Insulator-to-Proton-Conductor Transition in a Dense Metal–Organic Framework

Abstract

Metal-organic frameworks (MOFs) are prone to exhibit phase transitions under stimuli such as changes in pressure, temperature, or gas sorption because of their flexible and responsive structures. Here we report that a dense MOF, ((CH3)2NH2)2[Li2Zr(C2O4)4], exhibits an abrupt increase in proton conductivity from <10(-9) to 3.9 × 10(-5) S/cm at 17 °C (activation energy, 0.64 eV) upon exposure to humidity. The conductivities were determined using single crystals, and the structures were analyzed by X-ray diffraction and X-ray pair distribution function analysis. The initial anhydrous structure transforms to another dense structure via topotactic hydration (H2O/Zr = 0.5), wherein one-fourth of the Li ions are irreversibly rearranged and coordinated by water molecules. This structure further transforms into a third crystalline structure by water uptake (H2O/Zr = 4.0). The abrupt increase in conductivity is reversible and is associated with the latter reversible structure transformation. The H2O molecules coordinated to Li ions, which are formed in the first step of the transformation, are considered to be the proton source, and the absorbed water molecules, which are formed in the second step, are considered to be proton carriers.

Coordination environments and π-conjugation in dense lithium coordination polymers

“…8 There are several families of LCPs and most of them are dense structures with simple mono-carboxylate linkers, such as formate 9 and acetate, 10,11 or dicarboxylate linkers such as oxalate, 12 tartrate [13][14][15] and terephthalate. 21 This is analogous to that in transition metal coordination polyhedra in classical oxides. [17][18][19] Li-O bonds are ionic in nature in oxides, and this is considered to be true also in LCPs.…”

“…45 By using other crystallization routes, we can obtain different coordination environments. 21 The former was synthesized by a solvothermal reaction and the ECoN values of the Li-O coordination environments are 3.99 and 4.00, while the values in the latter are extremely high, 4.78, 4.54, 4.50 and 4.87. 21 The former was synthesized by a solvothermal reaction and the ECoN values of the Li-O coordination environments are 3.99 and 4.00, while the values in the latter are extremely high, 4.78, 4.54, 4.50 and 4.87.…”

Impact of Defects on Pyrazolate Based Metal Organic Frameworks

“…Noteworthy, the hydroxide ions mobility becomes very close to the reported values for hydroxide ions in solution . These results can be explained on the basis of a Grotthuss mechanism with the water mediated mobility of hydroxide ions inside the hydrated pore channels (Figure c).…”

Metal–Organic Frameworks as a Versatile Platform for Proton Conductors

“…Oxalate‐based MOF is a typical class of proton conductors that have been extensively studied. [ 73–77 ] Among these, ferrous oxalate dihydrate [Fe(ox)·2H 2 O] [ 73 ] is worth mentioning, which was constructed by the Fe(II) ions, oxalate ions and coordinated water molecules. At 25 °C and 98% RH, Fe(ox)·2H 2 O exhibited a high proton conductivity, with the σ value of 1.3 × 10 −3 S cm −1 .…”