Note: Descriptions are shown in the official language in which they were submitted.
CA 02244969 1998-07-31
WO 97/28571 PCT/EP97/00422
SPECIFICATION
Process for the Production of an Electrode for a Fused Carbonate Fuel Cell,
Electrode
Produced According to this Process and Fused Carbonate Fuel Cell Provided With
an
Electrode Produced According to this Process
The invention relates to a process for making an electrode for a carbonate
melt fuel cell, an
electrode made according to the process, and a carbonate melt fuel cell with
an electrode
made by the process.
The production of cathodes for carbonate melt fuel cells from lithium
cobaltite (LiCoO~ is
known. For making such cathodes, lithium cobaltite powder is mixed with a
binder. A
dispersant can be added to the binder. A foil is made from the mixture which
is divided into
plates. The plates are sintered at high temperatures in an air-carbon dioxide
atmosphere.
Lithium cobaltite is made by reacting cobalt with lithium compounds (EP 0 473
236 A2).
Making lithium cobaltite by reacting cobalt oxide (iron oxide) with lithium
hydroxide vapor
as a powder in a high-temperature reaction is also known. This powder is made
into brittle
electrode plates with small dimensions by a ceramic sintering process (JP
0636, 770).
Finally, making a lithium cobaltite layer from a ductile cobalt layer whose
pores are filled
with lithium carbonate is known. Conversion to the lithium cobaltite layer is
preferably done
after combination with a matrix layer and an anode layer and after
installation together with
current collector plates in a cell holder of a fuel cell during the start-up
phase of the fuel cell.
The structure of the lithium cobaltite electrode plate made in this way
corresponds to the
structure of the original porous cobalt electrode plate which has a relatively
high polarization
resistance (DE 43 03 136 Cl).
The invention is based on the problem of creating a process for producing a
porous lithium
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cobaltite electrode plate with a large internal surface area and a low
polarization resistance
and producing an electrode plate made according to the process.
2
The problem is solved for the process according to the invention by mixing
cobalt metal
powder and lithium carbonate powder with each other homogenously then
producing foils
from the mixture and plates from the foils, said plates being sintered into
porous electrode
precursor plates, then exposing the electrode precursor plates to an air flow
for several hours
at a temperature of between 400°C and 488°C, until the electrode
precursor plates have been
converted into lithium cobaltite electrode plates with extremely large
internal surface areas. In
the process according to the invention, a lithium cobaltite formation reaction
that determines
the structure takes several hours. Initially, cobalt in the porous
cobalt/lithium carbonate
precursor electrode plate is oxidized in the atmosphere of air. Then lithium
cobaltite and
lithium oxide are formed at the points where cobalt oxide contacts lithium
carbonate, releasing
carbon dioxide which is carried away with the air current. Because of its high
vapor pressure,
lithium oxide changes to the gas phase in which it reacts with cobalt oxide
that has not
contacted lithium carbonate to form lithium cobaltite.
While the lithium cobalt is being formed, it is preferable to keep the
temperature at 420°C to
480°C. It has been shown that in this temperature range the above-
described reactions take
place under favorable conditions, influenced by atmospheric oxygen.
In particular, the quantity of air admitted and the air flowrate are adjusted
such that the
carbon dioxide level in the air is no higher than approximately 1 % and the
air is allowed to
act for approximately 10 hours. Under these conditions, an electrode
consisting of lithium
cobaltite with a very large internal surface area of 2 to 6 mZ/g is obtained,
that no longer
contains any lithium carbonate.
The reactions that take place in the above-described process during the
various phases are
described in detail below:
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Mechanism of Solid-Gas Reaction (400°C-488°C)
Oxidation of cobalt:
Co+ 3 Oz--~ 3
Solid reaction at cobalt oxide/lithium carbonate contact points:
j CU3~4 + L12CO3 + I2 O2 ~ LiCoO2.+ Cp2 + 1 Li-,.O
(solid)
LizOtr°S~> -~ Li20ts~>
Gas diffusion of Li20ig~~:
Li20~s~~ (reaction location 1 ) -~ diffusion ~ Li20~g~~ (reaction location 2)
Gas-solid reaction at reaction location 2:
_1 _1
a) 3 Co304 + LlzOis~i + 12 02 ..-~ LiCo02
When the carbon dioxide component of the air is small, not exceeding a value
of I%, an
electrode consisting of lithium cobaltite with a very large internal surface
area of 2-6 m2/g is
obtained, which no longer has any LizC03 after ten hours. The structure formed
during this
combined oxidation/activation process is retained when the electrode is used
in a carbonate
melt fuel cell.
Li20 diffusion into oxidized CO particles that have not contacted Li2C03
particles is not
hindered by increasing the COZ level of the air since LiZC03 forms from the
Li20 and COZ
once again. Consequently the rate of LiCo02 formation decreases with a rising
COa level in
the air and the desired fine structure with a large internal surface area
cannot form.
3
When moist air is used with vapor levels of over 2%, the activation process
for forming a
larger internal surface area of over 2 m2/g can also be carried out with COz
levels of over I%
in the activation gas atmosphere. In this case, LiCo02 is formed by reacting
oxidized cobalt
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with lithium hydroxide according to the following mechanism.
LiCoO~ formation in the presence of water vapor:
Li2C03 + H20ts~ -.~ 2LiOHtg~ + COZ
Li20ts,s~ + H20tsI --~ 2LiOH~s~
m~.s~a~~
LIOHts~, (Reaction location 1) "~-~ LiOI-I(s~ (Reaction location 2)
3 Co3O4 + LiOHIts> + I Z 02 --~ LiCo02 + ~ HZOts>
When the vapor Ieve1 is raised to over 2%, the reaction is not accelerated
further.
The process according to the invention can be carried out after the electrode
precursor plates
have been placed in an oven under the conditions described above, removing the
lithium
cobaltite electrodes after the oven has cooled and assembling them with a
matrix layer
saturated with the molten electrolyte and an anode as well as with current
collectors to form a
fuel cell. ,
It is also favorable to combine the electrode precursor plate in question with
a matrix layer
filled with carbonate melt into a layer arrangement corresponding to the fuel
cell and then to
build it into a fuel cell together with the latter, and carry out the process
according to the
invention after installation in the fuel cell. Under the conditions of the
process according to
the invention, the lithium cobaltite cathode is formed during a fuel cell
start-up procedure.
The lithium cobaltite can also be produced as a thin, adhesive layer,on a
porous nickel
substrate, which thus becomes oxidized.
The example below will further illustrate the invention
- ~ CA 02244969 1998-07-31
S
Embodiment Example
Fine cobalt powder with a particle size of less than 3 ~c and Li2C03 powder
with a particle
size of between 1 ~ and 10 ~c, as starting components, are processed into a
viscous slurry in a
proportion of 66 wt.% CO and 34 wt.% Li2C03 with addition of an organic binder
dissolved
in a nonaqueous solvent, a softener, and other organic additives, and said
slurry is cast into a
foil by the tape casting process. Plates are made from the foil after the
drying process and
these plates are sintered in a protective gas oven at a temperature below the
melting point of
LiZC03, preferably at 650°C, for 30 minutes in a reducing atmosphere.
After this procedure
the CO and Li2C03 grains are in intimate contact in the electrode precursor
plates. After the
plates have been sintered, they are cooled at a rate of 200 K per hour to
460°C and, after
sufficient flushing with nitrogen, exposed to atmospheric air that is
exchanged. At this
temperature, complete oxidation of CO occurs within 10 hours and at the same
time lithium
cobaltite forms from the cobalt oxide and the Li2C03 formed in a solid and gas
reaction. The
lithium cobaltite electrode thus formed has an extremely large internal
surface area, depending
on the rate of formation, which is retained after it has been installed in a
carbonate melt fuel
cell and the latter has been operated. The rate of activation is determined by
(i) the carbon
dioxide and vapor content and (ii) the cobalt and Li2C03 powder particle size.
An electrode made by the process according to the invention has a structure
that is typical of
the process with an extremely large internal surface area. Because of this,
such a cathode has
a very low polarization resistance in a fuel cell and this increases its
performance.
