Trojan Horse Thiocyanate: Induction and Control of High Proton Conductivity in CPO-27/MOF-74 Metal–Organic Frameworks by Metal Selection and Solvent-Free Mechanochemical Dosing

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 [M 2 (dobdc); M = Mg 2+ , Zn 2+ , Ni 2+ ; dobdc = 2,5-dioxido-1,4-benzenedicarboxylate] materials loaded with various stoichiometric amounts of NH 4 SCN. 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.

Solvent-Free Mechanochemical Dense Pore Filling Yields CPO-27/MOF-74 Metal–Organic Frameworks with High Anhydrous and Water-Assisted Proton Conductivity

“…In addition to high anhydrous proton conductivity at temperatures exceeding 100 °C, the modified materials CPO-27-NCS FA and CPO-27-NCS MA also show high conductivity at lower temperatures (25−60 °C) under mild humidification (Figures S28−S29, S32−S34 and Tables S3−S4). Importantly, compared to our previous studies involving AM cations, 43 the mechanochemical reactions carried out in this work lead to the twice higher loading of charge carriers (6:6 metal:cation ratio) for CPO-27-NCS FA or CPO-27-NCS MA and, as a consequence, to the highest humidity-assisted proton conductivities in the whole CPO-27-NCS series, exceeding 10 −3 S/cm already at low humidity (30−50% RH) and reaching 10 −2 S cm −1 at 60 °C and 70% RH for CPO-27(Mg)-NCS MA (Tables S3−S4). Within the…”

“…Regardless of the modification, however, the CPO-27-NCS materials are stable in high temperatures and after EIS measurements at anhydrous conditions (Figures S15−S17 and S24−S25). It is noteworthy that the concentration and type of proton carriers have a key role for conductance since the modified materials with significantly lower amount of FASCN or MASCN (e.g., loaded at 6:3 or 6:1 metal/thiocyanate level) as well as those filled with NH 4 SCN (maximal loading was 6:3 metal/thiocyanate) 43 show negligible anhydrous proton conductivity on the order of ∼10 −9 S cm −1 or lower under the same conditions (Figures S26−S27). Thus, dense pore filling with proton carriers capable of forming waterfree extended networks of hydrogen bonds inside pores is crucial for achieving MOFs with high anhydrous proton conductivity, which are still few and far between.…”

Dual-Functional Metal–Organic Framework for Luminescent Detection of Carcinoid Biomarkers and High Proton Conduction

“…To further investigate the mechanism of proton conduction, the activation energy ( E a ) was fitted using the Arrhenius equation σT = σ 0 exp­(− E a / k B T ), where σ, σ 0 , E a , k B and T are the proton conductivity, pre-exponential factor, proton transport activation energy, Boltzmann constant, and the test temperature, respectively. , The best fit gave E a = 0.47 eV, indicating that the vehicle mechanism (>0.4 eV) is dominant for the proton transfer of 1 (Figure b). ,,,, The numerous water molecules as proton carriers in the hydrophilic channels need to absorb heat and ionize into hydration ions (H 3 O + ), achieving the transfer of protons based on the vehicle mechanism. In addition, considering the complexity of the mechanism of proton conduction and the crystal structure information, we speculate that continuous hydrogen-bonding networks can be formed among the uncoordinated carboxylic oxygen atoms, sulfonate oxygen atoms, protonated nitrogen atoms, and adsorbed water molecules under high-temperature and high-RH conditions, which can promote proton hopping along the hydrogen-bonding networks.…”

“…Metal–organic frameworks (MOFs) serve as an important class of crystalline functional materials, synthesized by coordination bonds between metal nodes and various multifunctional linkers, and have exhibited numerous important applications such as catalysis, sensing, separation, magenetism, proton conduction, etc. − In recent years, the construction of multifunctional MOF materials has attracted more and more attention, − especially in luminescence sensing and proton conduction for the dual-functional materials. − Among them, lanthanide-based MOFs (Ln–MOFs) can provide attractive candidatse for the construction of luminescence sensors, because lanthanide centers can exhibit excellent luminescence performances, including extremely high color purity and large Stokes shift as well as a relatively long luminescence lifetime . On the basis of these advantages, luminescent Ln–MOFs have been extensively exploited for the luminescence detection of multiple types of analyses, such as metal cations, inorganic anions, small molecules, amino acid molecules, solvents, gases, explosives, and so forth. − In addition, the organic component of Ln–MOFs is also very important in determining the properties of MOFs.…”

The unique opportunities of mechanosynthesis in green and scalable fabrication of metal–organic frameworks

“…Lupa et al used a solvent-free mechanosynthesis process to obtain a tunable proton conductivity in MOFs. 234 Simple grinding of CPO-27/MOF-74(Zn, Ni, Mg) with ammonium thiocyanate (NH 4 SCN) in the presence of small amounts of ethanol in an agate mortar for about 10 min led to the formation of three different CPO-27 compounds with coordinated thiocyanates. IR spectra confirmed the coordination of thiocyanate within the frameworks via the obvious chemical shift of ν CN from 2072 cm −1 to longer wavenumbers.…”