The invention generally relates to an electrochemical cell stack assembly.
A fuel cell is a type of electrochemical cell, which converts chemical energy directly into electrical energy. For example, one type of fuel cell includes a proton exchange membrane (PEM) that permits only protons to pass between an anode and a cathode of the fuel cell. Typically PEM fuel cells employ sulfonic-acid-based ionomers, such as Nafion, and operate in the room temperature to 90° Celsius (C) temperature range. Another type employs a phosphoric-acid-based polybenziamidazole, PBI, membrane that operates in the 150° to 200° temperature range. At the anode, diatomic hydrogen (a fuel) is reacted to produce protons that pass through the PEM. The electrons produced by this reaction travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the protons to form water. The anodic and cathodic reactions are described by the following equations:
Anode: H2→2H++2e− Equation 1
Cathode: O2+4H++4e−2H2O Equation 2
A typical fuel cell has a terminal voltage near one volt DC. For purposes of producing much larger voltages, several fuel cells may be assembled together to form an arrangement called a fuel cell stack, an arrangement in which the fuel cells are electrically coupled together in series to form a larger DC voltage (a voltage near 100 volts DC, for example) and to provide more power.
The fuel cell stack may include flow plates (graphite composite or metal plates, as examples) that are stacked one on top of the other, and each plate may be associated with more than one cell of the stack. The plates may include various surface flow channels and orifices to, as examples, route the reactants and products through the fuel cell stack. Electrically conductive gas diffusion layers (GDLs) may be located on each side of a catalyzed PEM to form the anode and cathodes of each fuel cell. In this manner, reactant gases from both the anode and cathode flow-fields may diffuse through the GDLs to reach the catalyst layers.
The PEM fuel cell is only one type of fuel cell. Other types of fuel cells include direct methanol, alkaline, phosphoric acid, molten carbonate, and solid oxide fuel cells.
In contrast to a fuel cell, an electrochemical cell may alternatively be configured to function as an electrolyzer, which produces hydrogen and oxygen from electricity and water. More specifically, the following reactions occurring at the anode and cathode of the cell:
Anode: 2H2O→O2+4H−+4e− Equation 3
Cathode: 4H−+4e−2H2 Equation 4
The electrochemical cell may also be configured to function as gas purifier, or pump. In this configuration, electrical energy is provided to the electrochemical cell to cause a gas species (such as hydrogen) at the anode side of the cell to be selectively transported to the cathode side of the cell.
Within the next decade, the demand for purified, compressed reactant gas is expected to increase dramatically. One factor that is driving this demand is the expected shift from oil-based fuels and internal combustion engines to hydrogen fuel and fuel cells.
Hydrogen production will likely be conducted by a variety of means. Examples include water electrolysis, methane reformation, propane reformation, alcohol reformation, sugar reformation, and/or oil and gasoline reformation. In the case of each of these examples, the product of the generation process is most frequently an impure, low pressure stream, which contains hydrogen gas as one of many constituents. The ideal product, however, would be a pure, dry pressurized hydrogen stream, which can be used directly by either the end application or which can be easily stored in pressurized gas containers.
Another factor driving an increased demand for purified, compressed reactant gases is the continued rapid increase in the industrialization of processes, which utilize reactant gases for materials microstructure processing. The semiconductor industry, as an example, uses large quantities of extremely pure, compressed reactant gases, such as hydrogen and oxygen.
In an embodiment of the invention, a technique includes providing a loading force member to extend between end plates to compress flow plates of an electrochemical cell stack assembly together. The technique includes positioning the loading force member in a region of the stack near where a maximum expansion force is exerted on the flow plates due to the operation of the stack.
In another embodiment of the invention, a technique includes compressing flow plates of an electrochemical cell stack between end plates and extending a loading force member through the stack to compress the flow plates. The technique includes positioning the loading force member to extend substantially along a center line of the stack.
In an another embodiment of the invention, an apparatus includes a stack of flow plates to form electrochemical cells, which generate a pressure force on the flow plates due to operation of the cells. The apparatus includes end plates and a loading member that extends between the end plates to maintain compression of the flow plates. The loading force member is positioned to maximize a compression force on the flow plates in a region of the stack in which the pressure force is maximized.
In yet another embodiment of the invention, an apparatus includes a stack of flow plates to form electrochemical cells and a loading force member. The loading force member extends substantially along a center line of the stack to maintain compression of the flow plates.
Advantages and other features of the invention will become apparent from the following drawing, description and claims.
An electrochemical cell may be configured to produce a significantly pure and compressed gas flow. For example, referring to
Turning now to more specific details, the electrochemical cell 12 routes the incoming gas mixture (received at the anode inlet 14) over an anode catalyst/electrode assembly of the cell 12. At this assembly interface, the gas species to be selectively transported is broken down and ionized. Following this, the species is transported through an electrolyte layer of the electrochemical cell 12. The voltage source 20 provides the electromotive force for this transport. At a cathode catalyst/electrode assembly of the cell 12, the species is re-assembled and neutralized. The gas species, which has been selectively transported exits the electrochemical cell 12 at the cathode outlet 18; and the gas species which were not transported exit the electrochemical cell 12 at the anode exhaust outlet 16.
Selective hydrogen gas transport is achieved using a proton conductor of the cell 12. This proton conductor might be an alkaline electrolyte; liquid or solid acid; or a proton-exchange-membrane (PEM). In the case of the PEM, the membrane may be catalyst coated Nafion.
As a more specific example, the electrochemical cell 12 may include a PEM and may be configured to produce a substantially pure hydrogen flow. For this cell, a gas mixture that contains hydrogen enters the anode side of the cell. At the anode catalyst/electrode, the hydrogen is broken down and ionized. Electrons are moved by the voltage produced by the voltage source 20 to the cathode side of the device, and protons are transported across the PEM electrolyte. At the cathode catalyst/electrode layer the hydrogen molecules re-assemble. The external voltage source 20 provides the energy to drive both the transportation of electrons and protons. Purified hydrogen is then free to exit the cathode of the electrochemical cell 12 at the cathode exhaust outlet 18. Species which are not transported exit the electrochemical cell 12 as an anode exhaust waste stream at the anode exhaust outlet 16.
The anode catalyst layer may be selected to prevent build-up of contaminant species. For example, when carbon monoxide (CO) is present as one of species in the incoming gas mixture, a Pt—Ru catalyst mixture or alloy may be used for the anode catalyst layer. Then, when oxygen is present in sufficient volume, CO is oxidized by the Ru catalyst layer, preventing contamination of the Pt catalyst. As an example, air flow rates equal to four to eight percent of the hydrogen flow rate are sufficient to oxidize CO levels of about 100 parts per million (ppm). Such methods may be very useful in the case of hydrogen-rich gas mixtures, which are produced via fossil fuel reforming processes, because these mixtures often contain trace amounts of CO.
The electrochemical cell 12, when operated as a pump, may compress the purified product to the desired use or storage pressure. Therefore, need for additional mechanical compression may be eliminated. Compression is achieved by placing a back-pressure on the cathode side of the cell 12. When this is done, the species being transported (hydrogen, for example) becomes compressed upon transport from the anode side to the cathode side of the cell 12.
Hydrogen is not the only species that may be transported across the membrane of the electrochemical cell 12 and thus, purified. For example, when the electrolyte material is an oxygen ion conductor, such one of the solid oxide electrolytes that may be used in a solid oxide fuel cell, oxygen pumping may be performed to achieve the same functions listed above for hydrogen, except with oxygen as the process gas.
The electrochemical cell may be combined with additional cells to further purify and compress the end product. For example, an electrochemical cell pump 25, which is depicted in
The efficiency of electrochemically compressing a purified hydrogen stream is far higher than can be achieved with the use of a mechanical compressor. Therefore, it is desirable to compress the gas that is produced by an electrochemical cell pump, without passing the gas through a mechanical compressor. For example,
As another example of the application of an electrochemical pump that directly furnishes a compressed and purified gas flow,
In response to the incoming hydrogen flow and an oxidant flow that is received from an oxidant source 84, the fuel cell stack 90 produces electrical power for a load 98. The fuel cell stack 90 may also include an anode exhaust outlet 92, as well as a cathode exhaust outlet 94. It is noted that the fuel cell system 70 may include power conditioning circuitry 96 that conditions the DC stack voltage that is provided by the fuel cell stack 90 into the appropriate form for the load 98.
Thus, because the electrochemical cell pump 74 directly compresses the purified fuel for the fuel cell stack 90, the overall efficiency of the fuel cell system 70 is increased, as compared to an arrangement in which a mechanical compressor is used to further pressurize the incoming fuel flow to the fuel cell stack 90 to the appropriate level.
Referring to
A difficulty with the stack assembly 120 of
Alternatively, a fuel cell stack assembly 150 (see
It has been discovered that by locating a compression loading force mechanism, such as one or more tie rods, near the center line of the electrochemical cell stack, bowing of the flow plates may be minimized, if not eliminated. More specifically, referring to
As a more specific example,
As depicted in
In accordance with some embodiments of the invention, the stack may use a one flow plate cell design, in which each electrochemical cell is formed from the upper surface of one flow plate and the lower surface of the adjacent flow plate. More particularly, one surface, or side, of the flow plate may be a cathode side of the flow plate and thus may be used to form the cathode chamber of the cell; and the opposite side of the flow plate may be the anode side and thus, be used to form the anode chamber of the adjacent cell. Membrane electrode assemblies (MEAs) are situated between each adjacent pair of flow plates of the stack.
As a more specific example,
The flow plate 250 is formed from a disk-shaped flow plate body 252 that is generally symmetrical with respect to a center 251 of the flow plate 250. Thus, the center line of the flow plate stack extends through the center 251 of the flow plate 250 and through the centers of the other flow plates of the stack. In accordance with some embodiments of the invention, an opening 270, which is concentric with respect to the center line of the stack, is formed in the center of the flow plate 250. The opening 270 receives the loading force member that extends through the stack along the stack's center line. In accordance with some embodiments of the invention, the opening 270 also serves to form part of the cathode exhaust plenum passageway for the stack. In this regard, when the flow plate 250 is assembled in the stack, the openings 270 of the flow plates align to form the cathode exhaust plenum passageway.
The flow plate 250 includes other openings 294 and 296, which form portions of other plenum passageways of the stack. In general, the openings 294 and 296 each are eccentric with respect to the center line 251, are located near the outer perimeter of the flow plate 250 and partially circumscribe the center line 251. In accordance with some embodiments of the invention, the openings 294 and 296 may be associated with anode flows. In this regard, the opening 294 may form a portion of the plenum passageway to communicate an incoming anode flow to the electrochemical cell, and the opening 296 may form part of the plenum passageway to communicate an anode exhaust from the electrochemical cell. Therefore, because the openings 294 and 296 are associated with anode flows, seals or gaskets 297 and 298 surround the openings 294 and 296, respectively, to isolate the anode flows from the cathode side (depicted in
The cathode side of the flow plate 250 includes an active region to communicate a cathode flow. For the case in which the electrochemical cell stack is an electrochemical pump, or purifier, this active region communicates the purified gas from the MEA of the cell. As depicted in
In accordance with some embodiments of the invention, a small number (four, for example) of openings 292 may be formed in the active region near the outer cell 290. The openings 292 effectively establish a bleed flow between the cathode and anode sides of the cell.
Other stack and plate designs are envisioned, all of which fall within the scope of the appended claims.
While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of the invention.
This application claims the benefit under 35 U.S.C.§119(e) to U.S. Provisional Patent Application Ser. No. 61/126,138, entitled, “ELECTROCHEMICAL CELL STACK ASSEMBLY,” which was filed on May 1, 2008, and is hereby incorporated by reference in its entirety.
Number | Date | Country | |
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61126138 | May 2008 | US |