Direct conversion of carbon dioxide into a solid fuel

2023-11-03 20:45:25

There is great interest in finding ways to extract carbon dioxide from the air or the exhaust gases of thermal power plants and other large emitters and turn it into something useful. One of the most promising ideas is to convert it into a stable fuel that can replace fossil fuels in some applications. But most of these conversion processes have had problems with low efficiency, or produce fuels that are difficult to handle, dangerously toxic or overly flammable.

Now, researchers from the Massachusetts Institute of Technology (MIT) and Harvard University, both institutions in the United States, have developed an efficient process that can convert carbon dioxide into formate, a solid material (also manageable in liquid form) that It can be used like hydrogen or methanol to power a fuel cell and generate electricity.

Sodium or potassium formate, which is already produced on an industrial scale and is commonly used as road and sidewalk antifreeze, is non-toxic and non-flammable, easy to store and transport, and can remain stable in ordinary steel tanks to be used months, or even years, after its production.

The new process has been developed by a team including, among others, Zhen Zhang and Ju Li from MIT, as well as Dawei Xi from Harvard University.

The entire process (which includes the capture and electrochemical conversion of the gas into a solid formate powder, which is then used in a fuel cell to produce electricity) has already been demonstrated on a small scale, in the laboratory. Now it needs to be adapted for operation on a larger scale, so that it can provide emissions-free heat and electricity to homes and even be used in industrial applications or on the electricity grid.

Other approaches to converting carbon dioxide into fuel typically involve a two-stage process: First, the gas is chemically captured and converted into a solid form such as calcium carbonate. That material is then heated to expel the carbon dioxide and convert it into a combustible raw material such as carbon monoxide. That second step has very low efficiency, typically converting less than 20% of the carbon dioxide gas into the desired product.

Instead, the new process achieves a conversion well above 90% and eliminates the need for the inefficient heating step by first converting the carbon dioxide into an intermediate form, essentially liquid bicarbonate. That liquid is then electrochemically converted into liquid sodium or potassium formate using a solar- or wind-powered machine. From the liquid form the solid can be converted by drying, for example using evaporation. The solid form is a very stable powder that can be stored in ordinary steel tanks for years or even decades.

Conceptual diagram of the process. Above, a home powered by a formate fuel cell is shown. The formate fuel is stored in an underground tank. In the middle, the fuel cell appears that uses the formate to supply electricity. Below right, the device that converts bicarbonate to formate is shown. (Image: Shuhan Miao, Harvard Graduate School of Design. CC BY-NC-ND 3.0)

Zhang and his colleagues present the technical details of the new process in the academic journal Cell Reports Physical Science, under the title “A carbon-efficient bicarbonate electrolyzer.” (Source: NCYT from Amazings)