Eutectic CsHSO₄-Coordination Polymer Glasses with Superprotonic Conductivity

Superprotonic phase transition in CsHSO₄ allows fast protonic conduction, but only at temperatures above the transition temperature of 141 °C (T_c). Here, we preserve the superprotonic conductivity of CsHSO₄ by forming a binary CsHSO₄-coordination polymer glass system, showing eutectic melting. Their anhydrous proton conductivities below T_c are at least 3 orders of magnitude higher than CsHSO₄ without compromising conductivity at higher temperatures or the need for humidification, reaching 6.3 mS cm^-1 at 180 °C. The glass also introduces processability to the conductor, as its viscosity below 10³ Pa·s can be achieved at 65 °C. Solid-state NMR and X-ray pair distribution functions reveal the oxyanion exchanges and the origin of the preserved conductivity. Finally, we demonstrate the preparation of a micrometer-scale thin-film proton conductor showing low resistivity with high transparency (transmittance >85% between 380-800 nm).