr. Gumaa El Nagar, a postdoctoral researcher in the team. "In the next step, we want to use operando and in situ measurements using X-rays to find out in detail how ion migration in the cell affects the chemical reaction processes," says Matthew Mayer.

arö


You might also be interested in

  • Green Deal Ukraina: HZB launches an Energy & Climate Project
    News
    07.06.2023
    Green Deal Ukraina: HZB launches an Energy & Climate Project
    Green Deal Ukraina, funded by the German Federal Ministry of Education and Research, is working with partner institutions in Ukraine and Poland to establish an energy and climate think tank in the capital, Kiev. The aim is to provide independent and evidence-based advice on rebuilding a sustainable energy system in Ukraine. After all, the implementation of energy and climate legislation is a prerequisite for Ukraine's accession to the EU. The project started on 1 June 2023 and will run for four years.
  • Spintronics at BESSY II: Domain walls in magnetic nanowires
    Science Highlight
    02.06.2023
    Spintronics at BESSY II: Domain walls in magnetic nanowires
    Magnetic domains walls are known to be a source of electrical resistance due to the difficulty for transport electron spins to follow their magnetic texture. This phenomenon holds potential for utilization in spintronic devices, where the electrical resistance can vary based on the presence or absence of a domain wall. A particularly intriguing class of materials are half metals such as La2/3Sr1/3MnO3 (LSMO) which present full spin polarization, allowing their exploitation in spintronic devices. Still the resistance of a single domain wall in half metals remained unknown. Now a team from Spain, France and Germany has generated a single domain wall on a LSMO nanowire and measured resistance changes 20 times larger than for a normal ferromagnet such as Cobalt.
  • Fractons as information storage: Not yet quite tangible, but close
    Science Highlight
    26.05.2023
    Fractons as information storage: Not yet quite tangible, but close
    A new quasiparticle with interesting properties has appeared in solid-state physics - but so far only in the theoretical modelling of solids with certain magnetic properties. An international team from HZB and Freie Universität Berlin has now shown that, contrary to expectations, quantum fluctuations do not make the quasiparticle appear more clearly, but rather blur its signature.