A soft processible polyoxometalate-zwitterion eutectic electrolyte for superprotonic conduction

Proton conductors are important for propelling the development of energy and electronic technologies. Polyoxometalates (POMs), as a class of crystalline solid superacids with nanocluster form and high proton conductivity, are intriguing candidates for this. However, the use of POMs as practical proton conductors is hindered by their low processability coming from inherent brittleness, and their humidity-dependent conductivity. Here, we report a deep eutectic strategy to convert brittle POM powders to soft processible electrolytes through supramolecular complexation with hydroxyl‑containing zwitterions. The resulting POM-zwitterion eutectics exhibit good malleability and high modulus over 1 MPa, promoting them easily processed into self-supporting eutectics. Due to the soft state and percolated hydrogen bonding networks, these eutectics can transport protons by both diffusing and hopping processes, that is, a Vehicle-Grotthuss synergistic mechanism, which enables them to show superprotonic conductivities up to 1.0 × 10^-3 S cm^-1 at 30 °C (25% RH) and 2.4 × 10^-2 S cm^-1 at 150 °C (anhydrous condition). This work paves a new route to designing advanced electrolyte materials based on the supramolecular eutectic concept.