Interview with Prof. Thomas Hannappel on decoding interfaces for the sustainable energy supply of the future | March 2021
"This could be a crucial building block for a 'green economy' in the near future"
Professor Hannappel, as a physicist at TU Ilmenau, you have been working on the fundamentals of energy materials for around ten years, in particular on the increasingly complex structures of semiconductors and their interfaces. Why are these structures so important for energy research?
Hannappel: Semiconductor materials are indeed quite crucial for the conversion of solar energy into other forms of energy, such as electrical power generated in an ordinary solar cell. The absorption of solar energy in the semiconductor and its conversion into so-called 'free energy' is the first, very crucial step towards solar energy conversion - crucial for whether we can become efficient at all.
For a very short time, the absorbed solar light energy is then stored in the semiconductor and can then be 'extracted' as electrical energy in the simplest next step. Different functional semiconductor layers and structures' help in this process, for example in the so-called charge separation. Our Thuringian Nobel Prize winner Herbert Krömer coined the phrase "the interface is the device", i.e. the challenge is the interfaces. So we can use suitable functional semiconductor layers and interfaces to absorb light one after the other, separate different charge carriers, so-called electrons and holes, from each other, extract them from the semiconductor as a charge current and thus generate electrical energy - all this is done by solar cells.
For solar fuel generation, there is now another step: the charge carriers are transferred from the semiconductor into the aqueous solution, the electrolyte, where they oxidize the water at the anode, reduce H+ ions at the cathode on the other side and ... oxygen is produced at the anode and hydrogen at the cathode.
There is a lot to do at all points: With so-called tandem cells, one can achieve higher efficiency than with completely normal solar cells. With three-dimensional semiconductor structures, we can change many other properties and save valuable ingredients such as indium or gallium. We can not only 'split' water, but also CO2, and thus reproduce artificial leaves that are much more efficient than the natural variant.
The DEPECOR joint project, which is coordinated by the TU Ilmenau, is concerned with precisely this, i.e. with the reduction of CO2 and the generation of higher-value fuels through artificial photosynthesis, the imitation of which is also referred to as the "Holy Grail" of photoelectrochemistry. How exactly does this process work and why is it so important?
Hannappel: With the basic structure of such a photoelectrochemical cell, we can also decompose CO2. The DEPECOR project (Direct Efficient Photoelectrocatalytic CO2 Reduction) aims at reducing CO2 entering the atmosphere by means of a closed carbon cycle using an artificial leaf. In this, we intend to develop with our collaborative partners a prototype that efficiently reduces CO2 by non-assisted, direct, sunlight-induced photoelectrochemistry in an integrated device and converts it into hydrocarbons as an energy carrier, which can then be more easily handled or even liquid.
In order to do this efficiently, we need tandem cells, i.e. cells with so-called multi-absorber structures, which optimally exploit the sunlight and simultaneously provide the necessary 'free energy', similar to what nature does. There, we do not speak of tandem structures, but of the so-called Z-scheme, which functions in an analogous way. In such a cell for CO2 reduction, water is oxidized again on one side and CO2 is decomposed into higher-value fuels on the other.
In other words, in the long term, these processes could solve world problems such as sustainable energy supply, storage and mobility and pave the way to a "green economy"?
Hannappel: Yes, that's right. Such components would be arbitrarily scalable, similar to solar cells, and could be used in a decentralized manner. As you say, in addition to regenerative energy conversion, the storage of energy for system stabilization would also be guaranteed. For mobility, hydrogen itself can be used in principle, but it is also the basis for electrochemical conversion into higher-grade fuel variants, including liquid ones. Assuming we are as successful with this as we have been with the rapid development of photovoltaics, this could be a decisive building block for a "green economy" in the near future.
The German magazine Der Spiegel recently wrote: "...but there is a crucial catch: the conversion of green electricity into hydrogen is not particularly efficient, about four-fifths of the energy is lost in the process. This leads [...] to the fact that hydrogen is far from being competitive. To what extent can your research contribute to solving this problem?
Hannappel: Photovoltaics has developed tremendously fast since the 80s, especially in the last ten to fifteen years. In the process, the price of solar modules has fallen by more than two orders of magnitude (!), from over EUR 20 per watt(peak) in the mid-1980s to less than 20 euro cents today. This is practically unprecedented. That means a similarly discouraging view could have been taken of solar cells in the 80s and 90s. However, this technology then developed at breakneck speed worldwide and clearly prevailed - not only ecologically, but also economically.
If we anticipate a similarly rapid development for artificial photosynthesis, we would indeed be on the way to finally solving the world's most pressing problems in the foreseeable future. The (solar-to-hydrogen) efficiency of almost 20 % achieved so far, as in our record-breaking water-splitting cell, is already quite respectable and many times higher than that of natural photosynthesis.
Together with great partners, we are now in the process of further developing direct water splitting on an industrial scale in the BMBF-funded H2Demo project. The aim is to produce larger demonstrators for direct solar hydrogen production for the first time. Another special feature is that III-V semiconductor materials are to be deposited on silicon, as a particularly inexpensive and promising variant of a tandem structure, and that machines with a particularly high throughput are also being developed for industrial scalability.
What role does research at the TU Ilmenau play in this?
Hannapel: The tandem structure used here consists on the one hand of the semiconductor silicon, the workhorse of the semiconductor industry, and on the other hand of so-called III-V semiconductor compounds, which practically refine the functionality of the silicon. Together they form a tandem structure, which is necessary to provide enough 'free energy', i.e. a high enough photovoltage, for water splitting - and to do so with high efficiency. Thanks to research funding, my Fundamentals of Energy Materials Group, together with Fraunhofer ISE and the University of Marburg, has been successfully tackling the difficult challenge of bringing these two semiconductor materials together as well as possible and understanding this important growth process. To do this, you have to proceed and look very precisely, on an atomic scale, so to speak, and we can do this at the TU Ilmenau.
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.
In the project, we are also taking a very close look at another particularly difficult interface, the solid-liquid interface between the semiconductor and the aqueous solution, the electrolyte. The aim here is to achieve stability and prevent corrosion. All in all, a whole range of work, possibilities and findings come to the fore, which we have worked out together with important partners in research projects and which we have created at the TU Ilmenau.
How could hydrogen produced in large quantities be stored and used?
Hannappel:If the goal were achieved, i.e. the competitive conversion of solar thermal radiation into hydrogen gas, the possibilities would be manifold. The hydrogen could be used in fuel cells or 'burned' to water in combustion engines for mobility or to stabilize the energy supply. It could also be used as a technical gas in many places, such as in large-scale industrial processes.
Hydrogen can be stored decentrally or in large storage reservoirs, and its distribution can take place in specific hydrogen pipelines as well as in natural gas networks, or of course in tanker vehicles. Hydrogen is also a component in almost all fuels and can thus be seen as a basic product for further utilization.
As a university partner in the H2Demo project, the training of young scientists is also of great importance for TU Ilmenau. What role does this aspect play in the context of the project and your research in general?
Hannappel: This is of course an extremely important point, a great topic to get young people, especially young women, excited about science and technology. Since the topic is very interdisciplinary in all its complexity, practically all topics at TU Ilmenau play a role here - physics and chemistry, of course, but also, for example, materials, mechanical engineering, electrical engineering and much more.
And the subject is future-oriented, with scientifically challenging and outstanding topics such as catalysis. The course of study 'Regenerative Energy Technology' naturally fits in like a glove, even if even here the entire scientific spectrum necessary to solve the scientific challenges cannot be dealt with. In many questions, which are highly attractive for young researchers, one moves at exciting borders of research.
In the DEPECOR and H2Demo projects, as in past projects, you are working with the best scientists from Germany and the USA, including the world-leading Joint Center for Artificial Photosynthesis in California (USA) and several industrial partners. How do the partners complement and support each other and what do you expect from the collaboration?
Hannappel: We have been in intensive exchange for several years now, among others with the California Institute of Technology, CalTech, where we have realized the currently world's best cell for direct water splitting with Prof. Harry Atwater and his team. This exchange is to be continued and expanded between the two countries, Germany and the USA, and we are currently working intensively on this. It is about the expansion of possibilities, of knowledge, the exchange of science. Of course, it is also about the transfer of knowledge and technology, e.g. to Fraunhofer ISE or to industry, such as the company Azur Space Solar Power GmbH or the cooperation partner Helmholtz-Zentrum Berlin.
To what extent is research in this area also important for related technologies?
Hannappel: Due to the complexity of the scientific challenges, there are a whole series of achievements that would fall away simultaneously if they were successfully realised. These include catalytic processes at interfaces, which are also critical, for example, in the coupling of electrolysers with the electrical provision of energy (power-to-X), opto-electronic components such as tandem solar cells, electrochemical processes as in batteries, the discovery of new materials and material structures for all optoelectronics and sensor technology, and much more. A variety of exciting research topics contribute to the subject, in which we ourselves are also involved in collaborations and projects, such as nanowire technology, new analytical methods and growth studies - all topics in which we have a great deal of research interest in addition to the general subject of solar energy conversion in our field.
Professor Hannappel, thank you very much for the interview!