WO2000065676A1 - Freeze tolerant fuel cell system and method - Google Patents

Abstract

A freeze tolerant fuel cell system and method of operating the freeze tolerant fuel cell system is disclosed. The freeze tolerant fuel cell system is realized by separating the coolant loop [25] from the active membrane [12] through the use of gaskets [20, 21] interposed between the collector cell plates [18, 19]. A method of operating the freeze tolerant fuel cell system is disclosed which comprises flowing a coolant fluid other than pure water having a sufficiently low freezing point through the coolant loop [25]. A method for startup and shutdown of the freeze tolerant fuel cell system is also disclosed.

Description

FREEZE TOLERANT FUEL CELL SYSTEM AND METHOD

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to and claims the benefit from U.S. Provisional Application 60/130,801 entitled "FUEL CELL AND SYSTEM WITH FREEZE TOLERANT COOLANTS" filed April 23, 1999, the entirety of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention The present invention relates to fuel cell systems and more particularly to

cooling systems for fuel cell systems.

2. Description of the Relevant Art

A fuel cell is a device that generates electrical energy by converting chemical

energy directly into electrical energy by an electrochemical reaction of fuel and

oxygen supplied to the cell. A typical fuel cell includes a casing which houses an

anode, a cathode and an electrolyte membrane. The electrolyte membrane is

disposed between the anode and cathode. A catalyst layer is disposed on the

electrolyte-facing surface of each electrode. Suitable catalysts include nickel, silver,

platinum and, in the case of the stabilized zirconium oxide electrolyte, base metal

oxides. Platinum is most commonly used. Appropriate fuel material and oxidant are

supplied respectively to the anodes and cathodes, the fuel and oxidant

electrochemically react to generate an electric current, and the reaction end product

is withdrawn from the cell. A relatively simple type of fuel cell (commonly called a PEM fuel cell) uses hydrogen and oxygen as the fuel and oxidant materials,

respectively. Hydrogen combines with oxygen to form water while at the same time

generating an electrical current and heat. More specifically, hydrogen is consumed

at the fuel cell anode releasing protons and electrons as shown in equation (1 )

below.

(1 ) H₂ — >2H+ +2e^' Anode Reaction

Protons produced are drawn into the fuel cell electrolyte. The electrons produced

travel from the fuel cell anode to the anode terminal, through an electrical load, back

to the cathode terminal, and into the cathode of the cell. A flow of ions through the

electrolyte completes the circuit. Chemical reaction rates vary with location on the

electrode and are dependent upon such local factors as reactant and product

concentrations and temperature. At the cathode oxygen, along with electrons from

the load and protons from the electrolyte combine to form water as shown in

equation (2) below.

(2) ¹/₂ 0₂ +2H+ +2e-> H₂0 Cathode Reaction

The main advantage of a fuel cell is that it converts chemical energy directly

to electrical energy without the necessity of undergoing any intermediate steps, for

example, combustion of a hydrocarbon or carbon based fuel as in a conventional

thermal power station. Fuel cells can be classified into several types according to

the electrolyte used. Relatively high performance fuel cells use electrolytes such as

aqueous potassium hydroxide, concentrated phosphoric acid, fused alkali carbonate

and stabilized zirconium oxide.

Since individual fuel cells may produce less than the desired voltage for a given application at full load, practical fuel cells stack several individual fuel cells in

series to attain the desired voltage level by electrically connecting the cathode of

one cell to the anode of an adjacent cell. Consequently, fuel cell stacks are

commonly used.

The overall reaction in the cell (i.e. formation of water) is highly exothermic.

The rate of heat generation is dependent upon the reaction rate and the heat flux

across a given area of the fuel cell and is proportional to the local reaction rate.

Consequently, a structure for cooling a fuel cell is generally required and is generally

designed based on the projected peak heat flux. Water is generally used for cooling fuel cells.

PEM fuel cells generally require humidification to maintain the moisture of the

electrolyte membrane which is required for efficient fuel cell operation. A single

water loop generally provides both humidification and cooling for fuel cells.

In some fuel cell applications, such as automotive applications, it may be

necessary to commence operation of a fuel cell stack having a core temperature

below the freezing temperature of water. For example, the SAE automotive

standard requires operation between -40°C and 53°C and survival (storage) of

between -46°C to 66°C. Numerous difficulties are encountered in attempting to

operate a fuel cell in applications below the freezing point of water. Water is known

to expand significantly upon freezing. Ice formation inside the fuel cell system may

destroy fuel cell components. Even if component damage does not occur upon

freezing, blockage of fuel system lines may occur which can delay the startup of the

fuel cell. Finally, for sub-freezing applications, coolants must be selected that freeze at temperatures below the freezing point of water. Most available coolants which

have freezing points below the freezing point of water are known to "poison" the

catalyst layer by binding to catalyst sites if such materials are allowed to come in

contact with the catalyst layer. What is needed is a freeze tolerant fuel cell design

and methodology that permits reliable fuel cell operation at temperatures well below

the freezing temperature of water.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a fuel cell system that

can be used in sub-freezing environments.

It is another object of the invention to provide a fuel cell system that can

utilize coolants other than a coolant compatible with the MEA.

It is yet another object of the invention to provide techniques for removing

water from the fuel cell on shut down and rapidly reheating the fuel cell upon start

up.

These and other objects of the invention are achieved by a number of

embodiments incorporating features and advantages of the invention. The invention

can include a freeze tolerant fuel cell system including at least one fuel cell made up

of a pair of collector plates having a series of channels for the flow of reactants. A

first and a second gas diffusion layer is disposed between said collector plates; and

a membrane electrode assembly (MEA) including a membrane sandwiched between

two electrode layers. The MEA is interposed between said gas diffusion layers, and

the reactant's on each side of the MEA are substantially sealed from leaking to the other side of the MEA. The fuel cell stack can further include at least one coolant

passage for flowing a coolant stream relative to the collector plates so that the

coolant stream does not contact said MEA while cooling the fuel cell.

According to another aspect of the invention, the coolant flowing in said

coolant passage is poisonous to the MEA and thus must be mechanically isolated

from the MEA. The surfaces of the coolant passage in contact with the coolant can

be electrically insulated or the coolant can be electrically non-conductive. The

isolation can be provided by a gasket arrangement around coolant ports running

through the collector plates. The gaskets are preferably spaced from the gas

diffusion layers. Alternatively, the cooling passage can be positioned outside the

conductive portion of the collector plates and surrounded by housing. This housing

can be molded integrally with the collector plates or otherwise connected to the

collector plates.

The fuel cell coolant loop can include edge cooling within the fuel cell, or a

coolant layer outside said collector plates can be provided. This coolant layer can

be directly adjacent each active fuel cell or provided at intermittent stages

throughout a fuel cell stack.

According to another aspect of the invention, the interface region between the

peripheral port gaskets and the active area membrane can be supported by bridging

sub-gaskets to avoid damaging mechanical and electrical edge effects in the

membrane. In preferred embodiments, the sub gaskets extend across the

uncovered membrane region between the port gaskets and the gas diffusion layers.

The sub-gaskets can be made of a number of materials, including FEP, TFE, ETFE, PFA, CTFE, E-CTFE, PVF2 and PVF.

To further equip a fuel cell system for use in sub-freezing environments, the

invention further contemplates improvements and techniques relating to shut down

and start up to avoid the presence of significant quantities of water in the fuel cell

when the fuel cell temperature falls below the freezing point of water, such as when

the fuel cell system is not operating.

During fuel cell operation, water can accumulate in the gas diffusion layers as

well as the flow channels of the collector plate. According to one aspect of the

invention, the reactant channels in the collector plates are discontinuous, whereby

flow fields are established through the gas diffusion layers. With this arrangement,

as part of a freeze tolerant shut down operation, purging dry gases can be forced

through the fuel cell, collecting and driving water not only out of the channels but

also the gas diffusion layers.

To further reduce the accumulation of residual water after shut down, the

surfaces of the channels are preferably essentially impermeable to water. Further,

the surfaces of the channels can be essentially impermeable to all fluids.

To further assist in the effective removal of water during shut-down, a fuel cell

system according to the invention can provide counterflow in the gas diffusion layers

on either side of the membrane to increase the moisture gradient and rate of water

transfer. The reactant channels of each collector plate can be arranged so the

direction of reactant flow in one gas diffusion layer is opposite the direction of

reactant flow in the other gas diffusion layer.

Another improvement to assist in water purging relates to the positioning of the outlet channels to use gravitational force. A fuel cell system according to the

invention can have reactant outlets in which at least one of the outlets is positioned

below the channels, whereby water removal is assisted by gravitational force.

The drained water can be removed from the system or collected in a

reservoir, such as a tank. The tank can be rendered freeze tolerant in a number of

ways, including insulation or heating from either a direct, dedicated source, or a

temporary source, such as heat production from the system's fuel processor or the

fuel cell. During dormant steps, watering in the tank can be allowed to freeze,

provided the tank is designed to permit expansion of freezing water.

To protect against the mechanical damage to the collector plates due to the

freezing expansion of any residual water, the walls of the channels can be tapered

and have rounded corners.

According to the invention, a fuel cell system can be made more freeze

tolerant by incorporating shut down procedures conducive to freezing environments.

The shut-down procedure can include the steps of: reducing the fuel cell system

temperature, whereby water vapor in said fuel stack is condensed; removing water,

liquid and gaseous, from said fuel cell; purging said the reactant gas lines with a

non-humidified gas; and reducing the system pressure to a pressure approximately

equal to atmospheric pressure. These steps can be performed in different orders,

and alternatively, can be begun simultaneously.

The shut down procedure can also include steps to further increase heat of

the fuel cell at or just before shut-down. One preferred step includes running said

fuel stack in a mode that results in a pulsed current output. The presence of substantial quantities of water during start-up, before the fuel

has heated to above the freezing point, can present a danger of damage by freezing

expansion of the water. So, techniques according to the invention also provide for

start-up procedures that introduce water so as to avoid freezing. These techniques

can include the steps of: flowing dry reaction gases through the reactant lines into

said fuel cell; measuring the temperature of said fuel cell; and initiating

humidification of said reactant gases after the fuel cell temperature is above a

predetermined temperature. The predetermined temperature can be the freezing

point of water at the ambient pressure. The start-up procedure can also preferably

include pressurizing to maximum operating pressure to increase heating of the fuel

cell and retain any thawing residual water.

The start-up procedures can also include: providing a humidifier for

humidifying reactant gas flows to the fuel cell; providing a water reservoir for the

supply of water for humidification of the reactant gas flows; heating the reservoir to

melt the water in the reservoir; melting water in the reservoir; and supplying water to

the humidifier for humidification of the gas flows.

The heat for melting the water in the reservoir can be obtained from the fuel

processor. The steps of an implementing method can include: operating a fuel

processor in an oxidant rich mode to increase heat output; transferring a portion of

said heat output to said reservoir to melt water in the reservoir; transferring a portion

of said heat output to said fuel cell to raise the fuel cell temperature; monitoring the

melting of the water in said reservoir and the fuel cell temperature; adjusting the

reactant mixture in the fuel processor to increase fuel production after at least a portion of the water in said reservoir is melted and the fuel cell temperature has

reached a predetermined temperature.

All the above constructions and operating methods can be employed in

various combinations. Alone and in combination, these features contribute to the

utilization of a fuel cell system in sub-freezing environments.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of the present invention will become apparent to

those skilled in the art from the following description with reference to the drawings,

in which:

Fig. 1 illustrates a breakaway side view of fuel cell and coolant system

according to the invention.

Fig. 2 illustrates a freeze tolerant fuel cell system schematic comprising a fuel

cell stack interfaced with a fuel processor and external cooling and humidification

systems used to implement Applicants' method of using a freeze tolerant fuel cell.

Fig. 3 illustrates a side view of a fuel cell having collector plates with non-

continuous flow field channels with tapered and rounded corners.

Fig. 4 illustrates a breakaway side view of a fuel cell having primary gaskets

and secondary sub-gaskets. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A novel freeze tolerant fuel cell structure is provided that is adapted for

operation in environments subject to sub-freezing temperatures, hereinafter referred

to as sub-freezing environments. As used throughout this specification, sub-freezing

environments refers also refers to ambient conditions that may be above freezing

during operation of a fuel cell, but reach sub-freezing temperatures are uring

dormant periods of the fuel cell. The cooling system and humidification systems are

preferably separated. The cooling system is also preferably isolated from the

electrochemically active region of the fuel cell through the use of gaskets. By

separating the cooling and humidification systems and isolating the cooling system

from the electrochemically active fuel ceil region, materials otherwise poisonous to

the fuel cell which have freezing points below the freezing point of water may be

used as coolant materials. A method for operating the freeze tolerant fuel cell

during sub-freezing conditions is also disclosed. During the startup of the fuel cell

from an idle mode during sub-freezing conditions, the fuel cell system is run in a

mode that maximizes the heat generated by the fuel cell. Humidification is delayed

until the fuel cell temperature is raised above the freezing temperature. Shutdown of

the fuel cell during sub-freezing conditions involves removing as much water as

possible from the fuel cell in a minimum period of time.

A novel freeze tolerant fuel cell structure having both gaskets and sub-

gaskets is also disclosed. Upon assembly of the freeze tolerant fuel cell, an interface

region between the gaskets and the edge of the electrochemically active region of

the fuel cell is formed. This region is subject to enhanced mechanical and enhanced electrical stress due to increased edge conduction relative to electrochemically

active areas of the fuel cell removed from the interface region. Use of sub-gaskets

in conjunction with gaskets in the fuel cell is disclosed which minimizes the

mechanical and electrical stress at the edges of the electrochemically active fuel cell

and leads to improved fuel cell reliability.

Referring to FIG. 1 , a fuel cell is identified generally by the reference numeral

10. A single fuel cell 10 depicted in the drawing, comprised of one cell unit 11.

However, it is to be understood that the invention can be utilized in conjunction with

fuel stacks having a plurality of cell units. Each cell unit 1 1 includes a membrane

electrode assembly (MEA) 12 comprised of a solid ion conducting membrane which

may have an anode 13 on one side and a cathode 14 on the other side of the

membrane, each formed by an electrochemically active catalyst layer attached

directly to the outside of the membrane. The MEA 12 is interposed between a first

gas diffusion layer 15 and a second gas diffusion layer 16.

Alternatively, the anode and cathode may be attached or integrated into the

gas diffusion layers 15 and 16, and pressed against the membrane. In the preferred

embodiments, the MEA includes an attached anode 13 and cathode 14 due to

greater fuel cell efficiency resulting from intimate contact of the electrodes with the

membrane compared to a pressure contact when the electrodes are mounted on the

gas diffusion layers 15 and 16. Preferably, the MEA 12 extends some minimum

distance beyond the outer periphery of the gas diffusion layers 15 and 16.

Alternatively, the MEA can terminate with the edges of the gas diffusion layers,

provided that edge is properly sealed to prevent the leakage of reactants around the MEA. The gas diffusion layers 15 and 16 are interposed between two electrically

conducting collector/separator plates 18 and 19 (collector plates), which are

commonly referred to as bipolar plates when two or more fuel cells are used to form

a fuel cell stack. The gas diffusion layers 15 and 16 are typically fabricated from

porous, electrically conductive materials, such as carbon/graphite fiber paper or

carbon/graphite cloth. When two or more fuel cells form a fuel cell stack, the

collector plates 18 and 19 are provided for separating the cathode of one cell unit 11

from the anode of adjoining cell units (not shown).

In the preferred embodiment of the invention, the fuel cell 10 is a proton

exchange membrane (PEM) fuel cell. Since individual PEM fuel cells produce less

than 1 volt at full load, practical PEM fuel cells stack several individual cells, such as

cell unit 11 in series to attain the desired voltage level by electrically connecting the

cathode of one cell to the anode of an adjacent cell (not shown).

The collector plates 18 and 19 are electrically conductive. In the preferred

embodiment of the invention, the collector plates 18 and 19 are formed from

electrically conductive polymer composites by filling a polymer with a plurality of

conductive particles, such as graphite. The collector plates 18 and 19 may be

designed to have a spectrum of water permeabilities from highly permeable to

essentially impermeable. In some applications, it may be desirable to have water

permeable collector plates. For example, water permeable collector plates may

provide water vapor to the fuel cell for membrane humidification. However, during

fuel cell operation, water permeable collector plates store significant quantities of

water. In fuel cell applications at temperatures below the freezing point of water, water impermeable collector plates 18 and 19 are generally required to avoid

damaging the collector plates 18 and 19 through expansive forces exerted by

freezing water contained therein. Accordingly, in the preferred embodiment of the

invention, collector plates 18 and 19 selected are essentially impermeable to water.

At a minimum, the surfaces of the channels are impermeable to water while the

remainder of the collector plate may be permeable to water.

The electrochemical reaction at the cathode 14 that forms water is highly

exothermic. Consequently, a cooling system is generally required for fuel cells. De-

ionized water is commonly used to cool fuel cells and also to maintain the hydration

of the membrane ("humidifying"), which is known in the art to be required for

efficient ionic transport across the membrane. Although this arrangement is quite

satisfactory in most fuel cell applications, in applications where the temperature may

reach freezing temperatures (at or below 0°C at one atmosphere pressure), pure

water cannot be used as a coolant because if water freezes the fuel cell will be

damaged or destroyed by the associated expansion of water during its phase

change to solid form. Membrane humidification, if required, must also be redesigned

to avoid freezing water in the fuel cell 1 1. If humidification is required, the invention

separates the humidification and the coolant systems. Separation of humidification

and cooling permits use of coolants other than pure water. This is important, since

the coolant fluid in a freeze tolerant fuel cell cannot be pure water since water

freezes at 0°C at 1 atmosphere pressure.

Humidification of the membrane may be derived from a source outside the

fuel cell stack, such as by humidifying incoming reactant gases through the use of misters or bubblers. In situations where fuel processors are used to produce

hydrogen fuel, the anode may not require humidification, due to moisture produced

as a by-product of the reforming process.

A dedicated coolant loop 25 has a coolant flow field through and between

collector plates, which form a cooling layer adjacent to the fuel cell 11. Electrically

conductive sealant 32 binds top collector plate and bottom collector plate. Coolant

flows both vertically though the passage and laterally through the cooling layer to

dissipate heat uniformly across the collector plate surface area of the adjacent fuel

cell 11. A coolant return path is provided but not shown.

Coolant loop 25 does not provide humidification to the fuel cell 11.

Additionally, the coolant loop 25 is isolated from the membrane by a minimum

distance "A." Depending on the anticipated operating temperature range the fuel

cell 11 will be subjected to, coolants other than pure water are needed as coolants

for the fuel cell 11. Since water freezes near 0°C at one atmosphere pressure, pure

water is not a viable coolant for sub 0°C fuel cell applications. The coolant system

is mechanically sealed during manufacture so that the coolant fluid is isolated from

the membrane. The distance "A" is chosen to avoid coolant contact with the

membrane and is based on the integrity of the overall fuel cell seal. In the preferred

embodiment of the invention, the distance "A" is at least approximately 0.1 inches.

Seal integrity is principally a function of the type of gasket material selected.

Many coolants other than pure water will contaminate the fuel cell if allowed

to contact the membrane by taking up catalyst sites. For example, hydrocarbons

are known to occupy catalyst sites. As used throughout the specification, these contaminating coolants are referred to as "poisonous." Poisonous coolants are

intended to refer to all non-pure water coolants that can bind the catalyst if allowed

to contact the MEA electrodes or otherwise interfere with the electro-chemical

reaction of the fuel cell.

In an alternate embodiment of the freeze tolerant fuel cell, the coolant

passage way is not part of the collector plate. For example, coolant may be flowed

through an external manifold in proximity to the periphery of the fuel cell collector

plates. In yet an another embodiment of the fuel cell, coolant channels are placed

on the periphery of collector plates in areas removed from the fuel cell active area.

In both of these alternate embodiments, coolant does not pass between the region

between adjacent collector plates.

Coolants other than pure water having a freezing point sufficiently below 0°C

to meet the expected minimum temperature of operation of the fuel cell 11 will be

generally useful for freeze tolerant fuel cells. Possible suitable coolants include:

(a) ethylene glycol alone, with any percentage of other coolants including

water, and/or lubricants, provided the freezing point of the solution is sufficiently low

for its intended application.

(b) propylene glycol alone, with any percentage of other coolants including

water, and/or lubricants, provided the freezing point of the solution is sufficiently low

for its intended application.

(c) other alcohols with similar freezing points and boiling points to propylene

glycol and ethylene glycol such as methanol, with any percentage of other coolants

including water, and/or lubricants, provided the freezing point of the solution is sufficiently low for its intended application; and

(d) gases under anticipated conditions of operation, such as nitrogen or

hydrogen.

The maximum allowable coolant ionization level depends on the design of the

coolant loop 25. If the coolant loop 25 is designed to be electrically isolated from the

collector plates 18 and 19, ionic coolants may be used. However, if the coolant loop

25 is not designed to be electrically isolated from the collector plates 18 and 19, the

coolant ionization level must be limited to avoid electrically coupling neighboring

collector plates 18 and 19 (not shown) through the fluid flowing through the coolant

loop 25. If neighboring collector plates 18 and 19 (not shown) are at different

electrical potentials and are electrically coupled through a coolant loop 25 that uses

a conductive coolant material, current will flow between neighboring collector plates

18 and 19 (not shown) through the conductive coolant. In the preferred embodiment

of the invention, the coolant loop 25 is not electrically isolated from collector plates

18 and 19 because coolant plate and flow field design is much more complex and

expensive if the coolant loop 25 is designed to be electrically isolated from collector

plates 18 and 19.

The coolant isolation can be provided by a gasket arrangement. The outer

edge of the MEA 12 is interposed between a first gasket 20 and a second gasket 21.

Gaskets 20 and 21 are preferably positioned so as to not overlap with gas diffusion

layers 15 and 16. Gaskets 20 and 21 may be made from polymer materials such as

EPDM rubber (also known as EP rubber), fluorinated hydrocarbon, butyl rubber,

fluorosilicone, polysiloxane, thermoplastic elastomers such as blends containing polypropylene and EP rubber, and or other similar materials. An interface region 22

is formed between the gas diffusion layers 15 and 16 and the gaskets 20 and 21.

The membrane at or near the interface region 22 is subjected to both

enhanced mechanical and electrical stress relative to other portions of the active

membrane. If a space between the gas diffusion layers 15 and 16 and the gaskets

20 and 21 results during the manufacture, of the fuel cell 11 , e.g. due to tolerances,

the membrane in the interface region 22 will be unsupported and will tend to sag or

pinch. Sagging or pinching may produce the undesirable result of a rupture which

could allow reactants to flow from one flow field into another flow field. The

magnitude of the mechanical stress at the interface region 22 may be reduced by

effectively splitting the interface region 22 into two regions. The interface region 22

may be effectively split by shifting the upper interface between the gas diffusion

layer 15 and gasket 20 and the lower interface between gas diffusion layer 16 and

gasket 21 during the manufacture of the fuel cell 11. Also, during manufacture, the

gas diffusion layers 15 and 16 may butted up against the gaskets 20 and 21 and

result in compression of the membrane in the interface region 22. In either case, the

membrane is subjected to additional mechanical stress in the interface region and

may result in early life failures of the fuel cell 11.

Further, since the edges of parallel conducting plates are known to generate

higher electrical field fluxes compared to interior portions of conducting plates, the

active membrane at or near the interface region 22 will experience higher current

densities in fuel cell 11 operation compared to interior portions. This phenomenon

enhances electrical stress to the membrane at the interface region, regardless of whether a space is formed as in Fig. 1 or the gas diffusion layers 15 and 16 butted

up against gaskets 20 and 21.

Even a well designed freeze tolerant fuel cell requires special startup,

shutdown and humidification processes to minimize damage resulting from the

formation of ice from water trapped therein during freezing conditions. In the

preferred embodiment of the invention, all operational aspects of the freeze tolerant

fuel cell system, such as reactant flow control, temperature monitoring and control

are controlled by a central computer using feedback control systems. Now referring

to Fig. 2, to begin startup of the fuel cell stack 10, heat from the fuel stream can be

first supplied to the fuel cell 10 by fuel processor 60 through anode inlet 62 with

minimum water content during startup to minimize the amount of ice formed. For

example, a partial oxidation based auto-thermal fuel processor propane into a hot

gas stream. The percentage of hydrogen in the reformate stream may be adjusted

to the desired level by varying the steam to carbon ratio as well as the stochiometric

ratio of air to fuel in the fuel processor 60. High water yield is accomplished by

feeding air to the fuel processor 60 at quantities close to or preferably exceeding

quantities required for complete combustion of the fuel supplied. The hot gas

stream produced by the fuel processor 60 is then cooled by the coolant in the anode

66, resulting in condensation of water which may be separated in the anode

separator 68 and directed to the process water tank 64. Combustion is usually a

very quick process and the fuel processor 60 is operated in this mode until sufficient

water is collected in the process water tank 64. Process water stored in the water

tank 64 will be used for steam reforming and cathode gas humidification when the operating temperatures in the system rise above freezing point of water. In start up

situations where the process water tank 64 initially contains ice, the fuel processor

60 will be operated in a combustion mode to produce maximum water as described

earlier and the hot gas stream produced by the fuel processor 60 can be used to

melt the ice in the freeze resistant process water tank 64. By following either of the

above startup procedures the fuel processor 60 will generate a hydrogen-rich hot

reactant stream to the fuel cell stack 10 with minimum moisture content during

startup. A hot reactant stream will also help thaw out the various fuel stack

components.

Humidification should be delayed until the stack and coolant temperatures are

above the freezing point of water. During the initial stages of startup, pressurized

dry air is fed to the cathode 70 without humidification. Pressurized dry air from the

cathode compressor is typically heated to a temperature in the range of 90-100°C

by pressurizing at 30 psig when the ambient air temperature is in the range of

approximately -10 to -20°C. The hot cathode air will also help thaw out the

membrane and other stack components that are exposed to freezing conditions.

Once hydrogen from fuel processor 60 is available for power generation the fuel cell

stack 10 will be operated in a low voltage/high current density mode to maximize

generation of heat. Heat generated will be used to raise the temperature of the

stack 10 and the coolant. As the stack 10 temperature increases, the stack will be

operated to begin producing higher output voltages. Thus, the system may be

transitioned to its more efficient electrical power generation mode as the fuel cell

stack 10 temperature increases. Cathode air humidification may be begun after the stack 10 and coolant temperature are well above freezing. Optionally, anode

humidification may be begun at the same time.

In an alternate embodiment of the freeze tolerant fuel cell system, the fuel

processor shown in Fig. 6 is replaced by an essentially pure hydrogen source.

Hydrogen is supplied to the anode of the fuel cell along with an oxygen source to the

cathode of the fuel cell to immediately generated heat. The heat generated raises

the temperature of the freeze tolerant coolant. If the water tank contains ice, the

heated freeze tolerant coolant is circulated through the water tank to melt the ice in

the water tank. Once the stack temperature rises above the freezing point of water,

humidification of the reactant lines may be begun.

A method for shutting down the freeze tolerant fuel cell is also required to

avoid formation of ice within the fuel cell system upon shutdown. In the event of a

system shut down, the main objective is to remove most water from within the fuel

cell stack and system components as quickly as possible. There are three steps

required to remove the water from the system. First, the system temperature must

be reduced to condense water vapor within the system. The temperature may be

reduced more rapidly by flowing a coolant through the coolant loop. Cooling the

system allows water contained in vapor phase fuel cell to condense. Once water is

in a liquid phase it can easily be separated from the gas streams and drained into a

freeze resistant storage tank 64. Second, the fuel cell stack 10 and system reactant

lines must be thoroughly purged with non-humidified gases using either an on-board

or supplemental purging system. Finally, the whole system must be returned to

ambient pressure. Following this three step procedure will significantly reduce the amount of condensed water that will remain in the fuel cell after shut down. If water

is left in the fuel cell during shutdown, it will expand upon freezing and may cause

damage to vital fuel cell components such as the anode water separator(s), cathode

water separator(s), fuel cell stack, fuel processor water storage tank (when used)

and reactant supply lines.

A similar method for anode shutdown is described below. If anode

humidification is used, humidification of the anode gas is stopped. The anode gas

lines 62 are then purged with dry cold anode gas. This will purge all humidified

anode gas from the fuel cell components and will also eliminate most liquid water

droplets from the fuel cell system 10 components.

Condensed water droplets will be separated in the anode separator(s) and

can either be drained into freeze tolerant storage tank 64 or be completely purged

from the system. If a process water storage tank is used, the process water storage

tank should be placed at the lowest possible level to permit gravity feeding into the

water storage tank.

A similar method for fuel processor shutdown is described below. The fuel cell

stack temperature is reduced to the ambient temperature. An anode cooler/chiller is

used to condense water out of the anode stream and transfer heat to the coolant.

This water is eliminated from the anode stream by an anode separator 74. The

separated water can either be drained into a freeze resistant storage tank 64 or be

completely purged from the system. The anode cooler/chiller holds the system

coolant, which is at the stack temperature, running through it. The stack

temperature is brought down to ambient such that the anode gas temperature can also be cooled to near ambient temperature.

A similar method for cathode shutdown is described below. Cathode shut¬

down should be initiated at the same time as the anode shut-down. Humidification

of cathode gas is terminated. The cathode system is then purged with dry cathode

gas. The compressor temperature and pressure are brought down to near ambient

conditions. All liquid water is separated in the cathode separator(s) and can either

be drained into the freeze resistant water storage tank 64 or be completely purged

from the system.

A similar method for coolant loop shutdown is described below. The system

layout should be such that the coolant storage tank 76 is at the lowest point in the

system. This allows the coolant to drain via gravity back into the coolant storage

tank 76. At the same time as the coolant is gravity fed into the coolant storage tank,

the system completes the dry reactant purge. Finally, the coolant pump is switched

off.

In an enhancement of the shutdown process, the fuel cell stack 10 is

operated in a mode that produces current through the stack sufficient to generate

large amounts of heat while supplying the fuel cell with dry reactants. This condition

will maximize the evaporation rate of water within the fuel cell stack 10 and result in

a faster shut down time.

The start up and shut down procedures described include a number of steps

that can be performed in different orders and can also be started simultaneously.

Referring to Fig. 3, In a further enhancement to the shutdown process, a

preferred embodiment of the fuel cell is shown having interdigitated discontinuous channels in the collector plates 18 and 19. Flow field anode inlet channel 26,

cathode inlet channel 28, anode outlet channel 27 and cathode outlet channel 29

are separated thus forcing flows through the gas diffusion layers. Channels shown

have rounded corners. Interdigitation and rounded corners of the flow fields need

not be combined to produce fuel cell improvements for freeze tolerant applications.

An interdigitated flow field configuration helps limit the amount of water trapped

within the gas diffusion layers 15 and 16 and speeds the purging of water out from

gas diffusion layers 15 and 16 during shut down of the fuel cell during freezing

conditions. In addition, the walls of the channels making up the flow field may be

tapered slightly and corners rounded to allow room for water to expand upon

freezing to minimize damage to the fuel cell 11 during freezing conditions.

Fig. 4 adds a pair of sub-gaskets 23 and 24 to Applicants' gasketed fuel cell

as shown in Fig. 1. Sub-gaskets 23 and 24 are positioned between first and second

gaskets 20 and 21 and extend into a position between the gas diffusion layers 15

and 16 and the membrane on the distal end of the region where the gas diffusion

layers 15 and 16 overlap the membrane. Sub-gaskets 23 and 24 provide extra

support for the fuel cell in the interface area reducing mechanical stress on the

membrane at or near the interface region 22. In addition, enhanced edge

conduction across the membrane at the interface 22 is eliminated by shifting the

effective edge of the active membrane to the distal end of sub-gaskets 23 and 24.

Thus, the use of sub-gaskets 23 and 24 results in improved fuel cell reliability. In the

preferred embodiment of this invention, sub-gaskets 23 and 24 are made from

strong acid resistant materials such as FEP, TFE, ETFE, PFA, CTFE, E-CTFE, PVF and PVF and have a thickness of approximately 1/10 the thickness of gaskets 20

and 21.

As shown, the coolant loop 25 passes through gaskets 20 and 21 as well as

sub-gaskets 23 and 24. As discussed earlier, sub-gaskets 23 and 24 reduce

mechanical and electrical stress near the edge of the active membrane in the

interface region 22. Sub-gaskets 23 and 24 need not extend to be co-terminus on

the side opposite the active membrane with the edge of primary gaskets 20 and 21.

In the preferred embodiment of the invention, sub-gaskets 23 and 24 are co-

terminus on the side opposite the active membrane with the edge of primary gaskets

20 and 21 due to ease of manufacture coupled with the low relative cost of the sub-

gasket 23 and 24 material as compared to the added labor cost to construct

comparatively short sub-gaskets 23 and 24 that do not extent significantly outside

the active membrane region.

While the preferred embodiments of the invention have been illustrated and

described, it will be clear that the invention is not so limited. Numerous

modifications, changes, variations, substitutions and equivalents will occur to those

skilled in the art without departing from the spirit and scope of the present invention

as described in the claims.

Claims

What is claimed is:

1. A freeze tolerant fuel cell system, comprising at least one fuel cell, said at

least one fuel cell system comprising:

a pair of collector plates having a series of channels for the flow of reactants

from ports formed through the collector plates;

a first and a second gas diffusion layer disposed between said collector

plates; and

a membrane electrode assembly (MEA) including a membrane sandwiched

between two electrode layers, said MEA being interposed between said gas

diffusion layers, a seal being provided to substantially prevent the transfer of

reactance gases around the MEA;

said fuel cell system further comprising at least one coolant passage for

flowing a coolant stream relative to said collector plates to cool said fuel cell and

wherein said coolant stream does not contact said MEA.

2. The fuel cell system as recited in claim 1 , further comprising a coolant flowing

in said coolant passage, said coolant being poisonous to the MEA.

3. The fuel cell system as recited in claim 1 , wherein surfaces of the coolant

passage in contact with the coolant are electrically insulated.

4. The fuel cell system as recited in claim 1 , wherein the coolant is electrically

non-conductive.

5. The fuel cell system as recited in claim 1 , further comprising a coolant loop,

said at least one coolant passage forming part of said coolant loop.

6. The fuel cell system as recited in claim 5, wherein said coolant loop includes a coolant layer outside said collector plates.

7. The fuel cell system as recited in claim 6, wherein said coolant layer is

adjacent one of said collector plates.

8. The fuel cell system as recited in claim 5, wherein the coolant loop includes

coolant channels in said collector plates, separate from said reactant channels.

9. The fuel cell system as recited in claim 1 , wherein said collector plates

provide separate coolant ports for the transfer of coolant apart from the reactant

flows and further comprising a pair of gaskets surrounding the coolant ports and

disposed between the collector plates.

10. The fuel cell system as recited in claim 1 , wherein said coolant passage is

housed in structure outside the conductive region of said collector plates.

11. The fuel cell system as recited in claim 9, wherein the pair of gaskets is

separated from the gas diffusion layers.

12. The fuel cell system as recited in claim 9, wherein at least said membrane

extends beyond at least a portion of the periphery of said gas diffusion layers and

the pair of gaskets overlap the membrane.

13. The fuel cell system as recited in claim 9, further comprising a pair of sub-

gaskets, each sub-gasket positioned between said gaskets and extending into a

position between said gas diffusion layers and said MEA, whereby said membrane is

supported.

14. The fuel cell system as recited in claim 13, wherein said sub-gaskets are

made from a fluoro-polymer.

15. The fuel cell system as recited in claim 14, wherein said sub-gaskets are made materials selected from the group consisting of FEP, TFE, ETFE, PFA, CTFE,

E-CTFE, PVF₂ and PVF.

16. The fuel cell system as recited in claim 1 , wherein said fuel cell is a proton

exchange membrane (PEM) fuel cell and said ion exchange membrane is water

permeable.

17. The fuel cell system as recited in claim 1 , wherein said reactant channels are

discontinuous, whereby flow of reactant gases are established through the gas

diffusion layers.

18. The fuel cell system as recited in claim 17, wherein said reactant channels in

each collector plate are arranged so that the direction of reactant flow in one gas

diffusion layer is opposite the direction of reactant flow in gas diffusion layer on the

opposite side of the MEA, whereby a greater moisture gradient is established to

facilitate the transfer of water during shutdown, purging operations.

19. The fuel cell system as recited in claim 17, wherein said reactant channels in

each collector plate are arranged so that relatively dry portions of the reactant flow

on one side of the MEA oppose relatively wet portions of the reactant flow on the

other side of the MEA, whereby a greater moisture gradient is established to

facilitate the transfer of water during shutdown, purging operations.

20. The fuel cell system as recited in claim 1 , wherein surfaces of the channels

are essentially impermeable to water.

21. The fuel cell system as recited in claim 20, wherein surfaces of the channels

are essentially impermeable to all fluids.

22. The fuel cell system as recited in claim 1 , wherein walls of said channels are tapered and have rounded corners, whereby any residual water in the fuel cell that

freezes can expand freely and avoid damage to the collector plates.

23. The fuel cell system as recited in claim 1 , wherein the reactant channels have

reactant outlets, at least one of said outlets being positioned below the channels,

whereby water removal is assisted by gravitational force.

24. The fuel cell system as recited in claim 1 , further comprising a water reservoir

positioned outside the stack, wherein the reactant channels have reactant outlets

operatively connected to the water reservoir to permit water from the fuel cell to

drain to water reservoir.

25. The fuel cell system as recited in claim 24, wherein the water reservoir is

freeze tolerant, including at least one of insulation and a heating source.

26. The fuel cell system as recited in claim 25, wherein the heating source is a

heater for the reservoir.

27. The fuel cell system as recited in claim 25, wherein the heating source is a

fuel processor.

28. The fuel cell system as recited in claim 25, wherein the heating source is the

fuel cell.

29. A method of using a fuel cell system in environments subject to sub-freezing

temperatures, said fuel cell system having at least one fuel cell, said fuel cell

including a membrane, a pair of gas diffusion layers, a pair of electrodes, a pair of

collector plates, at least two reactant flow lines, comprising the steps of:

creating at least one coolant flow passage relative to said fuel cell so as to

avoid coolant contact with said membrane while cooling the fuel cell; flowing a coolant stream poisonous to the membrane through said at least

one passage, whereby said coolant stream does not contact said membrane such

that a coolant having a freezing point below that of water can be used.

30. The method as recited in claim 29, wherein said coolant material is a fluoro-

polymer.

31. A method of shutting down a fuel cell system in environments subject to sub-

freezing temperatures, said fuel cell system including at least one fuel cell having a

membrane, a pair of gas diffusion layers, a pair of electrodes, a pair of collector

plates, and at least two reactant flow lines, comprising the steps of:

reducing the fuel cell system temperature, whereby water vapor in said fuel

cell is condensed;

removing water, liquid and gaseous, from said fuel cell;

purging at least one reactant gas line with a non-humidified gas; and

reducing the system pressure to a pressure approximately equal to atmospheric

pressure.

32. The method as recited in claim 31 , wherein the water removal step includes

purging from the system.

33. The method as recited in claim 31 , wherein the water removal step includes

draining water to a water reservoir.

34. The method as recited in claim 31 , wherein said shutdown further comprises

the step of running said fuel stack in a mode that results in a pulsed current output.

35. A method of starting-up a fuel cell system in environments subject to sub-

freezing temperatures, said fuel cell system including at least one fuel cell having a membrane, a pair of gas diffusion layers, a pair of electrodes, a pair of collector plates,

and at least two reactant flow lines, comprising the steps of:

flowing dry reactant gases through said at least one reactant line into said fuel

cell;

measuring the temperature of said fuel cell; and

initiating humidification of said reactant gases after said fuel cell temperature is

above predetermined temperature.

36. The method of claim 35, wherein said predetermined temperature is the freezing

point of water at the ambient pressure.

37. The method of claim 36, further comprising the steps of :

providing a humidifier for humidifying reactant gas flows to the fuel cell stack;

providing a water reservoir for the supply of water for humidification of the

reactant gas flows;

heating the reservoir to melt the water in the reservoir;

melting water in the reservoir; and

supplying water to the humidifier for humidification of the gas flows.

38. The method of claim 31 wherein the steps of heating the reservoir includes:

operating a fuel processor in an oxidant rich mode to increase heat output;

transferring a portion of said heat output to said reservoir to melt water in the

reservoir;

transferring a portion of said heat output to said fuel cell to raise the fuel cell temperature;

monitoring the melting of the water in said reservoir and the fuel cell

temperature;

adjust the reactant mixture in the fuel processor to increase fuel production after

at least a portion of the water in said reservoir is melted and the fuel cell temperature

has reached a predetermined temperature.

39. A method of using a gasketed proton exchange membrane (PEM) fuel cell stack

in sub-freezing environments, said fuel cell stack comprising a plurality of fuel cells

each having a membrane, a pair of gas diffusion layers, a pair of electrodes, a pair of

collector plates and at least a pair of gaskets at temperatures below the freezing

temperature of water, comprising:

creating at least one flow passage through said pairs of collector plates and said

pairs of gaskets of each said fuel cell not through said membrane;

flowing a coolant stream poisonous to the membrane through said at least one

passage, whereby said coolant stream does not contact said membrane;

a start up including:

flowing dry reaction gases through said at least two reactant lines into said fuel cell;

measuring the temperature of said fuel stack; and

initiating humidification of said reactant gases after said fuel cell temperature is above

predetermined temperature; and

a shutdown step including: reducing the fuel cell system temperature;

purging reactant gas lines with one or more non-humidified gases; and

reducing the system pressure to a pressure approximately equal to atmospheric

pressure.

40. The method of using a gasketed proton exchange membrane (PEM) fuel cell

stack as recited in claim 39, said shutdown step further comprising the step of running

said fuel stack in a mode that results in a pulsed current output.

41. A fuel cell stack, comprising at least one gasketed fuel cell, said at least one

gasketed fuel cell comprising:

a first and a second gas diffusion layer each comprising a sheet of electrically

conductive material and a pair of oppositely facing planar surfaces, wherein said gas

diffusion layers are positioned approximately parallel to one another;

a membrane electrode assembly (MEA) interposed between said gas diffusion

layers, said MEA having a pair of oppositely facing planar surfaces, an ion exchange

membrane and a pair of electrodes interposed between said ion exchange membrane

and said gas diffusion layers, said MEA extending beyond the length of said gas

diffusion layers;

a pair of gaskets, wherein said MEA is interposed between the distal ends of

said gaskets, said gaskets not overlapping said gas diffusion layers, wherein an

interface region is produced;

said gas diffusion layers, said gaskets and said MEA all interposed within a pair of collector plates; and,

said collector plates provide channels for reactant streams.

42. The fuel cell stack as recited in claim 41 , wherein said fuel cell is a proton

exchange membrane (PEM) fuel cell and said ion exchange membrane is water

permeable.

43. The fuel cell stack as recited in claim 42, wherein surfaces of said channels are

essentially impermeable to fluids.

44. The fuel cell stack as recited in claim 43, further comprising a pair of sub-

gaskets, each sub-gasket positioned between said gaskets and extending into a

position between the said gas diffusion layers and said MEA on the distal end of region

where said gas diffusion layers overlaps said MEA, whereby said membrane interface

region is supported.

45. The fuel cell as recited in claim 44, wherein said sub-gaskets are made

materials selected from the group consisting of FEP, TFE, ETFE, PFA, CTFE, E-CTFE,

PVF₂ and PVF.