A water-stable metal-organic framework with highly acidic pores for proton-conducting applications

Article abstract of DOI:10.1021/ja310435e

Metal-organic framework (MOF) materials are a nontraditional route to ion conductors, but their crystallinity can give insight into molecular-level transport mechanisms. However, some MOFs can be structurally compromised in humid environments. A new 3D metal-organic framework, PCMOF-5, is reported which conducts protons above 10^-3 S/cm at 60 C and 98% relative humidity. The MOF contains free phosphonic acid groups, shows high humidity stability, and resists swelling in the presence of hydration. Channels filled with crystallographically located water and acidic groups are also observed.

Full text of DOI:10.1021/ja310435e

Communication
pubs.acs.org/JACS
A Water-Stable Metal−Organic Framework with Highly Acidic Pores
for Proton-Conducting Applications
Jared M. Taylor, Karl W. Dawson, and George K. H. Shimizu*
Department of Chemistry, University of Calgary, Calgary, Alberta T2N 1N4, Canada
*
S Supporting Information
extends to retention of guest proton carriers and resistance to
swelling of the material, as macroscopic size deviations would
disrupt electrode/electrolyte interfaces.
ABSTRACT: Metal−organic framework (MOF) materi-
als are a nontraditional route to ion conductors, but their
crystallinity can give insight into molecular-level transport
mechanisms. However, some MOFs can be structurally
compromised in humid environments. A new 3D metal−
organic framework, PCMOF-5, is reported which conducts
protons above 10 S/cm at 60 °C and 98% relative
humidity. The MOF contains free phosphonic acid groups,
shows high humidity stability, and resists swelling in the
presence of hydration. Channels filled with crystallo-
graphically located water and acidic groups are also
observed.
Here we present a MOF that conducts protons at 4 × 10−
S/cm (at 98% relative humidity (RH) and 335 K) and shows
structural stability to high RH and dilute acidic solutions. The
robustness and resistance to swelling of [La(H L)(H O) ]
3
5
2
4
−3
(
PCMOF-5, L = 1,2,4,5-tetrakisphosphonomethylbenzene) are
attributed to a three-dimensionally connected M(III) phos-
phonate framework. The high proton conductivity is enabled by
free phosphonic acid groups lining the pores and crystallo-
graphically located intra-pore water molecules. With over 1200
metal phosphonate structures in the Cambridge Structural
Database, this represents the first 3D phosphonate with free
diprotic acid groups as pendants. Synthetic conditions were
employed to give free acid groups and promote “error
checking” to yield a crystalline material.
he development of new materials with targeted properties
T
requires an iterative cycle of design, execution, and
assessment. A key aspect of enhancing design is to establish
clear structure−activity relationships for the materials under
study. With regard to proton conductors, an excellent material
should offer an array of proton binding sites of equivalent pKa
with a low activation energy pathway linking them. As many of
the best electrolytes are polymeric solids with noncrystalline
structures, exact analysis of a structure−activity relationship is
Some recent reports have demonstrated the inclusion of
acidic hydrogen phosphonate protons within a MOF to create a
7
proton-conducting material. Also, we have demonstrated that
it is possible to produce a proton-conducting material from a
crystalline phosphonate with no phosphonic acid protons but
8
with acidic coordinated water. We sought to design a proton-
conducting phosphonate MOF through synthetic controls in
order to include diprotic phosphonic acid groups within the
framework. To achieve this goal, we chose to use a known
tetraphosphonic acid ligand, H L, to build the MOF. Two
8
9
1
often a challenge. Metal−organic frameworks (MOFs) are a
structurally diverse platform for a wide range of potential
2
applications. Relative to the immense effort directed at gas
capture and storage applications, ion conduction has received
previous reports have shown that this ligand adopts a dense
pillared-layered motif with divalent transition metals, with a 2:1
M:L ratio, where a hydrophilic metal phosphonate layer is
pillared by a hydrophobic organic layer. We believed that more
acidic conditions during synthesis would facilitate the exclusion
of some metal centers and replace them with acidic channels.
To synthesize the MOF, pH was reduced during synthesis
3
very little attention. That said, the hybrid nature of MOFs can
be viewed as an opportunity, as they can be perceived as
compositionally intermediate to ubiquitous polymeric electro-
lytes and inorganic solid acids; there has been a recent surge of
proton-conducting coordination polymer/MOF materials re-
4
ported.
through the addition of H SO (pH < 1), and La(III) was
Hallmark properties of MOFs are that they have modular
synthetic routes, open structures and are crystalline. The
crystallinity of MOFs enables direct visualization of structure
and enhances design prospects. This includes not just the
framework but also guest molecules which are often integral to
the proton conduction pathway. Even if not a commercial
candidate, a crystalline ion conductor can provide a firm
starting point for modeling studies. This point is germane to
the value of MOFs as proton electrolytes: even though a
particular MOF may not be water stable, it can still provide
insight to develop structure−activity relationships. Although
2
4
chosen to be able to bind to the phosphonate ligand under the
acidic conditions. Upon mixing an aqueous solution of
La(HSO ) /H SO with an aqueous solution of H L, a
4 3
2
4
8
colorless solution was formed, from which single crystals of
PCMOF-5 grew within a day upon standing at room
temperature. The addition of acid to the synthesis helps
modulate crystal growth, with greater equivalents of acid
increasing the time for crystal growth. Addition of a sulfate
source to the preparation of a MOF can represent a potentially
very stable competing phase. La(III) was chosen, in part,
5
some highly robust MOF materials have emerged recently, the
6
large majority of MOFs are not water stable. Beyond structural
Received: October 22, 2012
integrity, stability of a MOF proton conductor to hydration
©
XXXX American Chemical Society
A
dx.doi.org/10.1021/ja310435e | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
because its hydrogen sulfate salt is water soluble. The crystal
growth synthesis could be scaled-up to produce PCMOF-5 in
bulk, and PXRD measurements on a ground powder matched
the simulated pattern from the single-crystal solution (Figure
S1).
Before proton conductivity was measured for PCMOF-5,
structural stability to the highly humid conditions during
conductivity testing was assessed. A fresh sample of PCMOF-5
was boiled in water for 7 days without significant loss in
crystallinity, or mass loss, indicating the material was insoluble
and stable to water (Figure S2). For completeness, although
this is an unlikely scenario in a fuel cell where water is
omnipresent, the effect of complete dehydration was examined.
Full dehydration gives a reversible shift to a structure lacking
significant long-range order. Thermogravimetric analysis
showed a gradual mass loss of 19.9 wt% in four steps (Figure
S3). The first two steps (25−145 °C) are attributed to the loss
PCMOF-5 (Figure 1) crystallizes with a formula of
La(H L)(H O) and adopts a modified pillared-layered motif,
5
2
4
of pore and coordinated water, respectively (4 H O/formula
2
unit: 11.2% obs, 10.9% calc). The next step, up to 300 °C, is
attributed to condensation of free phosphonic acids; a formula
of La C H (P O )(P O ) is expected, with a loss of 6.5
1
10 10
2
5
4
9 0.5
equiv of water compared to the hydrated structure (17.6 wt%
obs, 17.7 wt% calc). Mass loss >300 °C is likely the onset of full
decomposition. The amorphization and condensation likely
occur with some retention of similar local environment, as a
sample heated to release all structural water regained
crystallinity by boiling in water (without dissolution) (Figure
S4).
To characterize the proton conductivity of PCMOF-5, AC
impedance analysis was performed on multiple samples with
varying % RH in air. The narrow 1D channels in PCMOF-5
necessitated multiple heating and cooling cycles for samples to
equilibrate; equilibrium was faster at higher RH (Figure S5).
Representative Nyquist plots are shown in Figure 2. Bulk and
Figure 1. (a) Structure of PCMOF-5 as viewed down the a-axis. Free
phosphonic acid groups and uncoordinated water molecules are
located in each pore. (b) View showing the one-dimensional
hydrogen-bonding array formed between phosphonic acid groups
and free water molecules in the direction of the a-axis.
where a hydrophobic molecule L pillars hydrophilic 1D
columns of La(III) phosphonate along the a-axis. Alternating
between the columns along the a-axis are acidic channels
housing a single column of water molecules. The 1D La(III)
phosphonate chains are connected into a 3D framework
through three of the four phosphonate groups on L, leaving the
fourth group, a diprotic phosphonic acid, uncoordinated and
protruding into the hydrated channel. The hydrated channel is
completely lined with phosphonic acid groups, with one
coordinated monodentate hydrogen phosphonate per formula
unit along the b-axis and one uncoordinated phosphonic acid
per formula unit along the c-axis; three coordinated water
molecules do not line the channel and appear to be involved in
structural hydrogen bonds with the 1D La(III) phosphonate
columns. Free water molecules fill the channel, alternating
between and bridging the phosphonic acid groups through
hydrogen bonds. The highly acidic, hydrated nature of the
channel, along with the existence of a potential hydrogen bond
pathway, suggested this material may be a good proton
conductor.
Figure 2. Nyquist plots for PCMOF-5 at 90% RH and various
temperatures.
grain boundary contributions to the resistivity are not resolved
into separate semicircles, consistent with an ion-conductive
interfacial region. This is to be expected as the particle
peripheries would contain phosphonic acid groups and
hydrated metal ions. The low-frequency tail is consistent with
blocking effects at the electrode, as would be expected for ionic
conduction. Analysis on PCMOF-5 at 98% RH gave very good
−3
conductivity of 1.3 × 10 S/cm at 21.5 °C that rose to 2.5 ×
−3
10 S/cm at 60.1 °C (Figure 3). The activation energy
measured was 0.16 eV, a very low value on par with solid acids
1
0a
(cf. H PMo O ·29H O (0.15 eV),
Sb O ·5.4H O (0.17
2 5 2
10c
3
12 40
2
1
0b
eV), and β-Al10.34Mg0.66O (H O) (0.17 eV) ), indicat-
17
3
1.66
ing a highly efficient Grotthuss mechanism. Indeed, this is the
lowest activation energy for total conduction (both bulk and grain
B
dx.doi.org/10.1021/ja310435e | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
phosphonates) that possessed free phosphonic acid groups, and
13
all of these were either 1D or 2D coordination networks.
Although these materials were not studied as electrolytes, even
if the backbone of the material was water stable, the solid would
likely experience swelling with hydration and present issues for
interfacial stability of a membrane electrode assembly. PCMOF-
5
represents the first coordination polymer with a 3D structure to
have uncoordinated phosphonic acid groups. For proton
conduction via a Grotthuss mechanism, beyond acidity, the
fact that these groups are uncoordinated and able to reorient is
notable.
The realization of the free phosphonic acid groups in a water-
stable network was a direct result of a synthetic strategy
employing highly acidic conditions and minimizing competing
phases. Kitagawa recently reported HPO42 - and H PO -
−
−
2
4
containing Zn azole coordination polymers that conduct in the
−4
−4
4o,p
Figure 3. Log of conductivity as a function of inverse temperature for
PCMOF-5 as measured at 90% and 98% RH.
range 1.2 × 10 −2.6 × 10 S/cm at 130 °C.
These
compounds are 1D and 2D networks, so they may be
susceptible to swelling/dissolution, but these authors were
targeting anhydrous conduction. Demadis and Cabeza also
recently reported Gd 2-hydroxyphosphonoacetic acid and La
tetramethylenediamine-N,N,N′,N′-tetrakis(methylenephos-
phonic acid) MOFs with coordinated hydrogen phosphonate
where acidic conditions were employed to mediate protic state
boundary effects) observed in any proton-conducting MOF
material. Costantino et al. have reported a lower value, but
11
this was derived from only the bulk resistivity. Reducing the
−
5
humidity to 90% gave conductivity values between 2 × 10
and 2 × 10−4 S/cm from 20 to 85 °C, with an activation energy
−
4
−3
and obtained conductivities of 3.2 × 107 and 8.0 × 10 S/cm
calculated at 0.32 eV (Figure 3). This is still within the range of
Grotthuss behavior but indicating a less organized pathway. To
ensure that conductivity was not due to residual H SO from
a,b
at 98% RH and ambient temperature.
The value of 2.5 ×
−3
1
0
S/cm at 60 °C and 98% RH for PCMOF-5 is among the
2
4
highest conductivities reported for a MOF material, with the
the synthesis, a second sample was washed with water multiple
times and then boiled in water for 1 day prior to measurement.
This sample also gave conductivity values in the same range as
the untreated sample with the same activation energy of 0.32
eV (Figure S6). Lowering the humidity gave an exponential loss
in conductivity due to the loss of water within the framework,
but conductivity could be recovered after dehydration, as a
sample which was heated to 105 °C fully regained conductivity
upon exposure to high RH (Figures S7 and S8). Phase integrity
was confirmed by PXRD after impedance analysis and
corroborated by the linearity of the temperature dependence
highest reported values being 8 × 10− S/cm at ∼20 °C for the
3
La material above and for (NH ) (adipic acid)-
4
2
4e
−2
[
°
×
Zn (oxalate) ]·3H O by Kitagawa, 1 × 104 S/cm at 150
2
3
2
q
C and low RH for H SO loaded in MIL-101, and finally 2.1
2
4
−2
1
4r
10 S/cm at ∼85 °C and 90% RH in PCMOF2 / .
Better proton-conducting materials are needed that can
2
operate both at higher temperatures and in humid environ-
ments. Humidity commonly augments conduction, provided
the integrity of the electrolyte and the interfacial contacts with
electrodes can be retained. Compositionally robust electrolytes
should also be resistant to swelling. Water stability is a
challenge for MOF materials in general, but this is a critical
factor for proton electrolyte applications. Here we have
presented PCMOF-5, a 3D MOF with pores lined with
uncoordinated acid groups. PCMOF-5 shows proton con-
ductivity on the order of the best MOF materials with the
lowest activation energy for proton transfer in any MOF.
Moreover, it is stable to water in terms of both the framework
connectivity but also with regard to swelling. The ordered
structure provides direct visualization of the proton transfer
pathway and opportunities for material improvements.
(
Figure S9).
The high conductivity, with corresponding low activation
energy at 98% RH, is likely due to complete occupancy of the
intra-channel water sites within PCMOF-5 under the highly
humid conditions. As the uncoordinated water molecules are
integral to the proton transfer pathway, any loss in these
molecules leads to an increase in activation energy due to a
longer average distance for the proton to hop between sites.
Especially given that the pathway is 1D, this should have a
significant effect on conductivity. This hypothesis is supported
qualitatively by the observation that the activation energy
doubles to 0.32 eV, and the conductivity drops 1 order of
magnitude at 90% RH. Below 50% RH, conductivity was
negligible, likely representing loss of the hydrogen bond
pathway in PCMOF-5. The uncoordinated phosphonic acid
groups make the structure reminiscent of phosphonic acid
ASSOCIATED CONTENT
■
*
S
Supporting Information
Details of synthesis, powder XRD, gas sorption, impedance
analysis, and CIF file for PCMOF-5. This material is available
free of charge via the Internet at http://pubs.acs.org.
12
containing proton-conducting polymers. The crystallographic
location of the atoms within the structure offers a useful handle
for further design or modeling of the conductivity of better
proton-conducting materials.
AUTHOR INFORMATION
■
Owing to the excellent ligating ability of organophospho-
nates, the incorporation of free phosphonic acid groups into a
coordination polymer is a significant challenge. In a search of
the Cambridge Structural Database, only six metal organo-
phosphonate compounds were found (of 1258 polymeric metal
Corresponding Author
gshimizu@ucalgary.ca
Notes
The authors declare no competing financial interest.
C
dx.doi.org/10.1021/ja310435e | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
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Vittadini, A.; Gross, S.; Serre, C. Angew. Chem., Int. Ed. 2012, 51, 9267.
ACKNOWLEDGMENTS
■
(f) Feng, D.; Gu, Z.-Y.; Li, J.-R.; Jiang, H.-L.; Wei, Z.; Zhou, H.-C.
We thank the Natural Sciences and Engineering Research
Council (NSERC) of Canada for support of this research.
J.M.T. and K.W.D. acknowledge scholarship support from the
Alberta Ingenuity Fund.
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