U.S. patent application number 11/355249 was filed with the patent office on 2007-08-16 for direct oxidation fuel cell systems with regulated fuel concentration and oxidant flow.
Invention is credited to Masahiro Takada, Chao-Yang Wang.
Application Number | 20070190378 11/355249 |
Document ID | / |
Family ID | 38007590 |
Filed Date | 2007-08-16 |
United States Patent
Application |
20070190378 |
Kind Code |
A1 |
Takada; Masahiro ; et
al. |
August 16, 2007 |
Direct oxidation fuel cell systems with regulated fuel
concentration and oxidant flow
Abstract
A direct oxidation fuel cell (DOFC) system, comprises at least
one fuel cell assembly including a cathode and an anode with an
electrolyte positioned therebetween; a source of liquid fuel in
fluid communication with an inlet of the anode; an oxidant supply
in fluid communication with an inlet of the cathode; a liquid/gas
(L/G) separator in fluid communication with outlets of the anode
and cathode for: (1) receiving unreacted fuel and liquid and
gaseous products, and (2) supplying a solution of fuel and liquid
product to the anode inlet; and a control system for measuring the
amount of liquid product and controlling oxidant stoichiometry of
the system operation in response to the measured amount of liquid
product. Alternatively, the control system controls the
concentration of the liquid fuel in the solution supplied to the
anode inlet, based upon the system operating temperature or output
power.
Inventors: |
Takada; Masahiro; (Shizuoka,
JP) ; Wang; Chao-Yang; (State College, PA) |
Correspondence
Address: |
MCDERMOTT WILL & EMERY LLP
600 13TH STREET, N.W.
WASHINGTON
DC
20005-3096
US
|
Family ID: |
38007590 |
Appl. No.: |
11/355249 |
Filed: |
February 16, 2006 |
Current U.S.
Class: |
429/410 ;
429/442; 429/447; 429/449 |
Current CPC
Class: |
H01M 8/04291 20130101;
H01M 8/1009 20130101; H01M 8/04395 20130101; H01M 8/04164 20130101;
H01M 8/04798 20130101; Y02E 60/50 20130101; H01M 8/04313 20130101;
H01M 8/04447 20130101; H01M 8/04365 20130101 |
Class at
Publication: |
429/022 ;
429/034; 429/013 |
International
Class: |
H01M 8/04 20060101
H01M008/04 |
Claims
1. A direct oxidation fuel cell (DOFC) system, comprising: (a) at
least one fuel cell assembly including a cathode and an anode with
an electrolyte positioned therebetween; (by a source of liquid fuel
in fluid communication with an inlet of said anode; (c) an oxidant
supply in fluid communication with an inlet of said cathode; (d) a
liquid/gas (L/G) separator in fluid communication with outlets of
said anode and cathode for: (1) receiving unreacted fuel, liquid
product, and gases, and (2) supplying a solution of liquid fuel in
liquid product to said inlet of said anode; and (e) a control
system for measuring the amount of said liquid product and
controlling oxidant stoichiometry of said DOFC system during
operation at an appropriate value in response to said measured
amount of liquid product.
2. The DOFC system as in claim 1, wherein: said control system
includes a sensor for measuring said amount of liquid product.
3. The DOFC system as in claim 2, wherein: said sensor measures the
amount of said liquid product contained in said L/G separator.
4. The DOFC system as in claim 1, wherein: said control system is
capable of periodically or continuously controlling said oxidant
stoichiometry.
5. The DOFC system as in claim 1, wherein: said control system
comprises an electronic control unit (ECU).
6. The DOFC system as in claim 5, wherein: said ECU, comprises an
electronic computer programmed for: (1) comparing said measured
amount of liquid product with a predetermined amount for
determining whether said measured amount is greater than, smaller
than, or the same as said predetermined amount; (2) determining a
calculation factor based upon said comparison; (3) calculating said
appropriate value of oxidant stoichiometry utilizing said
calculation factor; and (4) regulating said oxidant supply to
achieve said appropriate value of oxidant stoichiometry.
7. A direct oxidation fuel cell (DOFC) system, comprising: (a) at
least one fuel cell assembly including a cathode and an anode with
an electrolyte positioned therebetween; (b) a source of liquid fuel
in fluid communication with an inlet of said anode; (c) an oxidant
supply in fluid communication with an inlet of said cathode; (d) a
liquid/gas (L/G) separator for: (1) receiving unreacted fuel,
liquid product, and gases from said cathode and anode, and (2)
supplying a solution of liquid fuel in liquid product to said inlet
of said anode; and (e) a control system for controlling the
concentration of said liquid fuel in said solution supplied to said
inlet of said anode.
8. The DOFC system as in claim 7, wherein: said control system is
capable of regulating supply of said liquid fuel to said inlet of
said anode from said source of liquid fuel and from said L/G
separator.
9. The DOFC system as in claim 8, wherein: said control system is
capable of periodically or continuously controlling said oxidant
stoichiometry and comprises an electronic control unit (ECU).
10. The DOFC system as in claim 9, wherein: said control system
includes a sensor for measuring the operating temperature of said
at least one fuel cell assembly, and said ECU comprises an
electronic computer programmed for: (1) determining an appropriate
concentration of said liquid fuel in said solution supplied to said
inlet of said anode based upon the operating temperature of said at
least one fuel cell assembly measured by said sensor; and (2)
regulating said supply of said liquid fuel to said inlet of said
anode from said source of liquid fuel and from said L/G separator
to achieve said appropriate concentration.
11. The DOFC system as in claim 10, wherein: said computer is
programmed with a predetermined relationship between said
concentration of said liquid fuel supplied to said inlet of said
anode and said operating temperature of said at least one fuel cell
assembly.
12. The DOFC system as in claim 9, wherein: said ECU comprises an
electronic computer programmed for: (1) determining an appropriate
concentration of said liquid fuel in said solution supplied to said
inlet of said anode based upon a desired output power of said at
least one fuel cell assembly; and (2) regulating said supply of
said liquid fuel to said inlet of said anode from said source of
liquid fuel and from said L/G separator to achieve said appropriate
concentration.
13. The DOFC system as in claim 12, wherein: said computer is
programmed with a predetermined relationship between said
concentration of said liquid fuel in said solution supplied to said
inlet of said anode and said output power of said at least one fuel
cell assembly.
14. A method of operating a direct oxidation fuel cell (DOFC)
system comprising at least one fuel cell assembly including a
cathode and an anode with an electrolyte positioned therebetween, a
source of liquid fuel in fluid communication with an inlet of said
anode, an oxidant supply in fluid communication with an inlet of
said cathode; and a liquid/gas (L/G) separator in fluid
communication with outlets of said anode and cathode for: (I)
receiving unreacted fuel, liquid product, and gases, and (2)
supplying a solution of liquid fuel in liquid product to said inlet
of said anode, comprising: measuring the amount of said liquid
product and controlling oxidant stoichiometry of said DOFC system
during operation at an appropriate value in response to said
measured amount of liquid product.
15. The method according to claim 14, comprising: utilizing a
sensor capable of measuring the amount of said liquid product
contained in said L/G separator.
16. The method according to claim 15, further comprising: utilizing
an electronic computer programmed for: (1) comparing said measured
amount of liquid product with a predetermined amount for
determining whether said measured amount is greater than, smaller
than, or the same as said predetermined amount; (2) determining a
calculation factor based upon said comparison; (3) calculating said
appropriate oxidant stoichiometry utilizing said calculation
factor; and (4) regulating said oxidant supply to achieve said
appropriate oxidant stoichiometry.
17. A method of operating a direct oxidation fuel cell (DOFC)
system comprising at least one fuel cell assembly including a
cathode and an anode with an electrolyte positioned therebetween, a
source of liquid fuel in fluid communication with an inlet of said
anode, an oxidant supply in fluid communication with an inlet of
said cathode; and a liquid/gas (L/G) separator in fluid
communication with outlets of said anode and cathode for: (1)
receiving unreacted fuel, liquid product, and gases, and (2)
supplying a solution of liquid fuel in liquid product to said inlet
of said anode, comprising: controlling the concentration of said
liquid fuel in said solution supplied to said inlet of said
anode.
18. The method according to claim 17, comprising: regulating supply
of said liquid fuel to said inlet of said anode from said source of
liquid fuel and from said L/G separator.
19. The method according to claim 18, comprising: utilizing a
sensor for measuring the operating temperature of said at least one
fuel cell assembly and an electronic computer programmed for: (1)
determining an appropriate concentration of said liquid fuel in
said solution supplied to said inlet of said anode based upon the
operating temperature of said at least one fuel cell assembly
measured by said sensor; and (2) regulating said supply of said
liquid fuel to said inlet of said anode from said source of liquid
fuel and from said L/G separator to achieve said appropriate
concentration.
20. The method according to claim 19, comprising: utilizing a
computer programmed with a predetermined relationship between said
concentration of said liquid fuel in said solution supplied to said
inlet of said anode and said operating temperature of said at least
one fuel cell assembly.
21. The method according to claim 18, comprising: utilizing an
electronic computer programmed for: (1) determining an appropriate
concentration of said liquid fuel in said solution supplied to said
inlet of said anode based upon a desired output power of said at
least one fuel cell assembly; and (2) regulating said supply of
said liquid fuel to said inlet of said anode from said source of
liquid fuel and from said L/G separator to achieve said appropriate
concentration.
22. The method according to claim 21, comprising: utilizing a
computer programmed with a predetermined relationship between said
concentration of said liquid fuel in said solution supplied to said
inlet of said anode and said output power of said at least one fuel
cell assembly.
Description
FIELD OF THE INVENTION
[0001] The present disclosure relates generally to fuel cells, fuel
cell systems, and methods of operating same. More specifically, the
present disclosure relates to direct oxidation fuel cells (DOFC's),
such as direct methanol fuel cells (DMFC's), and their systems and
operating methods.
BACKGROUND OF THE DISCLOSURE
[0002] A direct oxidation fuel cell (DOFC) is an electrochemical
device that generates electricity from electrochemical oxidation of
a liquid fuel. DOFC's do not require a preliminary fuel processing
stage; hence, they offer considerable weight and space advantages
over indirect fuel cells, i.e., cells requiring preliminary fuel
processing. Liquid fuels of interest for use in DOFC's include
methanol (CH.sub.3OH), formic acid, dimethyl ether (DME), etc., and
their aqueous solutions. The oxidant may be substantially pure
oxygen (O.sub.2) or a dilute stream of oxygen, such as that in air.
Significant advantages of employing a DOFC in portable and mobile
applications (e.g., notebook computers, mobile phones, PDA's, etc.)
include easy storage/handling and high energy density of the liquid
fuel.
[0003] One example of a DOFC system is a direct methanol fuel cell
(DMFC). A DMFC generally employs a membrane-electrode assembly
(hereinafter "MEA") having an anode, a cathode, and a
proton-conducting membrane electrolyte positioned therebetween. A
typical example of a membrane electrolyte is one composed of a
perfluorosulfonic acid--tetrafluorethylene copolymer, such as
Nafion.RTM. (Nafion.RTM. is a registered trademark of E.I. Dupont
de Nemours and Company). In a DMFC, a methanol/water solution is
directly supplied to the anode as the fuel and air is supplied to
the cathode as the oxidant. At the anode, the methanol (CH.sub.3OH)
reacts with the water (H.sub.2O) in the presence of a catalyst,
typically a Pt or Ru metal-based catalyst, to produce carbon
dioxide (CO.sub.2), protons (H.sup.+ ions), and electrons
(e.sup.-). The electrochemical reaction is shown as equation (1)
below: CH.sub.3OH+H.sub.2O.fwdarw.CO.sub.2+6H.sup.++6e.sup.-
(1)
[0004] During operation of the DMFC, the protons migrate to the
cathode through the proton-conducting membrane electrolyte, which
is non-conductive to electrons. The electrons travel to the cathode
through an external circuit for delivery of electrical power to a
load device. At the cathode, the protons, electrons, and oxygen
(O.sub.2) molecules, typically derived from air, are combined to
form water. The electrochemical reaction is given in equation (2)
below: 3/2O.sub.2+6H.sup.++6e.sup.-.fwdarw.3H.sub.2O (2)
[0005] Electrochemical reactions (1) and (2) form an overall cell
reaction as shown in equation (3) below:
CH.sub.3OH+3/2O.sub.2.fwdarw.CO.sub.2+2H.sub.2O (3)
[0006] One drawback of a conventional DMFC is that the methanol
partly permeates the membrane electrolyte from the anode to the
cathode, such permeated methanol being termed "crossover methanol".
The crossover methanol chemically (i.e., not electrochemically)
reacts with oxygen at the cathode, causing a reduction in fuel
utilization efficiency and cathode potential, with a corresponding
reduction in power generation of the fuel cell. It is thus
conventional for DMFC systems to use excessively dilute (3-6% by
vol.) methanol solutions for the anode reaction in order to limit
methanol crossover and its detrimental consequences. However, the
problem with such a DMFC system is that it requires a significant
amount of water to be carried in a portable system, thus
diminishing the system energy density.
[0007] The ability to use highly concentrated fuel is desirable for
portable power sources, particularly since DMFC technology is
currently competing with advanced batteries, such as those based
upon lithium-ion technology. However, even if the fuel cartridge
with highly concentrated fuel (e.g., pure or "neat" methanol)
carries little to no water, the anodic reaction, i.e., equation
(1), still requires one water molecule for each methanol molecule
for complete electro-oxidation. Simultaneously, water is produced
at the cathode via reduction of oxygen, i.e., equation (2).
Therefore, in order to take full advantage of a fuel cell employing
highly concentrated fuel, it would be desirable to: (a) maintain a
net water balance in the cell where the total water loss from the
cell (mainly through the cathode) preferably does not exceed the
net production of water (i.e., two water molecules per each
methanol molecule consumed according to equation (3)), and (b)
transport some of the produced water from the cathode to anode.
[0008] Two approaches have been developed to meet the
above-mentioned goals in order to directly use concentrated fuel. A
first approach is an active water condensing and pumping system to
recover cathode water vapor and return it to the anode (U.S. Pat.
No. 5,599,638). While this method achieves the goal of carrying
concentrated (and even neat) methanol in the fuel cartridge, it
suffers from a significant increase in system volume and parasitic
power loss due to the need for a bulky condenser and its
cooling/pumping accessories.
[0009] The second approach is a passive water return technique in
which hydraulic pressure at the cathode is generated by including a
highly hydrophobic microporous layer (MPL) in the cathode, and this
pressure is utilized for driving water from the cathode to the
anode through a thin membrane (Ren et al. and Pasaogullari &
Wang, J. Electrochem. Soc., pp A399-A406, March 2004). While this
passive approach is efficient and does not incur parasitic power
loss, the amount of water returned, and hence the concentration of
methanol fuel, depends strongly on the cell temperature and power
density. Presently, direct use of neat methanol is demonstrated
only at or below 40.degree. C. and at low power density (less than
30 mW/cm.sup.2). Considerably less concentrated methanol fuel is
utilized in high power density (e.g., 60 mW/cm.sup.2) systems at
elevated temperatures, such as 60.degree. C. In addition, the
requirement for thin membranes in this method sacrifices fuel
efficiency and operating cell voltage, thus resulting in lower
total energy efficiency.
[0010] Thus, there is a prevailing need for DOFC/DMFC systems that
maintain a balance of water in the fuel cell and return a
sufficient amount of water from the cathode to the anode under
high-power and elevated temperature operating conditions. There is
an additional need for DOFC/DMFC systems that operate with highly
concentrated fuel, including neat methanol, and minimize the need
for external water supplies or condensation of electrochemically
produced water.
[0011] A further need exists for DOFC/DMFC systems and operating
methods therefor which facilitate operation under various and
dynamically changing conditions and scenarios, e.g., as where
variable control of the operating (output) current, hence fuel
conversion efficiency, is required for matching with dynamically
changing requirements of the electrical load.
[0012] In view of the foregoing, there exists a need for improved
DOFC/DMFC systems and methodologies which facilitate variable
(i.e., dynamic) control of the operating parameters of such systems
for obtaining optimal performance with very highly concentrated
fuel and high power efficiency.
SUMMARY OF THE DISCLOSURE
[0013] An advantage of the present disclosure is improved direct
oxidation fuel cell (DOFC) systems including control systems
adapted for measuring the amount of a product formed during
operation and controlling the oxidant stoichiometry in response to
the measured amount.
[0014] Another advantage of the present disclosure is improved DOFC
systems including control systems adapted for controlling the
concentration of liquid fuel in a solution supplied to an anode of
an electrode assembly of the system.
[0015] Still another advantage of the present disclosure is an
improved method of operating DOFC systems, including measuring the
amount of a product formed during operation and controlling the
oxidant stoichiometry in response to the measured amount.
[0016] Yet another advantage of the present disclosure is an
improved method of operating DOFC systems, including controlling
the concentration of liquid fuel in a solution supplied to an anode
of an electrode assembly of the system.
[0017] Additional advantages and features of the present disclosure
will be set forth in the disclosure which follows and in part will
become apparent to those having ordinary skill in the art upon
examination of the following or may be learned from the practice of
the present disclosure. The advantages may be realized and obtained
as particularly pointed out in the appended claims.
[0018] According to an aspect of the present disclosure, the
foregoing and other advantages are achieved in part by an improved
direct oxidation fuel cell (DOFC) system, comprising:
[0019] (a) at least one fuel cell assembly including a cathode and
an anode with an electrolyte positioned therebetween;
[0020] (b) a source of liquid fuel in fluid communication with an
inlet of the anode;
[0021] (c) an oxidant supply in fluid communication with an inlet
of the cathode;
[0022] (d) a liquid/gas (L/G) separator in fluid communication with
outlets of the anode and cathode for: (1) receiving unreacted fuel,
liquid product, and gases, and (2) supplying a solution of liquid
fuel and liquid product to the inlet of the anode; and
[0023] (e) a control system for measuring the amount of the liquid
product and controlling oxidant stoichiometry of the DOFC system
during operation at an appropriate value in response to the
measured amount of liquid product.
[0024] According to embodiments of the present disclosure, the
control system includes a sensor for measuring the amount of liquid
product; the sensor measures the amount of the liquid product
contained in the L/G separator; the control system is capable of
periodically or continuously controlling the oxidant stoichiometry;
and the control system comprises an electronic control unit
(ECU).
[0025] Preferably, the ECU comprises an electronic computer
programmed for: (1) comparing the measured amount of liquid product
with a predetermined amount for determining whether the measured
amount is greater, smaller, or the same as the predetermined
amount; (2) determining a calculation factor based upon the
comparison; (3) calculating the appropriate value of oxidant
stoichiometry utilizing the calculation factor; and (4) regulating
the oxidant supply to achieve the appropriate value of oxidant
stoichiometry.
[0026] Another aspect of the present disclosure is an improved
direct oxidation fuel cell (DOFC) system, comprising:
[0027] (a) at least one fuel cell assembly including a cathode and
an anode with an electrolyte positioned therebetween;
[0028] (b) a source of liquid fuel in fluid communication with an
inlet of the anode;
[0029] (c) an oxidant supply in fluid communication with an inlet
of the cathode;
[0030] (d) a liquid/gas (L/G) separator for: (1) receiving
unreacted fuel, liquid product, and gases from the cathode and
anode, and (2) supplying a solution of liquid fuel in liquid
product to the inlet of the anode; and
[0031] (e) a control system adapted for controlling the
concentration of the liquid fuel in the solution supplied to the
inlet of the anode.
[0032] According to the present disclosure, the control system is
capable of regulating supply of the liquid fuel to the inlet of the
anode from the source of liquid fuel and from the L/G separator;
the control system is capable of periodically or continuously
controlling the oxidant stoichiometry and comprises an electronic
control unit (ECU); and the control system includes a sensor for
measuring the operating temperature of the at least one fuel cell
assembly.
[0033] In accordance with certain embodiments of the present
disclosure, the ECU comprises an electronic computer capable of:
(1) determining an appropriate concentration of the liquid fuel in
the solution supplied to the inlet of the anode based upon the
operating temperature of the at least one fuel cell assembly
measured by the sensor; and (2) regulating the supply of the liquid
fuel to the inlet of the anode from the source of liquid fuel and
from the L/G separator to achieve the appropriate concentration;
wherein the computer is programmed with a predetermined
relationship between the concentration of the liquid fuel supplied
to the inlet of the anode and the operating temperature of the at
least one fuel cell assembly.
[0034] According to other embodiments of the present disclosure,
the ECU comprises an electronic computer programmed for: (1)
determining an appropriate concentration of the liquid fuel in the
solution supplied to the inlet of the anode based upon a desired
output power of the at least one fuel cell assembly; and (2)
regulating the supply of the liquid fuel to the inlet of the anode
from the source of liquid fuel and from the L/G separator to
achieve the appropriate concentration; wherein the computer is
programmed with a predetermined relationship between the
concentration of the liquid fuel supplied to the inlet of the anode
and the output power of the at least one fuel cell assembly.
[0035] Still another aspect of the present disclosure is an
improved method of operating a direct oxidation fuel cell (DOFC)
system comprising at least one fuel cell assembly including a
cathode and an anode with an electrolyte positioned therebetween, a
source of liquid fuel in fluid communication with an inlet of the
anode, an oxidant supply in fluid communication with an inlet of
the cathode; and a liquid/gas (L/G) separator in fluid
communication with outlets of the anode and cathode for: (1)
receiving unreacted fuel, liquid product, and gases, and (2)
supplying a solution of liquid fuel in liquid product to the inlet
of the anode, comprising:
[0036] measuring the amount of the liquid product and controlling
oxidant stoichiometry of the DOFC system during operation at an
appropriate value in response to the measured amount of liquid
product.
[0037] According to embodiments of the present disclosure, the
method comprises utilizing a sensor adapted for measuring the
amount of the liquid product contained in the L/G separator, and
utilizing an electronic computer programmed for: (1) comparing the
measured amount of liquid product with a predetermined amount for
determining whether the measured amount is greater, smaller, or the
same as the predetermined amount; (2) determining a calculation
factor based upon the comparison; (3) calculating the appropriate
oxidant stoichiometry utilizing the calculation factor; and (4)
regulating the oxidant supply to achieve the appropriate oxidant
stoichiometry.
[0038] Yet another aspect of the present disclosure is an improved
method of operating a direct oxidation fuel cell (DOFC) system
comprising at least one fuel cell assembly including a cathode and
an anode with an electrolyte positioned therebetween, a source of
liquid fuel in fluid communication with an inlet of the anode, an
oxidant supply in fluid communication with an inlet of the cathode;
and a liquid/gas (L/G) separator in fluid communication with
outlets of the anode and cathode for: (1) receiving unreacted fuel,
liquid product, and gases, and (2) supplying a solution of liquid
fuel in liquid product to the inlet of the anode, comprising:
[0039] controlling the concentration of the liquid fuel in the
solution supplied to the inlet of the anode.
[0040] According to certain embodiments of the disclosure, the
method comprises regulating supply of the liquid fuel to the inlet
of the anode from the source of liquid fuel and from the L/G
separator; wherein the method includes utilizing a sensor for
measuring the operating temperature of the at least one fuel cell
assembly and an electronic computer programmed for: (1) determining
an appropriate concentration of the liquid fuel in the solution
supplied to the inlet of the anode based upon the operating
temperature of the at least one fuel cell assembly measured by the
sensor; and (2) regulating the supply of the liquid fuel to the
inlet of the anode from the source of liquid fuel and from the L/G
separator to achieve the appropriate concentration. Preferably, a
computer is utilized which is programmed with a predetermined
relationship between the concentration of the liquid fuel supplied
to the inlet of the anode and the operating temperature of the at
least one fuel cell assembly.
[0041] According to other embodiments of the present disclosure,
the method comprises utilizing an electronic computer programmed
for: (1) determining an appropriate concentration of the liquid
fuel in the solution supplied to the inlet of the anode based upon
a desired output power of the at least one fuel cell assembly; and
(2) regulating the supply of the liquid fuel to the inlet of the
anode from the source of liquid fuel and from the L/G separator to
achieve the appropriate concentration. Preferably, a computer is
utilized which is programmed with a predetermined relationship
between the concentration of the liquid fuel supplied to the inlet
of the anode and the output power of the at least one fuel cell
assembly.
[0042] Additional advantages of the present disclosure will become
readily apparent to those skilled in this art from the following
detailed description, wherein only the preferred embodiments of the
present disclosure are shown and described, simply by way of
illustration but not limitation. As will be realized, the
disclosure is capable of other and different embodiments, and its
several details are capable of modification in various obvious
respects, all without departing from the spirit of the present
disclosure. Accordingly, the drawings and description are to be
regarded as illustrative in nature, and not as restrictive.
BRIEF DESCRIPTION OF THE DRAWINGS
[0043] The various features and advantages of the present
disclosure will become more apparent and facilitated by reference
to the accompanying drawings, provided for purposes of illustration
only and not to limit the scope of the invention, wherein the same
reference numerals are employed throughout for designating like
features and the various features are not necessarily drawn to
scale but rather are drawn as to best illustrate the pertinent
features, wherein:
[0044] FIG. 1 is a simplified, schematic illustration of a direct
oxidation fuel cell (DOFC) system capable of operating with highly
concentrated methanol fuel, i.e., a DMFC system;
[0045] FIG. 2 is a simplified, schematic illustration of a
DOFC/DMFC system according to embodiments of the present
disclosure;
[0046] FIG. 3 is a simplified, schematic illustration of a
DOFC/DMFC system according to other embodiments of the present
disclosure;
[0047] FIG. 4 is a graph for illustrating the variation of the
supply fuel concentration of a DOFC/DMFC system according to the
present disclosure, as a function of the operating temperature of
the system;
[0048] FIG. 5 is a graph for illustrating the variation of the
supply fuel concentration of a DOFC/DMFC system according to the
present disclosure, as a function of the of the desired output
power of the system; and
[0049] FIG. 6 is a graph for illustrating the variation of the
output voltage of a DOFC/DMFC system according to the present
disclosure as a function of the output current of the system, for
several examples of system output power.
DETAILED DESCRIPTION OF THE DISCLOSURE
[0050] The present disclosure relates to high power conversion
efficiency, dynamically controllable, direct oxidation fuel cells
(DOFC) and DOFC systems operating with highly concentrated fuel,
e.g., direct methanol fuel cells (DMFC) and systems, and operating
methods therefor.
[0051] Referring to FIG. 1, schematically shown therein is an
illustrative embodiment of a DOFC system adapted for operating with
highly concentrated fuel, e.g., a methanol-based DMFC system 10,
which system maintains a balance of water in the fuel cell and
returns a sufficient amount of water from the cathode to the anode
under high-power and elevated temperature operating conditions. (A
DOFC/DMFC system is disclosed in co-pending, commonly assigned U.S.
patent application Ser. No. 11/020,306, filed Dec. 27, 2004).
[0052] As shown in FIG. 1, DMFC system 10 includes an anode 12, a
cathode 14, and a proton-conducting electrolyte membrane 16,
forming a multi-layered composite membrane-electrode assembly or
structure 2 commonly referred to as an MEA. Typically, a fuel cell
system such as DMFC system 10 will have a plurality of such MEAs in
the form of a stack; however, FIG. 1 shows only a single MEA for
illustrative simplicity. Frequently, the membrane-electrode
assemblies 2 are separated by bipolar plates that have serpentine
channels for supplying and returning fuel and by-products to and
from the assemblies (not shown for illustrative convenience). In a
fuel cell stack, MEAs and bipolar plates are aligned in alternating
layers to form a stack of cells and the ends of the stack are
sandwiched with current collector plates and electrical insulation
plates, and the entire unit is secured with fastening structures.
Also not shown in FIGS. 1-3, for illustrative simplicity, is a load
circuit electrically connected to the anode 12 and cathode 14.
[0053] A source of fuel, e.g., a fuel container or cartridge 18
containing a highly concentrated fuel 19 (e.g., methanol), is in
fluid communication with anode 12 (as explained below). An oxidant,
e.g., air supplied by fan 20 and associated conduit 21, is in fluid
communication with cathode 14. The highly concentrated fuel from
fuel cartridge 18 is fed directly into liquid/gas ("L/G") separator
28 by pump 22 via associated conduit segments 23' and 25, or
directly to anode 12 via pumps 22 and 24 and associated conduit
segments 23, 23', 23'', and 23'''.
[0054] In operation, highly concentrated fuel 19 is introduced to
the anode side of the MEA 2, or in the case of a cell stack, to an
inlet manifold of an anode separator of the stack. Water produced
at the cathode 14 side of MEA 2 or cathode cell stack via
electrochemical reaction (as expressed by equation (2)) is
withdrawn therefrom via cathode outlet or exit port/conduit 30 and
supplied to liquid/gas separator 28. Similarly, excess fuel, water,
and CO.sub.2 gas are withdrawn from the anode side of the MEA 2 or
anode cell stack via anode outlet or exit port/conduit 26 and
supplied to liquid/gas separator 28. The air or oxygen is
introduced to the cathode side of the MEA 2 and regulated to
maximize the amount of electrochemically produced water in liquid
form while minimizing the amount of electrochemically produced
water vapor, thereby minimizing the escape of water vapor from
system 10.
[0055] During operation of system 10, air is introduced to the
cathode 14 (as explained above) and excess air and liquid water are
withdrawn therefrom via cathode exit port/conduit 30 and supplied
to L/G separator 28. As discussed further below, the input air flow
rate or air stoichiometry is controlled to maximize the amount of
the liquid phase of the electrochemically produced water while
minimizing the amount of the vapor phase of the electrochemically
produced water. Control of the oxidant stoichiometry ratio can be
obtained by setting the speed of fan 20 at a rate depending on the
fuel cell system operating conditions or by an electronic control
unit (ECU) 40, e.g., a digital computer-based controller. ECU 40
receives an input signal from a temperature sensor in contact with
the liquid phase 29 of L/G separator 28 (not shown in the drawing
for illustrative simplicity) and adjusts the oxidant stoichiometric
ratio (via line 41 connected to oxidant supply fan 20) so as to
maximize the liquid water phase in the cathode exhaust and minimize
the water vapor phase in the exhaust, thereby reducing or obviating
the need for a water condenser to condense water vapor produced and
exhausted from the cathode of the MEA 2. In addition, ECU 40 can
increase the oxidant stoichiometry beyond the minimum setting
during cold-start in order to avoid excessive water accumulation in
the fuel cell.
[0056] Liquid water 29 which accumulates in the L/G separator 28
during operation may be returned to anode 12 via circulating pump
24 and conduit segments 25, 23'', and 23'''. Exhaust carbon dioxide
gas is released through port 32 of L/G separator 28.
[0057] As indicated above, cathode exhaust water, i.e., water which
is electrochemically produced at the cathode during operation, is
partitioned into liquid and gas phases, and the relative amounts of
water in each phase are controlled mainly by temperature and air
flow rate. The amount of liquid water can be maximized and the
amount of water vapor minimized by using a sufficiently small
oxidant flow rate or oxidant stoichiometry. As a consequence,
liquid water from the cathode exhaust can be automatically trapped
within the system, i.e., an external condenser is not required, and
the liquid water can be combined in sufficient quantity with a
highly concentrated fuel, e.g., greater than about 5 molar (M), for
use in performing the anodic electrochemical reaction, thereby
maximizing the concentration of fuel and storage capacity and
minimizing the overall size of the system. The water can be
recovered in any suitable existing type of L/G separator 28, e.g.,
such as those typically used to separate CO.sub.2 gas and aqueous
methanol solution.
[0058] The DOFC system 10 shown in FIG. 1 comprises at least one
MEA 2 which includes a polymer electrolyte membrane 16 and a pair
of electrodes (an anode 12 and a cathode 14) each composed of a
catalyst layer and a gas diffusion layer sandwiching the membrane.
Typical polymer electrolyte materials include fluorinated polymers
having perfluorosulfonate groups or hydrocarbon polymers such as
poly-(arylene ether ether ketone) ("PEEK"). The electrolyte
membrane can be of any thickness as, for example, between about 25
and about 180 .mu.m. The catalyst layer typically comprises
platinum or ruthenium based metals, or alloys thereof. The anodes
and cathodes are typically sandwiched by bipolar separator plates
having channels to supply fuel to the anode and an oxidant to the
cathode. A fuel cell stack can contain a plurality of such MEA's 2
with at least one electrically conductive separator placed between
adjacent MEA's to electrically connect the MEAs in series with each
other, and to provide mechanical support.
[0059] As has been indicated above, ECU 40 adjusts the oxidant flow
rate or stoichiometric ratio so as to maximize the liquid water
phase in the cathode exhaust and minimize the water vapor phase in
the exhaust, thereby eliminating the need for a water condenser.
ECU 40 adjusts the oxidant flow rate, hence stoichiometric ratio,
according to a specific equation, illustratively equation (4) given
below: .xi. c = 0.42 .times. ( .gamma. + 2 ) 3 .times. .times.
.eta. fuel .times. p p sat ( 4 ) ##EQU1## wherein .xi..sub.c is the
oxidant stoichiometry, .gamma. is the ratio of water to fuel in the
fuel supply, p.sub.sat is the water vapor saturation pressure
corresponding to the cell temperature, p is the cathode operating
pressure, and .eta..sub.fuel is the fuel efficiency. Such
controlled oxidant stoichiometry automatically ensures an
appropriate water balance in the DMFC (i.e. enough water for the
anode reaction) under any operating conditions. For instance,
during start-up of a DMFC system, when the cell temperature
increases from e.g., 20.degree. C. to the operating point of
60.degree. C., the corresponding p.sub.sat is initially low, and
hence a large oxidant stoichiometry (flow rate) should be used in
order to avoid excessive water accumulation in the system and
therefore cell flooding by liquid water. As the cell temperature
increases, the oxidant stoichiometry (e.g., air flow rate)
decreases according to equation (4).
[0060] In the above, it is assumed that the amount of liquid (e.g.,
water) produced by electrochemical reaction in MEA 2 and supplied
to L/G separator 28 is essentially constant, whereby the amount of
liquid product returned to the inlet of anode 12 via pump 24 and
conduit segments 25, 23'', and 23''' is essentially constant, and
is mixed with concentrated liquid fuel 19 from fuel container or
cartridge 18 in an appropriate ratio for supplying anode 12 with
fuel at an ideal concentration.
[0061] However, a number of factors or conditions may result in
deviation of system operation from ideal. For example, changes in
temperature, load requirement, operating current error, oxidant
efficiency change, etc., may incur deviation from ideal operating
conditions. In addition, deviation from ideal operation may occur
as a result of extended duration of storage of the fuel cell(s) and
from vaporization of a portion of the liquid product (e.g.,
water).
[0062] Accordingly, sustained optimal operation of DOFC/DMFC
systems at high fuel efficiencies and high power output requires
control and/or regulation systems and methodologies for determining
and controlling system operating parameters at appropriate levels
or values, e.g., fuel supply concentration, in a dynamically
changing manner.
[0063] Referring to FIG. 2, shown therein is a simplified,
schematic illustration of a dynamically controllable DOFC/DMFC
system 50 according to embodiments of the present disclosure.
System 50 is similar to system 10 of FIG. 1, and therefore only
those components and features pertaining to the dynamic control
aspect of system 50 are described in the following.
[0064] As illustrated, system 50 includes a liquid level sensor
device 42 adapted for sensing/determining the amount of liquid
product (e.g., water) contained in L/G separator 28 at any given
instant. While sensor device 42 is shown in the figure as located
exteriorly of L/G separator 28, i.e., positioned along an exterior
wall thereof, the sensor device 28 is not limited to such
placement, and may be located interiorly of the L/G separator 28.
Suitable sensor devices 42 for use according to the present
disclosure include a variety of conventional liquid level sensing
devices, such as, but not limited to, photoelectric devices and
float devices. According to the illustrated embodiment, an output
signal from the sensor device 42 is supplied as an input to ECU 40
via line 43.
[0065] According to these embodiments, equation (4) given above for
relating oxidant stoichiometry (i.e., air flow rate to cathode 14
of MEA 2) with ratio of fuel (e.g., methanol) to liquid (e.g.,
water) in the fuel solution supplied to anode 12 of MEA 2, is
modified to include a calculation factor A taking into account
deviation of the amount of liquid (e.g., water) product from a
predetermined (i.e., expected) value, as follows: .xi. c .times. =
.times. 0.42 .times. .times. ( .gamma. .times. + .times. 2 ) 3
.times. .times. .eta. fuel .times. .times. p p sat .times. A ( 5 )
##EQU2## where: A<1 when the amount of liquid product is lower
than the predetermined amount;
[0066] A>1 when the amount of liquid product is greater than the
predetermined amount; and
[0067] A=1 when the amount of liquid product is the same as the
predetermined amount.
[0068] As shown in FIG. 2, DOFC/DMFC system 50 includes a L/G
separator 28 in fluid communication (via conduits 26 and 30) with
outlets of each of the anode 12 and cathode 14 for: (1) receiving
unreacted fuel, liquid product, and gases, and (2) supplying a
solution of liquid fuel in liquid product to the inlet of the anode
(via conduit segments 25, 23'', and 23''' and pumps 22 and 24); and
system 50 further includes a control system comprised of sensor
device 42 and ECU 40 adapted for measuring the amount of the liquid
product formed by the selected electrochemical reactions at the
cathode and anode and contained in the L/G separator 28, and
controlling oxidant (e.g., air) stoichiometry during operation of
system 50 at an appropriate value (via control of fan 20) in
response to the measured amount of liquid product.
[0069] Preferably, ECU 40 comprises an electronic computer
programmed for: (1) comparing the measured amount of liquid product
with a predetermined amount of liquid product for determining
whether the measured amount is greater than, smaller than, or the
same as the predetermined amount; (2) determining a calculation
factor based upon the comparison; (3) calculating the appropriate
oxidant stoichiometry utilizing equation (5) above including the
calculation factor A; and (4) continuously or periodically
regulating the oxidant supply (i.e., fan 20 speed) to achieve the
appropriate oxidant stoichiometry.
[0070] Adverting to FIG. 3, shown therein is a simplified,
schematic illustration of DOFC/DMFC systems according to further
embodiments of the present disclosure. As before, the illustrated
systems are similar to system 10 of FIG. 1, and therefore only
those components and features pertaining to the dynamic control
aspect of the systems are described in the following.
[0071] According to certain embodiments of the present disclosure,
system 60 includes sensor device 44 for sensing the temperature of
MEA 2 and supplying an input signal indicative of the measured
temperature to ECU 40. System 60 further includes lines 46 and 47
from ECU 40 respectively connected/communicating with circulating
pump 24 and concentrated fuel supply pump 22 for regulating the
ratio of concentrated fuel to liquid product in the solution or
mixture of fuel supplied to the inlet of anode 12 via conduits 23,
23', 23'', 23''', and 25.
[0072] In accordance with these embodiments, DOFC/DMFC system 60
includes a liquid/gas (L/G) separator 28 in fluid communication
(via conduits 26 and 30) with outlets of each of the anode 12 and
cathode 14 for: (1) receiving unreacted fuel, liquid product, and
gases, and (2) supplying a mixture/solution of the unreacted liquid
fuel and liquid product to the inlet of the anode (via conduits 23,
23', 23'', 23''', and 25); and system 60 further includes a control
system for controlling the concentration of the unreacted liquid
fuel in the mixture/solution supplied to the inlet of the anode
from the source 18 of concentrated liquid fuel and from the L/G
separator 28.
[0073] Preferably, ECU 40 comprises an electronic computer
programmed for: (1) determining an appropriate concentration of the
liquid fuel in the solution supplied to the inlet of the anode
based upon the operating temperature of the at least one fuel cell
assembly 2 measured by sensor 44; and (2) regulating the supply of
the liquid fuel 19 to the inlet of the anode 12 from the source of
liquid fuel 18 and from the L/G separator 28 to achieve the
appropriate concentration. The computer is programmed with a
predetermined relationship between the concentration of the liquid
fuel supplied to the inlet of the anode and the operating
temperature of the fuel cell assembly.
[0074] Referring to FIG. 4, shown therein is a graph illustrating
an example of a predetermined relationship between supply fuel
concentration of a DOFC/DMFC system (such as system 60) and
operating temperature of the system. As indicated above, ECU 40 is
programmed with such predetermined relationship between the
concentration of the liquid fuel supplied to the inlet of the anode
and the operating temperature of the fuel cell assembly.
[0075] According to still another embodiment of the present
disclosure, the DOFC/DMFC system does not require measurement of
the operating temperature of the MEA 2 for determining an
appropriate value of the fuel/liquid product ratio of the fuel
mixture/solution supplied to the anode 12 of the MEA. Rather, ECU
40 preferably comprises an electronic computer programmed for: (1)
determining an appropriate concentration of the liquid fuel in the
solution/mixture supplied to the inlet of the anode based upon a
desired (i.e., preselected) output power of the fuel cell assembly;
and (2) continuously or periodically regulating the supply of the
liquid fuel to the inlet of the anode 12 from the source of liquid
fuel 18 and from the L/G separator 28 to achieve the appropriate
concentration. The computer is programmed with a predetermined
relationship between the concentration of the liquid fuel supplied
to the inlet of the anode and the output power density of the fuel
cell assembly.
[0076] By way of illustration, FIG. 5 is a graph showing an example
of a predetermined relationship between the supply fuel
concentration and the desired output power density of a DOFC/DMFC
system according to the present disclosure. As indicated above, ECU
40 is programmed with such predetermined relationship between the
concentration of the liquid fuel supplied to the inlet of the anode
and the operating temperature of the at least one fuel cell
assembly.
[0077] Referring to FIG. 6, shown therein is a graph illustrating
the variation of the output voltage of a DOFC/DMFC system according
to the present disclosure as a function of the output current of
the system, for several examples of system output power density. As
is evident from the figure, the optimal operating point with best
fuel and liquid (water) supply depends upon the power output
density of the DOFC/DMFC system.
[0078] In summary, the present disclosure offers a number of
advantages in operating DOFC/DMFC systems, including variation of
the oxidant (air) stoichiometry in response to deviation of the
quantity of produced liquid (water) from an expected (preselected)
amount, and variation of the supply fuel concentration in response
to changes/deviations in fuel cell operating temperature or output
power density from desired (preselected) values.
[0079] In the previous description, numerous specific details are
set forth, such as specific materials, structures, reactants,
processes, etc., in order to provide a better understanding of the
present disclosure. However, the present disclosure can be
practiced without resorting to the details specifically set forth.
In other instances, well-known processing materials and techniques
have not been described in detail in order not to unnecessarily
obscure the present disclosure.
[0080] Only the preferred embodiments of the present disclosure and
but a few examples of its versatility are shown and described in
the present disclosure. It is to be understood that the present
disclosure is capable of use in various other combinations and
environments and is susceptible of changes and/or modifications
within the scope of the inventive concept as expressed herein.
* * * * *