97
activation energy of 0.22 eV. Another common example of a protic conductor that
operates via this mechanism is HUO 2 PO 4 ·4H 2 O (E a = 0.32 eV). This hopping mechanism can be contrasted to the vehicle mechanism, wherein direct diffusion of additional protons associated with water molecules or ammonia molecules (i.e. the
“vehicle”), for example, occurs. The counter diffusion of unprotonated vehicles
(e.g. H 2 O) results in the net transport of protons [29]. In 2016, a new mechanism
class was discussed that applies exclusively to MOFs. Intrinsic “free diffusion
inside a sphere” was observed (in contrast to the “jump diffusion between sites”) via
quasi-elastic neutron scattering (QENS) in a phosphonate-based MOF. The measured activation energy lies between that of the Grotthuss and vehicle mechanisms,
and the authors suggest this process might be considered as an intermediate between
the two in porous systems [30].
Polymer Nanocomposites
Proton exchange membrane fuel cells (PEMFCs) were first developed using the
prototypical Nafion
®
, widely considered the first “ionomer,” or ion conducting polymer. This is essentially a substituted Teflon, with perfluorovinyl ether groups terminated with sulfonate groups grafted onto a tetrafluoroethylene backbone. The
microstructure of this polymer allows for cationic movement by hopping from one
sulfonic acid group to the next. Significant work has been devoted to this and related
polymers. Nafion
®
has been shown to conduct a variety of cations, with the proton
conductivity reaching 0.2 S cm
−1
at high relative humidity. However, this value
depends strongly on humidity and temperature, as high water content is needed to
keep the sulfonic acid groups dissociated and therefore active conductors.
Furthermore, the polymer remains permeable to gases and it cannot operate at high
temperatures due to the loss of mechanical strength and water content. An important
attribute of Nafion
®
is that it very chemically stable, similar to Teflon. This robustness, coupled with the high proton conductivity, continue to make this the standard
proton conducting material of use [31]. An alternative is polybenzimidazole (PBI),
which can be formed into thinner films than Nafion
®
and shows more stability at
high temperatures. It must be complexed with phosphoric acid to be effective, but
one drawback is in the reactivity of imidazole groups with the platinum catalyst [32].
Various alterations have been made to improve the performance of Nafion
®
and
PBI. The combination of inorganic nanomaterials provides for a robust and more
stable system, while maintaining the properties of the polymers. Carbon Nanotubes
(CNTs) have a high aspect ratio and extremely high mechanical stability, making
them a good candidate for mechanical reinforcement. Unfortunately, they often
form tight bundles and are difficult to disperse. However, they can be functionalized
so as to prevent these strong intermolecular forces and result in individual, separated nanometer-diameter tubes [33]. Also, CNTs must be passivated to prevent
electrical conduction through them. For example, amine-functionalized CNTs that
had first been coated with a resistive layer of siloxane were mixed with Nafion
®
,
whereby the sulfonates of Nafion
®
interact with the amines of the CNTs. The
Polymer Nanocomposites for Ion Transport
activation energy of 0.22 eV. Another common example of a protic conductor that
operates via this mechanism is HUO 2 PO 4 ·4H 2 O (E a = 0.32 eV). This hopping mechanism can be contrasted to the vehicle mechanism, wherein direct diffusion of additional protons associated with water molecules or ammonia molecules (i.e. the
“vehicle”), for example, occurs. The counter diffusion of unprotonated vehicles
(e.g. H 2 O) results in the net transport of protons [29]. In 2016, a new mechanism
class was discussed that applies exclusively to MOFs. Intrinsic “free diffusion
inside a sphere” was observed (in contrast to the “jump diffusion between sites”) via
quasi-elastic neutron scattering (QENS) in a phosphonate-based MOF. The measured activation energy lies between that of the Grotthuss and vehicle mechanisms,
and the authors suggest this process might be considered as an intermediate between
the two in porous systems [30].
Polymer Nanocomposites
Proton exchange membrane fuel cells (PEMFCs) were first developed using the
prototypical Nafion
®
, widely considered the first “ionomer,” or ion conducting polymer. This is essentially a substituted Teflon, with perfluorovinyl ether groups terminated with sulfonate groups grafted onto a tetrafluoroethylene backbone. The
microstructure of this polymer allows for cationic movement by hopping from one
sulfonic acid group to the next. Significant work has been devoted to this and related
polymers. Nafion
®
has been shown to conduct a variety of cations, with the proton
conductivity reaching 0.2 S cm
−1
at high relative humidity. However, this value
depends strongly on humidity and temperature, as high water content is needed to
keep the sulfonic acid groups dissociated and therefore active conductors.
Furthermore, the polymer remains permeable to gases and it cannot operate at high
temperatures due to the loss of mechanical strength and water content. An important
attribute of Nafion
®
is that it very chemically stable, similar to Teflon. This robustness, coupled with the high proton conductivity, continue to make this the standard
proton conducting material of use [31]. An alternative is polybenzimidazole (PBI),
which can be formed into thinner films than Nafion
®
and shows more stability at
high temperatures. It must be complexed with phosphoric acid to be effective, but
one drawback is in the reactivity of imidazole groups with the platinum catalyst [32].
Various alterations have been made to improve the performance of Nafion
®
and
PBI. The combination of inorganic nanomaterials provides for a robust and more
stable system, while maintaining the properties of the polymers. Carbon Nanotubes
(CNTs) have a high aspect ratio and extremely high mechanical stability, making
them a good candidate for mechanical reinforcement. Unfortunately, they often
form tight bundles and are difficult to disperse. However, they can be functionalized
so as to prevent these strong intermolecular forces and result in individual, separated nanometer-diameter tubes [33]. Also, CNTs must be passivated to prevent
electrical conduction through them. For example, amine-functionalized CNTs that
had first been coated with a resistive layer of siloxane were mixed with Nafion
®
,
whereby the sulfonates of Nafion
®
interact with the amines of the CNTs. The
Polymer Nanocomposites for Ion Transport