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non-protonated sulfonate groups, which should augment proton conduction. A
series of MOF-polymer composite membranes exhibited an enhanced low-humid-
ity proton conductivity, compared with that of pure MOF submicrometer crystals,
{[Ca(D-Hpmpc)(H
2
O)
2
] 2HO
0.5
}
n
, at 25 °C and about 53 % RH [
62
]. It was
found that the available proton carriers in the MOF structure provided a basis for
the conductivity, and the large humidification effect of PVP with adsorbed water
molecules greatly contributed to the proton transport in the composite membrane.
Phosphonate-based MOFs have gradually attracted scientists' interest. As dis-
cussed above, the efficient preparation of porous crystalline metal phosphonates
still presents many difficulties. If metal phosphonates are to serve as proton con-
ductors for practical application, it would be preferable that they function under
relatively mild conditions (e.g., at low temperatures and in anhydrous conditions).
It is considered that this goal may be achieved through preprotection or post-func-
tionalization of the phosphonic bridging groups.
Notably, the considerable ion-exchange capability of metal phosphonates has
been confirmed [
63
,
64
]. Zirconium tetraphosphonates possess an open frame-
work structure with 1D cavities decorated with polar and acidic P
=
O and P-OH
groups [
65
]. In addition to the excellent proton conductivity, the hybrid was fully
protonated by adding HCl and then subjected to several acid-base ion-exchange
reactions with alkaline metal ions, such as Li
+
, Na
+
, and K
+
. Anionic MOF of
Zn
2.5
(H)
0.4-0.5
(C
6
H
3
O
9
P
3
)(H
2
O)
1.9-2
(NH
4
)
0.5-0.6
was synthesized with the use
of urea and 1,3,5-benzenetriphosphonic acid [
66
], in which ammonium ions are
exchangeable with Li
+
. Due to a certain degree of flexibility of the hybrid frame-
work, a reversible insertion/desertion of Li
+
through the pores and elastic network
can be envisioned, showing potential for secondary batteries. Although this aspect
is not extensively studied, the intrinsic porosity within the conductive hybrid mate-
rials (ions or protons) remains largely unknown but worthy of research effort.
Crystalline porous MOFs with hydrated water possess interesting proton-con-
ducting properties, but only at ambient temperatures [
67
,
68
]. The synthesis of
coordination polymers with high-temperature proton conductivity can be gener-
ally divided into two distinct approaches. First, inherently acidic frameworks can
be obtained either by self-assembly of the corresponding functionalized ligands
or by post-synthetic modifications of the MOFs. These result in reticular struc-
tures with covalently attached acidic groups decorating the pores of the extended
coordination network [
69
,
70
]. The alternative way is to imbue the pores of coor-
dination polymers with nonvolatile guest molecules as a medium that provides
multiple proton delocalization pathways for efficient proton transport. Generally
speaking, the ionic conductivity depends on the amount and mobility of charge
carriers (protons). Therefore, the inclusion of stronger acids into porous structures
should greatly improve the proton-conducting properties of such hybrid materials.
Ponomareva et al. [
71
] reported the impregnation of the mesoporous MIL-101 by
nonvolatile acids H
2
SO
4
and H
3
PO
4
. Such a simple approach afforded solid mate-
rials with potent proton-conducting properties at moderate temperatures, which
was critically important for the proper function of onboard automobile fuel cells.
These hybrid compounds demonstrate high proton conductivity (
˃
) over a broad
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