The use of solar-powered heating is one of the effective ways to save energy and reduce emissions and reduce haze pollution. However, one of the most thorny problems plaguing solar power generation is the dark night. With the increase of hazy weather, even daylight solar applications have been challenged. In this context, an extremely common substance, rust, has become the new darling of the international scientific and technological community. As long as the electronic action of rust can be freely made, it can become a magical material, providing an effective way for solar energy to be cheaply produced.
Break through the bottleneck of solar energy application into the research field
Even sporadic rust on the electrodes can make most engineers panic. However, this pair of partners, Kenneth Hardy and Allen Bard, has used this cheapest material to explore cheap solar power generation methods. Their general principle is to induce current to the rust, and to make the electrode. When the weak visible light is touched, the electrode will produce a small but still available current.
This episode took place in 1975, when it came to a time when silicon was emerging as the "blanker" of emerging materials. The extraordinary efficiency of silicon makes it the pillar of solar photovoltaic cells, and has been the market leader since then, and rust has not matched the electrical properties. The small breakthrough made at the University of Texas was shelved. The only occasion when people think of rust is when they find it necessary to eliminate it.
However, in the past few years, the spotlight has again begun to project onto this magical material. Although the efficiency of converting solar energy into electricity, iron dioxide (iron rust) cannot be compared with silicon, but it can do something that silicon cannot do, such as helping solar energy storage. One of the most thorny problems plaguing solar power generation is the dark night, and flaky rust may be playing a role in breaking through this bottleneck.
Solar energy studies have focused almost exclusively on efficiency. Every day the sun bathes the earth in enough energy for us to consume for a year, but harvesting this “gift from heaven†is not a breeze. Even the most advanced technology available will only convert 46% of solar energy into electricity, and that depends on the ideal conditions for normalization. The International Space Station spends billions of dollars on solar panels made of expensive rare earth elements. This is an example. Silicon-based, cheaper solar photovoltaic panels placed on the ground, at best, are only 15% to 20% efficient.
What is urgently needed now is to try to store excess energy so that it can be used without sunlight. To a certain extent, it is precisely because it can only be used instantaneously. This seemingly endless resource has a negligible share of all renewable energy, and its cost is 20 times that of fossil fuels.
The most obvious solution is the battery, but due to the low energy density and the high cost of the entire building's power supply system that need to be renewed every few years, they are only an option for a few rich people. A much more conservative solar energy storage method uses it to make hydrogen. The chemical bond of this element is said to be extremely powerful, and its storage capacity per kilogram is 170 times that of a standard lithium-ion battery. In addition, hydrogen is a true "generalist" and you can use it any time you capture it. If it is incorporated into a fuel cell, it can be recombined with oxygen to generate electricity according to actual needs; it will become methanol biofuel when combined with carbon monoxide; it will be stored well and will even burn like any other gaseous fuel.
The easiest way to convert the power of photovoltaic cells into hydrogen is to use the power to operate the electrolyzers to break the water (H2O) into hydrogen and oxygen. Simple or perhaps simple, but not necessarily effective. Of the 15% of solar radiation that can be captured by standard photovoltaic cells, another 30% is lost during the conversion process. When you're done, it's probably worthwhile to have a rechargeable battery.
Rust "inventory" rich in electrolysis to obtain water power
A more secure option is to identify cheap electrically conductive materials that can completely bypass photovoltaic cells and easily use the sun's photons to electrolyze water and produce hydrogen.
To allow a material to directly electrolyze water, it must release electrons of suitable energy when hit by photons. When these electrons are stimulated enough to evacuate the material, they will leave a hole called a hole. To fill these holes, water molecules donate an electron that belongs to itself. In this way, the electrons and the holes cooperate to allow water to oxidize and convert into hydrogen and oxygen.
Silicon is not a tool suitable for this work. Its electrons lack suitable energy. All materials need their own different, accurate energy in order to allow electrons to break free from the bondage of atoms. The silicon atom needs only 1.11 eV (electron volts) to release one electron, but the activation of water fission requires electrons to have at least 1.23 eV of energy.
For materials that are urgently needed in practice, they can be developed with heterogeneous compounds. For example, engineers at Bowling Green State University in the United States combined zinc selenide and cadmium sulfide crystals with platinum catalysts to release suitable electrons. However, devices made from complex processes and scarce materials are difficult to apply due to the high cost of commercialization.
As a result, researchers began to look back at rust. Iron dioxide has an energy "hit rate" of up to 2.1 eV, it is also not toxic, and it is very cheap. More importantly, its "inventory" is simply rich to the point where it is ubiquitous.
The low cost of nanotechnology to manipulate physical structures makes silicon obsolete
The stability of rust is also very important. Many materials are deformed after being eroded by water cracking, but iron dioxide can last up to a year in corrosive environments. Some people think that its corrosion resistance time may be longer, as Krauss of Imperial College London Helgaard described it as "It does not look like it will rust and rust."
Rust is not the "big player" in the world that is best at converting solar energy into hydrogen. Recent research results show that its theoretical limit is 16.8%. However, simply relying on its abundant sources means that quantity can compensate for the lack of quality.
However, this "Cinderella-style" material still lacks a crystal shoe. "It has not performed well so far," said Nat Lewis of the California Institute of Technology. "That doesn't mean we can't make it better."
Rust has physical properties suitable for electrolyzing water, but this alone does not mean that it can accomplish its mission alone. Therefore, most of the rust research in the past decade has focused on inducing electrons in water.
The first problem to be solved is the same obstacle that put Hardy and Buddy in a dilemma. The conductivity of iron dioxide is not very good. It cannot rely on its own power to send enough electrons out to the more effective edge. It needs to be stimulated from the outside world. One way is to obtain extra solar energy from the so-called tandem-type battery device. In 1991, Michael Gretzel, an engineer at the Swiss Federal Institute of Technology (EPFL) in Lausanne, developed a dye-sensitized solar cell using a thin layer of titanium dioxide treated with dyes to enhance photon absorption. In the absence of silicon, this is a shortcut that generates current that is both simple and inexpensive. As long as the resulting current is fed back to the underlying rust layer, they can develop the right electrons for electrolysis of water.
Gretel's device has achieved 4% efficiency as never before. However, that requires the use of two additional tandem cells. The extra energy is essential to enhance the electrons to form higher energy levels. Without these energies, rust will pump electrons back into the crystal array and reabsorb them before they have become pure.
The only solution is to form a layer of rust thin enough to allow the electrons to escape. The thickness is about tens of nanometers. This was not possible in 1975 or even later in the early 1990s. However, nanotechnology has made great progress after entering the 21st century, which not only made it possible to manipulate the physical structure of materials, but also gave birth to a number of exquisite and surprising solutions.
Jordan Katz of Denison University developed a thin coating consisting of several nanometers wide rust rods. Such a narrow width gives the device a very large surface area while allowing water to penetrate into the nano-gap between the rust rods. This allows electrons and holes to escape from the material and then merge with the surrounding water. However, Katz said that he is far from finding a material that can meet market demands.
Swiss Polytechnic researchers found a practical way. In order to assist electron escaping, Kevin Schiffer uses "cloud" precipitates to create nanoscale rust, which involves spraying the surface with a mist of iron solution. This precipitation method promotes the growth of iron dioxide into a large forest of cauliflower-like microscopic “trees†that form fractal surface areas that allow electrons to escape but can also be produced in large quantities.
Last year, the Shifra Group developed a working device that used nothing more than glass, which was not an expensive material. Its efficiency reaches 3.6%, which is comparable to that of Katz, but it does not need to rely on extra string type batteries. Shivra claims that he can improve efficiency to 10% within two or three years.
However, his goal may be frustrated when he encounters a paradoxical problem when the rust layer is very thin. The selection of any electrolytic material will face a fundamental contradiction, which is to be as thick and thin as possible. If you want the electronics to have any escaping momentum, I'm afraid it's better to be thin. To absorb as many photons as possible, the rust layer needs to be slightly thicker. The 20-nanometer electrolyte layer absorbs only 18% of the total amount of absorbable photons. After the thickness is increased to 1 micron, they can be captured almost completely, but in that case it will inevitably become clogged.
To solve this problem, Ivy Rotschild of the Israel Institute of Technology and his team turned to quantum physics. Their device traps input light onto a 30-nm rust film. After the photons enter the device, they are forced into a small chamber with V-shaped mirrors, where the mirrors refract them back and forth until they are absorbed. What's more, the light waves that propagate backwards and forwards, as well as the interference between them that further enhance the absorption effect, especially on the parts near the surface of the film, the electrons and holes may easily reach the surface before recombination. Fortunately, thanks to this fine-tuning, the device was able to absorb 71% of the input light while being thin enough to allow the electrons to escape, resulting in a theoretical efficiency of 4.9%.
According to the low standard of ferric oxide, it is already impressive enough, but it is not entirely suitable for use as a commodity raw material. Here finally touched the real "talent" of rust, although its efficiency is so low, why ultimately will make silicon eclipsed? Shivra said that even if it absolutely does not reach the maximum value of 16%, it is still cheap and can be prepared in large quantities. "In the final analysis, what matters is not efficiency but the cost of electricity per watt," Katz said. He said that even if the efficiency is only 10%, as long as the "price fair" will be stronger than 50% of the photovoltaic cells, because each surface coated with rust can not spend much money.
This is exactly what the researchers are looking for. Shifra thinks that he can apply his iron “cauliflower†mixture to a wallpaper-like object and print out the solar cells in slices to generate hydrogen anywhere. Lonesome outposts in the desert will become beautiful homes, and the process can use filtered wastewater.
Providing non-grid energy supply is expected to become a global renewable energy source
Of course, there are still some problems that must be solved before this dream comes true. For example, once the water is fissile, "actually it's equal to creating a bomb," Helgaard said because oxygen and hydrogen react in an explosive manner. A milder but equally bad outcome is that hydrogen and oxygen combine to form slightly hotter water than before.
Helgaard's design was to "eat" oxygen with low grade wastewater. Instead of being turned into gas, it is combined with organic compounds in the water, and hydrogen is bubbled back into the storage tank safely.
There is still one last remaining difficulty in obtaining solar energy with rust: While hydrogen production can be stored by solar energy, the storage itself also brings certain problems. The difficulty of sealing a gas is well known, and explosions can occur at any time without relying on materials that are expensive, strong, and resistant to corrosion. This problem even caused the much-anticipated hydrogen economy to ruin the entire future.
Researchers are always exploring a series of countermeasures to solve this problem. In addition to continuously improving the quality of fuel cells, there are many new ways are being brewed. For example, researchers at the University of New South Wales in Australia recently used sodium borohydride for storage. Salts must be heated to 550°C by convention to release the hydrogen stored in the chemical bonds, but it will do so when induced to 50°C at the nanometer scale. This is a promising advancement for carrying hydrogen in multiple sizes.
Although promising, it may not necessarily have this actual demand. The simplified hydrogen tanks that are stored on-site and burn like campfires also have the same purpose. Brian Holkerofft, manager of the Solar Energy Company, believes that this is where it is expected to find immediate use in a country like Kenya where there is a lack of sunshine and energy infrastructure. The result of his collaboration with Swiss Polytechnic was to use tandem-type batteries and a ferric oxide plant to provide the company with a non-grid-based energy solution. He hopes that these devices can also be extended to the top of houses in developed countries so that those who own such devices can obtain hydrogen fuel and electricity without having to go through the power grid.
Maybe, they don't need tandem batteries. The decades-long insight into the process of electronically induced fission of rust has ensured that Hardy and Bard's daring dream will surely continue from the past to the future, relying on rust that is not yet highly efficient but equipped with appropriate energy storage facilities. Optoelectronic devices.
"If you don't care about efficiency at all, then rust batteries can be used to make fuel or generate electricity through operations, or both." Katz said, "It can generate electricity during the day when electricity demand peaks. When demand is low, it's generation. It is fuel production.†Considering the economic reality of solar energy, the subtle currents that Hardy and Bard excavated in 1975 may also be transformed into a renewable energy source that covers a global scale. Perhaps it is time to enter the era of "rust."
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