New medium-temperature proton electrolytes based on CsH2PO4 and silicophosphate matrices

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Silicophosphate gels ranging widely in P 2 O 5 content and specific surface area have been synthesized by a sol-gel process. We have demonstrated the possibility of producing medium-temperature high-conductivity systems based on silicophosphate matrices and CsH 2 PO 4 . The thermal, structural, and transport properties of composite proton electrolytes have been investigated. The results indicate that the electrical conductivity of the composites based on matrices with Si : P = 1 : 0.5 increases by up to three and half or four orders of magnitude and that their proton conductivity is ∼10 −3 to 3 × 10 −2 S/cm at temperatures from 90 to 220°C and a water vapor content of ≃0.6–1 mol % in air. The additive suppresses the superionic phase transition of CsH 2 PO 4 . The increase in conductivity at low contents of the heterogeneous component is due to both CsH 2 PO 4 dispersion and the presence of protonated centers on the matrix surface. When the mole fraction of the additive exceeds 0.3, the composites contain CsH 5 (PO 4 ) 2 , a compound with a lower thermal stability, which is responsible for their high conductivity in a limited temperature range.