Achieving Superprotonic Conduction in Metal-Organic Frameworks through Iterative Design Advances

Article abstract of DOI:10.1021/jacs.7b11364

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, PCMOF2¹/₂(P

Full text of DOI:10.1021/jacs.7b11364

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Article
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
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Achieving Superprotonic Conduction in Metal-Organic Frameworks
through Iterative Design Advances
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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 ma-
terials, 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
ers can be achieved by introducing acidic moieties such as
H2SO4, RSO3H, H3PO4, water/hydronium, RCO2H, or proto-
nated N-heterocycles into the framework or as guests in the
proton conduction channels.46-56 Only about 50% of the
charge carriers should be protonated as proton hopping re-
quires non-protonated sites to act as proton acceptors. Final-
ly, a proton carrier will need to reorient itself following a
proton transfer and the ease with which this occurs is deter-
mined 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.
Metal−organic frameworks (MOFs) have received growing
interest as proton conducting materials in recent years.1-14
This is mainly because of the modular nature of MOF design
and synthesis as well as their crystallinity allowing structure
activity relationships to be developed.15-21 Many proton-
conducting MOFs have focused on incorporating proton
transfer agents within the pores, both to improve conductivi-
ty and the upper operating temperature, or functionalizing
co-ordinatively unsaturated metal sites or tuning the acidity
of the pores by incorporating specific functional groups.22-36
The following Arrhenius equation, which is derived from the
Nernst-Einstein relation, provides a fundamental guideline in
designing high proton conducting MOFs.37-45
ꢄꢅꢆ ꢇꢈꢉꢊꢋꢌꢍꢏ
ꢎ
ꢀ ꢁ ꢂꢃ
exp ꢊꢒꢓꢔꢏ → ꢀ ꢁ ꢕꢆ exp ꢊꢒꢐꢓꢑꢔꢏ ∙ ∙ ∙ ∙ ∙ ∙ (1)
ꢐꢑ ꢑ
ꢐꢑ
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
Figure 1. (Left) The space-filling cross section structure of β-
PCMOF2 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).
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-
A proton conducting MOF named β-PCMOF2 (Figure 1 left)
was reported in our previous study and the conductivity val-
ue was found to be 1.3 × 10-3 Scm-1 at 90% relative humidity
and 85 °C.57 β-PCMOF2 consists of a trisodium 2,4,6-
trihydroxy-1,3,5-trisulfonate benzene(Na3L1) complex ar-
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ranged in a 1D columnar structure where the pores are lined
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 me-
chanically shaken. The resulting mixture was placed into a
silicon tube, sealed and pressed under a hydrostatic pressure
of 10,000 pounds/inch2 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 mois-
ture, and ground into a fine powder using a mortar and pes-
tle. The resultant powder was analyzed using powder X-ray
diffraction (PXRD), thermogravimetric analysis (TGA), ele-
mental analysis, and 1H and 31P NMR to verify the completion
of the isomorphous ligand replacement as well as the chemi-
cal composition of the mixed ligand-heterocycle loaded
PCMOFs. Detailed compositional analysis on all PCMOFs is
presented in the supplementary information. We have ob-
served that higher phosphonate ligand loadings than 33%
give new phases in the PXRD.
with sulfonate oxygen atoms.57 The pores are 5.6 Å in diame-
ter and are hydrophilic in nature. Further, the proton con-
ductivity 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 (1,3,5-
benzenetriphosphonic acid, Na3H3L2) and the resultant
MOF named β-PCMOF2½ showed 2.1 × 10-2 Scm-1 at 90%
relative humidity and 85 °C.57 This enhancement in conduc-
tivity is due to increase the number of freely available acidic
protons from the backbone.57,58 In another study, we suc-
cessfully enhanced the anhydrous proton conductivity of β-
PCMOF2 by loading the pores with less volatile, amphiprotic
guest molecules (Figure 1 right). The report describes a β-
PCMOF2(1,2,4-Triazole) system where β-PCMOF2 was load-
ed with 0.3 equivalent of triazoles in the pores and showed a
five orders higher anhydrous conductivity of magnitude
relative to unloaded β-PCMOF2 to reach 5 × 10-4 Scm-1 at 150
°C.49 Thus combination of nonvolatile heterocyclic com-
pound loading and isomorphous ligand replacement in β-
PCMOF2 would be an excellent strategy to further enhance
both the water assisted and anhydrous proton conductivity
for MOF materials.
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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 humidi-
ty 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
data was collected by cycling between 106 and 1 Hz with 10-
200 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 de-
creased from 90% to the minimum of 30% to measure the
humidity dependent proton conductivity of the samples.
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 report-
ed and compared under common conditions to PCMOF2(Tz)
and PCMOF2½. The ligand H3L2 had previously been re-
ported in a structure named PCMOF359 and so the mixed
ligand systems were named PCMOF2½. The systematic vari-
ation of components provides insights to the extent different
factors can affect total proton conduction and activation
energy. To obtain good heterocycle loadings in these sys-
tems, solid state syntheses are employed, a method that is
potentially applicable to other MOF syntheses where compe-
tition from solvent is problematic. Ultimately, the applica-
tion of the two design strategies results in the synergistic
enhancement of the β-PCMOF2’s proton conductivity to
yield MOFs that conduct protons over 10-1 Scm-1.
RESULT AND DISCUSSIONS
Three new crystalline compounds, PCMOF2½(Pz),
PCMOF2½(Tz) and PCMOF2(Pz) were synthesized and
characterised through PXRD, elemental analysis, thermogra-
vimetry and NMR analyses (See supporting information).In
synthesizing PCMOF2½(Pz) and PCMOF2½(Tz), a three
component mixed ligand MOF, it was critical to employ ap-
propriate synthetic parameters to enable mutual compatibil-
ity between the two design strategies. At first the syntheses
of PCMOF2(heterocycle) (heterocycle= pyrazole and tria-
EXPERIMENTAL PROCEDURE
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zole) were carried out. This synthesis processes of
crease of ~0.1 & 0.1 Å and 0.03 & 0.04 Å in the d-spacing, re-
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PCMOF2(heterocycle) were tailored to kinetically capture
pyrazole within the β-PCMOF2 framework via a solution
spectively (Figure S11 and Table S1). The presence of hetero-
cycles were demonstrated by H-NMR studies, where the
1
precipitation.
In
the
second
design
strategy
digested samples of PCMOF2(heterocycle) showed peaks
corresponding to the heterocycle molecules in the frame-
work (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.
PCMOF2(heterocycle) were then mixed with Na3H3L2 and
placed under solid state reaction conditions to yield corre-
sponding 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 constitu-
ents. This synthetic approach is conceptually similar to that
of Accelerated Aging which describes the natural mineral
weathering process.60 Figure 2 shows the powder XRD analy-
sis 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), fine-
ly 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 con-
ditions
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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 ob-
tained 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.
[v]
Name
Conduc-
tivity
Scm-1
1.1 × 10-1
1.23 × 10-3
7.2 × 10-7
T
R
H
Ea
°C
eV
%
90
40
0
PCMOF2½(Pz)[i]
PCMOF2½(Pz)[i]
PCMOF2½(Pz)[i]
85
25
0.16
n/a
0.20[vi]
0.98[vii]
150
PCMOF2½(Tz)[i]
PCMOF2½(Tz)[i]
PCMOF2(Pz)[ii]
PCMOF2(Pz)[ii]
PCMOF2(Pz)[ii]
PCMOF2(Tz)[ii]
PCMOF2(Tz)[ii]
PCMOF2(Tz)[iii]
1.17 × 10-1
1.08 × 10-2
4.6 × 10-2
3.28 × 10-4
1.3 × 10-3
1.9 × 10-2
3.18 × 10-5
5 × 10-4
2.1 × 10-2
2.4 × 10-5
1.3 × 10-3
1.8 × 10-6
1 × 10-9
85
25
90
40
90
40
0
0.22
n/a
85
25
0.10
n/a
0.42[viii]
150
85
25
Figure 2. Powder XRD patterns of (A) PCMOF2½(Pz), post-
90
40
0
0.10
impedance; (B) PCMOF2½(Tz), post-impedance;
(C)
n/a
0.34[ix]
PCMOF2½, post-impedance; (D) PCMOF2(Pz), post-
impedance; (E) PCMOF2(Tz), post-impedance; (F) β-
PCMOF2, post-impedance; (G) PCMOF2½(Pz), pre-
impedance; (H) PCMOF2½(Tz), pre-impedance; (I) Mechan-
ical 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.
150
85
20
85
20
100
[iv]
PCMOF2½
90
50
90
50
0
0.21
[iv]
PCMOF2½
n/a
β-PCMOF2 [ii,iv]
β-PCMOF2 [iv]
β-PCMOF2 [iii]
PCMOF2(Im)[i]
PCMOF2½(Im)[i]
0.28
n/a
n/a
to achieve isomorphous ligand replacement, a thermody-
namically driven solid state reaction.57 The resultant powder
XRD of PCMOF2½(Pz) and PCMOF2½(Tz) resembled that
of β-PCMOF2 with a slight 2θ shift for the peaks that corre-
spond to the (2,-1,0) plane and the (0,0,1) plane with a de-
These PCMOFs exhibited proton
conductivity below 10-6 S cm-1 at
85 °C and 90% RH.
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i. New compounds and new conductivity data. ii. Com-
ty of the sample was maintained, as suggested by the repro-
ducible conduction data, and corroborated by the pre- and
post-impedance PXRD patterns (Figure 2). Notably, all
PCMOFs investigated showed the same PXRD pattern as β-
PCMOF2 both before and after impedance measurements,
confirming the preservation of the structure. The conductivi-
ty was found to be highly dependent on the humidity, which
suggests that the water plays a critical role in proton conduc-
tion. 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 con-
ductivity of PCMOF2(Pz) and PCMOF(Tz) experienced a
much more abrupt decrease (Table 1 & figure S8).
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pounds previously synthesized by Hurd et al; its conductivity
was measured under new conditions.49 iii. Compound and
conductivity previously reported by Shimizu et al (the au-
thor).49
iv. Compounds and conductivity previously reported by
Shimizu et al (the author).57 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.
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The activation energy for proton transfer in the PCMOF2
series under humid condition is between 0.10-0.28 eV,58
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).
As a control, PCMOF2(Pz) and PCMOF2(Tz) were measured
and their proton conductivities reproduced previously meas-
ured values on samples prepared by a different co-worker:
PCMOF2(Pz) (4.6 × 10-2 Scm-1) and PCMOF2(Tz) (1.9 × 10-2
Scm-1) 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 ×
10-1 Scm-1 and 1.17× 10-1 Scm-1 for PCMOF2½(Pz) and
PCMOF2½(Tz), respectively, at 85 °C and 90% RH.
Figure 4. Proton conductivity data (90% RH) for various
samples. Lines are present only as visual guides
The activation energy (Ea) of proton transfer is dependent on
the facility of H+ transfer and the reorientation of proton
carriers post-transfer.58 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 con-
ductivity, 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 do-
nor/acceptor sites, pKas, host/guest interactions, and the size
of the heterocycles – need to be addressed since they all af-
fect the resultant conductivity and the activation energy.
Figure 3. Nyquist plot for a) PCMOF2½(Pz) and b)
PCMOF2½(Tz) at 90% RH.
Since both PCMOF2½(Pz) and PCMOF2½(Tz) showed pro-
ton conducting properties over 10-1 Scm-1 (at the time of ini-
tial 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-
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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10-7 S cm-1 at 150 °C in air, a much lower value than the anhy-
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drous conductivity of PCMOF2(Pz) and PCMOF2(Tz), yet a
higher value than that of pure β-PCMOF2 (10-8 S cm-1) (Fig-
ure S11). It is speculated that the loaded heterocycle in
PCMOF2½(Pz) still imparts some degree of enhanced anhy-
drous 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 acti-
vate/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.
Table 2. A list of pKas relevant to this work.
Name
pKa
-0.60
1.88
Benzenesulfonic acid
Phenylphosphonic acid
1H-pyrazol-2-ium
2.50
2.39
6.90
-1.74
1H-1,2,4-triazol-2-ium
1H-imidazol-3-ium
9
Hydroxonium
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PCMOF2(Pz) and PCMOF2(Tz) has an activation energy for
proton transfer of 0.10 eV, a very low value indicative of high-
ly 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
activation energy than PCMOF2½.57 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 activa-
tion 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 re-
sulting in enhanced conductivity. Concurrently, the greater
difference in pKa will also lead to greater host-guest interac-
tion between Na3H3L2 and the heterocycles, inhibiting reori-
entation and increasing the activation energy. pKa is an em-
pirical 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 proficien-
cy and host/guest interactions (which in turn affects the mo-
tional entropy and the rotational freedom of the heterocy-
cles) also impact the resultant mobility and conductivity.
Being similar in terms of pKa and molecular size, both tria-
zole & 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), ena-
bling a more facile reversible proton transfer between the
two conjugate acids. The pKa values indicates that the tria-
zole-triazolium or pyrazole-pyrazolium pair would form a
more balanced mix of protonated and unprotonated mole-
cules – it is just as important to provide sufficient unproto-
nated 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-
ported now with accounts of values exceeding 10-1 Scm-1.45
These advances are enabled by design advances in parallel
with increasingly more robust MOF structures being report-
ed. 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 struc-
tures 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.
CONCLUSION
We have successfully applied two design strategies - isomor-
phous ligand replacement and heterocycle doping - to en-
hance the proton conductivity of a proton conducting metal-
organic 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 control-
lably 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
proton conduction values (>1 × 10-1 Scm-1) at 85 °C and 90%
RH, while maintaining the parent MOF structure. The prin-
ciples outlined are applicable to other proton conducting
systems beyond MOF materials.
ASSOCIATED CONTENT
Details of synthesis, powder XRD, impedance analysis. This
material is available free of charge via the Internet at
http://pubs.acs.org.
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 conductivi-
ties. Neither sample showed significant proton conductivity
at 85 °C and 90% RH. We hypothesized the minimal con-
ductivity to be caused by the much higher pKa of imidazoli-
um meaning much more localized protons as well as the
resulting stronger host-guest interaction between the imid-
azole molecule and the MOF framework limiting reorienta-
tion.
AUTHOR INFORMATION
Corresponding Author
gshimizu@ucalgary.ca
Notes
The authors declare no competing financial interest.
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
ACKNOWLEDGMENT
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We thank the Natural Science and Engineering Research
R.; Murray, C. A.; Tang, C. C.; Yang, S. H.; Schroder, M. J. Am.
Chem. Soc. 2016, 138, 6352.
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Council (NSERC) of Canada for support of this research and
for graduate scholarships to S. K. and B. S. G.. We thank Al-
berta Innovates Technology Futures for a doctoral scholar-
ship to K.W.D. and a postdoctoral fellowship to B. J.
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