‘Crazy idea’ may lead to high-density, low-cost fuel cell
Scientists have designed a fuel cell they say could deliver three times as much energy per unit of weight as existing lithium-ion batteries used in electric vehicles.
Unlike a battery, the liquid sodium-air device needs only to be refuelled, not recharged.
Luckily, sodium (a constituent of table salt) is inexpensive and one of the most abundant elements on Earth.
The researchers envision a fully developed version of the technology could help electrify hard-to-decarbonise transport systems, such as aircraft, ships and trains.
The chemical reaction also produces useful sodium hydroxide (NaOH) as a discharge product, which could be saved and used commercially as caustic soda.
Crucially, NaOH has another property: It readily combines with carbon dioxide (CO2) to form sodium carbonate, which in turn forms sodium bicarbonate or baking soda.
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NaOH has been explored as a potential vehicle for carbon capture and use (CCU). But its high cost and carbon footprint associated with production have remained barriers to widespread adoption.
In an enclosed system, the researchers envision NaOH could be retained and later used to capture and sequester CO2 from point sources. In an open system, such as on an aircraft, NaOH could instead be discharged during operation to combine with CO2 in the atmosphere.
If the resulting sodium bicarbonate ends up in the ocean, the researchers say it would help towards deacidifying it to counter another damaging effect of CO2.
“We expect people to think that this is a totally crazy idea,” said Yet-Ming Chiang, senior author of the study and professor of Materials Science and Engineering at Massachusetts Institute of Technology in the United States.
“If they didn’t, I’d be a bit disappointed because if people don’t think something is totally crazy at first, it probably isn’t going to be that revolutionary.
“People have been aware of the energy density you could get with metal-air batteries for a very long time, and it’s been hugely attractive, but it’s just never been realised in practice.”
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In their study published in the journal Joule, Chiang and colleagues outline the production of two lab-scale prototypes of their new system.
In the H cell prototype, two vertical glass tubes are connected by a tube across the middle.
Liquid sodium metal fills the tube on one side and humid air flows through the other. The connecting horizontal tube contains a solid ceramic electrolyte, which allows sodium ions to pass freely through, and a porous air-facing electrode that helps the sodium to chemically react with oxygen to produce electricity.
The other prototype uses a horizontal design, with a tray of the electrolyte material holding the liquid sodium fuel. The porous air electrode, which facilitates the reaction, is in the bottom of the tray.
Tests using an air stream with a carefully controlled humidity level produced nearly 1700 watt-hours per kilogram at the level of an individual “stack. Chiang said that would translate to more than 1000 watt-hours at the full system level.
“The threshold that you really need for realistic electric aviation is about 1000 watt-hours per kilogram,” he said.
Reaching this, he said, would enable electric aviation for regional flights, which account for about 80 per cent of domestic flights and 30 per cent of emissions from aviation.
The technology could be an enabler for other sectors as well, including marine and rail transportation.
“They all require very high energy density, and they all require low cost,” Chiang said.
“That’s what attracted us to sodium metal.”
While the prototype is a small, single cell, Chiang said the system should be quite straightforward to scale up to practical sizes for commercialisation.
Members of the research team have formed a company, Propel Aero, to further develop the technology.
This article was first published in Cosmos. Read the original here
