Revealing the Proton Slingshot Mechanism in Solid Acid Electrolytes through Machine-Learning Molecular Dynamics

However, the precise role of polyanion rotation and its interplay with proton hopping remains unresolved. Nanosecond-scale molecular dynamics simulations, driven by equivariant neural-network force fields, reveal a nuanced proton slingshot mechanism: a rotating polyanion first carries the proton, O–H bond reorientation then extends its displacement, and the combined motion enables long-range jumps.

This mechanism refines the conventional Grotthuss-type picture, revealing that long-range transport arises from significant O–H bond reorientation assisted by only limited polyanion rotation. Despite their structural similarity, CsH₂PO₄ and CsHSO₄ exhibit qualitatively different transport behavior associated with their distinct proton concentrations. In CsH₂PO₄, two rotation rates with different activation energies emerge, while CsHSO₄ displays a single-rate process with faster productive polyanion dynamics.