Photocatalytic water splitting with 9.2% solar-to-hydrogen efficiency

University of Michigan researchers developed a new photocatalytic water splitting system that is reportedly able to reach a 9.2% solar-to-hydrogen (STH) efficiency.

The proposed system uses the higher energy part of the solar spectrum to split water and the lower part of the spectrum to provide heat that encourages the reaction. The extra heat also allows the hydrogen and oxygen to remain separate, rather than renewing their bonds and forming water once more.

It works in an indoor environment and uses pure water, concentrated solar light, and an indium gallium nitride photocatalyst. The semiconductor catalyst, a forest of nanowires of indium gallium nitride grown onto a silicon surface, is able to become more efficient during use, according to the scientists. The device absorbs photons and converts them into electrons, which are used to split water into hydrogen and oxygen.

The nanowires are peppered with nanoscale balls of metal, 1/2000th of a millimeter across, that use those electrons and holes, positively charged gaps left behind when electrons are liberated by the light, to help direct the reaction.

“A simple insulating layer atop the panel keeps the temperature at a toasty 75 C, or 167 F – warm enough to help encourage the reaction while also being cool enough for the semiconductor catalyst to perform well,” the scientists said.

They claim that the system is nearly 10 times more efficient than other solar water-splitting systems of the same kind. They said that the final cost of hydrogen could fall with a bigger semiconductor.

“We reduced the size of the semiconductor by more than 100 times compared to some semiconductors only working at low light intensity,” said researcher Peng Zhou.

The research group described the system in the study “Solar-to-hydrogen efficiency of more than 9% in photocatalytic water splitting,” published in Nature.

“This temperature-dependent strategy also leads to an STH efficiency of about 7 % from widely available tap water and sea water and an STH efficiency of 6.2% in a large-scale photocatalytic water-splitting system with a natural solar light capacity of 257 watts,” they said.

They claimed that the next challenge is to further improve the efficiency and generate ultra-high purity hydrogen that can be directly fed into fuel cells.