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. 2021 Jun 17;13(25):29820–29826. doi: 10.1021/acsami.1c06346

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

Magdalena Lupa 1, Paweł Kozyra 1, Gabriela Jajko 1, Dariusz Matoga 1,*
PMCID: PMC8289229  PMID: 34137584

Abstract

graphic file with name am1c06346_0006.jpg

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.

Keywords: synthesis, mechanochemistry, metal−organic frameworks, proton transport, adsorption

Introduction

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, however, has serious limitations regarding inducing and control of conductivity in resultant frameworks. Direct introducing of linkers with acidic substituents is often hindered by their strong affinity toward metallic centers, which may consequently lead to undesired coordination and nonconducting products.7 Similarly, direct de novo introducing of charged guests or terminal ligands as proton carriers is often serendipitous as highly dependent on uncontrolled pH changes during framework assembly. A 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 Within this approach, several proton-conducting MOFs were obtained via impregnation of parent MOFs with inorganic acids9 or protic organic molecules.10 Other distinct examples include organic linker modifications such as oxidation of thiol groups11 or ring-opening reactions with sultones.12 Finally, the combined de novo and PSM approaches can lead to a significant rise in conductivity, like in the MIL-101 platform in which pendant SO3H groups were first installed (MIL-101-SO3H)13 and then nonvolatile strong acid was introduced to form H2SO4@MIL-101-SO3H of excellent superprotonic conductivity.14 It is noteworthy, however, that all the abovementioned synthetic approaches toward proton-conducting MOFs usually involve complicated steps, require large amounts of solvents and energy, are difficult to upscale, and do not allow for control of proton conductivity.

As a remedy for typically complicated, costly, and nonecological syntheses in solution, a facile solvent-free mechanochemical approach, which is established in organic,15 organometallic16 and main group chemistry,17 and also 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. Among initial mechanochemical syntheses of proton-conducting MOFs were those described by Horike, Kitagawa, and others who carried out grinding of zinc oxide with azoles and orthophosphates, followed by the demonstration of intrinsic proton conduction of the resulting frameworks.25,26 Using the same mechanochemical approach for analogous systems, these researchers also reported the formation of a glassy state of a MOF that exhibited enhanced proton conductivity by 2 orders of magnitude as compared to its crystalline form.27 In 2015, we carried out a unique postsynthetic modification of {[Mn2(ina)4(H2O)2]·2EtOH}n28 (JUK-1) by grinding it with an ionic compound that yielded a proton-conducting {(NH4)2[Mn(ina)2(NCS)2]}n·xH2O (JUK-2).29 This fast and stoichiometric solid-state reaction led to a significant structural reconfiguration of the parent framework involving unzipping of crystalline JUK-1 bilayers to JUK-2 monolayers.30

While mechanochemical syntheses of MOFs are gaining impetus, the use of mechanochemical methods for tuning of either ionic or electronic conductivity of these materials has not been reported so far. In the literature, a remarkable tuning of electrical conductivity in MOFs was previously demonstrated for HKUST-1 infiltrated by a redox-active 7,7,8,8-tetracyanoquinodimethane (TCNQ) molecule.31 However, the tuning was achieved by controlling of exposure (immersion) time of the MOF in a saturated TCNQ/CH2Cl2 solution. It is worth noting that tunability of conductivity is not only necessary to obtain the value as high as possible (e.g., for PEMFCs), it is also important to tailor materials conductivity for some specific applications, e.g., in devices for energy conversion and information transfer, such as sensors, capacitors, memristors, transistors, and batteries.

In this work, we report a convenient and facile strategy for realizing postsynthetic tunable proton conductivity in MOFs by scalable solvent-free mechanochemistry. 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. Contrary to our precedent case study involving JUK-2,29 the mechanochemical reactions carried out in this work lead to virtually intact networks, and the highest proton conductivities of the modified frameworks 10-fold exceed that of JUK-2. The structures for the modified MOFs are provided by density functional theory (DFT) calculations and are corroborated by vibrational spectroscopy and powder X-ray diffraction (PXRD). The conducting properties of all materials are compared and discussed. 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.

Results and Discussion

The family of CPO-27(M) MOFs (M = Mn, Mg, Fe, Co, Ni, Cu, and Zn) is known for relatively stable structures containing one-dimensional channels with open metal sites.32N,N′-dimethylformamide (DMF) molecules coordinated to metallic centers in the as-synthesized materials can be replaced with methanol or water and removed by heating under vacuum.33 The channels can be decorated by functional molecules for the application of interest. Recent literature examples show that the coordinative insertion of N,N′-dimethylethylenediamine enhances CO2 adsorption,34 whereas the insertion of urea enables highly stable superprotonic conductivity.35 In this work, we have carried out new scalable modifications of CPO-27 series and prepared three compounds with coordinated thiocyanates: (NH4)3[Mg6(dobdc)3(NCS)3(H2O)3]·xH2O (CPO-27(Mg)-NCS), (NH4)3[Zn6(dobdc)3(NCS)3(DMF)(H2O)2]·xH2O (CPO-27(Zn)-NCS), and (NH4)3[Ni6(dobdc)3(NCS)3(H2O)3]·xH2O denoted as CPO-27(Ni)-NCS (Figures 1 and S1). Regardless of the metal used, after grinding of CPO-27 and NH4SCN with the addition of a small amount of EtOH, rigid frameworks remain virtually intact (as confirmed by PXRD patterns), whereas the evidence for the coordination of thiocyanate is provided by the infrared (IR) shift of the strong ν(CN) band from 2072 (for the initial NH4SCN) to 2076, 2089, and 2101 cm–1 for CPO-27(Mg)-NCS, CPO-27(Zn)-NCS, and CPO-27(Ni)-NCS, respectively (Figures 1, S1, and S2). The mechanochemical reactions are fast, stoichiometric, and occur with retainment of crystallinity. The maximal stoichiometry of these reactions was determined to be 6:3 (metal/thiocyanate ratio) for the three MOFs, as evidenced collectively by powder X-ray diffraction (XRD) and IR spectroscopy (Figures 1, S2, and S3). The maximally loaded CPO-27-NCS materials show practically no uptake of nitrogen and argon (Figures S4 and S5), as compared to high uptakes reported in the literature for the CPO-27 materials, synthesized both in water and by solvent-free grinding, which indicates filling the pores of CPO-27 by ammonium thiocyanate.36,37 The reactions carried out at higher stoichiometric ratios (6:4 and 6:6) gave mixtures of uncoordinated thiocyanate and CPO-27-NCS (observable by the appearance of extra X-ray reflections of NH4SCN and/or extra v(CN) IR bands), whereas the use of lower stoichiometric ratios (6:2 and 6:1) led to partial filling of channels with coordinated NCS– and extra-framework NH4+ ions. The unique reactivity of the CPO-27 materials opens possibilities for tuning of their properties by mechanochemical dosing of thiocyanates with other counterions.

Figure 1.

Figure 1

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Solid-state reaction of CPO-27 with NH4SCN: (a) general scheme for the mechanochemical synthesis of CPO-27-NCS; (b)  PXRD patterns of mixtures after grinding CPO-27(Mg) with various amounts of NH4SCN; 6:1, 6:2, 6:3, 6:4, and 6:6 (given as CPO-27(Mg) to NH4SCN stoichiometric ratio). Characteristic reflections of NH4SCN are labeled with “*”; and (c) FT-IR spectra of CPO-27(Mg)-NCS, CPO-27(Zn)-NCS, and CPO-27(Ni)-NCS. Numbers indicate wavenumbers (in cm–1) of the characteristic CN stretches.

To verify the possibilities of both formation of CPO-27-NCS and tuning of their proton conductivities in solution, CPO-27(Mg, Zn, Ni) MOFs were immersed in alcohol solutions (methanol or ethanol) containing NH4SCN at various SCN– to MOF ratios (see the Supporting Information for details, Figures S6 and S7). Under no conditions we have tried the formation of CPO-27-NCS was observed, which proves the necessity of mechanical grinding in yielding CPO-27-NCS. Furthermore, given the recent demonstration of efficient gram-scale mechanosynthesis of CPO-27(Zn),36 it is noteworthy that CPO-27(Zn)-NCS can be entirely prepared from simple nonpolymeric precursors in a two-step mechanosynthesis. The second mechanochemical step can be reversed by immersing the modified materials in alcohols for several days at elevated temperature, which leads to recovery of initial CPO-27 materials upon removal of coordinated thiocyanates (Figure S8).

Insights into the structures of the modified MOFs were provided by periodic DFT+D calculations (see the Supporting Information for details). Our experimental XRD data did not allow us to unambiguously locate the positions of NH4+ and SCN– ions in the structure using Rietveld refinement (see the Supporting Information for details). We have taken under consideration the structures of CPO-27(Mg, Zn, Ni) with SCN– anions inserted inside the [001] channels at the metal-to-thiocyanate ratio of 6:3. DFT optimizations were carried out for two possible linkage isomers with either S- or N-bound thiocyanate. In the simplified model, the CPO-27 framework atoms were frozen. The calculated total energies for CPO-27-NCS and CPO-27-SCN optimized structures proved similar (for a given metal) and did not allow for the unambiguous discrimination between the two binding modes. Thus, full structural optimizations, which allowed a vibrational analysis, were performed for CPO-27(Mg)-NCS, CPO-27(Mg)-SCN, CPO-27(Zn)-NCS, and CPO-27(Zn)-SCN. Since the energies for the two linkage isomers within this approach were similar again, we have taken CN stretch wavenumbers as an indicator of the coordination mode of thiocyanate. The calculations clearly demonstrate that these wavenumbers are higher for M-NCS structures, with shorter CN bonds. To compare experimental IR wavenumbers for the modified CPO-27 MOFs with those calculated for the two binding variants, we have scaled our method by performing vibrational analysis for the selected monomeric thiocyanate complexes containing either of the three Mg, Zn or Ni metals. The Cambridge Structural Database (CSD) database search gave a few structurally characterized compounds including two complexes of [Zn(NCS)4]2– (CSD codes: MENBEM, MUDPIM, NESMOP)38−40 and [Ni(NCS)6]4– (CSD codes: FILVAA, YIGGIH01)41,42 whose averaged experimental CN wavenumbers were 2085 and 2095 cm–1, respectively. Their comparison with the values calculated by our method (2113 and 2119 cm–1, respectively) allowed us to estimate the scaling factor for the method as 0.988 (experimental/calculated), which was applied for the calculated raw wavenumbers in the modified CPO-27, and the scaled values were compared with the experimental wavenumbers (see Table S1). The scaled CN stretches for Mg- and Zn-based structures with N-bound thiocyanates match the experimental values very well. Similarly, the CN stretches estimated for the modified CPO-27(Ni) indicate that the nickel-based structure contains the N-bound thiocyanate. All three CPO-27-NCS structures, optimized by DFT methods, are presented in Figure 2.

Figure 2.

Figure 2

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Structures of CPO-27-NCS optimized by periodic DFT+D methods, view along the [001] channels (Mg, Zn—full optimization; Ni—framework atoms frozen). Hydrogen bonds involving NH4+ ions are indicated by red (N–H···O) and cyan (N–H···S) dotted lines between donor and acceptor atoms. Color codes: C—gray, H—light gray, N—blue, O—red, and S—yellow.

The optimized structures show significant differences in the location of ammonium ions inside the [001] channels (Figure 2). In all of the modified frameworks, ammonium ions are hydrogen-bonded with sulfur atoms of N-coordinated thiocyanates. These hydrogen bonds increase in strength as the average donor–acceptor distances (N···S) decrease in the order: Ni (3.35 Å) < Zn (3.27 Å) < Mg (3.14 Å), and the corresponding average N–H bonds are elongated: 1.066 Å (Ni) < 1.068 Å (Zn) < 1.089 Å (Mg) accordingly (Table 1). Apart from interactions with sulfur atoms, ammonium ions in CPO-27(Zn, Mg)-NCS are additionally stabilized by strong hydrogen bonds with intraframework oxygen atoms, which are also accountable for N–H bond elongations. The longer N–H bonds are, the higher proton mobility is inside MOF channels and, consequently, material conductivity.

Table 1. Hydrogen Bond Distances (Averaged, in Å) Involving NH4+ Ions Inside CPO-27-NCS.

    Mg Zn Ni
N–H···S d(NS) 3.14 3.27 3.35
d(NH) 1.089 1.068 1.066
N–H···O d(NO) 2.92 2.74  
d(NH) 1.046 1.071  
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In the literature, the recently reported studies for a single crystal of CPO-27(Co) showed that proton conduction of the material was anisotropic and occurred predominantly through one-dimensional channels (intrachannel proton conductivity at 25–30 °C was ∼10–4 S cm–1 in contrast to perpendicular directions with σ ∼ 10–6 S cm–1).43,44 It is justified to assume that the same intrachannel pathways are mainly responsible for proton conduction of other isoreticular CPO-27 MOFs and their modifications with retained framework structures, such as the CPO-27-NCS materials in this work. During solvent-free grinding of CPO-27(Co) and ammonium thiocyanate, we have observed partial decomposition of CPO-27(Co) and the formation of [Co(NCS)4]2– ions (the Vogel reaction),45,46 identifiable by the appearance of blue color (see Figure S11). However, the solvent-free reactions of CPO-27(Mg, Zn, Ni) MOFs with NH4SCN yielded CPO-27-NCS with ammonium proton carriers inside their unidirectional channels and induced high proton conductivity in the modified materials. This functionalization was enabled using thiocyanates as Trojan horse anions “carrying” counterbalancing protic cations. For proton conduction studies of the modified MOFs, alternating current (AC) impedance measurements were carried out using powdered samples pressed between metallic electrodes in a poly(tetrafluoroethylene) (PTFE) tube under different RH values from 30 to 90%, and variable temperatures from 25 to 60 °C (Figures 3 and S12–S16). The highest proton conductivities in the series, reaching 10–2 S cm–1 at high relative humidity (60 °C, 90% RH) and 10–4 S cm–1 already at 30% RH, were observed for CPO-27(Mg)-NCS. Despite the same type and concentration of proton carriers, the other two materials, CPO-27(Zn, Ni)-NCS showed lower values (Figure 3 and Table S2) and the whole series can be arranged in the decreasing order of conductivity: Mg > Zn > Ni. The series correlates well with the decreasing hydrophilicity of the MOFs, confirmed by their water vapor uptake isotherms (Figure 4). The isotherms also show the stability of the materials during adsorption–desorption cycling (Figure S24) as well as significant differences between the three MOFs including highest uptakes and conductivities for CPO-27(Mg)-NCS and the lowest for CPO-27(Ni)-NCS. The nanoconfinement of NH4SCN within the [001] channels of CPO-27 materials has a double effect on conductivity: (i) it induces high proton conductivity of CPO-27 through introducing extra-framework proton carriers and (ii) it modulates the ability of CPO-27 to adsorb water molecules that are essential elements of conduction pathways. The latter strongly depends on the selection of a metal center in the CPO-27 material. By metal selection (Mg, Zn, or Ni) and RH adjustment, proton conductivity of CPO-27-NCS can be controlled over 3 orders of magnitude (Figure 3c). It is likely that further finetuning might be achieved using various mixed-metal CPO-27 frameworks, whose mechanochemical syntheses have been recently reported in the literature.47 Interestingly, apart from appropriate metal selection, proton conductivities of CPO-27-NCS can be additionally controlled by mechanochemical stoichiometric dosing of thiocyanate ions within the 6:1–6:3 metal-to-thiocyanate ratio range. Unattainable by immersion in solution, tunable proton conductivity over 1 order of magnitude was achieved for all CPO-27-NCS at 90% RH and 60 °C (Tables S3–S5 and Figures S19–S21). At lower temperatures, the tuning range depends on a metal center and is either slightly narrower or wider, with up to 2- (Zn), 6- (Mg), and 40-fold (Ni) increases of conductivity upon increase of the dose of NH4SCN from 6:1 to 6:3. The conductivities achieved for the largest 6:3 doses are among the highest reported for similar materials and significantly higher as compared to initial CPO-27 MOFs (Table S6).

Figure 3.

Figure 3

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Proton conduction for CPO-27-NCS: (a) Arrhenius plots with activation energies indicated as numbers (in eV) after mechanochemical dosing of NH4SCN in CPO-27(Ni)-NCS (with 6:1, 6:2, and 6:3 metal/thiocyanate ratios); samples conditioned at 90%  RH. (b) Temperature-dependent AC impedance plots for CPO-27(Zn)-NCS at 45% RH (maximal NH4SCN loading, numbers indicate temperatures in °C) with equivalent circuit used for fitting plots. (c) Proton conductivities at 60 °C and different RH values, all MOFs with maximal NH4SCN loading.

Figure 4.

Figure 4

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Proton conductivity versus water vapor adsorption–desorption isotherms for CPO-27-NCS (with maximal NH4SCN loading) at 25 °C.

The linear regression of Arrhenius plots of the temperature-dependent proton conductivities of CPO-27-NCS provided activation energies (Ea) for proton transport inside their microporous channels at various RH values (Figures 3, S17, and S22). For two materials in the series, that is CPO-27(Mg, Ni)-NCS, the activation energies did not exceed 0.4 eV in the whole RH range, which shows that proton conduction occurs by a Grotthuss-type hopping mechanism, involving water molecules and ammonium cations. By contrast, proton conduction of CPO-27(Zn)-NCS is characterized by the increased Ea values between 0.46 and 0.86 eV, which indicates the different mechanism of proton transport involving translational movements of NH4+ and H3O+ ions as well as their deprotonated counterparts (vehicle mechanism). In general, NH4+ conductors are promising materials for electrochemical devices such as fuel cells utilizing ammonia or ammonium salts.48,49 The different mechanism for CPO-27(Zn)-NCS is likely associated with partial coordination of zinc metal centers with DMF molecules, whose presence disrupts the hydrogen bond network inside channels for proton hopping.

Conclusions

We have reported a convenient strategy for realizing tunable proton conductivity in metal–organic frameworks in which micropores are loaded with protic guest cations by solvent-free mechanochemistry. This scalable and facile approach is demonstrated using a series of CPO-27/MOF-74 materials loaded with ammonium thiocyanate. Tunable proton conductivities over 4 orders of magnitude are achieved in the whole series, with values as high as 10–2 S cm–1. The strategy could be extended with other salts composed of protic ions and those capable of complexation. The targeted MOFs for such postsynthetic modifications should preferably contain undercoordinated open metal sites. The modified proton-conducting MOFs of good stability and high performance may have applications in devices such as fuel and solar cells as well as other devices for energy conversion and information transfer.

Acknowledgments

The National Science Centre (NCN, Poland) is gratefully acknowledged for the financial support (Grant no. 2015/17/B/ST5/01190). The authors thank Prof. W. Makowski for the access to a volumetric gas adsorption analyzer.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.1c06346.

  • DFT optimized structure for CPO-27(Mg)-NCS (CIF)

  • DFT optimized structure for CPO-27(Zn)-NCS (CIF)

  • DFT optimized structure for CPO-27(Ni)-NCS (CIF)

  • Details of synthetic procedures, sample handling, physical measurements, and computational studies; additional IR, PXRD, EIS, and adsorption data (PDF)

Author Contributions

M.L.: investigation, formal analysis, visualization, and writing—original draft preparation. P.K.: investigation (DFT calculations). G.J.: investigation (sorption measurements). D.M.: conceptualization, formal analysis, writing—original draft preparation, writing—review and editing, supervision, project administration, and funding acquisition.

The authors declare no competing financial interest.

Supplementary Material

am1c06346_si_001.cif (22.2KB, cif)
am1c06346_si_002.cif (27.4KB, cif)
am1c06346_si_003.cif (27.1KB, cif)
am1c06346_si_004.pdf (3.6MB, pdf)

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

am1c06346_si_001.cif (22.2KB, cif)
am1c06346_si_002.cif (27.4KB, cif)
am1c06346_si_003.cif (27.1KB, cif)
am1c06346_si_004.pdf (3.6MB, pdf)

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