ROBUST VOLTAGE MANAGEMENT IN ELECTROCHEMICAL HYDROGEN CELLS

Abstract

Apparatus and operating methods are provided for integrated electrochemical hydrogen separation and compression systems. In one possible embodiment, an electrical potential is provided across an electrochemical cell. A portion of the potential is shunted to an electrical load when the potential is higher than a predetermined threshold. As an example, a Zener diode can be used as a suitable shunting mechanism. The invention can be used with individual cells or stacks of cells. Various methods, features and system configurations are discussed.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 USC 119(e) from U.S. Provisional Application Nos. 60/797,269, filed May 3, 2006, naming Gasda and Eisman as inventors, and titled “PASSIVE CELL PROTECTION SCHEME.” This application is hereby incorporated herein by reference in their entirety and for all purposes.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to apparatus and operating methods for electrochemical hydrogen separation and compression systems. Various methods, features and system configurations are discussed.

BACKGROUND

Electrochemical technologies are of increasing interest, due in part to advantages provided in efficiency and environmental impact over traditional mechanical and combustion based technologies.

A variety of electrochemical fuel cell technologies are known, wherein electrical power is produced by reacting a fuel such as hydrogen in an electrochemical cell to produce a flow of electrons across the cell, thus providing an electrical current. For example, in fuel cells utilizing proton exchange membrane technology, an electrically non-conducting proton exchange membrane is typically sandwiched between two catalyzed electrodes. One of the electrodes, typically referred to as the anode, is contacted with hydrogen. The catalyst at the anode serves to divide the hydrogen molecules into their respective protons and electrons. Each hydrogen molecule produces two protons which pass through the membrane to the other electrode, typically referred to as the cathode. The protons at the cathode react with oxygen to form water, and the residual electrons at the anode travel through an electrically conductive path around the membrane to produce an electrical current from anode to cathode. The technology is closely analogous to conventional battery technology.

Electrochemical cells can also be used to selectively transfer (or “pump”) hydrogen from one side of the cell to another. For example, in a cell utilizing a proton exchange membrane, the membrane is sandwiched between a first electrode (anode) and a second electrode (cathode), a gas containing hydrogen is placed at the first electrode, and an electric potential is placed between the first and second electrodes, the potential at the first electrode with respect to ground (or “zero”) being greater than the potential at the second electrode with respect to ground. Each hydrogen molecule reacted at the first electrode produces two protons which pass through the membrane to the second electrode of the cell, where they are rejoined by two electrons to form a hydrogen molecule (sometimes referred to as “evolving hydrogen” at the electrode).

Electrochemical cells used in this manner are sometimes referred to as hydrogen pumps. In addition to providing controlled transfer of hydrogen across the cell, hydrogen pumps can also be used to separate hydrogen from gas mixtures containing other components. Where the hydrogen is pumped into a confined space, such cells can be used to compress the hydrogen, at very high pressures in some cases.

There is a continuing need for apparatus, methods and applications relating to electrochemical cells.

SUMMARY OF THE INVENTION

Apparatus and operating methods are provided for integrated electrochemical hydrogen separation and compression systems. In one possible embodiment, an electrical potential is provided across an electrochemical cell. A portion of the potential is shunted to an electrical load when the potential is higher than a predetermined threshold. As an example, a Zener diode can be used as a suitable shunting mechanism. The invention can be used with individual cells or stacks of cells. Numerous optional features and system configurations are provided.

Various aspects and features of the invention will be apparent from the following Detailed Description and from the claims.

DETAILED DESCRIPTION OF THE INVENTION

It will be appreciated that the apparatus, methods, and applications of the invention can include any of the features described herein, either alone or in combination.

Electrochemical cells such as fuel cells, hydrogen pumping cells, electrolyzers, etc., often use carbon-supported precious metal catalysts at the electrodes, particularly the anode. The carbon support generally provides the advantage of lowering the catalyst loading required for satisfactory performance of the cell. However, the carbon support also provides a disadvantage in that it can be oxidized under various conditions that can be periodically encountered during cell operation, thereby degrading cell performance, generally on a permanent basis.

For example, under a condition where electrical current is supplied across a cell and insufficient hydrogen is present at the anode (sometimes referred to as a “fuel starvation” condition), the electrical potential across the cell can thereby increase to a point where oxidation of the electrode occurs. The electrical potential at which such oxidation can occur will depend on the configuration of a given cell, but it typically occurs over a threshold of 0.6 volts or 0.8 volts, as examples.

It is thus desirable to limit the potential that can be placed across a given cell. The present invention is generally discussed with respect to individual cells, but it will be appreciated that the invention is also applicable to stacks of cells. For example, an individual cell can form part of a stack of cells. As well known in the art, stacks of cells are generally configured such that they are connected in electrical series, though other configurations are possible. The flow field plates of cells in a stack are generally electrically conductive and stacked adjacently to each other in a bi-polar relationship such that the anode of one cell is adjacent to the cathode of another cell, and so forth.

Sophisticated control schemes are often used to monitor the voltage of each cell in a stack, and to remove or reduce the potential across the cell if it becomes too high. As an example, a cell can be shorted with a load contactor, or can be disconnected from a power supply (as in the case of hydrogen pumping), or an electrical load on the cell can be removed (as in the case of fuel cells). Such voltage scanning systems can be expensive, cumbersome and unreliable. They can also be undesirably slow to react. Another disadvantage of scanning configurations for stacks can be that a single cell failure can render the entire stack inoperable if the cell is unable to be isolated from the stack. Under the present invention, individual cells can be bypassed.

Under one possible embodiment of the present invention, a method is provided for managing voltage across an electrochemical cell, comprising at least the following steps: providing an electrical potential across an electrochemical cell; and shunting a portion of the potential to an electrical load when the potential is higher than a predetermined threshold.

As previously discussed, the electrochemical cell can be any type of electrochemical cell, such as a hydrogen pumping cell, a fuel cell, an electrolyzer, etc. In the case of hydrogen pumping cells, operation of a cell under such methods can be further characterized wherein the cell has an anode in contact with hydrogen, and a cathode in contact with hydrogen.

In the context of the present invention, “shunting” refers to any means of reducing electrical potential, as is the case where electrical charge or current is drained or diverted from a circuit such as the electrodes of an electrochemical cell. For example, a cell can be connected to an electrical load that will absorb current flow between the electrodes thereby reducing the electrical potential between them. “Electrical load” refers to any means of absorbing electrical current or charge. For example, this can refer to simply grounding or shorting a cell. It can also refer to a circuit capable of storing electrical charge, such as a capacitor or battery. It can also refer to other electrochemical cells, including those adjacent to the shunted cell in a stack configuration. For example, as discussed below, for an overly high potential cell in a stack, current can be shunted around the cell to the adjacent cells in the stack.

In one possible embodiment, a diode can be used as a suitable shunting mechanism. In this context, a diode is a component that restricts the direction of flow of electrical charge carriers. It allows an electric current to flow in one direction, but blocks it in the opposite direction. Thus, the diode can be thought of as an electronic version of a check valve. A diode such as a Zener diode can be selected that has a breakdown voltage that allows reverse flow if the potential exceeds a particular threshold. The diode can be wired in parallel with the cell such that, while the voltage across the cell is below the breakdown voltage of the diode, current is not allowed to pass through the diode, and so it passes through the cell instead. However, when the cell potential exceeds the breakdown voltage of the diode, current will begin to flow through the diode, and the cell potential will be thereby reduced until it is at or below the breakdown voltage of the diode. The current flowing through the diode can be configured to simply bypass the cell, or to flow to ground or any other electrical load. A circuit configured to flow electrical current to ground can be referred to as an electrical grounding circuit.

In such a configuration, if it is desired to limit the cell to below 0.6 volts, as an example, a diode can be selected having a breakdown voltage of 0.6 volts. As a further example, if there are two cells in series and it is desired to keep the potential across each below 0.6 volts, a diode having a breakdown voltage of 1.2 volts can be placed across both cells in parallel with the cells. Diodes can thus be used in this manner to provide protection for individual cells, or groups of cells.

In another possible embodiment, the invention provides a method of managing voltage across an electrochemical cell, comprising at least the following steps, which can be selectively paired with any of the features described above, alone or in combination: providing an electrical potential across a first electrochemical cell and a second electrochemical cell, wherein the first cell and second cell are electrically connected in series; connecting a diode in an electrically parallel configuration with the first and second cells; and shunting a portion of the potential through the diode to an electrical load when the potential is higher than a predetermined threshold.

In another possible embodiment, the invention provides a voltage management system for electrochemical cells, comprising an electrochemical cell; a power supply adapted to provide an electrical potential across the electrochemical cell; and a shunting mechanism adapted to shunt a portion of the potential to an electrical load when the potential is higher than a predetermined threshold. Any of the features described herein can be used with such a system, either alone or in combination.

As previously discussed, the shunting mechanism can be any means of reducing electrical potential across the cell. As an example, a diode can be used as discussed above. As additional examples, the shunting mechanism can include a physical switch or electrical switch wired to divert current from the cell. Suitable electrical switches can include transistors such as field effect transistors, MOSFETs, etc. The system can include a controller adapted to actuate such a switch on receipt of a control signal. For example, the controller can measure the potential across a cell and actuate a switch to short or isolate the cell if the potential across the cell exceeds a predetermined threshold.

As another possible example, a diode can be configured as described above, but to where reverse current across the diode is communicated to a controller, indicating an over-voltage condition (i.e., the potential across the cell is higher than a predetermined threshold).

In another possible embodiment, the electrical load can include a circuit configured to indicate any over-voltage condition occurring at a cell or group of cells. For example, a diode can be configured as described above, but to where reverse current across the diode powers a condition indicator, such as a light emitting device.

It will be appreciated that many other configurations are possible.

The inventive concepts discussed in the claims build on traditional electrochemical cells technologies that are well known in the art. As examples, various suitable designs and operating methods that can be used as a base to implement the present invention are described in the teachings of U.S. Pat. Nos. 4,620,914; 6,280,865; 7,132,182 and published U.S. patent application Ser. Nos. 10/478,852, 11/696,179, 11/737,730, 11/737,733, and 11/737,737 which are each hereby incorporated by reference in their entirety.

While the invention has been shown or described in only some of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention.

Claims

1. A method of managing voltage across an electrochemical cell, comprising:

providing an electrical potential across an electrochemical cell; and

shunting a portion of the potential to an electrical load when the potential is higher than a predetermined threshold.

2. The method of claim 1, wherein the cell is a hydrogen pumping cell.

3. The method of claim 1, wherein the cell has an anode in contact with hydrogen, and a cathode in contact with hydrogen.

4. The method of claim 1, wherein the potential is shunted to the electrical load through a diode.

5. The method of claim 1, wherein the potential is shunted to the electrical load through a Zener diode.

6. The method of claim 1, wherein the electrical load is an electrical grounding circuit.

7. The method of claim 1, wherein the electrical load is an second electrochemical cell.

8. The method of claim 1, wherein the predetermined threshold is 0.6 volts.

9. The method of claim 1, wherein the predetermined threshold is 0.8 volts.

10. The method of claim 1, wherein the cell forms part of a stack of cells electrically connected in series.

11. A method of managing voltage across an electrochemical cell, comprising:

providing an electrical potential across a first electrochemical cell and a second electrochemical cell, wherein the first cell and second cell are electrically connected in series;

connecting a diode in an electrically parallel configuration with the first and second cells; and

shunting a portion of the potential through the diode to an electrical load when the potential is higher than a predetermined threshold.

12. The method of claim 11, wherein the cell is a hydrogen pumping cell.

13. The method of claim 11, wherein the cell has an anode in contact with hydrogen, and a cathode in contact with hydrogen.

14. The method of claim 11, wherein the diode is a Zener diode.

15. The method of claim 11, wherein the electrical load is an electrical grounding circuit.

16. The method of claim 11, wherein the electrical load is an second electrochemical cell.

17. The method of claim 11, wherein the predetermined threshold is 0.6 volts.

18. The method of claim 11, wherein the predetermined threshold is 0.8 volts.

19. The method of claim 11, wherein the cell forms part of a stack of cells electrically connected in series.

20. A voltage management system for electrochemical cells, comprising:

an electrochemical cell;

a power supply adapted to provide an electrical potential across the electrochemical cell; and

a shunting mechanism adapted to shunt a portion of the potential to an electrical load when the potential is higher than a predetermined threshold.

21. The system of claim 20, wherein the cell is a hydrogen pumping cell.

22. The system of claim 20, wherein the cell has an anode in contact with hydrogen, and a cathode in contact with hydrogen.

23. The system of claim 20, wherein the shunting mechanism is a diode that is electrically connected in parallel to the cell.

24. The system of claim 20, wherein the shunting mechanism is a Zener diode that is electrically connected in parallel to the cell.

25. The system of claim 20, wherein the electrical load is an electrical grounding circuit.

26. The system of claim 20, wherein the electrical load is an second electrochemical cell.

27. The system of claim 20, wherein the predetermined threshold is 0.6 volts.

28. The system of claim 20, wherein the predetermined threshold is 0.8 volts.

29. The system of claim 20, wherein the power supply has a potential limit that is higher than the predetermined threshold.

30. The system of claim 20, wherein the cell forms part of a stack of cells electrically connected in series.