105
of 1.45 × 10
−3
S cm
−1
at 180 °C, one of the highest values among reported
intermediate- temperature proton conducting materials. When assembled into a H 2 /
O 2 fuel cell, a high electrical power density of 12 mW cm
−2
at 180 °C was measured,
among the highest values reported for cells assembled from crystalline solid electrolytes [75].
A phosphonate MOF that can operate over a wide temperature range was reported
in 2010. Low-temperature proton conductivity was seen, even at −20 °C, which was
facilitated with ordered, Zn-ligated water molecules. The phosphonate oxygen
atoms assisted in anchoring and ordering water molecules into chains. Though proton conductivity was not particularly high (3.5 × 10
−5
S cm
−1
at 98% RH, 25 °C), a
very low activation energy of 0.17 eV for proton transfer was measured through
ordered interlayers [71]. Water-assisted proton conductivities of 1.45 × 10
−2
S cm
−1
at 363 K/95% RH and intrinsic, sustainable anhydrous proton conductivity of
1.1 × 10
−5
S cm
−1
at 503 K was measured for another zirconium phosphonate
MOF. The protons can be transported with decent conductivity over a wide temperature range from 293 to 503 K [76].
Crystalline structure may change during transport of protons such to assist the
conductivity in a variety of ways. In many cases, ions or molecules may be released
from the structure with application of heat. These transitions are less frequently
reported, but are nonetheless essential to gain an understanding of the proton transfer mechanism. Single crystal-to-single crystal (SCSC) transformations may be
associated with water coordination. A Co-Ln MOF undergoes a crystalline phase
transition at 25 °C and 95% RH, upon which a proton is released to increase the free
charges and enhance the conductivity to reach 3.05 × 10
−4
S cm
−1
, which is two
orders of magnitude larger than that for the powder sample, demonstrating that the
direction of hydrogen-bonded modes is the preferred proton conduction pathway in
the crystal structure. A SCSC transformation was seen in an anionic Eu-based MOF,
upon which a proton is transferred between the anionic form and the neutral version
of the identical framework [51]. Here, phosphonate groups form parallel hydrogenbonded chains, resulting in a proton conductivity of 1.25 × 10
−3
S cm
−1
at 150 °C,
and water-assisted proton conductivity for a compacted pellet of micro-sized crystals attains 3.76 × 10
−3
S cm
−1
at 100 °C and 98% RH. Other unique attempts to
determine the mechanism of transport in MOFs have been reported, such as the
luminescent visualization of conductivity via shift in emission wavelength from a
ruthenium (II) metallo-ligand [57]. A Zn-based MOF also showed SCSC transformations that were induced by loading of guests, resulting in structures that operated
over a wide temperature range (−40 to 125 °C) [42].
In some cases, a nonporous network may be preferred to allow for more, closely
spaced sites for ions to hop between. In 2013, it was shown that a MOF could be
made such that it retained properties of porosity and the density inherent in solids to
provide the elements of both to a useful proton conductor. The structure was formed
around a bulky template, benzimidazole, and the authors demonstrate conductivity
along the MOFs interior surfaces, rather than directly through the pore. Once water
was removed and the crystalline shape changed, methanol was introduced, resulting
in a 24× increase in conductivity due to accessibility of the methanol. This is in spite
Polymer Nanocomposites for Ion Transport
of 1.45 × 10
−3
S cm
−1
at 180 °C, one of the highest values among reported
intermediate- temperature proton conducting materials. When assembled into a H 2 /
O 2 fuel cell, a high electrical power density of 12 mW cm
−2
at 180 °C was measured,
among the highest values reported for cells assembled from crystalline solid electrolytes [75].
A phosphonate MOF that can operate over a wide temperature range was reported
in 2010. Low-temperature proton conductivity was seen, even at −20 °C, which was
facilitated with ordered, Zn-ligated water molecules. The phosphonate oxygen
atoms assisted in anchoring and ordering water molecules into chains. Though proton conductivity was not particularly high (3.5 × 10
−5
S cm
−1
at 98% RH, 25 °C), a
very low activation energy of 0.17 eV for proton transfer was measured through
ordered interlayers [71]. Water-assisted proton conductivities of 1.45 × 10
−2
S cm
−1
at 363 K/95% RH and intrinsic, sustainable anhydrous proton conductivity of
1.1 × 10
−5
S cm
−1
at 503 K was measured for another zirconium phosphonate
MOF. The protons can be transported with decent conductivity over a wide temperature range from 293 to 503 K [76].
Crystalline structure may change during transport of protons such to assist the
conductivity in a variety of ways. In many cases, ions or molecules may be released
from the structure with application of heat. These transitions are less frequently
reported, but are nonetheless essential to gain an understanding of the proton transfer mechanism. Single crystal-to-single crystal (SCSC) transformations may be
associated with water coordination. A Co-Ln MOF undergoes a crystalline phase
transition at 25 °C and 95% RH, upon which a proton is released to increase the free
charges and enhance the conductivity to reach 3.05 × 10
−4
S cm
−1
, which is two
orders of magnitude larger than that for the powder sample, demonstrating that the
direction of hydrogen-bonded modes is the preferred proton conduction pathway in
the crystal structure. A SCSC transformation was seen in an anionic Eu-based MOF,
upon which a proton is transferred between the anionic form and the neutral version
of the identical framework [51]. Here, phosphonate groups form parallel hydrogenbonded chains, resulting in a proton conductivity of 1.25 × 10
−3
S cm
−1
at 150 °C,
and water-assisted proton conductivity for a compacted pellet of micro-sized crystals attains 3.76 × 10
−3
S cm
−1
at 100 °C and 98% RH. Other unique attempts to
determine the mechanism of transport in MOFs have been reported, such as the
luminescent visualization of conductivity via shift in emission wavelength from a
ruthenium (II) metallo-ligand [57]. A Zn-based MOF also showed SCSC transformations that were induced by loading of guests, resulting in structures that operated
over a wide temperature range (−40 to 125 °C) [42].
In some cases, a nonporous network may be preferred to allow for more, closely
spaced sites for ions to hop between. In 2013, it was shown that a MOF could be
made such that it retained properties of porosity and the density inherent in solids to
provide the elements of both to a useful proton conductor. The structure was formed
around a bulky template, benzimidazole, and the authors demonstrate conductivity
along the MOFs interior surfaces, rather than directly through the pore. Once water
was removed and the crystalline shape changed, methanol was introduced, resulting
in a 24× increase in conductivity due to accessibility of the methanol. This is in spite
Polymer Nanocomposites for Ion Transport