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posted on 2020-10-16 16:36:00 .
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When it comes to making efficient fuel cells, it’s all about the catalyst. A good catalyst will result in faster, more efficient chemical reactions and, thus, increased energy output. Today’s fuel cells typically rely on platinum-based catalysts. But scientists at American University believe that spinach—considered a “superfood” because it is so packed with nutrients—would make an excellent renewable carbon-rich catalyst, based on their proof-of-principle experiments described in a recent paper published in the journal ACS Omega. Popeye would definitely approve.
Spinach has a surprisingly long history in science; the notion of exploiting its photosynthetic and electrochemical properties has been around for about 40 years now. Spinach is plentiful, cheap, easy to grow, and rich in iron and nitrogen. Many (many!) years ago, as a budding young science writer, I attended a conference talk by physicist Elias Greenbaum (then with Oak Ridge National Labs) about his spinach-related research. Specifically, he was interested in the protein-based “reaction centers” in spinach leaves that are the basic mechanism for photosynthesis—the chemical process by which plants convert carbon dioxide into oxygen and carbohydrates.
There are two types of reaction centers. One type, known as photosystem 1 (PS1), converts carbon dioxide into sugar; the other, photosystem 2 (PS2), splits water to produce oxygen. There is a great deal of scientific interest in PS1, which acts like a tiny photosensitive battery, absorbing energy from sunlight and emitting electrons with nearly 100-percent efficiency. PS1s are capable of generating a light-induced flow of electricity in fractions of a second.
Granted, it’s not a huge amount of power, but it is sufficient to one day run small molecular machines. Greenbaum’s work held promise for building artificial retinas, for instance, replacing damaged retinal cells with light-sensitive PS1s to restore vision in those suffering from a degenerative eye condition. Since PS1s can be tweaked to behave like diodes, passing current in one direction but not the other, they could be used to construct logic gates for a rudimentary computer processor if one could connect them via molecule-sized wires made of carbon nanotubes.
Greenbaum is just one of many researchers who are interested in spinach. For instance, in 2012, scientists at Vanderbilt University combined PS1s with silicon to get current levels nearly 1,000 times higher than achieved when depositing the protein centers onto metals, along with a modest increase in voltage. The goal was to eventually build “biohybrid” solar cells that could compete with standard silicon solar cells in terms of voltage and current levels.
Spinach also has other interesting properties beyond its reaction centers. For instance, a 2014 paper by Chinese researchers reported on experiments to collect activated carbon from spinach for capacitor electrodes, while just last December, another group of Chinese scientists examined the potential of making nanocomposites based on spinach to serve as photocatalysts.
Spinach has also shown promise as a catalyst in fuel-cell technology. Greenbaum’s early patented take on using spinach as a catalyst involved sprinkling metallic platinum onto PS1s to produce pure hydrogen gas to power fuel cells. The leafy green could be a less toxic and cheaper catalyst for the oxygen-reduction reaction in fuel cells, according to the authors of this latest paper, which takes a different tack, making carbon-rich nanosheets out of spinach.
The first step of the AU scientists’ process is similar to the one employed by Greenbaum all those years ago. It starts with a common kitchen blender, packed full of freshly washed spinach leaves and then pureed. The AU team freeze-dried the resulting juice and ground it into a fine powder with a mortar and pestle. Next, they added salts (sodium chloride, potassium chloride) and a dash of melamine to promote nitrogen content. “At this point, [our method] does require us to add a little bit more nitrogen into the starting material, because even though [spinach] has a lot of nitrogen to begin with, during the preparation process, some of this nitrogen gets lost,” coauthor ShouZhong Zou, a chemistry professor at American University, told Spectrum.
The salts are key to producing pores in the final nanosheets, thereby increasing the surface area available to optimize chemical reactions. “Even though we call them nanosheets, when they are stacked together, it’s not like a stack of paper that is very solid,” Zou explained to Spectrum. “We need to make it porous enough that all the active sites can be use.”
Finally, the AU team used a couple rounds of pyrolysis (a thermal decomposition process) at temperatures of 900 degrees Celsius to produce the carbon-rich nanosheets.
They found that the spinach-derived catalysts were more efficient than the platinum-based ones. “This work suggests that sustainable catalysts can be made for an oxygen reduction reaction from natural resources,” said Zou. “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.”
Obviously, this is just proof of principle; what works well in an ideal laboratory setting doesn’t necessarily transfer easily into a real-world practical application. The next step is to build a complete prototype that uses the spinach-based catalyst in an actual hydrogen fuel cell. That will require collaboration with other laboratories, according to Zou. Spinach could also be a good catalyst for metal-air batteries used to power electric vehicles.
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