Renewable energy has one decisive disadvantage: It depends on current weather conditions. Solar cells produce energy when the sun shines, and wind turbines depend on wind to function. The energy that is produced must be stored so that it is available throughout the day at the same level of output. But that is a challenge: Conventional storage technologies are not designed to deliver energy rapidly and flexibly as needed. Additionally, power is ideally stored near the location where it is generated, minimising unnecessary losses during transport.
With solar cells, energy can be chemically stored directly on site: by splitting off hydrogen from water. Storing light with water Solar cells must be made ready to do this – but how can this be done with modern cells whose composition is already highly complex? A group of TU researchers have joined Wolfram Jaegermann and Bernhard Kaiser from the Surface Science group to study this. They are investigating third-generation solar cells – layered semiconductor structures on the surfaces of which solar-driven water splitting takes place. But the crucial reaction mechanisms are not yet sufficiently understood.
To prepare the solar cell system to work efficient and stable for water splitting, scientists must do meticulous detective work: What materials are suitable as semiconductors, catalysts or for electrolysis? What happens when the materials are combined? What effects occur externally? This research is part of a priority programme ‘Regenerative fuel generation using light-driven water splitting’ of the Deutsche Forschungsgemeinschaft (German Research Foundation). The programme involves a total of 19 German research institutions.
Nature as a model
The basic idea for this type of energy storage comes from nature: The leaf of a plant absorbs sunlight and takes in carbon dioxide from the air and water from the soil. These are converted to high-energy hydrocarbon compounds such as sugar. Researchers are experimenting with mimicking these processes – they are developing a solar cell technique that works like artificial leaves. ‘The basic idea is the conversion of photons into chemical storage materials,’ explains Bernhard Kaiser. ‘The artificial cells consist of semiconductor electrodes that convert light energy into electrical charge carriers. Instead of using them immediately as electricity, the photoelectrodes will be used to split water molecules into hydrogen and oxygen at the surface.’ The resultant hydrogen can be stored near the cell. Energy reclamation occurs in a fuel cell through a controlled reaction of hydrogen with oxygen. This recreates the starting material pure water. The result: a closed cycle without further waste products.
However, the process is not as simple as it sounds – especially because solutions have thus far been inefficient and unstable. And this is the challenge researchers face: They want to determine exactly how the relevant processes work and what material combinations are optimal. The process of water splitting requires a cell system with a voltage of 1.6 to 1.9 volts. A simple silicon solar cell with a voltage of only 0.7 is insufficient. And so, the research centre Jülich, one of the partners in the priority programme, has combined several layers of amorphous and icrocrystalline silicon into one cell. It absorbs different wavelengths of light and increases the photovoltage, e.g. a quadruple cell generates 2.5 volts, so that water molecules can be split successfully.
The petroleum of the future
TU scientists are especially interested in studying the interactions of such a multiple cell with protective layers and electrocatalysts. They determine among others how electrically conductive – and thus, efficient – the cell is. The Darmstadt scientists investigate the processes taking place at the atomic level when these materials grow together layer by layer. For example, atoms on the surface have different properties than atoms inside the same material. ‘This can lead to reconstruction on the surface, a shift in the position of the atoms; and thus, a change in electronic properties,’ says Kaiser. ‘This, and the high reactivity with molecules from the atmosphere, can lead to significant deterioration of the desired material properties.’
Successful detective work. Using triple cells with platinum as catalyst layer and ruthenium oxide as counter electrode, the researchers have achieved an efficiency of 9.5 percent for the conversion of sunlight into hydrogen. ‘This is an excellent yield at this early stage of research,’ says Kaiser. In the future, promising improvements towards higher efficiencies could be achieved by the combination of the tested cells with solar cells of other materials, and by replacement of the precious metal catalysts.
Besides searching for ideal photo-absorbers and electrocatalysts, scientists are also developing a better understanding of the electrochemical principles underlying photocatalytic systems. Using efficient and economical artificial leaves, hydrogen produced in a future energy scenario could be directly converted with carbon dioxide into gaseous or liquid fuels. These could be used like conventional hydrocarbon compounds – making water, as it were, the petroleum or coal of the future.
Dates and facts
Priority Programme 1613 of the German Science Foundation with the title ‘Fuels Produced Regeneratively Through Light-Driven Water Splitting: Clarification of the Elemental Processes Involved and Prospects for Implementation in Technological Concepts´ has been underway since 2012 and will be funded until March 2019. It is based with a subproject at TU Darmstadt. Coordinator is Professor Wolfram Jaegermann (Department of Materials and Geosciences of TU Darmstadt).
Important publications by the working group:
J. Ziegler, F. Yang, S. Wagner, B. Kaiser, W. Jaegermann, F. Urbain, J.-P. Becker, V. Smirnov, and F. Finger, Interface engineering of titanium oxide protected a-Si:H/a-Si:H photoelectrodes for light induced water splitting, Applied Surface Science 389, 73 (2016).
F. Urbain, V. Smirnov, J.-P. Becker, A. Lambertz, F. Yang, J. Ziegler, B. Kaiser, W. Jaegermann, U. Rau, and F. Finger, Multijunction Si photocathodes with tuneable photovoltages from 2.0 V to 2.8 V for light induced water splitting, Energy & Environmental Science 9, 145 (2016).
The working group also addresses fundamental mechanisms of semiconductor interfaces. Among other things, a team have investigated the suitability of hematite and other transitionmetal oxides for the regenerative generation of hydrogen. Their results were published in Nature Communications recently:
Christian Lohaus, Andreas Klein & Wolfram Jaegermann: Limitation of Fermi level shifts by polaron defect states in hematite photoelectrodes, Nature Communications (2018)9:4309,