One of the more promising ideas for carbon capture, usage and storage (CCUS) is to make it into a stable fuel that can replace fossil fuels in some applications, an MIT announcement said. Most conversion processes have problems with low carbon efficiency however, or they produce fuels that can be hard to handle, toxic, or flammable.
Potassium or sodium formate, already produced at industrial scales and commonly used as a runway de-icer, is nontoxic, nonflammable, easy to store and transport, and can remain stable in ordinary steel tanks to be used months and even years after its production.
The new process – including capture and electrochemical conversion of CO2 to a solid formate powder, which is then used in a fuel cell to produce electricity – was demonstrated at a small, laboratory scale in the MIT-Harvard project. The researchers expect it to be scalable, so that it could provide emissions-free heat and power to individual homes, or even be used in industrial or grid-scale applications.
Other approaches to converting CO2 into fuel usually involve a two-stage process, said MIT Professor Ju Li. First, the gas is chemically captured and turned into a solid form as calcium carbonate. Later, that material is heated to drive off the CO2 and convert it into a fuel feedstock such as carbon monoxide. That second step has very low efficiency however, typically converting less than 20% of the gaseous CO2 into the desired product, Professor Li said.
By contrast, the new process reportedly achieves 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, such as nuclear, wind, or solar power.
The highly concentrated liquid potassium or sodium formate solution produced can then be dried, for example by solar evaporation, to produce a solid powder that is highly stable and can be stored in ordinary steel tanks for years or even decades, Professor Li said.
The capture and conversion process first involves an alkaline solution-based capture that concentrates CO2, either from concentrated streams such as power plant emissions or from very low-concentration sources such as open air into a liquid metal-bicarbonate solution. A cation-exchange membrane electrolyser is then used to electrochemically convert that bicarbonate into solid formate crystals with a carbon efficiency of greater than 96%, as confirmed in the team’s lab-scale experiments.
These crystals have an “indefinite shelf life”, the MIT announcement said, remaining so stable that they could be stored for years, or even decades, with little or no loss. By comparison, even the best available hydrogen storage tanks allow the gas to leak out at a rate of about 1% per day, precluding any uses that would require year-long storage, Professor Li said.
Methanol, another widely explored alternative for converting CO2 into a fuel usable in fuel cells, is a toxic substance that cannot easily be adapted for use in situations where leakage could pose a health hazard. Formate, on the other hand, is widely used and considered benign, according to US national safety standards.
Several improvements account for the “greatly improved” efficiency of the new process, the announcement said. A “careful” design of the membrane materials and their configuration overcame a problem that previous attempts encountered, where a build-up of certain chemical by-products changed the pH, causing the system to lose efficiency over time.
The new system can therefore continue operating efficiently over long periods, the researchers said. In their tests, it ran for over 200 hours with no significant decrease in output. The whole process can be done at ambient temperatures and “relatively” low pressures, they said – about five-times atmospheric pressure.
Initial household applications for the formate fuel might involve an electrolyser unit about the size of a refrigerator to capture and convert the carbon dioxide into formate, which could be stored in an underground or rooftop tank. Then, when needed, the powdered solid would be mixed with water and fed into a fuel cell to provide power and heat. “This is for community or household demonstrations,” said MIT doctoral student Zhen Zhang, “but we believe that also in the future it may be good for factories or the grid.”
The work was supported by the US Department of Energy Office of Science. It was published in an open-access paper in Cell Reports Physical Science.
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