A Bold New Invention That Could Electrify Air Travel!
Interview with an innovator, and speculations on scaling up.
Air travel is freaking amazing. Flying was an archetypal impossible dream of humanity for thousands of years, a power marking gods and saints as beyond the ken of ordinary folk, and now it’s something that most people in developed countries can just choose to do for less than a month’s wages, to visit their grandmother or explore a new culture or enjoy a warm beach in the winter. Musician Christopher Tin’s modern classic Sogno di Volare, a hymn to human ingenuity that sets to music lyrics adapted from the writings of Leonardo da Vinci on the dream of flight, brings an upwelling of emotion every time I listen to it. I’ve traveled a lot in the last few years, and I try my best to remember every time that any inconveniences are minuscule compared the incredible privilege of living in a civilization where I can go see Earth from the sky.
But air travel also has some pretty gigantic negative externalities, with escalating consequences for the whole biosphere. As of 2019, air travel accounted for about 2.5% of humanity’s carbon emissions, and 4% of total human-caused global warming due to additional non-carbon factors like plane contrails trapping more heat in the atmosphere.
And that share is likely to keep going up, with air travel becoming a larger and larger contributor to climate change. That’s actually for a lot of good reasons: one reason is the world becoming wealthier, with rapid development in places like India and Indonesia meaning many more can afford air travel.
Another reason is that much of the rest of human civilization’s technology stack has become profitably electrifiable in the last twenty years. Humans can now quite easily drive cars, heat homes, and power grids with clean electrons generated by renewable energy and arbitraged across time by batteries — the barriers to deployment of this are generally political and procedural, not technological or even economic. Energy-related global carbon emissions might have peaked in 2024, and will likely enter a long-term decline, replaced by mass adoption of cleaner technologies that we need to accelerate as fast as possible.
But planes are among the least amenable to electrification of all major human activities, mostly because batteries are heavy. As electric vehicles, heat pumps, and grid-scale batteries continue to advance, air travel would become a bigger and bigger percentage of carbon emissions even if it was standing still — when in fact it’s growing fast.
People aren’t going to give up air travel, not even devoted climate activists. “Essay in which someone feels guilty about air travel emissions but ends up flying anyway” is practically its own genre at this point. Relatively affordable access to other continents in hours is too amazing to pass up. And seriously restricting or rationing air travel would clearly be politically doomed to fail, at least without providing a plausible alternative.
So what plausible alternatives do we have?
Fast-improving battery technology is already being used for a multitude of fascinating new developments in short-haul aviation, like electric heavy-lifter drones, city-level passenger-carrying eVTOLs and multiple experiments with electric small planes. It may well someday progress enough to power an industry standard-sized transatlantic jetliner — long-term, I certainly wouldn’t bet against it! But it can’t quite yet.
For routes with good overland transport links of comparable speed, cost, and quality to airports, banning some air travel routes for climate purposes can be politically feasible. In 2023, France banned air travel on three routes from Paris to Nantes, Bordeaux, and Lyon, but that didn’t cause much uproar because French high-speed rail links cover those routes in just 2.5 hours. We should definitely do a lot more high-speed rail buildouts, but that still leaves a lot of time and space uncovered.
There’s lots of innovative new designs to reduce the emissions of fuel-burning planes, often with more efficient and aerodynamic “blended wing” or “flying V” shapes.
Slightly tweaking flight paths to avoid zones where contrails are more likely to form could be a cheap and effective solution as well. This is a very new research finding that’s a highly promising candidate for corporate and policy-level activism, and contrail-minimizing flight paths should definitely be scaled up fast!
All is this is incredibly valuable work and needs to keep advancing. But it’s also all “nibbling around the edges” of the problem, addressing contrails and efficiency and routes already covered by rail lines without proposing anything that could feasibly replace jet fuel to power a fast flight. It would be great if we had another arrow in our quiver when trying to decarbonize air travel. A best-case scenario to aim for and build towards, a clear all-purpose win-win, maybe even something like the incredible solar-and-batteries pairing that provides a clear path forward for decarbonizing most of the rest of civilization.
The beginning spark of something like that may have just arrived, with one truly extraordinary innovation: a brand-new lab-scale sodium-air fuel cell1 invented just a few months ago by MIT engineers. It reacts liquid sodium with oxygen to produce electricity, potentially solving the battery weight problem by collecting much of its fuel mass (the oxygen) from the air in-flight. That effectively results in an energy density three times higher than the current best crop of EV batteries!
Furthermore, the emissions from this process — sodium hydroxide — naturally react with atmospheric carbon dioxide to form sodium carbonate, which can further react with water and carbon dioxide to form sodium bicarbonate, commonly known as baking soda, which when falling into the ocean will help counteract ocean acidification. This opens the tantalizing possibility that air travel could someday switch from a major emissions source to become another electrified industry, with a scalable climate action and ocean health project as a free by-product!
If this can scale up and commercialize, it could become a very big deal very fast. MIT is already helping new startup Propel Aero to further develop the fuel cell with drones, and they’ve already been awarded a federal ARPA-E grant through 2026 (which appears to still be extant) to support work on this “Redox Engine.”
Here’s the full study, as published in Joule.
When I first reported on this research in June, a lovely bit of serendipity happened: one of the authors, MIT materials science PhD student Saahir Ganti-Agrawal, mentioned in the comments that he was both a longtime newsletter subscriber and a coauthor of the new study describing the novel sodium-air fuel cell.
I started corresponding with Saahir via email and he was kind enough to write up his first-person account of the lab’s innovative work!
The Weekly Anthropocene Interviews: Saahir Ganti-Agrawal, Sodium-Air Fuel Cell Co-Inventor
In the interview below, this writer’s questions and comments are in bold, Mr. Ganti-Agrawal’s words are in regular text, and extra clarification (links, etc) added after the interview are in bold italics or footnotes.
Can you tell me the story of how you and your team invented this sodium-air fuel cell? How did this research start? Was there a "eureka" moment? Were there challenges to overcome in making it work? What was it like to be in "the room where it happened" on this?
In 2023 my advisor, Yet-Ming Chiang, was discussing potential ideas for metal-air battery systems with his collaborators Billy Woodford and Kailash Raman at the battery startup Form Energy. [That’s the renowned iron-air battery startup!] He had the idea that sodium-air cells could be designed to form sodium oxides on the outside of the cell, which would make it possible to remove them during cell operation.
With this system, sodium could be used as an electrochemical fuel, where sodium is fed into a cell and sodium oxides are removed. The sodium-air system also has about 10 times the theoretical energy density of a lithium-ion battery, meaning that achieving even 50% of the theoretical energy density would be a massive improvement compared to lithium-ion.
Since sodium is one of the most abundant elements on Earth (there’s a virtually inexhaustible supply of salt in the ocean), a cell that takes sodium as an input and discharges sodium oxide as an output could avoid recharging, which is one of main roadblocks preventing the viability of sodium-air batteries. Instead, once the sodium supply in the cell is exhausted, additional sodium metal could be loaded into the cell.
Conveniently, sodium metal has a very low melting point of 98 degrees Celsius, so it can be loaded into a cell as a liquid. Another convenient fact about sodium is that there are multiple commercially available sodium-conducting solid electrolytes, which are ceramic materials that can transport sodium metal through their crystal lattice.
The third convenient fact about sodium is that it will react with CO2 in the environment to produce sodium carbonate (soda ash) or sodium bicarbonate (baking soda), so we knew that operating the cell could have the added co-benefits of capturing CO2 and counteracting ocean acidification (since sodium carbonate and sodium bicarbonate are alkaline compounds).
When Prof. Chiang explained this research idea to Karen Sugano, Sunil Mair, and me (the three graduate students working on this project), we had a general idea of what the cell should look like: it would have a sodium metal electrode (the anode), a sodium-conducting solid electrolyte layer, and an air electrode (the cathode). On discharge, sodium ions would flow out of the sodium anode, through the solid electrolyte, and react with oxygen at the air electrode to form a sodium-oxygen discharge product.
The cell would be operated at above 98C so that the sodium anode is liquid – this would help us avoid one of the main issues encountered in solid-state batteries, where running large currents can form voids between the anode and the electrolyte, which impede further current flow and cause cell failure. Liquid sodium will easily flow into any void forming between it and the solid electrolyte, which prevents a well-known problem with solid metal electrodes, that of void buildup at the interface which eventually leads to open-circuit failure.
While we had a good idea of our cell’s anode and electrolyte design, we did not know initially what the best cathode would be. The cathode needed to be some material that was electrically conductive, helped catalyze reactions between sodium and oxygen, and formed sodium-oxygen products in such a way that these products could be removed from the cell.
The cathode design was the main challenge that Karen, Sunil, and I had to work on. Our initial cells used a coating of gold on top of the solid electrolyte as a cathode – the gold was electrically conductive, allowing us to connect wires to the cell, and it acted as a catalyst to help oxygen react with sodium ions (provided by the solid electrolyte).
In our initial tests, we observed that operating the cell with humid oxygen resulted in much better power output than dry air, and we noticed that the surface of the gold cathode was visibly wet when it was exposed to humidity. We also observed that the cell produced sodium hydroxide as a discharge product in the presence of any humidity/water vapor.
We figured out that the reason the cell looked wet was because sodium hydroxide exhibits a property called “deliquescence”, where it is capable of absorbing so much moisture from the air around it that it forms a liquid solution. Furthermore, this liquid solution (depending on its concentration) can have a much higher boiling point than pure water, which means that if sufficient humidity is supplied, sodium hydroxide can form a liquid solution even at temperatures like 120 C.
This was the main “eureka” moment for our work, because forming sodium hydroxide solution meant that there was now a clear way to remove the cell’s discharge product during operation – the liquid could simply drain out of the cell. Furthermore, forming a liquid hydroxide product meant that we could use air electrodes that are similar to those used in preexisting fuel cell technology (which operate in aqueous solutions).
Thus, using humidified air, an off-the-shelf fuel cell air electrode, and operating above the melting point of sodium metal, we demonstrated a sodium-air fuel cell with power performance that was roughly 100 times better than previous literature for rechargeable sodium-air batteries, and energy density that was around 50% of the theoretical maximum for pure sodium-air.
Working on this technology has been an incredible experience for me as a young engineer. I really enjoy working with the whole team and exercising my scientific creativity. At the start of the project, when Yet told me about the idea and I started working on it with Karen and Sunil, I had no idea how we would ever make a device that demonstrates high energy and power densities. Spending the past two years working on making this technology work has been one of my most satisfying intellectual experiences.
Sam’s Commentary
The sodium-air fuel cell that Saahir worked on is incredibly new, still a lab-scale invention with lots and lots more work to be done to equip it as a power source for vehicles. Like other commonly used energy materials including lithium and gasoline, sodium metal is famously explosive, so careful safety engineering will be required (although that shouldn’t be too much of a problem). It’s still very, very early days.
Nevertheless, I think this moment is akin to Bell Labs’ invention of the first modern silicon solar cell in the 1950s or John Goodenough’s lithium-ion battery breakthrough in 1980. Small and underreported now, but a world-changing technology in the future!
Saahir’s and his teammates’ paper conducts a fascinating techno-economic analysis of the economic prospects for sodium-air fuel cells. There isn’t a lot of sodium metal commercially produced right now, but it’s incredibly abundant and quite cheap and easy to acquire: sodium is the fifth-most abundant metal on Earth and you can isolate it with simple electrolysis of common sea salt, of which the oceans provide a functionally limitless supply. Back in the leaded gasoline era2, we even used to produce lots of sodium metal as part of the process for making tetraethyl lead, so we know how to make and ship large volumes already. The MIT researchers estimate that sodium could be mass-produced again for $0.80 to $1 per kilogram, compared to jet fuel’s current production at an equivalent of $0.94 per kilogram. Sodium isn’t quite as energy dense as jet fuel, so you’d need a bit more of it, but in the realm of decarbonization tech it’s amazing for a lab-scale alternative to be this close in cost to the fossil fuel standard right at the start!
There’d still be big upfront costs in developing and deploying sodium-air fuel cell-powered vehicles, but once you have that, the major fuel source would be close to cost-competitive with jet fuel right off the bat!
Once sodium-air fuel cell technology is fine-tuned and optimized enough to be ready for field deployments (which is a lot to do, I’m certainly not trying to minimize the vast amount of hard work engineers will be putting into this!), I think it will also be able to “slot in” to existing energy and civil infrastructure systems in a variety of useful ways.
The headline result of this breakthrough is the possibility of electric aviation, which is eminently understandable because it’s *finally* a fossil fuel-free technology with the kind of energy density we need to power a large plane, while most potential land applications are already well served by batteries. But there are many other enticing potential applications! As the original paper (and an early commentator on my previous newsletter) point out, ships are a similar hard-to-decarbonize sector, and the immediate ocean access could make it a natural fit for early fuel cell deployments. I could readily imagine this becoming an exciting new addition to commercial, research, and perhaps even recreation vessels, providing extra clean power while giving the feel-good (and brand-boosting) knowledge that you’re helping fight ocean acidification with every voyage.
I suspect that generating electricity on land will be overwhelmingly dominated by solar plus batteries by the time sodium-air fuel cells get to market, but it would likely still find a useful niche — perhaps as an “emergency backup” if grids go down akin to small-scale generators today.
Plus, as desalination becomes a more important and more economic way of getting fresh drinking water due to a warming climate and cheaper clean electricity, electrolyzing sodium from the remaining brine instead of sending it back to the ocean could be an efficient co-utilization of existing infrastructure, once there’s a market for sodium as a fuel. And if you’re already making sodium on site, perhaps you could set up a sodium-air fuel cell to provide some local power and then drop the sodium hydroxide by-product into the nearby ocean — turning the desalination plant into an anti-ocean acidification plant as well.
Over the years of writing newsletters on climate action, I’ve been fortunate enough to watch from a distance and report on year-by-year progress as promising world-bettering technologies grow from wild ideas to commercial realities. Two such examples now reaching fruition are nano-engineered Superwood and Fervo Energy’s advanced geothermal. I will definitely be checking for Propel Aero and Redox Engine news in the upcoming months and years, and I’m excited and hopeful for the rise of this incredible new technology!
What makes a fuel cell different from a battery? A battery can recharge with electricity while staying about the same mass, while a fuel cell uses up its fuel (gasoline, sodium, hydrogen, whatever) and needs to be refueled with more mass.
The last country to use neurotoxin-spreading leaded gasoline (Algeria) finally ended pump sales in 2021 — another reminder that we have, can, and will fix environmental problems!
this was hugely interesting Sam - thanks for the read!
Incredibly interesting! It’s so much fun to follow the nascent technologies that end up making it all the way through - will be exciting to follow developments here. Does the paper say anything about the safety aspects of planes dumping NaOH at massive scale if this technology goes all the way? Thanks!