Hydrogen fuel cells offer a promising route toward low-emission energy conversion, as they generate water as a benign byproduct. Their performance critically depends on the proton exchange membrane (PEM), which must combine high proton conductivity with thermal and chemical stability under operating conditions. This has motivated the search for solid-state proton-conductive materials, among which hydrogen-bonded organic frameworks (HOFs) have recently emerged as attractive PEM candidates due to their intrinsic hydrogen-bond networks that can support water-mediated proton transport. Phosphonate-based HOFs are particularly appealing because strong hydrogen bonding between phosphonic acid groups enhances structural stability under ambient conditions. Two such frameworks, UPC-H5a and GTUB-5, have been reported to exhibit high proton conductivities via a Grotthuss-like mechanism, yet UPC-H5a outperforms GTUB-5 by roughly four orders of magnitude. This work aims to clarify how nanoconfinement modulates hydrogen bond network dynamics and, in turn, proton transport, thereby providing a mechanistic explanation for this striking disparity.

We first employed Grand Canonical Monte Carlo (GCMC) simulations and isosteric enthalpy calculations to characterize water adsorption in both frameworks. Despite comparable accessible pore volumes and similar adsorption enthalpies over a wide loading range, GTUB-5 exhibits higher water uptake at intermediate to high pressures, indicating that neither pore volume nor overall adsorption capacity alone accounts for the conductivity differences. To uncover the relevant microscopic factors, we performed Ab Initio Molecular Dynamics (AIMD) simulations to probe the hydrogen bond network within the hydrated frameworks. In UPC-H5a, water molecules approach phosphonate oxygen atoms more closely and form a more persistent yet dynamically reconfiguring hydrogen bond array than in GTUB-5, with frequent hydrogen bond breaking and reforming, which are hallmarks of Grotthuss-type proton transport. These results show that differences in pore geometry and the resulting nanoconfinement control the structure and dynamics of the hydrogen bond network, providing a molecular-level rationale for the superior proton conductivity of UPC-H5a over GTUB-5 and offering design principles for next-generation solid-state PEM materials.