107
Although MOFs can be engineered with some degree of certainty, a prediction of
the type and the packing of guest molecules in the coordination space of MOFs can
often be difficult. A unique approach was taken by Nagarkar et al. Since a large
number of MOFs have been reported in the Cambridge Structural Database (CSD),
the compounds within CSD were screened for the presence of high-boiling proton
carriers, higher carrier concentrations, the extent of hydrogen bonding, inexpensive
starting materials, and ease of synthesis. An oxalate-based type of MOF {[(Me 2 NH
2 ) 3 (SO 4 )] 2 [M 2 (oxalate) 3 ]} n was found to be the most suitable candidate. Inspired by
this, a new complex, {[(Me 2 NH 2 ) 3 (SO 4 )] 2 [Zn 2 (oxalate) 3 ]} n was synthesized solvothermally. The compound consists of an anionic framework [Zn 2 (oxalate) 3 ]
2−
n that
is interpenetrated with a cationic supramolecular net [(Me 2 NH 2 ) 3 SO 4 ]
+ n , which is
formed by electrostatic and hydrogen-bonding interactions between sulfate anions
and dimethyl ammonium cations. The proton conductivity was found to be
7 × 10
−5
S cm
−1
at 30 °C under inert atmosphere, increasing to a maximum of
1.0 × 10
−4
S cm
−1
at 150 °C with an activation energy of 0.129 eV. Furthermore, the
conductivity reached a maximum value of 4.2 × 10
−2
S cm
−1
at 98% RH [74].
4.2 Hydroxide Ion Conductivity
Currently, alkaline fuel cells utilize the electrolytes KOH and NaOH for transport of
hydroxide ions. The required chemical sources here are hydrogen as the fuel at the
anode (just as in proton conducting membranes) and both oxygen gas and water at
the cathode. The hydroxide is stripped from water at the cathode from an input of
oxygen and water. At the anode, hydrogen gas combines with hydroxide to reform
water (See Fig. 1). This was utilized successfully on the Apollo Mission, even
before proton conducting membranes were used, despite the lower ionic mobility of
the bulkier hydroxide ions. Since hydroxide ions in the electrolyte are compatible
with less expensive, nonprecious metals (such as nickel electrodes versus platinum
required in protonic systems), these would offer a significant economic advantage
compared to proton conducting fuel cells. Furthermore, the kinetics of the reaction
in alkaline fuel cells are faster and require less total catalyst.
Polymer Nanocomposites
There are fewer reports of hydroxide-conducting membranes, as organic polymeric
systems that are chemically stable to the required conditions are rare, but interest in
them remains. A review of hydroxide-conducting polymers in 2011 explains that
there are three classes of polymeric materials which have been utilized as anionexchange membranes in alkaline fuel cells [86]. These include polymer blends/
composites based on PEO-type materials or polybenzimidazole, organic–inorganic
hybrid membranes (of interest here), and semi-interpenetrated networks. The following cations have been utilized as functional groups to bind to hydroxide in these
Polymer Nanocomposites for Ion Transport
Although MOFs can be engineered with some degree of certainty, a prediction of
the type and the packing of guest molecules in the coordination space of MOFs can
often be difficult. A unique approach was taken by Nagarkar et al. Since a large
number of MOFs have been reported in the Cambridge Structural Database (CSD),
the compounds within CSD were screened for the presence of high-boiling proton
carriers, higher carrier concentrations, the extent of hydrogen bonding, inexpensive
starting materials, and ease of synthesis. An oxalate-based type of MOF {[(Me 2 NH
2 ) 3 (SO 4 )] 2 [M 2 (oxalate) 3 ]} n was found to be the most suitable candidate. Inspired by
this, a new complex, {[(Me 2 NH 2 ) 3 (SO 4 )] 2 [Zn 2 (oxalate) 3 ]} n was synthesized solvothermally. The compound consists of an anionic framework [Zn 2 (oxalate) 3 ]
2−
n that
is interpenetrated with a cationic supramolecular net [(Me 2 NH 2 ) 3 SO 4 ]
+ n , which is
formed by electrostatic and hydrogen-bonding interactions between sulfate anions
and dimethyl ammonium cations. The proton conductivity was found to be
7 × 10
−5
S cm
−1
at 30 °C under inert atmosphere, increasing to a maximum of
1.0 × 10
−4
S cm
−1
at 150 °C with an activation energy of 0.129 eV. Furthermore, the
conductivity reached a maximum value of 4.2 × 10
−2
S cm
−1
at 98% RH [74].
4.2 Hydroxide Ion Conductivity
Currently, alkaline fuel cells utilize the electrolytes KOH and NaOH for transport of
hydroxide ions. The required chemical sources here are hydrogen as the fuel at the
anode (just as in proton conducting membranes) and both oxygen gas and water at
the cathode. The hydroxide is stripped from water at the cathode from an input of
oxygen and water. At the anode, hydrogen gas combines with hydroxide to reform
water (See Fig. 1). This was utilized successfully on the Apollo Mission, even
before proton conducting membranes were used, despite the lower ionic mobility of
the bulkier hydroxide ions. Since hydroxide ions in the electrolyte are compatible
with less expensive, nonprecious metals (such as nickel electrodes versus platinum
required in protonic systems), these would offer a significant economic advantage
compared to proton conducting fuel cells. Furthermore, the kinetics of the reaction
in alkaline fuel cells are faster and require less total catalyst.
Polymer Nanocomposites
There are fewer reports of hydroxide-conducting membranes, as organic polymeric
systems that are chemically stable to the required conditions are rare, but interest in
them remains. A review of hydroxide-conducting polymers in 2011 explains that
there are three classes of polymeric materials which have been utilized as anionexchange membranes in alkaline fuel cells [86]. These include polymer blends/
composites based on PEO-type materials or polybenzimidazole, organic–inorganic
hybrid membranes (of interest here), and semi-interpenetrated networks. The following cations have been utilized as functional groups to bind to hydroxide in these
Polymer Nanocomposites for Ion Transport