NMR transport studies of liquid redox-active organic materials

• Applicant: Dr. Sven Jovanovic
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry ; Analytical Chemistry
• Term: from 2023 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 539484433

Redox flow batteries using liquid redox-active organic molecules (L-ROMs) enable energy storages with flexible scaling of energy and power density, while being also non-toxic and price sensitive. In their uncharged state, L-ROMs exist as a neutral, molecular liquid. However, during the charging process, L-ROMs are converted to an ionic liquid. In order to both increase conductivity as well as to ensure charge neutrality during charging, conductive salts are added to L-ROMs. Additionally, in case of highly viscous systems, inert solvent may be added. Thus, L-ROMs systems contain up to three components, where the state of charge adds additional complexity to the compound. Charge transport, which is usually defined by ionic mobility, is an essential parameter of electrochemical materials. Low mobility results in low ionic conductivity and thus high internal resistance and high loss of efficiency in an electrochemical cell. Characterization of ionic mobility is thus pivotal for battery research. In the case of L-ROMs, two separate charge transport behaviors are expected due to their two limiting cases of a molecular and ionic liquid, as well as a non-trivial transition between them. Thus, in order to enable optimal operation of a redox flow battery, a complete characterization of charge transport across the whole charging cycle is essential. The aim of this project is the research of charge transfer behavior of L-ROM systems using pulsed field gradient (PFG) and exchange spectroscopy (EXSY) NMR methods. PFG NMR permits the direct quantification of diffusivity for different ions as well as the study of existing transport limitations. By means of the EXSY method, (self) exchange reactions and dynamic equilibria can be characterized, which are likely to be relevant for charge transfer in highly concentrated ionic systems. Both diffusion and exchange will be studied in L-ROM systems of different composition and state of charge in order to discern relevant mechanisms and transitions between different states of mobility.

• DFG Programme: WBP Fellowship
• International Connection: Austria
• Host: Professor Stefan A. Freunberger