Capturing carbon dioxide for the chemical industry
High-resolution neutron imaging can uncover the workings of an electrolytic cell designed to turn the greenhouse gas carbon dioxide into useful chemicals.
Mitigating uncontrolled climate change due to carbon dioxide (CO2) emissions from burning fossil fuels is probably the major challenge for the 21st century. An exciting approach that is gaining increasing interest is to capture and re-use CO2 by converting it into useful chemicals like carbon monoxide. This gas, when mixed with hydrogen for example, is a traditional feedstock for making a wide variety of high-value products such as pharmaceuticals.
This process can be achieved by utilising a clean, renewable source of electricity in a so-called electrolyser. This is a device, which is rather like a reverse fuel cell, consists of two electrodes – a cathode and an anode – coated with a suitable catalyst and immersed in an electrically conducting electrolyte. Introducing CO2 at the cathode in the presence of water, results in the formation of carbon monoxide gas together with negative hydroxide (OH-) ions, when an electrical current is passed through; oxygen and water are generated from the hydroxide ions at the anode. The reaction is best carried out using an alkaline electrolyte, in particular potassium hydroxide (KOH), so as to suppress a competing chemical reaction at the electrodes generating hydrogen and oxygen from the water.
The most promising design for this electrochemical reactor is the zero-gap electrolyser. This is a slimline device with two flat, multilayer electrodes directly separated by a special membrane that allows the transport of negative ions like hydroxide but hinders the crossing through of the positive potassium ions. Each electrode consists of a porous layer that gases can pass into and a thin layer of a metal catalyst sitting next to the membrane. On the outer sides of the assembly are current-collecting metal plates and units containing channels to allow gases to flow to and from the electrodes. This so-called zero-gap design offers a much improved performance over conventional cells, especially at the high currents needed for commercial operation. They are already reaching pilot scale. However, there are still design issues to be overcome.
One problem is that the hydroxide ions rapidly react with the CO2 to form negative carbonate ions at the cathode which travel through the membrane dragging water molecules through as well, thus drying out the cathode. At the same time, some potassium ions slip through to the cathode, and in the dry conditions created, form small crystals of potassium carbonate and bicarbonate within the cathode pores. This prevents the CO2 from reaching the catalyst sites where the carbon monoxide formation happens. Furthermore, the carbonate particles attract water which also clogs up the pores. The performance of the cell is thus reduced, especially at high current operation.
The power of neutron radiography
To find solutions requires investigating how the water travels through the cell and where it ends up, as well as establishing how the solid carbonates build up in the pores over time and at different current intensities. Neutron radiography is an excellent technique for investigating water transport and distribution in devices like electrochemical cells. Neutrons are attenuated to varying degrees depending on a material’s composition to give an image, just as in X-ray imaging but with a different contrast. Because neutrons are particularly sensitive to hydrogen, they can locate water and other hydrogen-containing compounds inside a sample, revealed as a darker area in the neutron radiograph collected. Materials that absorb or scatter fewer neutrons are seen as differentially lighter.
We were therefore able to employ neutron imaging to investigate the internal workings of a small, specially-made cell with a silver catalyst coating on the cathode and iridium dioxide on the anode. We could evaluate the performance of the cell by operando imaging with highest spatial resolutions down to the scale of some micrometers at different currents and over time, referencing the images obtained with no current. The high-resolution images obtained at NeXT clearly revealed darker areas indicating water, and allowed us to infer where the solid carbonates were likely deposited (seen as lighter areas) with micrometre-scale resolution.
These studies thus demonstrated that neutron imaging is a powerful analytical tool in developing the optimum architecture for an effective zero-gap CO2 electrolyser.
Instruments used: ILL’s NeXT neutron and X-ray tomograph, and an intensified neutron microscopy detector from the FRM II neutron source at the Technical University of Munich.
Research paper: Joey Disch et al., “High-resolution neutron imaging of salt precipitation and water transport in zero-gap CO2 electrolysis”, Nature Communications, 2022, 13, 6099, https://doi.org/10.1038/s41467-022-33694-y.
Research team: Joey Disch, Luca Bohn, Susanne Koch, Matthias Breitwieser and Severin Vierrath (University of Freiburg, Germany), Michael Schulz and Yiyong Han (Technical University of Munich, Germany), and Alessandro Tengattini and Lukas Helfen (University of Grenoble and the ILL).