Environmental Engineering Reference
In-Depth Information
tended to increase with the Fe(CN)
6
3-
concentration, but for the solid system the
C
dl
values
were only weakly dependent on the Fe(CN)
6
3-
concentration, and similar to that in an aqueous
solution at 5 mM Fe(CN)
6
3-
concentration.
Thus, it was elucidated that the polysaccharide solid can work well as a new solid
medium in place of liquid water for conventional electrochemistry and electrochemical
measurements.
Ionically Conductive Solid of Polysaccharide
Overview for Ionically Conductive Solid
Electrochemical reaction in solid electrolyte can be utilized for various devices such as
sensor, energy conversion, etc. Many groups have reported charge transport by redox
molecules in polymer thin films modified on an electrode, including our group [16-26].
However, in redox polymer solid phases, the charge transport is usually very slow, showing
that other heterogeneous media should be developed where charge transport takes place in the
same way as in a solution. It was shown in the last section 3 that molecular diffusion takes
place in tight and elastic polysaccharide solids containing excess water in the same way as in
a liquid water. This type of solid can offer not only a new solid medium for electrochemical
measurement, but also an excellent medium for electrochemical sensors, and devices. For
these purposes it is of importance to investigate the fundamental ionic conductivity of such a
solid as a medium.
The ionically conductive polymer is applied to the lithium battery, the fuel cell, etc.
These polymer electrolytes have high ionic conductivity. However, general electrochemical
measurement cannot be preformed in such polymer electrolyte. Electrochemical measurement
in such polymer electrolyte has been possible only by special electrode systems described
before. The reason is that the diffusion of ions or redox molecules and the rate of electron
transfer are slow in these polymer electrolytes.
In this section 4, it is shown that the polysaccharide solids containing excess water can be
investigated by a conventional three electrode system because of the high ionic conductivity
of the solids. The study was carried out by investigating the ionic conductivity of the
polysaccharide solids as a function of pH or hardness of the solid.
Ionic Conductivity of Polysaccharide Solids
Impedance spectra of agarose and κ-carrageenan solids containing excess 0.1 M KNO
3
aqueous solution and of a 0.1 M KNO
3
aqueous solution are shown in figure 10.
All the spectra show almost vertical linear relations characteristic for the circuit
composed of serial resistance and capacitance at all the frequencies. Impedance spectra of
agarose and κ-carrageenan solids, and an aqueous solution containing other electrolyte are
also similar to those in figure 10. The current is a non-Faradaic one because no redox species
are present.
The conductivity of polysaccharide solids containing various electrolytes aqueous
solution obtained from the impedance spectra are summarized in table 1. The conductivity in
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