Gas diffusion electrodes (GDE) are of critical importance to a number of energy conversion and storage devices, such as fuel cells and batteries. They possess high porosities to increase the reactivity per unit volume of material, namely the oxygen reduction/evolution reactions (ORR/OER) occurring at the triple-phase boundary (TPB) between the porous electron-conducting support, ion-conducting electrolyte and the gas phase. The electrochemical performance of the system is thus strongly influenced by the GDEs morphology, which affects the formation of a TPB. Additionally, the polarity/wetting of the electrolyte determines the formation of the TPB.
With the advent of ionic liquids (IL) a new class of “green solvents” has been introduced with the potential to overcome the drawbacks of classical aqueous electrolytes such as KOH. Since ILs are liquid salts solely composed of ions, they can cause complex diffusion processes in porous hosts that can be contradictive to what is commonly exhibited by fluids. Moreover, properties of ILs are difficult to predict, since theories for property estimations usually do not compensate strong Coulombic effects as observed in IL’s.
In this contribution the dynamics of pyrrolidinium and choline based IL confined to pressed Vulcan carbon black and electrospun GDE frameworks were assessed by PFG NMR. The results indicate diffusive motion inside the porous medium over length scales of μm. The distribution of the diffusion coefficients indicates exchange between two distinct environments inside the sample. Furthermore, diffusive diffraction-like patterns were found for certain IL confined to the GDE frameworks. Therefore, the classical approach for the interpretation of NMR in porous media needs to be rethought for IL in carbon based GDE frameworks.
Finally, based on our findings, the potential and limitations of PFG NMR for studying the dynamics of IL electrolytes in GDE frameworks are discussed and evaluated.
We gratefully acknowledge funding by the “German Federal Ministry of Education and Research” (BMBF, grant number 03SF0499F, project “LuZi”)