Integrated fuel cell and pressure swing adsorption system

Details Integrated fuel cell and pressure swing adsorption system Integrated fuel cell and pressure swing adsorption system

2000-12-21

2003-09-30

Gulakowski, Randy (Department: 1746)

Chemistry: electrical current producing apparatus, product, and

Having magnetic field feature

C429S010000, C429S006000

Reexamination Certificate

active

06627338

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to fuel cell systems operating on reactant streams that have been enriched by a pressure swing adsorption method. In particular, the present invention relates to solid polymer electrolyte fuel cell systems operating on oxygen enriched air or hydrogen enriched reformate.
BACKGROUND OF THE INVENTION
Fuel cell systems are currently being developed for use as power supplies in numerous applications, such as automobiles and stationary power plants. Such systems offer promise of economically delivering power with environmental and other benefits.
Fuel cells convert reactants, namely fuel and oxidant, to generate electric power and reaction products. Fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. A catalyst typically induces the desired electrochemical reactions at the electrodes. Preferred fuel cell types include solid polymer electrolyte fuel cells that comprise a solid polymer electrolyte and operate at relatively low temperatures.
A broad range of reactants can be used in solid polymer electrolyte fuel cells. 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, for example, substantially pure oxygen or a dilute oxygen stream such as air.
During normal operation of a solid polymer electrolyte fuel cell, fuel is electrcchemically oxidized at the anode catalyst, typically resulting in the generation of protons, electrons, and possibly other species depending on the fuel employed. The protons are conducted from the reaction sites at which they are generated, through the electrolyte, to electrochemically react with the oxidant at the cathode catalyst. The catalysts are preferably located at the interfaces between each electrode and the adjacent electrolyte.
Solid polymer electrolyte fuel cells employ a membrane electrode assembly (“MEA”), which comprises the solid polymer electrolyte or ion-exchange membrane disposed between the two electrodes. Separator plates, or flow field plates for directing the reactants across one surface of each electrode, are disposed on each side of the MEA.
Each electrode contains a catalyst layer, comprising an appropriate catalyst, located next to the solid polymer electrolyte. The catalyst may, for example, be a metal black, an alloy or a supported metal catalyst, for example, platinum on carbon. The catalyst layer typically contains ionomer that may be similar to the ionomer used for the solid polymer electrolyte (for example, Nafion®). The catalyst layer may also contain a binder, such as polytetrafluoroethylene. The electrodes may also contain a substrate (typically a porous electrically conductive sheet material) that may be employed for purposes of reactant distribution and/or mechanical support.
In operation, the output voltage of an individual fuel cell under load is generally below one volt. Therefore, in order to provide greater output voltage, numerous cells are usually stacked together and are connected in series to create a higher voltage fuel cell stack. (End plate assemblies are typically placed at each end of the stack to hold it together and to compress the stack components together. Compressive force is generally needed for effecting seals and making adequate electrical contact between various stack components.) Fuel cell stacks can then be further connected in series and/or parallel combinations to form larger arrays for delivering higher voltages and/or currents.
Difficulties may arise with the management of water in a solid polymer fuel cell. For instance, in order to function properly, the ion exchange membrane needs to remain adequately hydrated. However, the inlet reactant streams as supplied may be relatively dry and thus may dry out the membrane in the vicinity of the reactant inlets. Thus, one or both inlet reactant streams may need to be humidified. On the other hand, a substantial amount of product water may be generated at the cathode as a result of the electrochemical reaction therein which can result in flooding downstream in the cathode flow field plate thereby obstructing access of oxidant to the cathode catalyst. As described in U.S. Pat. No. 5,935,726, it may therefore be advantageous to periodically reverse the flow direction of a reactant stream, in particular the oxidant stream, to reduce the likelihood of forming overly wet and overly dry regions in the fuel cell and to reduce or eliminate the need for external humidification of the reactant streams.
For greater output voltages, it is also advantageous to supply fuel cells with concentrated reactant streams and preferably with pure reactant streams (for example, pure hydrogen and oxygen reactants). This is an advantage because the presence of relatively large amounts of non-reactive components in the reactant streams can significantly increase kinetic and mass transport losses in the fuel cells. However, in many applications it may be impractical to store and provide the desired reactants in pure form. For instance, hydrogen gas may be stored in high pressure cylinders, liquefied in a cryogenic container, or alloyed in a metal hydride alloy. Such storage options can all add substantial weight and cost to a fuel cell system. In a like manner, options for storing and providing oxygen gas (for example, in high pressure cylinders or cryogenic containers) also add cost and weight. Instead, hydrogen is frequently obtained by reforming a supply of methanol, natural gas, or the like, on-site or on-board. However, a significant amount of carbon dioxide is also generated in the reforming and it typically becomes a substantial non-reactive component in the reformed fuel stream. Oxygen is typically obtained from the air surrounding the fuel cell system. However, non-reactive nitrogen then typically becomes the major component in the dilute oxidant stream.
Increasing the concentration of the reactant in reformed fuel and/or air streams, that is, enrichment, has thus been considered in the art as a way of improving fuel cell performance. Several enrichment methods are commonly known that involve separating out a component from the reactant stream, including cryogenic, membrane, and pressure swing adsorption methods. In a cryogenic method, component separation is achieved by preferentially condensing a component out of a gaseous stream. In a membrane method, component separation is achieved by passing the stream over the surface of a membrane that is selectively permeable to a component in the stream. In a pressure swing adsorption method, a gas component is separated from a gas stream by preferential adsorption onto a suitable adsorbent under pressure. (The ability of a suitable adsorbent to adsorb a desired gas component is dependent on the partial pressure of that component but also may be dependent on the nature of and partial pressure of any other components present since these other components may also be adsorbed to some extent and/or may interact with the desired component.) The adsorbed component is then subsequently desorbed by reducing the pressure and is removed. By exposing the adsorbent to cyclic swings in pressure, a cyclical adsorption and desorption takes place at the adsorbent, and saturation of the adsorbent may be prevented. The gas stream remaining over the adsorbent (that is, the raffinate) is enriched in the component or components that are not adsorbed by the adsorbent. The gas stream that is later desorbed from the adsorbent (that is, the extract) is enriched in the component that was adsorbed by the adsorbent. Thus, an enriched stream may be derived from either the raffinate or the extract.
In a pressure swing adsorption system however, the desired enriched stream is only provided during one part of the two part pressure swing cycle. Thus, a pressure swing adsorption system typically comprises two portions (or more) of adsorbent in order to provide a continuous stream of enriched gas. The system is operated such that

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