Keenan Smith, Antonela Gallastegui, Zixuan Yu, Yuliana Pairetti, Andrew Seel, Jacques Ollivier, Victoria Garcia Sakai, Bob C Schroeder, Maria Forsyth, Aurelie Gueguen, David Mecerreyes, Fabrizia Foglia
Polymer electrolyte membranes composed of ionic liquids (IL), capable of conducting protons efficiently at elevated temperatures, without external humidification, could transform fuel cell technology. However, the molecular origins of such proton transport remain poorly understood, especially in the polymerized state. Here, we directly visualize a hierarchy of coupled elementary proton motion steps spanning picosecond to nanosecond timescales in polyIL membranes using pulse field gradient (PFG) NMR and multi-resolution quasi-elastic neutron scattering (QENS). Polymer dynamics comprise three-site jumps within methanesulfonate coordination shells, two-site hops along hydrogen-bond chains, and out-of-plane backbone flips which dynamically reconfigure the proton transfer pathway. The latter facilitates a correlated polymer-proton hopping mechanism above 60°C enabling rapid nano- and microscale proton transport at operational temperatures. Even trace water plasticizes the polymer and remarkably lowers this proton hopping barrier by nearly half. A critical transition occurs near 245 K, where water forms a continuous hydrogen-bonded network of acid-base pairs, enabling sub-10 ps Grotthuss proton hopping, approaching liquid water dynamics. This molecular-level understanding provides fundamental insight into polyIL conduction mechanisms and the long-standing question of how solid polymers achieve liquid-like proton mobility, providing a roadmap for polyILs in next-generation electrochemical energy technologies.