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Technology Uses Liquid Sodium and Air, Triples Energy Density of Current Batteries and Eliminates Carbon Emissions.
The search for more efficient and sustainable energy sources has just made an important advancement. Researchers from MIT have developed a sodium fuel cell that can not only power electric airplanes, but also capture carbon from the atmosphere.
Utilizing liquid sodium and ambient air, the system offers an energy density up to three times greater than current lithium-ion batteries, while eliminating carbon dioxide emissions.
Challenge of Current Batteries in Heavy Transportation
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This creates a challenge for the advancement of electric transportation in high energy consumption sectors, such as aviation, trains, and ships.
The new sodium fuel cell, developed by a team of MIT researchers and collaborators, emerges as a solution to this challenge.
Unlike conventional batteries, this fuel cell generates electricity through chemical reactions, but with the advantage of being quickly refueled without the need for long recharging periods.
Liquid metallic sodium, an abundant and inexpensive material, is the basis of the system, which uses common air as the second necessary element for the reaction.
Impressive Results in Laboratory Tests
In initial tests, the sodium-air fuel cell prototype showed surprising performance. It stored more than three times the energy per kilogram compared to current lithium-ion batteries used in electric vehicles.
The findings were published on May 27 in the journal Joule by doctoral students Karen Sugano, Sunil Mair, Saahir Ganti-Agrawal, and Professor Yet-Ming Chiang and colleagues.
According to Chiang, this new technology has the potential to be revolutionary, especially in aviation, where weight is a crucial factor.
For regional flights, which represent the majority of domestic trips and about 30% of aviation emissions, an energy density of 1,000 watt-hours per kilogram would be sufficient. Current lithium-ion batteries reach about 300 watt-hours per kilogram.
Potential for Marine and Rail Transportation
In addition to aviation, the sodium-air fuel cell also has potential for marine and rail transportation.
These sectors require high energy density and low cost, characteristics that the new system can offer.
The choice of metallic sodium was made precisely because it is a material that is easy to obtain and widely available.
Fundamental Difference: Fuel Cell Instead of Battery
For decades, scientists have sought to develop lithium-air and sodium-air batteries but have encountered difficulties in making them efficiently rechargeable.
The innovation from MIT lies in applying the concept as a fuel cell rather than a traditional battery.
In a fuel cell, the energy materials enter and exit the system during operation, unlike sealed batteries.
Prototypes with Functional Designs
The team developed two laboratory prototypes. The first, called Cell H, uses two vertical glass tubes connected by a central tube containing solid ceramic electrolyte and a porous air electrode.
Liquid metallic sodium fills one of the tubes, while air circulates through the other, providing oxygen for the reaction.
The second prototype utilizes a horizontal design, with a ceramic electrolyte tray that receives the liquid metallic sodium. The porous air electrode is fixed at the bottom of the tray, facilitating the reaction.
During tests, with a controlled humidity airflow, the system reached levels of up to 1,500 watt-hours per kilogram in the localized reaction, indicating more than 1,000 watt-hours per kilogram on a total scale.
Carbon Neutral Emissions and CO2 Capture
In addition to not emitting carbon dioxide, the MIT fuel cell has an additional advantage: it captures CO2 from the atmosphere.
The byproduct of the reaction of sodium with air generates sodium oxide, which quickly turns into sodium hydroxide upon contact with moisture.
The hydroxide, in turn, reacts with carbon dioxide to form sodium bicarbonate, the well-known household bicarbonate.
Chiang highlights that the process occurs spontaneously during the operation of the cell. Furthermore, if sodium bicarbonate is discarded in the ocean, it may help neutralize the acidity of the water, contributing to the mitigation of another impact of global warming.
Environmental Benefit at No Additional Cost
The use of sodium hydroxide to capture CO2 has been proposed before, but always with high production costs. In the new system, the hydroxide arises naturally as a byproduct of the fuel cell, eliminating the cost of the capture process.
Greater Safety Than Conventional Batteries
Another important point highlighted by Chiang is the safety of the system. Although metallic sodium is highly reactive, the design of the fuel cell keeps the reagents separated. Since there is only air on one side, there is no risk of uncontrolled reaction in case of a leak, unlike high energy density batteries where both reagents are in close contact.
Although it currently exists only at laboratory scale, the system has been designed to be easily scaled up. The research team has already created the startup Propel Aero, based in the MIT incubator, to develop the technology commercially.
The feasibility of producing metallic sodium has also been considered. In the past, the United States produced 200,000 tons of metallic sodium per year, mainly for the manufacture of leaded gasoline additives. Since sodium comes from salt, its abundance and wide global distribution facilitate extraction and use.
Rechargeable Cartridges for Practical Refueling
The proposed system envisions the use of rechargeable cartridges containing liquid metallic sodium. These cartridges would be refueled at special stations by melting the sodium at temperatures near 98°C, just below the boiling point of water, facilitating the process.
The first commercial step should be the construction of a fuel cell the size of a brick, capable of providing around 1,000 watt-hours. This would be sufficient to power large agricultural drones, serving as a proof of practical concept. The team hopes to conduct this test next year.
Importance of Humidity in the Process
Karen Sugano, responsible for much of the experiments, identified that the humidity of the air was crucial for the efficient operation of the cell. Tests with dry air and humid air showed that, under controlled humidity, the byproducts of the reaction remain liquid, making their removal easier through the airflow.
According to Saahir Ganti-Agrawal, the project is the result of the combination of various research areas such as fuel cells, high-temperature batteries, and sodium-air systems. This combination of knowledge has led to the significant performance gain achieved by the team.
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