Corrosion resistant PEM fuel cell

Abstract

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 passivating, protective metal layer intermediate the core and the titanium nitride. The protective layer forms a barrier to further oxidation/corrosion when exposed to the fuel cell's operating environment. Stainless steels rich in CR, Ni, and Mo are particularly effective protective interlayers.

Description

The Government of the United States of America has rights in this invention pursuant to contract No. DE-AC02-90CH10435 awarded by the United States Department of Energy.

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., H₂ & O₂ /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 H₂ -air PEM fuel cell environment, the bipolar plates are in constant contact with highly acidic solutions (pH 3.5) containing F-, SO₄^--, SO₃-, HSO₄-, CO₃^--, and HCO₃-, 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 H₂ and O₂. 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 orth forth hereafter in the claims which follow.

Claims

1. In a pem fuel cell having at least one cell comprising a pair of opposite polarity electrodes, a membrane electrolyte interjacent said electrodes for conducting ions therebetween, and an electrically conductive contact element engaging at least one of said electrodes for conducting electrical current from the electrode it engages, the imnprovement comprising said contact element comprising a metal core selected from the group consisting of aluminum and titanium, a protective coating on said core, and a microdiscontinuous titanium nitride topcoat atop said protective coating, said titanium nitride topcoat having a plurality of defects therein exposing said protective coating to a corrosive operating environment within said fuel cell, said protective coating being susceptible to oxidative passivation by said corrosive operating environment so as to form a barrier to further corrosion on the portions of said coating exposed to said environment, whereby said core is protected from corroding by said protective coating underlying said defects.

2. A fuel cell according to claim 1 wherein said protective coating comprises Cr/Ni/Mo-rich stainless steel.

3. In a bipolar pem fuel cell having a plurality of cells each comprising an anode, a cathode, a membrane electrolyte interjacent said anode and cathode for conducting ions therebetween, and a plurality of electrically conductive contact elements engaging said anodes and cathodes for conducting electrical current therefrom, the improvement comprising said contact elements each comprising a metal core selected from the group consisting of aluminum and titanium, a stainless steel protective coating on said core, and a titanium nitride topcoat atop said stainless steel protective coating, a plurality of defects in said topcoat exposing said protective coating to a corrosive operating environment within said fuel cell, said stainless steel coating containing sufficient chromium, nickel and molybdenum as to be susceptible to oxidative passivation by said corrosive operating environment so as to form a barrier to further oxidation/corrosion on the portions of such coating as are exposed to said environment, whereby said core is protected from corroding by said stainless steel underlying said defects.

4. The fuel cell according to claim 3 wherein at least one of said contact elements is a septum engaging the anode of one said cell and the cathode of the next adjacent cell while separating said adjacent cells from each other.

5. The bipolar fuel cell according to claim 1 3 wherein said stainless steel comprises by weight at least 16% Cr, at least 20% Ni, and at least 3% Mo.

6. The fuel cell according to claim 1 wherein said titanium nitride is sputtered onto said protective coating at a bias of at least about 60 V.

7. The fuel cell according to claim 2 wherein said stainless steel is sputtered onto said core at a bias of at least about 270 V.

8. The fuel cell according to claim 1 wherein said protective coating comprises electrolessly deposited nickel-phosphorous alloys.