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NREL research advances photoelectrochemical production of hydrogen using molecular catalyst

Researchers at the Energy Department’s National Renewable Energy Laboratory (NREL) have made advances toward affordable photoelectrochemical (PEC) production of hydrogen. A paper on their work is published in Nature Materials.

The PEC process 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. The NREL scientists took a different approach.

The NREL researchers decided to examine molecular catalysts outside of the liquid solution in which they are normally studied to see if the catalyst could be attached 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.

Producing hydrogen through solar water splitting requires the coverage of large land areas. Abundant metal-based molecular catalysts offer scalability, but only if they match noble metal activities. We report on a highly active p-GaInP2 photocathode protected through a 35-nm TiO2 layer functionalized by a cobaloxime molecular catalyst (GaInP2–TiO2–cobaloxime).

This photoelectrode mediates H2 production with a current density of ~9 mA cm−2 at a potential of 0 V versus RHE under 1-sun illumination at pH 13. The calculated turnover number for the catalyst during a 20-h period is 139,000, with an average turnover frequency of 1.9 s−1.

Bare GaInP2 shows a rapid current decay, whereas the GaInP2–TiO2–cobaloxime electrode shows ≤5% loss over 20 min, comparable to a GaInP2–TiO2–Pt catalyst particle-modified interface.

—Gu et al.

Corresponding author John 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.

—John Turner

This work was supported by the Department of Energy’s Office of Science.

Resources

  • Jing Gu, Yong Yan, James L. Young, K. Xerxes Steirer, Nathan R. Neale & John A. Turner (2015) “Water reduction by a p-GaInP2 photoelectrode stabilized by an amorphous TiO2 coating and a molecular cobalt catalyst” Nature Materials doi: 10.1038/nmat4511

Comments

SJC

Neat

solarsurfer

i don't see how this different then research at JCAP By Caltech. They use tiO2 sealed catalyst on their prototype artifical photosynthesis cells. HyperSolar also has a solar prototype baggie that segregates the Hydrogen from the Oxygen using non precious metals.

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