An international team has succeeded in considerably increasing the efficiency for direct solar water splitting. Using a tandem solar cell whose surfaces have been selectively modified, the old record of 12.4 percent that stood for 17 years was overtaken to 14 percent.  The group’s research paper has been published in Nature Communications in front of the paywall for now.

Record Setting Solar Water Splitting Cell. Image Credit: M. Mays. Click image for the largest view.

Record Setting Solar Water Splitting Cell. Image Credit: M. Mays. Click image for the largest view.

Solar energy, abundantly available globally, is unfortunately not a constant resource and not necessarily everywhere. One especially interesting solution for harvesting this energy into fuel is artificial photosynthesis. This is what every leaf can do, namely converting sunlight to chemical energy. That can take place with artificial systems based on semiconductors as well. These use the electrical power that sunlight creates in individual semiconductor components to split water into oxygen and hydrogen. Hydrogen possesses a high energy density, can be employed in many ways and could replace fossil fuels.

Using straight hydrogen for combustion has no carbon dioxide released to the atmosphere and the only effluent is water.

Until now, manufacturing of solar hydrogen at the industrial level has failed due to the costs, however. That’s because the efficiency of artificial photosynthesis, being the energy content of the hydrogen compared to that of sunlight, has simply been too low to produce hydrogen from the sun economically.

Scientific facilities worldwide have been researching for many years on how to break the existing record for artificial photosynthesis of 12.4 %, which has been held for 17 years by NREL in the USA.

Now a team from TU Ilmenau, Helmholtz-Zentrum Berlin (HZB), the California Institute of Technology as well as the Fraunhofer ISE has considerably exceeded this record value.

Lead author Matthias May, active at TU Ilmenau and the HZB Institute for Solar Fuels, processed and surveyed about one hundred samples in his doctoral dissertation to achieve the new record. The fundamental components are tandem solar cells of what are known as III-V semiconductors. Using a now patented photo-electrochemical process, May could modify certain surfaces of these semiconductor systems in such a way that they functioned better in water splitting.

May explained, “We have electronically and chemically passivated in situ the aluminium-indium-phosphide layers in particular and thereby efficiently coupled to the catalyst layer for hydrogen generation. In this way, we were able to control the composition of the surface at sub-nanometre scales.”

There was enormous improvement in long-term stability as well. At the beginning, the samples only survived a few seconds before their power output collapsed. Following about a year of optimizing, they remain stable for over 40 hours. Further steps toward a long-term stability goal of 1000 hours are already underway.

Prof. Thomas Hannappel, from the photovoltaics group at TU Ilmenau, who was academic advisor for the work said, “Forecasts indicate that the generation of hydrogen from sunlight using high-efficiency semiconductors could be economically competitive to fossil energy sources at efficiency levels of 15 % or more. This corresponds to a hydrogen price of about four US dollars per kilogram.”

Prof. Hans-Joachim Lewerenz from the Joint Center for Artificial Photosynthesis at the California Institute of Technology, who worked closely with May, said, “We are nearly there. If we are successful now in reducing the charge carrier losses at the interfaces somewhat more, we might be able to chemically store more than even 17 % of the incident solar energy in the form of hydrogen using this semiconductor system.”

Chances are that 15 percent or even 17 percent efficiencies aren’t going to make major market progress with oil so very cheap right now. Seventeen percent is likely only a point that holds for a time, as those kind of efficiencies are going to make some market inroads as oil climbs back up. And the storage issues are still out there for hydrogen.

This is great work, getting closer to the sunlight direct to power solution. The 1000 hour stability goal is still 2 or more orders of magnitude short, investment costs remain unknown, but we are again, closer and this is good.

Keep in mind the actual price to consumers for work accomplished is going to have to be cheaper than oil’s average over time for this kind of tech to get its market legs.


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