Interfacial functionalization steps.
An international team has succeeded in considerably increasing the efficiency for direct solar water splitting with a tandem solar cell whose surfaces have been selectively modified. The new record tops the previous 12.4%. Until now, manufacturing of solar hydrogen at the industrial level has failed due to the costs, however. This is because the efficiency of artificial photosynthesis, i.e. the energy content of the hydrogen compared to that of sunlight, has simply been too low to produce hydrogen from the sun economically.
Matthias May at TU Ilmenau and the HZB Institute for Solar Fuels, processed and surveyed about one hundred samples in his excellent doctoral dissertation to achieve this. The fundamental components are tandem solar cells of 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.
Energy schematic of the tandem layer structure under illumination.
“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,” explains May. 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 optimising, they remain stable for > 40 hours. Further steps toward a long-term stability goal of 1000 hours are already underway.
“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 4 US dollars/kg,” says Prof. Thomas Hannappel, academic advisor Prof.
Hans-Joachim Lewerenz from the Joint Center for Artificial Photosynthesis at the California Institute of Technology: “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.” http://www.nature.com/ncomms/2015/150915/ncomms9286/full/ncomms9286.html
http://www.helmholtz-berlin.de/pubbin/news_seite?nid=14294&sprache=en&typoid=49880
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