Solid acids are considered as a promising electrolyte for intermediate temperature fuel cells (ITFCs), but their application is hindered by their inherent shortcomings, including poor mechanical properties, limited chemical stability, low proton conductivity at low temperature and narrow high-conductivity temperature range. Despite much work devoted to addressing these shortcomings, none of them can tackle these issues simultaneously. In view of this, we reported a composite electrolyte membrane based on CsH2PO4, sulfonated poly(ether ether ketone) and phosphosilicate sol. The electrolyte membrane was fabricated by using a simple sol-gel process combined with mechanical ball-milling. The prepared electrolyte membrane exhibits excellent mechanical performance with a tensile strength of 23.5 MPa, despite an inorganics content of 70 wt%. The electrolyte membrane can keep intact even if it is heated to 260 degrees C. The measurement of proton conductivity shows that the electrolyte membrane can maintain a high conductivity of above 10(-3) S cm(-1) over a broad temperature range of 130-230 degrees C under ambient condition without humidifying. The successful fabrication of the electrolyte membrane not only provides a promising membrane for lilts, but also opens fresh insights for the development of electrolytes with superior comprehensive performance and the application of solid acids in ITFCs. (C) 2015 Elsevier B.V. All rights reserved.