Communication
pubs.acs.org/cm
Direct Observation of Two Types of Proton Conduction Tunnels
Coexisting in a New Porous Indium−Organic Framework
Xiang Zhao,† Chengyu Mao,† Xianhui Bu,*,‡ and Pingyun Feng*,†
†
Department of Chemistry, University of California, Riverside, California 92521, United States
Department of Chemistry and Biochemistry, California State University, Long Beach, 1250 Bellflower Boulevard, Long Beach,
California 90840, United States
‡
S Supporting Information
*
F
ligand approach is able to create proton conduction tunnels
with varied chemical and structural environments, which
represents a new direction for designing proton conducting
materials. One of the recent studies involving mixed trisulfonate
and triphosphonate anions has resulted in conductivity over
10−2 S cm−1.31
Here, we focus on a unique combination between oxalate
and 4,5-imidazoledicarboxylate (imdc). The use of imdc is
because it is more versatile than oxalate. H3imdc has three
deprotonatable protons and can provide an extra proton to
promote conduction (compared to H2ox), without compromising its ability to form the chelating mode similar to that of
oxalate (Supporting Information Figure S1). Even though imdc
has been studied in MOFs,34 this is the first time that it is
introduced for the purpose of enhancing proton conduction.
Instead of commonly used divalent metal ions,24−26 In3+ is
chosen in this work because In-MOFs are well-known for their
tendency to form a charged framework with mobile chargebalancing species such as NH4+ and H3O+, and such features
may enhance the proton conductivity. Another consideration is
that the new material should be moisture stable both for watermediated proton conduction enhancement and for tolerance
toward moisture under operating conditions. Taking these
factors into consideration, our syntheses were performed in a
highly basic aqueous solution with the addition of ammonia, in
which indium nitrate hydrate, oxalic acid, and 4,5-imidazoledicarboxylic acid were dissolved. Square shaped colorless crystals
were obtained.
The structure of the compound (denoted CPM-102) was
determined by single crystal crystallography under low
temperature (∼150 K), which gives a formula of [In(imdcH)(ox)]·(NH4)(H2O)1.5. The crystal structure can be understood
as negatively charged [In(imdcH)(ox)]− layers with charge
balancing cation NH4+ and neutral H2O guests located at the
interlayer space. Each indium ion is eight-coordinated in a
chelating manner with two 4,5-imidazoledicarboxylate and two
oxalate ligands (Supporting Information Figure S9).
Due to the coexistence of two kinds of ligands, three types of
4-membered rings are found in each layer alternatively, with the
ring compositions of In4(ox)4 (I), In4(imdc)4 (II), and
In4(ox)2(imdc)2 (III), respectively, in a 1:1:2 ratio (Figure
1a). These layers adopt the ABCDABCD stacking sequence
ast ion conductors lie at the foundation of many energyrelated applications such as batteries and fuel cells.1−3
Proton conduction is a particular class of ionic conduction and
is especially relevant to fuel cell applications. The state-of-theart proton conduction materials are mainly based on sulfonated
polymers such as Nafion.4 There has been an increasing interest
in other types of materials that may offer advanced applications
at lower cost, higher efficiency, or different operating
conditions.5
Metal−organic frameworks (MOFs) have been widely
studied for its porosity and the capability to adsorb and
separate gas molecules.6−10 Recently, the interest in MOFs has
been broadened to many other areas such as ionic
conduction.11−23 The well-defined structure of MOFs makes
it possible to pinpoint the proton conduction pathway, which
can lead to a better understanding of the proton conduction
mechanism. Furthermore, the designable architecture, the ease
with which functional ligands can be introduced into framework
design, and the capability to incorporate different guest species
provide numerous opportunities to improve the proton
conduction property by materials design.
Metal oxalates are known to exhibit high proton conduction,
especially under high humidity environment.24−29 One example
is Humboldtine, which is a 1-D coordination polymer with the
formula of Fe(ox)·2H2O. It is believed that the strong
hydrogen bonds between water molecules and oxygen atoms
from oxalate ligands provide a pathway for proton conduction.24 Another example is a honeycomb layered structure
of zinc oxalate with the formula of (NH4)2(adp)[Zn2(ox)3]·
3H2O.25 In this structure, an extensive hydrogen bonding
network was formed between oxalate oxygens and guest species
including NH4+ and adipic acid. A recent advance was observed
for a closely related compound, which also contains similar
[Zn2(ox)3]2− honeycomb structure, but interpenetrated with
another cationic net with the formula of [(Me2NH2)3SO4]+.
This compound has shown the highest water assisted
conductivity so far with a value comparable to that of Nafion.26
Oxalate, phosphonates, and sulfonates are also among the
fast proton conducting materials.30−33 The common feature of
these ligands is a high density of oxygen atoms. These oxygen
sites contribute to the formation of extended H-bonding
network, and also exposed Lewis base sites can function as
relays in proton transport. However, the above-mentioned
ligands only represent a small portion of the large pool of
potential useful ligands. Many opportunities can be realized
through the combination of two or more ligands. The mixed
© 2014 American Chemical Society
Received: February 8, 2014
Revised: February 28, 2014
Published: April 7, 2014
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conduction (Figure 1c). The type I proton tunnel goes
through the interlayer space between type I and type III rings,
in a direction where two oxalate ligands from adjacent layers are
stacked to each other. In contrast, the type II proton tunnel
goes through the interlayer space between type II and type III
rings, in a direction where two adjacent imdc ligands are
stacked. The two types of proton tunnels are arranged
alternatively in the same layer, but adopt the perpendicular
orientation from those in the adjacent layer.
As illustrated in Figure 2a, the type I channel is highly
hydrophilic, with 16 oxygen atoms surrounding four guest
molecules. The guest species are so well-defined that even the
hydrogen atoms can be precisely located from X-ray data. This
allows us to probe the detailed mechanism for the proton
conduction. Two NH4+ cations and two H2O molecules are
found within the interlayer space between a type I ring and a
type III ring. These four guest molecules are connected with
each other through H-bonds to form a square. The hydrogen
atoms pointing away from the square form 12 additional
hydrogen bonds with the oxygen atoms from surrounding
oxalate and imdc ligands (Figure 2b). In fact, the distance
between the closest N and O atoms (N3···O9c ∼ 3.37 Å) from
adjacent squares also allows the formation of a hydrogen bond,
although the hydrogen atoms observed at 150 K are not at
appropriate positions to form hydrogen bonds between
adjacent squares (N3···O9c) because they tends to be located
at their most stable position in the “frozen” condition.
However, at elevated temperatures, the higher thermal energy
may activate these guest species and allow them to vibrate and
rotate. Thus, all the NH4+ and H2O guest molecules in the
channel form railway-shaped double chains through hydrogen
bonding (Figure 2c), which provide a straight pathway for
proton conduction.
Figure 1. Illustration of (a) the periodical pattern of each layer
constructed from three types of 4-membered rings; (b) the ···
ABCDABCD··· layer stacking sequence and (c) two types of proton
tunnels embedded in the interlayer space and their spatial distribution.
(Figure 1b). It is found that the type III ring is sandwiched
between a type I ring and a type II ring from adjacent layers. In
contrast, both type I and type II rings are sandwiched between
two type III rings.
An analysis of the structure revealed an extensive H-bonding
network which includes both guest NH4+ and H2O species and
oxygen atoms from the host layer (Supporting Information
Table S1 and Figure S4). In fact, these H-bonds help connect
adjacent layers together to create a 3-D network. Interestingly,
there are two distinctly different groups of hydrogen bonds,
both of which forms tunnels that contribute to proton
Figure 2. (a) Perspective view along [110] direction of type I proton tunnel; (b) the interlayer space between type I and type III rings and related
hydrogen bonds; (c) hydrogen bonds along type I channel; (d) perspective view along [110] direction of type II proton tunnel; (e) the interlayer
space between type II and type III rings; and (f) hydrogen bonds along type II channel. (Gold: In; red: O; blue: N; gray: C; yellow: H.)
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crystal data, we are able to get a close look for the possible
proton conduction pathway, which is quite valuable for us to
understand its proton conduction mechanism. Therefore a
detailed analysis of the H-bonding networks in both channels
has been performed. Interestingly, this compound has
integrated two types of proton carriers in its structure, the
guest counterion NH4+ and a nondeprotonated carboxylic acid
group from imdc ligand in the host.
In the type I tunnel, the major proton carrier is NH4+. Under
applied electrical field, a proton from the ammonium ion can
hop to an adjacent water molecule and form a hydronium ion
(Supporting Information Figure S11). Then a proton can be
dissociated from the hydronium ion and further hop to another
ammonia molecule, either on the side or in front. Alternatively,
the proton from the ammonium ion can also hop to the
coordinated oxygen atoms on the host and then the proton can
either migrate along the surface of the host or hop back to the
guest chains through hydrogen bonding. Compared to the
guest−host pathway, the guest−guest direct pathway is
straighter but the longer distance between adjacent nonhydrogen atoms requires higher energy to activate the proton
to cross over the barrier. Thus, it is expected that both
pathways will contribute to the proton conduction, and they
will achieve a balance with each other. With increased
temperature, the guest−guest direct pathway will ultimately
dominate the conduction.
In the type II tunnel, the major proton carrier becomes the
carboxylic acid group. When electrical field is applied, the
proton can be activated and can hop between carboxyl groups
on adjacent imdc ligands and achieve conduction. Alternatively
it can also hop from a carboxyl group to a guest water molecule
to form a hydronium ion, and then the proton from the
hydronium ion can either hop back to another carboxyl group
or hop to an oxalate ligand and migrate on the surface of the
layer. In Supporting Information Figure S3, the hydrogen bond
networks are illustrated under different hydrogen bond lengths,
which can give a simple map for the proton conduction
pathway (the shorter, the easier). It is necessary to point out
that the proton conduction is a complicated phenomenon and
that the distance between donor and acceptor atoms is only one
factor. Other important factors such as the electronegativity of
the element and the coordination geometry also need to be
considered.
In summary, we have created a unique layered compound in
which two types of proton tunnels coexist. This phenomenon
has been never reported before and results from our use of the
mixed oxygen-rich ligand strategy that helps to create two
distinct tunnels. Impedance spectroscopy studies of both bulk
sample and single crystals have shown good conductivity. The
proton conduction pathway through H-bonding has been
mapped out, and the possible conduction pathway is proposed.
The introduction of imdc ligand for building proton conducting
MOFs has been demonstrated as successful for its dual role: it
helps to create an extensive H-bond network as well as to serve
as a proton carrier in its dinegative form. We believe that this
research will open up new opportunities for developing
advanced MOFs with superb proton conducting properties by
taking advantage of properly designed or chosen oxygen-rich
ligands.
Type II channel is based on the dangling carboxylate oxygens
from imdc ligands and the guest water molecule and is
narrower than the type I channel. Still, it has a high density of
oxygen atoms and is hydrophilic (Figure 2d). Unlike in type I
channel, the positions of hydrogen atoms in the type II channel
are not well-defined, likely due to the orientational disorder of
H2O, and are thus modeled into statistic distribution, which
correlates well with its thermal ellipsoids (Supporting
Information Figure S10). Still, the location of the water
molecule is always between a type II ring and type III ring, near
the type III ring end (Figure 2e). The distance between
adjacent carboxylate oxygens and water oxygen allow the
formation of hydrogen bonding, which is responsible for the
second type of proton conduction pathway (Figure 2f). Since
the imdc ligand only lost two protons in coordination with
indium ion, it contributes an extra hydrogen for proton
conduction, by hopping from one oxygen site to another
through the H-bonding network.
AC impedance spectroscopy was performed on a compressed
pellet of the crystalline powder sample coated with Pt/Pd
electrodes. The Nyquist plot is shown below. The semicircle in
the high frequency region is likely to be attributed to bulk and
grain boundary resistance, and the tail at low frequency is
related to the limited diffusion of mobile ions at the electrode−
electrolyte interface. Under 98.6% relative humidity (RH) and
23.5 °C, the proton conductivity reaches 0.82 × 10−3 S cm−1
(Figure 3), which is comparable to the best known proton
Figure 3. Nyquist plots of (a) the pellet sample at 23.5 °C and 98.6%
relative humidity and (b) the single crystal along the ab plane at 22.5
°C and 98.5% relative humidity.
conducting MOF materials developed so far (Supporting
Information Table S2). The Nyquist plots obtained under
different humidity conditions (from 70% to 98.6% RH) at
room temperature show that the proton conductivity is strongly
humidity dependent (Supporting Information Figure S7). To
further investigate the conductivity along specific directions in
the crystalline solid, single crystal conductivity measurement
was performed on a square plate shaped crystal with
dimensions of about 0.35 mm × 0.35 mm × 0.075 mm. The
electrodes are made by applying a melting-resolidification cycle
of gallium metal (Supporting Information Figure S8). The inplane conductivity was measured as 1.11 × 10−2 S cm−1 under
98.5% RH and 22.5 °C. The strong humidity dependence was
also observed for the single crystal sample.
Due to the small channel, it is unlikely for the guest
ammonium and water species to transport for a long distance
within the channel. Instead, local molecular motions such as
vibration and rotation are possible. Thus the vehicle mechanism
for proton conduction can be excluded, and it is expected that
the proton conduction within this compound is dominated by
the Grotthuss mechanism.35,36 Thanks to the precisely refined
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ASSOCIATED CONTENT
S Supporting Information
*
Experimental details, powder X-ray diffraction, thermal analysis,
IR spectrum, additional figures and tables, crystallographic
table, and CIF files. This material is available free of charge via
the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Authors
*(X.B.) E-mail: [email protected].
*(P.F.) E-mail: [email protected].
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This work was supported by the U.S. Department of Energy,
Office of Science, Basic Energy Sciences, under Award No. DEFG02-13ER46972.
■
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