Surface
modification strategies for the p-GaInP2 photoelectrodes. (nature.com)
(December 22, 2015) Researchers
at the Energy Department's National Renewable Energy Laboratory (NREL) have
made advances toward affordable photoelectrochemical (PEC) production of
hydrogen.
NREL's scientists took a different approach to the PEC
process, which uses solar energy to split water into hydrogen and oxygen. The
process requires special semiconductors, the PEC materials and catalysts to
split the water. Previous work used precious metals such as platinum, ruthenium
and iridium as catalysts attached to the semiconductors. A large-scale
commercial effort using those precious metals wouldn't be cost-effective,
however.
The use of cheaper molecular catalysts instead of precious
metals has been proposed, but these have encountered issues with stability, and
were found to have a lifespan shorter than the metal-based catalysts.
Instead, the NREL researchers decided to examine molecular
catalysts outside of the liquid solution they are normally studied in to see if
they could attach the catalyst directly onto the surface of the semiconductor.
They were able to put a layer of titanium dioxide (TiO2) on the surface of the
semiconductor and bond the molecular catalyst to the TiO2.
Their work showed molecular catalysts can be as highly
active as the precious metal-based catalysts.
Their research, "Water Reduction by a p-GaInP2
Photoelectrode Stabilized by an Amorphous TiO2 Coating and a Molecular Cobalt
Catalyst," has been published in Nature Materials. Jing Gu and Yong Yan
are lead authors of the paper. Contributors James Young, Nathan Neale and John
Turner are all with NREL's Chemistry and Nanoscience Center. Contributor K.
Xerxes Steirer is with NREL's Materials Science Center.
Turner points out that although the molecular catalysts
aren't as stable as the metal-based catalysts, PEC systems are shut down each
evening as the sun sets. That leaves time to regenerate a molecular catalyst.
"Hopefully you would not have to do that every day, but
it does point to the fact that low stability but highly active catalysts could
be viable candidates as a long-term solution to the scalability issue for PEC
water splitting systems," Turner said.