Interfaces between proton-conducting ionic liquids and catalytic electrodes

• Structural and electrochemical analysis of the electrochemical double layer
• In situ investigations using optical spectroscopy
• Modeling of ion dynamics and proton transport

To understand the mechanisms of oxygen reduction and proton transport in the medium-temperature polymer electrolyte fuel cells we develop, we analyze the interface between ionic liquids and catalytic surfaces. Since ionic liquids consist of charged organic molecules, between which electrostatic forces and steric effects occur, their properties are fundamentally different from those of conventional aqueous solutions. In particular, the electrochemical double layer, which mainly determines the effectiveness of the reactions in the fuel cell, must therefore be analyzed using high-resolution methods.

We use atomic force microscopy in force spectroscopy mode which allows us to verify that an ordered crystalline structure consisting of alternating anion and cation layers is formed at electrically polarized electrodes. This structure can extend several nanometers into the electrolyte.

The order and proton conductivity of the boundary layer depends on the water content, electrical polarization, and temperature, and is therefore characterized in electrochemical in situ test cells. To this end, we use fourier transform infrared spectroscopy (FTIR), surface-enhanced Raman microscopy, as well as electrochemical impedance spectroscopy. These investigation methods are complemented by theoretical calculations using density functional theory and molecular dynamics. On this basis, we further optimize the use of the developed ionic liquids in fuel cells.

References

[1] Rodenbücher, C.; Wippermann, K.; Korte, C. Atomic Force Spectroscopy on Ionic Liquids. Appl. Sci. 2019, 9, 2207, https://doi.org/10.3390/app9112207.