With these developments, new interest arose in PEC as a solid material, partly
due to the similarity to PEM concerning the local molecular structure so that they
could act as reference material. The structural similarity was shown by solid state
NMR, where the interatomic distances of the complexed monomers were found to
be identical in PEC and in PEM [17]. Furthermore, employing regions of stability in
dependence on electrolyte conditions, the phase diagrams of PEC and PEM systems
are comparable to each other [18, 19]. The newer work on PEC was recently
reviewed [20].
Renewed interest in solid PEC was, however, also strongly triggered by the
rising demand for novel materials with tailored properties for electrochemical
applications. Both types of competing devices, i.e. fuel cells and Li ion batteries
require suitable materials acting as the polymer electrolyte membrane, which
separates the electrodes and is required to exhibit a high ionic (Li
+ or protons,
respectively) and negligible electronic conductivity. At the same time, the material
has to allow thin film processing and exhibit high mechanical and chemical
stability. In fuel cells, Nafion has long been the benchmark material, as it shows
a microphase separation into hydrophobic, stabilizing domains and hydrated
channels, where the interface (carrying free sulfate groups) provides the proton
conduction properties [21]. In Li ion batteries, the classical state-of-the-art polymeric materials is poly(ethyleneoxide) (PEO) and many modifications thereof
attempt to achieve a compromise between high mechanical stability and high
ionic conductivity. One example is to apply salt-in-polymer electrolytes made
from comb-shaped copolymers, which exhibit short oligoether side chains that
can solubilize and transport the ions [22–24], while the backbone can be
crosslinked to provide mechanical stability. Typically, such bulk polymers are
then prepared as a thin membrane with a thickness in the micrometer range.
Aiming at applications in electrochemical energy devices, PEM fulfill two of the
three main requirements because of their generic material properties: firstly, PEM
layer thickness can be tuned in the nano- to micrometer range. A low film thickness
is of advantage to yield low overall resistance and fast loading. Secondly, in spite of
the low thickness, PEM are tremendously stable, which is an effect of the multiple
electrostatic interactions between subsequent layers. Their mechanical properties
have been extensively investigated in free-standing geometries, and Young’s
moduli of the order of gigapascal (GPa) have been found [25, 26].
A challenge remains, however, to fulfill the third requirement, which is to
achieve large ionic conductivities in these films. In several publications, dc
conductivities of PEM ranging from 10
À12 S cm
À1 to 10
À5 S cm
À1 have been
reported [27–30]. Studies of the conductivities of PEM started with the seminal
work by Durstock and Rubner who investigated films of PSS/PAH and PAA/PAH,
where PAA denotes poly(acrylic acid), PSS is poly(styrene sulfonate sodium salt),
and PAH is poly(allylamine hydrochloride). They found dc conductivities, σ dc , in
the range of 10
À12 to 10
À7 S cm
À1 [27], where the maximum of 10
À7 S cm
À1 was
achieved only at strong hydration. Further studies of conductivities in PEM
suggested them as potential ion conductive materials for battery applications
[28, 29]. When employing poly(2-acrylamido-2-methyl-1-propansulfonic acid)
Ion Conduction in Solid Polyelectrolyte Complex Materials
99
due to the similarity to PEM concerning the local molecular structure so that they
could act as reference material. The structural similarity was shown by solid state
NMR, where the interatomic distances of the complexed monomers were found to
be identical in PEC and in PEM [17]. Furthermore, employing regions of stability in
dependence on electrolyte conditions, the phase diagrams of PEC and PEM systems
are comparable to each other [18, 19]. The newer work on PEC was recently
reviewed [20].
Renewed interest in solid PEC was, however, also strongly triggered by the
rising demand for novel materials with tailored properties for electrochemical
applications. Both types of competing devices, i.e. fuel cells and Li ion batteries
require suitable materials acting as the polymer electrolyte membrane, which
separates the electrodes and is required to exhibit a high ionic (Li
+ or protons,
respectively) and negligible electronic conductivity. At the same time, the material
has to allow thin film processing and exhibit high mechanical and chemical
stability. In fuel cells, Nafion has long been the benchmark material, as it shows
a microphase separation into hydrophobic, stabilizing domains and hydrated
channels, where the interface (carrying free sulfate groups) provides the proton
conduction properties [21]. In Li ion batteries, the classical state-of-the-art polymeric materials is poly(ethyleneoxide) (PEO) and many modifications thereof
attempt to achieve a compromise between high mechanical stability and high
ionic conductivity. One example is to apply salt-in-polymer electrolytes made
from comb-shaped copolymers, which exhibit short oligoether side chains that
can solubilize and transport the ions [22–24], while the backbone can be
crosslinked to provide mechanical stability. Typically, such bulk polymers are
then prepared as a thin membrane with a thickness in the micrometer range.
Aiming at applications in electrochemical energy devices, PEM fulfill two of the
three main requirements because of their generic material properties: firstly, PEM
layer thickness can be tuned in the nano- to micrometer range. A low film thickness
is of advantage to yield low overall resistance and fast loading. Secondly, in spite of
the low thickness, PEM are tremendously stable, which is an effect of the multiple
electrostatic interactions between subsequent layers. Their mechanical properties
have been extensively investigated in free-standing geometries, and Young’s
moduli of the order of gigapascal (GPa) have been found [25, 26].
A challenge remains, however, to fulfill the third requirement, which is to
achieve large ionic conductivities in these films. In several publications, dc
conductivities of PEM ranging from 10
À12 S cm
À1 to 10
À5 S cm
À1 have been
reported [27–30]. Studies of the conductivities of PEM started with the seminal
work by Durstock and Rubner who investigated films of PSS/PAH and PAA/PAH,
where PAA denotes poly(acrylic acid), PSS is poly(styrene sulfonate sodium salt),
and PAH is poly(allylamine hydrochloride). They found dc conductivities, σ dc , in
the range of 10
À12 to 10
À7 S cm
À1 [27], where the maximum of 10
À7 S cm
À1 was
achieved only at strong hydration. Further studies of conductivities in PEM
suggested them as potential ion conductive materials for battery applications
[28, 29]. When employing poly(2-acrylamido-2-methyl-1-propansulfonic acid)
Ion Conduction in Solid Polyelectrolyte Complex Materials
99