The combustion of oil, coal, or natural gas produces carbon dioxide, or carbon dioxide2. This notorious greenhouse gas is a major driver of global warming, but it is also a raw material. It is technically possible to convert carbon dioxide2 into useful carbon compounds, a process that requires energy, water, suitable electrodes, and special catalysts.
co2 It can be electrochemically converted into carbon monoxide, formate or methane, but also into ethylene, propanol, acetate and ethanol. However, industrial processes must be designed to be highly selective and highly efficient to produce only the desired products and not a mixture of products.
by electrolytic carbon dioxide reduction2 To useful hydrocarbons, we can produce new fuels without using fossil resources. So we put CO2 Back in the cycle, just like recycling,” explains Dr. Matthew Meyer, Head of the Helmholtz Young Investigator Group “Electrochemical Conversion” at HZB. Electrical energy for electrolysis can be supplied by renewable energy from wind or solar energy, making the process continuous.
Zero-gap cell: a sandwich of several layers
From school we know that electrolysis can be done in a simple beaker of water; A further twist to this is the H cell, which is H-shaped. However, these cells are not suitable for industrial use. Instead, the industrial electrolyzer is designed with a sandwich structure consisting of several layers: on the right and left are current-conducting electrodes coated with catalysts, a copper gas diffusion layer that allows carbon dioxide to enter2 gas and membrane separation.
The electrolyte (supplied here at the anode and called anolyte) consists of dissolved potassium compounds and allows the ions to pass between the electrodes. The membrane is designed to allow negatively charged ions to pass through and to prevent positively charged potassium ions.
The problem: potassium crystals
However, potassium ions from the electrolyte pass through the membrane and form small crystals at the cathode, which clogs the pores. “This shouldn’t be happening,” says Flora Haun, Ph.D. Student on Matthew Meyer’s team.
Using electron microscopy and other imaging techniques, the scientists were able to study in detail the process of crystal formation in the negative electrode. “By analyzing energy-dispersive X-rays, we were able to pinpoint the location of individual elements and show exactly where potassium crystals form,” explains Flora Haun.
Investigations showed that the more potassium the electrolyte contained, the greater the cathode blockage. But there is no simple way to solve the problem: reducing the concentration of potassium is good on the one hand, but bad on the other, since the equilibrium of the reaction also changes: instead of the required ethylene, carbon monoxide is produced.
The electrolyte is key
“The most important observation is that cations can still permeate the anion exchange membrane, but to an extent that depends on the electrolyte concentration. By the concentration of the electrolyte, we simultaneously upregulate the products that are formed from carbon dioxide.2says Dr. Jumaa Al-Najjar, a postdoctoral researcher on the team.
“In the next step, we want to use Obrando’s and in situ X-ray measurements to find out in detail how the migration of ions in the cell affects chemical reaction processes,” says Matthew Meyer.
The study has been published in the journal Nature Communications.
Juma A. Al-Najjar et al., Unintentional cation crossover affects the selectivity of CO reduction in a copper-based zero-gap electrolyzer, Nature Communications (2023). DOI: 10.1038/s41467-023-37520-x
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