109
such that a membrane can operate at low humidity, which is also aided by the hydrophilic PVA. The ZIFs behaved simply as fillers here, and relatively high conductivities were measured at 33% RH at either 25 °C or 60 °C, with the highest conductivity
measured as 9.8 × 10
−4
S cm
−1
at 60 °C [85].
In 2015, PSM with KOH was used to form defects in MOFs by replacing a linker
with hydroxides, and these “missing linker defects” resulted in a four orders of
magnitude increase due to the increased basicity and hydrophilicity of the resulting
frameworks [81]. The conductivity values of these Ni-based MOFs increased up to
1.16 × 10
−2
S cm
−1
, and the activation energy dropped down to 0.20 eV at 100% RH
for the MOF modified with KOH. PSM was also utilized to form cationic MOFs. In
2014, Feng et al. showed that stripping a neutral framework of anions in a Cr-based
MOF leads to cationic frameworks and mobile hydroxide ions [79]. Using this procedure, another cationic MOF preparation was reported. (Fe-MIL-101-NH 2 )
+
Cl
−
was shown to have excellent hydroxide conductivity and alkaline stability properties.
An anion-exchange membrane was prepared from MOFs entrapped within porous
bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide) to yield a cationic MOF
with a polyvinyl alcohol (PVA) coating. Upon soaking in NaOH, the entrapped
cationic MOFs behave as the active OH
−
conductors, while the PVA coating is used
to prevent crossover of fuels. A high hydroxide conductivity of 0.145 S cm
−1
was
measured at 353 K [80].
In 2016, Ghosh et al. showed the first MOF exhibiting conductivity via in situ
incorporated hydroxides through the matrix, which were introduced as part of the
initial synthesis. The highest hydroxide conductivity in a MOF was measured at
8 × 10
−5
S cm
−1
at 27 °C and 99% RH, and the activation energy was found to be
0.19 eV, which is reported to be similar to the transport of hydroxide ions in solution
with no interference. The frameworks contained one-dimensional hexagonal channels extending through the structures, which were filled with hydroxide ions and
water molecule to form an extended, hydrogen-bonded supramolecular chain of
hydroxides and water molecules, which was theorized to be the reason for the OH
−
ion conductivity of the material [82].
5 Batteries
Compared to fuel cells, batteries are currently the leaders in the energy storage sector. Batteries are considered to be of vital importance for portable energy solutions
due to their high energy density and specific energy. In traditional aqueous batteries,
such as 1.5 V alkaline or 2.0 V lead-acid batteries, the electrolyte is an aqueous solution of KOH or H 2 SO 4 , respectively. In primary batteries, the device only undergoes
a discharge reaction, as the chemistry does not readily lend itself to operate in
reverse. In secondary batteries, the cycle may be reversed for rechargeability and
reuse. However, there is a limit to how many times a battery can be cycled as the
chemistry at the electrodes is not infinitely reversible and, in general, the charge and
discharge process degrades the electrodes over time. Furthermore, water breaks
Polymer Nanocomposites for Ion Transport
such that a membrane can operate at low humidity, which is also aided by the hydrophilic PVA. The ZIFs behaved simply as fillers here, and relatively high conductivities were measured at 33% RH at either 25 °C or 60 °C, with the highest conductivity
measured as 9.8 × 10
−4
S cm
−1
at 60 °C [85].
In 2015, PSM with KOH was used to form defects in MOFs by replacing a linker
with hydroxides, and these “missing linker defects” resulted in a four orders of
magnitude increase due to the increased basicity and hydrophilicity of the resulting
frameworks [81]. The conductivity values of these Ni-based MOFs increased up to
1.16 × 10
−2
S cm
−1
, and the activation energy dropped down to 0.20 eV at 100% RH
for the MOF modified with KOH. PSM was also utilized to form cationic MOFs. In
2014, Feng et al. showed that stripping a neutral framework of anions in a Cr-based
MOF leads to cationic frameworks and mobile hydroxide ions [79]. Using this procedure, another cationic MOF preparation was reported. (Fe-MIL-101-NH 2 )
+
Cl
−
was shown to have excellent hydroxide conductivity and alkaline stability properties.
An anion-exchange membrane was prepared from MOFs entrapped within porous
bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide) to yield a cationic MOF
with a polyvinyl alcohol (PVA) coating. Upon soaking in NaOH, the entrapped
cationic MOFs behave as the active OH
−
conductors, while the PVA coating is used
to prevent crossover of fuels. A high hydroxide conductivity of 0.145 S cm
−1
was
measured at 353 K [80].
In 2016, Ghosh et al. showed the first MOF exhibiting conductivity via in situ
incorporated hydroxides through the matrix, which were introduced as part of the
initial synthesis. The highest hydroxide conductivity in a MOF was measured at
8 × 10
−5
S cm
−1
at 27 °C and 99% RH, and the activation energy was found to be
0.19 eV, which is reported to be similar to the transport of hydroxide ions in solution
with no interference. The frameworks contained one-dimensional hexagonal channels extending through the structures, which were filled with hydroxide ions and
water molecule to form an extended, hydrogen-bonded supramolecular chain of
hydroxides and water molecules, which was theorized to be the reason for the OH
−
ion conductivity of the material [82].
5 Batteries
Compared to fuel cells, batteries are currently the leaders in the energy storage sector. Batteries are considered to be of vital importance for portable energy solutions
due to their high energy density and specific energy. In traditional aqueous batteries,
such as 1.5 V alkaline or 2.0 V lead-acid batteries, the electrolyte is an aqueous solution of KOH or H 2 SO 4 , respectively. In primary batteries, the device only undergoes
a discharge reaction, as the chemistry does not readily lend itself to operate in
reverse. In secondary batteries, the cycle may be reversed for rechargeability and
reuse. However, there is a limit to how many times a battery can be cycled as the
chemistry at the electrodes is not infinitely reversible and, in general, the charge and
discharge process degrades the electrodes over time. Furthermore, water breaks
Polymer Nanocomposites for Ion Transport