WO2001028022A1

WO2001028022A1 - Method and apparatus for managing hydration level of fuel cell electrolyte

Info

Publication number: WO2001028022A1
Application number: PCT/US2000/027472
Authority: WO
Inventors: Ronald J. Kelley; Steven D. Pratt; Sivakumar Muthuswamy; Robert W. Pennisi
Original assignee: Motorola Inc.
Priority date: 1999-10-14
Filing date: 2000-10-05
Publication date: 2001-04-19

Abstract

A method and apparatus for managing the hydration level of the electrolyte in a fuel cell system (100) using a humidification means (150). The method of hydrating the electrolyte involves, measuring hydration parameters (110) of the electrolyte, comparing the hydration parameters (120) against target values, selecting a control method (130) from a set of available control methods based on the result of comparison and using that control method to initiate and control a hydration cycle, and actuating (140) a humidification means using the selected control method so as alter the measured hydration parameters (160).

Description

METHOD AND APPARATUS FOR MANAGING HYDRATION LEVEL OF FUEL CELL ELECTROLYTE

TECHNICAL FIELD This invention relates to fuel cells in general and a method of managing the hydration level of electrolyte in a fuel cell in particular.

BACKGROUND

Fuel cells are electrochemical cells in which a free energy change resulting from a fuel oxidation reaction is converted into electrical energy. A typical fuel cell consists of a fuel electrode (anode) and an oxidant electrode (cathode), separated by an ion-conducting electrolyte. The electrodes are connected electrically to a load (such as an electronic circuit) by an external circuit conductor. In the circuit conductor, electric current is transported by the flow of electrons, whereas in the electrolyte it is transported by the flow of ions, such as the hydrogen ion (H+) in acid electrolytes, or the hydroxyl ion (OH-) in alkaline electrolytes. In theory, any substance capable of chemical oxidation that can be supplied continuously (as a gas or fluid) can be oxidized galvanically as the fuel at the anode of a fuel cell. Similarly, the oxidant can be any material that can be reduced at a sufficient rate. Gaseous hydrogen has become the fuel of choice for most applications, because of its high reactivity in the presence of suitable catalysts and because of its high energy density. Similarly, at the fuel cell cathode the most common oxidant is gaseous oxygen, which is readily and economically available from the air for fuel cells used in terrestrial applications. When gaseous hydrogen and oxygen are used as fuel and oxidant, the electrodes are porous to permit the gas-electrolyte junction to be as great as possible. The electrodes must be electronic conductors, and possess the appropriate reactivity to give significant reaction rates. At the anode, incoming hydrogen gas ionizes to produce hydrogen ions and electrons. Since the electrolyte is a non-electronic conductor, the electrons flow away from the anode via the metallic external circuit. At the cathode, oxygen gas reacts with the hydrogen ions migrating through the electrolyte and the incoming electrons from the external circuit to produce water as a byproduct. The byproduct water is typically extracted as vapor. The overall reaction that takes place in the fuel cell is the sum of the anode and cathode reactions, with part of the free energy of reaction released directly as electrical energy. The difference between this available free energy and the heat of reaction is produced as heat at the temperature of the fuel cell. It can be seen that as long as hydrogen and oxygen are fed to the fuel cell, the flow of electric current will be sustained by electronic flow in the external circuit and ionic flow in the electrolyte.

In practice, a number of these unit fuel cells are normally stacked or 'ganged' together to form a fuel cell assembly. A number of individual cells are electrically connected in series by abutting the anode current collector of one cell with the cathode current collector of its nearest neighbor in the stack. Fuel and oxidant are introduced through manifolds into respective chambers. An alternate style of fuel cell has been recently proposed (U.S. Patent 5,783,324) which is a side-by-side configuration in which a number of individual cells are placed next to each other in a planar arrangement. This is an elegant solution to the problem of gas transport and mechanical hardware.

Water balance in the electrolyte during operation represents a particular problem in the case of the fuel cells. The operability of the fuel cell is closely linked to the water content in the fuel cell and, in particular, in the electrolyte. An excessively high water content in the electrolyte leads to the available power from the fuel cell being reduced, as a result of the excessively high dilution of the electrolyte. An excessively low water content of the electrolyte likewise leads to the electrical power from the fuel cell being reduced as a result of the increase in the internal resistance.

Fuel cells used in portable power applications typically use a Perfluorosulfonic ion exchange membrane, such as those sold by DuPont under its Nafion trade designation. The ion conductivity through these ion exchange membranes generally requires the presence of water molecules between the surfaces of the membrane. As ions are transported through such perfluorosulfonic membranes, water molecules associated with those ions are also transported. This phenomenon is sometimes referred to as "water pumping" and results in a net flow of water from the anode side of the membrane to the cathode side. Thus, membranes exhibiting the water pumping phenomenon can dry out on the anode side if water transported along with hydrogen ions (protons) is not replenished.

The fuel and oxidant gases are therefore humidified prior to introducing them to the fuel cell to maintain the saturation of the membrane . Ordinarily, the fuel and oxidant gases are humidified by flowing each gas on one side of a water vapor exchange membrane and by flowing deionized water on the opposite side of the membrane. Deionized water is preferred to prevent membrane contamination by undesired ions. In such membrane-based humidification arrangements, water is transferred across the membrane to the fuel and oxidant gases. Other non-membrane based humidification techniques could also be employed, such as exposing the gases directly to water in an evaporation chamber to permit the gas to absorb evaporated water.

It is generally preferred to humidify the fuel and oxidant gases at, or as close as possible to, the operating temperature and pressure of the fuel cell. The ability of gases such as air to absorb water vapor varies significantly with changes in temperature, especially at low operating pressures. Humidification of the air (oxidant) stream at a temperature significantly below fuel cell operating temperature could result in a humidity level sufficiently low to dehydrate the membrane when the stream is introduced to the cell. Consequently, it is preferable to integrate the humidification function with the active section of the fuel cell stack, and to condition the fuel and oxidant streams to nearly the same temperature and pressure as the active section of the stack. In such an integrated arrangement, the coolant water stream from the active section, which is at or near the cell operating temperature, is normally used as the humidification water stream. One of the prior art schemes for humidifying the oxidant or the fuel stream is to pass them through a heated bottle of water. The degree of humidification is determined by the temperature of the water. The hotter the water, the more water vapor is introduced into the gas stream. In order for the water vapor to reach the fuel cell, the entire gas stream must be kept at the temperature of the water bottle or the vapor will condense out of the air onto the surface of the tubes that transfer the gas to the fuel cell. Thus, the gas stream going into the fuel cell must be maintained at an elevated temperature (usually around 60 degree C). The fuel cell is heated by the incoming gas stream. Heating the fuel cell hinders the humidification of the membrane. Raising the temperature of the cell tends to dehydrate the membrane. In addition, if the fuel cell is not heated, the water vapors tend to condense in the gas channels and gas diffusion layers in the fuel cell. As a result, portions of the cell tend to flood while other portions tend to dry out. Alternatively, the entire fuel cell can be heated so that no moisture condenses in the cell, but the elevated temperature reduces the rate of membrane hydration.

In the prior art (see, for example, U.S. Patents 5,543,238), some of these issues are addressed by partially recirculating the exhaust gas which occurs on the cathode side of the fuel cell.

Although prior art techniques successfully keep the ion exchange membrane electrolyte sufficiently hydrated, they do so at the expense of overall fuel cell performance. It would therefore be an advancement in the art of fuel cell systems to have a electrolyte hydration system that uses ambient temperature water vapor and thus obviates the need for heating the fuel cell, the water vaporizer or both.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a process flow diagram in accordance with the first embodiment of the hydration management method of the present invention. FIG. 2 is a process flow diagram in accordance with the second embodiment of the hydration management method of the present invention.

FIG. 3 is a schematic representation of an apparatus to implement the hydration management method in accordance with the present invention.

FIG. 4 is a schematic representation of a second embodiment of the apparatus to implement the hydration management method in accordance with the present invention.

FIG. 5 is a schematic representation of a third embodiment of the apparatus to implement the hydration management method in accordance with the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A method and apparatus for managing the hydration level of electrolyte in a fuel cell using a humidification means is disclosed. The method of hydrating the electrolyte involves, measuring hydration parameters of the electrolyte, comparing the hydration parameters against target values, selecting a control method from a set of available control methods based on the result of comparison and using that control method to initiate and control a hydration cycle, and actuating a humidification means using the selected control method so as alter the measured hydration parameters.

FIG. 1 shows a typical flow chart of the process used to manage the hydration level of electrolyte in a fuel cell according to a first embodiment of the present invention where the rectangular boxes represent structural entities in the process, and boxes with rounded corners represent process steps to achieve the various structural entities. Referring now to FIG. 1, the hydration management process starts with measuring hydration parameters 110 of a fuel cell system 100. Typical hydration parameters measured are temperature, internal resistance, dielectric properties, thickness, acoustical attenuation properties and optical properties of the electrolyte. The hydration parameters are then compared 120 to a predetermined set of ideal target values designed to provide peak electrolyte performance. The goal of this comparison is to assess how close the performance of the electrolyte under observation is to an ideal or "optimized" fuel cell electrolyte.

Although the preferred embodiment has listed some of the more commonly used operational and performance parameters of a fuel cell system, the present invention is not necessarily limited by the use of these parameters. Any set of measurable or computable hydration parameters can be used within the structure described in the preferred embodiment. Following the comparison step 120, a control method is selected 130 from a list of available control methods. The control method has the necessary parameters and logic to define a humidification initiation process 140 which in turn actuates a humidification means 150. Some of the key parameters defined in the control method include size of vaporized water particles, quantity of water vaporized per unit time, rate of vaporization, length of hydration cycles, frequency of hydration cycles, and target fuel cell sides on which to initiate the hydration cycle. The humidification means is disposed in the fuel stream, oxidant stream or both streams of the fuel cell. Alternately, the humidification means can be disposed on the anode side, the cathode side or both sides of the fuel cell. Actuation of the humidification means hydrates the electrolyte and alters its hydration parameters 160. The humidification of fuel and/or oxidant stream delivers moisture to the anode and/or cathode sides of the fuel cell. The added moisture diffuses to the electrolyte surfaces and adds water to the electrolyte. This additional water assists in maintaining the water balance of the electrolyte by replenishing water that was pumped across the membrane from the anode side and water that was removed by the oxidant flow on the cathode side. In addition, the humidification step cools the surfaces of the fuel cell, thus reducing the rate of evaporation of water from the membrane. The preferred humidification means is one or more piezoelectric elements vibrating an inert membrane, which is just below the surface of the water, at ultrasonic frequency range. The ultrasonic vibration imparts enough mechanical energy into the water so as to vaporize it. A unique feature of the humidification means used in the preferred embodiment is that it generates water vapor that has substantial water paniculate content as opposed to water molecules. Prior art humidification schemes using evaporation of water, such as passing fuel/oxidant through a hot water bottle, produced water vapor substantially made up of water molecules. In published literature, the term vapor is defined in different ways and sometimes used interchangeably with the term gas. The reader should understand that the term "water vapor" is used in this disclosure according to the following definition : "Water vapor is a mixture of water in particulate and molecular forms with particulate form comprising a substantial portion of the mixture." Some alternate humidification means that will produce water vapor according the present definition are mechanical agitation humidifiers, atomizers, sprayers and water vapor pressure lowering humidifiers. The various humidification means referred to above are described and known in the literature, and since one of ordinary skill in the art is assumed to be familiar with these, they will not be further elaborated upon here. In addition to the humidification means described above, many other humidification means can be used to implement this fuel cell electrolyte hydration management method without deviating from the spirit of the invention. The second embodiment of the performance management method shown in FIG. 2 uses a closed-loop configuration with a feedback loop 170, wherein the change in hydration parameters as a result of actuation of humidification means is fed back to the step of selecting the control method. The parameters and logic of the selected control method are fine-tuned based on the feedback information. This updated control method is used to update the actuation process which in turn actuates the humidification means for a second time. This feedback and update process is repeated as necessary.

FIG. 3 shows a schematic view of an apparatus for implementing the fuel cell electrolyte hydration management method. The apparatus 200 consists of a plurality of fuel cells, each fuel cell 210 having a membrane electrode assembly 220 having two opposing major sides. Each of the membrane electrode assemblies comprises a solid electrolyte 230 disposed between and in intimate contact with an anode 240 and a cathode 250. The fuel cell also has a humidification means 260 which provides water vapor at the temperature and pressure of the surrounding environment to at least one portion of the fuel cell. The water vapor hydrates the electrolyte 230 and alters the hydration parameters of the membrane.

FIG. 4 shows a schematic view of an alternate embodiment of an apparatus for implementing the fuel cell electrolyte hydration level management method. The apparatus 300 consists of a plurality of fuel cells 310 with one or more humidification means 320 disposed on the fuel and/or oxidant entry point. The humidification means 320 imparts water vapor to the fuel and/or oxidant streams. When the humidified fuel or oxidant passes over the electrolyte membrane the moisture is transferred to the electrolyte to keep it hydrated. A schematic view of a yet another embodiment of an apparatus for implementing the fuel cell electrolyte hydration level management method is depicted in FIG 5. This embodiment is well suited for hydrating the electrolyte of a fuel cell that has not been operational for a significant period of time. In this configuration the entire fuel cell and the humidification means are disposed inside a chamber. The apparatus 400 consists of a chamber 410 with one or more humidification means 440 disposed along with the fuel cell 430 inside the chamber. The humidification means 440 imparts water vapor to the electrolyte in the fuel cell. Depending on the level of hydration desired, the length of residence of the fuel cell in the chamber, and various other parameters of the humidifier such as size of vaporized water particles, quantity of water vaporized per unit time, rate of vaporization, length of hydration cycles, and frequency of hydration cycles are adjusted. The present invention enhances the performance of a fuel cell by properly managing the hydration level of the electrolyte. It achieves these results by using water vapor generated at the temperature and pressure of the surrounding environment (ambient conditions). This method of using ambient condition water vapor eliminates the problems encounter with the prior art evaporation schemes such as water condensation, electrolyte flooding and electrolyte dry-out. Use of water vapor consisting primarily of particulate form of water makes it possible to precisely control the level of hydration of the electrolyte. Thus the present invention provides a method and an apparatus for managing the performance of a fuel cell which overcomes the disadvantages of the prior-art methods and devices of this general type. This unique hydration management method is simple to implement and control.

While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not so limited, and other equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined by the appended claims. What is claimed is:

Claims

1. A method of managing hydration level of electrolyte in a fuel cell having an anode side and a cathode side, comprising: measuring hydration parameters of the electrolyte in the fuel cell; comparing the measured hydration parameters of the electrolyte against target values to produce a result; selecting at least one control method from a plurality of control methods based on the value of said result; initiating a hydration cycle to actuate a humidification means using the selected control method so as alter the hydration level of the electrolyte; and wherein the humidification means generates water vapor at the temperature and pressure of the surrounding environment.

2. The method of claim 1, wherein the control method comprises size of vaporized water particles, quantity of water vaporized per unit time, rate of vaporization, length of hydration cycles, frequency of hydration cycles, and target fuel cell sides on which to initiate the hydration cycle.

3. The method of claim 1, wherein the humidification means is selected from the group consisting of ultrasonic humidifiers, mechanical agitation humidifiers, atomizers, sprayers and water vapor pressure lowering humidifiers.

4. The method of claim 1, wherein the measured hydration parameters are selected from the group consisting of temperature, internal resistance, dielectric properties, thickness, acoustical attenuation properties and optical properties of the electrolyte.

5. The method of claim 1, wherein the humidification means is disposed in fuel stream, oxidant stream or both streams of the fuel cell.

6. The method of claim 1, wherein the humidification means is disposed on the anode side, the cathode side or both sides of the fuel cell.

7. A method of managing hydration level of electrolyte in a fuel cell having an anode side and a cathode side, comprising: measuring hydration parameters of the electrolyte selected from the group consisting of temperature, internal resistance, dielectric properties, thickness, 5 acoustical attenuation properties and optical properties of the electrolyte; comparing the measured hydration parameters of the electrolyte against target values to produce a result; selecting at least one control method from a plurality of control methods based on the value of said result; and 10 initiating a hydration cycle to actuate a humidification means selected from the group consisting of ultrasonic humidifiers, mechanical agitation humidifiers, atomizers, sprayers and water vapor pressure lowering humidifiers, using the selected control method so as alter the hydration level of the electrolyte; wherein the humidification means generates water vapor at temperature and 15 pressure of the surrounding environment.

8. A method for hydrating electrolyte of a fuel cell having an anode side and a cathode side, comprising selecting and using a control method, initiating a hydration cycle to actuate a humidification means using the selected control method, sufficient to alter hydration level of the electrolyte in the fuel cell.

9. A method of managing hydration level of electrolyte in a fuel cell having an anode side and a cathode side, comprising the following steps in the order named: a) measuring hydration parameters of the electrolyte in the fuel cell; b) comparing the measured hydration parameters of the electrolyte against target values to produce a result; c) selecting at least one control method from a plurality of control methods based on the value of said result; and d) initiating a hydration cycle to actuate a humidification means using the selected control method so as alter the hydration level of the electrolyte; wherein the humidification means generates water vapor at temperature and pressure of the surrounding environment.

10. A fuel cell system, comprising: at least one fuel cell; each fuel cell further comprising at least one membrane electrode assembly, having two major sides, comprising a solid electrolyte with two major sides disposed between and in intimate contact with an anode forming first major side of the membrane electrode assembly and a cathode forming second major side of the membrane electrode assembly ; and an humidification means to provide humidification to at least one portion of the fuel cell system; wherein operation of the humidification means causes hydration of at least one portion of one major side of the electrolyte.