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Researchers develop a more efficient process to make fuel from CO2
8/11/2023
Researchers at MIT and Harvard University in the US report that they have developed an efficient process that can convert CO2 into formate, a liquid or solid material that can be used to power a fuel cell and generate electricity.
Previous processes to convert the greenhouse gas (GHG) into a stable fuel that can replace fossil fuels in some applications have had problems with low carbon efficiency, or they produce fuels that can be hard to handle, toxic or flammable, explain the researchers.
According to MIT Professor Ju Li, other approaches to converting CO2 into fuel usually involve a two-stage process – first the gas is chemically captured and turned into a solid form as calcium carbonate, then later that material is heated to drive off the CO2 and convert it to a fuel feedstock such as carbon monoxide (CO). That second step has very low efficiency, typically converting less than 20% of the gaseous CO2 into the desired product.
By contrast, the new process is reported to achieve a conversion of well over 90% and eliminates the need for the inefficient heating step by first converting the CO2 into an intermediate form, liquid metal bicarbonate. That liquid is then electrochemically converted into liquid potassium or sodium formate in an electrolyser that uses low-carbon electricity. The concentrated liquid potassium or sodium formate solution can then be dried, for example by solar evaporation, to produce a solid powder that is highly stable and can be stored in steel tanks for years or even decades.
Several steps of optimisation developed by the team made all the difference in changing an inefficient chemical-conversion process into a practical solution, comments Li.
The new process is described in the journal Cell Reports Physical Science.
The process of carbon capture and conversion involves first an alkaline solution-based capture that concentrates CO2, either from concentrated streams such as from power plant emissions or from very low-concentration sources, even open air, into the form of a liquid metal-bicarbonate solution. Then, using a cation-exchange membrane electrolyser, the bicarbonate is converted into solid formate crystals with a carbon efficiency of greater than 96%.
These crystals have an indefinite shelf life. By comparison, even the best available hydrogen storage tanks allow the gas to leak out at a rate of about 1%/d, precluding any uses that would require year-long storage, Li says. Methanol, another widely explored alternative for converting CO2 into a fuel for fuel cells, is a toxic substance. Formate, on the other hand, is considered benign according to national safety standards.
According to the researchers, the formate fuel produced using their new process ‘can potentially be adapted for anything from home-sized units to large scale industrial uses or grid-scale storage systems’.
The whole process has been demonstrated at a small, laboratory scale. However, the researchers expect it to be scalable to provide emissions-free heat and power to individual homes and even be used in industrial or grid-scale applications in the future.
