MIT develops game-changing H fuel process
Scrap aluminum may have just transformed the playing field Metal Tech News - August 18, 2021
Last updated 8/31/2021 at 3:12pm
As the world aspires to move away from fossil fuels and minimize carbon emissions, researchers at the Massachusetts Institute of Technology have developed a new method to generate clean hydrogen fuel using scrap aluminum and water.
Hydrogen fuel cells function by combining hydrogen and oxygen in an electrochemical process to produce electrical energy and water. While an incredible source of energy, with the only byproduct being water vapor, the extraction of hydrogen is not green.
Almost all hydrogen is produced using fossil fuel-based processes that generate more than 2% of all global greenhouse gas emissions. Additionally, it is often made in one location and consumed at another, presenting several costly logistical challenges that have diminished this clean fuel's adoption.
Also, the storage of hydrogen is more complex than that of a simple barrel of crude and still manages to be more dangerous, as the gas can burn in air at oxygen concentrations ranging from 4% to 75%.
Furthermore, raw materials such as platinum and iridium are typically required as catalysts in fuel cells and some types of water electrolyzers, making them costly. Combine that with the lack of defined regulatory frameworks, and investors struggle to see the worth without applicable commercial deployment models.
To solve some of these issues, researchers have long discovered using aluminum to react with water to generate hydrogen gas. However, this presents its own slew of problems.
Two of the most prominent issues that have kept aluminum from being employed as a safe, economical source for hydrogen power are unsoiled aluminum (meaning it is clean so it can better react with water) and the production of pure aluminum, as it is energy-intensive to mine and produce, contradicting the benefits.
Therefore, researchers have sought to use scrap aluminum as a workaround, but once again, this provided new difficulties.
As scrap aluminum is usually in an alloyed form, it reacts with water differently depending on the other elements added to change the properties or characteristics of aluminum. For example, adding magnesium increases strength and corrosion resistance, and adding silicon lowers the melting point.
Despite considerable research on aluminum as a source of hydrogen, two key questions remain:
What is the best way to prevent the adherence of an oxide layer on the surface of aluminum (the thin layer that covers pure aluminum when it comes into contact with oxygen), and how do alloying elements in a piece of scrap aluminum affect the total amount of hydrogen generated and the rate at which it is generated?
"If we're going to use scrap aluminum for hydrogen generation in a practical application, we need to be able to better predict what hydrogen generation characteristics we're going to observe from the aluminum-water reaction," said Laureen Meroueh, Ph.D. in mechanical engineering, who helped develop this latest method of hydrogen fuel production.
Testing the aluminum waters
To prepare, samples of pure aluminum and specific alloys – combinations of silicon and magnesium common in scrap aluminum – were used by the MIT researchers to perform tests.
They started by combining two metals – gallium and indium – in specific proportions to create a "eutectic" mixture, that is, a mixture that would remain in liquid form at room temperature. This cocktail of critical metals was then painted on the aluminum samples, allowing them to permeate through the grain boundaries (solid aluminum is made up of tiny grains that are packed together, creating imperfections that do not line up entirely).
The gallium-indium mixture helps to prevent the formation of an oxide layer that would block the water molecules from reacting with the aluminum and creating hydrogen.
"You don't have to apply any energy for the gallium-indium eutectic to work its magic on aluminum and get rid of that oxide layer," said Meroueh. "Once you've activated your aluminum, you can drop it in water, and it'll generate hydrogen – no energy input required."
Even better, the gallium-indium mixture does not chemically react with aluminum.
"It just physically moves around in between the grains," she added. "At the end of the process, I could recover all of the gallium and indium I put in and use it again."
A considerable advantage, as gallium and indium are costly and in relatively short supply.
As for the storage of hydrogen, the aluminum-water reaction solves that problem for any location with available water, simply move the aluminum and then react it with water on-site.
"Fundamentally, the aluminum becomes a mechanism for storing hydrogen – and a very effective one," said Douglas Hart, professor of mechanical engineering at MIT and contributing scientist to this discovery. "Using aluminum as our source, we can 'store' hydrogen at a density that's 10 times greater than if we just store it as a compressed gas."
Fundamentally, the researchers at MIT were able to devise a method to use recycled aluminum with plain water to create hydrogen that can be used to generate clean electricity, and in a way that does not require intensive logistical costs, production costs, or energy costs.
Further insights, a potential game-changer
After discovering a method to eliminate the oxide layer on aluminum cleanly and efficiently, the team needed to know just how the varying degrees of alloying elements would influence hydrogen generation.
It would not be practical if the generation of hydrogen could not at least match conventional methods.
As it turns out, the presence of 0.6% silicon (the percentage used in the sample during testing) increased the hydrogen yield for a given weight of aluminum by 20% compared to pure aluminum. In contrast, the presence of magnesium at 1% (test sample percentage) produced far less hydrogen. Finally, a mixture of both (often found in some aluminum alloys and was furthermore part of the test samples) pushed the yield up, but not to the level of pure aluminum.
Setting that aside, the researchers also experimented with grain sizes. As mentioned before, microscopic imperfections exist in the structure of pure aluminum, forming grains. These boundaries exist in most metals as they are generally found in a polycrystalline state.
Through careful engineering, new samples were acquired with processed aluminum to determine if the size of the grains influenced hydrogen production, and it was quickly discovered that this too affects the chemical process.
The team found that reducing the grain size increased the efficiency yet decreased the duration of the reaction to varying degrees in the different samples, while the presence of either alloying element – silicon and magnesium – had a major effect on the outcome once again.
With different grain sizes, the effects of silicon-doped aluminum increased by as much as 100 times and from the other three compositions by 10 times (magnesium, pure, and silicon-magnesium).
The research team went on to stress the benefits of developing a better fundamental understanding of the underlying chemical reactions involved.
In addition to guiding the design of practical systems, it might help them find a replacement for the expensive indium in their pre-treatment mixture.
"At this point, we know that the indium in our eutectic is important, but we don't really understand what it does, so we don't know how to replace it," said Hart.
Regardless, Hart, Eager, and Meroueh have demonstrated new practical ways of tuning the hydrogen reaction rate. In combination, these approaches, grain sizes and doping, could possibly deliver the ingredients to the ideal recipe for cheap hydrogen fuel – a discovery that could change energy forever.
"If you go from magnesium-containing aluminum with the largest grain size to silicon-containing aluminum with the smallest grain size, you get a hydrogen reaction rate that differs by two orders of magnitude," added Meroueh. "That's huge if you're trying to design a real system that would use this reaction."