Bipolar plates for fuel cell and fuel cell comprising same

Details Bipolar plates for fuel cell and fuel cell comprising same Bipolar plates for fuel cell and fuel cell comprising same

2001-06-28

2003-11-18

Bell, Bruce F. (Department: 1746)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S006000, C429S006000, C029S623100, C029S825000, C029S729000, C029S730000

Reexamination Certificate

active

06649297

ABSTRACT:

DESCRIPTION
The present invention relates to a bipolar plate for a fuel cell.
The invention also concerns a fuel cell device, in particular of the solid electrolyte type, comprising at least one of said bipolar plates.
BACKGROUND OF THE INVENTION
The field of the invention may be defined as that of fuel cells, in particular fuel cells of the solid polymer electrolyte type.
Solid polymer electrolyte type fuel cells in particular, find their application in electrical vehicles which are presently the subject of many development programs in order to bring a solution to pollution caused by thermal engine vehicles.
With solid polymer electrolyte fuel cells playing the role of an electrochemical energy converter associated with an onboard energy tank, for example hydrogen or an alcohol, problems may be overcome, notably problems with motor vehicles, charging times and costs, related to the use of batteries in electrical vehicles.
The schematic assembly of a fuel cell for producing electrical energy is partly illustrated in enclosed FIG.
1
.
A membrane of the ion exchanger type formed of a polymer solid electrolyte (
1
), is used for separating the anodic compartment (
2
) where oxidation of the fuel such as hydrogen H
2
(
4
), occurs according to the equation:
 2H
2
→4H
+
+4e
−
,
from the cathodic compartment (
3
), where the oxidizer, such as atmospheric oxygen O
2
(
5
), is reduced according to the equation:
O
2
+4H
+
+4e
−
43 2H
2
O,
with production of water (
6
), while the anode and cathode are connected through an external circuit (
10
). The thereby produced water flows between both compartments by electro-osmosis and by diffusion (arrows
11
,
12
).
The ionic conducting membrane is generally an organic membrane of the perfluorated ionomer type containing ionic groups which, in the presence of water, provide conduction of protons (
9
) produced at the anode by oxidation of hydrogen.
The thickness of this membrane is from a few tens to a few hundred of microns and it results from a compromise between mechanical resistance and ohmic drop. This membrane also enables separation of gases. The chemical and electrochemical resistance of the membranes generally provide battery operation over periods greater than 1,000 hours.
The bulk electrodes (
13
) placed on both sides of the membrane, generally comprise an active area (
14
) and a diffusion area (
15
). The active area comprises porous graphite covered with noble metal grains (
16
), such as platinum, and a thin coating of an ionic conducting polymer, with a similar structure to that of the membrane, provides ionic transport. The diffusion area (
15
) comprises a porous material hydrophobized by applying a hydrophobic polymer, such as graphite coated with PTFE. The hydrophobicity allows the liquid water to be discharged.
Protons produced at the anode, through oxidation e.g. of hydrogen at the surface of the platinum grains, are transported (
9
) through the membrane to the cathode where they recombine with ions produced by the reduction e.g. of atmospheric oxygen giving water (
6
).
The thereby generated electrons (
17
), may then power for example, an electric motor (
18
) placed in the external circuit (
10
), with water as sole byproduct of the reaction.
The set of membrane plus electrodes is a very thin assembly with a thickness of the order of a millimeter and each electrode is supplied with gases from behind, for example by means of a fluted plate.
The power densities obtained through this recombination and which are generally of the order of 0.5-2 W/cm
2
, if hydrogen and oxygen are used, require the combination of several of these bulk electrode/membrane/bulk electrode structures in order to obtain, for example the 50 kW necessary for a standard electrical vehicle.
In other words, a large number of these structures must be assembled, the elementary surfaces of which may be of the order of 20×20 cm
2
, in order to obtain the desired power, notably if the fuel cell is used in an electrical vehicle.
For this purpose, each set formed of two electrodes and a membrane, defining an elementary cell of the fuel cell, is thus positioned between two impermeable plates (
7
,
8
), which, on the one hand, provide hydrogen distribution on the anode side and on the other hand oxygen distribution on the cathode side. These plates are called bipolar plates.
Bipolar plates used in fuel, cells must fulfil several functions; they should i.a. meet the following criteria or requirements:
provide mechanical resistance of the bulk electrodes/membranes sets in the assemblies of the filter/press type, while limiting the thickness to a few millimeters in order to obtain an overall volume of compatible cell, notably for application in an electrical vehicle;
provide electronic and thermal conduction between the successive bulk electrodes/membranes sets, by obtaining the largest possible electronic and thermal conductivities in order to limit ohmic drops detrimental to the cell's operation (excessive heating) and efficiency;
provide gas tightness while withstanding thermal and electrochemical corrosion associated with the specific operating conditions for a cell;
integrate diffusion paths providing homogeneous distribution of supply gases on the electrodes;
integrate components for removing excess water;
integrate cooling components.
Gas distribution channels with a square or rectangular section (
19
), for example with a side of about 1 to 2 millimeters, are machined on the bipolar plates for gas distribution. These channels are for supplying electrodes with gas in the most uniform manner as possible as they are laid out so as to meander over the whole surface of the electrode. They also enable the produced water to be recovered and discharged outside. These channels usually consist of horizontal sections separated by 180° downward bends.
In order to minimize pressure losses between the gas inlet and outlet and to avoid imposing a too strong pressure difference between both faces of the membrane, several channels may be positioned on a same bipolar plate or distributor plate, for example in parallel.
It has been noted that the performances of cells provided with such bipolar plates, including gas distribution channels with a square or rectangular section, were still unsatisfactory, in particular because the attained relatively low maximum voltage, which is about 0.5 V to 0.7 V cm
2
while operating in H
2
/air, may be considered as insufficient.
Further, the delivered voltage exhibits strong instability over time, and it is impossible to maintain the highest voltage level over a long period without their occurring occasionally large and totally unexpected variations.
These problems apparently seem to be related to the flow of various gas and liquid fluids flowing in the fuel cell device, and, in particular, in the gas distribution channels of the bipolar plates.
On the other hand, the presently used bipolar plates are either in graphite impregnated with resins, or in stainless steel. In both cases, it is necessary to resort to lengthy, costly and complicated machining for forming gas supply grooves, channels or flutes for the bipolar plates.
It was thus demonstrated that the cost of these plates may account for about 60 to 70% of the total cost for existing prototypes, a large, if not essential portion of the cost of the plates being associated with their machining.
Thus, if during recent years consequent progress has been made and has provided a reduction in fuel cell costs, by reducing the amounts of platinum used on the one hand and on the other hand to a lesser extent, the manufacturing costs for the required membranes, substantial progress still remains to be made as regards the plates, providing simplified implementation, notably by suppressing machining operations, in particular for obtaining gas diffusion paths.
Such a simplification in their manufacturing leading to a reduction in the plates' costs would have repercussions on the cost of fuel cells bringing them into a price

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