RH-dependent scaling procedure proposed here. In materials where THSP, but not
humidity-dependent Summerfield scaling, applies the scaling procedure proposed
here could, therefore, be used to quantify the increase in mobile ions resulting from
the absorption of water. This would allow a distinction between the increase in
mobility and number density of the mobile ions with RH, which is impossible from
σ dc alone. In summary, the newly found THSP for frequency-dependent
conductivities can help in identifying the extent to which the increase in conductivity with RH is due to an increase in mobility and/or to an increase in the number
density of mobile ions.
6 Summary and Outlook
We have presented here the state of the art in impedance spectroscopy performed on
PEC materials and the implications for ion dynamics and transport as concluded
from such data. The major findings include identification of the main contribution
to charge transport, which is the cationic counterion for both cation-rich and anionrich PEC. In dry PEC, the mobility of smaller alkali cations is greater, whereas in
humidified PEC the hydration shell causes larger alkali cations to exhibit a greater
mobility, an indication that transport involves the hydration shell.
Furthermore, scaling concepts such as time–temperature and time–humidity
superposition describe the frequency-dependent dynamics in PEC. From these
concepts, conclusions about the dependence of charge carrier concentration on
temperature and humidity can be drawn. The overall picture arising from the
work reviewed here is consistent with PEC forming a dense polymeric glass with
very low matrix mobility, but with fairly mobile charge carriers, the mobility of
which (and also the activation energy of ionic motion) is dependent on their size.
Thus, ion motion takes place in a rather static potential energy landscape by
Arrhenius-activated jumps of the ions. Model approaches to describe such jump
contributions to the conductivity spectrum could be successfully transferred from
other disordered ion-conducting materials.
So far, impedance spectroscopy has been applied to only a few PEC materials.
Mostly, studies have been performed on the standard polyelectrolytes also
employed in layer-by-layer formation, such as PSS, PDADMAC and only a few
more. Therefore, how far the properties depend on the molecular structures
involved is still an open question.
Another interesting aspect is the crossover between cation-dominated conductivity
and proton-dominated conductivity that should occur with increasing water content
for PEC. So far, only PEM with their intrinsically low concentration of residual
counterions were identified as proton conductors. However, for strongly humidified
stoichiometric PEC with nominally no residual counterions, a similarly dominating
proton contribution to the conductivity can be expected. Such knowledge is relevant
for potential applications of PEM either as proton or as Li
+ conductors, in particular
134
C. Cramer and M. Scho ¨nhoff
humidity-dependent Summerfield scaling, applies the scaling procedure proposed
here could, therefore, be used to quantify the increase in mobile ions resulting from
the absorption of water. This would allow a distinction between the increase in
mobility and number density of the mobile ions with RH, which is impossible from
σ dc alone. In summary, the newly found THSP for frequency-dependent
conductivities can help in identifying the extent to which the increase in conductivity with RH is due to an increase in mobility and/or to an increase in the number
density of mobile ions.
6 Summary and Outlook
We have presented here the state of the art in impedance spectroscopy performed on
PEC materials and the implications for ion dynamics and transport as concluded
from such data. The major findings include identification of the main contribution
to charge transport, which is the cationic counterion for both cation-rich and anionrich PEC. In dry PEC, the mobility of smaller alkali cations is greater, whereas in
humidified PEC the hydration shell causes larger alkali cations to exhibit a greater
mobility, an indication that transport involves the hydration shell.
Furthermore, scaling concepts such as time–temperature and time–humidity
superposition describe the frequency-dependent dynamics in PEC. From these
concepts, conclusions about the dependence of charge carrier concentration on
temperature and humidity can be drawn. The overall picture arising from the
work reviewed here is consistent with PEC forming a dense polymeric glass with
very low matrix mobility, but with fairly mobile charge carriers, the mobility of
which (and also the activation energy of ionic motion) is dependent on their size.
Thus, ion motion takes place in a rather static potential energy landscape by
Arrhenius-activated jumps of the ions. Model approaches to describe such jump
contributions to the conductivity spectrum could be successfully transferred from
other disordered ion-conducting materials.
So far, impedance spectroscopy has been applied to only a few PEC materials.
Mostly, studies have been performed on the standard polyelectrolytes also
employed in layer-by-layer formation, such as PSS, PDADMAC and only a few
more. Therefore, how far the properties depend on the molecular structures
involved is still an open question.
Another interesting aspect is the crossover between cation-dominated conductivity
and proton-dominated conductivity that should occur with increasing water content
for PEC. So far, only PEM with their intrinsically low concentration of residual
counterions were identified as proton conductors. However, for strongly humidified
stoichiometric PEC with nominally no residual counterions, a similarly dominating
proton contribution to the conductivity can be expected. Such knowledge is relevant
for potential applications of PEM either as proton or as Li
+ conductors, in particular
134
C. Cramer and M. Scho ¨nhoff