Corrosion resistant PEM fuel cell

Details Corrosion resistant PEM fuel cell Corrosion resistant PEM fuel cell

2000-02-14

2001-07-17

Kalafut, Stephen (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S006000

Reissue Patent

active

RE037284

ABSTRACT:

This invention relates to PEM fuel cells and more particularly to corrosion-resistant electrical contact elements therefor.
BACKGROUND OF THE INVENTION
Fuel cells have been proposed as a power source for electric vehicles. One such fuel cell is the PEM (i.e., Proton Exchange Membrane) fuel cell as it has potentially high energy and low weight, both of which are highly desirable for mobile electric vehicles. PEM fuel cells are well known in the art, and include a so-called “membrane-electrode-assembly” comprising a thin, solid polymer membrane-electrolyte having an anode on one face of the membrane-electrolyte and a cathode on the opposite face of the membrane-electrolyte. The membrane-electrode-assembly is sandwiched between a pair of electrically conductive elements which serve as current collectors for the anode/cathode and often contain appropriate channels and openings therein for distributing the fuel cell's gaseous reactants (e.g., H
2
& O
2
/air)over the surfaces of the respective anode and cathode. The anode and cathode themselves typically comprise finely divided carbon particles, very finely divided catalytic particles supported on the internal and external surfaces of the carbon particles, and proton conductive material intermingled with the catalytic and carbon particles. One such membrane-electrode-assembly and fuel cell is described in U.S. Pat. No. 5,272,017 issued Dec. 21, 1993 and assigned to the assignee of the present invention.
It is also known to construct bipolar PEM fuel cells wherein a plurality of the membrane-electrode-assemblies are stacked together in electrical series while being separated one from the next by an impermeable, electrically conductive contact element often referred to as a bipolar plate or septum. The bipolar septum/plate electrically conducts current between the anode of one cell to the cathode of the next adjacent cell in the stack.
In an H
2
-air PEM fuel cell environment, the bipolar plates are in constant contact with highly acidic solutions (pH 3.5) containing F
−
, SO
4
—
, SO
3
−
, HSO
4
−
, CO
3
—
, and HCO
3
−
, etc. Moreover, the cathode ms polarized to a maximum of about +1 V vs. the normal hydrogen electrode and exposed to pressurized air, and the anode is exposed to pressurized hydrogen or methanol reformat. Hence, metal contact elements (including bipolar plates/septums) are subject to anodic dissolution at the cathode, and hydrogen embrittlement at the anode. Accordingly, contact elements are often fabricated from graphite which is light-weight, corrosion-resistant, and electrically conductive in the PEM fuel cell environment. However, graphite is quite fragile which makes it difficult to mechanically handle and process contact elements made therefrom. Moreover, graphite is, quite porous making it virtually impossible to make very thin gas impervious plates. The pores in graphite often lead to gas permeation under the fuel cell's operating pressure which could lead to the undesirable mixing of H
2
and O
2
. Finally, the electrical and thermal conductivity of graphite is quite low compared with light weight metals such as aluminum and titanium and their alloys. Unfortunately, such light weight metals are either not corrosion resistant in the PEM fuel cell environment, and contact elements made therefrom deteriorate rapidly, or they form highly electronically resistive oxide films on their surface that increases the internal resistance of the fuel cell and reduces its performance.
SUMMARY OF THE INVENTION
The present invention contemplates a PEM fuel cell having electrical contact elements (including bipolar plates/septums) comprising a titanium nitride coated light weight metal (e.g., Al or Ti) core, having a protective metal layer intermediate the core and the titanium nitride. The protective layer is susceptible to oxidation in the operating environment of the fuel cell so as to form an barrier to further corrosion at sites where the layer is exposed to such environment. Oxides formed on the protective metal layer have relatively low electrical resistivity so as not to substantially increase the internal resistance of the fuel cell. A particularly effective such protective layer comprises stainless steels rich in chromium, nickel and molybdenum (hereinafter Cr/Ni/Mo-rich). By Cr/Ni/Mo-rich stainless steel means a stainless steel containing at least about 16% by weight Cr., at least about 20% by weight Ni, and at least 3% by weight Mo. Another material potentially useful as a protective interlayer between the core and the titanium nitride topcoat is a nickel-phosphorus alloy formed by electroless chemical deposition from nickel hypophosphite solutions
uing

using
techniques well known to those skilled in the art.
DETAILED DESCRIPTION OF THE INVENTION
The invention will be better understood when considered in the light of the following detailed description thereof which is given hereafter in conjunction with the following drawings of which:


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