Proton Conductivity of SO3H-Functionalized Benzene-Periodic Mesoporous Organosilica

The proton conductivity of benzene-periodic mesoporous silica (PMO) materials functionalized with sulfonic acid groups is investigated using experimental and theoretical techniques. The SO3H functionalization of pristine benzene-PMO is realized by three different pathways based on a grafting method in which surface silanol groups and/or benzene rings are used to anchor SO3H groups for enhanced proton conductivity. The functionalized material is experimentally characterized using X-ray diffraction, small-angle neutron scattering, and argon adsorption isotherms. After pressing the functionalized benzene-PMOs into pellets, the proton conductivity is deduced from Bode plots of impedance spectra taken in the temperature range of 333-413 K at 100% relative humidity. Using quantum mechanical approaches for selected proton-conduction mechanisms, the free energy barriers for proton transport as well as the local water environment at the surface are calculated. These calculations indicate that different mechanisms from purely bulk water transport are important for the benzene-PMO proton conduction, in agreement with experimental data.