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. The proton conductivity of two metal-organic frameworks can be tuned via the close packing of water clusters within their pores, in which one of the MOFs shows a dielectric relaxation phenomenon and giant dielectric permittivity.