Liquid Fuels from Carbon Dioxide

Challenge: CO₂ to C₂₊ Products

Most of our global energy demands are still being met by burning fossil fuels, which contributes to the greenhouse effect through the release of CO₂. To reduce global warming, we must look for opportunities to use CO₂ as a raw material for basic chemicals.

Through electrocatalytic conversion of CO₂ using renewable energy, a climate-neutral, artificial carbon cycle could be established. Excess energy produced by photovoltaics and wind energy could be stored through the electrocatalytic production of fuels from CO₂. These could then be burned as needed. Conversion into liquid fuels would be advantageous because they have high energy density and are safe to store and transport. However, the electrocatalytic formation of products with two or more carbon atoms (C₂₊) is very challenging.

Electrocatalyst Converts CO₂ into Multicarbon Products

A team from Foshan University, Guangdong, China, the University of Science and Technology of China, Hefei, and Xi’an Shiyou University, China, led by Fei Hu, Tingting Kong, Jun Jiang, and Yujie Xiong has developed an electrocatalyst (a-CuTi@Cu) that efficiently converts CO₂ to liquid fuels with multiple carbon atoms (C_2–4). The primary products are ethanol, acetone, and n-butanol.

To make the electrocatalyst, thin ribbons of a copper/titanium alloy are etched with hydrofluoric acid to remove the titanium from the surface. This results in a material named a-CuTi@Cu, with a porous copper surface on an amorphous CuTi alloy.

It has catalytically active copper centers with remarkably high activity, selectivity, and stability for the reduction of CO₂ to C₂₊ products (total faradaic efficiency of about 49 % at 0.8 V vs. reversible hydrogen electrode for C_2–4, and it is stable for at least three months). In contrast, pure copper foil produces C1 products but hardly any C²⁺ products.

The reaction involves a multistep electron-transfer process via various intermediates. In the new electrocatalyst, the inactive titanium atoms below the surface actually play an important role; they increase the electron density of the Cu atoms on the surface. This stabilizes the adsorption of *CO, the key intermediate in the formation of multicarbon products, allows for high coverage of the surface with *CO, and lowers the energy barrier for di- and trimerization of the *CO as new carbon–carbon bonds are formed.