104
two isomorphous, dense metal-terephthalate (m = Mg, Cd) MOFs, and one related
Nd MOF using post-synthetically anchored proton transporters. Zwitterionic
4- pyridinol was included in the original synthesis, which provides a coordinating
proton source and the MOF was then additionally loaded with ethylene glycol molecules via partial exchange of the pyridinol groups, allowing for conductivity of
protons via hydrogen-bonding type interactions. Not only did the hydrogen- bonding
environment result in increased conductivities (up to 10
−3
S cm
−1
for Cd), the E a was
one of the lowest reported thus far, at 0.11 eV [41].
When designing a MOF that will conduct protons “as-is,” it is important to consider what the overall structure is and, specifically, the orientation of the acidic or
basic groups, which ideally are pointing toward the continuous porous connections.
Furthermore, these can provide water-assisted conductivity via channels for water
to travel through. MOFs often have anisotropic packing that can allow for observation of directional transport that is not as readily achieved in standard organic polymer composites. A few Cu-based MOFs serve as examples, including a
polyoxymetalate MOF containing a 1-D water channel [50] and an anisotropic,
homochiral 2D MOF containing hydrogen-bonded waters [47]. Also, a lanthanide
MOF, which has two distinct channels decorated with pendant hydroxyls was studied. One channel has more water per area, and correspondingly, higher transport
along this axis was predicted via computation [55]. Also a Zn-based MOF containing imidazolium groups aligned inside the channel along the crystallographic a and
b axes showed low conductivity up to the tetrahydrate stage, but once the fifth water
molecule is introduced, it shows high proton conductivity with a low activation
energy, both of which are comparable to those of Nafion
®
[69]. Isostructural functionalized metal–organic nanotubes have been synthesized using 5-triazole isophthalic acid with proton conductivity along channels, measured as 5.35 × 10
−5
and
3.61 × 10
−3
S cm
−1
for In and Cd, respectively [40]. Magnetic MOFs were also used
to arrange the pores in MOFs. Cu allowed for ferrimagnetic ordering and pores
containing water to mediate proton transport via proton exchange along ammonium
and hydronium groups [49].
Phosphonates and phosphates are finding application in proton conducting
MOFs. Phosphonate groups typically offer high water stability and the potential for
ligating as a hydrogen phosphonate, yielding additional acidic pores [59]. The first
intrinsic proton conductivity in a MOF was observed in 2012. Here, Zn
2+
, triazole,
and orthophosphates result in a 2D layered structure with a conductivity of
>10
−4
S cm
−1
at 150 °C parallel to the layers with a low activation energy [68]. A
water stable carboxylate La MOF containing phosphonic acid groups that resists
swelling upon proton incorporation was reported by Shimizu et al. The conductivity
was measured to have an activation energy of 0.32 eV, which the authors suggest is
still within the Grotthuss range, but indicates less ordering [58]. Also, isomorphous
ligand replacement of trisulfonate with bis(hydrogen phosphonate) in a sodium
MOF leads to 1.5× enhancement in proton conduction with greater acidity, having
pores partially lined with hydrogen phosphonates [63]. Most recently in 2018, a
MOF containing zirconium phosphate in one-dimensional anionic chains, which is
charge-balanced with NH 4
+
cations yielded a stable anhydrous proton conductivity
C. A. Bauer
two isomorphous, dense metal-terephthalate (m = Mg, Cd) MOFs, and one related
Nd MOF using post-synthetically anchored proton transporters. Zwitterionic
4- pyridinol was included in the original synthesis, which provides a coordinating
proton source and the MOF was then additionally loaded with ethylene glycol molecules via partial exchange of the pyridinol groups, allowing for conductivity of
protons via hydrogen-bonding type interactions. Not only did the hydrogen- bonding
environment result in increased conductivities (up to 10
−3
S cm
−1
for Cd), the E a was
one of the lowest reported thus far, at 0.11 eV [41].
When designing a MOF that will conduct protons “as-is,” it is important to consider what the overall structure is and, specifically, the orientation of the acidic or
basic groups, which ideally are pointing toward the continuous porous connections.
Furthermore, these can provide water-assisted conductivity via channels for water
to travel through. MOFs often have anisotropic packing that can allow for observation of directional transport that is not as readily achieved in standard organic polymer composites. A few Cu-based MOFs serve as examples, including a
polyoxymetalate MOF containing a 1-D water channel [50] and an anisotropic,
homochiral 2D MOF containing hydrogen-bonded waters [47]. Also, a lanthanide
MOF, which has two distinct channels decorated with pendant hydroxyls was studied. One channel has more water per area, and correspondingly, higher transport
along this axis was predicted via computation [55]. Also a Zn-based MOF containing imidazolium groups aligned inside the channel along the crystallographic a and
b axes showed low conductivity up to the tetrahydrate stage, but once the fifth water
molecule is introduced, it shows high proton conductivity with a low activation
energy, both of which are comparable to those of Nafion
®
[69]. Isostructural functionalized metal–organic nanotubes have been synthesized using 5-triazole isophthalic acid with proton conductivity along channels, measured as 5.35 × 10
−5
and
3.61 × 10
−3
S cm
−1
for In and Cd, respectively [40]. Magnetic MOFs were also used
to arrange the pores in MOFs. Cu allowed for ferrimagnetic ordering and pores
containing water to mediate proton transport via proton exchange along ammonium
and hydronium groups [49].
Phosphonates and phosphates are finding application in proton conducting
MOFs. Phosphonate groups typically offer high water stability and the potential for
ligating as a hydrogen phosphonate, yielding additional acidic pores [59]. The first
intrinsic proton conductivity in a MOF was observed in 2012. Here, Zn
2+
, triazole,
and orthophosphates result in a 2D layered structure with a conductivity of
>10
−4
S cm
−1
at 150 °C parallel to the layers with a low activation energy [68]. A
water stable carboxylate La MOF containing phosphonic acid groups that resists
swelling upon proton incorporation was reported by Shimizu et al. The conductivity
was measured to have an activation energy of 0.32 eV, which the authors suggest is
still within the Grotthuss range, but indicates less ordering [58]. Also, isomorphous
ligand replacement of trisulfonate with bis(hydrogen phosphonate) in a sodium
MOF leads to 1.5× enhancement in proton conduction with greater acidity, having
pores partially lined with hydrogen phosphonates [63]. Most recently in 2018, a
MOF containing zirconium phosphate in one-dimensional anionic chains, which is
charge-balanced with NH 4
+
cations yielded a stable anhydrous proton conductivity
C. A. Bauer