Close-packing effect of water clusters within metal-organic framework pores on proton conductivity: a dielectric relaxation phenomenon in loose space and colossal dielectric permittivity.

Proton-conducting metal-organic frameworks (MOFs) have attracted tremendous attention for their promising application in proton-exchange membrane fuel cells. Water clusters play an extremely important role in the proton-conduction process and affect the proton conductivity of host materials. To date, the close-packing effect of water clusters within pores on proton conductivity due to the amorphous structure of commercial proton-exchange membranes is unclear. Herein, we prepared two crystalline MOFs containing different water clusters, namely, [Sm ₂ (fum) ₃ (H ₂ O) ₄ ]·3H ₂ O (Sm-fum-7H ₂ O) and [Er ₂ (fum) ₃ (H ₂ O) ₄ ]·8H ₂ O (Er-fum-12H ₂ O) (H ₂ fum = fumaric acid), and regulated their proton conductivities by changing the water clusters. As expected, Sm-fum-7H ₂ O showed a high proton conductivity of 6.89 × 10 ^-4 S cm ^-1 at 333 K and ∼97% RH because of the close packing of the water clusters within the pores triggered by a lanthanide contraction effect, outperforming that of Er-fum-12H ₂ O and some previously reported MOFs. Additionally, Sm-fum-7H ₂ O and Er-fum-12H ₂ O demonstrated high dielectric functions, reaching 2.22 × 10 ³ and 1.42 × 10 ⁵ at 10 ^2.5 Hz, respectively, making Er-fum-12H ₂ O a highly dielectric material. More importantly, broadband dielectric spectroscopy measurements indicated that there was a dielectric relaxation process in Er-fum-12H ₂ O with an activation energy of 0.59 eV. The present findings provide a better understanding of the crucial role of confined water clusters in proton conductivity and the novel phenomenon of the coexistence of proton conduction and dielectric relaxation in crystalline MOF materials.