Detailed Simulation and Characterization of Highly Proton Conducting Sulfonic Acid Functionalized Mesoporous Materials under Dry and Humidified Conditions

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Two different synthesis routes to prepare highly ordered and highly proton conductive sulfonic acid (SO 3 H) functionalized mesoporous silica materials are presented. The highest loading of Si-MCM-41 with SO 3 H groups was reached via a co-condensation route, which turned out to be advantageous compared to grafting, leading to hybrid materials exhibiting proton conductivities up to 0.2 S/cm at 100% relative humidity and 413 K. Especially at low humidities and under dry conditions, we show that the high group density of co-condensed materials is crucial for the proton transport. Experimental characterization, via impedance spectroscopy and water adsorption, and theoretical investigations of a simplified model system concordantly demonstrate the most important dependencies of the proton transport properties on the water content and on the density of SO 3 H groups. Computational studies were performed including classical molecular dynamic (MD) simulation and free energy calculation with a quantum mechanical method, which are showing comparable trends to the performed proton conductivity measurements of the material. Furthermore, the water uptake of the functionalized material was studied by MD simulation, which revealed the water density inside the porous system to be inhomogeneous in the case of low relative humidity.