Issue Date: April 24, 2017
Quantum dots are superefficient at generating hydrogen
Getting a two-for-one deal is always appealing. That math is particularly beneficial for scientists when it comes to generating electrons from light in solar cells. Using the right materials, researchers can generate two or more electrons for every sufficiently energetic photon of light absorbed, which computes to greater than 100% quantum efficiency.
Building on this capability, Yong Yan of New Jersey Institute of Technology, Matthew C. Beard of the National Renewable Energy Laboratory, and coworkers have constructed a superefficient quantum-dot-based photoelectrochemical cell that produces hydrogen (Nat. Energy 2017, DOI: 10.1038/nenergy.2017.52).
“As far as we know, this is the first time that hydrogen has been produced photoelectrochemically under visible light with a quantum yield greater than 100%,” Yan says.
The new system relies on a process called multiple exciton generation, or MEG. During MEG, two or more electron-hole pairs, known as excitons, are created within quantum dots from the absorption of one high-energy photon. The team’s photoelectrochemical cell includes an anode constructed of a lead sulfide quantum dot layer deposited on a fluorine-doped tin oxide base. The cathode is a platinum mesh.
When light strikes the anode, electrons and holes are generated within the lead sulfide layer. The holes oxidize sulfide in a sodium sulfide solution in the anode compartment of the cell to form sulfur. Meanwhile, the electrons make their way to the platinum cathode in a phosphate-buffered solution in the other compartment, where hydrogen ions are reduced to H2. A salt bridge separating the compartments enables hydrogen ions and sodium ions to migrate from one side to the other. Pushing the efficency threshold over 100% with a photoelectrochemical cell provides new opportunities to capture excess photon energy to produce solar fuels such as H2, the researchers note.
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