Bipolar plates for fuel cell stacks

Details Bipolar plates for fuel cell stacks Bipolar plates for fuel cell stacks

2001-01-10

2004-03-16

Kalafut, Stephen (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C252S511000

Reexamination Certificate

active

06706437

ABSTRACT:

BACKGROUND OF THE INVENTION
1 Field of the Invention
The present invention relates to surface treated bipolar plates for electrochemical cells, particularly polymer electrolyte membrane fuel cells (PEMFC), a process for improvement of the surface properties of such bipolar plates, and fuel cells made with such bipolar plates.
2 Description of the Related Art
A fuel cell converts a fuel such as hydrogen, and an oxidant, typically oxygen or air, in an electrochemical reaction into electricity, reaction products and excess heat. As shown in
FIG. 1
, a single fuel cell
1
is typically constituted of an electrolyte layer
2
sandwiched between two typically flat porous electrodes
3
and
4
, individually referred to as the anode
3
and the cathode
4
.
A single polymer electrolyte membrane fuel cell (PEMFC) comprises a thin polymer membrane with high proton conductivity as electrolyte placed between two porous electrodes. The electrode surfaces adjacent to the electrolyte are covered with thin porous layers containing the electrocatalysts typically comprising metals from the platinum group.
Oxidation of hydrogen at the anode
3
catalyst layer generates protons and electrons. The protons are transferred across the electrolyte to the cathode. The electrons travel via an external circuit to the cathode
4
. At the cathode
4
, oxygen is reduced by consumption of two electrons per atom to form oxide anions which react with the protons that have crossed the electrolyte layer to form water.
A plurality of single cells is usually assembled in a stack to increase the voltage and hence, the power output. Within the stack, adjacent single cells are electrically connected by means of bipolar plates (BPP)
5
and
6
positioned between the surfaces of the electrodes opposite to those contacted with the electrolyte membrane. These BPP must be impermeable for the reactants to prevent their permeation to the opposite electrode, mixing and uncontrolled chemical reaction. With respect to this function, the BPP is often referred to as separator, too. Those BPP or separators can be made of metals, particulate carbon and graphite materials, impregnated graphite or lately also by moulding compounds consisting of graphite and a polymer binder (cf. U.S. Pat. No. 4,214,969). Flow channels or grooves on the surfaces of the BPP provide access for the fuel to the adjacent anode
3
and for the oxidant to the adjacent cathode
4
and removal of the reaction products and the unreacted remnants of fuel and oxidant. These flow channels reduce the useful surface of the BPP, as the electrical contact area is limited to the part of the surface between the channels.
The electrodes
3
and
4
comprise a porous structure referred to as gas diffusion layer (GDL). These GDL have to provide an efficient entry passage for both fuel and oxidant, respectively, to the catalyst layer as well as an exit for the reaction products away from the catalyst layer into the flow channel of the adjacent BPP. To facilitate the mass transfer between the flow channels and the GDL pores, the GDL surface area exposed to the channels should be as large as possible. It is preferred, therefore, that a large portion of the BPP surface is consumed by the flow channels with only a small portion remaining for the electrical contact. Reduction of the electrical contact area is limited, however, by the high contact resistance between BPP and GDL. The contact area between these two must be sufficiently large to avoid local overheating at high current densities which would finally lead to destruction of the assembly. Only a significantly reduced contact resistance between BPP and GDL would allow for a larger channel area and thus better transfer of fuel and oxidant to the electrodes thereby increasing the power output of the fuel cell.
Several suggestions have been made to improve the electronic contact between BPP and GDL, many of them resulting in rather complicated layered structures of the BPP. Those structures (cf. e.g. U.S. Pat. No. 4,956,131) generally comprise an inner layer made of metal or a gas-impermeable conductive carbon material to prevent gas leakage and provide mechanical stability, and outer contact layers made of a porous soft conductive material such as carbon fibres, thermal expansion graphite (cf. EP-A 0 955 686) or carbonaceous dispersed particles (cf. EP-A 1 030 393) to provide good electrical contact to the GDL. It is obvious that the manufacturing of multi-layer BPP is a rather time-consuming and expensive process requiring a more complex technology, compared to the production of a monolithic separator with uniform composition. Therefore it is preferable to create the desired surface properties of the BPP by a rather simple physical or chemical treatment following the process of shaping/moulding or machining.
In the European Patent Application EP-A 0 949 704, a method is described to improve the surface contact between BPP and GDL by immersion of the BPP in acidic solutions. This method, however, involves the utilisation of 30 wt % sulfuric acid and is carried out at 90 □C. over a long period of time. Such a treatment can attack the polymer binder as well as the graphite material of a BPP and is not suitable for mass production.
Other methods to modify the surface of the BPP as disclosed in EP-A 0 975 040 comprise plasma treatment, corona-discharge treatment and ultraviolet-irradiation treatment each in an atmosphere of hydrophilicising gas. While aimed mainly on improving the hydrophilicity of the BPP surface, most of the examples described there clearly show that the resistivity of the BPP (as measured with the four-probe-method) is negatively affected by the plasma treatment. The resistivity of BPP made by moulding a mixture of phenolic resin and scaly graphite and then subjected to plasma treatment with varying time, output power and hydrophilicising gas was comparable or even significantly higher than that of the untreated BPP made of the same material. Only with increased plasma output power and rather long treatment time a slight decrease of the resistivity (from 15 to 12 m&OHgr;·cm) was achieved. Further shortcomings of this method are the expensive and complex equipment necessary for the plasma or irradiation treatment and the possible destruction of resin particles not only at the surface but also in the bulk of the BPP due to local overheating during plasma treatment.
Consequently a method is required that allows reliable and persistent improvement of the state of the BPP surface employing relatively simple and low cost technique. In the European Patent Application EP-A 0 933 825, a manufacturing method for BPP is disclosed which includes grinding of the press-moulded BPP in order to reduce the contact resistance and to improve the hydrophilicity of the BPP surface. This is not the method of choice since the BPP surface is likely to be contaminated by the grinding agent.
SUMMARY OF THE INVENTION
It is an object of the present invention to enhance the conductivity of a BPP especially in the surface region, and thereby minimise the contact resistance between BPP and GDL in a fuel cell assembly. It is a further object to provide an inexpensive method to manufacture BPP with enhanced surface conductivity.
It has now been found that bipolar plates for electrochemical cells comprising a polymer bound conductive material which are devoid of a skin of binder material and which exhibit a through-plane resistivity of not more than 1m&OHgr;·m, preferably less than 0.9 m&OHgr;·m, and especially preferred less than 0.85 m&OHgr;·m, and a surface roughness as measured with a 3 &mgr;m front end diameter probe is at least 1.5 &mgr;m, and not more than 9 &mgr;m have the desired low contact resistance, or high conductivity.
A bipolar plate is said to have a skin of binder material if the partial density of the conductive material in the outer layers which form the flat surface of the plates with a thickness of 5 &mgr;m is considerably less than the average partial density of the said conductive material in the overall p

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