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.

Abstract

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 102.5 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.