103
Table 1: PSM refers to post-synthetic modification for conductivity, inh. refers to
inherent conductivity of the unmodified framework, and guests refers to use of
incorporated guests as charge carriers. Kitagawa demonstrated a combination of
two of the concepts by introducing NH 4
+
ions using an anionic framework and putting carboxyl end groups of adipic acid into honeycomb-shaped voids. This design
resulted in superprotonic conductivity of 10
−2
S cm
−1
at ambient temperature, comparable to Nafion
®
. The activation energy (E a ) for this MOF was found to be
0.63 eV. The mechanism of proton conduction is expected to be influenced by both
the Grotthuss and vehicle-type mechanisms, whereby there is direct diffusion of
protons. As the conductivity remains high in spite of this activation energy, one
might expect there to be a high concentration of charge carriers [70].
Kitagawa et al. also later reported an isostructural series, whereby changes to
functional group ligands on the organic linkers were done in a systematic way to
observe effects on proton transport [39]. Here, a series of iron- or aluminum-based
MOFs were made with alterations to the ligand’s acidity. It was found that the
smaller the pKa, the better the proton transport properties, owing to the increased
number of protons. In another study utilizing a Ni-based MOF, the acidified MOF
exhibits a proton conductivity of 2.2 × 10
−2
S cm
−1
at 80 °C and 95% RH at pH 1.8
with low activation energy [65]. Since proton conductivity is determined partly by
the amount of protonic charge carriers, it is advantageous to develop methods to
increase the proton concentration in a material. A strong acid can be fully ionized in
an aqueous solution and H
+
can be directly supplied to the MOF of interest, which
requires that the framework should be stable under such acidic conditions. However,
in contrast, there are no strongly acidic groups in the one-dimensional coordination
polymer ferrous oxalate dihydrate, which showed proton conductivity on the order
of 1.3 × 10
−3
S cm
−1
at ambient temperatures, suggesting other mechanisms are at
play [53].
The permanent and tunable porosity of MOFs has been exploited in a number of
ways. As the structures remain crystalline upon evacuation of the pores, these empty
spaces can be filled with guest molecules. In 2009, it was first shown that a MOF
could be incorporated with guests to exhibit anhydrous proton conductivity.
Na 3 (2,4,6-trihydroxy-1,3,5-benzenetrisulfonate), which has regular onedimensional pores that are lined with sulfonate groups, was modulated by the controlled loading of 1H-1,2,4-triazole (Tz) guests within the pores, reaching
5 × 10
−4
S cm
−1
at 150 °C. Furthermore, this showed to be gas tight, an improvement
over polymer membranes [64]. Another group demonstrated the enhanced proton
conductivity in a 3D MOF by the cooperation of guest [(CH 3 ) 2 NH 2 ]
+
cations with
water molecules and the carboxylates in the host frameworks. The low activation
energy of 0.25 eV suggests a Grotthuss mechanism, and is attributed to high proton
carrier concentration and adsorbed water molecules [72].
PSM of MOFs has been used to introduce histidine, a protic amino acid in biologically inspired membranes. Although the observed anhydrous conductivity is not
high (4.3 × 10
−9
S cm
−1
at 146 °C), the ion-conductive range of temperatures is
largely shifted from 40–80 °C to 120–146 °C compared with the bulk histamine
[67]. A more recent paper shows 1000-fold enhancement in proton conductivity of
Polymer Nanocomposites for Ion Transport
Table 1: PSM refers to post-synthetic modification for conductivity, inh. refers to
inherent conductivity of the unmodified framework, and guests refers to use of
incorporated guests as charge carriers. Kitagawa demonstrated a combination of
two of the concepts by introducing NH 4
+
ions using an anionic framework and putting carboxyl end groups of adipic acid into honeycomb-shaped voids. This design
resulted in superprotonic conductivity of 10
−2
S cm
−1
at ambient temperature, comparable to Nafion
®
. The activation energy (E a ) for this MOF was found to be
0.63 eV. The mechanism of proton conduction is expected to be influenced by both
the Grotthuss and vehicle-type mechanisms, whereby there is direct diffusion of
protons. As the conductivity remains high in spite of this activation energy, one
might expect there to be a high concentration of charge carriers [70].
Kitagawa et al. also later reported an isostructural series, whereby changes to
functional group ligands on the organic linkers were done in a systematic way to
observe effects on proton transport [39]. Here, a series of iron- or aluminum-based
MOFs were made with alterations to the ligand’s acidity. It was found that the
smaller the pKa, the better the proton transport properties, owing to the increased
number of protons. In another study utilizing a Ni-based MOF, the acidified MOF
exhibits a proton conductivity of 2.2 × 10
−2
S cm
−1
at 80 °C and 95% RH at pH 1.8
with low activation energy [65]. Since proton conductivity is determined partly by
the amount of protonic charge carriers, it is advantageous to develop methods to
increase the proton concentration in a material. A strong acid can be fully ionized in
an aqueous solution and H
+
can be directly supplied to the MOF of interest, which
requires that the framework should be stable under such acidic conditions. However,
in contrast, there are no strongly acidic groups in the one-dimensional coordination
polymer ferrous oxalate dihydrate, which showed proton conductivity on the order
of 1.3 × 10
−3
S cm
−1
at ambient temperatures, suggesting other mechanisms are at
play [53].
The permanent and tunable porosity of MOFs has been exploited in a number of
ways. As the structures remain crystalline upon evacuation of the pores, these empty
spaces can be filled with guest molecules. In 2009, it was first shown that a MOF
could be incorporated with guests to exhibit anhydrous proton conductivity.
Na 3 (2,4,6-trihydroxy-1,3,5-benzenetrisulfonate), which has regular onedimensional pores that are lined with sulfonate groups, was modulated by the controlled loading of 1H-1,2,4-triazole (Tz) guests within the pores, reaching
5 × 10
−4
S cm
−1
at 150 °C. Furthermore, this showed to be gas tight, an improvement
over polymer membranes [64]. Another group demonstrated the enhanced proton
conductivity in a 3D MOF by the cooperation of guest [(CH 3 ) 2 NH 2 ]
+
cations with
water molecules and the carboxylates in the host frameworks. The low activation
energy of 0.25 eV suggests a Grotthuss mechanism, and is attributed to high proton
carrier concentration and adsorbed water molecules [72].
PSM of MOFs has been used to introduce histidine, a protic amino acid in biologically inspired membranes. Although the observed anhydrous conductivity is not
high (4.3 × 10
−9
S cm
−1
at 146 °C), the ion-conductive range of temperatures is
largely shifted from 40–80 °C to 120–146 °C compared with the bulk histamine
[67]. A more recent paper shows 1000-fold enhancement in proton conductivity of
Polymer Nanocomposites for Ion Transport