Effects of porous SiO ₂ on protonic conductivity of RbH ₂PO ₄-based composite electrolytes

The properties of RbH ₂PO ₄/SiO ₂ composites with different SiO ₂ pore sizes, prepared by mechanical milling, have been investigated here by various methods. The protonic conductivity is found to depend significantly on SiO ₂ pore size: the smaller the pore size, the stronger the conductivity enhancement in low-temperature (LT) region, comparing with pure RbH ₂PO ₄. 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 RbH ₂PO ₄ phase, which exhibits the orientation-disorder of H ₂PO ₄ ^- anions induced by the weakening of H-bonds. The interfacial layer exists on the surfaces or inside the pores of SiO ₂ particles, creating a continuous proton-conductive pathway and leading to the faster proton movement and greater conductivity even at LT.