U.S. patent application number 11/888244 was filed with the patent office on 2007-11-22 for liquid anode electrochemical cell.
This patent application is currently assigned to SRI International. Invention is credited to Iouri I. Balachov, Steven Crouch-Baker, Lawrence H. Dubois, Marc D. Hornbostel, Alexander S. Lipilin, Michael C. McKubre, Angel Sanjurjo, Francis Louis Tanzella.
Application Number | 20070269688 11/888244 |
Document ID | / |
Family ID | 34981681 |
Filed Date | 2007-11-22 |
United States Patent
Application |
20070269688 |
Kind Code |
A1 |
Lipilin; Alexander S. ; et
al. |
November 22, 2007 |
Liquid anode electrochemical cell
Abstract
An electrochemical cell is provided which has a liquid anode.
Preferably the liquid anode comprises molten salt and a fuel, which
preferably has a significant elemental carbon content. The supply
of fuel is preferably continuously replenished in the anode. Where
the fuel contains or pyrolizes to elemental carbon, the reaction
C+2O.sup.2-.fwdarw.CO.sub.2+4e.sup.- may occur at the anode. The
electrochemical cell preferably has a solid electrolyte, which may
be yttrium stabilized zirconia (YSZ). The electrolyte is connected
to a solid or liquid cathode, which is given a supply of an
oxidizer such as air. An ion such as O.sup.2- passes through the
electrolyte. If O.sup.2- passes through the electrolyte from the
anode to the cathode, a possible reaction at the cathode may be
O.sub.2+4e.sup.-.fwdarw.2O.sup.2-. The electrochemical cell of the
invention is preferably operated as a fuel cell, consuming fuel and
producing electrical current.
Inventors: |
Lipilin; Alexander S.;
(Ekaterinburg, RU) ; Balachov; Iouri I.; (Menlo
Park, CA) ; Dubois; Lawrence H.; (Palo Alto, CA)
; Sanjurjo; Angel; (San Jose, CA) ; McKubre;
Michael C.; (Menlo Park, CA) ; Crouch-Baker;
Steven; (Palo Alto, CA) ; Hornbostel; Marc D.;
(Palo Alto, CA) ; Tanzella; Francis Louis; (San
Carlos, CA) |
Correspondence
Address: |
MINTZ, LEVIN, COHN, FERRIS, GLOVSKY AND POPEO, P.C
1400 PAGE MILL ROAD
PALO ALTO
CA
94304-1124
US
|
Assignee: |
SRI International
Menlo Park
CA
|
Family ID: |
34981681 |
Appl. No.: |
11/888244 |
Filed: |
July 30, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11134555 |
May 19, 2005 |
|
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11888244 |
Jul 30, 2007 |
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60572900 |
May 19, 2004 |
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Current U.S.
Class: |
429/430 ;
429/495; 429/503; 429/519; 429/523; 429/534 |
Current CPC
Class: |
H01M 8/1253 20130101;
H01M 8/1233 20130101; H01M 8/225 20130101; H01M 2250/405 20130101;
Y02P 70/56 20151101; H01M 2004/8684 20130101; Y02B 90/10 20130101;
H01M 8/1009 20130101; Y02E 60/525 20130101; H01M 8/143 20130101;
Y02B 90/16 20130101; H01M 2008/1293 20130101; H01M 2008/147
20130101; Y02E 60/50 20130101; H01M 4/8626 20130101; H01M 8/2455
20130101; H01M 8/086 20130101; H01M 8/22 20130101; Y02P 70/50
20151101; Y02E 60/526 20130101 |
Class at
Publication: |
429/013 ;
429/033; 429/034; 429/030 |
International
Class: |
H01M 2/02 20060101
H01M002/02; H01M 8/10 20060101 H01M008/10 |
Claims
1. A fuel cell comprising (a) a conductor serving as an anode
current collector, (b) a liquid anode, (c) fuel distributed in the
liquid anode, (d) a solid oxygen ion-conductive electrolyte, (e) a
solid gas diffusion cathode, (f) a first inlet for gaseous
oxidizer, (g) an outlet for gas evolved during the operation of the
fuel cell.
2. The fuel cell of claim 1, further comprising an anode
recirculation system for recirculating the liquid anode.
3. The fuel cell of claim 1, further comprising a fuel
replenishment system for replenishing the supply of fuel in the
liquid anode.
4. The fuel cell of claim 1, further comprising a liquid anode
cleanup system for removing from the liquid anode fuel oxidation
products and impurities accumulated during cell operation.
5. The fuel cell of claim 1, wherein the anode current collector
forms a channel for the passage of the liquid anode.
6. The fuel cell of claim 1, wherein the fuel is selected from one
of carbon-containing materials, a metal, or hydrocarbons.
7. The fuel cell of claim 6, wherein the fuel comprises tar,
elemental carbon, coal, coke, biomass, carbon-containing waste, or
aluminum.
8. The fuel cell of claim 1, wherein the fuel is solid.
9. The fuel cell of claim 8, wherein the fuel is in the form of
solid particles.
10. The fuel cell of claim 1, wherein the fuel is a liquid or
molten hydrocarbon.
11. The fuel cell of claim 1, further comprising a system for
confining some or all of the gases evolved during the operation of
the fuel cell so as to avoid their dispersal into the
atmosphere.
12. The fuel cell of claim 1, comprising a liquid anode outlet
which is connected to a heat exchanger for utilizing the heat in
liquid anode and gaseous fuel oxidation products leaving the fuel
cell.
13. The fuel cell of claim 1, further comprising a second outlet
for used oxidizer gases, wherein the second outlet is connected to
a heat exchanger for utilizing the heat in the used oxidizer gases
leaving the fuel cell.
14. The fuel cell of claim 1, wherein a power density of at least
50 mW/cm.sup.2 can be obtained using coal and coke as a fuel.
15. The fuel cell of claim 1, wherein a power density of at least
35 mW/cm.sup.2 can be obtained using biomass, tar,
carbon-containing waste, or a metal as a fuel.
16. The fuel cell of claim 1, wherein a power density of at least
50 mW/cm.sup.2 can be obtained using acetylene black as a fuel.
17. The fuel cell of claim 1, wherein the liquid anode comprises
molten salts, molten oxides, or mixtures thereof.
18. The fuel cell of claim 17, wherein the liquid anode comprises
alkali salts.
19. The fuel cell of claim 1, wherein the liquid anode mixed with
fuel is electronically conductive.
20. The fuel cell of claim 1, wherein the liquid anode is oxygen
ion conductive.
21. A process for supplying electric current to a load having
terminals, comprising the steps of: (a) mixing fuel with a liquid
anode, (b) causing the liquid anode to contact a solid oxygen
ion-conductive electrolyte, (c) causing the fuel to react with
oxygen ions entering the liquid anode from the electrolyte,
releasing electrons, (d) collecting released electrons via an anode
current collector, (e) supplying an oxidizer to a cathode connected
to the solid electrolyte, (f) causing a reduction reaction to occur
at the cathode by supplying electrons via cathode current
collector, thus producing oxygen ions which move through the
electrolyte, (g) electrically connecting the cathode and liquid
anode current collectors to terminals of the load, wherein the
current supplied is sufficient to transfer energy to the load at a
cell power density of 50 mW/cm.sup.2.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 11/134,555, filed May 19, 2005, which claims
priority under 35 U.S.C. .sctn. 119(e)(1) to Provisional U.S.
Patent Application Ser. No. 60/572,900, filed May 19, 2004. The
disclosures of which are incorporated by reference in their
entirety.
TECHNICAL FIELD
[0002] This invention relates generally to electrochemical cells,
and specifically to electrochemical cells capable of operation as
fuel cells, using directly fuels other than hydrogen.
BACKGROUND
[0003] A typical device for direct (one-step) conversion of
chemical energy into electricity utilizes fuel and oxidizer as
reagents. Both reagents may be in gas, liquid, or solid (including
paste) forms.
[0004] Batteries are electrochemical devices that irreversibly
consume the reagents while supplying current to an external
circuit. Rechargeable batteries are devices that reversibly consume
the reagents, such that the initial reagents may be restored by
supplying a current to the device from an external source. The
major limitation of all batteries is their limited capacity,
usually expressed in Ampere-hours. Rechargeable batteries have a
limited number of charge-discharge cycles and thus eventually
fail.
[0005] The fuel cell is another type of electrochemical device for
generating electricity. Fuel cells are characterized by having open
anode and cathode reaction chambers. Fuels cells operate when fuel
is supplied into the anode chamber and oxidizer is supplied into
the cathode chamber. Fuel cells do not have such disadvantages as
limited capacity and a limited number of charge-discharge cycles.
The efficiency of the electrochemical fuel cell increases with
temperature for the practical temperature ranges. Typical fuel
cells may have electric outputs ranging from under 1 kW up to
megawatts.
[0006] Schematically, a fuel cell may be described as a multi-layer
system: fuel/current collector/anode/electrolyte/cathode/current
collector/oxidizer. A typical solid oxide fuel cell operating on
hydrogen may be described as hydrogen/nickel cermet/yttria
stabilized zirconia/lanthanum strontium manganite/air. Current
collectors are embedded in the anode and cathode.
[0007] A major disadvantage of conventional fuel cell design is
that the electrode reactions proceed using an inefficient
three-phase boundary. To elaborate, the fuel cell electron flow is
generated by an electrochemical reaction of fuel oxidation with
release of electrons. Conventional oxidation reactions proceed at a
three-phase boundary: electrode-electrolyte-gaseous reactant. The
actual working surface of the electrodes in this case is very small
and does not exceed 1-4% of the apparent electrode surface.
Accordingly, more than 95% of the electrode area does not
participate in the electricity generation process. Multiple
attempts have been made to increase the useful area of the
electrodes by introducing mixed (electronic and ionic) conductors
into the three-phase boundary. When this is done, working area may
increase up to 5-10%. Still, at best, about 90% of the electrode
area is not being used.
[0008] Fuel cell developers devote major attention to cells
operating on gaseous fuel (hydrogen, natural gas, CO). Cells
operating on a solid fuel, such as carbon-containing materials
(coal, biomass, or waste--both municipal and from the petrochemical
industry) have received much less attention. At the same time,
operation on solid fuel may have such advantages as: much safer
operation (fuel is not flammable or explosive), easier
transportation, generally low cost, high power density, and, in
some cases such as when carbon-containing fuel is used, much higher
efficiency of energy conversion. The last is a result of the
near-zero entropy loss in complete electrochemical oxidation of
carbon. This translates to efficiency above 70%, while the
efficiency of gas-fueled cells is in the 30-50% range.
[0009] Use of carbon-containing fuel for electricity generation in
electrochemical fuel cells (Direct Carbon Fuel Cell or DCFC) opens
an opportunity to eliminate release of fuel oxidation products and
contaminants into atmosphere, which is the major problem associated
with coal combustion power plants.
[0010] For general background on fuel cells, please refer to James
Larminie & Andrew Dicks, Fuel Cell Systems Explained (Wiley 2d
ed. 2003), and to EG&G Services et al., Fuel Cell Handbook
(U.S. Department of Energy, 5th ed. 2000).
[0011] A variety of schemes have been proposed for a direct carbon
fuel cell. None have as yet come to commercial fruition. For
example, U.S. Pat. No. 5,298,340 to Cocks and LaViers stated that
"[t]hermodynamic factors favor a solid carbon fuel cell over other
fuel cell designs." They proposed "the dissolution of carbon into a
solvent" which would "act[ ] as an anode." In their subsequent U.S.
Pat. No. 5,348,812, Cocks and LaViers taught that "[f]uel cells
containing an anode of molten metal into which carbon has been
dissolved, and a carbon-ion electrolyte, can be improved by making
the molten metal the same as that used as the cation on the solid
carbon-ion-electrolyte."
[0012] U.S. Pat. No. 6,607,853 to Hemmes discusses fuel cells based
on the oxidation of carbon and carbon-containing materials
contained in a molten corrosive salt. The possibility of internal
reformation is included, but not explicitly required. In Hemmes'
disclosure the molten corrosive salt contacts both the solid
electrolyte and the anode. Hemmes discloses that the anode may be
porous, made of nickel, and in contact with the solid electrolyte.
Hemmes also discloses that the anode may be positioned at a
distance from the solid electrolyte.
[0013] U.S. Pat. No. 6,692,861 to Tao addresses a fuel cell with a
carbon-containing anode and an electrolyte having a melting
temperature of between about 300.degree. C. and about 2000.degree.
C. in contact with the anode. U.S. Pat. No. 6,200,697 to P.
Pesavento of SARA, Inc., Cypress, Calif., describes a concept for
generating electricity using a carbon-containing consumable anode.
Because that concept employs a consumable anode, it is in essence a
large nonrechargeable battery. Subsequently, J. Cooper of Lawrence
Livermore National Laboratory has sought to develop a fuel cell
employing carbon nanopowder as fuel using molten carbonate
electrolyte, similar to the concept developed by Robert D. Weaver
at SRI International in the 1970s.
[0014] There is thus a need in the art for a direct carbon fuel
cell which can be scaled up effectively to a commercially viable
size, at a minimum to the hundreds of kilowatts of present-day
commercial phosphoric acid cogeneration fuel cell plants or molten
carbonate fuel cells, and which can operate efficiently with
naturally available fuels such as coal, coke, tar, biomass, and
various forms of carbon-containing wastes.
SUMMARY OF THE INVENTION
[0015] The patent describes fuel cells for converting fuel chemical
energy into electricity. The concept is based on the replacement of
the traditional three phase reaction boundary
(electrode-gas-electrolyte) with a two-phase boundary concept:
liquid electrode(s) mixed with fuel or oxidizer separated by a
solid electrolyte.
[0016] A preferred embodiment of the invention includes a fuel cell
which comprises an electronic conductor serving as an anode current
collector, a liquid anode, a fuel distributed in the liquid anode,
a solid oxygen ion-conductive electrolyte, a gas diffusion cathode,
an inlet for gaseous oxidizer, and an outlet for gas evolved during
the operation of the fuel cell.
[0017] A further preferred embodiment of the invention
interconnects a number of fuel cells in order to obtain higher
voltage and power. An electrical load, for example an DC/AC
inverter, having two terminals is connected with one terminal
connected to the anode current collector of the first fuel cell and
the other terminal connected to the cathode current collector of
the last fuel cell. Fuel cells may also be connected in parallel or
in series and in parallel.
[0018] A further preferred embodiment of the invention is a method
for supplying an electric current to a load. Fuel is mixed with a
conductive liquid anode. The fuel may be, for example, a
carbon-containing material (coal, coke, biomass, tar, or various
waste forms) or a metal (for example aluminum) in a powder form.
The electronically conductive mixture of the liquid anode with fuel
contacts a solid oxygen ion-conductive electrolyte. The fuel is
oxidized by oxygen ions entering the liquid anode from the
electrolyte, releasing electrons. An oxidizer is supplied to a
cathode connected to the opposite side of solid electrolyte,
causing an oxygen reduction reaction to occur to produce a flux of
oxygen ions through the electrolyte. The liquid anode is
electrically connected to a terminal of the load via an
electronically conductive current collector. With this method
executed in a preferred manner as discussed below, the current
supplied will be sufficient to transfer to the load at least 100 mW
for each square centimeter of cell working surface area.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a general schematic diagram of a liquid anode
electrochemical cell.
[0020] FIG. 2 is a schematic design for a liquid anode
electrochemical cell.
[0021] FIG. 3 illustrates a continuous operation liquid anode fuel
cell in a further embodiment of the present invention.
[0022] FIG. 4 illustrates a fuel cell stack assembly formed of a
number of fuel cells having liquid anode and cathode.
[0023] FIG. 5 shows cell power density as a function of current
density for an electrochemical cell of the invention.
[0024] FIG. 6 shows cell power density as a function of temperature
for an electrochemical cell of the invention operating with coal,
biomass, and tar.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0025] Before describing the present invention in detail, it is to
be understood that this invention is not limited to specific fuels,
materials, or device structures, as such may vary. It is also to be
understood that the terminology used herein is for the purpose of
describing particular embodiments only, and is not intended to be
limiting.
[0026] It must be noted that, as used in this specification and the
appended claims, the singular forms "a," "an," and "the" include
both singular and plural referents unless the context clearly
dictates otherwise. Thus, for example, reference to "an outlet"
includes a plurality of outlets as well as a single outlet,
reference to "an inlet" includes a plurality of inlets as well as
single inlet, and the like.
[0027] The present invention introduces electrochemical devices for
electrochemical energy conversion, which are believed to have
two-phase reaction zones. A two-phase reaction zone for oxidation
reactions is established by using a liquid anode. A two-phase
reaction zone for reduction reactions is established by using a
liquid cathode.
[0028] One embodiment of the present invention teaches a liquid
electrode fuel cell having a solid electrolyte with a gas diffusion
cathode on one side, and a liquid anode on the other side. The
liquid anode is electronically conductive media, based, for
example, on molten salts mixed with electronically conductive fuel
particles. The liquid anode also plays the role of the fuel
carrier. The liquid anode may be stagnant or it may recirculate
through the fuel cell constantly supplying fuel and removing fuel
oxidation products and fuel impurities. Electronic conductivity of
the liquid electrode makes the current collection scheme much more
efficient because the liquid electrode acts as if it were a part of
a distributed current collector.
[0029] In fuel cells of the invention with a liquid anode, the
anode will tend to conduct electrons reasonably well. Thus, the
electrochemical reaction will take place at the boundary between
the liquid anode mixed with fuel and an oxygen ion conducting
electrolyte (a two-dimensional "two-phase boundary"). In the case
of an ionically conductive liquid anode, that reaction zone may be
expanded to the bulk of the liquid anode (a three-dimensional
reaction zone). If the fuel is a finely dispersed solid within the
liquid anode, the electrochemical reaction can take place at a
large surface area. As a result, the liquid anode fuel cell will
have higher power density and will be able to generate more
electricity while having smaller geometrical dimensions. A liquid
electrode fuel cell will scale up to hundreds of kW or even tens of
MW of electric output more easily than conventional fuel cells with
a three-phase reaction boundary, such as Solid Oxide Fuel Cells
(SOFC).
[0030] Similar advantages in terms of reaction area may be obtained
for the reduction of oxygen if the cathode is an ionic liquid.
[0031] There are other technical, manufacturing, and operating
advantages besides a large reaction area to having one or both of
the electrode(s) in an electrochemical system be liquid. From a
heat transfer perspective, a liquid anode carrying solid fuel will
tend to have much higher heat capacity than gaseous fuel, reducing
heat differentials and allowing efficient transfer of heat
generated in the electrochemical system. Liquids do not raise the
same concerns with thermal expansion mismatch that solid electrodes
do, and so there may be longevity advantages to the use of liquid
electrodes. Recirculation of the liquid anode provides a means to
achieve close to complete utilization of the fuel. It is also
simpler with liquid anode fuel cells to provide an exit path for
the gases evolved and reduce accumulation of impurities. In
contrast, in gas-fueled cells, fuel oxidation gases will dilute the
incoming fuel stream so that portions of the fuel cell which are
downstream in the anode gas flow may be comparatively poorly
supplied with fuel, adversely affecting efficiency.
[0032] A fuel oxidation reaction which has a high reaction surface
area in a liquid anode can run without catalyst. This is an
important positive attribute for liquid anodes because commonly
available fuels such as coal and biomass may contain impurities
that would poison a catalyst.
[0033] Sealing requirements to separate fuel and oxidizer are not
so stringent as in the case of gas fueled cells, which helps
achieve low cost, reliability, and scalability.
[0034] The fuel cells of the invention may be operated with static
or flow-through modes of operation.
[0035] In a static mode of operation, the liquid electrolyte (or
liquid/solid composite electrolyte/electron-conductor/fuel)
undergoes no net motion during cell operation. Stirring may be
used, however, to facilitate particle-particle and/or
particle-electrode contact, enhance diffusional mass transport, or
to dislodge trapped gas bubbles. Furthermore, the liquid anode may
be periodically drained to remove suspended and dissolved
impurities, and replaced with fresh liquid.
[0036] In a flowing mode of operation, liquid anodes and/or
cathodes are caused to flow during cell operation. The flow may be
gravitationally induced from an upper reservoir to a lower.
Alternatively the fluid can be pumped in a continuously
recirculating flow down, along, or up through the cell. The purpose
of the flow may be to enhance diffusional mass transport, to
dislodge trapped gas bubbles or to remove suspended and dissolved
impurities in continuous recirculation.
[0037] The electrochemical devices of the invention may be, for
example, of tubular, planar, or monolith configurations.
[0038] FIG. 1 depicts in schematic form an exemplary configuration
of the liquid anode concept of the invention. The liquid anode 10
contains fuel particles depicted as black circles 12. The liquid
anode also contains gases evolved by the anode reaction, depicted
as bubbles 14. Immersed within the liquid anode is an anode current
collector 16. O.sup.2- ions pass through electrolyte 20 entering
the liquid anode 10. On the other side of electrolyte 20, there is
a cathode 24. It is in contact with a cathode current collector 26.
Passing over the cathode is gaseous oxidizer 28, which is reduced
at the cathode creating the O.sup.2- ions.
[0039] A preferred embodiment of the invention includes a fuel cell
which comprises an electronic conductor serving as an anode current
collector, a liquid anode, a fuel distributed in the liquid anode,
a solid oxygen ion-conductive electrolyte, a gas diffusion cathode,
an electronic conductor serving as a cathode current collector, an
inlet for gaseous oxidizer, a second inlet for replenishing or
recirculating the liquid anode, an outlet for used gaseous
oxidizer, with an optional second outlet for recirculating fuel and
gas evolved during the operation of the fuel cell.
[0040] In the normal operation of such a fuel cell, the two
terminals of an electrical load are connected to the anode current
collector and the cathode current collector. Preferably, a number
of such fuel cells would be interconnected, with one terminal of
the electrical load connected to the anode current collector of the
first cell and the other terminal connected to the cathode current
collector of the last cell. The electrical load will typically
include a DC/AC converter.
[0041] The anode current collector may be any suitable metal or
other electronic conductor compatible with the conditions of use.
Preferably the anode current collector is in the form of a mesh or
a spiral. Alternatively, the anode current collector may have a
solid shape defining channels in which the liquid anode exists. The
channels may serve to channel the liquid anode when it
recirculates.
[0042] The liquid anode is any liquid which is electronically
conductive when mixed with fuel particles and compatible with
conditions such as operating temperatures. Preferably the liquid
anode is a composition of molten salts and molten oxides. Examples
of suitable salts include eutectic mixtures of K.sub.2CO.sub.3,
Li.sub.2CO.sub.3, and/or Na.sub.2CO.sub.3.
[0043] It is preferred that the liquid anode containing fuel be
recirculated using natural circulation or a pump. Such
recirculation has, for example, the benefit that it achieves a more
uniform distribution of the fuel within the liquid anode.
[0044] Suitable solid fuels, for example, are those containing
carbon, which undergoes the reaction
C+2O.sup.2-.fwdarw.CO.sub.2+4.sup.-e given a suitable supply of
O.sup.2- and a suitable sink for electrons e.sup.- or metals, such
as aluminum.
[0045] Usable fuels containing carbon can be, for example, coal,
coke, tar, biomass, and plastic waste. Preferably the fuel
comprises solid particles.
[0046] The concentration of fuel in the liquid anode will vary over
time as the fuel cell is operated and fuel is consumed and
replenished. The choice of fuel concentration can influence the
efficiency of the fuel cell. As it was observed experimentally,
anode ohmic losses can be reduced significantly by a higher
concentration of fuel.
[0047] The electrolyte may be selected from a group of solid oxide
oxygen ion conductive materials, stable in the expected operating
conditions, and can be fabricated in the form of thin layers on a
supporting substrate. Among suitable electrolytes are
ytrria-stabilized zirconia (YSZ) and lanthanum gallate with doping
of the lanthanum sublattice with strontium from about 0% to about
30% and of the gallium sublattice with lithium oxide from about 0%
to about 30%. Additives, such as alumina, may be included in modest
quantities to stabilize the electrolyte further.
[0048] In a preferred embodiment, the electrolyte may be a thin
film electrolyte with thickness of, for example, 1-50 microns
deposited on a supporting cathode. Alternatively, it may be a
self-supporting solid electrolyte with thickness, for example, up
to 0.3-0.8 mm.
[0049] The cathode may be any suitable gas permeable mixed
conductive material with the coefficient of thermal expansion (CTR)
compatible with the CTR of the electrolyte. The cathode may be of
the type referred to as "gas diffusion" cathodes. A preferred
cathode is strontium doped lanthanum manganite.
[0050] In an alternative embodiment of the invention, the cathode
may be an ionic liquid carrying oxidizer.
[0051] The cathode current collector is an electronically
conductive material such as metal or alloy stable under given
oxidizing conditions. The cathode current collector preferably
contacts the cathode at a number of points. When the cathode
current collector is metallic, a mesh or a spiral are preferred
shapes of the cathode current collector.
[0052] A preferred oxidizer is air.
[0053] The fuel cell is enclosed in a vessel of a suitable material
adapted to the conditions of operation. A number of geometric
arrangements of the fuel cell are possible. The common arrangement
for fuel cells is as a large number of planar or tubular fuel
elements which are connected together into a "stack."
[0054] The temperatures employed in fuel cells of the invention, in
order to cause the anode and cathode reactions to proceed a
desirable rate, mean that the gases leaving the fuel cells (for
example CO.sub.2 where the anode reaction is
C+2O.sup.2-.fwdarw.CO.sub.2+4e.sup.-) will contain considerable
usable thermal energy. The operation of the fuel cell will evolve
heat, for example through ohmic loss, which will be transmitted to
the gases as well as to the liquid anode. In such an arrangement
preferably the heat of the exiting gases would be made use of in
some way. It could be conveyed to other fluids, for example
incoming oxidizer, water needing to be heated, or fluid to be used
for heating a structure, by means of a heat exchange mechanism. The
use of the heat of the exiting gases or, in some cases, the heat of
the circulating liquid anode, would permit the fuel cells of the
invention to be used in a combined heating and power plant, in the
manner that existing phosphoric acid, molten carbonate electrolyte,
and solid oxide fuel cells are employed.
[0055] There is considerable concern with CO.sub.2 emissions into
the atmosphere based on the belief that they give rise to a
greenhouse effect which warms the earth. The CO.sub.2 produced
where the anode reaction is C+2O.sup.2-.fwdarw.CO.sub.2+4e.sup.- is
therefore readily confined and may be sequestered in some manner or
utilized further, for example in the cathode gas stream of a
conventional hydrogen fuelled molten carbonate electrolyte fuel
cell. The relative purity of the CO.sub.2 produced when the anode
reaction is C+2O.sup.2-.fwdarw.CO.sub.2+4e.sup.- facilitates its
sequestration.
[0056] In the fuel cells of the invention it is possible to provide
fuel in a continuous or batch fashion to the liquid anode. In some
cases it may be possible to replenish the fuel periodically in a
batch fashion through an inlet giving access to the liquid anode.
It may also be desirable, when naturally occurring fuels such as
biomass are used, to remove from the liquid anode unreacted residue
of the fuel. Furthermore, gases generated by the anode reaction and
fuel impurities may evolve or dissolve in the liquid anode material
and have to be removed to prevent contamination of the liquid
anode. For these purposes, it is preferred that the circulation of
the liquid anode material be such that some fraction of that
material is at a location where it can conveniently be accessed for
purposes of replenishment or, if desired, removal of fuel residue
and/or dissolved gases.
[0057] In a particularly preferred embodiment of the invention, the
fuel is a ground solid, for example carbon, coal, wood, or
aluminum. The anode is a liquid, for example a eutectic mixture of
suitable molten salts. The anode current collector is a metal
member immersed in the liquid anode and adjacent to the surface of
solid electrolyte. The electrolyte is an O.sup.2- conducting solid,
for example yttria-stabilized zirconia. The cathode is a suitable
porous mixed conductor, for example lanthanum strontium manganite.
The cathode current collector is a metal member contacting the
cathode at a considerable number of points. The operating
temperature is between 600.degree. C. and 1000.degree. C.
[0058] FIG. 2 depicts a configuration of this preferred embodiment
used to study experimentally the performance of the liquid anode
fuel cell. As may be seen, a tubular arrangement was adopted, with
the platinum anode current collector 48 and liquid anode 40, which
comprises a molten mixture of
Li.sub.2CO.sub.3+K.sub.2CO.sub.3+Na.sub.2CO.sub.3, enclosed by a
containing tube 50. The solid YSZ electrolyte closed end tube with
wall thickness 0.3-0.8 mm 42 is immersed into the anode, and the
LSM cathode 46 is deposited as a 1 mm layer on the inner surface of
the solid electrolyte tube. The cathode current collector 49 lies
inward of the cathode itself. The tube containing the fuel cell is
closed on both ends. Air is supplied through the inner portion of
the tube 45 and exhausted through a concentric tube 44. A version
of FIG. 2 has been constructed with a YSZ tube diameter 10 mm, the
height of the cathode 10 mm, and employing a containing tube inner
diameter of 27 mm.
[0059] FIG. 3 illustrates a continuous operation fuel cell 500 in a
further embodiment of the present invention. The fuel cell 500
includes liquid electrodes 502 and 504 equipped with current
collectors (not shown) separated by a solid ion conductive
electrolyte 506, a fuel dispensing module 508, oxidizer supplying
module 510, a module for separation of the reaction products 512,
an anode circulation module 514 (e.g., a pump), a cathode
circulation module 516, and two heat exchangers 518 and 520.
[0060] FIG. 4 illustrates a fuel cell stack assembly 550 having a
plurality of liquid electrode planar fuel cells 552 according to a
different embodiment of the present invention. It is seen that both
anode and cathode are liquid in this embodiment, and that the
respective anode and cathode current collectors form channels for
the flow of the liquids. The liquid electrodes are circulated by
means of one or more electrically insulated pumps (not shown) in
order to establish a uniform distribution of fuel and oxidizer over
the reaction zone.
[0061] FIG. 5 depicts power density as a function of current
density for the fuel cell shown in FIG. 2 at 950.degree. C. with
PRB coal as fuel. Volumetric fuel content in the liquid anode was
about 40%. An electromotive force of about 1.4 V was observed in
this experiment. A maximal power density above 100 mW/cm.sup.2 was
observed. This level of power density achieved with real fuel
suggests that the inventive fuel cell has considerable commercial
potential, as one may conclude from comparison with commercially
available molten carbonate fuel cells, which have power outputs
above 100 kW and power densities close to 100 mW/cm.sup.2. (As is
normal in describing fuel cell operation, power density was
obtained in FIG. 5 dividing power by cell working surface area. For
the configuration of FIG. 2, this working surface area is the area
of a cylinder with diameter equal to the outer diameter of the YSZ
tube. The height of the cylinder is the minimal height among the
cathode, cathode current collector, and anode current
collector.)
[0062] FIG. 6 depicts the peak power density as a function of
operating temperature observed at similar conditions for coal and
other real fuels--biomass (pine saw dust) and tar (Maya atmospheric
tower bottom) with the preferred fuel cell shown in FIG. 2 and
described above.
[0063] It is contemplated that fuel cells of the invention will be
interconnected and assembled in a stack. Sufficient number of
stacks will be interconnected to achieve desired power output from
a standalone unit. The modular principle and the resulting
scalability are an attractive feature of the invention. For
example, this plant could produce hundreds of kilowatts as a
distributed power generation unit or be scaled up to tens of
megawatts for centralized power generation.
[0064] It is to be understood that while the invention has been
described in conjunction with the preferred specific embodiments
thereof, that the foregoing description and the examples that
follow are intended to illustrate and not limit the scope of the
invention. Other aspects, advantages, and modifications within the
scope of the invention will be apparent to those skilled in the art
to which the invention pertains.
[0065] All patents, patent applications, and publications mentioned
herein are hereby incorporated by reference in their entireties.
However, where a patent, patent application, or publication
containing express definitions is incorporated by reference, those
express definitions should be understood to apply to the
incorporated patent, patent application, or publication in which
they are found, and not to the remainder of the text of this
application, in particular the claims of this application.
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