Electricity and hydrogen, from one cell

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Many leading research institutes the world over are looking into artificial photosynthesis processes, which can use energy from sunlight to drive a chemical reaction that splits water to generate hydrogen.

Such devices have not yet achieved the type of efficiency that would make them commercially viable, and often require an additional power source to drive the reaction, meanwhile powering an electrolyzer to produce hydrogen from solar generated electricity is a concept that is rapidly gaining commercial ground.

Now, however, researchers at the Lawrence Berkeley Lab (LBL) in California, working alongside the Joint Center for Artificial Photosynthesis have successfully demonstrated a device which combines a hydrogen producing photoelectrochemical cell with a silicon PV cell.

The device developed by LBL is described in the paper Hybrid photoelectrochemical and photovoltaic cells for simultaneous production of chemical fuels and electrical power, published in the journal Nature Materials. The researchers calculate that such a device could achieve 6.8% hydrogen efficiency, and 13.4%, putting a combined 20.2% of sunshine hitting the cell to use.

The cell’s extra outlet allows current to be split, with one part contributing to hydrogen production and the extracted as electricity. Image: Berkeley Lab/JCAP

Image: Berkeley Lab/JCAP

The problem with such devices in the past, according to LBL, is that the lower performance of the other materials in the stack limit the current that can be generated in the silicon cell. “It’s like always running a car in first gear,” explains Gideon Segev, the study’s lead author. “This is energy that you could harvest, but because silicon isn’t acting at its maximum power point, most of the excited electrons in the silicon have nowhere to go, so they lose their energy before they are utilized to do useful work.”

The researchers got around this problem by simply adding an additional contact to the rear of the silicon cell, which could split the current in two – with one part contributing to the hydrogen production, and the other free to harvest more of the power generated in the silicon cell.

“The concept that photogenerated charge carriers can be controllably directed to produce electricity and chemical fuel provides an opportunity to significantly increase the energy return on energy invested in solar fuels systems,” reads the paper’s abstract, “and can be adapted to a variety of architectures assembled from different materials.” Segev says that the researchers plan to continue their collaboration and look into further applications for the hybrid device.

This article was updated on 01.11.2018, to correct a statement that prototype devices had actually achieved the quoted efficiencies, which are in fact only the result of calculations.