108
anion-exchange membranes: ammonium, imidazolium, guanidinium, pyridinium,
tertiary sulfonium, phosphonium, benzimidazolium, and pyrrolidinium. These
classes of ions are all sensitive to OH
−
chemical attack, however. Fumapem (perfluorinated sulfonic acid/PTFE) is currently the go-to polymeric membrane used for
this purpose as it demonstrates chemical stability similar to that of Teflon. The interested reader is directed toward a contemporary review that discusses perfluorinated
sulfonic acid polymers and provides new insights [87].
A few polymer nanocomposites have been made and tested for hydroxide
exchange.
For
example,
cross-linked
polyvinyl
alcohol/
poly(diallyldimethylammonium) chloride was infiltrated with nanosized graphene
oxide. The resulting composite exhibited both an increased mechanical stability and
a hydroxide conductivity that increased by 80–120%, having values of 0.0121 S cm
−1
@ 30 °C and 0.021 S cm
−1
@ 80 °C [88]. Another example from 2017 is found in a
pre-designed, hybrid core–shell nanoarchitecture, having nanoparticles composed
of SiO 2 cores and quaternary ammonium-functionalized polystyrene shells. The
hydroxide-conducting groups are locally concentrated on the high surface area of
the functionalized nanoparticle composite. Upon embedding these nanocomposites
(20–70 wt %) into a polysulfone matrix, which is itself nonionic and resists aggregation, an extremely high hydroxide conductivity value was measured at
0.1881 S cm
−1
at 80 °C. The well-connected ion channels that result are expected to
be the reason for such high values. Additionally, favorable mechanical properties
are observed, comparable to that of Nafion
®
[89].
MOFs
Hydroxide ion conducting MOFs are rare. ZIFs are the prototypical MOFs used for
hydroxide conduction, owing to their stability at high pH as described earlier. The
first report in 2014 showed that the inclusion of alkylammonium hydroxides as
ionic carriers into the pores was successful. Though the conductivity was not significantly high, an increase by four orders of magnitude (2.3 × 10
−8
S cm
−1
at 25 °C)
vs. the unfilled ZIF-8 was shown. Hence, an otherwise insulating and neutral ZIF-8
became a hydroxide conductor. However, the E a value of 0.70 eV is much higher
than that of hydrated OH
−
ions in liquid (less than 0.2 eV), and suggests that there
are some unfavorable features in the conducting pathways. For example, it was
noted that the pore apertures are smaller than other MOFs at 3.4 Å in ZIF-8 [83].
Another report in 2014 showed that an aminated polymer could be threaded through
ZIF-8. This was a slow process, as the monomer was incorporated by diffusion over
a 1-month period before in situ polymerization was initiated, which was followed
by in situ amination. Essentially, an anion-exchange membrane was formed within
ZIF-8, which allowed for fast ion exchange under anhydrous conditions, although
conductivity values were not reported [84]. In 2015, an ionic liquid (choline hydroxide) was incorporated into the pores of ZIFs and made into PVA composite membranes, which were studied for their hydroxide-conducting properties in low
humidity conditions (33% RH). The ionic liquid is capable of maintaining hydration
C. A. Bauer
anion-exchange membranes: ammonium, imidazolium, guanidinium, pyridinium,
tertiary sulfonium, phosphonium, benzimidazolium, and pyrrolidinium. These
classes of ions are all sensitive to OH
−
chemical attack, however. Fumapem (perfluorinated sulfonic acid/PTFE) is currently the go-to polymeric membrane used for
this purpose as it demonstrates chemical stability similar to that of Teflon. The interested reader is directed toward a contemporary review that discusses perfluorinated
sulfonic acid polymers and provides new insights [87].
A few polymer nanocomposites have been made and tested for hydroxide
exchange.
For
example,
cross-linked
polyvinyl
alcohol/
poly(diallyldimethylammonium) chloride was infiltrated with nanosized graphene
oxide. The resulting composite exhibited both an increased mechanical stability and
a hydroxide conductivity that increased by 80–120%, having values of 0.0121 S cm
−1
@ 30 °C and 0.021 S cm
−1
@ 80 °C [88]. Another example from 2017 is found in a
pre-designed, hybrid core–shell nanoarchitecture, having nanoparticles composed
of SiO 2 cores and quaternary ammonium-functionalized polystyrene shells. The
hydroxide-conducting groups are locally concentrated on the high surface area of
the functionalized nanoparticle composite. Upon embedding these nanocomposites
(20–70 wt %) into a polysulfone matrix, which is itself nonionic and resists aggregation, an extremely high hydroxide conductivity value was measured at
0.1881 S cm
−1
at 80 °C. The well-connected ion channels that result are expected to
be the reason for such high values. Additionally, favorable mechanical properties
are observed, comparable to that of Nafion
®
[89].
MOFs
Hydroxide ion conducting MOFs are rare. ZIFs are the prototypical MOFs used for
hydroxide conduction, owing to their stability at high pH as described earlier. The
first report in 2014 showed that the inclusion of alkylammonium hydroxides as
ionic carriers into the pores was successful. Though the conductivity was not significantly high, an increase by four orders of magnitude (2.3 × 10
−8
S cm
−1
at 25 °C)
vs. the unfilled ZIF-8 was shown. Hence, an otherwise insulating and neutral ZIF-8
became a hydroxide conductor. However, the E a value of 0.70 eV is much higher
than that of hydrated OH
−
ions in liquid (less than 0.2 eV), and suggests that there
are some unfavorable features in the conducting pathways. For example, it was
noted that the pore apertures are smaller than other MOFs at 3.4 Å in ZIF-8 [83].
Another report in 2014 showed that an aminated polymer could be threaded through
ZIF-8. This was a slow process, as the monomer was incorporated by diffusion over
a 1-month period before in situ polymerization was initiated, which was followed
by in situ amination. Essentially, an anion-exchange membrane was formed within
ZIF-8, which allowed for fast ion exchange under anhydrous conditions, although
conductivity values were not reported [84]. In 2015, an ionic liquid (choline hydroxide) was incorporated into the pores of ZIFs and made into PVA composite membranes, which were studied for their hydroxide-conducting properties in low
humidity conditions (33% RH). The ionic liquid is capable of maintaining hydration
C. A. Bauer