Guest-tuned proton conductivity of a porphyrinylphosphonate-based hydrogen-bonded organic framework

Article abstract of DOI:10.1039/d0ta07207a

Hydrogen-bonded organic frameworks (HOFs), similar to their MOF analogues, exhibit great potential in proton conduction applications. Herein, a porous HOF namely [(NiH₄TPPP)(Me₂NH₂)₄(DMF)(H₂O)₄] (UPC-H5) was synthesized from phosphonate-based porphyrinato nickel (NiH₈TPPP), and its proton conductivity is regulated through a two-step guest change. Firstly, immersing UPC-H5 in CH₂Cl₂to exchange lattice solvent molecules for 24 h followed by heating under vacuum afforded the lattice solvent molecule-free HOF [(NiH₄TPPP)(Me₂NH₂)₄] (UPC-H5a) with the pristine framework still retained. Secondly, exposing UPC-H5a to vapors of 25% aqueous ammonia for 24 h at room temperature gave a new derivative UPC-H5a@NH₃·H₂O with the molecular formula [(NiH₄TPPP)(Me₂NH₂)₂(NH₄)₂(H₂O)₄] according to elemental and thermal analyses. At 30 °C and 95% R.H., the proton conductivity of UPC-H5, UPC-H5a, and UPC-H5a@NH₃·H₂O amounts to 5.59 × 10^?4, 7.00 × 10^?3, and 1.47 × 10^?2S cm^?1, respectively, which increases to 1.85 × 10^?3, 3.42 × 10^?2, and 1.59 × 10^?1S cm^?1at 80 °C and 99% R.H., clearly showing the effect of guest regulation on the proton conductivity of the HOF-based materials. In addition, this result is also helpful towards understanding the important role of guests in the formation of their proton conduction pathways.

Full text of DOI:10.1039/d0ta07207a

Journal of
Materials Chemistry A
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Guest-tuned proton conductivity of
a porphyrinylphosphonate-based hydrogen-
bonded organic framework†
Cite this: J. Mater. Chem. A, 2021, 9,
683
2
Received 23rd July 2020
Accepted 11th January 2021
Yijie Wang, Jianbo Yin, Di Liu, Chengqi Gao, Zixi Kang, Rongming Wang, *ab
a
a
a
a
b
ab
ac
Daofeng Sun
*
and Jianzhuang Jiang
*
DOI: 10.1039/d0ta07207a
rsc.li/materials-a
Hydrogen-bonded organic frameworks (HOFs), similar to their MOF energy sources such as nuclear energy, solar energy, geothermal
2
analogues, exhibit great potential in proton conduction applications. energy, hydropower, and hydrogen energy. Among them
4 2 2 4 2 4
Herein, a porous HOF namely [(NiH TPPP)(Me NH ) (DMF)(H O) ]
hydrogen seems to be an ideal candidate to resolve the corre-
(
UPC-H5) was synthesized from phosphonate-based porphyrinato sponding problems because of its abundance, high energy
3
nickel (NiH
8
TPPP), and its proton conductivity is regulated through density, and environmentally benign combustion product. As
Cl to a consequence, fuel cell technologies in particular proton
exchange lattice solvent molecules for 24 h followed by heating under exchange membrane fuel cells (PEMFCs) that are able to utilize
vacuum afforded the lattice solvent molecule-free HOF [(NiH hydrogen as a raw material have been recognized as a crucial
TPPP)(Me NH ] (UPC-H5a) with the pristine framework still retained. solution to alternative energy sources due to their high effi-
Secondly, exposing UPC-H5a to vapors of 25% aqueous ammonia for ciency and low emission. Among the important components
a two-step guest change. Firstly, immersing UPC-H5 in CH
2
2
4
-
2
2 4
)
4
2
4 h at room temperature gave a new derivative UPC-H5a@NH
3 2
$H O
constituting PEMFCs, the proton exchange membrane (PEM)
with the molecular formula [(NiH
4
TPPP)(Me
2
NH
ꢀ
2
)
2
(NH
4
)
2
(H O)
2
4
]
plays a key role in the cell’s performance enhancement.
according to elemental and thermal analyses. At 30 C and 95% R.H., Therefore, developing efficient and stable proton conducting
the proton conductivity of UPC-H5, UPC-H5a, and UPC-H5a@NH
3
-
materials suitable for PEMs has been one of the important
research topics in this eld.
O amounts to 5.59 ꢁ 10ꢂ4, 7.00 ꢁ 10ꢂ3, and 1.47 ꢁ 10 S cm
ꢂ2
ꢂ1
,
5
$
H
2
ꢂ3
ꢂ2
respectively, which increases to 1.85 ꢁ 10 , 3.42 ꢁ 10 , and 1.59 ꢁ
Naon and Naon-like polymers showing excellent proton
ꢂ1
ꢂ1
ꢀ
1
0
S cm at 80 C and 99% R.H., clearly showing the effect of guest conducting properties have been employed as proton exchange
regulation on the proton conductivity of the HOF-based materials. In membranes for PEMFCs. However, some disadvantages of these
addition, this result is also helpful towards understanding the impor- organic polymers including fabrication difficulty, narrow
tant role of guests in the formation of their proton conduction working conditions, and high cost restrict their further wide
pathways.
range of applications. In particular, their amorphous nature
prevents researchers from clearly understanding the structure-
property relationship as well as the proton conduction mecha-
nism. This in turn limits further optimization over their proton
With the increasing consumption of fossil energy, humankind
is facing not only the energy depletion situation but also envi-
ronmental pollution. Over the past few decades, great efforts
6
conduction pathways through a rational manner. In the past
1
two decades, rapidly developing crystalline solid materials,
have been devoted to developing new clean and renewable
7
metal organic frameworks/coordination polymers (MOFs/CPs),
8
and covalent organic frameworks (COFs) have shown great
a
application potential in proton conduction. Their highly
College of Science, China University of Petroleum (East China), Qingdao, Shandong
2
66580, China. E-mail: rmwang@upc.edu.cn; dfsun@upc.edu.cn
ordered and well-dened porous structures are very helpful in
b
State Key Laboratory of Heavy Oil Processing, School of Materials Science and investigating and understanding the proton conduction mech-
Engineering, China University of Petroleum (East China), Qingdao, Shandong
9
anism towards further improving their proton conductivity.
2
c
66580, People's Republic of China
Thus far, three main strategies have been revealed and
employed to improve the proton conductivity of MOFs and
COFs on the basis of chemical modication over either
Beijing Key Laboratory for Science and Application of Functional Molecular and
Crystalline Materials, Department of Chemistry, University of Science and
Technology Beijing, Beijing 100083, China. E-mail: jianzhuang@ustb.edu.cn
†
Electronic supplementary information (ESI) available: Synthetic details, FTIR precursors or frameworks: (1) introducing acidic groups such as
images, PXRD patterns, TGA results, etc. CCDC 1976307. For ESI and –COOH, –SO
3 2
H, and –PO H into the framework to improve the
3
crystallographic data in CIF or other electronic format see DOI:
9e,10
proton density of materials;
(2) introducing a conducting
1
0.1039/d0ta07207a
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3 4
medium such as water, N-heterocyclic compounds, and H PO
11
to improve the proton carrier density in the pores or channels;
3) tuning the hydrogen bonded networks to optimize the
(
12
proton transfer pathways. It is however worth noting that thus
far most excellent MOF- and COF-based proton conductors with
high proton conductivity employ water as the proton-
conducting medium depending on the degenerate conjugate
+
+
ꢂ
acid–base system (H
2
O + H
3 3 2 2
O # H O + H O or H O + OH
ꢂ
9f,13
2
#OH + H O).
In good contrast, ammonia-mediated proton
conducting MOF/COF materials still remain rarely reported,
despite the recent demonstration of ammonia as an excellent
proton-conducting medium depending on conjugate acid–base
+
+
proton transfer (NH
4
+ NH
3
#NH
3
+ NH
4
) reported by Kita-
1
4
gawa and co-authors.
As the counterparts of MOFs and COFs, hydrogen bonded
organic frameworks (HOFs) that are constructed mainly by NiH TPPP showing 1D channels, Me NH2 , DMF and H O are
Fig. 1 Crystal structure of UPC-H5. (a) 3D anionic framework formed
4ꢂ
+
4
2
2
omitted for clarity; (b) the stacking of 2D layers showing the distances
depending on hydrogen-bond interactions have also shown
of p–p interactions; (c) the local enlarged 2D layers; (d) four types of
great application potential in diverse elds including gas
4ꢂ
hydrogen bonds between NiH
4
TPPP
.
1
5
16
17
storage and separation, catalysis, molecular recognition,
18
and chemical sensing. As can be expected, efforts have also
been made towards exploring the proton conducting properties
of this kind of material. In this work, we report the fabrication
and proton conduction properties of a porous porphyrin-based
HOF, [(NiH TPPP)(Me NH ) (DMF)(H O) ] (UPC-H5, NiH -
4 2 2 4 2 4 8
TPPP: 5,10,15,20-tetrakistetra(4-phosphonophenyl) porphyrin
+
19
the 2D layers, Me
2
NH
2
moieties and lattice water molecules
˚
under the assistance of p–p interactions (3.72 A), Fig. 1b. When
viewed along the a axis, UPC-H5 possesses three different sizes
of 1D channels. The biggest sized channel scales 11.0827 A˚
˚
(
benzenyl C–C) ꢁ 11.3948 A (phosphonate O–O), in which the
nickel(II), Me NH: dimethylamine, DMF: N,N-dimethylforma-
2
lattice DMF molecules exist. The medium- and smallest-sized
mide). The proton conductivity of the HOF-based material
could be further regulated through a two-step guest-tuned
strategy, Scheme S1.† First, removing the lattice DMF mole-
cules that cannot form hydrogen bond networks in the channels
of the framework to accommodate more guest water molecules
˚
˚
channels are of the scale 9.8735 A (benzenyl C–C) ꢁ 10.3008 A
˚
(
˚
phosphonate O–O) and 5.8385 A (phosphonate O–O) ꢁ 7.9652
+
2
A (phosphonate O–O), in which Me
2
NH
and lattice water
molecules reside, respectively, Fig. 1a. The total solvent-
accessible volume of UPC-H5 is calculated to be 33.2%
to form more smoother proton-conducting pathways; second,
20
+
according to the SQUEEZE module of the PLATON routine.
Aer immersing UPC-H5 in CH Cl to exchange lattice solvent
molecules for 24 h followed by heating under vacuum, [(NiH
TPPP)(Me NH ] (UPC-H5a) was obtained. UPC-H5a was then
exposed to vapors of 25% aqueous ammonia for 24 h at room
temperature, giving UPC-H5a@NH $H O.
As shown in Fig. S1,† the powder X-ray diffraction (PXRD)
pattern of the as-synthesized UPC-H5 is in good agreement with
the one simulated from the single crystal diffraction data,
revealing the high phase purity of the freshly prepared samples.
This is further supported by the elemental analysis result as
listed in the ESI†. The PXRD pattern of UPC-H5a does not show
signicant changes in comparison with that for UPC-H5, except
for the slight shi of the peak at 5.7 to 6.0 , indicating the
slight shrink of the fundamental HOF framework aer losing
the lattice DMF and water molecules in UPC-H5. This, however,
is not the case for UPC-H5a@NH $H O. A big change in the
PXRD pattern of UPC-H5a@NH $H O, Fig. S1,† reveals the
signicant transformation occurring in the framework struc-
partially replacing the Me
2
NH
2
moieties with a larger volume
+
2
2
and higher pK value (10.75) with NH with a smaller volume
a
4
4
-
and lower pKa value (9.26) to increase the pore volume and
hydrophilicity of channels also to form more and smoother
proton-conducting pathways with guest water molecules. Such
a two-step guest regulation provides about two orders of
magnitude increase in the proton conductivity.
2
2 4
)
3
2
8
UPC-H5 was prepared by the assembly of NiH TPPP in the
mixed solution of acetone, DMF, and H O. According to single-
2
crystal X-ray diffraction structural analysis, UPC-H5 crystallizes
ꢀ
in a triclinic P1 space group, Table S1.† The asymmetric unit
4
ꢂ
+
contains one NiH TPPP , four Me NH , one lattice DMF, and
4
2
2
4
four lattice water molecules. As shown in Fig. 1, each NiH -
ꢀ
ꢀ
4
ꢂ
TPPP provides four H atoms from four –PO
accepts four H atoms of four –PO H groups to form eight
O–H/O hydrogen bonds with six surrounding NiH
building blocks, leading to the formation of a two-dimensional
2D) hydrogen-bonded network containing two layers of NiH -
3
H groups and
3
4
ꢂ
4
TPPP
3
2
3
2
(
4
4
ꢂ
˚
TPPP with p–p interactions (3.67 A), Fig. 1b and c. Mean-
while, the eight hydrogen bonds include four types of O3–H3/
O11, O5–H5/O7, O8–H8/O4, and O10–H10/O9 with the
ture. However, the framework remains uncollapsed as shown in
its PXRD analysis, and the crystal shape of UPC-H5a@NH $H O
3 2
has no change compared with that of UPC-H5. In addition, the
in situ PXRD analysis discloses that the structures of UPC-H5,
UPC-H5a, and UPC-H5a@NH
20 C, Fig. S2.† Correspondingly, TG-MS analyses indicate that
˚
O/O distances of 2.54, 2.56, 2.47 and 2.60 A, respectively and
ꢀ
ꢀ
ꢀ
ꢀ
the bond angles of 170 , 166 , 175 and 166 , Table S2† and
Fig. 1d. Then a stable 3D structure is further formed depending
on the hydrogen-bonding and electrostatic interactions among
3
$H
2
O show good stability up to
ꢀ
1
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ꢀ
UPC-H5 loses 5.8% weight from room temperature to 120 C, molecules at 94% RH on the basis of the initial 4 lattice water
which is attributed to the loss of four lattice water molecules molecules, and UPC-H5a can adsorb 5 more water molecules
+
(
calculated value 5.5%), and then Me NH ions begin to aer removing DMF molecules. Compared with UPC-H5a, UPC-
2
2
decompose along with the loss of lattice DMF molecules. UPC- H5a@NH $H O can take up 15 more water molecules because
3
2
ꢀ
+
+
H5a shows almost no weight loss below 125 C because of the part of Me
2
2 4
NH is replaced by NH , resulting in a bigger pore
lack of lattice solvents. The structure, however, slowly gets volume and stronger hydrophilicity.
collapsed aer that along with the decomposition and loss of The proton conduction performance of UPC-H5, UPC-H5a,
O loses 6.7% weight and UPC-H5a@NH $H O was explored by the impedance
+
Me
2
NH
2
. In contrast, UPC-H5a@NH
3
$H
2
3
2
ꢀ
from room temperature to 95 C, corresponding to the loss of technique using their pressed pellets under different relative
about four water molecules (calculated value 6.1%), and then humidities (R.H.) (from 40 to 99%). Towards revealing the
+
+
NH4 and Me NH2 ions begin to decompose, Fig. S3.† In guest-tuned effect, the proton conductivity of these materials
2
combination with the elemental analysis result listed in the was studied under exactly the same testing conditions. At rst,
ESI,† the molecular formula of UPC-H5a@NH
lated to be [(NiH TPPP)(Me NH (NH (H O)
about two Me NH
3
$H
2
O is specu- the time-dependent proton conductivity was calculated
4
2
2
)
2
4
)
2
2
4
]. That is to say, (Fig. S9†), showing that the equilibrium time of UPC-H5, UPC-
moieties in UPC-H5a are replaced by two H5a and UPC-H5a@NH
+
2
2
3
$H
2
O is almost 170, 120, and 100
+
ꢀ
NH
4
moieties in every structural unit of UPC-H5a@NH
3
$H
2
O, minutes at 80 C and 40% R.H., and 100, 80, and 70 minutes at
ꢀ
and four water molecules re-enter the channels during the 80 C and 99% R.H, respectively. As shown in Fig. S10 and Table
ammonia-exchange process of UPC-H5a.
S3,† UPC-H5 shows only a low proton conductivity of 1.04 ꢁ
ꢂ6 ꢂ1
ꢀ
To further clarify the structures of UPC-H5, UPC-H5a, and 10 S cm at 25 C and under 40% R.H., indicating its rela-
UPC-H5a@NH $H O, their infrared spectra were recorded and tively weak active proton conducting nature under low
3
2
are shown in Fig. S4.† Compared with NiH
8
TPPP, UPC-H5 temperature and humidity conditions, based on the hydrogen-
+
exhibits a C]O characteristic vibration peak of lattice DMF bonding networks formed by –PO
3
H, Me
2
ꢀ
NH
2
, and H
at along the a and b axes, Fig. S11.† Also at 25 C and under 40%
2
peak at R.H., the conductivity of UPC-H5a and UPC-H5a@NH $H O
2
O
ꢂ1
+
9b–f
molecules at 1661 cm , a C–H bending vibration of Me
2
NH
2
ꢂ1
ꢂ1
+
1
1
469 cm , and a C–N stretching vibration of Me
2
NH
2
3
ꢂ6
ꢂ5
ꢂ1
050 cm , well consistent with the single crystal X-ray analysis was found to be 8.23 ꢁ 10 and 1.68 ꢁ 10 S cm , respec-
ꢂ1
result. However, the peak at 1661 cm disappeared in the IR tively, indicating the slight inuence of eliminating the lattice
spectrum of UPC-H5a, indicating the complete removal of the DMF molecules in UPC-H5 and ammonia-exchange in UPC-
lattice DMF. As can be found, a new peak belonging to the N–H H5a@NH $H O on their proton conduction performance under
3
2
+
ꢂ1
bending vibration of NH
spectrum of UPC-H5a@NH
replacement of Me
4
appears at 1634 cm in the IR low humidity, because UPC-H5a and UPC-H5a@NH
3
$H
2
O can
3
$H
by NH
2
4
O, demonstrating the partial take up ca. 1.3 and 2.9 more water molecules than UPC-H5
.
+
+
2
NH
2
under these conditions, respectively. With increasing the
To assess the permanent porosity of UPC-H5a and UPC- humidity to 99% R.H., the proton conductivity of UPC-H5, UPC-
ꢀ
ꢂ1
ꢂ4
H5a@NH
3
$H
2
O, both samples aer CH
2
Cl
2
exchange were H5a, and UPC-H5a@NH
3
$H
2
ꢂ2
O at 25 C increases to 4.28 ꢁ 10
,
ꢀ
ꢂ3
activated to eliminate lattice solvents by heating at 80 C for 6.24 ꢁ 10 , and 1.39 ꢁ 10 S cm , respectively. This is also
2
4 h under vacuum. Then gas uptake tests were conducted at consistent with the results of water adsorption experiments that
different temperatures. Because of the hydrophilic characteris- they can adsorb more water molecules under higher humidity,
tics of the channels with a large number of –PO H and –NH showing the important role of water in the construction of
groups, both materials absorb only a small amount of N under proton transfer pathways.
a high pressure even at 77 K, Fig. S5.† However, they can adsorb With the increasing of temperature, the proton conductivity
3
2
3
ꢂ1
ꢂ6
ꢂ1
ꢀ
99 and 131 cm g of CO
2
at 196 K, Fig. S6.† The total pore of UPC-H5 increases to 9.28 ꢁ 10 S cm at 40 C and 40%
3 ꢂ5 ꢂ1
ꢂ1
ꢀ
volume is then calculated to be 0.18 and 0.23 cm g , respec- RH, and 4.68 ꢁ 10 S cm at 80 C and 40% RH, Fig. 2, S12
tively, with the Brunauer–Emmett–Teller surface areas of 237.5 and Table S4.† Correspondingly, UPC-H5a exhibits a proton
2
ꢂ1
ꢂ5
ꢂ1
ꢀ
and 371.1 m g , according to the CO isotherm (P/P ¼ 0.98), conductivity of 5.22 ꢁ 10 S cm at 80 C and 40% RH.
2
0
Fig. S7.† These results indicate that UPC-H5a@NH $H O keeps Interestingly, ammonia-exchange shows an obvious inuence
3
2
the porosity aer ammonia-exchange, but actually possesses on enhancing the proton conductivity, increasing the proton
ꢂ3
ꢂ1
larger porosity and specic surface area than UPC-H5a despite conductivity of UPC-H5a@NH
3
$H
2
O to 1.43 ꢁ 10 S cm at
ꢀ
the signicant structural change. In addition, the water 80 C and under 40% R.H., an almost two orders of magnitude
adsorption isotherms of three HOFs were recorded at 298 K increase. As shown in Fig. 2, along with increasing the humidity
aer removing lattice water and free water molecules that may to 99% R.H., the proton conductivity of UPC-H5, UPC-H5a, and
ꢀ
be adsorbed in air. As shown in Fig. S8, UPC-H5, UPC-H5a and UPC-H5a@NH $H O at 80 C gets increased in a rapid manner
3
2
ꢂ3
ꢂ2
ꢂ1
ꢂ1
UPC-H5a@NH $H O can take up ca. 3.5, 4.8, and 7.7 water to 1.85 ꢁ 10 , 3.42 ꢁ 10 , and 1.59 ꢁ 10 S cm , respec-
3
2
molecules at 40% RH, and ca. 4.8, 8.7, and 16.4 water molecules tively, further disclosing the important role of water in the
at 85% RH, and rapidly increase to ca. 9.3, 14.3 and 29.4 water construction of proton transfer pathways. As can be seen, at
ꢀ
molecules per formula unit at 94% RH, respectively. This indi- 80 C and under 99% R.H., the conductivity of UPC-H5a
cates that both the removal of DMF and ammonia-exchange amounts to about 18 times higher than that for UPC-H5, sug-
have bigger inuence on the water adsorption capacity under gesting the increased inuence of eliminating the lattice DMF
higher humidity, and means that UPC-H5 can take up 5 water molecules of UPC-H5 on the proton conduction performance
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Fig.
H5a@NH
3
Nyquist plots of UPC-H5 (a), UPC-H5a (b) and UPC-
Fig.
H5a@NH
2
Nyquist plots of UPC-H5 (a), UPC-H5a (b) and UPC-
3 2
$H O (c) under 95% R.H. at different temperatures; and
ꢀ
3 2
$H O (c) at 80 C under different R.H values; and the
Arrhenius plots of UPC-H5, UPC-H5a and UPC-H5a@NH
3 2
$H O at
humidity-dependent proton conductivity of UPC-H5, UPC-H5a and
UPC-H5a@NH $H O (d).
ꢀ
80 C and under 95% R.H. (d).
3
2
performance even at ambient temperature under high
humidity. With increasing the temperature, the conducting
under high humidity. Similar to the low humidity situation, the
proton conductivity of UPC-H5a@NH $H O still remains
3
2
ꢂ3
ꢂ2
values increase gradually and reach 1.71 ꢁ 10 , 1.82 ꢁ 10
,
almost two orders of magnitude higher than that for UPC-H5
under 99% R.H., conrming the signicant role of ammonia-
exchange. Meanwhile, six cyclic tests were carried out to
ꢂ2
ꢂ1
ꢀ
and 8.68 ꢁ 10 S cm at 80 C and 95% R.H., respectively.
Obviously, the effect of temperature on proton conductivity is
much smaller than that of humidity as detailed above.
measure the proton conductivities of UPC-H5, UPC-H5a, and
ꢀ
According to the Arrhenius equation, activation energies (E
.23, 0.20, and 0.40 eV for UPC-H5, UPC-H5a, and UPC-
H5a@NH $H O were obtained by tting the temperature-
a
) of
UPC-H5a@NH
3
$H
2
O between 40% and 99% R.H. at 80 C,
0
showing a negligible change in the conductivity values, indi-
cating their good structure and property stability under the
testing conditions, Fig. S13.† In order to further verify the
3
2
dependent conductivity data, suggesting their possible Grot-
thuss transport mechanism. In addition, we also carried out ve
contrasting tests using ve different pellets for measuring the
temperature-dependent proton conductivity of each sample at
structural stability of UPC-H5a@NH
pellet were placed at 80 C and 99% R.H. for one week. The EDS
3
$H
2
O, its crystals and
ꢀ
3 2
elemental mapping images of the UPC-H5a@NH $H O crystals
95% R.H. As exhibited in Fig. S16,† the conducting values
before and aer placing do not show an obvious change,
ꢀ
remain consistent in a good manner for different pellets, indi-
cating their good proton conductivity reproducibility. Besides,
the PXRD patterns before and aer the cyclic temperature- and
humidity-dependent impedance tests for UPC-H5, UPC-H5a
and UPC-H5a@NH $H O do not show an obvious change,
3 2
Fig. S17,† indicating their structural stability.
demonstrating their long-term structural stability at 80 C and
99% R.H., Fig. S14.† Moreover, the proton conductivity of the
UPC-H5a@NH $H O pellet was measured every day for one
3
2
week, and the conducting values remained well consistent,
indicating that it also has good long-term property stability
under these conditions, Fig. S15.† At the end of this section, it is
worth noting that these results clearly show the guest-tuned
effect on the proton conductivity of HOF-based materials for
the rst time. Meanwhile, the proton conductivity of UPC-
H5a@NH $H O ranks as one of the highest values among the
3 2
excellent porous proton-conducting materials reported to date,
Table S5.†
As can be seen, despite the existence of a high density of
+
–
PO H, Me NH , and H O in the channels, the proton
3
2
2
2
conductivity of UPC-H5 is still poor under low humidity, sug-
gesting that the proton conduction of UPC-H5 did not mainly
originate from its intrinsic hydrogen bond networks but from
the free water molecules which enter the channels to reorganize
the proton transfer pathways. However, the existence of lattice
DMF molecules in UPC-H5 greatly hinders the free water
molecules from entering the channels, Fig. 4a. As a result of
removing the lattice molecules, free water molecules can enter
the empty type III channels in UPC-H5a to form more proton
conduction pathways, in addition to entering the types I and II
channels just the same as in UPC-H5, Fig. 4b, S11, and S18,†
To further understand the relationship between proton
conductivity and temperature, the temperature-dependent
proton conductivity of UPC-H5, UPC-H5a, and UPC-
H5a@NH
3
$H
2
O was also tested under 95% R. H. in a varying
ꢀ
temperature range from 30 to 80 C. As displayed in Fig. 3 and
Table S6,† the conductivity of UPC-H5, UPC-H5a, and UPC-
ꢀ
ꢂ4
H5a@NH $H O at 30 C and 95% R.H. is 5.59 ꢁ 10 , 7.00 ꢁ
3
2
ꢂ3
ꢂ2
ꢂ1
inducing an improved proton conducting performance.
10
, and 1.47 ꢁ 10 S cm , respectively, verifying the effec-
+
However, the methyl groups of Me
2
NH
2
in UPC-H5a still block
tiveness of regulating guests on the proton conduction
the formation of better hydrogen bond networks in multiple
2686 | J. Mater. Chem. A, 2021, 9, 2683–2688
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Communication
Journal of Materials Chemistry A
+
smooth proton transfer pathways formed by NH
4
and more
11a
water molecules, leading to high proton conductivity.
In summary, a porous HOF with a high density of –PO H,
3
+
2
Me NH and H O and a characteristic tunable structure was
2
2
constructed based on a phosphonate-based porphyrin nickel
building block. This material exhibits guest-responsive proton
conductivity. In particular, the ammonia-exchanged material
shows high proton conductivity in a wide temperature range, up
ꢂ1
ꢂ1
ꢀ
to 1.59 ꢁ 10 S cm at 80 C and 99% R.H., representing one
of the highest values among the excellent porous proton-
conducting materials reported thus far. The result demon-
strates that guest regulation is an effective method to improve
the proton conductivity of HOF-based materials. This is surely
helpful in designing and developing new HOF-based proton
conductors with excellent performance.
Fig. 4 (a and b) The structures showing the guests viewed along the
a axis for UPC-H5 and UPC-H5a, respectively; (c and d) enlarged local
+
structure showing Me
2
NH
2
viewed along the a and b axes, respec- Conflicts of interest
tively. Note: the structure of UPC-H5a is illustrated using the crystal
data of UPC-H5 owing to the slight change of the structure as indi- There are no conicts to declare.
cated by PXRD tests.
Acknowledgements
directions, Fig. 4b–d. Therefore, partial replacement of the This work was nancially supported by the National Natural
+
2
+
Me
2
NH
groups by NH
4
with tetrahedral N–H conguration in Science Foundation of China (No. 21771193, 21631003, and
UPC-H5a@NH
3
$H O further promotes the formation of smooth 21875285), Taishan Scholar Foundation (No. ts201511019), Key
2
proton transfer pathways, resulting in further optimization of Research and Development Projects of Shandong Province
+
the proton conducting properties. In addition, NH4 with the (2019JZZY010331), and Fundamental Research Funds for the
lower pKa value (9.26) and smaller volume enables UPC- Central Universities (No. 18CX05020A).
3 2
H5a@NH $H O to donate more protons and provide a larger
pore volume as demonstrated by gas uptake tests in comparison
Notes and references
+
with Me
2
NH
2
(pK
a
¼ 10.75), enriching more free water mole-
cules and further forming more smooth proton transfer path-
ways. This also contributes to the optimized proton conducting
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