Cluster of Excellence e-conversion : Interfaces and interferences

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The current level of energy consumption is estimated to be equivalent to around 14 billion tons of oil. According to experts, in order to generate this amount of energy from renewable sources alone, the power output of sustainable energy systems will have to be increased 100-fold. This assessment provides much of the motivation for the new Cluster of Excellence e-conversion. Its goal is to find ways of increasing the efficiency of the renewable energy systems currently in use. As modern civilization is utterly dependent on access to such enormous amounts of energy, solving the problems associated with sustainable energy conversion is not just an urgent, but an existential task. Although there are many ideas and technologies already out there, none of them can be readily scaled up to the extent that they could produce the approximately 30 terawatt-hours of power required annually, say professors Thomas Bein, Ulrich Heiz and Karsten Reuter, the Coordinators of the new Cluster: “We need a new approach, which integrates important insights and strategies from diverse modes of energy conversion ranging from photovoltaic systems to catalysis and batteries.” The new research network is a joint initiative of LMU and the Technical University of Munich (TUM), in which the Max Planck Institutes for Chemical Energy Conversion (Mülheim/Ruhr) and Solid-State Research (Stuttgart) will also participate.

“The participating scientists plan to use a bottom-up approach,” as Thomas Bein, who holds a Chair in Physical Chemistry at LMU, explains. “The basic idea is to use precisely controllable model systems to understand the fundamental challenges at the atomic level, and the reasons for the current limits in the various systems now in use. Then we can take appropriate steps to overcome them.” Among the parameters of interest are low efficiency, inadequate long-term stability, deleterious chemical transformations and excessive dependence on rare or toxic materials. The potential for synergies is considerable. Because comparable materials and processes are employed in different production systems, similar problems are encountered in different settings. – And these often turn up at the interfaces between different materials.

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It can be argued that research on sustainable energy generation has been quite successful, and new technologies for energy production and storage have established themselves on the market. The conversion of solar energy into electricity has already become a pillar, albeit a small one, of sustainable energy generation. But in practice many daunting challenges must be met before energy production as a whole can be placed on a sustainable basis. The variability of solar radiation levels makes it necessary to find ways of storing the energy derived from it. In addition, mobile power consumers need a degree of independence from the grid, and electric vehicles must be able to store energy at high densities. However, a buffer system for cars must be configured very differently from one intended to store the wind energy harvested across a whole country. “System specifications are inherently very diverse,” says Heiz, who holds a Chair in Physical Chemistry at the TUM. “So we need a variety of very different, as well as completely new solutions. That is the challenge that energy research now faces.”

Another problem is that many of the elements that form essential parts of solar cells, LEDs, semiconductors and batteries – such as indium, gallium and lithium – are quite scarce and often toxic. Much of the lithium now used comes from salt lakes in South America and it too is in short supply. Annual production of iridium, which is a prime candidate for use as a catalyst in the sustainable electrolytic production of hydrogen, is in the region of 10 metric tons. “If one were to stack this amount of iridium in one spot, it would only occupy the volume of a small refrigerator,” explains Reuter, who holds a Chair in Theoretical Chemistry at the TUM. This is only one of the inconvenient facts which demonstrate that current technologies are not going to solve the energy problem.

Climate change is forcing humanity to rethink its consumption of fossil fuels, which further underlines the need to develop new concepts in the field of sustainable energy technologies. “It is increasingly obvious that the principle of empirical and incremental advances in the efficiency of solar cells or the quest for ‘the ideal material’ is no longer adequate,” says Bein. Current energy research often focuses on individual technologies and the materials used in them. “There is no clear way forward, in part because the processes involved are so complex.” It has also become apparent that in a variety of systems – whether solar cells, fuel cells, electro- or photocatalysis or energy storage in batteries – the atomic details are vital. “In virtually all cases, interfaces play a decisive role,” says Heiz. “They are largely responsible for limited efficiencies.” Critical phenomena such as overpotentials, recombination losses and unwanted resistances arise because undesirable microscale excitation and energy conversion processes at interfaces between materials cannot be fully suppressed.

Structures built up layer by atomic layer

The participating researchers therefore plan to develop model systems that allow them to explore, on the nanoscale, the interactions that take place at interfaces, with a view to resolving and ultimately controlling, at the atomic level, the mechanisms involved. With the aid of nanotechnologies they want to reduce the sizes of the active structural elements so as to cut down the amounts of expensive materials required significantly. They intend to investigate new molecular interactions, some of which take place between extremely thin surface layers. And they plan to image the structures of interest in different systems under operating conditions, using high-resolution microscopy and spectroscopic techniques, in order to clarify and ultimately optimize the crucial processes. “This will entail resolving events on extremely short temporal scales down to the femtosecond – 0.000000000000001 of a second – level,” Heiz explains.

“Modern chemistry and nanosciences work together,” says Bein. Insights from the nanosciences are needed to fabricate and tune the properties of the systems found at different types of interface – between solid phases, between a liquid and a solid, and between a solid and a thin molecular film. Indeed the Cluster itself is organized according to the nature of the interfaces to be investigated. This allows researchers to focus on characterizing the known physical properties of the interface or on seeking and studying novel effects. “e-conversion can be thought of as a platform for the development of entirely new microscopic concepts,” says Reuter. “The combination of solar cells with catalysts to yield an ‘artificial leaf’ is one example.”

As mentioned above, the limitations of renewable energy technologies often lie in the microscopic details. Selecting the appropriate model structure for the problem at hand enables one to tease out the factors that limit its performance, and one can then tinker with the model to optimize efficiency. For example, suppose one discovers that the ionic mobility between the electrodes in a solid-state battery is too low. To enhance it one can either reduce the thickness of the electrolyte layer or alter its crystalline structure. However, both approaches require precise control of the fabrication process. This often necessitates the use of elaborate and expensive procedures like molecular beam epitaxy (MBE) or atomic layer deposition (ALD), which allow structures to be successively built up layer by atomic layer. In this way, researchers can fabricate solid-state electrolyte layers as extremely thin films, thus producing well-characterized interfaces whose structure and electrochemical behavior can be analyzed in detail. In this way, one can construct model systems for each interface of interest. The Cluster will fabricate a library of models that can be used to test specific architectures and applications, some of which can perhaps be effectively combined with one another.

Searching for novel materials

The Cluster will ultimately consist of 40 research groups, including teams of theorists such as that led by Karsten Reuter, which specializes in designing model structures in silico, i.e. on the computer. This is where new ideas for novel molecular structures are often likely to emerge, which can then be fabricated and tested within the Cluster. Compounds that exhibit what is called the ‘perovskite’ structure, in which the repeating unit consists of one negatively and two positively charged ions, are currently of great interest for use in solar energy conversion. Methylammonium lead triiodide is such a substance. When stacked in thin sheets, it forms the basis of an efficient solar cell, but it is toxic and its long-term stability is problematic. It should be possible to develop a high-throughput method to test novel materials of this structural class that are amenable to synthesis. Such a system would greatly speed up the search for compounds that lack the drawbacks of the perovskite architectures currently favored. It would also shorten the route from a model system whose interfacial characteristics have been dissected and precisely understood to a commercially viable product. “Modern supercomputers allow us to theoretically assess the suitability of huge collections of potentially interesting materials on the basis of such a model, before selecting the most promising candidates for chemical synthesis,” says Reuter.

Its Coordinators emphasize the importance of the support structures incorporated into the “e-conversion” Cluster. These include links with selected industrial firms which envisage the exchange of experts, measures to facilitate the setting up of spin-off firms by individual researchers or the formation of a network with international laboratories working on related issues, as well as support for researchers and their families. “Real collaborations often require an exchange of personnel”, says Heiz. “We have faith in the dynamic development of our researchers.” The planned cooperations with commercial firms and specialized research institutions around the world are symptomatic of how the Cluster views its future. In the second phase, more weight will be placed on the transformation of new concepts and materials into marketable products, say all three Coordinators.

For further information on the work of the e-conversion Cluster:

A wonder in blue

Nanophysicists at LMU have fabricated ultrathin perovskite nanocrystals suitable for use in tunable and energy-efficient LEDs.

Hydrogen for all seasons

LMU chemists have developed novel porous materials called “covalent organic frameworks”, which provide a basis for the design of polymeric photocatalysts with tunable physical, chemical and electronic properties.