Researchers have identified fuel cell solutions that will help electrify transportation systems and overcome the limitations of traditional batteries.
Fuel cells triple the energy per pound of today’s best electric car batteries, providing a lightweight option to power trucks, planes, or ships.
In contrast to batteries, fuel cells can be refueled immediately, rather than being charged. In this case, the fuel is liquid sodium metal, which is inexpensive and widely available.
In a series of experiments using prototype devices, researchers have demonstrated that the cells can carry more than three times the energy they weigh more than three times the weight of the lithium-ion batteries used in almost every electric vehicle today.
Create a breakthrough with electric aviation
Researchers have discovered that fuel cells could be revolutionary in the aviation sector. In aviation where weight is particularly important, such improvements in energy density could ultimately be a breakthrough that makes electric flight practical on a large scale.
“The threshold really needed for realistic electric aviation is about 1,000 watt hours per kilogram,” explains Ming Chiang, professor of materials science and engineering at MIT.
“Today’s lithium-ion batteries for electric vehicles are about 300 watts-hour batteries per kilogram. There’s nothing close to what you need. Even 1,000 watts-hours per kilogram is not enough to allow for transcontinental or transatlantic flights.”
While this is still out of reach of known battery chemistry, Chiang said achieving 1,000 watts per kilogram would be an effective technology for electric aviation in the region, which accounts for around 80% of domestic flights and 30% of air emissions.
Fuel cell technology also serves as an enabler for other sectors, such as marine and rail transport.
Fuel cell technology and traditional batteries
Over the past 30 years, a considerable amount of research has been carried out to develop lithium or sodium air batteries. However, achieving full rechargeability has proven challenging.
“People have known the energy density you can get from metal air batteries for a very long time, and while it was very appealing, it has never actually been realized,” Chen said.
Applying the same basic electrochemical concept, but using fuel cells instead of batteries, researchers were able to exploit the benefits of high energy density in a practical way. Unlike batteries, where the material is once incorporated and sealed in a container, the fuel cell’s energy-carrying material is replenished.
Can fuel cells be expanded for commercial use?
The researchers have envisioned that using the system, fuel packs containing stacks of cells will be inserted into the fuel cell, similar to racks of food trays in cafeterias. The sodium metals in these packs undergo chemical conversion to provide power.
A stream of chemical byproducts is then released, and in the case of an aircraft, this is released from the rear, like exhaust from a jet engine.
However, there are very big differences. There is no carbon dioxide emissions. Instead, the emissions made up of sodium oxide actually absorb carbon dioxide from the atmosphere.
This compound quickly binds to moisture in the air to form sodium hydroxide, and reacts easily with carbon dioxide to form the solid material sodium carbonate. This forms sodium bicarbonate, also known as baking soda.
An additional benefit is that when the final product, sodium bicarbonate, reaches the ocean, it helps to remove water and counteracts another damaging effect of greenhouse gases.
Importantly, Chen said the new fuel cells are inherently safer than many other batteries.
So far, devices have only existed as small, single-cell prototypes, but Chiang says that the system should be very simple to expand to a practical size for commercialization.
Members of the research team have already set up companies to develop technology. It is currently housed in the engine, the startup incubator at MIT.
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