High Proton Conduction at above 100 °C Mediated by Hydrogen Bonding in a Lanthanide Metal–Organic Framework

Abstract

A lanthanide metal-organic framework (MOF) compound of the formulation [Eu2(CO3)(ox)2(H2O)2]·4H2O (1, ox = oxalate) was prepared by hydrothermal synthesis with its structure determined crystallographically. Temperature-dependent but humidity-independent high proton conduction was observed with a maximum of 2.08 × 10(-3) S cm(-1) achieved at 150 °C, well above the normal boiling point of water. Results from detailed structural analyses, comparative measurements of conductivities using regular and deuterated samples, anisotropic conductivity measurements using a single-crystal sample, and variable-temperature photoluminescence studies collectively establish that the protons furnished by the Eu(III)-bound and activated aqua ligands are the charge carriers and that the transport of proton is mediated along the crystallographic a-axis by ordered hydrogen-bonded arrays involving both aqua ligands and adjacent oxalate groups in the channels of the open framework. Proton conduction was enhanced with the increase of temperature from room temperature to about 150 °C, which can be rationalized in terms of thermal activation of the aqua ligands and the facilitated transport between aqua and adjacent oxalate ligands. A complete thermal loss of the aqua ligands occurred at about 160 °C, resulting in the disintegration of the hydrogen-bonded pathway for proton transport and a precipitous drop in conductivity. However, the structural integrity of the MOF was maintained up to 350 °C, and upon rehydration, the original structure with the hydrogen-bonded arrays was restored, and so was its high proton-conduction ability.

Imidazole-doped nanocrystalline cellulose solid proton conductor: synthesis, thermal properties, and conductivity

“…This fact explains the difference in activation energies. On the other hand, our results coincide with those obtained for the materials measured at lower humidity levels and/or containing heterocyclic molecules (Kreuer et al 1993;Tang et al 2014;Pandey et al 1999;Yamada et al 2012;Yamada et al 2004;Yamada and Ogino 2015). Generally, the higher activation energy (i.e.…”

Highly Water‐Stable Lanthanide–Oxalate MOFs with Remarkable Proton Conductivity and Tunable Luminescence

“…Although MOF‐based proton conductors have been widely investigated over the last decade, only a small number of these materials contained lanthanide elements and only a few showed high room‐temperature proton conductivity and the highest record obtained is 1.3 × 10 −3 S cm −1 (Table S4, Supporting Information) . To the best of our knowledge, MOF‐ 1 discussed in this work displays so far the best room‐temperature proton conductivity of 3.42 × 10 −3 S cm −1 and possesses an added benefit of remarkable water stability, which is due to the high coordination number of the lanthanide ions introduced in the framework.…”

Achieving Amphibious Superprotonic Conductivity in a CuI Metal–Organic Framework by Strategic Pyrazinium Salt Impregnation

“…This could be attributed to the threshold kinetic energy required for rapid release of the protons from the proton carriers. However, this is not a new phenomenon for MOF‐based materials exhibiting anhydrous proton conductivity in a wide range of temperatures starting from low temperature …”