When Shouzhong Zou and his team at the Department of Chemistry, American University, decided to try spinach as way to improve the performance of fuel cells, even they were a little surprised at how well it worked. In their proof-of-concept experiments, they used spinach—bought from local supermarkets—to make a carbon-rich catalyst that can be used in fuel cells and metal-air batteries.
The spinach was a used a precursor for high-performance catalysts required for the oxygen reduction reactions (ORRs) in fuel cells. Traditionally, fuel cells have used platinum-based catalysts, but not only is platinum very expensive and difficult to obtain, it can be vulnerable to chemical poisoning in certain conditions. Consequently, researcher have looked into biomass-derived, carbon-based, catalysts to replace platinum, but there have been bottlenecks in preparing the materials in a cost-effective and non-toxic way. “We were a little bit lucky to pick up spinach,” says Zou, because of its high iron and 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.”
Zou and his team weren’t the first to discover the electrochemical wonders of spinach, of course, even though other studies have used the leafy greens for other purposes. For example, a 2014 study harvested activated carbon from spinach to create capacitor electrodes, while a more recent paper tackled spinach-based nanocomposites as photocatalysts. Spinach, apart from being abundant in iron and nitrogen (both essential in ORRs), is easy to cultivate, and “definitely cheaper than platinum,” Zou adds.
The preparation of the spinach-based catalyst sounds as first suspiciously like a smoothie recipe at first—wash fresh leaves, pulverize into a juice, and freeze-dry. This freeze-dried juice is then ground into a powder, to which melamine is added as a nitrogen promoter. Salts like sodium chloride and potassium chloride—“pretty much like the table salt that we use in our kitchen,” says Zou—are also added, necessary for creating pores that increase the surface area available for reactions. Nanosheets are produced from the spinach–melamine–salt composites by pyrolyzing them at 900 C a couple of times. “Obviously…we can optimize how we prepare this material [to make it more efficient].”
An efficient catalyst means a faster, more efficient reaction. In the case of fuel cells, this can increase the energy output of batteries. This is where the porosity of the nanosheets helps. “Even though we call them nanosheets,” Zou says, “when they are stacked together, it’s not like a stack of paper that is very solid.” The addition of salts to create tiny holes that allows oxygen to penetrate the material rather than access only the outer surfaces. “We need to make it porous enough that…all the active sites can be used.”
The other factor that favorably disposed the American University team towards spinach was that it is a renewable source of biomass. “Sustainability is a very important factor in our consideration,” says Zou. The big question to explore, he adds, is how can we avoid competition "with the dinner table". (Biofuel production has already raised concerns about food crops being diverted away from hungry mouths.) “And the second is, how do we keep the carbon footprint down in terms of his catalyst preparation…because currently we do use high temperatures in our preparation procedure?… If we can find different ways to do these to achieve the same type of material, that will cut back the energy consumption and reduce significantly the carbon footprint.”
Even though the results are promising, there is yet a long way to go. Zou cautions that the study so far is only a proof-of-principle. “We need to be very careful when we talk about practical applications because something that shows excellent performance in [lab] conditions could become more challenging when we implement them in the real device.” Another aspect that needs further study, he adds, is that while the spinach-derived catalyst outperforms platinum-based catalysts in alkaline conditions, the performance in an acidic medium is not as efficient. “So obviously, there is still some tuning we need to do to see if they can work through a range of pH.”
A complete prototype is obviously a next step—testing the catalyst derived from spinach in a fuel cell. “That’s the kind of expertise I don’t have in my lab at this point,” Zou admits. “We are thinking about collaborating with other groups, or we can build up our expertise in this area, because it’s a necessary step.”
This article appears in the December 2020 print issue as “Spinach Gives Fuel Cells a Power-Up.”