Effect of acidity and structure on the anhydrous proton conduction of acid-doped inorganic–organic hybrid membranes

Homogeneous acid-doped polysiloxane (PS) electrolyte membranes were prepared by the sol–gel reaction from 3-aminopropyltriethoxysilane (APTES) and different acids, such as sulfuric acid (SA), methanesulfonic acid (MSA), ethanesulfonic acid (ESA), and 1-propanesulfonic acid (PSA). The effect of the acidity as well as the structure on the proton transport has been investigated. The interaction between the acid and the basic nitrogen of PS made by APTES has been confirmed by FT-IR and ab initio calculation. The thermal stability depends on the stability of the doping acid. The structure of acid-doped PS was determined by X-ray diffraction pattern and changed with the kind of acid as well as the doping amount. The proton conductivity of annealed (1:1) PS/SA membrane having high acidity as well as the hexagonal-like structure reached 1.1 × 10−3 S/cm at 120 °C under no (or low) humidity, which is more than one order of magnitude higher than those of PS/acid membranes without structure. The structural effect was studied by PS/ESA membrane having different structure with the amount of acid doping. The proton conductivity of (3:1) PS/ESA membrane showing hexagonal-like structure is 3.82 × 10−4 S/cm, whereas it is 9.6 × 10−6 S/cm at 120 °C for amorphous (1:1) PS/ESA membrane. This result implies that the main requirements for high proton transport of electrolyte membranes prepared by acid–base interaction under low humidity are the acidity and the proton transport structure. If the acidity is fixed, then the presence of the ionic channel structure is essential to design the electrolyte membrane having high proton conductivity.