As we look to power tomorrow's devices sustainably and economically, there's a great deal of research going into the development of better, cleaner battery technology. Showing particular promise are fuel-cell and metal-air battery technologies. Though both continue to operate using the anode/cathode/electrolyte paradigm of current batteries, the race is on to find more sustainable materials that can replace those in current use. A new study published in the American Chemical Society's open-source journal ACS Omega proposes that one solution may be Popeye's favorite superfood: spinach.
Cathodes and anodes, oh my
Flow of energy when battery is in use, discharging
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Electrons travel within a battery from one electrode, called the anode, through the battery's electrolyte — either a powder or liquid barrier — to another electrode, called the cathode. The anode releases these electrons through a chemical process called oxidation, while the cathode accepts them through another, an oxygen reduction reaction. Together, this exchange is called a "redox."
The electrons' return trip back to the anode, however, requires a "load" provided by an external device, which is fine, since that device — a flashlight, a phone, or a car, for example — operates on the energy produced by the battery's electrons passing through.
The electrons travel out from the cathode's positive terminal to the device then return to the battery's negative anode terminal. In this way the energy travels round and round the battery-device circuit. (When charging a battery, electrons go in the opposite direction connected to a charger.)
The new study is concerned with the catalyst that produces the cathode's oxygen reduction reaction.
Replacing a problematic, pricey catalyst
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Fuel cell batteries and metal-air batteries use the surrounding air outside the battery as their cathode. It's clean, free, plentiful, and it works, as long as there's a catalyst that can adequately prompt the requisite oxygen reduction reaction.
The most commonly used catalysts for such batteries have been based on platinum. There are problems with these, though. Of course, platinum is expensive. Also, as the study notes, "the lack of long-term stability and the vulnerability to surface poisoning by various chemicals such as methanol and carbon monoxide, call for the development of non-Pt group metal (NPGM) catalysts."
Researchers have therefore been exploring non-toxic, carbon-based catalyst alternatives since they may be more stable and exhibit resistance to surface poisoning. And because carbon is everywhere, they'd be inexpensive to produce. However, some of the materials being investigated don't do the job as well as platinum-based catalysts. The chemical reaction they produce is slow, posing a speed bottleneck to the flow of electrons.
Enter spinach
Credit: Liu, et al./ACS Omega 2020, 5, 38, 24367-24378
Shouzhong Zou, of American University's Department of Chemistry, is the paper's senior author. The lead author is Xiaojun Liu, with Wenyue Li as co-author. Professor Zou reports:
"The method we tested can produce highly active, carbon-based catalysts from spinach, which is a renewable biomass. In fact, we believe it outperforms commercial platinum catalysts in both activity and stability. The catalysts are potentially applicable in hydrogen fuel cells and metal-air batteries."
While other catalyst research has involved plants such as rice and cattails, Zou believes spinach has a few things that make it a superior candidate as a catalyst material. For one thing, it's rich in iron and nitrogen, both essential catalyst ingredients. In addition, it's easy and inexpensive to grow, and it's abundant.
Zou and his students developed spinach-based carbon nanosheets a thousand times thinner than a human hair. The process is complex, a combination of basic and advanced techniques.
To begin, the researchers washed, juiced, and freeze-dried the vegetable before grinding it by hand into a fine powder using a mortar and pestle. Next, the spinach powder was dissolved and mixed with melamine, sodium chloride, and potassium chloride in water and cooked together at 120°C. This mixture was then rapid-cooled in liquid nitrogen and freeze-dried. Then it was pyrolized twice.
It may well have been worth the effort. Measurements of the resulting nanosheet indicated that it can out-perform platinum as a catalyst in both speed and stability. Of course, that's on top of being made from such an unassuming, inexpensive, and widely available plant.
"This work," says Zou, "suggests that sustainable catalysts can be made for an oxygen reduction reaction from natural resources." The next step for Zou and his students is to try out their spinach catalyst in prototype fuel cells to assess its performance in action. They're also looking into the use of other plant materials for catalysts.
Finally, Zou understandably hopes to develop a simple, less energy-intensive way to make their catalyst nanosheets.
Cheaper, more efficient batteries. That's what Tesla says will allow it to offer a $25,000 electric car within the next three years. The announcements came at the company's "Battery Day" event on Tuesday afternoon in Palo Alto, California.
"One of the things that troubles me the most is that we don't yet have a truly affordable car, and that is something that we will make in the future," Tesla CEO Elon Musk told a socially distanced audience, who were sitting in cars in a parking lot. "But in order to do that, we've got to get the cost of batteries down."
How to cut costs? Tesla is working on a design update for its batteries, and the company plans to begin manufacturing them in-house. (Panasonic currently produces Tesla batteries.) A key design update is removing a tab within the battery that connects the cell to what it powers.
"You actually have a shorter path length [for the electron to travel] in a large tabless cell than you have in the smaller cell with tabs," Musk said. "So even though the cell is bigger, it actually has more power."
Tesla also plans to lower costs by using nickel instead of cobalt in its cathodes. The company said its new cathode design would reduce costs by about 75 percent, and also remove waste water from the manufacturing process.
What's more, the international cobalt supply is limited, and most of it comes from the Democratic Republic of Congo, where adult and child miners are known to be exploited.
Screenshot of Tesla's 'Battery Day' presentation
Tesla
It's unclear when Tesla will stop using cobalt, or when it will stop sourcing its batteries from Panasonic. But Tesla claims that its new battery design and manufacturing changes will allow it to cut the cost per kilowatt-hour in half. If Tesla can successfully scale up production, the company could hit its goal of $100 per kilowatt-hour sooner than expected.
Hitting that mark could usher in the electric-car revolution, considering $100 per kilowatt-hour is generally regarded as the threshold the industry needs to reach in order to make electric vehicles cost competitive with gas-powered cars.
A $25,000 electric car would also be Tesla's cheapest offering by far. The company had previously promised a $35,000 car, but only offered one at that price for a limited time. Tesla's website says its Model 3, its cheapest car, starts at about $39,000.
Photo of Tesla's new battery design
Tesla
To be sure, Musk is known for promising big on his projects, but not always following through on the promised timetable. But despite having an "insanely hard" 2020, as Musk said, Tesla's had a good past couple years.
"In 2019, we had 50% growth," Musk said at the event. "And I think we'll do really pretty well in 2020, probably somewhere between 30 to 40 percent growth, despite a lot of very difficult circumstances."
It's said to be delicious, but you probably haven't encountered any durian fruit-scented candles. That's because it smells, as the late gourmand Anthony Bourdain put it, "indescribable, something you will either love or despise…Your breath will smell as if you'd been French-kissing your dead grandmother."
Eye-watering odor aside, Vincent G. Gomes of the University of Sydney and his colleagues have discovered that durian fruit has an amazing and potentially useful property: It's a natural supercapacitor. In a paper published in the Journal of Energy Storage, Gomes explains that supercapacitors are "promising for energy storage due to their superior cycling stability and excellent charge–discharge ability." Unfortunately, they also often suffer from low capacitance and stability. Stinky durian fruit and its cousin, jackfruit, don't have those issues.
We need a better battery
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Researchers have been trying to move away from existing lithium-ion batteries that contain chemicals whose interactions produce electricity. When those chemicals are depleted, what's left is a little bundle of toxic waste.
A capacitor, on the other hand, stores energy by building up a static electricity charge on the surfaces of two metal plates. (You might think of how static electricity builds up on your hair when you rub a balloon against your head, for a sense of how this works.) However, capacitors can't hold a lot of energy, nor can they hold it for long. Still, they are infinitely rechargeable, unlike lithium-ion solutions.
Supercapacitors begin to address some of these problems. They typically contain metal plates which have more surface-area and are coated with a second layer of activated charcoal or a similar material. This makes them better at soaking up and holding a charge. Still, supercapacitors are expensive to produce and have their own stability issues.
So now imagine one made of durian fruit or jackfruit. Gomes' paper describes the potential:
"The structural precision of natural biomass with their hierarchical pores, developed over millions of years of biological evolution, affords an outstanding resource as a template for the synthesis of carbon-based materials. Their integrated properties of high surface area, in-plane conductivity and interfacial active sites can facilitate electrochemical reactions, ionic diffusion and high charge carrier density."
Jacking into durian fruit
Image source: Pakjira Rongtong/pukao/Shutterstock
The bodies of the fruit batteries are made of aerogels: durian carbon aerogel (DCA) and jackfruit carbon aerogel (JCA). The process of deriving them seems complicated, but hey, science is hard.
First, the researchers scrubbed small bio samples from the spongy core of each fruit. Next, the samples were rinsed with ionized water several times to clean them. Placed in autoclaves, they were steamed hydrothermally for 10 hours at 180° C. After cooling off, they were rinsed again and then freeze-dried in a -80° C vacuum over the course of 24 hours. Following that, they were heated to 800° C and held at that temperature for an hour. Overnight ambient cooling yielding black, highly porous, ultralight aerogels. Easy-peasy.
For electrodes, each DCA and JCA battery was fitted with two electrodes, and two different electrode arrays were tested.
The first array, designed to allow an electrochemical measurement of the batteries' performance, incorporated a pair of glass substrates, each coated with an ink comprised of either DCA or JCA powder, respectively, and mixed with carbon black, polyvinylidene fluoride, or a PVDF binder.
The second electrode array used a pair of glass substrates coated with indium tin oxide, with a prepared PP (Celgard) separator between them. This architecture allowed appraisal of the battery's gravimetric capacitance.
The authors' conclusions
The paper concludes that "both electrodes are attractive candidates for the next generation, high performance, yet low-cost supercapacitors for energy storage devices derived from biowastes." In both the DCA and JCA variants, "the electrodes…displayed long-term cycling stability, and rapid charge–discharge processes. " It turns out that the durian fruit battery has a bit more power-storage capacity than its jackfruit cousin. The paper makes no mention of the final olfactory personality of the batteries.
In addition to offering proof of the potential for using durian fruit and jackfruit for energy storage, the authors point out that for the first time, they've demonstrated the development of carbon aerogels "via a facile, chemical-free, green synthesis procedure."
Carbon dioxide is a really inconvenient little molecule. It's bad to breathe, makes the oceans and rain acidic, and traps heat in the atmosphere, raising the global temperature. It also happens to be locked away in one of the most easily accessible forms of fuel. We've long known that the carbon dioxide we produce from burning fossil fuels is contributing to climate change, but there hasn't been any practical way for us to stop doing so. Fortunately, new research from MIT has identified a way for us to turn the dangerous waste product into a useful part of batteries.
The high cost of keeping CO2 out of the atmosphere
This is a big step up from prior efforts to reduce our carbon dioxide emissions. While the best way to reduce emissions is to simply produce or use less power, this option isn't quite palatable (or profitable) to most people. Instead, much of our efforts have been focused on capturing carbon dioxide before it leaves the power plant.
Generally, carbon-capture processes like this use solutions containing amine, a derivative of ammonia, to bind with carbon dioxide, preventing it from entering the atmosphere. But the problem with these solutions is that the amines and CO2 need to be separated again—this way, the amines can be reused and the CO2 can be safely stored. Unfortunately, doing so costs about 30% of the energy a power plant produces. Even if this process becomes more efficient, it will still come at the cost of lost energy and won't produce any benefit—aside from a healthier planet.
A recent article published in Joule by Betar Gallant and her research team offers a more attractive alternative: Rather than sequestering CO2 deep underground, why not make use of it to produce more energy in a clean way?
Carbon-capturing coal plants (sometimes referred to as "clean coal" plants) use amines to capture CO2 before it enters the atmosphere. This plant, the American Electric Power's Mountaineer coal plant, plans to store 100,000 tons of CO2 7,200 feet underground.
SAUL LOEB/AFP/Getty Images
Building a better battery
Battery systems are made of three primary components: a cathode, which provides electrons; an anode, which receives electrons; and an electrolyte, a substance that conducts electricity between the anode and the cathode. Researchers have had the smart idea of using CO2 as a component of the electrolyte before, but they always ran into a snag. CO2 just isn't very reactive and requires high voltages in order to conduct electricity, which is too inefficient for use as a battery. Other studies have incorporated metal catalysts into CO2-based electrolytes to make it more reactive, but these metals are expensive, and the reactions aren't very controllable.
Here's where the Gallant and her team's innovation comes in. They used the same trick from carbon-capture processes to make a CO2-based electrolyte and an associated battery system that carried a voltage comparable to modern lithium-gas batteries. Essentially, they took CO2 gas and bonded it with an amine-based solution, turning the gas into a liquid.
In this system, the anode was made of lithium, and the cathode was made of carbon. When the CO2-based electrolyte reacted with the carbon cathode, the amine was cleaved from the CO2, and CO2-derived compounds built up on the cathode. To state this simply, the battery system both used up CO2 to generate electricity and produced recycled amines that could be loaded with CO2 again.
In theory, this system could be installed in power plants and continuously fed the CO2 gas that would otherwise be emitted to the atmosphere. As in traditional carbon-capture processes, an amine solution would bond with the CO2 gas, but then it could be fed into this battery system to act as an electrolyte. As the electrochemical reaction occurs, CO2-derived compounds build up on the cathode, and the amine solution can scrub new CO2 gas, repeating the process.
This sidesteps both the expensive process of separating amines and CO2 in regular carbon-capture processes and produces a more sustainable and practical CO2 battery than has been produced before.
This scanning electron microscopy (SEM) image compares the carbon cathode before and after its use in the battery. The inset image shows the cathode in pristine condition (note that the scales in the two images are the same). The outer image shows the same cathode coated in material derived from CO2 produced during the electrochemical reaction. In a real-world situation, this material would have come from CO2 that would otherwise be emitted to the atmosphere.
MIT/courtesy of the researchers
What's next?
While this is very exciting stuff, it's important to remember that this was a proof of concept. In theory, such a system could be put into place in a power plant. But the system that the researchers built was limited in how often it could be charged and discharged. This system began to fail after about 10 charge-discharge cycles. In contrast, most lithium-ion batteries—the kind used in your smartphone—are supposed to last for about 500 cycles.
The researchers told MIT News that "lithium-carbon dioxide batteries are years away" from being used in power plants and other CO2-producing facilities. "Future challenges will include developing systems with higher amine turnover to approach near-continuous operation or long cycle life, and to increase the capacity attainable at higher powers," the researchers said.
Despite the work that must be done to make this kind of battery a reality, Gallant and her team have provided a major insight that required creative, inter-disciplinary thinking. This proof-of-concept battery represents the first time that carbon-capturing amines have been applied to a battery system, and, if future research can make similar leaps forward, we'll have greenhouse gas–powered batteries in no time.
