Enhancement of Mobile Protons Induced by Ammonia in Isostructural Hydrogen-Bonded Organic Frameworks

Superprotonic conductivity is achieved by simple compositional tuning of hydrogen-bonded organic frameworks (HOFs). We compare two isostructural HOFs, H2O_DOBDC [(C8H6O6)(H2O)] and NH4+_DOBDC [(C8H5O6)(NH4+)], each undergoing a reversible transformation from monoclinic to triclinic symmetry with incorporation of additional lattice water molecules under high temperature and humidity conditions. The ammonium-containing triclinic phase, NH4+_DOBDC-TS [(C8H5O6)(NH4+)(H2O)], displays the highest conductivity, reaching 2.55 × 10–3 S cm–1 at 363 K and 95% relative humidity, in contrast to 1.15 × 10–6 S cm–1 for H2O_DOBDC-TS [(C8H6O6)(H2O)2]. 1H-Pulsed-field-gradient nuclear magnetic resonance (1H-PFG-NMR) reveals similar diffusion coefficients for both NH4+_DOBDC-TS and H2O_DOBDC-TS. However, the mobile proton concentrations are significantly different, calculated as 4.88 × 1025 ions m–3 for NH4+_DOBDC-TS and 1.23 × 1023 ions m–3 for H2O_DOBDC-TS. This distinction accounts for the significantly enhanced conductivity in the ammonium-based structure. These findings underscore the pivotal role of guest ions in modulating hydrogen-bonded networks and establish a design strategy for soft crystalline proton conductors.