U.S. patent application number 11/775710 was filed with the patent office on 2008-01-24 for water filter for an electrochemical fuel cell system.
This patent application is currently assigned to NUCELLSYS GMBH. Invention is credited to Myles L. BOS, Ralph FISCHER.
Application Number | 20080020249 11/775710 |
Document ID | / |
Family ID | 35908659 |
Filed Date | 2008-01-24 |
United States Patent
Application |
20080020249 |
Kind Code |
A1 |
FISCHER; Ralph ; et
al. |
January 24, 2008 |
Water Filter For An Electrochemical Fuel Cell System
Abstract
Silica may form as a degradation product in an electrochemical
fuel cell system and may be found within the water management
subsystem thereof. The silica may polymerize and/or react to form
insoluble metal silicates which may lead to reduced lifetime or
performance of individual components within the fuel cell system.
These problems can be eliminated or reduced by adding a silica
absorber such as aluminum, either as alumina granulate or an
aluminum plate to the water management subsystem, for example in,
upstream and/or downstream of the water filter. In addition, the
silica absorber may be in, upstream and/or downstream of the water
tank.
Inventors: |
FISCHER; Ralph; (Stuttgart,
DE) ; BOS; Myles L.; (Burnaby, CA) |
Correspondence
Address: |
CROWELL & MORING LLP;INTELLECTUAL PROPERTY GROUP
P.O. BOX 14300
WASHINGTON
DC
20044-4300
US
|
Assignee: |
NUCELLSYS GMBH
Neue Str. 95
Kirchheim/Teck-Nabern
DE
73230
|
Family ID: |
35908659 |
Appl. No.: |
11/775710 |
Filed: |
July 10, 2007 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10924728 |
Aug 23, 2004 |
7250230 |
|
|
11775710 |
Jul 10, 2007 |
|
|
|
Current U.S.
Class: |
429/410 ;
429/414; 429/437; 429/450 |
Current CPC
Class: |
H01M 8/04291 20130101;
H01M 8/04171 20130101; H01M 2008/1095 20130101; Y02E 60/50
20130101; H01M 8/04126 20130101; H01M 8/04029 20130101 |
Class at
Publication: |
429/019 |
International
Class: |
H01M 8/18 20060101
H01M008/18 |
Claims
1. A method for operating a water management subsystem in a fuel
cell system, said method comprising: recovering water from other
subsystems in said fuel cell systems; removing silica from said
water recovered from the other subsystems, by passing said water
through a silica absorber; supplying said water, with silica
removed, to at least one of a humidification subsystem for an
oxidant stream, a humidification subsystem for a fuel stream, a
fuel processing subsystem and a fuel cell cooling subsystem.
2. The method according to claim 1, wherein said other subsystems
comprises at least one of the fuel cell subsystems comprise at
least one of the fuel cell cooling subsystem fuel processing,
subsystem, an oxidant subsystem, an anode exhaust and a cathode
exhaust.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to water filters for
electrochemical fuel cells and more particularly for water filters
in a water management subsystem for an electrochemical fuel cell
system.
[0003] 2. Description of the Related Art
[0004] Electrochemical fuel cells convert reactants, namely fuel
and oxidant fluid streams, to generate electric power and reaction
products. Electrochemical fuel cells employ an electrolyte disposed
between two electrodes, namely a cathode and an anode. The
electrodes each comprise an electrocatalyst disposed at the
interface between the electrolyte and the electrodes to induce the
desired electrochemical reactions. The location of the
electrocatalyst generally defines the electrochemically active
area.
[0005] Polymer electrolyte membrane (PEM) fuel cells generally
employ a membrane electrode assembly (MEA) consisting of an
ion-exchange membrane disposed between two electrode layers
comprising porous, electrically conductive sheet material as fluid
diffusion layers, such as carbon fiber paper or carbon cloth. In a
typical MEA, the electrode layers provide structural support to the
ion-exchange membrane, which is typically thin and flexible. The
membrane is ion conductive (typically proton conductive), and also
acts as a barrier for isolating the reactant streams from each
other. Another function of the membrane is to act as an electrical
insulator between the two electrode layers. The electrodes should
be electrically insulated from each other to prevent
short-circuiting. A typical commercial PEM is a sulfonated
perfluorocarbon membrane sold by E.I. Du Pont de Nemours and
Company under the trade designation NAFION.RTM..
[0006] The MEA contains an electrocatalyst, typically comprising
finely comminuted platinum particles disposed in a layer at each
membrane/electrode layer interface, to induce the desired
electrochemical reaction. The electrodes are electrically coupled
to provide a path for conducting electrons between the electrodes
through an external load.
[0007] In a fuel cell stack, the MEA is typically interposed
between two separator plates that are substantially impermeable to
the reactant fluid streams. The plates act as current collectors
and provide support for the electrodes. To control the distribution
of the reactant fluid streams to the electrochemically active area,
the surfaces of the plates that face the MEA may have open-faced
channels formed therein. Such channels define a flow field area
that generally corresponds to the adjacent electrochemically active
area. Such separator plates, which have reactant channels formed
therein are commonly known as flow field plates. In a fuel cell
stack a plurality of fuel cells are connected together, typically
in series, to increase the overall output power of the assembly. In
such an arrangement, one side of a given plate may serve as an
anode plate for one cell and the other side of the plate may serve
as the cathode plate for the adjacent cell. In this arrangement,
the plates may be referred to as bipolar plates.
[0008] The fuel fluid stream that is supplied to the anode
typically comprises hydrogen. For example, the fuel fluid stream
may be a gas such as substantially pure hydrogen or a reformate
stream containing hydrogen. Alternatively, a liquid fuel stream
such as aqueous methanol may be used. The oxidant fluid stream,
which is supplied to the cathode, typically comprises oxygen, such
as substantially pure oxygen, or a dilute oxygen stream such as
air. In a fuel cell stack, the reactant streams are typically
supplied and exhausted by respective supply and exhaust manifolds.
Manifold ports are provided to fluidly connect the manifolds to the
flow field area and electrodes. Manifolds and corresponding ports
may also be provided for circulating a coolant fluid through
interior passages within the stack to absorb heat generated by the
exothermic fuel cell reactions.
[0009] In conventional solid polymer fuel cell stacks, cooling of
the fuel cells is typically accomplished by providing cooling
layers disposed between adjacent pairs of stacked fuel cells. Often
the cooling layer is similar in design to a reactant flow field
plate wherein a coolant, typically water, is fed from an inlet
manifold and directed across the cooling plate in channels to an
outlet manifold. This type of fuel cell stack typically requires
three plates between each adjacent MEA, namely an anode plate, a
cathode plate and a cooling plate. The coolant channels thus
superpose the active area of the fuel cell. In operation, heat
generated in the fuel cells is drawn away from each fuel cell by
the coolant through the thickness of the plates perpendicular to
the plane of the fuel cell assemblies. Heat is then transferred to
and carried away by a circulating coolant. Cooling with an
additional coolant layer can be called "interstitial" cooling.
[0010] It is desirable to seal reactant fluid stream passages to
prevent leaks or inter-mixing of the fuel and oxidant fluid
streams. U.S. Pat. No. 6,057,054, incorporated herein by reference
in its entirety, discloses a sealant material impregnating into the
peripheral region of the MEA and extending laterally beyond the
edges of the electrode layers and membrane (i.e. the sealant
material envelopes the membrane edge).
[0011] For a PEM fuel cell to be used commercially in either
stationary or transportation applications, a sufficient lifetime is
necessary. For example, 5,000 hour operations may be routinely
required. In practice, there are significant difficulties in
consistently obtaining sufficient lifetimes as many of the
degradation mechanisms and effects remains unknown. Accordingly,
there remains a need in the art to understand degradation of fuel
cell components and to develop design improvements to mitigate or
eliminate such degradation. The present invention fulfills this
need and provides further related advantages.
BRIEF SUMMARY OF THE INVENTION
[0012] A possible degradation product in fuel cell systems is
silica that can be found in soluble or insoluble forms within the
water management subsystem of an electrochemical fuel cell system
and may react with metal particulates or metal surfaces to form
metal silicates. In particular, silica scale may be found within
the humidification subsystem of the fuel cell system. Such
degradation products building up and collecting on fuel cell
components may lead to reduced performance and/or reduced lifetime
of the fuel cell components or the system as a whole. The water
management subsystem may supply deionized water to one or all of
the humidification subsystem for humidifying the oxidant stream,
the fuel stream or both; the fuel processing subsystem; or the
electrochemical fuel cell stack for cooling purposes.
[0013] To remove silica from the fuel cell system, a silica
absorber may be present. More particularly, an electrochemical fuel
cell system may comprise an electrochemical fuel cell stack and a
water management subsystem which comprises a water tank, a water
filter, a silica absorber and a pump all fluidly connected.
[0014] In an embodiment, the silica absorber is within a separate
compartment of the water filter. Alternatively, the silica absorber
may be in a separate cartridge and either upstream, downstream or
both from the water filter. The silica absorber may comprise, for
example, aluminum. In particular, the silica absorber may be
aluminum oxide (also known as alumina). In a more specific
embodiment, the silica absorber may be alumina granulate with a
specific surface area of 100 to 240 m.sup.2/g. The silica absorber
may also comprise activated carbon. If a combination of alumina and
activated carbon is used, then the particles can be mixed within a
single compartment or isolated in separate compartments of the same
or different cartridge.
[0015] Similarly, the silica absorber may be in the water tank, for
example as an aluminum plate located within the water tank.
Alternatively or in addition, a silica absorber may be upstream,
downstream or both of the water tank.
[0016] These and other aspects of the invention will be evident
upon reference to the attached figures and following detailed
description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a schematic of a fuel cell system.
[0018] FIG. 2 is a schematic of a water management subsystem for a
fuel cell system.
[0019] FIG. 3 is a scanning electron microscope image of an oxidant
humidifier after operation in a fuel cell.
[0020] FIG. 4 is a cross-sectional illustrative view of a water
filter of the present invention.
[0021] FIG. 5 is a scanning electron microscope image of a typical
water filter after operation in a fuel cell system.
[0022] FIG. 6 is a scanning electron microscope image of a water
filter of the present invention after operation in a fuel cell
system.
[0023] FIG. 7 is a scanning electron microscope image of alumina
granulate particles from a water filter of the present invention
after operation in a fuel cell system.
[0024] FIG. 8 is a scanning electron microscope image of activated
carbon particles from a water filter of the present invention after
operation in a fuel cell system.
[0025] In the above figures, similar references are used in
different figures to refer to similar elements.
DETAILED DESCRIPTION OF THE INVENTION
[0026] A hydrocarbon fueled proton exchange membrane fuel cell
electric power generation system is the subject of commonly-owned
U.S. Pat. Nos. 5,360,679 and 6,316,134 which are hereby
incorporated by reference in their entirety. FIG. 1 is a schematic
of the fuel cell system 1 as described in the '679 patent. In
particular, the elements of fuel cell system 1 comprise: [0027] an
electric power generation subsystem 10 for producing electricity,
heat, and water from a hydrogen-containing fuel stream and an
oxidant stream; [0028] a fuel processing subsystem 20 for producing
a hydrogen-rich fuel for the electric power generation subsystem
10; [0029] an oxidant subsystem 30 for delivering pressurized
oxidant to the electric power generation subsystem 10; [0030] a
water management subsystem 40 for recovering the water produced in
the electric power generation subsystem 10 and optionally for
cooling the electric power generation subsystem 10; [0031] a power
conversion subsystem 50 for converting the electricity produced
into utility grade electricity; and [0032] a control subsystem 60
for monitoring and controlling the supply of fuel and oxidant
streams to the electric power generation subsystem.
[0033] FIG. 2 shows a schematic of water management subsystem 40 in
more detail. Specifically, water management subsystem 40 comprises
a water tank 42, water filter 44 and pump 46. Water management
subsystem 40 recovers water by collecting excess water from the
streams in other subsystems (for example from electric power
generation subsystem 10, fuel processing subsystem 20, oxidant
subsystem 30, anode exhaust 70 and cathode exhaust 80) and
returning the recovered water to water tank 42. Water separators
and water traps may be used to recover water from the relevant
subsystems as required. Other water filters (not shown) may also be
used in addition to or instead of water filter 44 prior to
introduction of water into water tank 42.
[0034] Pump 46 can then pump recovered water from water tank 42
through water filter 44 to provide a purified water stream to the
following subsystems: [0035] electric power generation subsystem 10
for cooling the fuel cell stack as shown by arrows 12 and 14;
[0036] the fuel processing subsystem 20 (see arrow 22) for use in
the hydrocarbon reforming process and for humidifying the fuel
stream fed to the fuel cell stack (see arrow 24); and [0037] the
oxidant subsystem 30 (see arrow 32) for humidifying the oxidant
stream fed to the fuel cell stack (see arrow 34).
[0038] In other embodiments, each subsystem may be supplied by an
independent water management subsystem instead of having one
central water management system provide water to the various
subsystems. In yet other embodiments, hydrogen gas is supplied
directly without the use of a fuel processing subsystem. In which
case, the water management subsystem may provide a purified water
stream for humidifying the hydrogen gas fuel prior to being fed to
the fuel cell stack. In yet further embodiments, a separate cooling
subsystem is used to supply a coolant other than water (for example
polyethylene glycol) to the fuel cell stack, in which case the
water management subsystem is independent of the cooling of the
fuel cell stack. In any event, most, if not all fuel cell systems
will contain a water management subsystem for at least one of the
functions of cooling, fuel processing and humidifying.
[0039] Degradation pathways present in the fuel cell system can
result in contaminants that reduce the lifetime of the various
components. FIG. 3 is a scanning electron microscope image of an
oxidant humidifier after continued operation of a fuel cell system.
Particles can be clearly seen on the surface of the humidifier. The
smallest particles observed were 7 to 17 .mu.m though many of the
particles were hundreds of microns long. In particular, these
particles formed downstream of a 40 .mu.m particulate filter. Thus
it is insufficient to simply rely on a particulate filter to
eliminate contamination of the humidifier. Without being bound by
theory, these particles have formed by the growth of polymeric
silica on the aluminum surfaces of the humidifier to form
aluminosilicates. Particle formation would also be expected on
other components of the fuel cell system and would not be specific
to the humidifier.
[0040] Silica is a polymer with the basic repeating unit of
SiO.sub.2. There are both polymeric and monomeric forms of silica
and can be represented as: ##STR1## Of the monomeric forms of
silica, formula 1 H.sub.2SiO.sub.3 is also known as mono-silicic
acid and formula (2) H.sub.4SiO.sub.4 is also known as
ortho-silicic acid.
[0041] Silica can be a difficult family of compounds to remove from
water and can be present in three forms: dissolved, colloidal or
suspended, or a combination thereof. Silica will not necessarily
stay in one form in solution and may convert to another form by
polymerization depending on the water conditions (temperature, pH,
total alkalinity and metals concentration). Monomeric silica tends
to be soluble whereas polymeric silica may be colloidal and
granular silica may be suspended.
[0042] In addition silica can form insoluble metal silicates with
some trace metals in solution or on metal surfaces. Basic "ortho"
silicates are of the form M.sub.2SiO.sub.4 where M can be a
divalent metal such as Mg.sup.2+ or Fe.sup.2+. Aluminum silicates
are also very common though their structure is more complex.
Further, metal silicates tend to be chemically stable, particularly
within the temperature and pH conditions typically found within a
fuel cell system.
[0043] Without being bound by theory, silica and silicates may be
observed as a result of degradation of silicone used in other
components within the fuel cell system, for example from silicone
seals.
[0044] Silica may be removed from the water management subsystem by
employing a silica absorber. For the purposes of this application,
a silica absorber comprises a metal that removes silica from an
aqueous solution thereof. Without being bound by theory, the
mechanism by which the silica absorber removes the silica may be
either chemically (for example, through the formation of metal
silicates) or by physical mechanisms (for example, through
adsorption on materials with high specific surfaces).
Representative examples of silica absorbers include magnesium, iron
and aluminum, their metal oxides and combinations thereof. In a
more specific embodiment, the silica absorber comprises aluminum
oxide (also known and referred to herein as alumina).
[0045] FIG. 4 is a schematic of a modified filter 44 comprising an
ion exchange resin 90, activated carbon 92 and alumina granulate
94. The different compartments for ion exchange resin 90, activated
carbon 92 and alumina granulate 94 are partitioned through the use
of partition filters 96. Filter 44 further comprises intake filter
95 and exit filter 98 at the water inlet and water outlet
respectively. Partition filters 96 and intake filter 95 may be, for
example 100 .mu.m filters whereas exit filter 98 may be, for
example, a 25 .mu.m filter. In the embodiment illustrated in FIG.
4, activated carbon 92 and alumina granulate 94 are 15 mm thick.
PVDF spacers may be used (not shown) in making filter 44 and easily
obtaining the correct thickness of layers 92 and 94. Each of the
layers 92 and 94 may represent about 10% of the filter cartridge
volume with the remaining 80% (approximately 120 mm) being filled
with ion exchange resin 90.
[0046] Ion exchange resin 90 is made up of anion and cation resins,
in approximately equal ratios. The resins remove both anionic and
cationic contaminants. A typical resin is based on the
styrene-divinyl benzene co-polymer though other resins such as
acrylic resins are also used. Typically the resin has a bead
structure composed of an inert skeleton with charged functional
groups throughout its structure. In such a resin, the difference
between the anion resin and the cation resin is the functional
group attached to the benzene group.
[0047] The activated carbon 92 typically has a surface with a
relatively high amount of polar functional groups that can attract
contaminants of a similar polarity. Alumina granulate 94 may be,
for example of the type Saint-Gobain-Norpro (SGN) SA62125. Alumina
granulate SGN SA 62125 in particular has a chemical composition of
.gamma.-alumina with a surface area of 100 to 240 m.sup.2/g and a
median pore diameter of 65 to 120 Angstroms. High surface areas for
both the activated carbon and the alumina granulate is desired in
order to increase the efficiency in which they remove contaminants
from the water.
[0048] Water arriving from the water inlet flows through alumina
granulate 94 and activated carbon 92. The large specific surface
area of the two granulate beds 94 and 92 and the surface reaction
between the aluminum ions and the silica causes the silica
contaminants to become absorbed to the surface of the granules.
Subsequently, the water flows through ion exchange resin 90 leading
to the removal of other contaminants and is again filtered at exit
filter 98. The purified water may then be used as needed in the
various fuel cell subsystems.
[0049] Silica can also lead to clogging of filter 44, particularly
from polymeric silica greater than 25 .mu.m in diameter. To
illustrate this and show the improvement of the modified water
filter as in FIG. 4, two fuel cell systems were operated for 16
hours under normal operating conditions. In fuel cell system A, a
conventional water filter was used without activated carbon 92 nor
alumina granulate 94. In comparison, in fuel cell system B, a water
filter as in FIG. 4 was used. Scanning electron microscope images
were then taken of exit filter 98. FIG. 5 is the scanning electron
microscope image for the filter used in system A whereas FIG. 6 is
the scanning electron microscope image for the filter used in
system B.
[0050] Even with a relatively short operation time of 16 hours,
silica particles can clearly be seen in FIG. 5. In comparison,
close inspection of FIG. 6 shows a significant reduction of silica
present when the modified filter cartridge of the present invention
is used. This would be expected to result in a significantly longer
service life of water filter 44. In addition, absorption of silica
at the water filter would be expected to result in reduced amounts
of solubilized silica in the water thereby leading to reduced
formation of metal silicates on downstream components.
[0051] FIG. 7 is a scanning electron microscope image of the
alumina granulate from the modified filter cartridge in system B.
The alumina granulate shows small surface bound silicon particles.
FIG. 8 is a scanning electron microscope image of the activated
carbon granulate from the same modified filter cartridge. As with
the alumina granulate, the activated carbon also shows significant
silica absorption. An SEM/EDX analysis (not shown) conducted on
both the alumina granulate and the activated carbon showed
significant silicon peaks on both materials.
[0052] The composition of the silica absorber can vary
significantly without departing from the scope of the present
invention. For example, only the alumina granulate may be present
in the water filter without the activated carbon. In another
embodiment (not shown), the alumina granulate and the activated
carbon are mixed together within a single compartment and not
separated as in FIG. 4.
[0053] In addition, the location of the silica absorber can also
vary significantly. For example, in other embodiments, the silica
absorber is in a separate cartridge from a conventional water
filter and upstream, downstream or both of the water filter.
Similarly, the silica absorber may be upstream, downstream or both
of water tank 42.
[0054] In another embodiment (not shown), replaceable exfoliated
high surface area aluminum plates could be placed in water tank 42.
This would allow the silica to plate out as an aluminosilicate
before entering the filtration system. The aluminum plates could be
replaced and washed in caustic or hydrofluoric acid periodically to
regenerate them. However, aluminum fabrics tend to be expensive and
not have as high a surface area as alumina granulate.
[0055] From the foregoing, it will be appreciated that, although
specific embodiments of the invention have been described herein
for purposes of illustration, various modifications may be made
without deviating from the spirit and scope of the invention.
Accordingly, the invention is not limited except as by the appended
claims.
[0056] All of the above U.S. patents, U.S. patent application
publications, U.S. patent applications, foreign patents, foreign
patent applications and non-patent publications referred to in this
specification and/or listed in the Application Data Sheet, are
incorporated herein by reference, in their entirety.
* * * * *