Achieving Superprotonic Conduction in MetalâOrganic Frameworks

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Achieving Superprotonic Conduction in Metal-Organic Frame-works through Iterative Design Advances SiRim Kim, Biplab Joarder, Jeff Hurd, Jinfeng Zhang, Karl W Dawson, Benjamin S. Gelfand, Norman E. Wong, and George K. H. Shimizu J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b11364 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 22, 2017

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Journal of the American Chemical Society

Achieving Superprotonic Conduction in Metal-Organic Frameworks through Iterative Design Advances SiRim Kim, Biplab Joarder, Jeff A. Hurd, Jinfeng Zhang, Karl W. Dawson, Benjamin S. Gelfand, Norman E. Wong, George K. H. Shimizu* Department of Chemistry, University of Calgary, Calgary, AB, Canada, T2N 1N4 ABSTRACT: Two complementary design strategies – isomorphous ligand replacement and heterocycle doping – have been applied to iteratively enhance the proton conductivity of a metal-organic framework: β-PCMOF2. The resulting materials, PCMOF2½(Pz) and PCMOF2½(Tz) (Pz= 1H-pyrazole, Tz= 1H-1,2,4-triazole), have their proton conduction raised almost two orders of magnitude compared to β-PCMOF2. The bulk conductivities of these materials are over 10-1 Scm-1 at 85 °C and 90% relative humidity (RH), while maintaining the parent MOF structure. A solid state synthetic route for doping 1-D channels is also presented.

INTRODUCTION Metal−organic frameworks (MOFs) have received growing 1-14 interest as proton conducting materials in recent years. This is mainly because of the modular nature of MOF design and synthesis as well as their crystallinity allowing structure 15-21 activity relationships to be developed. Many protonconducting MOFs have focused on incorporating proton transfer agents within the pores, both to improve conductivity and the upper operating temperature, or functionalizing co-ordinatively unsaturated metal sites or tuning the acidity 22-36 of the pores by incorporating specific functional groups. The following Arrhenius equation, which is derived from the Nernst-Einstein relation, provides a fundamental guideline in 37-45 designing high proton conducting MOFs.

exp

→

exp

∙∙∙∙∙∙

(1)

ers can be achieved by introducing acidic moieties such as H2SO4, RSO3H, H3PO4, water/hydronium, RCO2H, or protonated N-heterocycles into the framework or as guests in the 46-56 proton conduction channels. Only about 50% of the charge carriers should be protonated as proton hopping requires non-protonated sites to act as proton acceptors. Finally, a proton carrier will need to reorient itself following a proton transfer and the ease with which this occurs is determined by how the carriers interact with the pore and each other; this is intimately related to the structure of the MOF. Therefore, a desirable proton conducting metal organic framework would have a 3-dimensional pore structure with large number of available protons and proton carriers that have high rotational freedom. Proton carriers would interact weakly with the framework and they should be at a distance to allow for facile proton hopping but not be so close as to hinder reorientation following proton transfer.

σ = Ionic conductivity n = Number of charge carriers (temperature independent mobile site occupancy) e = Charge of the mobile ion Do = Constant which is related to the mechanism of ionic conductivity k = Boltzmann constant T = Temperature ΔSm = Motional entropy Ea = Motional enthalpy (activation energy for ion transport) According to the above equation, better and high conducting MOFs can be achieved via three different strategies: 1) by increasing the number of charge carriers, 2) by decreasing the activation energy of proton transfer, and 3) by increasing the motional entropy. Increasing the number of charge carri-

Figure 1. (Left) The space-filling cross section structure of βPCMOF2 with its 1-dimensional proton conduction channel illustrated with an arrow and the schematic illustration of isomorphous replacement. (Right) A 2-dimensional crystal layout of β-PCMOF2 where the pores are impregnated with heterocycles (teal pentagons). A proton conducting MOF named β-PCMOF2 (Figure 1 left) was reported in our previous study and the conductivity val-3 -1 ue was found to be 1.3 × 10 Scm at 90% relative humidity 57 and 85 °C. β-PCMOF2 consists of a trisodium 2,4,6trihydroxy-1,3,5-trisulfonate benzene(Na3L1) complex ar-

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ranged in a 1D columnar structure where the pores are lined 57 with sulfonate oxygen atoms. The pores are 5.6 Å in diameter and are hydrophilic in nature. Further, the proton conductivity value of β-PCMOF2 was improved 1.5 orders of magnitude by isomorphous ligand replacement with a more protic ligand of comparable size and shape (1,3,5benzenetriphosphonic acid, Na3H3L2) and the resultant -2 -1 MOF named β-PCMOF2½ showed 2.1 × 10 Scm at 90% 57 relative humidity and 85 °C. This enhancement in conductivity is due to increase the number of freely available acidic 57,58 protons from the backbone. In another study, we successfully enhanced the anhydrous proton conductivity of βPCMOF2 by loading the pores with less volatile, amphiprotic guest molecules (Figure 1 right). The report describes a βPCMOF2(1,2,4-Triazole) system where β-PCMOF2 was loaded with 0.3 equivalent of triazoles in the pores and showed a five orders higher anhydrous conductivity of magnitude -4 -1 relative to unloaded β-PCMOF2 to reach 5 × 10 Scm at 150 49 °C. Thus combination of nonvolatile heterocyclic compound loading and isomorphous ligand replacement in βPCMOF2 would be an excellent strategy to further enhance both the water assisted and anhydrous proton conductivity for MOF materials.

Scheme 1. Chemical structure of L1, H3L2, 1H-pyrazole and 1H-1,2,4-triazole. Herein, we report a study assessing the effect of heterocycle loading, isomorphous protic ligand replacement, and the effect of merging these approaches on proton conduction in a single family of isostructural MOFs. Three new compounds, PCMOF2½(Pz), PCMOF2½(Tz) and PCMOF2(Pz) are reported and compared under common conditions to PCMOF2(Tz) and PCMOF2½. The ligand H3L2 had previously been re59 ported in a structure named PCMOF3 and so the mixed ligand systems were named PCMOF2½. The systematic variation of components provides insights to the extent different factors can affect total proton conduction and activation energy. To obtain good heterocycle loadings in these systems, solid state syntheses are employed, a method that is potentially applicable to other MOF syntheses where competition from solvent is problematic. Ultimately, the application of the two design strategies results in the synergistic enhancement of the β-PCMOF2’s proton conductivity to -1 -1 yield MOFs that conduct protons over 10 Scm . EXPERIMENTAL PROCEDURE

General synthesis of mixed ligand-heterocycle loaded MOFs. As-synthesized PCMOF2(Heterocycle) and Na3H3L2 were individually ground into fine powders using a mortar and pestle and mixed in a 2:1 ratio with respect to Na3L1 and Na3H3L2. The ground powders were placed in a vial and mechanically shaken. The resulting mixture was placed into a silicon tube, sealed and pressed under a hydrostatic pressure 2 of 10,000 pounds/inch for 2 minutes. The resulting pellet was removed from the silicone tube and placed into a 23 mL Teflon autoclave, along with a vial of water (1.7 mL). The Teflon autoclave was then sealed in a stainless steel jacket and heated to 80 °C for 48 hours then cooled back to room temperature over 12 hours. The pellet was then placed in a desiccator for minimum of 12 hours to remove excess moisture, and ground into a fine powder using a mortar and pestle. The resultant powder was analyzed using powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), ele1 31 mental analysis, and H and P NMR to verify the completion of the isomorphous ligand replacement as well as the chemical composition of the mixed ligand-heterocycle loaded PCMOFs. Detailed compositional analysis on all PCMOFs is presented in the supplementary information. We have observed that higher phosphonate ligand loadings than 33% give new phases in the PXRD. AC Impedance Measurements. Powdered samples of PCMOF2½(Pz), PCMOF2½(Tz), PCMOF2(Pz), PCMOF2(Tz), (25 to 30 mg each) were placed in a glass cell and compressed between two solid titanium electrodes (0.3175 cm diameter). The sample length was measured by the difference between the empty cell and the filled cell, which was typically to 1 to 2 mm in length. The sample cells were placed inside a humidity and temperature controlled chamber (ESPEC BTL-433) and connected to a Princeton Applied Research VersaSTAT 3 impedance analyzer using a 2 probe setup. AC impedance 6 data was collected by cycling between 10 and 1 Hz with 10200 mV of applied potential using VersaStudio software. Samples were equilibrated for between 20 to 24 hours after each step in temperature (20 to 85 °C) and for 48 to 120 hours for each step in relative humidity (30% to 90% RH). These lengthy times are not arbitrary as short equilibration times, especially for RH, yield non-Arrhenius behavior and much lower conduction values. Exposure of the samples to humid environment was performed for conditions at and below 90% RH using the ESPEC BTL-433 humidity control oven. Proton conductivity of all samples was measured under 90% RH condition at various temperatures. The temperature was varied from 20 °C to 85 °C for minimum of two heating and cooling cycles with sufficient time for sample equilibration between each step. Following the heating and cooling cycles, the temperature was held at 25 °C and the humidity was decreased from 90% to the minimum of 30% to measure the humidity dependent proton conductivity of the samples. RESULT AND DISCUSSIONS Three new crystalline compounds, PCMOF2½(Pz), PCMOF2½(Tz) and PCMOF2(Pz) were synthesized and characterised through PXRD, elemental analysis, thermogravimetry and NMR analyses (See supporting information).In synthesizing PCMOF2½(Pz) and PCMOF2½(Tz), a three component mixed ligand MOF, it was critical to employ appropriate synthetic parameters to enable mutual compatibility between the two design strategies. At first the syntheses of PCMOF2(heterocycle) (heterocycle= pyrazole and tria-


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Journal of the American Chemical Society zole) were carried out. This synthesis processes of PCMOF2(heterocycle) were tailored to kinetically capture pyrazole within the β-PCMOF2 framework via a solution precipitation. In the second design strategy PCMOF2(heterocycle) were then mixed with Na3H3L2 and placed under solid state reaction conditions to yield corresponding PCMOF2½(heterocycle) (Supporting information). Pelletization accelerates the process because it increases the contact between the multi-phases. Additionally, the high temperature and humidity condition accelerates the mobility of the molecules and drives the reaction because it increases the positional and vibrational entropy of the solid constituents. This synthetic approach is conceptually similar to that of Accelerated Aging which describes the natural mineral 60 weathering process. Figure 2 shows the powder XRD analysis of the various PCMOFs featured in this work with the focus on PCMOF2½(Pz) and PCMOF2½(Tz). We performed powder XRD analysis throughout the synthesis to confirm the preservation of the original β-PCMOF2 structure (Figure 2F). In synthesizing PCMOF2½(Pz) and PCMOF2½(Tz), finely ground powders of PCMOF2(Pz)/PCMOF2(Tz) and Na3H3L2 were mechanically mixed. The PXRD pattern of the mechanical mixture exhibited a simple super positioning of the patterns of its two components (Figure 2I, J & M and 2K, L & M). The mechanical mixture was then pressed into a pellet and placed under high temperature and humidity conditions

crease of ~0.1 & 0.1 Å and 0.03 & 0.04 Å in the d-spacing, respectively (Figure S11 and Table S1). The presence of hetero1 cycles were demonstrated by H-NMR studies, where the digested samples of PCMOF2(heterocycle) showed peaks corresponding to the heterocycle molecules in the framework (supporting information). The molecular formula of all the compounds were calculated merging elemental analysis and thermogravimetric analysis (supporting information). The maximum ratio of H3L2 that can be incorporated with retention of structure is 33%. While we do not have direct structural support, a possible arrangement based on this ratio would be a phosphonate ligand between two sulfonate ligands. To characterize the proton conductivity, AC impedance measurements were performed on samples of PCMOF2, PCMOF2(heterocycle) and PCMOF2½(heterocycle) at 90% RH in air. Table 1 summarises the conductivity values and the corresponding activation energies of the various PCMOFs under their respective conditions. The Nyquist plots obtained from the second heating cycle, to ensure humidity equilibration, are shown in Figure 4 and it shows that PCMOF2½(heterocycle) exhibited very low resistances, resulting in only the tail end of a semi-circle being observed at high frequencies due to instrument limitations. At lower frequencies, the capacitive tail is also observed, as expected for blocking effects of the mobile charge at the electrode interface. The conductivity was calculated from the real-axis intercept of the Nyquist plot (Figure 3). Table 1. List of PCMOF samples featured in this work and their conductivity profiles. Name

Conductivity Scm

Figure 2. Powder XRD patterns of (A) PCMOF2½(Pz), postimpedance; (B) PCMOF2½(Tz), post-impedance; (C) PCMOF2½, post-impedance; (D) PCMOF2(Pz), postimpedance; (E) PCMOF2(Tz), post-impedance; (F) βPCMOF2, post-impedance; (G) PCMOF2½(Pz), preimpedance; (H) PCMOF2½(Tz), pre-impedance; (I) Mechanical mixture of PCMOF2(Pz) and Na3H3L2 (Resembles J + M); (J) PCMOF2(Pz); (K) Mechanical mixture of PCMOF2(Tz) and Na3H3L2 (Resembles L + M); (L) PCMOF2(Tz) and (M) Na3H3L2. to achieve isomorphous ligand replacement, a thermody57 namically driven solid state reaction. The resultant powder XRD of PCMOF2½(Pz) and PCMOF2½(Tz) resembled that of β-PCMOF2 with a slight 2θ shift for the peaks that correspond to the (2,-1,0) plane and the (0,0,1) plane with a de-

1.1 × 10

PCMOF2½(Pz)

[i]

1.23 × 10

PCMOF2½(Pz)

[i]

7.2 × 10

PCMOF2½(Tz)

[i]

1.17 × 10

PCMOF2½(Tz)

[i]

-1

90

0.16

40

n/a

-7

150

0

0.20 [vii] 0.98

-1

85

90

0.22

-2

25

40

n/a

-2

85

90

0.10

1.08 × 10

PCMOF2(Pz)

3.28 × 10

PCMOF2(Pz)

[ii]

1.3 × 10

PCMOF2(Tz)

[ii]

1.9 × 10

PCMOF2(Tz)

[ii]

[iii]

4.6 × 10

-4

25

40

150

0

-2

85

90

0.10

3.18 × 10

25

40

n/a

-4

150

0

-2

85

90

0.21

-5

-5

5 × 10

2.1 × 10

2.4 × 10

85

90

0.28

1.8 × 10

-6

20

50

n/a

-9

100

0

n/a

β-PCMOF2


[ix]

n/a

[iii]

PCMOF2½(Im)

0.34

50

1.3 × 10

[i]

[viii]

20

[iv]

PCMOF2(Im)

n/a 0.42

-3

β-PCMOF2

[i]

[vi]

-3

[ii,iv]

β-PCMOF2

eV

25

[ii]

PCMOF2½

[v]

85

PCMOF2(Pz)

[iv]

Ea

-3

[ii]

[iv]

R H %

[i]

PCMOF2½

°C

-1

PCMOF2½(Pz)

PCMOF2(Tz)

T

1 × 10

These PCMOFs exhibited proton -6 -1 conductivity below 10 S cm at 85 °C and 90% RH.


i. New compounds and new conductivity data. ii. Compounds previously synthesized by Hurd et al; its conductivity 49 was measured under new conditions. iii. Compound and conductivity previously reported by Shimizu et al (the au49 thor). iv. Compounds and conductivity previously reported by 57 Shimizu et al (the author). v. Activation energy measured between the temperature of 85 °C and 25 °C. vi. Activation energy measured between the temperature of 150 °C and 110 °C. vii. Activation energy measured between the temperature of 110 °C and 90 °C. viii. Activation energy measured between the temperature of 150 °C and 60 °C.ix. Activation energy measured between the temperature of 150 °C and 90 °C. As a control, PCMOF2(Pz) and PCMOF2(Tz) were measured and their proton conductivities reproduced previously measured values on samples prepared by a different co-worker: -2 -1 -2 PCMOF2(Pz) (4.6 × 10 Scm ) and PCMOF2(Tz) (1.9 × 10 -1 Scm ) at 85 °C and 90% RH (Table 1). The mixed ligand, loaded samples were then measured and gave data almost an order of magnitude higher under identical conditions: 1.1 × -1 -1 -1 -1 10 Scm and 1.17× 10 Scm for PCMOF2½(Pz) and PCMOF2½(Tz), respectively, at 85 °C and 90% RH.

ty of the sample was maintained, as suggested by the reproducible conduction data, and corroborated by the pre- and post-impedance PXRD patterns (Figure 2). Notably, all PCMOFs investigated showed the same PXRD pattern as βPCMOF2 both before and after impedance measurements, confirming the preservation of the structure. The conductivity was found to be highly dependent on the humidity, which suggests that the water plays a critical role in proton conduction. Interestingly, both PCMOF2½(Pz) and PCMOF2½(Tz) showed a steady decrease in conductivity when the relative humidity was lowered from 90% to 40%, where as the conductivity of PCMOF2(Pz) and PCMOF(Tz) experienced a much more abrupt decrease (Table 1 & figure S8). The activation energy for proton transfer in the PCMOF2 58 series under humid condition is between 0.10-0.28 eV, which is consistent with a Grotthuss transfer mechanism (Figure 3). β-PCMOF2 had the greatest activation energy, 0.28 eV among all the PCMOFs investigated in this study. PCMOF2½ showed activation energy of 0.21 eV. PCMOF2(Tz) and PCMOF2(Pz) showed very low activation energies (0.10 eV). PCMOF2½(Pz) and PCMOF2½(Tz) showed activation energies of 0.16 and 0.22 eV respectively (Table 1 and figure S6-7).

Figure 4. Proton conductivity data (90% RH) for various samples. Lines are present only as visual guides

Figure 3. Nyquist plot for a) PCMOF2½(Pz) and b) PCMOF2½(Tz) at 90% RH. Since both PCMOF2½(Pz) and PCMOF2½(Tz) showed pro-1 -1 ton conducting properties over 10 Scm (at the time of initial measurement, the highest in any proton conducting MOF), new samples were remade (by a second researcher) and conductivity remeasured for four heating and cooling cycles to confirm stability and reproducibility. Each cycle was measured at 90% RH and from 20 °C to 85 °C with at least an 8 hour equilibration time between temperatures. The integri-

The activation energy (Ea) of proton transfer is dependent on + the facility of H transfer and the reorientation of proton 58 carriers post-transfer. Subsequently, proton mobility is a function of activation energy, and proton conductivity is a function of mobility. It may seem that a PCMOF with the lowest activation energy would yield the highest proton conductivity, but based on the results of this work (Figure 4 and Table 1), lowest activation energy does not necessarily equal to highest conductivity. In order to address this, multiple factors that affect conductivity – amphiprotic proton donor/acceptor sites, pKas, host/guest interactions, and the size of the heterocycles – need to be addressed since they all affect the resultant conductivity and the activation energy. Empirical comparison of conductivity can be made using the relative pKa values (Table 2) of the groups lining the pores and the conjugate acid form of the heterocycles.


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Table 2. A list of pKas relevant to this work. Name

pKa

Benzenesulfonic acid

-0.60

Phenylphosphonic acid

1.88

1H-pyrazol-2-ium

2.50

1H-1,2,4-triazol-2-ium

2.39

1H-imidazol-3-ium

6.90

Hydroxonium

-1.74

-1

10 S cm at 150 °C in air, a much lower value than the anhydrous conductivity of PCMOF2(Pz) and PCMOF2(Tz), yet a -8 -1 higher value than that of pure β-PCMOF2 (10 S cm ) (Figure S11). It is speculated that the loaded heterocycle in PCMOF2½(Pz) still imparts some degree of enhanced anhydrous enhancement but nowhere as drastic as the 5 orders of magnitude increase as PCMOF2(Pz) versus β-PCMOF2. We hypothesized that the conductivity enhancement by Na3H3L2 requires sufficient amount of pore water molecules to activate/dissociate its free acidic protons. This is supported by the rapid decrease in PCMOF2½(Pz)’s and PCMOF2½(Tz)’s conductivity when the humidity was decreased from 90% to 40% RH.

PCMOF2(Pz) and PCMOF2(Tz) has an activation energy for proton transfer of 0.10 eV, a very low value indicative of highly facile proton transport pathway. PCMOF2½(Pz) and PCMOF2½(Tz), on the other hand, have activation energies of 0.16 eV and 0.22 eV, respectively, double that of PCMOF2(Pz) and PCMOF2(Tz). PCMOF2 itself has a higher 57 activation energy than PCMOF2½. This result is in accord with the pKa values in Table 2 as the sulfonate group has lower affinity for protons. However, despite the higher activation energy, in all three cases, the PCMOF2½(heterocycle) complexes show higher conductivity than the pure sulfonate counterparts (PCMOF2(heterocycle)) . Revisiting equation [1], the conductivity in the PCMOF2½(heterocycle) series is greater because the higher activation energy is offset by the increase in the number of freely available acidic protons resulting in enhanced conductivity. Concurrently, the greater difference in pKa will also lead to greater host-guest interaction between Na3H3L2 and the heterocycles, inhibiting reorientation and increasing the activation energy. pKa is an empirical solution measurement and may be a good predictor for the resultant conductivity but in the PCMOF framework, other factors such as their size, hydrogen bonding proficiency and host/guest interactions (which in turn affects the motional entropy and the rotational freedom of the heterocycles) also impact the resultant mobility and conductivity. Being similar in terms of pKa and molecular size, both triazole & pyrazole showed similar conductivity. Perhaps this is because of both the pKa of pyrazolium and triazolium cation is much closer to the pKa of hydronium cation (-1.74), enabling a more facile reversible proton transfer between the two conjugate acids. The pKa values indicates that the triazole-triazolium or pyrazole-pyrazolium pair would form a more balanced mix of protonated and unprotonated molecules – it is just as important to provide sufficient unprotonated sites that can act as facile proton acceptors in addition to maximizing the number of proton donors.

The field of proton conducting MOF materials has seen steady progress in the levels of proton conduction being re-1 -1 45 ported now with accounts of values exceeding 10 Scm . These advances are enabled by design advances in parallel with increasingly more robust MOF structures being reported. In this case, the MOF materials are not highly robust and would not survive a long-term fuel cell test; the value of this work is the design principles put forth. The fundamental chemistry governing proton transfer is identical regardless of the specific material or even class of solid. There must be a continuous proton transfer pathway with minimal potential wells to trap the protons. MOFs, with their crystalline structures offer the ability to correlate crystallographic structure and function. This coupled with the well-established routes for systematic structural variation, as we have shown here, make iterative cycles of design and assessment a rational rather than stochastic process.

As a contrast, we also investigated imidazole (Im, pKa1 = 7.18, pKa2 = 14.52) as a dopant. We synthesized PCMOF2(Im) and PCMOF2½(Im) and measured their respective conductivities. Neither sample showed significant proton conductivity at 85 °C and 90% RH. We hypothesized the minimal conductivity to be caused by the much higher pKa of imidazolium meaning much more localized protons as well as the resulting stronger host-guest interaction between the imidazole molecule and the MOF framework limiting reorientation.

Details of synthesis, powder XRD, impedance analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

Comparisons of the proton conductivity in these systems was also carried out under anhydrous conditions. PCMOF2½(Pz) was dehydrated and the conductivity was found to be 7.2 x

The authors declare no competing financial interest.

CONCLUSION We have successfully applied two design strategies - isomorphous ligand replacement and heterocycle doping - to enhance the proton conductivity of a proton conducting metalorganic framework: β-PCMOF2. The rational and sequential variation of structures gives proton conductivity data that correlates with expected performance based on molecular properties. We have also shown a solid state route to controllably loading 1-D pores in the MOFs with proton carriers. Of six PCMOFs investigated in this study, two materials, PCMOF2½(Pz) and PCMOF2½(Tz), show exceptionally high -1 -1 proton conduction values (>1 × 10 Scm ) at 85 °C and 90% RH, while maintaining the parent MOF structure. The principles outlined are applicable to other proton conducting systems beyond MOF materials.

ASSOCIATED CONTENT

AUTHOR INFORMATION Corresponding Author [email protected]

Notes

ACKNOWLEDGMENT



We thank the Natural Science and Engineering Research Council (NSERC) of Canada for support of this research and for graduate scholarships to S. K. and B. S. G.. We thank Alberta Innovates Technology Futures for a doctoral scholarship to K.W.D. and a postdoctoral fellowship to B. J.

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