Flexible and Robust CsH ₂ PO ₄ –Polytetrafluoroethylene Composite Electrolyte Membrane for Solid Acid Fuel Cell Application

Solid acid fuel cells (SAFCs) represent a promising clean energy technology, characterized by moderate operating temperatures (150–300 °C) and high energy conversion efficiency. While cesium dihydrogen phosphate (CsH 2 PO 4 ) exhibits high proton conductivity between 230 and 260 °C and serves as a key solid acid electrolyte, its practical use is limited by poor mechanical strength and high resistance, especially in pure membranes with thicknesses typically exceeding 250 μm. In this work, a CsH 2 PO 4 –polytetrafluoroethylene (PTFE) composite electrolyte membrane was fabricated using a simple and practical approach. The resulting composite electrolyte membrane with a thickness of only 80 μm, shows excellent flexibility. The microstructural density of the composite electrolyte has been tuned by the introduction of 5 wt % PTFE without affecting the crystal structure of CsH 2 PO 4 . At 260 °C, the obtained membrane achieves a proton conductivity of 30.4 mS·cm –1 and enables a peak power density of 147 mW·cm –2 in a SAFC, along with stable discharge performance over 50 h at 0.5 V. This composite membrane effectively overcomes the thickness and mechanical limitations of pure CsH 2 PO 4, offering a new strategy for the design of advanced electrolytes and facilitating the practical implementation of SAFCs.