softening of the PEC matrix caused by the absorption of water. Instead, the mobile
cations and their local environment also have to be considered. What implications
do the above observations have concerning the ion transport mechanisms in
humidified PEC? What are the charged species governing the ion transport?
In the case of dry PEC, alkali ions are the major conducting species [41, 47].
By contrast, for humidified PEM it was shown that the dominant conducting
species are protons [39]. This might be due to the low salt ion concentration in PEM
as compared to PEC, or due to a very high mobility of protons in hydrated samples.
Thus, in the humidified PEC investigated here, the charge carriers could either be
protons as well, or small cations, or a combination of both.
In PEC with x ¼ 0.50, the number of incorporated small ions should be theoretically zero, but small values close to zero are difficult to control. In PEC compositions
with x 6
¼ 0.50, the amount of alkali ions involved in extrinsic charge compensation is
well defined and determined by the ratio of polycations to polyanions. The number of
Na
+ or Cs
+ ions is therefore known and N
% ranges between 10% in the case of
x ¼ 0.55 and 50% in the case of x ¼ 0.75. From this, it follows that if protons were
the dominant conducting species in humidified PEC, differences in conductivity
between NaPEC and CsPEC with the same composition x would have to arise from
differences in the water content. However, the water content in NaPEC is generally
greater, wheres the conductivity (see Fig. 14) is lower than or similar to the conductivity of CsPEC. These observations are an indication against proton conduction in
humidified PEC. It is thus evident that, even in strongly hydrated PEC, alkali ions are
the charge carriers dominating the conductivity. Here, however, the mobility of the
larger alkali ion, Cs
+
, is higher than that of Na
+
. This indicates that, in contrast to dry
PEC, it is not the size of the bare ion itself, but the size of the hydration shell, being
larger for Na
+ than for Cs
+
, that controls the mobility.
More information on the conducting species in humdified PEC and the
influences of number density and mobility of ions on the conductivity can be
obtained on the basis of new scaling relations, which are described in the following
section.
5 Scaling of Conductivity Spectra
In the last section, the shapes of PEC conductivity spectra taken at a constant
composition, but at different external parameters, i.e., temperature [41] and RH [67]
were reviewed. In addition to modeling, further information is accessible from
scaling relations that holds for conductivity spectra. Before going into details, basic
scaling concepts for ion-conducting materials will be briefly summarized.
124
C. Cramer and M. Scho ¨nhoff
cations and their local environment also have to be considered. What implications
do the above observations have concerning the ion transport mechanisms in
humidified PEC? What are the charged species governing the ion transport?
In the case of dry PEC, alkali ions are the major conducting species [41, 47].
By contrast, for humidified PEM it was shown that the dominant conducting
species are protons [39]. This might be due to the low salt ion concentration in PEM
as compared to PEC, or due to a very high mobility of protons in hydrated samples.
Thus, in the humidified PEC investigated here, the charge carriers could either be
protons as well, or small cations, or a combination of both.
In PEC with x ¼ 0.50, the number of incorporated small ions should be theoretically zero, but small values close to zero are difficult to control. In PEC compositions
with x 6
¼ 0.50, the amount of alkali ions involved in extrinsic charge compensation is
well defined and determined by the ratio of polycations to polyanions. The number of
Na
+ or Cs
+ ions is therefore known and N
% ranges between 10% in the case of
x ¼ 0.55 and 50% in the case of x ¼ 0.75. From this, it follows that if protons were
the dominant conducting species in humidified PEC, differences in conductivity
between NaPEC and CsPEC with the same composition x would have to arise from
differences in the water content. However, the water content in NaPEC is generally
greater, wheres the conductivity (see Fig. 14) is lower than or similar to the conductivity of CsPEC. These observations are an indication against proton conduction in
humidified PEC. It is thus evident that, even in strongly hydrated PEC, alkali ions are
the charge carriers dominating the conductivity. Here, however, the mobility of the
larger alkali ion, Cs
+
, is higher than that of Na
+
. This indicates that, in contrast to dry
PEC, it is not the size of the bare ion itself, but the size of the hydration shell, being
larger for Na
+ than for Cs
+
, that controls the mobility.
More information on the conducting species in humdified PEC and the
influences of number density and mobility of ions on the conductivity can be
obtained on the basis of new scaling relations, which are described in the following
section.
5 Scaling of Conductivity Spectra
In the last section, the shapes of PEC conductivity spectra taken at a constant
composition, but at different external parameters, i.e., temperature [41] and RH [67]
were reviewed. In addition to modeling, further information is accessible from
scaling relations that holds for conductivity spectra. Before going into details, basic
scaling concepts for ion-conducting materials will be briefly summarized.
124
C. Cramer and M. Scho ¨nhoff