Proton mobility in sulfonic acid functionalized mesoporous materials studied by MAS PFG NMR diffusometry and impedance spectroscopy

Abstract

The proton mobility in sulfonic acid functionalized mesoporous MCM-41, MCM-48 and KIT-6 was studied by impedance spectroscopy and MAS PFG NMR diffusometry. Highest proton conductivities were found at 140 °C as 19 S m−1, 0.48 S m−1, and 37 S m−1, for MCM-41, MCM-48 and KIT-6, respectively. The self-diffusion coefficients of water molecules in functionalized MCM-41 and KIT-6 were found to be smaller than the diffusivity of the bulk water, however, for functionalized MCM-48 they appeared to be larger. For comparison, we calculated diffusion coefficients of the charge carrier from the conductivity values by means of the Nernst-Einstein equation. It turned out that for MCM samples the NMR water diffusivity includes inter-particle contributions and succeeds the charge carrier diffusivity by some order of magnitudes. The reason is that the root-square displacement of the water molecules within the diffusion observation time (20–100 ms) is larger than the mesoporous particle size, which amounts 1–4 μm for the MCM materials. In contrast, for KIT-6 consisting of larger particles of up to 50 μm, it became possible to detect the intra-particle diffusion within the material particles, because the root-square displacement of the species is smaller than the size of the particles. A two-component behavior of the NMR diffusivity can be observed, and the slower component describes the self-diffusion within the particles. The observed intra-particle NMR diffusivity is in agreement with the charge carrier diffusivity. From this, one may conclude that the NMR technique observes the charge carrier diffusivity of hydroxonium or Zundel ions without water fluctuations. Therefore, we claim that the intra-particle proton conduction in functionalized mesoporous materials is a “packed-acid mechanism” without water fluctuation.