Guest post by Ralph House of UNC’s Center for Solar Fuels
Developing a way to store solar energy when the sun is down, or not available, is a key element and the central mission of the University of North Carolina Energy Frontier Research Center: Center for Solar Fuels (UNC EFRC) which is funded by Basic Energy Sciences in the US Department of Energy. Inspired by natural photosynthesis where light is absorbed and stored as carbohydrates for later use, we designed an “artificial photosynthesis” device called a Dye Sensitized Photoelectrosynthesis Cell (DSPEC). Similar to photosynthesis, in the DSPEC, light is absorbed and used to drive chemical reactions leading to water splitting into its component parts, oxygen, protons and electrons, with the electrons and protons used to make a “solar fuel,” in this case hydrogen gas.
The UNC EFRC led by scientist Thomas Meyer, made news recently with a major breakthrough in solar energy storage: demonstration of the first functional DSPEC for hydrogen fuel production.
Improving the molecular machinery of DSPECs
At the heart of the DSPEC is a molecular assembly made up of a light-absorbing molecule (called a choromophore) that converts light into high-energy electrons, and a catalyst for splitting (oxidizing) water into its component parts. The assembly is tethered to the surface of a semiconductor material, in this case titanium dioxide (TiO2), layered on top of nanoparticles in a film of a transparent conducting oxide. The steps required to generate solar fuels are illustrated in the figure below (click to enlarge).
There were two hurdles to using DSPECs that the EFRC recently overcame. First, the molecular assemblies were only stable on the electrode surface in acidic environments. Figuring out how to keep them on the surface at a more basic pH (pH=11), would make the rate of water splitting (oxidation) a million times faster! Second, before the high-energy electrons, freed upon absorption of light by transfer to the semiconductor, could be transferred to the other side to make hydrogen, they found their way back to the chromophore-catalyst assembly halting water oxidation in its tracks.
The answer to both lay in a technique called atomic layer deposition (ALD). It provides a way to build ultrathin films, one layer at a time, over complex three-dimensional surfaces. In collaboration with the Parsons Research Group at NC State University, the EFRC team used ALD to apply an ultrathin layer of TiO2 on a conducting oxide surface after attaching a catalyst. This effectively protected the bonds that attached the catalyst to the electrode surface where it stayed put allowing for water oxidation to occur at basic pH, a million times faster. This solved problem number one.
The root cause of the second problem was the thickness of the semiconductor in the nanoparticle films – several microns – too thick to allow the injected electrons to escape to the other side. In this case, ALD was used to deposit an ultrathin layer of TiO2 to the surface of the conductor with the chromophore-catalyst assembly then tethered to it. The ultrathin layer of TiO2 reduced electron transit times significantly allowing them to escape to the other electrode where they made hydrogen.
Although these results look to be a major breakthrough in solar energy storage, much needs to be done to improve both efficiency and stability. A next step will be the use of ALD to simultaneously increase the stability of surface binding and enhance the rate of electron capture. Stay tuned….