Proton Conduction in a Nonporous One ... - ACS Publications

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Proton Conduction in a Nonporous One Dimensional Coordination Polymer Jolanta Stankiewicz,*,†,∥ Milagros Tomás,*,‡,∥ Isabel T. Dobrinovitch,§ Elena Forcén-Vázquez,§ and Larry R. Falvello§ †

Instituto de Ciencia de Materiales de Aragón and Departamento de Física de la Materia Condensada, ‡Instituto de Síntesis Química y Catálisis Homogénea and Departamento de Química Inorgánica, and §Instituto de Ciencia de Materiales de Aragón and Departamento de Química Inorgánica, CSIC−Universidad de Zaragoza, 50009 Zaragoza, Spain S Supporting Information *

ABSTRACT: Complex impedance measurements on single crystals and pellets of a CuCo-orotate coordination polymer, as a function of relative humidity and temperature, reveal proton conduction with a relatively high thermal activation energy, a negligible isotope effect, a large pre-exponential factor in the conductivity model, and a strong dependence on humidity, all of which are consistent with a vehicle mechanism. The proton conductivity found for this polymer compares favorably with that of MOFs and related porous materials even though it has no framework with pores and channels to provide proton-conduction pathways.

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INTRODUCTION Typical proton conducting materials are amorphous organic polymers with nanosized water channels, such as the extensively studied Nafion, or crystalline inorganic compounds such as perovskite-type oxides.1 Both are widely used in fuel cell technology because of their facile proton conduction. Nafionlike polymers are efficient at temperatures below 100 °C whereas ceramic oxides operate at temperatures higher than 600 °C. Metal−organic framework (MOF) compounds and coordination polymers (CP) with pores or channels can be considered as intermediate solids between these two extremes because they have channels, as do the organic polymers, but also show a crystalline nature with metal atoms, as do the oxides. Some MOFs and CPs are proton conductors. For instance, the b-PCMOF2 with 1H-1,2,4-triazole in the pores reaches a relatively high conductivity of 5 × 10−4 S cm−1 at 150 °C.2 These recent findings prompted a new and active effort within the field of proton-conducting materials.3−5 A few nonporous coordination polymers containing acidic groups, such as H2PO4, have also been studied, showing interesting conduction behavior.6 In water-mediated proton-conducting materials, the hydrogen-bonding networks serve as proton conduction pathways. For a well-defined hydrogen-bond network, proton jumps between neighboring molecules are accompanied by an additional reorganization of the proton solvating environment. This rearrangement (most often a rotation), in turn, enables the next jump. The rate of the transport is determined by the intra-bond proton transfer and the solvent reorganization. Such a process is usually called structural diffusion or, in the case of proton transport in water, the Grotthuss mechanism.7 Proton © XXXX American Chemical Society

conduction may also occur through molecular diffusion (“vehicle mechanism”) in which the proton diffuses together with a vehicle (e.g., H3O+ or NH4+).8 The counter diffusion of unprotonated vehicle (e.g., H2O) gives the net transport of protons. The rate of this process is entirely determined by the hydrodynamic properties of the medium. In some cases, a mixed conduction, in which the short- distance transport of hydronium ions (vehicle mechanism) bridges the percolation path of the hydrogen-bond network (Grotthuss-type hopping), seems to be observed.9,10 Few solid one dimensional (1D) coordination polymers have been reported as proton conductors,11 and these are mainly porous materials.2−5 We have recently reported the complete substitution of all of the water-bound hydrogen atoms by deuterium in a crystal of a 1D coordination polymer of Mn(II) citrate when the crystal, which presents an extended H-bonding network but no channels or pores, is exposed to a D2O atmosphere.12,13 In what follows, we report the results of structural and conduction studies on crystals of a 1D CuCoorotate copolymer that, similar to the polymer of Mn(II) citrate, has neither channels nor pores. Its electrical conductivity, which is ∼10−6 S cm−1 at 25 °C, varies strongly with humidity and temperature. Further, although the crystal is hydrated and possesses a hydrogen-bonding network, this network is discontinuousthat is, it does not form a water wire extending through an entire unit cell in any direction contrary to what is found for the Mn(II) citrate polymer. Received: June 12, 2014 Revised: August 19, 2014

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dx.doi.org/10.1021/cm502142a | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

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without H atom positions, and short of conducting a neutron diffraction analysis, this detailed treatment using X-ray data provides at least partial information. Crystal data are provided in Supporting Information Table S1, and a list of hydrogen bonds is given in Supporting Information Table S2. The Kitaigorodskii packing index was calculated by Platon.19 Conductivity Measurements. For conductivity measurements, pellets of 130 and 260 μm thickness and 10 mm in diameter were prepared by pressing polycrystalline powder at room temperature. We also selected a few single crystals of approximately 1 × 2 × 0.1 mm3 in size for impedance experiments. Silver paste or sputtered gold electrodes were deposited on parallel surfaces of the crystals or pellets. Impedance spectra were obtained for the frequency range from 20 Hz to 10 MHz. The excitation amplitude of 100 mV was applied to ensure conditions close to thermodynamic equilibrium. These measurements were performed in the temperature range from 10 °C up to approximately 60 °C in a small home-made chamber that was gastight. We also varied the relative humidity (RH) in our experiments. Since the conductivity is drastically affected by the partial water pressure, we allowed the samples to equilibrate with the surrounding atmosphere before taking measurements. The corresponding rate was approximately 2% RH/1 h. The RH values were determined by a capacitive humidity sensor that had been calibrated using tabulated values for various salts in equilibrium with saturated vapor at different temperatures. Both humidity and platinum temperature sensors were placed next to the sample. The morphological characterization of the samples was carried out using field-emission scanning electron microscopy (FE-SEM).

EXPERIMENTAL SECTION

Crystal Structure Determination. Crystallographic data (excluding structure factors) for the structure reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-1002422. Copies of the data can be obtained free of charge from www.ccdc.cam.ac.uk/conts/ retrieving.html or on application to the director. Single crystal X-ray diffraction data were measured at room temperature from a red rectangular plate-like crystal, using an Xcalibur S3 diffractometer. Measurements and data integration were managed by the program CrysAlis Pro.14 The structure was solved by direct methods using the program SIR-9215 and was refined by full-matrix least-squares analysis (program SHELXL-9716). Computer graphics were made using the program Diamond.17 Refinement of 823 parameters to 10534 data and 6 observational restraints converged with the residuals R1 = 0.0395 [for 9385 data with I > 2s(I)]; wR2 = 0.0949 (for all data), goodnessof-fit 1.038. The largest difference peak and hole had densities of 0.571 and −0.639 e·Å−3. The crystal is triclinic and acentric (Sohncke space group P1, which is also polar); the absolute structure parameter18 refined to a value of 0.142(10). The absence of a center of symmetry can be tested in the usual ways, for example using the program Platon.19 However, that the structure is polar can also be seen by visual inspection. The polar segment of the polymer shown in Figure 1,

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RESULTS AND DISCUSSION The compound is a polymer formed by two alternating building blocks, formally a copolymer, with formula {[Co(orot)2(bpy)][μ-Cu(bpy)(H 2 O)]} n [Co(orot) 2 (bpy)] n ·5nH 2 O, or C50H44Co2CuN14O22. (Bpy is 2,2′-bipyridyl, C10H8N2; H2orot is orotic acid, C5H4N2O4.) The backbone of the 1D polymer, Figure 1, is formed by alternating Cu and Co centers bonded by two crystallographically independent orotate groups. Both orotate groups chelate the cobalt centers, while the orotate coordination to copper involves only the oxygen atom of one of the carbonyl groups (O4) of one orotate and one oxygen atom (O18) of the carboxylate group of the other orotate. The cobalt centers complete their distorted octahedral coordination sphere with two N atoms from one 2,2′-bipyridyl ligand, while the copper atoms are coordinated by the two N atoms of another 2,2′bipyridyl ligand and by an H2O molecule (O4W). Together with O4 and O18 from the two orotate groups, these form a distorted square pyramidal coordination environment. One unit of the polymer, that is, one link consisting of Co, Cu, and their coordination environments, has a charge of +1. This is balanced by an independent [Co(orotate)2bipy]− anion. Also present are five unligated H2O molecules (O1W, O2W, O3W, O5W, O6W). The six crystallographically independent H2O molecules are all involved in hydrogen bonding but do not form an unbounded H-bonded chain (a construct commonly called a water wire). Although it was not possible to locate all of the hydrogen atoms of the water molecules, the oxygen··· oxygen distances permit us to identify probable H-bond locations. Importantly, as already noted, it is not possible to identify a continuous chain of water molecules consistent with proton transfer via the Grotthuss mechanism. There are at least three water molecules (O2W, O4W, O3W) connected by contiguous hydrogen bonds, Figure 2 [O3W···O4W 2.708(6) Å, O4W···O2W 2.995(5) Å]. Other water···water distances, however, are not indicative of hydrogen bonding [O1Wii··· O6Wi 3.849(7), O1Wii···O3W 4.171(7), O2W···O5W 3.696(6)

Figure 1. Backbone of the 1D polymer {[Co(orot)2(bpy)][μCu(bpy)(H2O)]}n-[Co(orot)2(bpy)]n·5nH2O. The symmetry operations represented by superscript Roman numerals on the atom labels are i: 1 + x, y, z; iv: −1 + x, y, z. which traverses more than one unit cell along [100], is parallel to all such segments; there are no antiparallel chains in the crystal. The absolute structure parameter signals a small degree of polar-axis twinning; the structure has no chiral centers. Because the hydrogen atoms play an important role in the properties of this crystalline substance, their treatment in the structure analysis was more detailed than is usually the case. All non-H atoms were found by direct methods and were refined anisotropically in the final least-squares cycles. H atoms bound to C and N were placed at calculated positions, refined as riders, and assigned U(iso) = 1.2 U(eq) of their respective parent atoms. Of the hydrogen atoms of the H2O fragments, those that could be located in difference maps were included in the structure model and refined as freely as the data permitted. Six H of the total of 12 were thus located. Those of the ligated water, O4W, were not found. Of the five free waters, both H atoms were found only for O2W and O5W. One H atom was found for each of O1W and O6W, and none was successfully located for O3W. U(iso) was constrained for these H atoms as for all of the others. For H1WA, attached to O1W, an antibumping restraint was used to keep the hydrogen atom at the location of maximum electron density as seen in an omit map. Similarity restraints were used for the O···H distances within each of the two H2O groups for which both H atoms were located. With this treatment, we hope to have located at least some of the H atoms correctly without having added any at incorrect positions. Proton conduction pathways cannot be proposed B



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Figure 2. Section of the hydrogen-bonding pattern in {[Co(orot)2(bpy)][μ-Cu(bpy)(H2O)]}n-[Co(orot)2(bpy)]n·5nH2O. Hbonds are shown as broken lines. The symmetry operations represented by superscript Roman numerals on the atom labels are i: 1 + x, y, z; ii: 1 + x, 1 + y, z; iii: x, 1 + y, z.

Figure 3. (a) FE-SEM image of CuCo-orotate polymer single crystals with two gold wire contacts for ac impedance measurements. The interface between crystal and sputtered gold contact is shown in the inset. FE-SEM images of the cross-section of pellet (b) and single crystal of CuCo−orotate polymer (c).

The complex impedance spectra for the CuCo-orotate polymer show two distinct contributions to the ac conductivity in single crystals and three in the pellets. This follows from the asymmetry of −Im(Z) vs Re(Z) plots (Nyquist plots), such as the ones shown in Figure 4. Usually, the impedance of the

Å; superscript Roman numerals represent symmetry codes defined in the caption to Figure 2]. The shortest path from O6Wi to O1Wii traverses the carboxylate group O7i/C7i/O8i and two hydrogen bonds. Similarly, O1Wii and O3W are connected through two contiguous hydrogen bonds in which carboxylate oxygen atom O8i forms the bridgehead. A path from O2W to O5W passes through the HNCO bridge (H53A/ N53/C52/O52). Together, all of these pathways form a chain involving H-bonds but punctuated by covalent bonds [O6Wi, O7i, C7i, O8i (O1Wii), O3W, O4W, O2W, N53, C52, O52, O5W]. Further, between one such discrete chain and the next the shortest water···water distance is greater than 5 Å [O1W(2 + x, 1 + y, z) ...O6Wi 5.468(8) Å], so these truncated chains can be described as isolated “hydrogen-bond clusters,” which obviates the working of a pure Grottuss mechanism. We used ac impedance spectroscopy, in addition to X-ray and morphological studies, in order to elucidate the mechanism behind the proton conduction observed in this polymer. An advantage in studying this system is the availability of both single crystals and powdered samples of the same compound. This provides a unique opportunity to separate the intrinsic (bulk) conductance from artifacts such as contributions from surface or grain boundaries. Figure 3a shows a typical contact set−up for the impedance measurements in single crystals. Two 25 μm gold wires are used for contacting the sample on parallel surfaces on which ∼30 nm of gold had been sputter-deposited. The surfaces to which the wires were attached were nearly parallel to the crystallographic a-axis, which is the propagation direction of the polymer. The electrical conductivity was measured along a direction nearly parallel to the b*-axis. Supporting Information Figures S2 and S3 show the placement of the electrodes with reference to the crystallographic axes. The interface between contact and sample is displayed in the inset of Figure 3a. We found that sputtered gold contacts were more permeable for hydrogen and water vapor than is silver paint, and consequently, we used gold contacts preferentially in our experiments. FE-SEM observations of the fracture surfaces of pellets show a dense microstructure with grains of various sizes in the 50−150 nm range (Figure 3b). An image of the singlecrystal cross-section, shown in Figure 3c, does not reveal any features of interest.

Figure 4. Nyquist plot for a CuCo-orotate polymer pellet. The inset shows the equivalent circuit, describing the different electrical responses observed.

dielectric material is represented by a circuit with two components connected in parallel. One component, R, brought about by the leakage of the material, is purely resistive. The other one, which is capacitive, C, accounts for the dielectric response. This description corresponds to the Debye model, characterized by an arc of a circle in the plot −Im(Z) vs Re(Z) and a unique relaxation time.20,21 In most cases (polymers, glasses, ceramics, etc.), however, the Nyquist plot does not correspond to an arc of a circle and is frequently interpreted in terms of a relaxation time distribution.22 To account for the non-ideal dielectric response, C is commonly replaced by a constant phase element (CPE). The CPE impedance is given by Z(CPE) = 1/(iω)αQ, where Q is a constant, i = √−1, α is the CPE exponent (0 ≤ α ≤ 1), and ω is the angular C



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frequency.23 For α = 1, the CPE is identical to a capacitance (Q = C) whereas for α = 0.5, Z(CPE) is equal to the so-called Warburg impedance. The α = 0 case corresponds to a purely resistive element. Introducing a relaxation time τ = (RQ)1/α, the complex impedance of a parallel R-CPE circuit can be expressed as Z = R/1 + (iωτ)α. Data, shown in Figure 4 for a 260 μm pellet, are fitted with three R-CPE parallel circuits connected in series as sketched: Z=

R gb αgb

1 + (iωτgb)

+

R cr Rc αcr + 1 + (iωτcr) 1 + (iωτc)αc (1)

where the subscripts gb, cr, and c refer to grain boundaries, crystalline, and contact contributions, respectively. In all samples studied, the different parts can be easily identified as they show quite distinct relaxation times. The contact contribution is seen at low frequencies (typically τc ≈ 10−1− 10−2 s), and behaves like the Warburg element (αc ≈ 0.5). This gives a linear spike in −Im(Z) vs Re(Z) plots. A depressed semicircle, seen in pellets at higher frequencies, comes from crystalline-bulk and grain- boundary contributions. Nyquist plots for single crystals, however, show only one semicircle, with negligible depression, brought about by a single conduction mechanism in bulk. In what follows, we will use and discuss mostly the results obtained for single crystals. Although proton-conductivity measurements using pellets are frequently used to provide data for the macroscopic proton conductivity of materials, they often fail to reveal their intrinsic conductivity, mainly because of grain boundary effects. Singlecrystal measurements allow us to overcome this problem. A more serious limitation of measurements on pellets is that they do not give information on the conductivity anisotropy, even though proton conductors are often highly anisotropic. How the real and imaginary parts of the impedance Z(ω) vary with the relative humidity for a CuCo-orotate polymer single crystal at room-temperature is shown in Figure 5. The ac conductivity decreases strongly with decreasing RH for frequencies ω ≤ 105 s−1 while a peak observed in −Im(Z) moves to lower frequencies. The inset in Figure 5 exhibits the variation of intrinsic τ and conductivity σ = (d/A)/R with RH obtained from numerical fittings of eq 1 to the experimental data. Here, d is the thickness of the sample and A is the area of the contacts, respectively. Only two parallel R-CPE circuits have been used in the fit: one corresponding to the contact contribution and the other corresponding to the intrinsic contribution. Systematically, our fits yielded α ≈ 1 (Q = C) for the crystalline part. We find that both τ and σ vary exponentially with RH, with like absolute values of their exponents. Such dependence can be easily understood for the vehicle mechanism (VM) of proton conduction, which involves the diffusion of H3O+. The dipolar nature of the water is essential for counterbalancing the internal electric field in the migration of H3O+, and therefore, higher hydration lowers the activation energy for proton conduction. Using the C values yielded by our fits, we obtain a value of about 50 for the dielectric permittivity of the crystalline material at 300 K, which shows that this CuCo-orotate polymer is a highly polarizable material. Further support for the vehicle mechanism of proton conduction in the CuCo-orotate polymer single crystals comes from the measured variation of the complex impedance with temperature. Figure 6 shows −Im(Z) vs Re(Z) plots

Figure 5. Frequency dependence of (a) the real and (b) imaginary parts of the impedance, measured at T = 295 K and for various values of relative humidity RH, for a CuCo-orotate polymer single crystal. The inset shows variation of the intrinsic conductivity and of the relaxation time with RH obtained from numerical fittings.

Figure 6. Nyquist plots at various temperatures for a CuCo-orotate polymer single crystal measured in N2−H2O atmosphere at RH = 96%.

corresponding to various temperatures T for a single crystal at RH = 96%. As mentioned above, only one semicircle, related to the bulk proton conduction, is observed in the plots. The fitted resistivity increases exponentially with decreasing temperature for T ≤ 310 K. Using the relation σ = σo exp(−EA/kT), where k is the Boltzmann constant, we obtain a value of approximately D



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0.95 ± 0.05 eV for the thermal activation energy EA from the Arrhenius plot in Figure 7. This is close to the values found for

to their hydrogenated counterparts, are obtained for Grotthusstype conduction,25 consistent with semiclassical theory.26 We observe a large conductivity anisotropy for the CuCoorotate polymer. The conductivity along the a-axis direction (which is also the propagation direction of the polymer) is almost 2 orders of magnitude higher than the conductivity in the perpendicular direction (which is discussed in our manuscript). Unfortunately, we were not able to grow sufficiently large crystals to study this anisotropy. The difficulty in preparing contacts on the presently available single crystals, for the [001] direction, prevented us from obtaining reliable results. Finally, we comment on results of impedance measurements in CuCo-orotate polymer pellets. We find that their complex impedance is dominated by the inter-grain contribution, which is much larger than that of the bulk single crystal. We attribute this to a large surface role in the grain-boundary conduction. Our fits of eq 1 to the data obtained for pellets of two different thicknesses yield an activation energy for proton transport in grain boundaries of about 50% higher than that for the bulk (see Figure 7), similar to the findings reported for dicarboxylic acids.27 No isotope effect was found in the pellets, as was the case for the single crystal. The proton conduction at grain boundaries, therefore, seems to proceed through molecular diffusion, the same mechanism as in the bulk. In sum, the results of structural and complex impedance studies in single crystals and powdered pellets of the CuCoorotate copolymer, varying relative humidity and temperature, can be consistently accounted for by a model in which the proton conduction occurs through a vehicle mechanism. This is deduced from (i) relatively high thermal activation energy, comparable to that of other protonic conductors with vehicle mechanisms, (ii) a large pre-exponential factor in the conductivity, coming from the large entropy involved in the vehicle conduction process, (iii) lack of an isotope effect in the conduction, (iv) strong conductivity variation with relative humidity, brought about by a polar environment,28 and (v) lack of a continuous network of hydrogen bonds in the CuCoorotate copolymer, as shown by structural studies. The proton conductivity found for this polymer compares favorably with that of MOFs and related porous materials2 even though it has no framework with pores and channels to provide protonconduction pathways. The Kitaigorodskii packing index, calculated as 68.9%, signals the absence of significant voids or channels in the structure. This finding is a challenge not only to a basic understanding of proton conduction but also to the design of new proton conductors.

Figure 7. Variation of the conductivity with 103/T at RH = 96% for a CuCo-orotate copolymer single crystal and 260 μm pellet measured in N2−H2O and N2−D2O atmospheres. The solid lines show the exponential fit to the data. The inset shows variation of the conductivity with T for a single crystal. The solid lines are to guide the eye.

other compounds with the vehicle proton transport mechanism. An activation enthalpy of 0.75 ± 0.07 eV has been obtained for solid lithium hydrazinium sulfate, LiN2H5SO4, in which one-dimensional vehicle proton conduction has been observed.7 The layered compound, H3OUO2AsO4·3H2O, one of the first known solid fast proton conductors, shows activated conduction with an energy of ∼0.8 eV.24 The proton conduction in this compound proceeds through a molecular diffusion mechanism though it was initially thought to occur through the Grotthuss mechanism. On the other hand, the activation energies found for structural diffusion are much lower, typically close to 0.3 eV.3,20,21,24 Other conductivity parameters for vehicle transport are also quite different from those for structural diffusion.24 The diffusion of H3O+ is correlated with that of H2O, which leads to significant entropy contributions for conduction. Therefore, not only is the overall (mobile ion plus solvent) activation enthalpy higher, but the large entropy of the conduction process also leads to a strong increase of the pre-exponential factor for the vehicular conductivity. Indeed, we find a rather high value of σo (≈ 4 × 109 S cm−1), in line with these observations. In addition, this agrees with a low thermal stability of the CuCo-orotate copolymer. For temperatures beyond approximately 310 K, this compound starts to lose water and its conductivity decreases (see inset of Figure 5). Samples in a D2O atmosphere have also been measured in the impedance experiments. There is almost no isotopic effect on the conductivity, as shown in Figure 7. This is expected for the vehicle mechanism. On the contrary, larger activation energy and lower conductivities in deuterated materials, as compared

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ASSOCIATED CONTENT

S Supporting Information *

Materials and methods, preparation of CuCo-orotate, crystal data, structure refinements, and hydrogen bonds. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Email: [email protected]. *Email: [email protected]. Author Contributions ∥

All authors have given approval to the final version of the manuscript. J.S. and M.T. contributed equally.

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Notes

(28) Kreuer, K. D. Chem. Mater. 1996, 8, 610.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge support from grants MAT2012-38213-C0201 and MAT2011-27233-C02-01 from the Ministerio de Economiá y Competividad of Spain and the European Regional Development Fund. Additional support from Diputación General de Aragón (DGA-CAMRADS, DGA-M4) is also acknowledged. E.F.V. thanks the Ministry of Education (Spain) for a predoctoral scholarship under the program “Becas y Contratos FPU” (reference AP2009-4211). The authors acknowledge the use of Servicio General de Apoyo a la Investigación-SAI, Universidad de Zaragoza.

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REFERENCES

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