Chemistry: electrical current producing apparatus – product – and – Having magnetic field feature
Reexamination Certificate
1998-11-12
2002-06-25
Brouillette, Gabrielle (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Having magnetic field feature
C429S010000, C429S006000
Reexamination Certificate
active
06410175
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to methods and apparatus for commencing operation of a solid polymer fuel cell system. In particular, the invention relates to starting fuel cell systems that include a reformer.
BACKGROUND OF THE INVENTION
Fuel cell systems are currently being developed for use as power supplies in numerous applications, such as transportation applications and stationary power plants. In some of these applications, the fuel cell system may operate more or less continuously for long periods, albeit at varying power levels. However, in other applications, the fuel cell system may be subjected to frequent on-off cycles and hence go through numerous starts from a shutdown condition. Automotive applications are an example of applications with such a duty cycle.
In general, electrochemical fuel cells convert reactants, namely fuel and oxidant fluid streams, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. An electrocatalyst is needed to induce the desired electrochemical reactions at the electrodes. In addition to electrocatalyst, the electrodes may also comprise an electrically conductive substrate upon which the electrocatalyst is deposited. Solid polymer electrolyte fuel cells employ a membrane electrode assembly (“MEA”). The MEA comprises a solid polymer electrolyte or ion-exchange membrane disposed between the two electrode layers. Solid polymer fuel cells operate at relatively low temperatures (circa 80° C.) compared to other fuel cell types.
A broad range of reactants can be used in electrochemical fuel cells. The oxidant is typically oxygen, delivered in a substantially pure oxygen stream or in a dilute oxygen stream such as air. The fuel is often molecular hydrogen, delivered as substantially pure hydrogen gas or in a hydrogen-containing gas stream such as a reformate stream. Other fuels, besides molecular hydrogen, may be oxidized directly at the fuel cell anode. For example, methanol, dimethyl ether, and methane may be delivered to the fuel cell anode where they are oxidized to produce protons. Such fuels may be delivered in gaseous streams. For methanol and dimethyl ether however, aqueous liquid streams are more commonly used.
A given solid polymer fuel cell can be expected to operate to some extent on most fuels, either in the gas phase or liquid phase, and therefore provide power. However, the design and operation of a solid polymer fuel cell system is typically adapted for the specific type of fuel stream (both the fuel and phase) which is to be used. Along with differences in the subsystems external to the fuel cells (e.g., fuel circulation, cooling, and/or humidification subsystems), there may also be differences in the fuel cells themselves. At this time, for instance, the anodes in direct methanol liquid feed fuel cells (i.e., cells which operate “directly” on unreformed aqueous methanol) typically employ different electrocatalysts and different electrode structures than do fuel cells supplied with hydrogen gas. Another difference between hydrogen gas and direct methanol liquid feed fuel cells might be the choice of solid polymer membrane. In direct methanol liquid feed fuel cells, there is often a problem with crossover of methanol fuel from the anode to the cathode side through the membrane. Improvements in crossover characteristics of membrane materials can be expected to lead to different membranes being preferred for each fuel cell type.
Hydrogen gas is presently a preferred fuel insofar as fuel cell operation and performance (output power) are concerned. However, it can be significantly more difficult to store and handle hydrogen than other fuels. Accordingly, in many fuel cell systems, a hydrogen-containing gaseous fuel stream is created from another fuel using a fuel processing subsystem. Typically, the fuel processing subsystem includes a reformer which generates a hydrogen-containing reformate stream from a fuel feedstock (such as methanol or natural gas), usually by reacting the fuel with steam at elevated temperature in the presence of a suitable catalyst. The fuel processing subsystem also typically includes various other components to assist the reforming process, to purify the reformate stream, and/or to introduce other desirable compounds into the gas stream (e.g., vaporizer, shift converter, selective oxidizer, hydrogen separator, humidifier, etc.).
While reformer-based fuel cell systems are preferred in some applications, there are some difficulties associated with the use of reformed fuel. For instance, aside from the need for and complexity of the fuel processing subsystem itself, it can be significantly more complicated and time consuming to start up the system. Both the solid polymer fuel cells and the reformer typically operate above ambient temperature and thus generally need to be heated before normal operation can begin. The reformer in particular may need to be heated to several hundred degrees Celsius and this can take several minutes to accomplish. Further, during warm-up, operation of the reformer is usually not as efficient and any reformate produced may contain large quantities of impurities such as carbon monoxide which can poison the electrocatalysts typically employed in fuel cell anodes. Thus, any reformate produced during the start-up period may not be of much use for purposes of generating electrical power from the fuel cells. Additionally, the power output of the fuel cells themselves may be relatively low until they have reached a certain operating temperature. Finally, any water supply used in the fuel processing subsystem or in humidification of the fuel cell reactant streams is subject to freezing when ambient conditions fall below 0° C., and thus represents an additional potential difficulty for system start-up. As a result, additional subsystems may be required to provide power and/or heat just during the start-up of a reformer-based fuel cell system. For instance, fuel from the fuel feedstock supply can be burned to heat up the reformer. Once the reformer is operating, hydrogen-containing reformate is available to start up the fuel cells. Also, the reformer may be used to heat the fuel cells. This procedure however may be undesirably slow for some applications. Alternatively, a supply of substantially pure hydrogen can be maintained in the system simply for start-up purposes. The hydrogen can be combusted (by burner or catalytic combustion) to provide heat for warming up the reformer and fuel cells. Hydrogen can also be directed to the fuel cell anodes to initiate operation of the fuel cells until a suitable supply of reformate is available. The supply of hydrogen can be stored, for example, as bottled compressed gas or absorbed in metal hydride compounds. However, the hydrogen supply must periodically be replenished. In another approach, reformer-based fuel cell systems can be started up using energy provided by storage batteries or using combinations of the preceding methods.
Direct methanol fuel cell systems (DMFCs) are not subject to the same problems relating to start-up. Direct methanol fuel cells show relatively good performance during the start-up phase and thus are capable of fairly rapid start-up and can provide some useful power output when starting from ambient temperatures. Further still, methanol has a freezing point that is well below the typical lower temperature limit to which the system is exposed in most applications. Thus, methanol and certain aqueous methanol mixtures may not pose a freezing concern (although typical aqueous methanol mixtures for DMFCs are too dilute to provide significant protection against freezing). However, at this time at least, the performance and efficiency of direct methanol fuel cells is not adequate to supplant reformer-based fuel cell systems in all applications.
SUMMARY OF THE INVENTION
A solid polymer fuel cell system which comprises a supply of fuel and a reformer can be started up quickly by including a port
Colbow Kevin M.
St.-Pierre Jean
Tillmetz Werner
Wilkinson David P.
Alejandro R.
Ballard Power Systems Inc.
Brouillette Gabrielle
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