High proton conductivity in cyanide-bridged metal–organic frameworks: understanding the role of water

Abstract

Supplementary Information (ESI) available: IR, PXRD, TGA and proton conductivity analysis for the as-synthesized NdMo−MOF and its activated forms at 80 °C, 130 °C and 150 °C, respectively. SeeWe investigate and discuss the proton conductivity properties of the cyanide-bridged metal-organic framework (MOF) [Nd(mpca)2Nd(H2O)6Mo(CN)8]·nH2O (where mpca is 5−methyl−2−pyrazinecarboxylate). This MOF is one of an exciting class of cyanide-bridged materials that can combine porosity with magnetism, luminescene, and proton conductivity. Specifically, we show that this material features highly hydrophilic open channels filled with water molecules. They enable a high proton conductivity, as much as 10 -3 S cm -1 . A rich hydrogen-bonding network, formed by the ligands' carboxylate groups with both coordinated and lattice water molecules, facilitates this high proton conductivity. Combined thermogravimetric studies, FTIR spectroscopy and PXRD analysis show that upon heating at 80 °C, the lattice water molecules are removed without any change in the framework. Further heating at 130 °C results in a partial removal of the coordinated water molecules, while still retaining the original framework. These activated MOFs shows an increasing conductivity from ~10 -9 S cm -1 to ~10 -3 S cm -1 when the relative humidity increases from 0% to 98%. Our studies show that the increase in proton conductivity is correlated with the re-hydration of the framework with lattice water molecules. The Arrhenius activation energy for the proton conductivity process is low (Ea = 37 kJ·mol -1 ), indicating that the protons "hop" through the channels following the Grotthuss mechanism. The fact that this MOF is remarkably stable both under high humidity conditions and at relatively high temperatures (up to 130 °C) makes it a good candidate for real-life applications.

Designed Synthesis of Multiluminescent Materials Using Lanthanide Metal‐Organic Frameworks and Carbon Dots as Building‐Blocks

“…Moreover, they are environmentally friendly and have excellent biocompatibility, therefore they are widely employed to make smart multifunctional materials. [44] We have also demonstrated that these materials retain their structural integrity during multiple dehydration/hydration cycles. [37][38][39][40] A few examples of CDs@MOFs composites were reported so far.…”

“…The most suitable method is the transmission electron microscopy (TEM). The bands in the range of 1550-1700 cm -1 , at 1500 cm -1 and 1100 cm -1 are the characteristic peaks of the EuW-MOF structure [44,45] and they indicate that the MOF framework is retained upon CDs uptake. [21] Fortunately, the reversibility of the dehydration-rehydration process provides the possibility to iden-tify the relative location of CDs within the MOF framework using photoluminescence measurements.…”

“…Eu-MOFs/N,S-CDs, CQDs/ ZIF-8 composites). [44][45][46] These materials are obtained using a molecular building-block approach which employs the self-assembly of mononuclear lanthanide complexes, [Ln(mpca) 2 ] + (Ln 3+ = Eu, Tb, Gd, mpca = 5-methyl-2-pyrazinecarboxylate) and octacyanometallate ions, [M(CN) 8 ] 4-(M 4+ = Mo, W) [44][45][46] in aqueous solution. Moreover, they are environmentally friendly and have excellent biocompatibility, therefore they are widely employed to make smart multifunctional materials.…”

“…[30][31][32] They are superior to the traditional quantum dots and dye materials in terms of their chemical inertness. [44] The 1D channels are filled with water molecules engaged in a rich hydrogen bonding network which is responsible for the high proton conductivities observed in these materials. [30,[33][34][35][36] Notably, the CDs containing heteroatoms such as N, O, S are also known to have multiple emissions.…”

Proton Conductive Zr-Phosphonate UPG-1—Aminoacid Insertion as Proton Carrier Stabilizer

“…Pseudo-activation energies (Ea) were calculated by fitting the conductivity data to the Vogel−Tamman−Fulcher (VTF) equation ( Figure S13 ) [ 42 ], which is widely used for non-Arrhenius polymeric ion conductors. The non-Arrhenius behavior has been already described for other MOFs, although is not fully understood [ 43 , 44 ].…”