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Interfacial processes in reaction technology
Prof. Dr. Hans-Georg Steinrück
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Research interest of the AK Steinrück

Energy storage and energy transformation materials are at the heart of the energy transition. Owing to the ever-increasing demand regarding performance metrics for modern materials, the underlying physics and chemistry is becoming increasingly complex, and their parameter space is enormous. Here, one example are ion batteries. While the electronic revolution was initiated by lithium ion batteries three decades ago, these materials still need to be improved in order to meet the world’s energy demand while conserving the planet. Furthermore, many basic physicochemical aspects of these energy storage devices still puzzle researchers.

We utilize advanced atomic/molecular-scale techniques to elucidate the basic physicochemical processes underlying the functionality of energy related materials. Our approach to tackle these questions is of reductionist nature, in which we take one step back from the device level, and investigate structural and chemical properties of materials employing simple and well-defined model systems. From this, we envision to obtain a predictive understanding that can be translated to provide rational knowledge-based design rules for improved materials. For a successful implementation of the latter approach, scalable knowledge spanning all time- and length-scales involved, ranging from picoseconds to years, and Ångstroms to meters, is desired.

Specific research topics of the Steinrück group can be found below:

Interfacial (electro)chemistry in energy storage materials

The arrangement of electrolyte species within the first few nanometers of electrodes in the electric double layer (EDL) is of fundamental importance for all electrochemical systems. For example, it governs interfacial interactions in catalysis, energy storage capacity and desalination capacity in supercapacitors and capacitive deionization, respectively, as well as electrolyte stability and cell kinetics in lithium ion batteries. In the latter, the EDL combined with the thermodynamic instability of electrolytes at the operating voltages of lithium ion batteries, leads to a passivation layer termed solid electrolyte interphase (SEI) which is an essential part for the functionality of batteries. Despite their importance, both EDL and SEI are presently not well understood, owing inter alia to their buried nature at the solid liquid interface, rendering direct probes sparse. Toward this end, we combine surface-sensitive measurements with atomic/molecular resolution with precision electrochemical characterization to unravel the structural and compositional properties found near interfaces in electrochemical systems with particular focus on ion batteries and electrochemical desalination technologies.

Development of model systems for advanced characterization of dynamic processes at the atomic scale
Many energy-related devices consist of an extremely complex assembly of materials, spanning many orders of length scales, ranging from the arrangement of atoms and molecules on that nanometer scale to the meter scale of full devices. Technologies include photovoltaic energy transformation systems such as the novel hybrid organic-inorganic perovskite system, desalination technologies, and metal ion batteries (Li, Na, Zn, Mg, etc.). For the latter, the inherent complexity is particularly apparent: Cathode and anode consist of primary and secondary particles, which are mixed with polymeric binders and conductive additive, comprising porous electrodes. These are infiltrated with electrolyte and sandwich a porous separator. Finally, these are arranged in single-layer, multi-layer, or cylindrical geometry to make up a single battery cell, which can then be arranged in various formats to construct battery packs. Accordingly, the times scales on which functionality-relevant phenomena occur range from picoseconds over which ions hope from one crystal site to another, over minutes during which ions diffuse through the electrolyte and electrodes during charging, to years over which materials degrade. This complexity inherently complicates approaches to unravel the properties and functionality of individual components, in particular on the atomic and molecular scale. However, this knowledge is imperative for a bottom-up approach in which materials, for example novel high capacity battery electrodes, are designed for specific purposes. Our approach is to develop and utilize simple and well-defined model systems for the investigation of dynamic processes occurring in energy-related materials, in particular at the atomic scale. Our philosophy is that such foundational knowledge is scalable and can be utilized for the knowledge-driven improvement of materials as fundamental insight can inspire novel concepts.

Understanding ion transport in electrolytes via advanced synchrotron techniques
Predictive and quantitative understanding of ion and mass transport in liquid and polymeric electrolytes is at the heart of electrochemistry. Here, the goal is to accurately simulate the performance of an electrochemical cell, for which the temperature- and concentration-dependent transport coefficients of a given electrolyte must be known. Such predictive knowledge is key towards the successful improvement of safety and energy density of lithium-ion batteries, which is necessary to meet the stringent and diverse market demands. Towards this end, our goal is to utilize advanced synchrotron techniques, such as X-ray photon correlation spectroscopy and X-ray absorption (spectro)microscopy to directly measure physical parameters in an electrochemical cell resultant from mass transport. These can be combined with microscopic and continuum level simulations to provide unambiguous insight into otherwise challenging to determine transport parameters.

Ion adsorption and intercalation in desalination technologies for clean water|
Water scarcity for potable, industrial, and agricultural use is a major challenge for our global society. Approximately one billion people lack access to clean water for at least for one month of the year. Accordingly, water purification/desalination is of importance in order to mitigate the issues accommodating the increase in society and industrial production. Two of the most common and mature technologies are membrane based reverse osmosis and capacity deionization. The working principle of the latter is ion storage in the electric double layer in nanosized pores in carbon electrodes upon application of a potential. Consequently, the intrinsic ion storage limit is the surface area. This issue can be overcome by moving towards faradaic reactions, i.e desalination batteries where ions are removed from feed water via intercalation into metal oxide hosts in redox reactions. These materials show great potential, in particular for selectivity of specific ions, which is important for removal of specific ions due to their disadvantageous effects water treatment technologies or the production of large quantities of raw materials (such as lithium). Towards this end, we utilize well-defined model electrodes to understand on a fundamental level how ion intercalation proceeds in these novel materials and aim to develop novel low-energy cost desalination batteries based on rational design principles. For this purpose, we use predictive understanding gained via operando synchrotron scattering and spectroscopy combined with electrochemical measurements which allows us to understand the structure and chemistry evolution of the respective materials.

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