Films of electrically conductive polymers

A film of an electrically conductive polymer consists of PA0 (A) a layer I which is treated on one side with an aqueous acid and contains incorporated counterions and whose thickness is 1-60% of the total thickness of the film, and PA0 (B) an electrochemically reversible oxidizable layer II whose thickness is 40-99% of the total thickness of the film.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to films of electrically conductive polymers 
and the use of such films as electrode material in electrochemical cells, 
and electrochemical cells containing such films. 
2. Discussion of the Background 
Over the past few years, the use of electrically conductive polymers as 
electrode material in primary and secondary cells has become increasingly 
important. For example, EP-A-36 118 describes electrochemical cells in 
which one or more electrodes consist of electrically conductive polymers. 
The electrically conductive polymers are particularly suitable for use in 
secondary cells, and it has been possible to show that the charge capacity 
and the cycle life of some polymers reach values which are of interest in 
practice (cf. H. Munstedt in H. Kuzmany et al., Electronic Properties of 
Polymers and Related Compounds, Springer-Verlag, 1985, page 13). 
Especially for use in secondary elements, it is necessary for the 
electrically conductive polymers to be capable of undergoing reversible 
oxidation. Electrochemical oxidation or reduction is accompanied by the 
reversible incorporation of counterions into the electrically conductive 
polymer or removal of these ions from the said polymer. However, all 
conductive polymers generally have the disadvantage that their 
conductivity falls with decreasing content of counterions or complexing 
agents. However, to achieve a high degree of discharge in a secondary cell 
or in an electrochemical cell, incorporation or removal must be as 
complete as possible, ideally to the neutral state of the polymer. The 
very low electrical conductivity of the polymers in the neutral or 
virtually neutral state necessitates the use of special conductors, as a 
rule metals or, for example, carbon fibers. This is necessary in order to 
ensure a conductivity sufficient for the speed of charge transport and 
hence also to ensure a good power density of the electrochemical cell. 
However, there are a number of disadvantages involved in the mounting of 
special conductors. Many metals exhibit corrosion in the electrolyte 
solvents used; frequently, the continuous fastening of the conductor to 
the polymer material of the film over a large area presents difficulties, 
and, not least, the use of conductors is fairly expensive in terms of 
construction. The last-mentioned disadvantage is particularly undesirable 
in large-scale series production. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide films of electrically 
conductive polymers which can be employed as electrode material in 
electrochemical cells without the use of a special conductor. 
We have found that this object is achieved, according to the invention, by 
a film of an electrically conductive polymer consisting of 
(A) a layer I which is treated on one side with an aqueous acid and 
contains incorporated counterions and whose thickness is 1-60% of the 
total thickness of the film, and 
(B) an electrochemically reversibly oxidizable layer II whose thickness is 
40-99% of the total thickness of the film.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Preferably, the thickness of layer I is from 2 to 10% of the total 
thickness of the film. The electrically conductive polymer can be a 
polymer of pyrrole, of furan, of thiophene or of aniline. 
The present invention also provides an electrochemical cell containing a 
film of the electrically conductive polymer provided by the present 
invention as an electrode material, and an organic electrolyte solvent. 
The present invention also provides a process for the preparation of a film 
of an electrically conductive, polymer provided by the present invention. 
In this process, the film is treated on one side with an aqueous acid. 
The acid-treated layer I and the reversibly electrochemically oxidizable 
layer II can be seen in FIG. 1. FIG. 2 shows that, when the novel films 
are installed in an electrochemical cell, the acid-treated layer I is 
directly in contact with the direct current source IV or the load IV, and 
the layer II is in direct contact with the electrolyte III. Reference 
symbol V denotes the second electrode of the electrochemical cell. 
The films according to the invention are distinguished in that they 
possess, along their cross-section, an acid-treated region having good 
conductivity and an untreated region whose electrical conductivity depends 
on the content of counterions. 
German Laid-Open Application DOS 3,346,935 discloses a process for 
improving the stability of the electrical conductivity of pyrrole 
polymers, wherein the polymers are treated with aqueous acids. In this 
treatment, ion exchange takes place in polymers containing counterions; 
under the action of sulfuric acid, the incorporated complexing agent 
(counterion) of the electrically conductive polymer is replaced by the 
anion of sulfuric acid (HSO.sub.4 -). 
If a film treated in this manner is used as electrode material in an 
electrochemical cell containing an aqueous electrolyte, the general 
problem of falling conductivity with decreasing content of counterions is 
encountered. Surprisingly, an acid-treated film in which complete ion 
exchange has taken place cannot be subjected to reversible electrochemical 
discharge in an organic electrolyte solvent. Consequently, the 
conductivity is retained unchanged. At the same time, however, such a film 
is not suitable as the sole electrode material for reversible 
electrochemical cells. 
Preferred monomers for the preparation of the electrically conductive 
polymers are 5-membered or 6-membered heterocycles containing oxygen, 
nitrogen or sulfur as hetero atoms and a conjugated .pi. electron system. 
Furan, thiophene and pyrrole and derivatives of these compounds may be 
mentioned here merely by way of example. Electrically conductive polymers 
of this type and processes for their preparation are described in the 
literature, so that further information is superfluous here. The polymer 
films can preferably be prepared by electrochemical polymerization of the 
monomers in the presence of complexing agents (counterions). Processes for 
electrochemical polymerization as well as suitable complexing agents are 
described in the literature (cf. German Laid-Open Applications DOS 
3,428,843 and DOS 3,328,636). In addition to the abovementioned monomers, 
all monomers which can be converted to electrically conductive polymers 
are in principle suitable, with the precondition that the polymers are 
sufficiently stable to acids. Polymers of aniline may be mentioned here 
merely as typical examples. 
It is of course also possible to prepare the polymers in solution in the 
presence of suitable catalysts and, if required, oxidizing agents and 
reducing agents, complexing agents once again advantageously being present 
during the polymerization. 
It is in principle also possible to prepare novel films of electrically 
conductive polymers from polymers which are free of counterions. In this 
case, only the anions of the acids used are incorporated in the polymer 
during the acid treatment, without ion exchange taking place. 
The acid-treated layer of the novel films is then prepared by treating one 
side or one surface of the film with an aqueous acid, as described in, for 
example, German Laid-Open Application DOS 3,346,935. Acids which have 
proven particularly useful are aqueous sulfuric acid or hydrochloric acid, 
whose pH should preferably be less than 4. The concentration of the acids 
used is in general from 0.1 to 50, preferably from 0.2 to 25, in 
particular from 0.2 to 10, % weight. In addition to sulfuric acid and 
hydrochloric acid, examples of other suitable acids are dilute phosphoric 
acid, acetic acid and trichloroacetic acid, to name but a few, provided 
the pH of the dilute solutions is less than 4. On the basis of past 
experience, it is important that aqueous acids are used for the treatment; 
if anhydrous, concentrated acids are used, the advantageous properties of 
the film are less pronounced. If necessary, the acids may furthermore 
contain organic solvents. The water content of the acids is preferably not 
less than 50%, in particular not less than 90%. 
As stated above, this treatment results in exchange of the complexing agent 
or counterion incorporated in the polymer for the anion of the acid used, 
e.g. HSO.sub.4.sup.- where sulfuric acid is used. The depth of penetration 
and hence the thickness of the acid-treated region can be controlled both 
by the acid concentration and by the treatment time. It is clear that 
these two parameters are dependent on one another, and a suitable 
combination must be chosen in order to achieve the desired thickness of 
the treated layer or penetration depth. In general, treatment times of 
from 1 to 30 minutes are sufficient at an acid concentration of from 0.1 
to 50% by weight in order to obtain acid-treated layers from 2 to 20 .mu.m 
thick, i.e. a 1% strength by weight acid leads to an acid-treated layer 
about 1 .mu.m thick when treatment is carried out for 1 minute. For a 
given acid concentration and a given treatment time, the depth of 
penetration is of course also dependent on the temperature, the physical 
conditions and the type of acid used. The atmospheric humidity in the 
system and any other components present may be mentioned merely as 
examples of physical conditions which influence the depth of penetration. 
The thickness of the acid-treated layer is from 1 to 60%, in particular 
from 2 to 10%, of the total thickness of the film of electrically 
conductive polymers used. In conjunction with the usual film thicknesses 
of from 10 to 150 .mu.m, this corresponds to thicknesses of acid-treated 
regions of, preferably, from 0.1 to 90 .mu.m, in particular from 1 to 15 
.mu.m. 
The film according to the invention are particularly advantageously used as 
electrode materials in electrochemical cells. Based on the effect of 
electrochemical irreversibility of the treated regions, it is possible to 
produce electrodes in which the sheet-like conductor is a direct part of 
the film. In fact, the acid-treated side I of the film is connected 
directly to a voltage source, while the untreated surface II is in direct 
contact with the electrolyte solvent used. The surface in direct contact 
with the electrolyte solvent can be reversibly charged and discharged 
electrochemically, i.e. reversible incorporation and removal of the mobile 
counterions is possible. Regardless of the electrochemical processes 
taking place in that region of the film which has not been acid-treated, 
the acid-treated part of the film remains highly electrically conductive 
since no incorporation and removal of acid anions occurs. It is important 
to point out that the discharge process is completely irreversible only in 
organic electrolyte solvents; in aqueous electrolytes or electrolyte 
solvents having a high water content, it is also possible to charge and 
discharge acid-treated layers, so that the advantageous effects of the 
novel films are not so pronounced here. 
Accordingly, the novel films of electrically conductive polymers can be 
used as electrodes with a directly integrated sheet-like conductor in 
electrochemical cells containing an organic electrolyte. The conductivity 
of the acid-treated layer is independent of the electrochemical charge 
state and is sufficiently high to achieve rapid charging and discharging, 
i.e. a satisfactory power density of the electrochemical cell. Since the 
reversibly oxidizable material and the integrated conductor are merely two 
parts of one and the same film, both intimate attachment (contacting) and 
excellent mechanical adhesion are ensured. Furthermore, the corrosion 
problems encountered when metals are used as conductors are likewise 
absent. 
Moreover, large-scale series production of electrodes of this type is 
substantially simpler since one component, i.e. the conductor, as well as 
one process step, i.e. producing contact between the conductor and the 
active electrode material over an area, are dispensed with. 
Depending on the embodiment, both electrodes of an electrochemical cell or 
only one of the electrodes may be composed of the films according to the 
invention. Suitable counterelectrodes are the conventionally used alkaki 
metal electrodes or carbon fibers, as are known per se. 
In addition to the use of the novel films as active electrode material in 
electrochemical cells, the differing conductivity of the novel films, 
which is due to the fact that they are composed of two regions, can also 
be utilized for other purposes. Layer elements consisting of a dielectric 
with an integrated conductor may be mentioned here merely by way of 
example. If the reversible chargeable and dischargeable part of a novel 
film is discharged, this part, as an insulator, possesses a high 
dielectric constant, whereas the acid-treated part has a comparatively 
high conductivity. 
EXAMPLE 1 
A 25 .mu.m thick polypyrrole film complexed with phenyl sulfonate and 
having an electrical conductivity of 100 S/cm was immersed in 10% strength 
by weight aqueous sulfuric acid. After treatment for ten minutes, the 
conductivity had increased to about 110 S/cm. When installed in an 
electrochemical cell, no discharge was possible, in contrast to the 
untreated film. 
EXAMPLE 2 
The polypyrrole film from Example 1 was treated only on one side with 10% 
strength by weight aqueous sulfuric acid. The conductivity of the film was 
unchanged thereafter. A platinum wire was used as the conductor, and this 
electrode was installed in an electrochemical cell containing lithium as 
the counterelectrode and propylene carbonate/LiClO.sub.4 as the 
electrolyte; the polypyrrole film could be discharged by applying an 
appropriate potential difference. The kinetics of the subsequent charging 
process were similar to those of a film contacted over an area with 
platinum sheet. 
EXAMPLE 3 
A 50 .mu.m thick polypyrrole film complexed with ClO.sub.4.sup.- was 
exposed to a 10% strength by weight aqueous sulfuric acid on one side. 
After treatment for five minutes, virtually complete exchange of the 
ClO.sub.4.sup.- anion for HSO.sub.4.sup.- had taken place in a layer about 
5 .mu.m thick, as was shown by energy-dispersive X-ray analysis. The film 
treated in this manner behaved similarly to the electrode described in 
Example 2.