With global economic growth comes a demand for more energy. But our planet is in the brink of succumbing to the ways of our kin. Right on this scene that efficient and green energy solutions come into play.
Scientists at Israel Institute of Technology have come up with a solar-to-fuel conversion technique with record efficiency. Their idea is to implement the mechanisms of photosynthesis to push energy conversion efficiency to new heights.
Ph.D. Lilac Amirav, the principal investigator of the study says "We want to fabricate a photocatalytic system that uses sunlight to drive chemical reactions of environmental importance." She and her group at Israel Institute of Technology are in the process of designing a photocatalyst that can strip away and isolate hydrogen from water.
She explains "When we place our rod-shaped nanoparticles in water and shine light on them, they generate positive and negative electric charges," and adds "The water molecules break; the negative charges produce hydrogen (reduction), and the positive charges produce oxygen (oxidation). The two reactions, involving the positive and negative charges, must take place simultaneously. Without taking advantage of the positive charges, the negative charges cannot be routed to produce the desired hydrogen"
Although, as we all know, opposites attract. If the positive and negative charges find an opportunity to merge, they rule each other out, leaving us nothing. So, keeping particles with different charge properties is necessary.
To do that, the team has devised unique heterostructures that include different semiconductors along with metal and metal oxide catalysts. They have built a model system to study the oxidation and reduction happening and optimized their heterostructures for better performance.
In a 2016 research, the same team designed another heterostructure. The cadmium-selenide quantum dot at the one end attracted positive charge while negative charge piled up on the other side.
Amirav said, "By adjusting the size of the quantum dot and the length of the rod, as well as other parameters, we achieved 100% conversion of sunlight to hydrogen from water reduction." In this system, one single photocatalyst nanoparticle could put out 360,000 hydrogen molecules per hour.
But in this older study, only the reduction part of the reaction was studied. For a functioning solar-to-fuel converter, we need to handle the other part, oxidation, as well. Amirav remarks "We were not converting solar energy into fuel yet," and elaborates, "We still needed an oxidation reaction that would continually provide electrons to the quantum dot."
It's a significant challenge to walk through the water oxidation process because it has multiple steps. Also, the byproducts of the reactions skew with the outcome by compromising the stability of the semiconductor.
In their latest study, they have gone with a different approach. This time, instead of water they utilized a compound called benzylamine for the oxidation part. This way water reduces to hydrogen and oxygen while benzylamine gets converted to benzaldehyde. The U.S. Dept. of Energy defines 5 to 10 percent as the "practical feasibility threshold". The maximum efficiency of this method was benchmarked at 4.2%.
The researchers are looking to other compounds that may prove viable for solar-to-chemical conversion. With AI on their side, they are searching for compounds that'd be a good fit for this process. Amirav notes that this process has been fruitful so far.
The results of the study will be presented at the Fall 2020 Meeting and Expo held by the American Chemical Society (virtually, of course).