Superprotonic Conductivity with Ultralow Activation Energy: Integration of HOF into Mixed-Matrix Membranes for Enhanced Proton Transport

Proton exchange layer (PEL) is a crucial element of bipolar membrane fuel cell (BPMFC) which is a promising alternative of nonrenewable energy sources. Framework materials, especially hydrogen-bonded organic frameworks (HOFs) are particularly useful to fabricate these PELs due to their long-range hydrogen-bonded network, solution processability, and absence of metal toxicities. Thus, in view of fabricating a cost-effective PEL with consistent performance over a wide range of temperatures, we present a bis(diaminotriazene)-functionalized HOF, namely, 1·2H2O, synthesized using a solvothermal reaction with 1,4-bis(2,4-diamino-1,3,5-triazine)-benzene in N,N-dimethylformamide (DMF) and water. Three-dimensional (3D) HOF consists of a continuous array of hydrogen bonds between −NH2 groups and water, typically favorable for proton hopping. The pristine HOF exhibits a proton conductivity (PC) of 10–3 S·cm–1 order at room temperature in a hydrated state. However, with 40% doping of HOF into a mixed-matrix membrane (MMM) of poly(vinylidene fluoride) (PVDF)–poly(vinylpyrrolidone) (PVP) polymer blend, named as MMM@1·2H2O-40, the PC value increased ∼10 times that of pristine HOF reaching superprotonic conductivity of 1.19 × 10–2 S·cm–1 at similar conditions. Additionally, it also achieved an ultralow activation energy of 0.044 eV, the lowest recorded for any HOF-based membrane reported until date. Thus, the reported HOF is a potential candidate to facilitate the advancement of fuel cell technologies.