was developed by a professor of mechanical engineering at the Massachusetts Institute of Technology. As the production of low-carbon hydrogen fuels has become increasingly important to global decarbonization efforts, a new method of using aluminum and water to produce hydrogen has been developed.
A promising response
One of the main challenges for hydrogen energy is clean production, but at the same time it is also economical. The new hydrogen fuel produced using aluminum and water may be very promising. The reason is that aluminum will react with water at room temperature, so there is no major technical challenge. The result of the reaction is hydrogen and aluminum hydroxide. In our daily life, both water and aluminum are widely used, but this reaction does not always occur. The reason is that the original metal aluminum is naturally covered with a layer of alumina . This oxide layer prevents the aluminum metal itself from directly contacting water.
uses aluminum and water to generate hydrogen, and the whole process does not emit any greenhouse gas. In addition, technologies like this can help overcome traffic challenges in any place with water. Any place where there is already water only needs to introduce aluminum, and it can react on-site to generate hydrogen.
The hydrogen production method using a combination of aluminum and water can make hydrogen more practical. Douglas P. Hart, Professor of Mechanical Engineering at MIT, said: “Fundamentally speaking, aluminum can become a mechanism for storing hydrogen, and it is a very effective mechanism. Using aluminum as our raw material, we can disguise it. The'storage' of hydrogen is 10 times more dense than we store in the form of compressed gas."
So far,There are two problems that hinder the use of aluminum for this purpose:
First, make sure that the surface of the metal aluminum is clean so that it can react with water. In this regard, we must first adopt a practical method to modify the aluminum oxide layer. Then, when the reaction occurs, there must be a way to prevent it from reforming.
Secondly, the problem with aluminum is that the mining and production process of this metal is energy-intensive. Therefore, any feasible method of using this metal requires concentrated use of aluminum scrap from various sources. Unfortunately, scrap metals are difficult to use as raw materials because they are usually an alloy containing other elements to meet their original purpose.
Dr. Laureen Meroueh
"If we need to use aluminum in actual applications to better observe the production of hydrogen from waste water, we will Hydrogen generation characteristics," said Dr. Laureen Meroueh, who received a PhD in mechanical engineering last year.
Since the basic steps of the reaction are not yet clear, it is difficult to predict the rate and volume of hydrogen formed from scrap aluminum, which may contain different types and concentrations of alloying elements. Therefore, Hart, Merueh and Thomas W. Eagar, professor of the Department of Materials Science and Engineering at MIT, decided to study the influence of these alloying elements on the aluminum-water reaction in a systematic way, as well as the technology to prevent the formation of oxide layers.
To prepare, they asked the experts of Novelis Inc. to make pure aluminum and specific aluminum alloy samples, which consist of commercial pure aluminum mixed with 0.6% silicon (by weight), 1% magnesium or both Made-This is a typical scrap aluminum composition.MIT researchers used these samples to conduct a series of tests to explore different aspects of the aluminum-water reaction.
Aluminum pretreatment
The first step is to show an effective way to penetrate the oxide layer formed on the surface of aluminum. Solid aluminum is composed of tiny particles that are not perfectly aligned with the boundary. In order to maximize hydrogen production, researchers need to prevent the formation of an oxide layer on the surface of all these internal grains.
The research team has tried various methods to keep the aluminum particles "activated" to react with water. Some people pulverize waste samples into very small particles, so that the oxide layer cannot adhere. But aluminum powder is dangerous because they react with moisture and explode. Another method is to grind waste samples and add liquid metal to prevent oxide deposition. But grinding is an expensive and energy-consuming process.
For Hart, Merueh, and Eagle, the most promising method (first introduced by Jonathan Slocum during the work of Hart’s research group) involves pretreating solid aluminum by coating the top with liquid metal and allowing them to penetrate To grain boundary.
In order to determine the effectiveness of this method, researchers need to confirm that liquid metal will reach the surface of the internal grains, regardless of the presence of alloying elements. They must determine how long it will take for the liquid metal to cover all the particles of pure aluminum and its alloys.
They first mixed two metals—gallium and indium—in a specific ratio,A " eutectic " mixture is formed; that is, a mixture that will remain liquid at room temperature. They coated the sample with eutectic and allowed it to penetrate for 48 to 96 hours. They then exposed the sample to water and monitored the hydrogen production and flow rate for 250 minutes. After 48 hours, they also took high-magnification scanning electron microscope (SEM) images to observe the boundaries between adjacent aluminum grains.
Based on hydrogen yield measurements and SEM images, the MIT team concluded that the gallium-indium eutectic will indeed penetrate naturally and reach the surface of the internal crystal grains. However, the rate and degree of penetration vary from alloy to alloy. The penetration rate of the silicon-aluminum sample is the same as that of the pure aluminum sample, but it is slower in the magnesium-doped sample.
Perhaps the most interesting is the result of the sample doped with silicon and magnesium-an aluminum alloy often found in recycled materials. Silicon and magnesium chemically bond to form magnesium silicide, which appears as a solid deposit on the surface of the internal grains. Meroueh hypothesized that when both silicon and magnesium are present in scrap aluminum, these deposits can act as a barrier to the flow of gallium-indium eutectic.
Experiments and images confirmed her hypothesis: solid sediments did act as a barrier, and images of samples pretreated for 48 hours showed incomplete penetration. Obviously, long-term pretreatment is essential to maximize the hydrogen production from aluminum scrap containing silicon and magnesium.
Meroueh listed several benefits of the process they used. "You don't need to apply any energy to the gallium-indium eutectic to perform its magic on aluminum and remove the oxide layer," she said. "Once you activate your aluminum, you can put it in water, and it will produce hydrogen — no energy input is required." Better yet, the eutectic does not chemically react with aluminum. "It just moves between particles," she said. "At the end of this process, I can recycle all the gallium and indium I put in and use them again"-this is a valuable feature,Because gallium and indium are expensive and relatively short in supply.
The influence of alloying elements on hydrogen production
They tested samples that had undergone eutectic treatment for 96 hours; by then, the hydrogen production and flow rates of all samples had stabilized.
Compared with pure aluminum, the presence of 0.6% silicon increases the hydrogen production rate of a given weight of aluminum by 20% even though the aluminum content of silicon-containing samples is lower than that of pure aluminum samples. In contrast, the presence of 1% magnesium produces much less hydrogen, and the simultaneous addition of silicon and magnesium will increase the yield, but it does not reach the level of pure aluminum.
The presence of silicon also greatly accelerates the reaction speed, resulting in a higher peak in the flow rate, but shortens the duration of hydrogen output. The presence of magnesium produces a lower flow rate, but allows the hydrogen output to remain stable over time. Once again, the flow rate of hydrogen produced by aluminum containing two alloying elements is between that of magnesium doped and pure aluminum.
These results provide practical guidance on how to adjust the hydrogen output to match the operating needs of the hydrogen consuming device. If the starting material is commercial pure aluminum, adding a small amount of carefully selected alloy elements can adjust the hydrogen production and flow rate. If the starting material is scrap aluminum, careful selection of raw materials may be the key. For high and short bursts of hydrogen, silicon-containing aluminum flakes from car dumps can work well. For lower but longer flows, magnesium-containing waste from the frame of demolished buildings may be better. For results in between, aluminum containing silicon and magnesium should work well; this material can be obtained in large quantities from scrapped cars and motorcycles, yachts, bicycle frames, and even smartphone cases.
Meroueh pointed out,You can also combine different aluminum alloy scraps to adjust the results. "If I have a sample of active aluminum that contains only silicon and another sample that contains only magnesium, I can put them both in a container with water and let them react," she said. "So I got a rapid rise in hydrogen production from silicon, and then magnesium took over and had a stable output."
This material can be removed from scrapped cars and motorcycles, yachts, and bicycles.
Another adjustment opportunity: reducing the grain size
Another practical way to reduce hydrogen production may be Aluminum grain size-this adjustment can increase the total surface area available for reaction.
In order to study this method, the researchers asked their suppliers to provide specially customized samples. Using standard industrial procedures, Novelis experts first feed each sample into two rollers and squeeze from the top and bottom to flatten the internal particles. Then they heated each sample until the long, flat particles were reorganized and shrunk to the target size.
In a series of carefully designed experiments, the MIT team found that in different samples, reducing the grain size can improve efficiency and shorten the reaction duration to varying degrees. Likewise, the presence of specific alloying elements has a major impact on the results.
unexpected results
throughout the experiment,The researchers encountered some unexpected results. For example, the standard corrosion theory predicts that pure aluminum produces more hydrogen than silicon-doped aluminum—contrary to what they have observed in experiments.
In order to clarify the potential chemical reaction, Hart, Merueh, and Eagle studied hydrogen "flux", which is the amount of hydrogen produced per square centimeter of aluminum surface over time, including internal grains. They examined the three grain sizes of each of the four components and collected thousands of data points for measuring hydrogen flux.
The results show that reducing the grain size has a significant effect. It increases the peak hydrogen flux of silicon-doped aluminum by 100 times, while the peak hydrogen flux of the other three components increases by 10 times. For pure aluminum and silicon-containing aluminum, reducing the grain size will also reduce the delay before peak flux and increase the rate of decline afterwards. For magnesium-containing aluminum, reducing the grain size will lead to an increase in peak hydrogen flux and lead to a slightly faster drop in hydrogen output rate. In the presence of silicon and magnesium at the same time, when the grain size is not controlled, the hydrogen flux is similar to the hydrogen flux containing magnesium and aluminum over time. As the grain size decreases, the hydrogen output characteristics begin to resemble the behavior observed in silicon-containing aluminum.
The researchers emphasized the benefits of a better understanding of the underlying chemical reactions involved. In addition to guiding the design of practical systems, it can also help them find alternatives to the expensive indium in the pretreatment mixture. Other work has shown that gallium naturally penetrates through the grain boundaries of aluminum. "At this point, we know that indium in the eutectic is important, but we don't really understand its role, so we don't know how to replace it," Hart said.
But Hart, Merueh and Eagle have demonstrated two practical methods for adjusting the hydrogen reaction rate: adding certain elements to aluminum and controlling the size of internal aluminum grains .Together, these methods can produce significant results. "If you go from magnesium-aluminum with the largest grain size to silicon-aluminum with the smallest grain size, you will get a hydrogen reaction rate of two orders of magnitude," Merueh said. "If you try to design a real system that uses this kind of reaction, it will be a huge improvement."
The comparison between hydrogen technology and Massachusetts, firstly from the chemical reaction formula, Purdue technology has obvious advantages. Purdue technology completely utilizes the hydrogen element in the water, and the by-product is alumina, which does not contain hydrogen. The aluminum hydroxide produced by the provincial technology wastes hydrogen. Purdue technology does not use any precious metals or rare metals as catalysts, while the Massachusetts technology requires gallium and indium, gallium and indium, which are trace elements. The world's output is very small, and future commercial applications are impossible. Purdue's technology is very mature, and a patent has been formed in 2015, and there are mature prototypes for car modification and power generation applications. Massachusetts technology is still in the laboratory stage. Economic Purdue technology’s hydrogen cost is less than US$4 per kilogram, while the Massachusetts technology’s hydrogen cost is unknown. Considering that the production of Massachusetts technology is very complex, the cost will not be low.
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