Rationally tuning host-guest interactions to free hydroxide ions within intertrimerically cuprophilic metal-organic frameworks for high OH- conductivity

Article abstract of DOI:10.1039/c6ta11242k

Hydroxide-anion-exchange membrane fuel cells (HEMFCs) are now considered as one of the most promising green energy-conversion technologies for stationary and mobile applications, showing high fuel conversion efficiency at high pH and low cost due to their ability to operate under basic conditions using non-precious metal catalysts. But one key impediment to commercialization is insufficient hydroxide ion (OH^-) conductivity of the central HEM component. Here, we report the development of free OH^- anion-containing metal-organic frameworks (FOMOFs) with rationally tunable host-guest interactions for HEMs with high OH^- conductivity. Among three solids obtained by post-synthesis treatment of ultrastable MOF FJU-66 with various bases, free OH^- anions are observed in FJU-66·[EVIm]OH with strong host-guest interactions between the MOF backbone and guest cations. Despite the lowest OH^- concentration, FJU-66·[EVIm]OH exhibits the highest OH^- conductivity close to 0.1 S cm^-1. The high OH^- conductivity achieved suggests the potential application of the FOMOFs for practical HEMs of fuel cells.

Full text of DOI:10.1039/c6ta11242k

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Journal of
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Zhang, Y. Ye, K. cai, F. Du, H. Zeng, J. Tao, Q. Lin and Y. Zheng, J. Mater. Chem. A, 2017, DOI:
1
0.1039/C6TA11242K.
Volume
4
Number
1
7
January 2016 Pages 1–330
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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS nanocages as a lithium ion battery anode material
2
rsc.li/materials-a

Page 1 of 19
Journal of Materials Chemistry A
View Article Online
Rationally Tuning Host-guest Interactions to Free Hydroxide Ions
DOI: 10.1039/C6TA11242K
within Intertrimerically Cuprophic Metal-organic Frameworks for
-
High OH Conductivity
1
1,2*
1
1
1
1
1
Ziyin Li, Zhangjing Zhang,
Yingxiang Ye, Kaicong Cai, Fenfen Du, Heng Zeng, Jin Tao,
1
1
1,2*
Quanjie Lin, Ying Zheng and Shengchang Xiang
1
College of Materials Science and Engineering, Fujian Provincial Key Laboratory of Polymer
Materials, Fujian Normal University, 32 Shangsan Road, Fuzhou 350007, China.
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of
2
Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, PR China. *Email:
zzhang@fjnu.edu.cn; scxiang@fjnu.edu.cn.
†
Electronic Supplementary Information (ESI) available: See DOI:10.1039/x0xx00000x
Abstract: Hydroxide-anion-exchange membrane fuel cell (HEMFC) is now considered as one of
the most promising green energy-conversion technologies for stationary and mobile applications,
showing up high fuel conversion efficiency at high pH and low cost due to their ability to operate in
basic conditions using non-precious metal catalysts. But one key impediment to commercialization
−
is insufficient hydroxide ion (OH ) conductivity of central HEM component. Here, we report the
-
development of free OH anions-containing metal-organic frameworks (FOMOFs) with rationally
−
tunable host-guest interactions for HEMs with high OH conductivity. Among three solids obtained
-
by post-synthetic treat of ultrastable MOF FJU-66 with various bases, free OH anions are observed
in FJU-66∙[EVIm]OH with strong host-guest interactions between MOF backbone and guest
cations. Despite the lowest OH concentration FJU-66∙[EVIm]OH exhibits the highest OH-
-
-
conductivity close to 0.1 S/cm. The high OH conductivity achieved suggests the potential
application of the FOMOFs for practical HEMs of fuel cells.
1
. Introduction
The control of ionic conductivity is important in chemistry, biology, and materials science due
1
to a fundamental interest in understanding ionic transport phenomena, as well as potential
2
3-5
applications in fuel cells. Recently, metal-organic frameworks (MOFs) with their advantages in
high regularity and structural visualization offer great potential for systematic design and
modification of ionic conduction, particularly proton conduction.6-9 The concentration or mobility
of proton carriers can be imparted or improved by pore filling,8 skeleton modification,9 phase

Journal of Materials Chemistry A
Page 2 of 19
-
transformation,6 and other methods.7 By contrast, research on hydroxide ion (OH ) conduction in
View Article Online
DOI: 10.1039/C6TA11242K
-
the MOFs is still in its initial stage. Cationic MOF with OH counter ions shows a low activation
energy E
of 0.19 eV.10 The MOFs post-synthetically treated with tetrabutyl ammonium hydroxide
[NBu
a
(
4
]OH),11 NaOH12 and KOH,13 or composited with organic ionomers,14 show enhanced OH-
-
conductivity in comparison with their mother materials. But the OH conductivities are still
insufficient, commonly lower than the conductivities for the proton exchange materials. Poor OH-
conduction of the central component, the hydroxide anion-exchange membranes (HEMs), is the key
impediment to commercialization of the HEM fuel cells, one of the most promising green
energy-conversion technologies for stationary and mobile applications, those show up the high fuel
conversion efficiency at high pH and low cost due to their ability to operate in basic conditions
using non-precious metal catalysts.
-
Basically, the OH conductivity is directly related to a combination of ion exchange capacity
(
IEC) and ion mobility.15 Dissociation of the bases presents a considerable theoretical barrier for
-
16
efficient OH IEC and improved conductivity, as the broadly used bases are weakly basic
quaternary ammoniums with the pK
b
values on the order of 4.17 Even in some HEMs with the
-
loaded strong bases like KOH, the cation-OH electrostatic force within the base may restrain the
-
-
OH mobility under electric field. Thus, how to free OH anions from the restraints of the
-
cation-anion electrostatic force or of the dissociation is critical to enhance the OH conductivity of
the HEMs. This goal may be probably achieved by the MOFs’ pore filling approach, which can
provide rationally tunable host-guest interactions between their robust skeleton and the loaded bases
-
to pull the cation-OH pairs apart via the choices of appropriate functional sites on both the MOFs
and the bases, but it is still unexplored.
-
In this paper, we report the development of free OH anions-containing metal-organic
frameworks (FOMOFs) with rationally tunable host-guest interactions for the HEMs with high OH−
-
conductivity at low OH concentration. [Cu
6
(NDI)
3
]∙2DMF∙6MeOH∙2H
2
O [FJU-66∙S, H NDI:
2
2
,7-bis(3,5-dimethyl) dipyrazol-1,4,5,8-naphthalene-tetracarboxydiimide, S: solvent molecules] is
the first example of microporous MOF made up of discrete [Cu (Pz) ] clusters (Pz, pyrazolate) with
6
6
intertrimer cuprophilicity, which endows FJU-66 with ultrastable immunity to attacks of heat and of
acid-base aqueous solutions. Naphthalene diimide on FJU-66 is an attractive class of expanded
π-electron-deficient (π-acidic) functional sites to offer specific affinity to the electron-rich (basic)
organic amines. 18 Under the consideration of the influence of functional sites and steric
configuration of alkaline cations, three bases with various cations, planar
+
2
1
-ethyl-3-vinylimidazolium ([EVIm] ) with sp C and N atoms as possible sites binding to the

Page 3 of 19
Journal of Materials Chemistry A
naphthalene diimide groups on FJU-66 via hydrogen bonding or lone pair- interactions, large
View Article Online
DOI: 10.1039/C6TA11242K
+
]+ without any binding sites, are chosen as the ionic carriers to be
multihydrated K , or large [NBu
4
loaded into FJU-66, resulting in three post-synthetically treated solids containing various
stoichiometric bases, FJU-66∙nXOH (n is 1 for [EVIm]OH, 3 for KOH, 0.9 for [NBu ]OH]),
4
-
respectively. Interestingly, the free OH anions are firstly observed by X-ray single crystal
+
diffraction, as a result of binding [EVIm] cations to the naphthalene diimide groups on the MOF
-
skeleton in FJU-66∙[EVIm]OH, which significantly improves its OH conductivity up to about 0.1
-
5
6
S/cm despite the lowest OH concentration of 0.34 mmol/g. Its conductivity is about 10, 10 and 10
fold those of FJU-66∙3KOH, FJU-66∙0.9NBu OH and the parent material, respectively, and 7.6
time the highest value reported ever among all MOF-based HEMs. Its activation energy E
4
1
3
a
of 0.11
eV is the lowest among all the HEMs, further demonstrated that FJU-66∙[EVIm]OH is a type of
-
FOMOF containing free OH anions.
2
. Experimental section
2
.1 Materials and Measurements
All reagents and solvents used in synthetic studies were commercially available and used as
supplied without further purification. H
2
NDI was synthesized similar to published procedures.19
3
Powder X-ray diffraction (PXRD) was carried out with PANalytical X’Pert powder diffractometer
equipped with a Cu-sealed tube (λ = 1.541874 Å) at 40 kV and 40 mA over the 2θ range of 5-30o.
-1
The Fourier transform infrared (KBr pellets) spectra was recorded in the range of 400~4000 cm on
a Thermo Nicolet 5700 FT-IR instruments. Thermal analysis was carried out on a METTLER TGA/
o
o
-1
SDTA 851 thermal analyzer from 30 to 800 C at a heating rate of 10 C min under nitrogen
atmosphere. Differential scanning calorimetry (DSC) was carried out with a Mettler Toledo DSC
o
-1
8
22e at the heating rate of 10 C min under nitrogen atmosphere. SEM images were obtained with
a JEOL JSM-7500F scanning electron microscope. The samples were sputtered with Au (nano-sized
1
film) prior to imaging. H NMR spectra were recorded on a Bruker advance III 400 (400 MHz)
spectrometer as solutions in D O. Elemental analyses (C, H and N) were performed on a
2
Perkin-Elmer 240C analyzer. X-ray photoelectron spectroscopy (XPS) measurements were
conducted on a VG Scientific ESCA Lab Mark II spectrometer using Al Kα as X-ray source. All
binding energies were referenced to the C1s peak of the surface adventitious carbonat 284.6 eV.
Solid state UV-vis spectra were recorded using Pekin Elmer Lambda 950. Luminescence spectra
were investigated on a FLS980 with a 450W Xenon lamp. Inductively coupled plasma (ICP)

Journal of Materials Chemistry A
Page 4 of 19
analysis was performed on a Ultima 2.
.2 Syntheses
Synthesis of FJU-66∙S: A mixture of Cu(NO
g, 0.05 mmol), H BTC (0.025 g, 0.12 mmol) , DMF/MeOH (4 mL/1 mL) and 1 mol/L HCl (0.5 mL)
View Article Online
DOI: 10.1039/C6TA11242K
2
1
9
3
)
2
∙3H
2
O (0.0242 g, 0.1 mmol), H NDI (0.0227
2
3
was stirred for 5 minutes and then transferred to teflon-lined 23 mL Parracid digestion bomb, heated
to 150 °C and kept at the temperature for 3 days. Red crystals were obtained, then collected and
washed with DMF and dried in air. Yield 45% (based on H
Cu 22) (%) Calcd: C 47.68, H 4.26, N 13.25; Found: C 47.08, H 4.45, N 12.82.
Activation of FJU-66·S sample. After the bulk of the solvent was decanted, the freshly
2
NDI). Elemental Analysis for FJU-66∙S
(
6
C
84
H
90
20
N O
prepared sample of FJU-66·S (~0.15 g) was soaked in ~10 mL acetone for 1 hours, and then the
solvent was decanted. Following the procedure of acetone soaking and decanting for ten times, the
o
solvent-exchange samples were activated by vacuum at 80 C for 24 hours till the pressure of 5
μmHg to get the activated sample FJU-66. X-ray single crystal diffraction of the sample FJU-66 in
2
N atmosphere at 297 K shows no electron density in the voids, confirming the effectiveness of the
activation procedure.
Synthesis of FJU-66∙[EVIm]OH. The activated sample FJU-66 sample (0.2 g) were
suspended in the mixed solution of 20 mL ethanol (EtOH) and [EVIm]OH (2 g, 0.014 mol) and then
was stirred under ambient temperature for 24 hours. The solids were collected by centrifugation,
then washed by ethanol solvent for several times and dried in air to yield FJU-66∙[EVIm]OH.
Elemental Analysis for FJU-66∙[EVIm]OH·12H
.42, C 47.26, H 6.69; Found: N 9.37, C 47.45, H 6.75.
Synthesis of FJU-66·3KOH. FJU-66·3KOH was prepared comparably to the synthesis of
FJU-66∙[EVIm]OH, except KOH (0.785g, 0.014 mol) instead of [EVIm]OH. Elemental Analysis
for FJU-66·3KOH·28H O·2EtOH (Cu 45) (%) Calcd: N 10.07, C 36.44, H 4.75;
2
O·19EtOH (Cu
6
C H
117 199
N
20
O44) (%) Calcd: N
9
2
6
3
K C
76
H
119
18
N O
Found: N 9.94, C 36.79, H 4.80. ICP analysis was performed to gauge relative contents of K and Cu
reveal that the K/Cu ratio is 1.00 : 2.07 which is in good agreement with the elemental analysis
(
Table S8).
Synthesis of FJU-66·0.9NBu
synthesis of FJU-66∙[EVIm]OH, except NBu
Elemental Analysis for FJU-66·0.9NBu OH·12H
C 46.70, H 4.66; Found: N 12.09, C 47.38, H 4.81.
4
OH. FJU-66·0.9NBu
4
OH was prepared analogously to the
OH (3.633 g, 0.014 mol) instead of [EVIm]OH.
O (Cu 24.9) (%) Calcd: N 11.99,
4
4
2
6
C
86.4
H
105.3
N
18.9
O

Page 5 of 19
Journal of Materials Chemistry A
2
.3 Single Crystal X-Ray Diffraction (SCXRD) Studies
View Article Online
DOI: 10.1039/C6TA11242K
Data collection and structural analysis of crystals FJU-66·S, the activated sample FJU-66 and
FJU-66∙[EVIm]OH were collected on an Agilent Technologies Super Nova Single Crystal
Diffractometer equipped with graphite monochromatic Cu Kα radiation (λ = 1.54184 Å). The
crystal was kept at 100 K during data collection. Using Olex2,20 the structure was solved with the
Superflip structure solution program using Charge Flipping and refined with the ShelXL21
refinement package using Least Squares minimization. All nonhydrogen atoms were refined with
anisotropic displacement parameters. The hydrogen atoms on the ligands were placed in idealized
positions and refined using a riding model.
2
2
23
Single-crystal X-ray crystallography. For FJU-66·S, we employed PLATON /SQUEEZE
to calculate the diffraction contribution of the solvent molecules and thereby produce a set of
solvent-free diffraction intensities. Crystal data: C36 ; Mr = 869.26; tetragonal; space
group I-4; a = 17.2079(3), c = 34.0361(10) Å; V = 10078.5(4) Å ; Z = 8; ρcalcd = 1.146 gcm ; μ =
H24Cu
3
N
9
O
6
3
-3
-1
1
.829 mm ; F(000) = 3504.0; GOF = 1.013. A total of 14517 reflections were collected, 8376 of
which were unique (Rint = 0.0618). R /wR = 0.0561/0.1215 for 493 parameters and 4320 reflections
I > 2σ(I)).
For the activated sample FJU-66, there is no electron density in the voids after we completed
1
2
(
the refinements of the skeleton. Crystal data: C36
H
24Cu
3
N
9
O
6
; Mr = 869.26; tetragonal; space group
3
-3
I-4; a = 17.4696(5), c = 33.7023(9) Å; V = 10285.5(7) Å ; Z = 8; ρcalcd = 1.128 gcm ; μ = 1.813
-1
mm ; F(000) = 3520.0; GOF = 1.036. A total of 15360 reflections were collected, 8840 of which
were unique (Rint = 0.0863). R
1
/wR
2
3
= 0.0805/0.2121 for 493 parameters and 4818 reflections (I >
-
2
σ(I)). The highest peak (0.69 eÅ ) is not a real atom. It is located 1.58 Å from the Cu1 atom,
-3
whereas the deepest hole (-1.10 eÅ ) is 0.90 Å from the Cu1 atom.
+
For FJU-66∙[EVIm]OH, after the skeleton and [EVIm] cation are assigned, the highest Q
-3
peak of 1.2 eÅ is located 3.083 Å away from the center of nanocage (Wyckoff site 2a), which is
-3
assigned as the hydroxide (O7) atom. Then the residual highest peak (0.91 eÅ ) is not a real atom.
-3
It is located 0.76 Å from the C2 atom, whereas the deepest hole (-0.37 eÅ ) is 1.82 Å from the C15
atom. Crystal data: C38.02 6.29; Mr = 907.06; tetragonal; space group I-4; a =
7.3245(3), c = 33.9396(6) Å; V = 10186.6(3) Å ; Z = 8; ρcalcd = 1.183 gcm ; μ = 1.839 mm ;
F(000) = 3659.0; GOF = 1.033. A total of 14071 reflections were collected, 8425 of which were
unique (Rint = 0.0400). R /wR = 0.0611/0.1618 for 548 parameters and 6277 reflections (I > 2σ(I)).
H
25.15Cu
3
N
9.58
O
3
-3
-1
1
1
2

Journal of Materials Chemistry A
Page 6 of 19
The detailed crystallographic data and structure refinement parameters for these compounds
View Article Online
DOI: 10.1039/C6TA11242K
are summarized in Table S1. CCDC-1475870 (FJU-66·S), -1475871 (FJU-66), and -1475872
(FJU-66∙[EVIm]OH) contain the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
2
.4 Gas Adsorption
Nitrogen, carbon dioxide, methane, ethylene, ethane and acetylene adsorption isotherms were
measured on Micromeritics ASAP 2020 HD88 surface area analyzer for the activated sample
FJU-66 and FJU-66∙[EVIm]OH. The sorption measurement was maintained at 77 K with liquid
nitrogen and at 273 K with an ice-water bath (slush), respectively.
2
.5 Proton conductivity measurement
The samples were pressed under a pressure of ~0.19 t to obtain pellets of the powder samples.
The sample length was 0.10~0.30 cm, with a 0.25 cm diameter. Measurements were performed
using an impedance and gain-phase analyzer (Solartron analytical ModuLab 1260) over a frequency
range from 100 Hz to 1MHz with an input voltage amplitude of 100 mV. Measurements were taken
under hydrated conditions and done at thermal equilibrium by holding for 30 min in the temperature
o
rang from 20 to 85 C Ionic conductivity was calculated using the following equation,
l


SR
,
2
where l and S are the length (cm) and cross-sectional area (cm ) of the samples, respectively, and R,
which was extracted directly from the impedance plots, is the bulk resistance of the sample (Ω). The
activation energy (E
a
) for the materials conductivity was estimated from the following equation,
Ea

T  exp(
)
0
k T
B
,
where σ is the ionic conductivity, σ
0
is the preexponential factor, k is the Boltzmann constant, and
B
T is the temperature. ZView software was used to extrapolate impedance data results by means of
an equivalent circuit simulation to complete the Nyquist plot and obtain the resistance values.

Page 7 of 19
Journal of Materials Chemistry A
3
. Results and discussion
View Article Online
DOI: 10.1039/C6TA11242K
3
.1 Crystal structures
Fig. 1 (a) [Cu
6
(Pz) ] cluster nodes in staggered mode with intra- (green sticks) and inter-trimer (red
6
sticks) cuprophilic interactions acting as octahedral nodes. (b) A single pcu-network with octahedral
nodes bridged by NDI linkers viewed down along the a axis. (c) Interpenetration of two
crystallographically dependent pcu networks (purple and yellow) to give a nanocage (grey-blue,
prolate ellipsoid) centered on Wyckoff site 2a (0.5, 0.5, 0.5) and surrounded by 12 adjacent
[
Cu
6
(Pz) ] clusters (d). In (c) and (d), methyl groups on the pyrazolates, O atoms on the imides and
6
all H atoms are omitted for clarity. Color code: Cu, bright green; C, gray; O, red; N, blue. Symmetry
code ’ in (a): 1−x, 1−y, z. (e) Cu 2p3/2 XPS spectra and (f) Cu LMM X-ray induced Auger emission
spectra of FJU-66. Contours of the frontier orbitals for the [Cu
6
(Pz) ] model compound with
6
single-crystal geometry, HOMO (g) and LUMO (h). To simplify the calculation, all methyl groups
on the pyrazolate rings are replaced by hydrogen atoms. Color codes: pink Cu, blue N, gray C and
H; the electron density is shown in red and green.
Air-stable red crystals of FJU-66 were synthesized by solvothermal reaction of a
Cu(NO
3
)
2
∙3H
2
O, H
at 150 C for 3 days. FJU-66 crystallizes in tetragonal space group I-4. It is a 3D open framework
comprising discrete [Cu (Pz) ] clusters as octahedral nodes (Fig. 1 and S3). The [Cu (Pz) ] node is
composed of two symmetry-dependent [Cu (Pz) ] trimer units, in which the central Cu ring is
nearly coplanar with a mean deviation of 0.074 Å and the separations between Cu ions ranging
from 3.146 to 3.250 Å, very close to those in the [Cu (Pz)
] monomers, 24 dimers25 and
3
BTC (benzene-1,3,5-tricarboxylic acid) and H
2
NDI in DMF/methanol solution
o
6
6
6
6
3
3
3
N
6
I
3
3
coordination polymers.26,27 Two trimers stack together in staggered pattern, giving two shorter
intertrimer Cu···Cu contacts (3.038 and 3.064 Å), the shortest values among all extended
[
Cu
3
(Pz) ]-base polymers reported (Table S3), indicating the intertrimer cuprophilic interactions
3

Journal of Materials Chemistry A
Page 8 of 19
within the [Cu
6
6
(Pz) ] nodes. Each [Cu
6
6
(Pz) ] node connects with six neighboring [Cu
6
(Pz) ] nodes
6
View Article Online
DOI: 10.1039/C6TA11242K
e
12
3
through NDI linkers to produce 3D pcu-type network8 with the topological point symbol of 4 .6 .
The triangular channels in a single net along a or b direction have a dimension of ca. 8.6 Å in
diameter (Fig. 1b). The spacious nature of the single network allows another identical network to
interlock each other, resulting in a two-fold interpenetrating array (Fig. 1c). The two fold
2
interpenentration produces a nanocage (29.1  11.2 Å ) centered on Wyckoff site 2a (0.5, 0.5, 0.5)
and surrounded by 12 adjacent [Cu
6
(Pz) ] clusters (Fig. 1d). Despite the interpenetration, the
6
framework of FJU-66 remains open and contains 1D channels of ca. 5.6 Å in diameter along the a
and b axes that are occupied by disordered solvent molecules. PLATON28 analysis showed that the
effective free volume of FJU-66 is 42.6% of the crystal volume. It is worthy to note that employing
H
2
NDI, the longest linear bi-pyrazolate ever with its length of ~14.048 Å, in FJU-66 makes the
Cu (Pz) ] clusters isolated, which avoid the formation of difinite intertrimeric Cu···Cu contacts
observed in the extended [Cu (Pz)
]-base polymers (Fig. S5 and Table S4).25-27 The intertrimer
cuprophilicity is further confirmed by bond valence calculations, X-ray photoelectron spectroscopy
XPS), density functional theory (DFT) calculations and luminescence (Figs. 1e-1h and S6-S7,
Tables S5-S6). In the XPS spectra, the Cu 2p core level spectra have the dominant Cu 2p3/2 peak at
33.2 eV, meanwhile, the position of the LMM-2 auger transition only centers at 570 eV, indicating
[
6
6
3
3
(
9
I
0
II 29
that all copper elements are Cu , not Cu or Cu . DFT calculations suggest that the highest
occupied molecular orbital (HOMO) for [Cu (Pz) ] trimer dimmer is markedly intra- and
inter-trimer Cu 3d-Cu 3d antibonding for the quadrangle made up of couple of Cu1 and Cu2 atoms
within the [Cu (Pz) ] cluster. While the lowest occupied molecular orbital (LUMO) is mainly
6
6
6
6
contributed from Cu 4s and Cu 4p of the Cu1-Cu2 quadrangle and from a small amount of N 3s of
the coordinated pyrazolate groups. The population of the HOMO and LUMO indicates the presence
of intertrimer cuprophilic interactions, which arises due to the attractive dispersive forces that
1
0
10
10 2
10 2
30
overcome the Pauli repulsion between d –d or even d s –d s closed shells. The energy (∆E)
-1
for the intertrimer cuprophilic interactions is of +4.66 kcal mol . FJU-66 with the shortest
intertrimer Cu···Cu contacts among the extended polymers is the first example of intertrimerically
cuprophic MOF based on discrete [Cu
6
(Pz) ] clusters.
6

Page 9 of 19
Journal of Materials Chemistry A
3
.2 Ultrastability
View Article Online
DOI: 10.1039/C6TA11242K
Fig. 2 Powder X-ray diffraction patterns of FJU-66·S under variable (a) temperature and (b) pH
conditions. (c) The comparison of thermal stability and chemical stability for FJU-66·S with
representative MOFs. The olive and and blue bars represent the MOFs with high-valence metal ions
or high pKa ligands, respectively.
The short intertrimer Cu···Cu contacts may bring FJU-66 with improved thermal and chemical
stability. From thermo-gravimetric analysis (TGA) and variable-temperature powder X-ray
diffraction (PXRD) as shown in Fig. 2 and Figs. S8-S9, FJU-66 can keep its structure intact up to
8
03 K. Such high thermal stability of FJU-66 is comparable to ZIF-8 (823 K),31 and superior to all
the other extended [Cu (Pz) ] polymers (Tables S3 and S4) and most of famous MOFs with
high-valence metal ions32 or high pK ligands33,34 (Fig. 2c and Table S7), demonstrating the extra
3
3
a
stability resulted from the intertrimer cuprophilicity. Chemical stability of FJU-66 was tested by
treating its samples under different conditions (Fig. 2b). The FJU-66 remains stable not only in
aqueous solutions with pH values in the range of 3-14, but also in 10 M sodium hydroxide solution
over 12 hours. Although some reported MOFs present water tolerance or chemical stability in acidic
or basic media, very few MOFs were found to be resistant to both acid and strong base (Fig. 2c and
Table S7).35 FJU-66 is one of most stable MOFs with microporous structure, which can survive in

Journal of Materials Chemistry A
Page 10 of 19
both high temperature over 800 K and an extensive pH range.
View Article Online
DOI: 10.1039/C6TA11242K
3
.3 Ionic conductivity
Fig. 3 (a) N sorption isotherms of FJU-66 and the [EVIm]OH treated sample FJU-66∙[EVIm]OH
2
at 77 K. (b) Pore-size distribution of FJU-66 and FJU-66∙[EVIm]OH calculated by the non-local
density functional theory (NLDFT). (c) Nyquist plots of FJU-66∙[EVIm]OH at different
temperatures under 95% RH during the heating process. Arrhenius plots for (d) FJU-66 (blue),
FJU-66∙[EVIm]OH (red), FJU-66∙3KOH (black) and FJU-66∙0.9NBu OH (green) under 95% RH.
4
The inset in (c) represents the ionic conductivity of FJU-66∙[EVIm]OH under 95% RH (red) in
terms of heating-cooling cycles and under 50% RH (black).
X-ray single crystal diffraction of the activated sample FJU-66 in N
shows no electron density in the voids, confirming the effectiveness of the activation procedure
Table S1). Its permanent porosity has also been established by N sorption at 77 K, displaying
typical type-I sorption behavior with BET (Langmuir) surface area of 843.5 (979.4) m g (Fig. 3a)
2
atmosphere at 100 K
(
2
2
-1
3
-1
and pore volume of 0.348 cm g , indicating FJU-66 is a robust micropous MOF. The pore size
distribution of the FJU-66 was calculated by employing non-local density functional theory
(NLDFT) showing that the major pores located at around 0.6 and 0.8 nm (Fig. 3b).
The permanent porosity and excellent alkaline stability of FJU-66 encourage us to develop
new HEM electrolytes by post-synthetic treat with various alkalines and to investigate the
relationship between the ionic conductivity and the alkaline cations-framework interactions. The
stoichiometry of the loaded alkalines to the parent material for the resulted FJU-66∙nXOH was

Page 11 of 19
Journal of Materials Chemistry A
-
confirmed by elementary analyses and inductively coupled plasma (ICP) (Table S8). The OH ionic
View Article Online
DOI: 10.1039/C6TA11242K
concentration of 0.34 mmol/g for FJU-66∙[EVIm]OH is lower than those for FJU-66∙3KOH (1.20
mmol/g) and FJU-66∙0.9NBu OH (0.41 mmol/g). After the alkaline is loaded into FJU-66, the
4
differential pore volume obviously decreased as observed in N adsorption at 77 K (Figs. 3a and 3b).
2
-
The temperature dependence of OH conductivities for both the parent material and the three solids
FJU-66∙nXOH were measured under various related humanity (RH) conditions at the temperatures
ranging from 293 to 358 K by using AC impedance spectroscopy (Figs. 3c and S17). In comparison
-8
-1
with a relatively low ionic conductivity of 2.1 × 10 S∙cm observed in the parent material FJU-66
−
2
-1
at 293 K and 95% RH, FJU-66∙[EVIm]OH shows a high value of 5.2 × 10 S∙cm at the same
condition, indicating that the loaded [EVIm]OH molecules in cages are responsible for such a
significantly increasing conductivity. Upon heating but keeping 95% RH, the conductivity for
-2
-1
FJU-66∙[EVIm]OH gradually increases to reach a maximum of 9.1 × 10 S∙cm at 358 K. As
-
shown in the inset of Fig. 3c, the OH conductivity of FJU-66∙[EVIm]OH is in a great degree
dependent on RH of environment. The conductivity at 50% RH is far lower than that at 95% RH
over the temperature from 293 to 358 K. Therefore, we can suppose that the high conductivity for
−
FJU-66∙[EVIm]OH is predominantly controlled by the diffusion of OH ions along the water
1
3
pathway within the pore of the framework. In addition, the differences of ionic conductivity for
FJU-66∙[EVIm]OH at the same temperature and RH between the heating and cooling processes
can be neglected, demonstrating the good durability, an essential for practical applications of the
material. The activation energy E
a
of FJU-66∙[EVIm]OH is 0.11 eV at 95% RH (Fig. 3d). The
value of FJU-66∙[EVIm]OH in the
-
observation of high OH conductivity and the lowest E
a
temperature range from 293 to 358 K suggest that it belongs to fast-ion conductors.

Journal of Materials Chemistry A
Page 12 of 19
Table 1 Comparison of hydroxide ionic conductivity for FJU-66∙[EVIm]OH, FJU-66∙3KOH and
View Article Online
DOI: 10.1039/C6TA11242K
FJU-66∙0.9NBu
4
OH with the MOF-based HEMs under the similar condition.
-
-
1
OH
Hydroxide conductivity (S cm )
Ea
eV)
HEMs
concentration
mmol g )
Ref.
(
-
1
(
303 K
.057
358 K
0
0.091
This
work
This
FJU-66∙[EVIm]OH
FJU-66∙3KOH
0.34
0.11
0.42
0.21
(
95% RH)
(95% RH)
0
.006
0.059
1.20
1.50
-
(
95% RH)
(95% RH)
work
0
.012
KOH@[Ni
8
(OH)
4
(H
2 2
O) (BDP-OH)
6
]
-
13
(
100% RH)
0
.009
IL@ZIF-8/IL/PVA
-
0.55 14a
0.38 14b
(
80% RH)
−
5
−4
4
(
2
(
.1× 10
98% RH)
.5 × 10
95% RH)
.3 × 10
99.9% RH)
.99 × 10
95% RH)
2.6 × 10
(98% RH)
4.89 × 10
(95% RH)
2.1 × 10
(99.9% RH)
PVA–ZIF8i-48
-
−
5
−5
[
Ni
2
(μ-pymca)
MIL-100-Cr-OH
FJU-66∙0.9NBu OH
NBu OH@ZIF-8
3
]OH∙nH
2
O
1.68
-
0.19
0.38
0.48
0.70
10
12
−
6
−5
7
(
-7
-6
1
(
2
(
3.44 × 10
This
work
4
0.41
0.21
(95% RH)
-
−
8
.3 × 10
99% RH)
4
11
A comparison of the conductivity of the three solid FJU-66∙nXOH with those of the
MOF-based HEMs under similar conditions is shown in Fig. 3d and Table 1. Among the three solid,
-
the conductivity of FJU-66∙[EVIm]OH with the lowest OH ionic concentration is higher than
o
those of the other two over the temperature range from 20 to 85 C, although the two have the
-
higher, even 3.5 times OH ionic concentration. The conductivity of FJU-66∙[EVIm]OH at 293 K
5
is about 10- and 10 -fold the values for FJU-66∙3KOH and FJU-66∙0.9NBu
4
OH, respectively,
-
indicating that [EVIm]OH may be a better OH carrier than KOH and NBu
4
OH for the ionic
conductors. The conductivity of FJU-66∙[EVIm]OH at 358 K and 95% RH is about 7.6 times the
highest value reported among all MOF-based HEMs (Table 1),10-14 and even higher than the values
for most of proton conductors.4b,7 Its activation energy E
is the lowest among the MOFs-based
a
HEMs (Table 1),10-14 indicating the OH anions in FJU-66∙[EVIm]OH could be tranported more
-
easily within the channels.

Page 13 of 19
Journal of Materials Chemistry A
3
.4 Ionic transport mechanism
View Article Online
DOI: 10.1039/C6TA11242K
+
Fig. 4 Multiple point interactions of the [EVIm] cation with the framework: hydrogen bonding d
+
+
[
O1(-imide)···C39 ([EVIm] )] = 3.127 Å and lone pair- interaction d [N ([EVIm] ···Center
-
+
of-naphthyl)] = 2.838 Å. The nearest distance between OH anion and the [EVIm] cation, d [Center
of- imidazolium···O7 (anion)] = 10.915 Å. Color code: Cu, bright green; C, gray; O, red; N, blue.
Hydrogen atoms are omitted for clarity.
The significantly improved OH conductivity of FJU-66∙[EVIm]OH but with the lowest OH-
-
-
ion concentration motivated us to investigate possible mechanism for OH ionic transport. The
common methods to determine the carriers, for examples imidazole,8 histamine,36 nonvolatile
d
c
11
acids8 and NBu OH, in the post-synthetically treated MOFs include NMR, synchrotron XRPD,
4
DSC, SEM, and TGA. FJU-66∙[EVIm]OH is the first example that determines the encapsulated
ionic carriers in the post-synthetically treated MOFs through single crystal X-ray diffraction
technique (Figs. 4 and S18). It has two striking features: (1) There is one crystallographically
+
independent [EVIm]OH molecule within the ellipsoidal cages. The planar cation [EVIm] is located
at the terminal of semi-major axis of the prolate-ellipsoid cage, and trapped by two naphthalimide
+
groups via O1(-imide)···H-C39 ([EVIm] ) interactions (3.127 Å) and lone pair- interaction (d
+
-
[
N([EVIm] ···Center of-naphthyl)] = 2.838 Å). (2) Each of OH (O7) is free and resides 3.083 Å
away from the center of nanocage (Wyckoff site 2a). Neither hydrogen bonding interactions nor van
der Waals interactions is observed between OH anions and the framework backbone or among OH-
anions. Cu2 is the nearest atom on the framework to O7 with thedistance of 4.179 Å, while the
distance between the neighboring OH anions is 6.174 Å. The nearest distance between the OH-
anion and the center of imidazolium cations in FJU-66∙[EVIm]OH is 10.915 Å, far longer than the
-
-

Journal of Materials Chemistry A
Page 14 of 19
normal cation-anion distance of 2.820 Å in ionic crystal (NaCl).37 According to the Coulomb's law,
View Article Online
DOI: 10.1039/C6TA11242K
-
+
the OH anions are almost free from the restraint of electrostatic force with [EVIm] cations of
loaded bases, as a result of the multipoint supramolecular interactions38 between the [EVIm]+
cations and the framework backbone. We did not achieve the location of KOH and NBu OH in
4
FJU-66 even after several attempts. In FJU-66∙3KOH, potassium ions may generally stay hydrated
O) distances ranging from 2.53 to 3.40 Å.39 Unlike the flat [EVIm] cations situated
+
with K-O (H
2
at the terminal of semi-major axis of the nanocage, the large size of the multi-hydrated potassium
+
-
ions press K ion away from the MOF backbone but close to the center OH ions, which may help to
+
-
keep the electrostatic interactions between K and OH ions in FJU-66∙3KOH. Thus, a small
conductivity of FJU-66∙3KOH, only one tenth of the value for FJU-66∙[EVIm]OH, is observed.
Besides the large size, [NBu
4
]+ cations even have no possible binding site to form the hydrogen
bonds with the framework. Thus, the electrostatic interactions between NBu +
and OH ions may be
-
4
preserved to the greatest content in FJU-66∙0.9NBu OH after the alkaline loading. Such a
4
-
maximum preservation of the cation-OH electrostatic interactions leads to the smallest conductivity
of FJU-66∙0.9NBu
OH, only ten per million of the value for FJU-66∙[EVIm]OH. PLATON28
4
analysis shows that [EVIm]OH loaded FJU-66 still remains 28% voids of the crystal volume,
which can be described as one-dimensional channels (4.8 Å in diameter) along the a and b
directions, centered at (-0.030, -0.014, 0.000) and 2.60 Å away from the hydroxide O7 atom (Fig.
S18b). These cross-overlapped channels provide a natural and open pathway to adopt water
-
molecules from outside environment, finally to realize the OH ion transfer among water molecules
via hydrogen bonding interactions (Fig. S18c). In a word, in FJU-66∙[EVIm]OH, the forming
+
multipoint supramolecular interactions between the planar [EVIm] cations and the robust
-
framework can consequently free the OH anions from the restraints of the dissociation and of the
-
electrostatic force with the alkaline cations to remarkably improve the OH conductivity.
4
. Conclusion
In summary, free OH- anions-containing metal-organic framework (FOMOF)
FJU-66∙[EVIm]OH was firstly achieved through rationally tuning host-guest interactions and
-
developed for the HEMs with high OH conductivity. FJU-66 shows a ultrastable immunity to
attacks of heat up to 803 K and of acid-base aqueous solutions from 1 mM HCl to 10 M NaOH,
superior to the other extended [Cu
3
3
(Pz) ] polymers and most of MOFs, which comes from the
intertrimeric cuprophicity within its [Cu
6
(Pz)
6
] nodes. FJU-66∙[EVIm]OH with a low OH-

Page 15 of 19
Journal of Materials Chemistry A
-1
-
-
concentration of 0.34 mmol g is a free OH anions- containing MOF exhibiting high OH
View Article Online
DOI: 10.1039/C6TA11242K
of 0.11 eV. The conductivity is
-1
conductivity close to 0.1 S∙cm with the lowest activation energy E
a
6
5
the highest value among all the MOF-based HEMs, about 10 , 10 and 10 fold the values of FJU-66,
FJU-66∙0.9NBu OH and FJU-66∙3KOH, respectively. The high conductivity in
FJU-66∙[EVIm]OH can be contributed to multipoint supramolecular interactions between the
4
+
+
-
robust framwork and planar [EVIm] cations, which pulls [EVIm] -OH pairs far apart and frees
-
-
-
OH anions. The restraint-free OH anions sigfinicantly improve the OH conductivity. We believe
-
that our findings will encourage further work in the FOMOFs with high OH conductivity for the
HEMFCs by adjusting the host-guest interaction between the alkaline cations and the framework in
the near future. In this regard, the integration of the studied material in membranes is under current
investigation.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China
(21273033, 21673039 and 21573042) and Fujian Science and Technology Department (2014J06003
and 2014H6007). S. X. gratefully acknowledges the support of the Recruitment Program of Global
Young Experts.
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DOI: 10.1039/C6TA11242K
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Free OH anions-containing metal-organic framework (FOMOF) FJU-66·[EVIm]OH was
-
developed for hydroxide-anion-exchange material with high OH conductivity close to 0.1
S/cm.