Effects of porous SiO2 on protonic conductivity of RbH2PO4-based composite electrolytes

The properties of RbH2PO4/SiO2 composites with different SiO2 pore sizes, prepared by mechanical milling, have been investigated here by various methods. The protonic conductivity is found to depend significantly on SiO2 pore size: the smaller the pore size, the stronger the conductivity enhancement in low-temperature (LT) region, comparing with pure RbH2PO4. The improved conductivity in composites arises from the formation of interface phase layer due to surface interaction between two components. This layer with thickness of ~ 4 nm is structurally disordered or amorphous RbH2PO4 phase, which exhibits the orientation-disorder of H2PO4− anions induced by the weakening of H-bonds. The interfacial layer exists on the surfaces or inside the pores of SiO2 particles, creating a continuous proton-conductive pathway and leading to the faster proton movement and greater conductivity even at LT.