Nickel catalyst opens door to sustainable, branched hydrocarbon fuels

A research team led by Associate Professor Boon Siang YEO from the Department of Chemistry at National University of Singapore (NUS) have developed a new way to turn carbon dioxide, a greenhouse gas, into valuable liquid hydrocarbons, which are the main components of fuels like gasoline and jet fuel. The research was conducted in collaboration with Professor Núria López, an expert in computational simulation from the Institute of Chemical Research of Catalonia, Spain, and Professor Javier Pérez-Ramírez from ETH Zürich, Switzerland, who brings extensive expertise in electro- and thermocatalytic fuel synthesis.

For years, scientists have searched for efficient ways to recycle carbon dioxide into energy-rich molecules, with the twin goals of cutting harmful emissions and creating sustainable fuels. Most efforts have focused on using copper as the catalytic material, as it has been shown to convert carbon dioxide into simpler products like ethylene or ethanol. However, copper has consistently fallen short in producing longer, branched hydrocarbon chains which are key components of high-quality fuels.

The team explored a different path in green fuel production by using a nickel-based material to catalyze the electrochemical reduction of carbon dioxide. By introducing a small amount of fluoride ions into the nickel structure as well as by applying pulsed potential electrolysis, they found that they could fine-tune the catalytic process. These strategies allowed them to have unprecedented control over the types of hydrocarbons produced, especially in determining whether the molecules are straight chains or have branches. Branched hydrocarbons are particularly valuable because they enable fuels to burn more efficiently and with higher performance, making them ideal for use in vehicles and aircraft.

Their findings were recently published in the journal of Nature Catalysis.

The study showcases new strategies to selectively promote the production of branched hydrocarbons. By applying a technique called pulsed potential electrolysis, where the electrical bias is varied in periodic cycles, the team was able to markedly increase the branch-to-linear ratio of hydrocarbons with five or more carbon atoms, achieving an over 400% improvement compared to standard methods. In addition, fluoride doping in the nickel catalyst helped maintain its oxidation state under reducing conditions, a key factor in promoting the formation of longer hydrocarbon chains.

Despite being extensively studied and modified over the last decade, a known limitation of copper-based catalysts is its inability to reduce carbon dioxide to appreciable amounts of long-chain hydrocarbons. A key insight from this study was understanding how nickel and copper catalysts behave differently at the molecular level. The team showed that nickel-based catalysts promote the removal of oxygen from reaction intermediates and favor asymmetric coupling between adsorbed carbon monoxide (*CO) intermediates and unsaturated hydrocarbon species. This contrasts with copper-based catalysts, which tend to convert oxygen-containing intermediates into alcohols, which halts the growth of longer hydrocarbon chains. These distinct properties mean that on nickel catalysts, the building blocks needed for longer and more complex hydrocarbons are more likely to form and link together, resulting in products that more closely resemble those made through traditional, high-temperature industrial processes such as Fischer-Tropsch synthesis.

Prof Yeo said, “This work brings together complementary expertise in catalyst synthesis, mechanistic investigation and computational modelling, which allows us to uncover new mechanisms and design strategies for carbon dioxide reduction to long-chain hydrocarbons. This work would not have been possible, if not for the intense collaboration between experimentalists and theoreticians.”

Prof Lopez indicates “None of our techniques individually is able univocal identifying key mechanistic steps it is only by combination of experimental and computational results .”

The impact of this study goes beyond advancing the fundamental understanding of carbon dioxide electroreduction mechanisms. By developing ways to precisely control the structure of hydrocarbons produced from carbon dioxide using electricity, this research opens new pathways for the development of on-demand, sustainable aviation fuels and chemical precursors. Such advances are crucial for supporting the global shift towards cleaner technologies.