98
measured proton conductivity was high at 2.8 × 10
−2
S cm
−1
at 30 °C, and it also
maintained the proton conduction at high temperatures (σ i = 6.3 × 10
−2
S cm
−1
at
130 °C) [34]. Other nanocomposite additives have been used with Nafion
®
as well,
including metal oxide nanoparticles and silica nanoparticles [35]. In these cases,
fuel permeability is reduced, mechanical strength improves, and the conductivity at
low relative humidity (RH) improves due to water retention by the nanofillers. But,
at high RH, the filler prevents movement of water through the water channels and
can lower the conductivity. Others have used anodized aluminum oxide membranes
(which contain nanopores that extend through the entire membrane) that had been
infiltrated with Nafion
®
as an electrolyte. Alignment of the Nafion
®
within the membranes was theorized to increase the in-plane conductivity via growing through the
1-dimensional pores. The authors report superior ionic conductivity and ion selectivity versus Nafion
®
alone [36].
PBI has been used very recently, specifically to avoid the hydration issue that is
present with Nafion
®
. In 2018, PBI nanocomposite membranes were described. The
authors note enhanced proton conductivity with low loading of amine- functionalized
silica nanoparticles. The amine functionalization is again used to enhance dispersibility, but also results in self-assembly due to repulsion between the PBI and
amines. The composite shows an affinity for phosphoric acid. The measured proton
conductivities reached as high as 0.260 S cm
−1
at 140 °C [37]. Others report on a
derivative of PBI, poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole], which is mixed
with ZrO 2 to form a high-temperature PEM fuel cell that resulted in conductivities
of 0.104 S cm
−1
at 185 °C upon infiltration with phosphoric acid [38].
Metal Organic Frameworks (MOFs)
As shown in Fig. 4, the growth of MOFs and PCPs for use in proton conduction has
been increasing steadily, along with an understanding of the mechanisms and features that can improve their performance. MOFs have been extensively used as proton conductors and a review article from 2014 “MOFs as proton
conductors – challenges and opportunities” [7] demonstrates the increasing potential in this area. In this chapter, the aim is to highlight the main methodologies used.
Due to the high accessibility of the organic groups in MOFs, they are amenable to
post-functionalization with high yield, which has been exploited frequently to
achieve proton conductivity. The ability to incorporate functionality via PSM or
guest incorporation, along with their synthetic tunability in shape and size, resulting
in alignment of pores for directional ion transport pathways, has opened up their
potential for designing tailored composites. Libraries of MOFs exist, whereby systematic functionalities can be made to understand specific phenomena.
Table 1 presents a summary of the metals and organic linkers that have been
used, including the conductivity values and measurement conditions, along with
activation energy (if known) for proton transport (when a series was studied, the
highest values are listed here). In the simplest cases, studies whereby MOFs and
ZIFs are combined with an ion conducting matrix have been reported. MOFs can be
C. A. Bauer
measured proton conductivity was high at 2.8 × 10
−2
S cm
−1
at 30 °C, and it also
maintained the proton conduction at high temperatures (σ i = 6.3 × 10
−2
S cm
−1
at
130 °C) [34]. Other nanocomposite additives have been used with Nafion
®
as well,
including metal oxide nanoparticles and silica nanoparticles [35]. In these cases,
fuel permeability is reduced, mechanical strength improves, and the conductivity at
low relative humidity (RH) improves due to water retention by the nanofillers. But,
at high RH, the filler prevents movement of water through the water channels and
can lower the conductivity. Others have used anodized aluminum oxide membranes
(which contain nanopores that extend through the entire membrane) that had been
infiltrated with Nafion
®
as an electrolyte. Alignment of the Nafion
®
within the membranes was theorized to increase the in-plane conductivity via growing through the
1-dimensional pores. The authors report superior ionic conductivity and ion selectivity versus Nafion
®
alone [36].
PBI has been used very recently, specifically to avoid the hydration issue that is
present with Nafion
®
. In 2018, PBI nanocomposite membranes were described. The
authors note enhanced proton conductivity with low loading of amine- functionalized
silica nanoparticles. The amine functionalization is again used to enhance dispersibility, but also results in self-assembly due to repulsion between the PBI and
amines. The composite shows an affinity for phosphoric acid. The measured proton
conductivities reached as high as 0.260 S cm
−1
at 140 °C [37]. Others report on a
derivative of PBI, poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole], which is mixed
with ZrO 2 to form a high-temperature PEM fuel cell that resulted in conductivities
of 0.104 S cm
−1
at 185 °C upon infiltration with phosphoric acid [38].
Metal Organic Frameworks (MOFs)
As shown in Fig. 4, the growth of MOFs and PCPs for use in proton conduction has
been increasing steadily, along with an understanding of the mechanisms and features that can improve their performance. MOFs have been extensively used as proton conductors and a review article from 2014 “MOFs as proton
conductors – challenges and opportunities” [7] demonstrates the increasing potential in this area. In this chapter, the aim is to highlight the main methodologies used.
Due to the high accessibility of the organic groups in MOFs, they are amenable to
post-functionalization with high yield, which has been exploited frequently to
achieve proton conductivity. The ability to incorporate functionality via PSM or
guest incorporation, along with their synthetic tunability in shape and size, resulting
in alignment of pores for directional ion transport pathways, has opened up their
potential for designing tailored composites. Libraries of MOFs exist, whereby systematic functionalities can be made to understand specific phenomena.
Table 1 presents a summary of the metals and organic linkers that have been
used, including the conductivity values and measurement conditions, along with
activation energy (if known) for proton transport (when a series was studied, the
highest values are listed here). In the simplest cases, studies whereby MOFs and
ZIFs are combined with an ion conducting matrix have been reported. MOFs can be
C. A. Bauer