The present invention relates to electrochemical devices and more particularly relates to the neutralization of fuel cell effluent in situ to provide a corrosion-resistant electrochemical device.
BACKGROUND OF THE INVENTION
Fuel cells are electrochemical devices that convert chemical potential energy into usable electricity and heat without combustion as an intermediate step. Fuel cells are similar to batteries in that both produce a DC current by using an electrochemical process. Two electrodes, an anode and a cathode, are separated by an electrolyte. Like batteries, fuel cells are combined into groups, called stacks, to obtain a usable voltage and power output. Unlike batteries, however, fuel cells do not release energy stored in the cell, running down when battery energy is gone. Instead, they convert the energy typically in a hydrogen-rich fuel directly into electricity and operate as long as they are supplied with fuel and oxidant. Fuel cells emit almost none of the sulfur and nitrogen compounds released by conventional combustion of gasoline or diesel fuel, and can utilize a wide variety of fuels: natural gas, coal-derived gas, landfill gas, biogas, alcohols, gasoline, or diesel fuel oil.
Fuel cells, such as the PEM (proton exchange membrane) fuel cell, have been proposed as a power source for electric vehicles, as a secondary power source in transportation applications, and for other portable and stationary power applications. The PEM fuel cell is generally well known in the art and comprises a membrane-electrode-assembly (MEA) comprising a thin, proton-conductive polymer membrane electrolyte having an anode electrode film formed on one face thereof and a cathode electrode film formed on the opposite face thereof Hydrogen and oxygen (either pure or in air) are supplied to the anode and cathode, respectively. The PEM prevents hydrogen and oxygen from directly mixing, while allowing ionic transport to occur. At the anode, hydrogen is oxidized to produce protons. These protons migrate across the membrane, or are exchanged with other protons within the membrane, to the cathode and react with oxygen to produce water. The overall electrochemical reduction-oxidation reaction is spontaneous, releasing energy. When several PEM cells are combined in a stack, higher voltages and significant power outputs can be obtained.
The MEA for each PEM cell is sandwiched between a pair of electrically conductive elements which serve as current collectors for the anode/cathode and contain an array of grooves in the faces thereof for distributing the fuel cell's gaseous reactants (hydrogen and oxygen/air) over the surfaces of the respective anode and cathode. In a fuel cell stack, a plurality of the cells are stacked together in electrical series while being separated one from the next by an impermeable, electrically conductive contact element known as a bipolar plate. The bipolar plate serves as an electrically conductive separator element between two adjacent cells. The bipolar plate electrically conducts current between the anode of one cell to the cathode of the next adjacent cell in the stack. The bipolar plate has reactant gas distributing grooves, or flow channels, on both of its external faces, and in most cases, has internal passages therein which are defined by internal heat exchange faces and through which coolant flows to remove heat from the stack.
In the PEM fuel cell environment, the exterior faces of the bipolar plates, which confront the membrane-electrode-assembly (MEA), are in constant contact with highly corrosive, acidic fuel cell effluent solutions (having a pH of about 3.5 to about 4.5) containing fluoride (F−), sulfate (SO4 −), sulfite (SO3 −), hydrogen sulfate (HSO4 −), carbonate (CO3 −), and hydrogen carbonate (HCO3 −), etc., counterions. To survive in such a corrosive environment, at least the exterior faces of the bipolar plates must be highly corrosion resistant.
One approach to the problem of bipolar plate corrosion is to use materials having the ability to withstand the oxidizing conditions within the electrochemical cell, such as gold, titanium, niobium, and tantalum. These metals, however, are prohibitively expensive. In addition, in highly acidic applications, such as PEM fuel cell applications, these metals are subject to anodic dissolution at the cathode, hydrogen embrittlement at the anode, and the formation of electronically resistive oxide films. The art also details pellicle techniques such as use of corrosion resistant coatings that could prevent the direct attack of the metal substrate. U.S. Pat. No. RE37,284 (Reissue of U.S. Pat. No. 5,624,769) to Li et al., discloses a PEM fuel cell having electrical contact elements (including bipolar plates/septums) having a titanium nitride coated light weight metal core and a passivating, protective metal layer intermediate the core, and the titanium nitride. The protective layer forms a barrier to further oxidation or corrosion when exposed to the fuel cell's operating environment.
For electrochemical devices, such as PEM fuel cells, to become a competitive energy technology, particularly in transportation applications, the power densities and operating lifetimes of the devices must be increased and the manufacturing and operating costs reduced. What is needed in the art is a fuel cell stack that is lightweight, compact, low cost, and non-corroding. For transportation applications, PEM bipolar plate durability requirements of about 5000 operating hours are highly desirable.
SUMMARY OF THE INVENTION
The present invention contemplates a device and method for neutralization of fuel cell effluent in situ providing a corrosion-resistant electrochemical device. The present corrosion-resistant electrochemical device comprises a plurality of individual fuel cells connected in electrical series, each fuel cell having a membrane-electrode-assembly (MEA) comprising a positive electrode, a negative electrode, and a separator that contains an electrolyte, and at least one bipolar plate disposed between adjacent individual fuel cells. A neutralization agent sufficient to neutralize corrosive species in fuel cell effluent is disposed within the electrochemical device at one or more locations.
In one embodiment, the neutralization agent is disposed in flow channels of the bipolar plate.
In an alternate embodiment, the neutralization agent is embedded within diffusion layers of the MEA.
Other locations for disposal of the neutralization agent are contemplated and constitute part of the present invention. For example, in another embodiment, the neutralization agent (or ion exchange media) may be included as an integral part of the material comprising the bipolar plate itself.
In another embodiment, the neutralization agent or ion exchange media may be combined with the catalyst layers on the membrane material. In this embodiment, a combination of neutralization agent and catalyst are disposed on the membrane material. This embodiment is particularly advantageous in that it addresses the problems associated with catalyst pooling that is known to occur on the membrane material resulting in portions of the membrane having excess catalyst and portions of the membrane being catalyst free. The combination neutralization agent or ion exchange media catalyst layer provides a substantially uniform layer with the neutralization agent or ion exchange media acting as a placeholder to prevent migration and associated clumping of catalyst.
The method for operating a corrosion-resistant electrochemical device comprises providing a plurality of individual fuel cells connected in electrical series, each fuel cell having a membrane-electrode-assembly comprising an anode catalyst layer and an anode diffusion layer disposed on one side of an electrolyte membrane and a cathode catalyst layer and cathode diffusion layer disposed on an opposite side of the electrolyte membrane; disposing at least one bipolar plate between adjacent individual fuel cells, the bipolar plate having oxygen flow channels and hydrogen flow channels; disposing a neutralization agent or ion exchange media within the electrochemical device, the neutralization agent or ion exchange media being sufficient to neutralize corrosive species present in fuel cell effluent; and operating the electrochemical device whereby corrosive species present in the fuel cell effluent are neutralized by the neutralization agent or ion exchange media in situ.
Electrochemical devices as used herein include, but are not limited to, PEM fuel cells, phosphoric acid fuel cells, alkaline fuel cells, direct methanol fuel cells, aluminum-air reserve cells and the like.
By providing environment neutralization of corrosive cell effluent within the fuel cell, the present invention eliminates the need for expensive corrosive resistant materials. Advantageously, the present invention enables the use of low cost plate materials and coatings without sacrificing performance or durability. In the present invention, environment neutralization is an advantageously robust process. Further, the present invention eliminates the need for sensitive coating processes that can result in pores and failed plates or cells. Advantageously, the neutralized cell effluent can be recirculated without requiring additional components in the system.
These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings.