26.03.2021

Green Hydrogen for Direct Solar Water Splitting

As chemical and thus storable energy carriers, hydrogen technologies are the link between energy and mobility transition. But there are some challenges – especially with regard to the sustainable and environmentally compatible production of large quantities of hydrogen. One possible solution could be the use of solar energy. In the joint project "H2Demo", funded by the German Federal Ministry of Education and Research (BMBF) and led by the Fraunhofer Institute for Solar Energy Systems ISE in Freiburg, eleven partners are working on demonstrators for direct solar water splitting – among them the TU Ilmenau which is involved with significant research contributions. The university‘s work in the area of developing the tandem structure as well as the solid-liquid interface is being funded with more than one million euros. The total funding for the project amounts to around 14 million euros over a period of five years, plus substantial contributions from the industrial partners in the project.

In water electrolysis, water is split into hydrogen and oxygen, a process that requires electrical energy. Renewable energies are used here for the production of green hydrogen. So far, this has been done by using solar or wind power to drive a water electrolyzer. One step further is the process of direct solar hydrogen production via photoelectrochemical processes. This involves the absorption of sunlight in a semiconductor material that itself generates a sufficiently large photovoltage (> 1.6 volts) to decompose water directly into hydrogen and oxygen. So-called tandem absorbers, in which two absorbing materials are electrically connected in series, are particularly suitable, analogous to tandem solar cells, which are already used in the highest-efficiency photovoltaics. On a small scale, this hydrogen production process has already been demonstrated. The goal of the "H2-Demo" research project is to produce larger demonstrators for the first time.

"The work packages in H2-Demo include optimizing the III-V tandem absorbers, which are deposited on silicon and whose properties need to be further improved and optimized for the specific application," says project coordinator Dr. Frank Dimroth, head of the III-V Photovoltaics and Concentrator Technology Department at Fraunhofer ISE. "In addition, processes and equipment will be scaled up for later industrial use and new processes with high throughput will be developed," he adds. Finally, a demonstrator with an area of 36x36 cm2 will be built and installed on a test field to measure solar hydrogen efficiency – H2 generation and module efficiency – in detail.

Porträtfoto Prof. Hannappel vor beschriebener TafelTU Ilmenau/Christoph Gorke

As part of the project, the TU Ilmenau is primarily concerned with the difficult interfaces that lie between silicon and the III-V semiconductors in the tandem structure and between the device, the "artificial leaf", and the aqueous electrolyte. Here, the application of so-called in situ analysis is a methodology specifically developed at the TU Ilmenau. The tandem structure used here consists on the one hand of the semiconductor silicon, the 'workhorse' of the semiconductor industry, and on the other of so-called III-V semiconductor compounds, which practically refine the silicon in terms of its functionality. "Together, they result in a tandem structure that is necessary to provide enough 'free energy', i.e. a sufficiently high photovoltage for water splitting – and to do so with high efficiency," says Prof. Thomas Hannappel, Head of the Fundamentals of Energy Materials Group and Deputy Director of the Thuringian Energy Research Institute (ThEFI), who is leading the subproject at TU Ilmenau. Together with Fraunhofer ISE and Phillips University Marburg, he has been successfully working for years on the difficult challenge of bringing these two semiconductor materials together as well as possible: "You have to proceed and look very precisely, on an atomic scale, so to speak, and we can do just that at TU Ilmenau," says Prof. Hannappel. "Likewise, in the project we are trying to achieve stability and prevent corrosion at another particularly difficult interface, the solid-liquid interface between the semiconductor and the aqueous solution, the electrolyte."

In addition, the TU Ilmenau is providing measurement technology in analytics for charge carrier lifetime measurement and developing novel measurement technology to analyze surface properties and charge carrier dynamics as well as performance characteristics in the environment of the photoelectrochemical cell.
Overall, the project brings to bear a range of work, possibilities and findings that the TU Ilmenau has developed together with partners in research projects and created at the university. Professor Hannappel: "The realization of sufficiently good III-V/Si tandem structures alone would be a breakthrough that could be profitably applied in many other places, for example in low-cost solar cells with high performance characteristics."  

Other "H2Demo" project partners
AZUR SPACE, Helmholtz Zentrum Berlin, HQ Dielectrics, LayTec AG, Plasmetrex GmbH, Philipps-Universität Marburg, SEMPA, Technische Universität München and the Universität Ulm.

Funding of the project
The "H2Demo" project is being funded with around 14 million euros over a period of five years and is one of several winners of the "Hydrogen Republic of Germany" ideas competition organized by the German Federal Ministry of Education and Research (BMBF) in the field of basic research. The projects complement the three industry-led hydrogen lead projects, which are also scheduled to start this spring.

More information on the project website:
https://www.wasserstoff-leitprojekte.de/grundlagenforschung/h2_produktion

Interview with Prof. Hannappel on decoding interfaces for the sustainable energy supply of the future:

https://www.tu-ilmenau.de/forschung/forschung/themenjahr-energie/interview-prof-hannappel

Contact:
Prof. Thomas Hannappel
Head of Fundamentals of Energy Materials Group
+49 3677 69-2566
thomas.hannappel@tu-ilmenau.de