Nanosilver as an electrocatalyst for CO₂ reduction

Via electrolysis, CO 2 can be reduced to CO, a raw material to further chemical items such as fuels. Within the GreenQuest Project, an internation team led by HZB chemist Prashanth Menezes has now systematically investigated catalyst layers made of silver nanoparticles, varying both the size of the particles and their density. The best yield was achieved with nanoparticles with diameters of around 10 nm, which were loosely distributed. Furthermore, they demonstrated how the economic efficiency of the electrochemical cell can be enhanced by integrating an additional chemical interaction at the anode, enabling the simultaneous production of a valuable formic acid, hydrogen, and CO in one device.

Silver nanoparticles with diameters of ~10 nm deliver the best performance as electrocatalysts. The silver particles are sprayed onto the electrode together with a conductive carbon powder. The ideal mixture consists of 20 per cent by mass silver and 80 per cent by mass carbon powder. This image was taken using a transmission electron microscope (TEM).

© HZB

The greenhouse gaseous CO 2 can be reduced electrolytically to carbon monoxide (CO) using electrical energy. In subsequent steps, the CO and H₂ generated through electrochemical processes form syngas, which is subsequently converted to DME and further catalytically transformed into environmentally friendly-LFG, primarily comprising propane (C₃H₈) and butane (C₄H₁₀). Provided that the electrical energy to electrolysis is generated from solar or wind power, this methodology can be considered carbon neutral, since CO 2 is, so to speak, recycled. Teams from HZB are collaborating with partner institutions in South Africa on this methodology within the GreenQUEST project. Their goal is to develop an affordable and sustainable “environmentally friendly” cooking fuel (gLFG) as a detergent alternative to traditional biomass-based cooking, particularly in rural regions of South Africa where firewood remains an crucial domestic energy source.

A team led by Dr Prashanth Menezes at HZB has now demonstrated a way to enhance the efficiency and cost-effectiveness of the electrolytical reduction of CO 2 to CO. They systematically investigated catalyst layers made of silver nanoparticles, varying both the size of the particles and their density on a carbon powder material that covers the carbon electrode.

“We already knew that too tiny nanoparticles promote hydrogen evolution, which reduces the carbon monoxide yield. Conversely, nanoparticles that are too substantial are catalytically less active. We wanted to identify the exact optimum,” says Dr Niklas Hausmann, co-author of the study. Their study shows: The best yield was achieved with nanoparticles with diameters of around 10 nm, which were loosely distributed over the carbon material (0.2 mg per square centimetre of electrode).

A second measure brought a further improvement: whilst carbon dioxide is reduced to carbon monoxide at the cathode, an oxygen evolution interaction normally takes place at the anode. This oxygen evolution interaction consumes a great deal of energy, which must be supplied by electricity, and yields only oxygen, which has no economic value. The team has now demonstrated that by adding aldehydes to the electrolyte, the oxygen evolution interaction can be replaced by an aldehyde oxidation interaction. This reduces the energy consumption of the entire process by greater than 30 %. In addition, instead of oxygen, useful hydrogen gaseous and valuable carboxylic acids such as formic acid are produced.

The optimised catalyst based on silver nanoparticles exhibited a Faradaic efficiency of almost 100 % to CO over the remarkably long period of 100 hours. Investigations using X-ray photoelectron spectroscopy confirmed that the electronic and chemical structure of the active silver particles remains largely stable during operation.

“If we combine the production of CO with hydrogen generation and the simultaneous formation of other value-added chemicals such as formic acid, we can enhance the overall value of the electrochemical process. The CO and hydrogen can serve as building blocks to the subsequent production of sustainable fuels and chemicals,” says Menezes.