Pore size effects on high-efficiency proton conduction in three stable 3D Al-based MOFs modified with imidazole

Abstract

Proton-conducting materials have become highly desirable for clean energy-related applications, and the design and synthesis of stable and efficient proton-conduction materials have become increasingly important but challenging. Three stable porous metal–organic frameworks (MOFs) based on the aluminum(III) cation (Al-MOFs), DUT-5 (1), DUT-4 (2), and NOTT-300 (3), were selected as imidazole (Im) supporters and were successfully used to obtain three proton conduction composites (namely Im@1, Im@2, and Im@3) with excellent proton conductivities (σ). At 80 °C and 100% RH, the proton conductivity values of 1–3 were respectively 1.39 × 10⁻², 2.98 × 10⁻², and 5.57 × 10⁻² S cm⁻¹, which increased significantly with the decrease in their aperture sizes (1, 11.1 Å; 2, 8.5 Å; 3, 6.5 Å). The proton conductivity values of Im@1–3 were significantly higher than those of the original MOFs 1–3. Especially, 3 has a suitable aperture size for loading imidazole molecules and restricting them in a limited space, which made it difficult for the imidazole molecules of Im@3 to escape from the pores of 3, eventually leading to the formation of strong continuous hydrogen-bonding networks. Therefore, at 80 °C and 100% RH, the proton conductivities (σ) of Im@3 can reach up to 2.55 × 10⁻¹ S cm⁻¹, which is close to the σ of the commercial benchmark Nafion under the same conditions. Finally, the related molecular simulation was accomplished to elucidate the proton conduction mechanism by using the Grand Canonical Monte Carlo (GCMC) method. All the above features make Im@3 one of the most promising candidates for further practical applications in fuel cells.