Highly proton conducting sulfonic acid functionalized mesoporous materials studied by impedance spectroscopy, MAS NMR spectroscopy and MAS PFG NMR diffusometry

Proton conducting mesoporous Si-MCM-41 materials with different amounts of SO3H-groups were prepared by the co-condensation method and investigated by NMR spectroscopy, NMR diffusometry and impedance spectroscopy. The successful incorporation of mercaptopropyltrimethoxysilane (MPMS) into the mesoporous framework was proven by 29Si NMR measurements. The deconvolution of the 29Si MAS NMR spectra shows that Si atoms of the functionalizing silane are linked to the MCM-41 framework via three or two ≡Si–O–Si≡ bonds. 29Si MAS NMR proves that almost all MPMS was incorporated into the mesoporous framework, since 36.9% of the signal belongs to T groups, whereas the concentration of MPMS in the synthesis solution amounts 40%. 13C CP MAS spectroscopy confirms that the majority of the organic functional groups remained intact after the oxidation in 30 wt.-% H2O2. The proton conductivity was investigated by impedance spectroscopy (IS). Drastic differences were found for different degrees of functionalization. The maximum conductivity was found for the sample with maximum loading of sulfonic acid groups (40% functionalization) as σ dc = 0.19 S cm−1 at 413 K. The large conductivity differences between 20% and 40% functionalization result in large differences in the diffusion coefficients of the charge carrier by application of the Nernst–Einstein equation. But only small differences of the water self-diffusion coefficients were measured by magic-angle spinning pulsed field gradient (MAS PFG) NMR diffusometry. The comparison of IS and MAS PFG NMR results allows the determination of the Haven factor, which amounts 5 660 for 20% and 329 for 40% functionalization. We explain the proton conductivity in functionalized MCM-41 by structure diffusion. The drastic increase of conductivity (at 353 K from 9.51 to 260 × 10−5 S cm−1) from 20% to 40% functionalization is caused by the reduction of the activation energy of the charge relocation in a denser lattice of proton donator sites.

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► Proton conducting functionalized Si-MCM-41 materials have a high conductivity.
► 1H, 13C and 29Si MAS NMR spectroscopy proved the functionalization.
► 1H MAS PFG NMR diffusometry gives information about the hydrogen mobility.