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, [Sm₂(fum)₃(H₂O)₄]·3H₂O (Sm-fum-7H₂O) and [Er₂(fum)₃(H₂O)₄]·8H₂O (Er-fum-12H₂O) (H₂fum = fumaric acid), and regulated their proton conductivities by changing the water clusters. As expected, Sm-fum-7H₂O 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-12H₂O and some previously reported MOFs. Additionally, Sm-fum-7H₂O and Er-fum-12H₂O demonstrated high dielectric functions, reaching 2.22 × 10³ and 1.42 × 10⁵ at 10^2.5 Hz, respectively, making Er-fum-12H₂O a highly dielectric material. More importantly, broadband dielectric spectroscopy measurements indicated that there was a dielectric relaxation process in Er-fum-12H₂O 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.