“`html

Battery technology is approaching its thresholds regarding the amount of energy it can hold relative to its weight. This presents a significant challenge for energy advancements and the exploration of alternative methods to power aircraft, trains, and ships. Currently, researchers at MIT and other institutions have devised a solution that may assist in electrifying these transport systems.

Rather than employing a conventional battery, the innovative idea is a type of fuel cell — which resembles a battery but can be swiftly refueled rather than needing to be recharged. In this instance, the fuel is liquid sodium metal, a cost-effective and abundantly accessible material. The opposing side of the cell utilizes regular air, which functions as a source of oxygen atoms. Between them, a layer of solid ceramic substance acts as the electrolyte, permitting sodium ions to move freely, while a porous air-facing electrode facilitates the chemical reaction between sodium and oxygen to generate energy.

In a series of trials with a prototype apparatus, the researchers demonstrated that this cell could store over three times the energy per unit weight compared to the lithium-ion batteries currently found in nearly all electric vehicles. Their discoveries are being released today in the journal Joule, in a paper co-authored by MIT doctoral candidates Karen Sugano, Sunil Mair, and Saahir Ganti-Agrawal; professor of materials science and engineering Yet-Ming Chiang; and five additional researchers.

“We anticipate that many will perceive this as a completely outrageous idea,” states Chiang, who holds the Kyocera Professorship in Ceramics. “If they don’t, I would be slightly disappointed because if individuals don’t consider something to be utterly outlandish initially, it likely won’t be that transformative.”

And this innovation does seem to have considerable revolutionary potential, he suggests. Specifically, for aviation, where weight is critically important, such an enhancement in energy density might be the breakthrough that finally renders electrically-powered flight feasible on a large scale.

“The benchmark needed for viable electric aviation is approximately 1,000 watt-hours per kilogram,” Chiang explains. Current lithium-ion batteries for electric vehicles reach about 300 watt-hours per kilogram — far from the necessary threshold. Even at 1,000 watt-hours per kilogram, he adds, that would still fall short for transcontinental or trans-Atlantic journeys.

That’s still unattainable with any existing battery technology, but Chiang mentions that achieving 1,000 watts per kilogram could enable regional electric aviation, which represents approximately 80% of domestic flights and 30% of aviation emissions.

This technology could also empower other industries, including marine and rail travel. “They all demand very high energy density, along with low costs,” he asserts. “That’s precisely why we were drawn to sodium metal.”

A substantial amount of research has been invested in developing lithium-air or sodium-air batteries over the last thirty years, yet making them fully rechargeable has proven challenging. “People have long understood the energy density potential of metal-air batteries, and it has been incredibly appealing, yet practical realization has remained elusive,” Chiang observes.

By implementing the same fundamental electrochemical principles, only as a fuel cell rather than a battery, the researchers managed to achieve the benefits of high energy density in a workable format. Unlike batteries, whose components are assembled once and sealed, a fuel cell allows the energy-carrying materials to flow in and out.

The team created two distinct versions of a laboratory-scale prototype of the system. One design, referred to as an H cell, consists of two vertical glass tubes connected by a central tube housing a solid ceramic electrolyte material and a porous air electrode. Liquid sodium metal fills one tube, while air circulates through the other, supplying oxygen for the electrochemical reaction at the center, which gradually consumes the sodium fuel. The alternative prototype features a horizontal setup, with a tray of electrolyte material holding the liquid sodium fuel. The porous air electrode, which enables the reaction, is attached to the tray’s base.

Tests conducted with an air stream featuring meticulously controlled humidity levels produced over 1,500 watt-hours per kilogram at the individual “stack” level, which would correspond to more than 1,000 watt-hours at the whole system level, Chiang states.

The researchers envision using this system in aircraft where fuel packs, containing stacks of cells comparable to cafeteria food trays, are inserted into the fuel cells; the sodium metal within these packs undergoes a chemical transformation as it supplies power. The resulting chemical byproduct is expelled, and in the case of aircraft, this would be released from the rear, similar to the exhaust of a jet engine.

However, a significant distinction exists: there would be no carbon dioxide emissions. Instead, the emissions, which consist of sodium oxide, would actively absorb carbon dioxide from the atmosphere. This compound would combine swiftly with moisture in the air to create sodium hydroxide — a substance frequently used as a drain cleaner — which readily reacts with carbon dioxide to form a solid, sodium carbonate, which subsequently forms sodium bicarbonate, commonly known as baking soda.

“A natural series of reactions unfolds when beginning with sodium metal,” Chiang states. “It’s entirely spontaneous. We don’t need to take any additional steps to initiate it; we merely need to operate the aircraft.”

As an additional advantage, if the end product, sodium bicarbonate, ends up in the ocean, it could aid in reducing the acidity of the water, mitigating another harmful impact of greenhouse gases.

The utilization of sodium hydroxide to capture carbon dioxide has been suggested as a carbon emission mitigation strategy; however, it has standalone economic limitations due to the compound’s expense. “In this instance, it’s a byproduct,” Chiang clarifies, making it virtually free and delivering environmental advantages without associated costs.

Critically, the new fuel cell is fundamentally safer than many existing batteries, he notes. Sodium metal is highly reactive and requires thorough protection. Like lithium batteries, sodium can ignite spontaneously when in contact with moisture. “Whenever a battery possesses a very high energy density, safety concerns arise, as membrane ruptures separating the two reactive components may lead to uncontrolled reactions,” Chiang explains. However, in this fuel cell, one side merely consists of air, “which is diluted and constrained. Consequently, there are not two concentrated reactants adjacent to one another. If one strives for exceptionally high energy density, a fuel cell is preferable to a battery for safety reasons.”

While the device currently exists solely as a small, single-cell prototype, Chiang asserts that scaling the system to practical sizes for commercialization should be reasonably uncomplicated. Members of the research team have already established a company, Propel Aero, to further develop the technology. The enterprise is presently situated within MIT’s startup incubator, The Engine.

Producing sufficient sodium metal to allow for widespread, large-scale global implementation of this innovation should be feasible since the material has been manufactured at significant scales previously. In the era of leaded gasoline, prior to its phase-out, sodium metal was employed to create tetraethyl lead as an additive, with production in the U.S. reaching capacities of 200,000 tons annually. “This reminds us that sodium metal was once produced in large quantities and safely managed and distributed throughout the U.S.,” Chiang mentions.

Furthermore, sodium primarily derives from sodium chloride,
“““html

or sodium, thus it is plentiful, broadly distributed globally, and easily sourced, unlike lithium and other substances utilized in contemporary EV batteries.

The design they foresee would feature a refillable cartridge, which would be filled with liquid sodium metal and sealed. When it reaches depletion, it would be taken back to a refill station and replenished with new sodium. Sodium melts at 98 degrees Celsius, just below the boiling point of water, making it straightforward to heat to the melting point to recharge the cartridges.

Initially, the strategy is to develop a brick-sized fuel cell capable of delivering around 1,000 watt-hours of energy, sufficient to power a sizable drone, to validate the concept in a practical format that could be employed in agriculture, for instance. The team aspires to have such a demonstration prepared within the coming year.

Sugano, who undertook much of the experimental research as part of her doctoral dissertation and will now be involved with the startup, mentions that a critical realization was the significance of humidity in the procedure. While she experimented with the device using pure oxygen, and subsequently with air, she discovered that the humidity level in the air was vital for optimizing the electrochemical reaction. The moist air caused the sodium to produce its discharge products in liquid form instead of solid, thereby facilitating their removal through the air flow in the system. “The crucial point was that we can generate this liquid discharge product and extract it easily, unlike the solid discharge that would occur in arid conditions,” she asserts.

Ganti-Agrawal remarks that the team integrated insights from various engineering disciplines. For instance, although there has been considerable research on high-temperature sodium, there has been no exploration of a system with regulated humidity. “We’re drawing on fuel cell research regarding our electrode design, incorporating older high-temperature battery studies as well as emerging sodium-air battery investigations, and somewhat merging it together,” which resulted in “the significant enhancement in performance” the team has accomplished, he explains.

The research group also comprised Alden Friesen, an MIT summer intern from Desert Mountain High School in Scottsdale, Arizona; Kailash Raman and William Woodford from Form Energy in Somerville, Massachusetts; Shashank Sripad of And Battery Aero in California, and Venkatasubramanian Viswanathan from the University of Michigan. The project was endorsed by ARPA-E, Breakthrough Energy Ventures, and the National Science Foundation, utilizing facilities at MIT.nano.

“`