Abstract
Proton-conducting metal–organic frameworks (MOFs) have been gaining attention for their role as solid-state electrolytes in various devices for energy conversion and storage. Here, we present a convenient strategy for inducing and tuning of superprotonic conductivity in MOFs with open metal sites via postsynthetic incorporation of charge carriers enabled by solvent-free mechanochemistry and anion coordination. This scalable approach is demonstrated using a series of CPO-27/MOF-74 [M2(dobdc); M = Mg2+, Zn2+, Ni2+; dobdc = 2,5-dioxido-1,4-benzenedicarboxylate] materials loaded with various stoichiometric amounts of NH4SCN. The modified materials are not achievable by conventional immersion in solutions. Periodic density functional theory (DFT) calculations, supported by infrared (IR) spectroscopy and powder X-ray diffraction, provide structures of the modified MOFs including positions of inserted ions inside the [001] channels. Despite the same type and concentration of proton carriers, the MOFs can be arranged in the increasing order of conductivity (Ni < Zn < Mg), which strongly correlates with amounts of water vapor adsorbed. We conclude that the proton conductivity of CPO-27 materials can be controlled over a few orders of magnitude by metal selection and mechanochemical dosing of ammonium thiocyanate. The dosing of a solid is shown for the first time as a useful, simple, and ecological method for the control of material conductivity.
Highlights
Development of proton-conducting materials and gaining control of the conductivity are important both for fundamental understanding of charge transport phenomena as well as potential applications in sensors and polymer electrolyte membrane fuel cells (PEMFCs).[1,2] Within the last decade, metal−organic frameworks (MOFs) emerged as a class of compounds suitable for these purposes, due to their high performance and designability.[3−6] The literature contains several competing methods for inducing proton conductivity in MOFs which generally fall into two main categories
Promising and effective alternative is offered by the second main approach toward proton-conducting MOFs that relies on postsynthetic modifications (PSMs) of preassembled frameworks.[8]
We demonstrate that grinding rigid CPO-27/MOF-74(Mg, Zn, Ni) materials with NH4SCN leads to a series of functionalized charged frameworks whose channels, filled with counterbalancing ions, become suitable for proton transport
Summary
Development of proton-conducting materials and gaining control of the conductivity are important both for fundamental understanding of charge transport phenomena as well as potential applications in sensors and polymer electrolyte membrane fuel cells (PEMFCs).[1,2] Within the last decade, metal−organic frameworks (MOFs) emerged as a class of compounds suitable for these purposes, due to their high performance and designability.[3−6] The literature contains several competing methods for inducing proton conductivity in MOFs which generally fall into two main categories. The first approach, known as the de novo synthesis, requires nonpolymeric building blocks and leads to the installation of Brønsted acidic centers either as linker pendant groups, guests, or terminal ligands This approach, has serious limitations regarding inducing and control of conductivity in resultant frameworks. As a remedy for typically complicated, costly, and nonecological syntheses in solution, a facile solvent-free mechanochemical approach, which is established in organic,[15] organometallic[16] and main group chemistry,[17] and enables large-scale synthesis,[18−23] has increasingly become an alternative consideration for the preparation of various functional materials including porous MOFs.[24] Even though mechanochemistry is not widely used to induce proton conductivity in MOFs, there are a few distinct reports in the literature. Mechanochemical stoichiometric dosing of an ionic compound is shown for the first time as a useful, simple, and scalable method for the control of material conductivity
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