Anisotropic Water-Mediated Proton Conductivity in Large Iron(II) Metal

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Anisotropic Water-Mediated Proton Conductivity in Large Iron(II) MetalOrganic Framework Single Crystals for Proton-Exchange Membrane Fuel Cells Hana Bunzen, Ali Javed, Danielle Klawinski, Anton Lamp, Maciej Grzywa, Andreas KalyttaMewes, Michael Tiemann, Hans-Albrecht Krug von Nidda, Thorsten Wagner, and Dirk Volkmer ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01902 • Publication Date (Web): 18 Dec 2018 Downloaded from http://pubs.acs.org on December 19, 2018

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ACS Applied Nano Materials

Anisotropic Water-Mediated Proton Conductivity in Large Iron(II) Metal-Organic Framework Single Crystals for Proton-Exchange Membrane Fuel Cells Hana Bunzen,†,* Ali Javed,‡ Danielle Klawinski,‡ Anton Lamp,† Maciej Grzywa,† Andreas Kalytta-Mewes,† Michael Tiemann,‡ Hans-Albrecht Krug von Nidda,§ Thorsten Wagner,‡ Dirk Volkmer†,* Chair of Solid State and Materials Chemistry, Institute of Physics, University of Augsburg, Universitätsstraße 1, D-86159 Augsburg, Germany †

Department of Chemistry, Faculty of Science, Paderborn University, Warburger Straße 100, D-33098 Paderborn, Germany ‡

Experimental Physics V, Center for Electronic Correlations and Magnetism, Institute of Physics, University of Augsburg, Universitätsstraße 1, D-86159 Augsburg, Germany §

ABSTRACT: Herein we present a new proton conducting iron(II) metal-organic framework (MOF) of an unusual structure formed by chains of alternating bistriazolate-p-benzoquinone anions and iron(II) cations with four axially coordinated water molecules. These chains assemble via π-π stacking between the aromatic units to form a threedimensional grid-like network with channel pores filled with water molecules. The material was structurally characterized by single crystal XRD analysis, and its water and thermal stability was investigated. The proton conductivity was studied by impedance measurements on needle-like single crystals. A simple but efficient measurement set-up consisting of interdigital electrodes was used. The influence of the crystal orientation, temperature and humidity was investigated. The iron(II)-MOF showed the highest proton conductivity of 3.3·10-3 S cm-1 at 22 °C and 94 % relative humidity. Contrary to most known structures, the conductivity in this material is controlled by chemical properties of the pore system rather than by grain boundaries. The presented material is the starting point for further tailoring the proton conducting properties, independent of morphological features which could find potential applications as membrane materials in proton-exchange membrane fuel cells. Keywords: metal-organic framework, proton conductivity, anisotropy, iron(II) MOF, water stability.

■ INTRODUCTION

Regenerative and sustainable energy production demands highly efficient conversion and storage technologies. Here fuel cells could play a major role, since (stationary) storage of liquids or gases is relatively simple and these fuels offer high energy densities compared to e.g. batteries.1 For the conversion of chemical energy back to electrical energy, the electrochemical devices rely on an ion conductive material, which determines the efficiency of the process. In a widespread type, the so called protonexchange membrane fuel cells (PEMFCs), electricity is generated by transporting protons through a membrane electrolyte whereas electrons are transported via an external circuit.2 The membrane materials of choice are often ionic polymers (ionomers),3,4 such as a sulfonated fluoropolymer called Nafion (developed and registered by DuPont).5 However, due to their amorphous nature, it is rather difficult to optimize the proton transport pathway

and to characterize the conduction behavior in these polymers. Therefore, crystalline porous materials, such as proton conducting metal-organic frameworks (MOFs), seem to be promising alternatives.6 Their highly crystalline nature facilitates the understanding of the conductive mechanism and the freedom in the choice of building components and active groups allows designing new and better performing materials.7 Due to the high surface areas, high porosities and extended crystalline structures, MOFs have already found various applications in many different fields such as gas storage and separation,8-10 and recently also as proton conducting materials.11-13 One of the first reported proton conducting MOFs was a 1D coordination framework formed by iron cations and oxalate anions.14 Others include, for instance, frameworks prepared by combing 2,5-dihydroxy-1,4-benzoquinone with different metal cations.15 In general, two main types of proton conducting MOFs have been reported, water-mediated proton

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recorded in the range of 150-4000 cm-1 on a Bruker Equinox 55 FT-IR spectrometer equipped with an ATR unit. Magnetic properties were investigated with a Quantum Design MPMS 5 SQUID magnetometer. Susceptibility measurements were performed in a temperature range from 2 to 345 K and in external fields H of 1 and 10 kOe. The molecular formula of CFA-17 with the occluded solvent molecules, i.e. [Fe(C6N6O2)(H2O)4]·5H2O, was used for the calculation. Thermogravimetric analysis (TGA) was performed with a Netzsch STA 409 PC analyzer in a temperature range of 25 800 °C under a nitrogen atmosphere at a heating grade of 10 K min-1. X-ray powder diffraction (XRPD) data were collected in the 5–50° 2θ range using a Seifert XRD 3003 TT – powder diffractometer with a Meteor1D detector operating at room temperature using Cu Kα1 radiation (λ=1.54187). VT XRPD data were collected on a Bruker D8 ADVANCE. The sample was ground and loaded into a quartz capillary. The patterns were recorded in a temperature range of 25 to 600 °C, in the 5–70° 2θ range. The XRPD data at relative humidity of 85 % and 93 % were collected on a Panalytical Empyrean diffractometer equipped with a CHC plus+ chamber and a Bragg-BrentanoHD mirror employing Cu-radiation. The patterns were recorded in a temperature range of 25 to 60 °C, in the 5–40° 2θ range. For the single crystal X-ray analysis, a crystal of CFA-17 was taken from mother liquor and mounted on a MiTeGen MicroMounts. The data for the structure determination were collected on a Bruker D8 Venture diffractometer. Intensity measurements were performed using monochromated (doubly curved silicon crystal) MoKα radiation (0.71073 Å) from a sealed microfocus tube. Generator settings were 50 kV, 1 mA. APEX3 software was used for preliminary determination of the unit cell.25 Determination of integrated intensities and a unit cell refinement were performed using SAINT.26 The structure was solved and refined using the Bruker SHELXTL Software Package.27 The positions of H-atoms of water molecules, which could not be determined from the difference Fourier map, were added by AFIX constraints. Complete crystallographic data have been deposited in the CIF format with the Cambridge Crystallographic Data Center, 12 Union Road, Cambridge CB21EZ, UK as supplementary publication no. CCDC 1839932.

conducting MOFs and anhydrous proton conducting MOFs.12 Herein we present a MOF of the first type. Microcrystalline MOF samples are usually used to estimate proton conductivity through alternating current (AC) impedance analysis, and thus, the characterization of anisotropic effects is precluded because of the random orientations. To solve these problems, single crystals with ordered and well-defined guest molecules are necessary. However, to produce and to measure MOF single crystals is not trivial. To date, many of reported conducting MOFs lacked crystals large enough, sufficiently regularly shaped, or adequately stable (with regard to durability at higher working temperatures and moisture levels). Therefore, reports on using MOF single crystals to study proton conductivity are scarce and often require a complex sample processing (Table 1).16-22 On the contrary, herein we propose a simple but efficient measurement set-up consisting of interdigital electrodes used to measure conductivity of MOF single crystals. The present study utilizes anisotropic conductivity of large iron(II)-MOF single crystals to investigate the influence of the pore network and grain boundaries on the proton transport. A pore dominated proton conductivity is important for further material design, since it allows controlling the conductivity by tuning the chemical properties of the pores, for instance, by selecting different ligands. Table 1. Proton conductivity of single crystals of selected coordination frameworks. Studied material

Conductivity and measurement conditions PCMOF-17 1.2∙10-3 S cm−1, 25 °C, 40% RH [Cu2(Htzehp)2(4,4′- 1.4∙10-3 S cm−1, bipy)]·3H2O 80 °C, 95% RH [Zn(H2PO4)2(TzH)2] 1.1∙10-4 S cm−1, 130 °C, dry N2 CoLa-II 3.1∙10-4 S cm−1, 25 °C, 95% RH [Fe(ox)(H2O)2] > 5.4∙10-9 S cm−1, 20 °C, 80% RH CPM-102 CFA-17

1.1∙10-2 S cm−1, 22.5 °C, 98.5% RH 2.1∙10-3 S cm-1, 22 °C, 95% RH

Material used to connect the crystal Silver paste

Ref.

Gold wires

17

Gold paste

18

Gold wires

19

Pressure sensitive adhesive Gallium metal

21

No material

This work

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16

Synthesis

of

[Fe(C6N6O2)(H2O)4]

(CFA-17).

Na2(C6N6O2)·4H2O (30.6 mg, 0.10 mmol) and FeCl2·4H2O (20 mg, 0.10 mmol) were dissolved in 4 mL H2O and placed in a 10 mL glass tube which was sealed. The mixture was placed into a heating block and kept at 90 °C for 1 hour. Then the heating was switched off and the mixture was cooled down very slowly (overnight). CFA-17 was obtained as yelloworange needle-like crystals (35 mg, yield calc. for [Fe(C6N6O2)(H2O)4]·5H2O: 86.2 %). IR: 𝜈 =3499, 3152, 1677, 1642, 1481, 1436, 1382, 1197, 1168, 1024, 991, 759, 701, 597, 495, 339 and 213 cm-1.

22

■ EXPERIMANTAL SECTION

Materials and methods. All reagents were of analytical grade and used as received from commercial suppliers, except for benzobistriazole which was prepared according a previously published procedure23 and used to synthetized Na2(C6N6O2)·4H2O.24 Micrographs of CFA-17 were recorded with an optical microscope (Olympus IX70) equipped with a camera and a scanning electron microscope (LEO Zeiss, Gemini 982). Fourier transform infrared (FTIR) spectra were

Proton conductivity measurements. Impedance measurements were carried out by using a Solartron SI1260 frequency response analyzer (FRA). The system was enhanced by a Chelsea Dielectric Interface because of the induced low signal-to-noise ratio. The interface acts as highimpedance buffer between sample and FRA and as a wide range band-width current-to-voltage converter which

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enables measurements of low currents (pA). The impedance measurements were done in sweep mode (frequency range: 1 MHz – 1 Hz) with an amplitude of 0.1 V. The samples were measured in a Faraday cage to reduce noise from the surroundings.28

anisotropic proton conductivity with a macroscopic setup (see below). CFA-17 crystallizes in the orthorhombic crystal system in the space group Pna21 (no. 33) with an empirical formula {[Fe(C6N6O2)(H2O)4]·5H2O} (Table 2). The framework is formed by one-dimensional chains of alternating bistriazolate-p-benzoquinone anions and iron(II) cations, which assemble to form a grid-like threedimensional framework with channel pores filled with lattice water molecules. The unit cell of CFA-17 viewed along the a axis and the diagonal axis of the bc plane is shown in Figure 1.

For contacting, the samples were arranged in different positions on interdigital electrodes (IDE, commercially available sensing substrates: UST GmbH, alumina based, 3 mm × 3 mm with interdigitated electrodes with 20 μm electrode gaps and Pt10 heater). The contact area of the crystal was evaluated by utilizing a confocal laser microscope (Olympus, Lext 3D Measuring Laser Microscope OLS4000).

Table 2. Crystallographic parameters and structure refinement details of CFA-17.

Defined atmospheres were provided by a custom built gas mixing equipment based on mass flow controllers. The dry nitrogen stream (50 mL/min) was humidified by flowing through a washing bottle with pure water. The relative humidity was measured in the gas stream with a Sensirion SHT2x humidity and temperature sensor. For measurements at higher temperatures (≥ 30 °C) a drying cabinet (Heratherm OMS60) was used. For low temperature characterization (< 30 °C) a custom-built climate chamber based on a cryostat (IKA®CBC5Control) was used. The temperature of the substrate was recorded by measuring the resistance of the integrated Pt10 heater with a digital multimeter (DMM, Agilent, 34972A). The crystals were equilibrated for 24 h at a certain temperature and humidity level.

Compound Empirical formula Formula weight T/K λ/Å Crystal system Space group a/ Å b/ Å c/ Å α = β = γ/ ˚ Volume/ Å3 Z Density/ Mg m-3 (calc.) Absorption coeff./ mm-1 F(000) Theta range/ ˚ Independent reflections Data/restraints/parameters Goodness of fit on F2 R1 (I >2s(I))a wR2 (all data)b Δρmax,min/eÅ-3

■ RESULTS AND DISCUSSION

Synthesis and structural refinement. To prepare a potentially proton conducting MOF, we selected bistriazolate-p-benzoquinone as a ligand (Scheme 1). This molecule was prepared by oxidizing benzobistriazole and was shown to be redox active in alkali aqueous solutions as we reported recently.24 If the starting material, benzobistriazole, is combined with iron(II) cations, a MOF with honeycomb-like structure is obtained, which was recently reported as a material for a cooperative adsorption of carbon monoxide on coordinatively unsaturated iron(II) sites.29 However, when sodium bistriazolate-p-benzoquinone (yellow solid) was combined with iron(II) cations in water (Scheme 1), a MOF {[Fe(C6N6O2)(H2O)4]}n (CFA-17; CFA stands for Coordination Framework Augsburg University) with a completely different and unexpected structure was obtained.

a

CFA-17·5H2O C6H18FeN6O11 406.11 100(2) 0.71073 Orthorhombic Pna21 (no. 33) 6.4238(2) 15.2359(6) 15.1142(6) 90 1479.26(9) 4 1.824 1.094 840 2.67 to 36.39 3707 3707/1/216 0.974 0.0352 0.1210 1.196, -1.154

R1 = Σ||Fo| – |Fc||/Σ|Fo|. b wR2 = {Σ[w(Fo2 – Fc2)2]/Σ[w(Fo2)2]}1/2.

Each iron cation is six-coordinated. It forms two coordinative bonds with N(2)-donor nitrogen atoms from two different ligand molecules, and the other four coordination sites are occupied by water molecules. This coordination motif is rather uncommon for bistriazolatebased ligands which are usually found in coordination frameworks as µ4- or µ6-bridging ligands.29-32 The bond length between the metal ion and the nitrogen donor atoms are 2.134(2) and 2.145(2) Å, and the bond length between the metal ion and the oxygen donor atoms are 2.104(2), 2.109(2), 2.130(2) and 2.2099(18) Å. The interatomic dihedral angles are in a range of 84.68(8) to 96.21(10) °, i.e. they deviate slightly from the 90° angles found in perfect octahedral symmetry, indicating a slightly distorted octahedral coordination of the iron ions. By comparing the Fe-O bond length values to the typical values of Fe-O interatomic distances determined by X-ray analysis in [Fe(H2O)6]2+ and [Fe(H2O)6]3+,33 which are 2.13 and 1.99 Å, respectively, it can be concluded that the distances in CFA-17 correspond to Fe(II)-O interatomic distances which is in agreement with the stoichiometric

Scheme 1. Synthesis of CFA-17 from sodium bistriazolate-p-benzoquinone and iron(II) chloride in water.

CFA-17 formed long (several hundred micrometers) needle-like orange single crystals (Figure S1), which were used to solve the MOF structure by single crystal X-ray analysis. In addition, the crystals allowed studying



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Figure 1. Molecular packing of CFA-17 viewed along the a axis (a) and along the diagonal of the bc plane (b), and a portion of the crystal structure of CFA-17 showing the coordination environment of the iron cations (c); Hydrogen atoms and non-coordinated water molecules inside the pores are omitted for clarity.

hydrogen bonds, bond lengths and angles can be found in Supporting Information. The experimental powder X-ray diffraction (XRPD) pattern of a bulk sample of CFA-17 shows good agreement with the pattern simulated on the basis of the crystal structure, affirming the phase purity of the sample (Figure S4).

charge-balance calculation in {[Fe(C6N6O2)(H2O)4]·5H2O}n. Iron(II) octahedral complexes can be either high-spin or low-spin. By comparing the Fe-N interatomic distances in CFA-17 to the average values of Fe(II)-N distances reported in literature for high-spin and low-spin iron(II) complexes,34 which are 2.1 - 2.3 Å and 1.9 - 2.1 Å, respectively, the iron(II) centers in CFA-17 are expected to be in the highspin state. This was further confirmed by a SQUID measurement (Figure 2). The analysis of the C-O interatomic distances in the ligand molecule revealed that the C-O bond length was for both cases 1.223(3) Å indicating a presence of two double bonds, and thus a quinone unit. This finding was further confirmed by an IR measurement, in which a strong C=O stretch band at 1677 cm-1 was observed (Figure S2).

Although the X-ray analysis revealed a coordination polymer with channel pores, despite all our effort we were unable to remove the solvent molecules completely from the pores without collapsing the structure. This clearly demonstrates the importance of the hydrogen bond network (see Figure S5 and Table S4) for the framework stability. Nevertheless, a computational structure analysis with the Poreblazer35 and PLATON36 software revealed that CFA-17 has 1D channels with a pore diameter of 4.30 Å and a pore limiting diameter of 3.65 Å, and the total potentially accessible void volume of 241.9 Å3 which corresponds to 16.3 % of the unit cell volume (1479.3 Å3).

The molecular packing in Figure 1 shows that in CFA-17, the one-dimensional coordination chains assemble via π-π stacking between the aromatic units (with a mean interplanar distance of 3.21 Å) to form a grid-like structure with channel-like pores along the a axis (see Figure S3). The framework is further stabilized by a complex network of hydrogen bonds, which also plays an important role in the material proton conductivity. The hydrogen bonds formed between the lattice water molecules (five molecules per formula unit), coordinating water molecules and triazolate and carbonyl functional groups array inside the one-dimensional pores and construct a supramolecular three-dimensional network. The network not only increases the framework stability but also facilitates the proton conductivity in this direction, as seen later. The complete crystal data, structure refinement, figure showing H-bond network and list of

Figure 2. Temperature-dependent susceptibility (T) of CFA-17 (solid circles) and its inverse (open triangles) in the


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range of 2 to 345 K. The red solid lines indicate a Curie-Weiss law (T) = C/(T-) with a Curie-Weiss constant C = 3.15 emu K / mol and a weakly antiferromagnetic Curie-Weiss temperature  = -1K.

was partly transformed into a new phase. The reason for the phase transition was the removal of water molecules from the pores, which was further confirmed by thermogravimetric (TG) analysis and by variable temperature (VT) XRPD measurements (Figure 4). To avoid the influence of the new phase on the results of our investigations, all the proton conductivity measurements were carried out ≤ 50°C. To check a long-term stability of the framework in water, XRPD patterns of CFA-17 single crystals immersed in water at ambient temperature for 0, 14, 30 and 60 days were recorded (Figure S6). The unchanged XRPD patterns indicated a high moisture durability of CFA-17 and the possibility to keep the material in water over a long period of time, which is an essential prerequisite of materials for utilization in proton conductive membranes.

Magnetic properties. To clarify the spin state of the iron centers in CFA-17, a magnetic susceptibility measurement from 2 to 345 K was carried out. The SQUID data showed normal paramagnetic behavior (Figure 2), with a linear slope for the 1/χ vs temperature curve. The Curie constant C, determined by the slope of the curve, revealed an effective magnetic moment of 5.02 µB, i.e. a g value of 2.05, which is a reasonable value for paramagnetic iron(II) centers in high-spin state. Thus, the orbital moment is quenched by the ligand field, but Hund’s first rule concerning maximization of spin is still valid.

Figure 3. XRPD patterns of CFA-17 recorded at relative humidity of 85 % (top) and 93 % (bottom) at various temperatures.

Figure 4. VT XRPD (top) measurements and TG analysis (bottom) of CFA-17 carried out under a nitrogen atmosphere.

Water and thermal stability. For the later presented study on proton conductivity under different humidity and temperature levels, knowing the material stability under these conditions is important. Figure 3 shows XRPD patterns of CFA-17 kept in air atmosphere of relative humidity (RH) of 85 and 93 % in a temperature range of 25 to 60 °C. When heated above 50 °C, CFA-17

The overall thermal stability of CFA-17 was investigated by VT XRPD measurement and TG analysis carried out under a nitrogen atmosphere (Figure 4). The TG analysis revealed four main weight lost steps. In the first two steps, water molecules were removed; firstly the noncoordinated solvent molecules from the pores (measured: -15.04 %, which corresponds to 3.1 water molecule per



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Figure 5. Microscope images of CFA-17 powder sample (left), single crystal perpendicular (middle) and parallel (right) to the IDE.

from 3.3·10-3 S/cm (CFA-17_2) to 2.4·10-7 S/cm (CFA-17_1) depending on the crystal orientation, measurement temperature and relative humidity (Table 3). These values are comparable to those of previously reported watermediated proton conducting MOFs (see Table 1 and Supporting Information of the ref. 16, 17 and 22).

formula unit), and then the four water molecules coordinated to the iron cations (measured: -19.27 %; calc. for four water molecules per formula unit: -19.40 %). When heated above 275 °C, the organic ligand decomposed in two steps resulting in a body-centered cubic (bcc) form of iron as observed by XRPD recorded at 600 °C [diffraction peaks at 44.6 (110) and 65.0 (200); Figure 4]. As seen from the VT XRPD measurement (Figure 4), removing the lattice water influenced the crystal structure and if the coordinated water molecules were removed, the framework collapsed. The diffraction peaks in the XRPD pattern recorded at 50 °C were shifted to the higher 2 Theta values compare to the pattern recorded at room temperature which indicated that upon removing the lattice molecules the unit cell of CFA-17 became smaller. Unfortunately, due to the low data quality, calculating lattice parameters of the new phase was not successful. When the temperature increased further, the coordinated water molecules were removed and the material lost its crystallinity and became amorphous. This clearly showed the importance of water molecules participating in the hydrogen bond network for the structure stabilization.

Table 3. Summary of proton conductivities of CFA-17 depending on orientation, temperature and relative humidity. Sample name

Orientation on IDE

Temp RH . [°C] [%]

CFA-17_1

powder, statistical orientation

CFA-17_2

single crystal, perpendicular

22 23 22 23 22 22 22 22 22

70 78 86 94 77 82 85 94 85

Proton conductivity [S/cm] 2.5∙10-7 8.4∙10-7 1.8∙10-5 1.1∙10-4 1.3∙10-5 5.9∙10-5 5.7∙10-4 3.3∙10-3 2.0∙10-5

22

95

2.1∙10-3

23

95

2.14∙10-5

22 30 40 50 23.26 24.28 24.44 24.91 25.09 26.27

86 86 85 84 85.42 85.92 86.05 85.98 86.24 85.99

5.2∙10-4 7.0∙10-4 6.3∙10-4 6.9∙10-4 1.00∙10-4 1.07∙10-4 1.13∙10-4 1.16∙10-4 1.15∙10-4 1.30∙10-4

CFA-17_2_II single crystal, parallel CFA-17_3 single crystal, perpendicular CFA-17_3_II single crystal, parallel CFA-17_4 single crystal, perpendicular

Proton conductivity. To investigate the influence of crystal orientation, temperature and relative humidity on the proton conductivity, AC impedance measurements on needle-like single crystals as well as on a powder sample were carried out by contacting the crystals on interdigital electrodes (IDEs, Figure 5). The empty IDEs’ impedance under humid conditions is typically more than 2 order of magnitudes higher than for the oriented crystals. Therefore the contribution of the electrode structure can be ignored for the following data evaluation (example see Supporting Information). By utilizing the IDE, no additives are needed to ensure the gas accessibility of the pore channels of CFA-17. Other methods reported in the literature16-22 use additives, such as conducting pastes or electric glues for connecting wires to the crystals (see Table 1). The contact area of the crystal on the IDE was evaluated from microscope images (Figure S8) and the proton conductivity was calculated by eq. S1 (see Supporting Information). The proton conductivity varied

CFA-17_5

single crystal, perpendicular

Anisotropic effects of the proton conductivity were studied by performing impedance measurements on single crystals in two different orientations with respect to the electrodes (perpendicular and parallel) at 22 °C and RH 85 % or 95 % (in nitrogen, 50 mL/min). For perpendicular orientation, the impedance was measured in the direction along the (011) plane of the crystal and for


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ACS Applied Nano Materials statistical orientation of the crystals on the IDE in the powder sample; only few crystals in the powder sample will be oriented perpendicular which is why the overall proton conductivity is lower than for the single crystals measured along the (011) plane. On the other hand, the proton conductivity of the powder sample is nearly the same as for the single crystals parallel on the IDE at RH 85 % (CFA-17_2_II) and higher at RH 94 % (CFA17_3_II). This, too, may be assigned to the statistical orientation and to grain boundary effects. In total, the proton conductivity measured in the powder sample may be assumed to be governed by intrinsic proton transport.21

parallel orientation in the direction along the (100) plane (Figure S3). The Nyquist plots (Figure 6) show the typical behavior for proton conductive materials, namely a (depressed) semicircle for high frequencies and a capacitive tail for low frequencies.18 The proton conductivity was deduced from the semicircle by fitting an equivalent circuit, that consists of a series resistance (R1) and a parallel circuit of a resistor (R2) and a constant phase element (CPE1), in the frequency range from 1kHz to 1 MHz as shown in the inset of Figure 6. R1 corresponds to wire and electrode resistances and R2 is assumed to account for proton resistance. The calculated proton conductivity of CFA-17_2 was 5.6·10-4 S/cm (Table 3) for the crystal oriented perpendicular on the IDE and 2.0·10-5 S/cm for the parallel orientation at 22 °C and RH 85 %. This means that the proton conductivity of CFA-17_2 is one order of magnitude higher in the direction along the (011) plane than in the direction along the (100) plane. The measurements were repeated with another crystal to affirm reproducibility of the results. The proton conductivity of CFA-17_3 was 2.1·10-3 S/cm for the crystal oriented perpendicular on the IDE and 2.1·10-5 S/cm for the parallel orientation at 22 - 23 °C and RH 95 %. For CFA-17_3 the proton conductivity is two orders of magnitude higher in the direction along the (011) plane than in the direction along the (100) plane. The different orders of magnitude of the values measured in the direction along the (011) plane for CFA-17_2 and CFA-17_3 may be assigned to different relative humidity levels and different defect concentration in the crystal structure of the respective sample. For both crystals, higher proton conductivity was observed in the direction along the (011) plane. Along this direction pore channels with H-bonds network exist, which facilitates the proton transport in this direction. This is consistent with the previously reported observations.17,18 To study the dependence of proton conductivity on relative humidity and temperature, impedance measurements on single crystals were performed at various relative humidity levels (RH 70 – 95 %) and temperatures (22 – 50 °C) in nitrogen flow (50 mL/min). The crystals were measured in a perpendicular orientation on the IDE [impedance measured in the direction along the (011) plane]. A powder sample that depicts the impedance behavior and proton conductivity of statistically oriented single crystals on the IDE was also measured at various relative humidity levels (RH 70 – 95 %).

Figure 6. Impedance spectra of CFA-17_2 single crystal perpendicular (black) and parallel (red) on IDE at 22 °C and RH 85 % (in nitrogen, 50 mL/min). Enlargement of the high frequency range with fitted curves (dashed lines, bottom).

The proton conductivity of both the powder sample and the CFA-17_2 single crystal (perpendicular on IDE) increases with increasing relative humidity (Table 3, Figure S9). This indicates a water-mediated proton transport in the pore channels. In this respect, the Hbonds network in CFA-17 seems to play an important role. The proton conductivity of the powder sample is lower than for the single crystals (CFA-17_2 and CFA-17_3) oriented perpendicular on the IDE, presumably due to the

Temperature effects on the proton transport were investigated for the CFA-17_4 and CFA-17_5 single crystals oriented perpendicular on IDE (Table 3). For CFA-17_4, the conductivity increases from 22 °C to 30 °C. This can be explained by enhancement of the proton mobility at higher temperature, which enhances the proton conductivity. At 40 °C the proton conductivity decreases and then slightly increases again at 50 °C. This may be assigned to the observed phase transformation of


ACS Applied Nano Materials the material at 40 °C and RH 85 %. For CFA-17_5, the activation energy of the proton transport was determined from an Arrhenius plot where ln(σT) is plotted as a function of inverse temperature (Figure 7).13 The resulting activation energy of 𝐸𝑎 = 0.68 ± 0.20 suggests, that the proton transport follows a vehicular type mechanism13, where protons transfer in the hydrated form through the crystal. The observed mechanism is also in agreement with the dependence of proton conductivity on various relative humidity levels (Table 3 and Figure S9). However, due to the relatively large error caused by the measurement in such a small temperature window also a Grotthus type mechanism13, mediated by water molecules forming an H-bonds network inside the channel pores of CFA-17, seems possible. Calculating the activation energy based on the two proton conductivity values recorded for sample CFA-17_4 reveals 𝐸𝑎 = 0.3 ± 0.2 eV. For clarification, a future study will focus on this issue. -3,20

Ln (σT) [S cm-1K]

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humidity, which is comparable to the best-known watermediated proton conducting MOF materials developed so far. The presented single crystal studies provide a basis for understanding the essence of complex conductive mechanism in MOFs, which can help to better understand and construct highly performing electrolyte materials. CFA-17, comprising channel pores filled with water molecules, is the starting point for tailoring the proton conductivity further, since the observed properties were mostly independent of morphological features, as e.g. grain size.

ASSOCIATED CONTENT Supporting Information. Microscopy images and IR spectrum of CFA-17, X-ray diffraction analysis and crystallographic data of CFA-17, additional data of proton conductivity measurements of CFA-17. This material is available free of charge via the Internet at http://pubs.acs.org.

CFA-17_5 linear fit

-3,25

AUTHOR INFORMATION

-3,30

Corresponding Author * E-mail: [email protected], [email protected].

-3,35

ORCID

-3,40

Hana Bunzen: 0000-0003-1824-0291 Ali Javed: 0000-0002-9947-0904 Michael Tiemann: 0000-0003-1711-2722 Thorsten Wagner: 0000-0002-4014-0185 Dirk Volkmer: 0000-0002-8105-2157

-3,45 -3,50 -3,55 0,00334

0,00335

0,00336

0,00337

0,00338

Temperature [K-1]

Notes

The authors declare no competing financial interest.

Figure 7. Plot of ln(σT) vs. 1/T for the CFA-17_5 single crystal (perpendicular on IDE) at about RH 85 % (in nitrogen, 50 mL/min). The red line is the linear fit.

ACKNOWLEDGMENT We thank Romy Ettlinger (Institute of Physics, University of Augsburg) for recording SEM micrographs and Dana Vieweg (EP V, University of Augsburg) for the SQUID measurements. Further, we thank Dr. Gerhard Berth and Peter Mackwitz (Prof. Dr. Artur Zrenner, Department of Physics, Paderborn University) for the confocal laser microscope measurements. This work was partially funded by the DFG priority program 1928 COORNETS (grant holder D.V.). H.B. is grateful to the program “Chancengleichheit für Frauen in Forschung und Lehre” from the University of Augsburg for financial support via a fellowship. T.W. thanks the Federal Ministry of Education and Research (BMBF, 13N12969) and the German Research Foundation (DFG, Project Number 316657024) for financial support. H.-A. K.v.N. acknowledges funding by the DFG within the transregional collaborative research centre TRR 80 “From electronic correlations to functionality” (Augsburg, Munich, Stuttgart).

■CONCLUSIONS

A new proton conducting iron(II)-MOF, named CFA-17, based on benzotriazole-p-benzoquinone ligands was synthesized and structurally characterized. CFA-17 consists of chains of alternating bistriazolate-pbenzoquinone anions and iron(II) cations with four axially coordinated water molecules, which assemble via π-π stacking between the aromatic units to form a threedimensional grid-like network with channel pores. In the channels, there is a structure-stabilizing network of hydrogen bonds between the coordinating and lattice water molecules and the benzotriazolate-p-benzoquinone ligands. Proton conductivity measurements carried out on single crystals revealed anisotropic proton conductivity. The preferred conductivity path was in the direction along the (011) plane which was consistent with the direction of the hydrogen bond chain array inside the channel pores. The proton conductivity of the material was up to 2.1·10-3 S cm-1 at 22°C and 95 % relative

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Anisotropic Water-Mediated Proton Conductivity in Large Iron(II) Metal

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