Controlling leakage in an electrochemical cell

An electrochemical cell includes a flow plate and a seal that is contact with the flow plate. The seal forms an outer boundary of a sealed region to contain a reactant flow. The flow plate includes an opening that is located out of the sealed region to communicate leakage from the sealed region to an exhaust flow of the cell.

BACKGROUND

The invention generally relates to controlling leakage in an electrochemical cell, such as a hydrogen pump cell or a fuel cell, as examples.

A fuel cell is an electrochemical device that 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 50° Celsius (C) to 75° 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

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.

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.

In general, a fuel cell is an electrochemical cell that operates in a forward mode to produce power. However, the electrochemical cell may be operated in a reverse mode in which the cell produces hydrogen and oxygen from electricity and water. More specifically, an electrolyzer splits water into hydrogen and oxygen with the following reactions occurring at the anode and cathode, respectively:
Anode: 2H2O→O2+4H++4e−Equation 3
Cathode: 4H++4e−→2H2Equation 4

An electrochemical cell may also be operated as an electrochemical pump. For example, the electrochemical cell may be operated as a hydrogen pump, a device that produces a relatively pure hydrogen flow at a cathode exhaust of the cell relative to an incoming reformate flow that is received at an anode inlet of the cell. In general, when operated as an electrochemical pump, the cell has the same overall topology of the fuel cell. In this regard, similar to a fuel cell an electrochemical cell that operates as a hydrogen pump may contain a PEM, gas diffusion layers (GDLs) and flow plates that establish plenum passageways and flow fields for communicating reactants to the cell. However, unlike the arrangement for the fuel cell, the electrochemical pump cell receives an applied voltage, and in response to the received current, hydrogen migrates from the anode chamber of the cell to the cathode chamber of the cell to produce hydrogen gas in the cathode chamber. A hydrogen pump may contain several such cells that are arranged in a stack.

The operation of an electrochemical cell typically is more efficient when its reactant flows are pressurized. In general, higher pressures translate to higher efficiencies. However, higher pressure fuel cell systems inherently have a greater potential for leaks. Thus, as the reactant pressures increase, so does the leakage rate. In general, the leakages may pose environment and safety challenges.

Thus, there is a continuing need for better ways to control the leakage in an electrochemical cell.

SUMMARY

In an embodiment of the invention, an electrochemical cell includes a flow plate and a seal that is contact with the flow plate. The seal forms an outer boundary of a sealed region to contain a reactant flow. The flow plate includes an opening that is located out of the sealed region to communicate leakage from the sealed region to an exhaust flow of the cell.

In another embodiment of the invention, a technique that is usable with an electrochemical cell includes forming a sealed region on a flow plate to contain a reactant flow of the cell and forming an opening in the flow plate out of the sealed region. The technique includes communicating leakage from the sealed region through the opening to an exhaust flow of the cell.

Advantages and other features of the invention will become apparent from the following drawing, description and claims.

DETAILED DESCRIPTION

FIG. 1depicts an electrochemical cell stack10in accordance with some embodiments of the invention. The electrochemical cell stack10may be used to form a fuel cell stack that produces electrical power, an electrolyzer or an electrochemical pump (a hydrogen pump, for example), depending on the particular embodiment of the invention. Regardless of the configuration of the electrochemical cell stack10, the stack10generally contains electrochemical cells, each of which is formed from a repeating cell unit12.

As an example, each cell unit12may be a two plate design that includes an anode cooler plate14and a cathode cooler plate16. When used as a fuel cell stack, the anode cooler plate14has a bottom surface that contains flow channels for purposes of communicating fuel to a membrane electrode assembly (MEA) of a particular fuel cell. An oxidant flow is communicated to the other side of the MEA through flow channels that are formed in an upper surface of the adjacent cathode cooler plate16. The bottom surface of the cathode cooler plate16and the upper surface of the anode cooler plate14contain coolant flow channels for purposes of circulating a coolant through the stack10.

It is noted that the above description is for purposes of example only, as electrochemical cells may be formed from other designs, such as a three flow plate design, for example, in accordance with some embodiments of the invention. Furthermore, as another example, coolant channels may be spaced farther apart in the stack10. Thus, many variations are possible and are within the scope of the appended claims.

FIG. 2depicts a side, or surface51, of an exemplary flow plate50. It is assumed for purposes of simplifying the discussion herein that the flow plate50is used for purposes of forming a fuel cell, although the flow plate50may also be used for other purposes, such as forming an electrochemical pump cell or an electrolyzer cell, depending on the particular embodiment of the invention. The surface51is associated with a reactant flow, such as an oxidant flow or a fuel flow (hydrogen flow, for example). As a more specific example, for the case in which the flow plate50forms part of the electrochemical cell stack10(seeFIG. 1), the surface51may be the lower surface of the anode cooler plate14(and thus, be associated with a fuel flow); or, alternatively, the surface51may be the upper surface of the cathode cooler plate16(and thus, may be associated with an oxidant flow).

Referring toFIG. 2, the flow plate50includes an active area70that has flow channels (serpentine flow channels, for example, which are not shown) to communicate the reactant flow to an MEA (not shown) that resides next to the active area70between the flow plate50and the adjacent flow plate.

Inside the fuel cell stack, the reactant and coolant flows are routed between inlet and outlet plenum passageways of the stack. The input and output plenums are formed from openings of the flow plates of the stack.

For example,FIG. 2depicts openings52,54and56that form respective portions of the inlet plenum passageways of the stack. As a more specific example, the opening52may form part of the oxidant inlet plenum passageway; the opening54may form part of the coolant inlet plenum passageway, and the opening56may form part of the fuel inlet plenum passageway.FIG. 2also depicts openings58,60and62that may be associated with the outlet plenum passageways of the stack. In this regard, the opening58may form part of the fuel outlet plenum passageway; the opening60may form part of the coolant outlet plenum passageway; and the opening62may form part of the oxidant outlet plenum passageway.

It is assumed for purposes of the example that is depicted inFIG. 2that the surface51is associated with a fuel flow, a fuel flow that is routed from the fuel inlet plenum passageway of the stack, through the active area70and into the fuel outlet plenum passageway. More specifically, an incoming fuel flow enters the active area70through an opening71(called a “dive-through”) that is in communication with the opening56. The incoming fuel flows through the flow channels of the active area70and exits the active area70through another opening72, or dive-through, to enter the fuel outlet plenum passageway, which is in communication with the opening58. The openings71and72are sized to work in conjunction with the incoming fuel flow to build up a relatively high pressure in the fuel inlet plenum passageway and in the active area70. For example, this pressure may be in the range of 600 to 10,000 pounds per square inch (psi), depending on the particular embodiment of the invention. It is noted that other pressures may be used in accordance with other embodiments of the invention.

With the relatively high pressure in the active region70and inlet plenum passageway, it is possible that leakage may occur around seals that isolate the high pressure fuel flow from the remaining regions of the flow plate50. For example, as depicted inFIG. 2, the flow plate50includes a seal68that generally surrounds the opening56and a seal74that generally surrounds the active area70. The seals68and74are primary seals that are designed to define outer boundaries so that the reactant flow is contained within. However, under relatively high pressure, leakage may occur, and thus, fuel may escape outside of the primary seals68and74. It is noted that the primary seals68and74, as well as other seals that are described herein, may be planar gasket-type seals, in accordance with some embodiments of the invention, although other seals are possible and are within the scope of the appended claims.

An outer secondary seal80is located on the flow plate50surrounds the primary seals68and74for purposes of forming a backup seal for the primary seals68and74. By itself, however, the secondary seal80may be insufficient in that if not for the features that are described herein, pressure may build up from accumulated leakage from the primary seals68and74to eventually cause leakage from the secondary seal80. However, such a pressure buildup does not occur because the flow plate50includes at least one opening92, or dive-through, that is located inside the region sealed by the secondary seal80and outside of the regions sealed by the primary seals68and74. The opening92extends through the flow plate50from the upper reactant surface51to the lower surface52, which is depicted inFIG. 3.

Referring toFIG. 3in conjunction withFIG. 2, the opening92is in fluid communication with the opening58. More specifically, as shown inFIG. 3, a seal100extends around the periphery of the opening58and the opening92. Therefore, any fuel leakage flows from the upper surface51through the opening92and into the opening58via the seal provided by the seal100. As a result, any leakage is communicated into the anode exhaust stream. Thus, leakage is contained internally within the stack, pressure on the second seal80is minimized and any leakage is routed to the stack's anode exhaust flow.

Referring back toFIG. 2, in accordance with some embodiments of the invention, for purposes of enhancing the communication of leakage to the opening92, a leakage channel90is formed in the upper surface51of the flow plate50. As depicted inFIG. 2, in accordance with some embodiments of the invention, the channel90is located inside the region defined by the secondary seal80and circumscribes the region that includes the primary seals68and74. Therefore, any fuel leakage from the primary seals68and74flows into the channel90and is routed (due to the relative low pressure present at the opening92) to the opening92.

Referring toFIG. 10in conjunction withFIG. 2, for purposes of preventing any leakage from being communicated to the reactant flow of the adjacent flow plate, in accordance with some embodiments of the invention, a layer93(depicted as being transparent for purposes of simplification) extends from the active area70and outside of the secondary seal80near the perimeter of the flow plate50. As examples, the layer93may be formed from an extension of the membrane of the MEA that extends to the edges of the flow plate50, in accordance with some embodiments of the invention. In other embodiments of the invention, the layer93may be a separate layer that is formed from a plastic border, such as kynar, which extends to the edges of the flow plate50. The layer93extends beneath the primary seal80to seal any leakage inside the seal80. In some applications, the seal that is formed by the layer93may be unnecessary, and for these embodiments of the invention, the layer93is not included in the stack.

Among the other features depicted inFIG. 2, in accordance with some embodiments of the invention, primary seals68may be formed around the other plate openings52,54,60and62.

Referring toFIG. 3, similar seals100may be formed around the openings53,56,58and62. Furthermore, another seal110surrounds the coolant openings54and60and a coolant flow area112. As also shown inFIG. 3, the seal100around the opening56extends around the opening71(see alsoFIG. 2) in the active area70. Thus, incoming fuel flow is routed through the opening56into the opening71and to the active region70. As also shown inFIG. 3, the exit opening72of the active region70is surrounded by the seal100that also surrounds the opening58. Thus, the fuel flow from the active area70flows through the opening72and into the opening58to enter the fuel exhaust flow. As also shown inFIG. 3, the opening92is also contained within the same seal100for purposes of routing any leakage into the fuel exhaust flow.

The above-described flow plates and seals may be used to form an electrochemical cell stack, such as an exemplary power producing fuel cell stack151that is depicted inFIG. 4. Referring toFIG. 4, the fuel cell stack151is part of a fuel cell system150. The fuel cell stack151includes an oxidant inlet152that receives an incoming oxidant flow from an oxidant source76. The incoming oxidant flow flows through the cathode chamber of the fuel cell stack151and to an oxidant exhaust158. In the context of this application, the “cathode chamber” refers to the oxidant inlet and outlet plenum passageways as well as the oxidant flow plate channels of the fuel cell stack16. The oxidant exhaust may be routed to an oxidizer, may be routed to a reformer, may be routed back to an anode inlet154(described below), etc., depending on the particular embodiment of the invention.

The fuel cell stack151also includes an anode inlet154that receives an incoming fuel flow from a fuel source180. In accordance with some embodiments of the invention, the fuel source180may include a reformer with a compressor; may be a pressurized hydrogen tank; etc., depending on the particular embodiment of the invention. The incoming fuel flow is communicated through the anode chamber of the fuel cell stack151and exits the stack10at an anode exhaust outlet160. In the context of this application, the “anode chamber” refers to the fuel inlet and outlet plenum passageways as well as the anode flow channels of the fuel cell stack151.

As depicted inFIG. 4, in accordance with some embodiments of the invention, the anode exhaust may be routed through an oxidizer170that produces an output flow and an outlet172of the oxidizer170. As also shown inFIG. 4, the fuel cell stack151may be coupled to a coolant subsystem175that circulates a coolant through the stack151for purposes of regulating its temperature. Thus, as shown inFIG. 4, the fuel cell stack151may be used to produce electrical power (delivered at output terminals177) to an electrical load180.

The above-described leakage control may be applied to other types of electrochemical cells, other than fuel cells that produce electricity. More specifically, referring toFIG. 5, in accordance with some embodiments of the invention, the above-described leakage control may be applied to an electrochemical cell pump200. The electrochemical pump200may be formed from a stack of electrochemical cells, similar to a fuel cell stack that produces electricity. However, unlike the fuel cell stack, the electrochemical pump200receives an applied voltage current (across its stack terminals201) and produces a significantly pure fuel flow at its cathode chamber outlet206in response to an incoming reformate flow at its anode inlet202.

Similar to the flow plates described above, the stack that forms the electrochemical pump200may use dive-through openings in its flow plates for purposes of controlling the leakage from the cathode chamber, the chamber that contains the relatively pure fuel flow. This leakage may, depending on the particular embodiment of the invention, either be directed to the anode exhaust or the cathode exhaust of the electrochemical pump200.

More specifically,FIG. 6depicts a side, or surface212of an exemplary flow plate210of the electrochemical cell pump200according to an embodiment of the invention. The flow plate210may generally have the same design as the flow plate50(seeFIG. 2), with like reference numerals being used. However, the flow plates50and210differ in the following manner. In particular, because the electrochemical cell pump200does not have a cathode inlet passageway in its input plenum, the flow plate210does not include the opening52or the associated seal68. As depicted inFIG. 6, instead of having an opening92that opens into the anode exhaust flow, the flow plate210has an opening204to remove leakage that might occur from the cathode chamber. Thus, any leakage outside the primary seal74or the seal68that surrounds the opening62is routed to the opening204, an opening that is in communication with the cathode exhaust. As depicted inFIG. 6, the flow plate210may also include a groove216that extends around the seals68and74and inside the for purposes of routing leakage to the opening204.

Referring also toFIG. 7which shows the opposite side of the flow plate200, the opening204is located inside a seal100that surrounds the cathode exhaust opening62. Thus, all leakage flow occurs to the cathode exhaust.

Referring toFIG. 8, alternatively, in accordance with some embodiments of the invention, leakage from the cathode chamber may be routed to the anode exhaust. In this regard, a flow plate250has a similar design to the flow plate210(seeFIG. 6) with like reference numerals being used, except that the flow plate250includes an opening260(that replaces the opening204) that communicates leakage from the cathode chamber into the anode exhaust. More particularly, the opening260is in communication with the anode exhaust opening58. A channel261is formed in the flow plate250for purposes of routing the leakage to the opening260. As depicted inFIG. 9of the other opposite side, or surface252of the flow plate250, the opening260is in fluid communication with the anode exhaust opening because the openings58and260are located inside a seal100.