Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2000-06-08
2003-01-21
Weiner, Laura (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S105000, C429S233000, C423S44500R
Reexamination Certificate
active
06509119
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a carbon electrode material used for a vanadium-based redox-flow type battery and, more particularly to, such a carbon electrode material which is excellent in the energy efficiency of the overall battery system and also which has less changes in its performance accompanied by a prolonged period of services.
2. Description of the Related Art
Conventionally, the electrode has been developed intensively as a key factor of the battery performance. Some types of electrodes do not act as an active material but act as a reaction field which promotes the electrochemical reaction of active materials, often coming in of carbon materials due to their good electro-conductivity and chemical resistance. In particular, as redox-flow type battery electrodes which have been developed popularly for power storing purposes, a carbon fiber assembly having a chemical resistance, electro-conductivity, and liquid flowability is us ed.
The redox-flow type battery has been changed from a type using an aqueous solution of iron and hydrochloric acid in its positive electrode and an aqueous solution of chromium and hydrochloric acid in negative to a type using in both electrodes an aqueous solution of vanadium sulfate having a higher electromotive force and thus improved to have a higher energy density. It is recently having a further enhanced energy density by the development for a higher concentration of active materials employed therein.
The redox-flow type battery mainly comprises, as shown in
FIG. 1
, external tanks
6
and
7
for storing an electrolyte solution and an electrolytic cell EC, in such a configuration that while pumps
8
and
9
are sending the electrolyte solution containing an active material from the external tanks
6
and
7
to the electrolytic cell EC, electrodes incorporated in the electrolytic cell EC performs electrochemical conversion, i.e. charge-discharge.
In charge-discharge in general, the electrolytic cell has such a liquid flow-through type structure as shown in
FIG. 1
, in order to circulate an electrolyte solution between the external tanks and the electrolytic cell itself. Such a liquid flow-through type electrolytic cell is called a single cell and used, as a minimum unit, independently or in a multi-layer stack. Since the electrochemical reaction within a liquid flow-through type electrolytic cell occurs as a non-uniform phase reaction on the surface of an electrode, it is generally accompanied by a two-dimensional electrolytic-reaction field. Since its electrolytic-reaction field is a two-dimensional one, the electrolytic cell suffers a problem that it has a smaller reaction amount per unit volume.
Therefore, the electrochemical reaction field has been three-dimentionalized in order to increase a reaction amount, i.e. current density per unit area.
FIG. 2
shows an exploded perspective view of a liquid flow-through type electrolytic cell having three-dimensional electrodes. In this electrolytic cell, between counter-opposing two collector plates
1
an ion exchange membrane
3
is interposed, flow passages
4
a
and
4
b
for an electrolyte solution along the inner surface of the collector plates
1
are formed by a spacer
2
. At least one of these flow passages
4
a
and
4
b
has therein an electrode material
5
such as a carbon fiber assembly, thus constituting a three-dimensional electrode. Note here that the collector plate
1
is provided with a liquid inlet
10
and a liquid outlet
11
for the electrolyte solution.
In the case of a redox-flow type battery using vanadium oxysulfate as its positive-electrode electrolyte solution and vanadium sulfate as its negative-electrode electrolyte solution, at the time of discharge, the solution containing V
2+
is supplied to the liquid flow passage
4
a
at the negative electrode and the electrolyte solution containing V
5+
(actually an ion containing oxygen) is supplied to the flow passage
4
b
at the positive electrode. At the flow passage
4
a
of the negative electrode, in the three-dimensional electrode
5
, V
2+
releases electrons, to be oxidized into V
3+
. These released electrons pass through an external circuit and reduces, within the three-dimensional electrode of the positive electrode, V
5+
into V
4+
(actually ions containing oxygen). As this oxidation-reduction reaction goes on, the electrolyte solution at the negative electrode runs short of SO
4
2−
and that at the positive electrode has an excessive amount of SO
4
2−
, so that SO
4
2−
moves from the positive electrode through the ion-exchange membrane to the negative electrode, thus maintaining a charge balance. Or H
+
moves from the positive electrode through the ion-exchange membrane to the negative electrode, thus maintaining the charge balance. At the time of charge, the reaction opposite to that for discharge proceeds.
The electrode materials for vanadium-based redox-flow type batteries must have the following performance as their properties:
1) No side reactions occurring other than desired reactions (high reaction selectivity), specifically, high current efficiency (&eegr;I).
2) High activity, specifically, small cell resistance. That is, high voltage efficiency (&eegr;v).
3) High battery energy efficiency (&eegr;E) related to the above-mentioned items 1) and 2).
&eegr;
E
=&eegr;I×&eegr;v
4) Less deterioration against repetitive use (prolonged service life), specifically, less deterioration of battery energy efficiency (&eegr;E).
Japanese Patent Publication No. Sho-60-232669, for example, suggests that a carbonaceous material should be used as an electrode material for ion-chromium-based redox-flow type batteries, which has an quasi-graphite microcrystal having an average of 3.70 Å or less of a <002> inter-facial spacing obtained with X-ray wide-angle analysis and an average of 9.0 Å or more of a c-axial crystallite size and also that has a total acid functional group amount of at least 0.01 meq/g.
Japanese Patent Publication No. Hei-5-234612 also suggests that carbonaceous fiber made from polyacrylonitrile-based fiber as a material should be used as an electrode material for an electrolytic cell of iron-chromium-based redox-flow type batteries, which has an quasi-graphite crystal structure with a <002> inter-facial spacing of 3.50-3.60 Å as obtained with X-ray wide-angle analysis, and in which the number of the combination oxygen atoms is 10-25% of that of the carbon atoms on the surface.
By Japanese Patent Publication Nos. Sho-60-232669 and Hei-5-234612, however, to effectuate proper wettability between the carbonaceous material surface and the electrolyte solution, the total acid functional group amount must be 0.01 meq/g or more or the number of the combination oxygen atoms must be 10% or more of that of the carbon atoms on the carbonaceous material surface. Therefore, as found out recently, a vanadium-based redox-flow type battery presently under development having a higher active material concentration and also a higher viscosity provides a higher contact resistance between the carbonaceous material surface and the collector plate or that between the fiber and the fiber, resulting in the cell resistance being too high to obtain a high energy efficiency.
Moreover, it was found out that since the vanadium-based redox-flow type battery has strong oxidization force of the penta-valent ions of vanadium, the above-mentioned electrode material cannot provide sufficient oxidization resistance, so that the cell resistance increases as the charge-discharge cycle is repeated, thus increasing a change in (deterioration ratio of) the energy efficiency.
In view of the above, it is an object of the present invention to provide such an electrode material for vanadium-based redox-flow type batteries that can enhance the total efficiency of those vanadium-based redox-flow type batteries and also that improve the charge-discharge cycle service life.
It is another
Inoue Makoto
Kobayashi Masaru
Takase Satoshi
Toyo Boseki Kabushiki Kaisha
Weiner Laura
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