Chemistry: electrical current producing apparatus – product – and – Having magnetic field feature
Reexamination Certificate
1999-06-25
2002-10-29
Ryan, Patrick (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Having magnetic field feature
C429S006000, C429S006000, C429S006000
Reexamination Certificate
active
06472090
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for operating an electrochemical fuel cell with periodic reactant starvation at an electrode. More particularly, the method comprises periodically momentarily fuel starving at least a portion of the anode of an operational fuel cell or periodically momentarily oxidant starving at least a portion of the cathode of an operational fuel cell or both. The method and apparatus may be used to improve fuel cell performance without suspending the generation of power by the fuel cell.
BACKGROUND OF THE INVENTION
Electrochemical fuel cells convert reactants, namely fuel and oxidant fluid streams, to produce electric power and reaction products. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly (“MEA”) comprising a solid polymer electrolyte or ion-exchange membrane disposed between two porous electrically conductive electrode layers. The anode and cathode each comprise electrocatalyst, which is typically disposed at the membrane/electrode layer interface, to induce the desired electrochemical reaction.
At the anode, the fuel moves through the porous anode layer and is oxidized at the anode electrocatalyst to produce protons and electrons. The protons migrate through the ion exchange membrane towards the cathode. On the other side of the membrane, the oxidant moves through the porous cathode and reacts with the protons at the cathode electrocatalyst. The electrons travel from the anode to the cathode through an external circuit, producing an electrical current.
Electrochemical fuel cells can operate using various reactants. For example, the fuel stream may be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, or methanol in a direct methanol fuel cell. The oxidant may be substantially pure oxygen or a dilute stream such as air containing oxygen.
The fuel stream may contain impurities that do not contribute to, and may actually inhibit, the desired electrochemical reaction at the anode. These impurities may, for example, originate from the fuel stream supply itself, or may be generated in situ in the fuel cell, for example, as intermediate species during the fuel cell reactions. Further, impurities may enter the fuel stream from elsewhere in the system. In a like manner, although less commonly, the oxidant stream may also contain impurities which may inhibit the desired electrochemical reaction at the cathode. Again, impurities may originate within the cathode stream, may be generated in situ, or may originate elsewhere in the system (e.g., fuel stream species may crossover from the anode to the cathode side of a solid polymer fuel cell by diffusion through the membrane electrolyte). Some of these impurities may be chemically adsorbed or physically deposited on the surface of the electrode electrocatalyst, blocking the active electrocatalyst sites and preventing these portions of the electrode electrocatalyst from inducing the desired electrochemical fuel oxidation or oxidant reduction reactions. Such impurities are known as electrocatalyst “poisons” and their effect on electrochemical fuel cells is known as “electrocatalyst poisoning”. Electrocatalyst poisoning thus results in reduced fuel cell performance, where fuel cell performance is defined as the voltage output from the cell for a given current density. Higher performance is associated with higher voltage for a given current density or higher current for a given voltage.
In the absence of countermeasures, the adsorption or deposition of electrocatalyst poisons may be cumulative, so even minute concentrations of poisons in a fuel stream, may for instance, over time, result in a degree of electrocatalyst poisoning which is detrimental to fuel cell performance.
Reformate streams derived from hydrocarbons or oxygenated hydrocarbons typically contain a high concentration of hydrogen fuel, but typically also contain electrocatalyst poisons such as carbon monoxide. To reduce the effects of anode electrocatalyst poisoning, it is known to pre-treat the fuel supply stream prior to directing it to the fuel cell. For example, pre-treatment methods may employ catalytic or other methods to convert carbon monoxide to carbon dioxide. However, known pretreatment methods for reformate streams cannot efficiently remove 100% of the carbon monoxide. Even trace amounts less than 10 ppm can eventually result in electrocatalyst poisoning which causes a reduction in fuel cell performance.
Substances other than carbon monoxide are also known to poison fuel cell electrocatalysts. Depending on the type of fuel and the fuel processing methods, impurities in the fuel stream may be present in quantities sufficient to poison the electrocatalyst and reduce fuel cell performance. Fuel cell components and other fluid streams in the fuel cell system may also be a source of impurities that may result in poisoning of the electrocatalyst on either or both electrodes. For example, fuel cell separator plates are commonly made from graphite. Organic impurities in the graphite may leach out and poison the electrocatalyst. Other poisons may be generated by the reaction of substances in the reactant streams with the fuel cell component materials. Alternatively, substances present in one reactant stream may diffuse through the electrolyte and thus crossover from one electrode to the other. The crossover substance may be acceptable at the first electrode but may represent a poison at the other (for instance, in principle, methanol crossover from the anode to the cathode in a direct methanol fuel cell can depolarize or otherwise adversely affect the cathode).
What constitutes a poison may depend on the nature of the fuel cell. For example, whereas methanol is the fuel in a direct methanol fuel cell, in a hydrogen fuel cell operating on a methanol reformate stream, traces of unreformed methanol can be detrimental to the electrocatalyst performance at the anode.
Conventional methods for addressing the problem of electrode electrocatalyst poisoning include purging the electrode with an inert gas such as nitrogen. However, such purging methods involve suspending the generation of power by the fuel cell. A secondary power source is therefore needed to provide power while the fuel cell electrode is being purged.
Another approach for removing carbon monoxide or other poisons from an electrocatalyst comprises introducing a “clean” reactant stream containing substantially no poisons to a poisoned fuel cell electrode. Where adsorption is reversible, an equilibrium process induced by introducing a clean reactant stream results in some rejuvenation of the electrocatalyst. However, a disadvantage of this approach is that it is generally not effective against irreversibly adsorbed poisons. Furthermore, the recovery of the electrode electrocatalyst by such an equilibrium process can be very slow, during which time the fuel cell is not able to operate at full capacity.
Another approach to counteract carbon monoxide electrocatalyst poisoning at the anode is to continuously introduce a low concentration of oxygen into the fuel stream upstream of the fuel cell, as disclosed in Gottesfeld U.S. Pat. No. 4,910,099. However, there are several disadvantages to Gottesfeld's method which influence fuel cell performance and efficiency. For example, an oxygen bleed results in parasitic losses, undesirable localized exothermic reactions at the anode, and dilution of the fuel stream.
U.S. patent application Ser. No. 08/998,133 filed Dec. 23, 1997, now U.S. Pat. No. 6,096,448, entitled “Method and Apparatus for Operating an Electrochemical Fuel Cell With Periodic Fuel Starvation At The Anode” is incorporated herein by reference in its entirety.
It is apparent from the prior art that there is a need for an improved method and apparatus for rejuvenating a fuel cell electrode electrocatalyst by removing poisons therefrom, which does not involve suspending the availability of the fuel cell to generate power.
SUMMARY OF THE INVENTION
A fuel cell is operated to produ
Colbow Kevin Michael
Geest Marian van der
Longley Cindy J.
Müller Jens
Roberts Joy
Ballard Power Systems Inc.
Crepeau Jonathan
McAndrews Held & Malloy Ltd.
Ryan Patrick
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