Sparse layers of 10-nanometer silver particles have been shown to enhance the production of carbon monoxide (CO) from carbon dioxide (CO₂) electrolysis. Within the GreenQUEST project, an international team led by HZB chemist Prashanth Menezes investigated catalyst layers made of silver nanoparticles, varying both particle size and density. Their work, published in the journal Advanced Functional Materials, found that the best yield came from loosely distributed nanoparticles with diameters of around 10 nm. The team also demonstrated that adding a chemical reaction at the anode can improve the economic efficiency of the electrochemical cell, enabling the simultaneous production of formic acid, hydrogen, and CO in one device. The greenhouse gas CO₂ can be reduced electrolytically to CO using electrical energy. In subsequent steps, the CO and H₂ generated through electrochemical processes form syngas, which is converted to dimethyl ether (DME) and then catalytically transformed into green-LFG, primarily comprising propane (C₃H₈) and butane (C₄H₁₀). If the electrical energy for electrolysis comes from solar or wind power, this technology can be considered carbon neutral because CO₂ is recycled. Teams from HZB are collaborating with partner institutions in South Africa on this technology within the GreenQUEST project. Their goal is to develop an affordable and sustainable "green" cooking fuel (gLFG) as a cleaner alternative to traditional biomass-based cooking, particularly in rural regions of South Africa where firewood remains an important household energy source. A team led by Menezes at HZB has now demonstrated a way to improve the efficiency and cost-effectiveness of the electrolytic reduction of CO₂ 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 large are catalytically less active. We wanted to identify the exact optimum," says Dr. Niklas Hausmann, co-author of the study. The study shows that the best yield came from nanoparticles with diameters of around 10 nm, loosely distributed over the carbon material (0.2 mg per square centimeter of electrode). While carbon dioxide is reduced to carbon monoxide at the cathode, an oxygen evolution reaction normally takes place at the anode. This reaction 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 adding aldehydes to the electrolyte can replace the oxygen evolution reaction with an aldehyde oxidation reaction. This reduces the energy consumption of the entire process by more than 30%. In addition, instead of oxygen, useful hydrogen gas and valuable carboxylic acids such as formic acid are produced. The optimized catalyst based on silver nanoparticles exhibited a Faradaic efficiency of almost 100% for CO over 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 improve the overall value of the electrochemical process. The CO and hydrogen can serve as building blocks for the subsequent production of sustainable fuels and chemicals," says Menezes. Publication details: Venkata S. R. K. Tandava et al, Decoupling the Size and Loading Effects in Silver Nanoparticles for Efficient Paired Carbon Dioxide and Formaldehyde Electrolysis, Advanced Functional Materials (2026). DOI: 10.1002/adfm.78365.