An H-cell modified with electrodes and an ion-conducting ceramic membrane to conduct sodium-air fuel cell experiments.
Researchers at the Massachusetts Institute of Technology (MIT) have developed a sodium-air fuel cell that could power future transportation, including electric aircraft. The fuel cell approach overcomes limitations of sodium-air batteries, but can also help us clean up carbon from the planet as fuel-cell-powered planes take flight, a press release said.
As the world moves away from fossil fuels, batteries have become a crucial component of the energy transition, storing renewable energy to meet energy demand when production is slow. Lithium-ion batteries are the best energy storage solution we have now. However, these batteries have also started reaching their limits regarding energy storage capacity, creating a bottleneck in developing fully electric solutions for heavy transport such as planes, ships, and trains.
Sodium-air or metal-air batteries have been touted as an alternative for higher power applications. Still, much of the technology is in the developmental stage.
Researchers at MIT, in collaboration with those from other institutes in the US, came up with a radically different approach for using this chemistry to power a fuel cell, overcoming the limitations of battery design.
Fuel cells over batteries
The researchers could extract higher energy density from the setup by switching to a fuel cell design instead of a battery.
The researchers used two vertical glass tubes in one of the lab-scale prototypes. They connected them across the middle using a solid ceramic electrolyte and a porous air electrode. One of the tubes contained liquid sodium metal while the other contained air, providing oxygen for the fuel cell reaction.
The reaction of the two components produces sodium oxide and energy. By controlling the humidity of the air stream, the researchers successfully generated 1,700 watt-hours per kilogram of each fuel cell stack. Realistically, an electric aircraft needs an energy density of 1,000 watt-hours per kilogram instead.
Unlike a battery, where reaction components are sealed inside the container, the fuel cell components go in and out. So, instead of recharging the fuel cell, it needs to be refueled, which can be attained faster, making it more feasible for commercial applications.
Although energy is dense, the reaction components are sodium and air, which are dilute but also limited. So, even unwanted interaction of the components will only result in a small reaction, unlike a thermal runaway seen in batteries, making the fuel cell a much safer option.
Representative stock image of carbon emissions that can be mopped up by sodium-air fuel cell-powered aircraft. Image credit: Schroptschop/iStock
Mopping up carbon emissions
The sodium oxide released by the fuel cell naturally reacts with the humidity in the air to produce sodium hydroxide. This, in turn, reacts with carbon dioxide in the atmosphere to form sodium carbonate and then sodium bicarbonate or baking soda, as it is commonly known.
Not only does this help mop up carbon dioxide from the atmosphere, but sodium bicarbonate can end up in oceans and help deacidify them, further negating the impact of carbon emissions. These solutions have been suggested for combating climate change, but haven’t been implemented due to high costs.
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With a sodium-air fuel cell, these can be deployed without much hassle. “There’s this natural cascade of reactions that happens when you start with sodium metal,” explained Yet-Ming Chiang, a professor of materials science and engineering at MIT, in the press release. “It’s all spontaneous. We don’t have to do anything to make it happen; we must fly the airplane.”
Unlike lithium in contemporary batteries, which is available in certain regions and needs to be extracted through extensive processing, sodium can be sourced from salt in any part of the world.
The researchers plan to build brick-sized fuel cells to power large drones to showcase their concept and then work toward powering larger aircraft and ships.
The research findings were published in the journal Joule.
Ameya Paleja Ameya is a science writer based in Hyderabad, India. A Molecular Biologist at heart, he traded the micropipette to write about science during the pandemic and does not want to go back. He likes to write about genetics, microbes, technology, and public policy.
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