Atomic-scale mapping of Grotthuss proton hopping in a single-crystal 3D COF via hydrogen-bond networks

Abstract

Proton conduction mechanisms within covalent organic frameworks (COFs) remain elusive due to interfacial impedance in polycrystalline samples and the absence of structurally defined single-crystal COFs. Here, we resolve this challenge using a high-quality single-crystal COF to definitively delineate proton transport pathways. Single crystal X-ray diffraction (SCXRD) demonstrates that dynamic lattice contraction and a well-defined hydrogen bonding network (O–H⋯O: 2.868 Å) are present in hydrated COF-300 (COF-300-H₂O). Under humid conditions, the superprotonic conductivity of COF-300-H₂O reaches 0.37 S cm⁻¹ at 85 °C and 98% relative humidity, with a low activation energy of 0.196 eV, comparable to commercial Nafion membranes. Under anhydrous conditions, COF-300-H₂O maintains hydrogen-bond network integrity up to 130 °C, with proton conductivity reaching 3.16 × 10⁻³ S cm⁻¹ (E_a = 0.253 eV). Using SCXRD with DFT calculations, we establish the first single-crystal COF model to map proton transport pathways and elucidate the Grotthuss hopping mechanism.