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After eating bananas, we always take it for granted that the banana peels are thrown in the trash. But there are always scientists who have to do something unusual, plus they have "tall" devices, so they decided to see what would happen just by shining a light on the xenon lamp. But before talking about what these scientists do, you have to know what they're really for, and you have to put banana peels in an oven to bake them.
Written by | Wang Yibo
Review | Twenty-seven
Seeing an electric car on the street, some people may think: it should use lithium batteries. After all, the one that each of us is most familiar with is the "lithium battery" (smartphone) that we hold in our hands or pockets. Moreover, even in the electric vehicle industry, lithium batteries occupy a leading position, such as the power lithium battery used by Tesla. But in fact, lithium batteries also have a strong opponent - hydrogen fuel cells.
Although hydrogen fuel cells and lithium batteries are still competing for the first place in the future electric vehicle industry, they do have one thing in common: "hydrogen" and "lithium" are in the "front row" of the periodic table, ranking first and third, respectively. Interestingly, these 2 simplest elements (removing helium) play an important role in the future of electric vehicles or new energy industries. However, compared with lithium batteries, the reaction of hydrogen fuel cell utilization, we need to be more familiar with it.
Hydrogen is the "power"
If you haven't forgotten the "electrolyzed water" reaction in a middle school chemistry textbook, it's easy to understand how hydrogen fuel cells work — the reverse process of electrolyzing water. In general, hydrogen and oxygen chemically react to form water, and this part of the chemical energy is converted into electrical energy.
In addition, hydrogen has a much older history in power applications than lithium. In 1766, Henry Cavendish first discovered the substance "hydrogen" in the laboratory. At that time, Cavendish dripped dilute hydrochloric acid on zinc flakes, creating bubbles. Not only did he analyze the "bubbles", but he also found that this bubble could generate water after burning.
Subsequently, this gas has left a strong mark in the dream of human flight and as a source of lift. In 1783, Jacques Charles first invented the hydrogen balloon. Hydrogen is very light in mass and less dense than air, so a balloon filled with hydrogen can float in the air. On this basis, Henri Giffard built the first airship powered by hydrogen in 1852. Such airships had safely transferred 35,000 people during World War I without any accidents. Unfortunately, in 1937, the German airship Hindenburg was destroyed in the air by a hydrogen explosion. Since then, the era of airships with hydrogen as lift has also come to an end.
But in recent years, "hydrogen can burn, and the combustion product is only water", which has once again attracted the attention of scientists, especially because water is not harmful to the environment. This time, when scientists want to use hydrogen as a driving force, they rely on its ability to burn.
However, hydrogen is largely absent in nature. Hydrogen on Earth is mostly found in the form of compounds, such as water and organic matter. But the good news is that it is not difficult to produce hydrogen in theory, such as when hydrogen was first discovered, the reaction of acid and metal iron or zinc produced hydrogen. However, to use hydrogen as an energy source, it is also necessary to find ways to produce it on a large scale.
Hydrogen production faces challenges
For hydrogen fuel cells, the production of hydrogen is one of the most challenging aspects. Given the environmental impact of fossil fuels, scientists have been looking for clean energy alternatives to fossil fuels. But the clean energy they think of, hydrogen, doesn't really get rid of the limitations of fossil fuels: at present, industrial hydrogen production is mainly through methane water vapor reforming, catalytic reforming of oil or coal gasification, but their raw materials are still fossil fuels. When thinking about how to make really clean hydrogen, the first thing that scientists think of is the "electrolyzed water" experiment in textbooks.
But electrolyzing water to produce hydrogen in industrial production is not as simple as drawing a schematic diagram or writing an equation in a book. From the selection of electrodes, electrolytes and catalysts to the proton exchange membrane, each step needs to be optimized, because the efficiency of electrolyzing water to hydrogen is still very low.
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In addition to electrolyzing water, there is another method that is also increasingly favored by scientists: biomass thermal cracking. Biomass refers to various organisms formed by photosynthesis, such as organic wastes such as corn, deciduous leaves, and fruit husks. These organisms have been absorbing carbon dioxide before becoming "waste", which is a "container" for natural storage of carbon dioxide. Therefore, if organic waste is directly discarded in the environment, it may lead to the release of a large amount of carbon dioxide. Carbon, hydrogen, and oxygen are the main components that make up organisms, so scientists are thinking that if organic waste is collected, it may be converted into useful substances. Obtaining hydrogen from it is also in the scientists' plans.
Recently, a team led by Hubert Girault of the Department of Basic Science at the Swiss Federal Institute of Technology (EPFL) has developed a new method of decomposing biomass. Using banana peels that are considered garbage, they get hydrogen and a substance called biochar. The findings were published in the journal Chemical Sciences.
What happens with banana peels?
In this study, the key to turning banana peels into hydrogen was the xenon lamp. A xenon lamp is an electric light source that emits light by discharging xenon. Xenon lamps are no strangers to the team, which has been used to prepare nanoparticles. But in the study, they decided to use a high-power xenon flash as a light source to light the banana peels that tend to be thrown away with photopyrolysis and then see what exactly would happen.
High-power xenon flashes not only provide higher power energy, but also short pulses. In other words, it is to produce a powerful flash of light in order to quickly trigger the photochemical reaction of the banana peel.
Before irradiating with light, they also need to dry the wet banana peel at 105 degrees Celsius, and then grind the dried banana peel into a powder. These powders are then transferred to stainless steel reactors filled with inert gases. It is worth noting that this reactor can withstand a certain amount of pressure and has a glass window, which allows researchers to see the changes that occur inside in real time. However, ordinary reactors do not have glass windows, because the combination of several atmospheric pressures and glass still discourages many scientists.
Graphical summary of the study. Image source: EPFL
Under this xenon flash, the transformation of the banana peel ends in 14.5 milliseconds. As a result, about 100 liters of hydrogen and 330 grams of biochar are produced per kilogram of banana peel (dry weight). In addition, the researchers believe that this biochar is also valuable, for example, it can be used to improve the soil, but also for the production of electrodes, because the cathodes of many batteries are carbon-based materials.
Bhawna Nagar (co-author of the study) believes the most important thing about the study is that they indirectly converted carbon dioxide "stored" by banana peels into a valuable substance. This is also the significance of biomass research.
The study did not use any catalysts, only xenon lamps. But it's clear that researchers need to do more to increase hydrogen production. The similarity between "biomass" and "electrolyzed water" is that advances in either area have the potential to drive new ideas.
Maybe one day, banana peel hydrogen production will no longer be a small discovery in the laboratory, but will really be used for large-scale hydrogen production?
Cover image: Ton Gregorio/flickr
Reference Links:
https://doi.org/10.1039/D1SC06322G
https://techxplore.com/news/2022-01-hydrogen-banana.html
https://periodic-table.com/hydrogen/#:~:text=Hydrogen%201%20Discovery%20and%20History%20The%20informal%20discovery,Hydrogen%20Gas%20...%208%20Isotopes%20of%20Hydrogen%20
https://en.wikipedia.org/wiki/Hydrogen
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Source: Global Science
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