U.S. patent application number 13/480114 was filed with the patent office on 2012-09-13 for direct oxidation fuel cells with improved cathode gas diffusion media for low air stoichiometry operation.
This patent application is currently assigned to The Penn State Research Foundation. Invention is credited to Takashi Akiyama, Guoqiang LU, Chao-Yang Wang.
Application Number | 20120231375 13/480114 |
Document ID | / |
Family ID | 39381995 |
Filed Date | 2012-09-13 |
United States Patent
Application |
20120231375 |
Kind Code |
A1 |
LU; Guoqiang ; et
al. |
September 13, 2012 |
DIRECT OXIDATION FUEL CELLS WITH IMPROVED CATHODE GAS DIFFUSION
MEDIA FOR LOW AIR STOICHIOMETRY OPERATION
Abstract
A cathode for use in a direct oxidation fuel cell (DOFC)
comprises a gas diffusion medium (GDM) including a backing layer
and a microporous layer comprising a fluoropolymer and an
electrically conductive material, wherein loading of the
fluoropolymer in the microporous layer is in the range from about
10 to about 60 wt. %. In use, a concentrated solution of a liquid
fuel is supplied to an anode and an oxidant to the cathode of the
fuel cell, and the fuel cell may be operated at a low oxidant
stoichiometry .xi..sub.c not greater than about 2.5.
Inventors: |
LU; Guoqiang; (Latham,
NY) ; Wang; Chao-Yang; (State College, PA) ;
Akiyama; Takashi; (Osaka, JP) |
Assignee: |
The Penn State Research
Foundation
University Park
PA
Panasonic Corporation
Osaka
|
Family ID: |
39381995 |
Appl. No.: |
13/480114 |
Filed: |
May 24, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12722081 |
Mar 11, 2010 |
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13480114 |
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11655867 |
Jan 22, 2007 |
7704629 |
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12722081 |
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Current U.S.
Class: |
429/530 ;
977/773; 977/948 |
Current CPC
Class: |
H01M 2004/8689 20130101;
H01M 8/1007 20160201; Y02E 60/523 20130101; H01M 4/8605 20130101;
H01M 8/1011 20130101; Y02E 60/50 20130101; Y10S 977/742
20130101 |
Class at
Publication: |
429/530 ;
977/948; 977/773 |
International
Class: |
H01M 4/86 20060101
H01M004/86 |
Claims
1-19. (canceled)
20. A cathode electrode for use in a direct oxidation fuel cell
(DOFC), the cathode comprising: a catalyst layer and a gas
diffusion medium (GDM); wherein the GDM comprises a microporous
layer (MPL) and a backing layer; wherein the MPL has a thickness of
about 10 .mu.m to about 50 .mu.m, an average pore size between 10
nm and 500 nm, and comprises a fluoropolymer and an electrically
conductive material, wherein the electrically conductive material
comprises carbon particles or carbon nanofibers, wherein loading of
the fluoropolymer is in the range of about 30 wt. % to 40 wt. %,
and loading of the carbon particles or carbon nanofibers is about
0.5 mg/cm.sup.2 to about 5 mg/cm.sup.2; wherein the backing layer
has a porosity greater than about 70%, and comprises carbon paper,
woven paper, or non-woven cloth, and includes loading of a
fluoropolymer from about 5 wt. % to about 30 wt. %; and wherein the
catalyst layer, the MPL, and the backing layer are arranged in
direct overlying sequence in the recited order.
21. The cathode as in claim 20, wherein the fluoropolymer in the
MPL comprises poly(tetrafluoroethylene) (PTFE).
22. The cathode as in claim 20, wherein said electrically
conductive material comprises carbon particles.
23. The cathode as in claim 20, further comprising: a hydrophobic
microporous layer between said MPL and said GDM.
24. A direct oxidation fuel cell comprising the cathode of claim
20.
Description
FIELD OF THE DISCLOSURE
[0001] The present disclosure relates generally to fuel cells, fuel
cell systems, and electrodes/electrode assemblies for same. More
specifically, the present disclosure relates to cathodes with
improved gas diffusion media, suitable for direct oxidation fuel
cells (hereinafter "DOFC"), such as direct methanol fuel cells
(hereinafter "DMFC"), and their fabrication methods.
BACKGROUND OF THE DISCLOSURE
[0002] A 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, formic acid,
dimethyl ether, etc., and their aqueous solutions. The oxidant may
be substantially pure oxygen 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,
personal data assistants, etc.) include easy storage/handling and
high energy density of the liquid fuel.
[0003] One example of a DOFC system is a 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 reacts with
water in the presence of a catalyst, typically a Pt or Ru
metal-based catalyst, to produce carbon dioxide, H.sup.+ions
(protons), and electrons. The electrochemical reaction is shown as
equation (1) below:
CH.sub.2OH+H.sub.2O.fwdarw.CO.sub.2+6H.sup.++6e.sup.31 (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
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.2OH+3/2O.sub.2.fwdarw.CO.sub.2+2H.sub.2O (.sup.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 is considered 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 (hereinafter "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 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] In order to utilize highly concentrated fuel with DOFC
systems, such as DMFC systems described above, it is preferable to
reduce the oxidant stoichiometry ratio, i.e., flow of oxidant (air)
to the cathode for reaction according to equation (2) above. In
turn, operation of the cathode must be optimized so that liquid
product(s), e.g., water, formed on or in the vicinity of the
cathode can be removed therefrom without resulting in substantial
flooding of the cathode.
[0011] Accordingly, 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
when operated with highly concentrated fuel and low oxidant
stoichiometry ratio, i.e., less than about 8. 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.
[0012] In view of the foregoing, there exists a need for improved
DOFC/DMFC systems and methodologies, including electrodes and gas
diffusion media, which facilitate operation 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 gas
diffusion media (GDM's) for use in cathode electrodes of fuel
cells.
[0014] Another advantage of the present disclosure is improved
cathode electrodes suitable for use in fuel cells, comprising
improved GDM's.
[0015] Still another advantage of the present disclosure is
improved direct oxidation fuel cells (DOFC's).
[0016] Yet another advantage is improved methods of operating
direct oxidation fuel cells (DOFC's).
[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 a cathode
electrode for use in a direct oxidation fuel cell (DOFC),
comprising an improved gas diffusion medium (GDM) including a
backing layer and a microporous layer, the microporous layer
comprising a fluoropolymer and an electrically conductive material,
wherein loading of the fluoropolymer in the microporous layer is in
the range from about 10 to about 60 wt. %.
[0019] According to preferred embodiments of the present
disclosure, the fluoropolymer comprises poly(tetrafluoroethylene)
(PTFE); the electrically conductive material comprises carbon
particles or nanofibers, loading of the carbon particles or
nanofibers in the microporous layer being in the range from about
0.5 to about 5 mg/cm.sup.2, the microporous layer having a
thickness ranging from about 10 to about 50 .mu.m and an average
pore size between about 10 and about 500 nm; and the backing layer
comprises carbon paper or woven or non-woven cloth having a
porosity greater than about 70% and fluoropolymer loading in the
range from about 5 to about 30 wt. %.
[0020] Further preferred embodiments of the present disclosure
include those wherein the cathode further comprises a catalyst
layer and a hydrophobic microporous layer (MPL), the catalyst
layer, hydrophobic MPL, and GDM being arranged in overlying
sequence in the recited order.
[0021] Another aspect of the present disclosure is a direct
oxidation fuel cell (DOFC) comprising the above-described
cathode.
[0022] Yet another aspect of the present disclosure is a method of
operating a direct oxidation fuel cell (DOFC) system, comprising
steps of: [0023] a) providing at least one fuel cell assembly
including a cathode and an anode with an electrolyte positioned
therebetween, the cathode comprising an improved gas diffusion
medium (GDM) including a backing layer and a microporous layer, the
microporous layer comprising an electrically conductive material
and a fluoropolymer, loading of the fluoropolymer in the
microporous layer being in the range from about 10 to about 60 wt.
%; [0024] (b) supplying a concentrated solution of a liquid fuel to
the anode and an oxidant to the cathode; and [0025] (c) operating
the at least one fuel cell assembly at a low oxidant stoichiometry
.xi..sub.c not greater than about 4.
[0026] In accordance with preferred embodiments of the present
disclosure, step (a) comprises providing at least one fuel cell
assembly wherein the fluoropolymer comprises
poly(tetrafluoroethylene) (PTFE); the electrically conductive
material comprises carbon particles or nanofibers, loading of the
carbon particles in the microporous layer being in the range from
about 0.5 to about 5 mg/cm.sup.2, the microporous layer having a
thickness ranging from about 10 to about 50 .mu.m and an average
pore size between about 10 and about 500 nm; and the backing layer
comprises carbon paper or woven or non-woven cloth having a
porosity greater than about 70% and fluoropolymer loading in the
range from about 5 to about 30 wt. %.
[0027] Further preferred embodiments of the present disclosure
include those wherein step (a) comprises providing at least one
fuel cell assembly wherein the cathode further comprises a catalyst
layer and a hydrophobic microporous layer (MPL), the catalyst
layer, hydrophobic MPL, and GDM are arranged in overlying sequence
in the recited order; step (b) comprises providing a 20 M solution
of methanol (CH.sub.3OH) to the anode and air to the cathode; and
step (c) comprises operating the at least one fuel cell assembly at
a low oxidant stoichiometry .xi..sub.c not greater than about 2.5,
a steady-state power density not less than about 70 mW/cm.sup.2,
and a temperature of about 60.degree. C.
[0028] 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
invention. Accordingly, the drawings and description are to be
regarded as illustrative in nature, and not as restrictive.
BRIEF DESCRIPTION OF THE DRAWING
[0029] 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:
[0030] FIG. 1 is a simplified, schematic illustration of a DOFC
system capable of operating with highly concentrated methanol fuel,
i.e., a DMFC system;
[0031] FIG. 2 is a schematic, cross-sectional view of a
representative configuration of a membrane electrode assembly
suitable for use in a fuel cell/fuel cell system such as the
DOFC/DMFC system of FIG. 1;
[0032] FIG. 3 is a graph for illustrating the dependence of
allowable fuel concentration as a function of cathode air
stoichiometry for various system operating temperatures; and
[0033] FIG. 4 is a graph for illustrating the steady-state
performance of a DMFC with cathode electrodes comprising the
improved gas diffusion layer (GDL) according to the present
disclosure.
DETAILED DESCRIPTION OF THE DISCLOSURE
[0034] The present disclosure relates to fuel cells and fuel cell
systems with high power conversion efficiency, such as DOFC's and
DOFC systems operating with highly concentrated fuel, e.g., DMFC's
and DMFC systems fueled with about 5 to about 25 M methanol
(CH.sub.3OH), and electrodes/electrode assemblies therefor.
[0035] 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 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).
[0036] 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 9 commonly referred to as an MEA. Typically, a fuel cell
system such as DMFC system 10 will have a plurality of such MEA's
in the form of a stack; however, FIG. 1 shows only a single MEA 9
for illustrative simplicity. Frequently, the MEA's 9 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 FIG.
1, for illustrative simplicity, is a load circuit electrically
connected to the anode 12 and cathode 14.
[0037] 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 (hereinafter
"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'''.
[0038] In operation, highly concentrated fuel 19 is introduced to
the anode side of the MEA 9, 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 9 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 carbon dioxide gas are withdrawn from the anode side of the MEA
9 or anode cell stack via anode outlet or exit port/conduit 26 and
supplied to L/G separator 28. The air or oxygen is introduced to
the cathode side of the MEA 9 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.
[0039] 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 (hereinafter "ECU") 40, e.g., a digital computer-based
controller or equivalently performing structure. 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.
[0040] 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.
[0041] 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, 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 carbon dioxide gas and
aqueous methanol solution.
[0042] The DOFC/DMFC system 10 shown in FIG. 1 comprises at least
one MEA 9 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) (hereinafter "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 9 with at least one electrically conductive separator
placed between adjacent MEA's to electrically connect the MEA's in
series with each other, and to provide mechanical support.
[0043] As has been indicated above, ECU 40 can adjust 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, and hence the
stoichiometric ratio, according to equation (4) given below:
.xi. c = 0.42 ( .gamma. + 2 ) 3 .eta. fuel p p sat ( 4 )
##EQU00001##
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, defined as the ratio of the operating current density,
I, to the sum of the operating current density and the equivalent
fuel (e.g., methanol) crossover current density, I.sub.xover, as
expressed by equation (5) below:
.eta. fuel = I I + I xover ( 5 ) ##EQU00002##
[0044] 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) can be
reduced according to equation (4).
[0045] In the above, it is assumed, though not required, that the
amount of liquid (e.g., water) produced by electrochemical reaction
in MEA 9 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.
[0046] Referring now to FIG. 2, shown therein is a schematic,
cross-sectional view of a representative configuration of a MEA 9
for illustrating its various constituent elements in more detail.
As illustrated, a cathode electrode 14 and an anode electrode 12
sandwich a polymer electrolyte membrane 16 made of a material, such
as described above, adapted for transporting hydrogen ions from the
anode to the cathode during operation. The anode electrode 12
comprises, in order from electrolyte membrane 16, a metal-based
catalyst layer 2.sub.A in contact therewith, and an overlying gas
diffusion layer (GDL) 3.sub.A, whereas the cathode electrode 14
comprises, in order from electrolyte membrane 16: (1) a metal-based
catalyst layer 2.sub.C in contact therewith; (2) an intermediate,
hydrophobic micro-porous layer (MPL) 4.sub.C; and (3) an overlying
gas diffusion medium (GDM) 3.sub.C. GDL 3.sub.A and GDM 3.sub.C are
each gas permeable and electrically conductive, and may be
comprised of a porous carbon-based material including a carbon
powder and a fluorinated resin, with a support made of a material
such as, for example, carbon paper or woven or non-woven cloth,
felt, etc. Metal-based catalyst layers 2.sub.A and 2.sub.C may, for
example, comprise Pt or Ru.
[0047] As graphically shown in FIG. 3, the allowable fuel
concentrations of DOFC's such as DMFC's significantly depend upon
the cathode air stoichiometry. As may be evident from FIG. 3, an
efficient way of utilizing a highly concentrated solution of fuel
(methanol) in the fuel reservoir or cartridge of the DMFC is to
reduce the cathode air stoichiometry for fixed temperature
operation of the DMFC. More specifically, in order to increase the
concentration of the fuel stored in fuel cartridge 18, it is
preferable that the oxidant stoichiometry ratio (flow rate),
.xi..sub.c, be reduced to less than about 8, preferably less than
about 4, more preferably not greater than about 2.5. As a
consequence, the cathode electrode must be optimized with respect
to liquid product (e.g., water) removal therefrom so as to prevent
flooding during operation at such low oxidant stoichiometery ratios
(flow rates). This is accomplished by means of hydrophobic MPL
4.sub.C interposed between catalyst layer 2.sub.C and GDM
3.sub.C.
[0048] Completing MEA 9 are respective electrically conductive
anode and cathode separators 6.sub.A and 6.sub.C for mechanically
securing the anode 12 and cathode 14 electrodes against polymer
electrolyte membrane 16. As illustrated, each of the anode and
cathode separators 6.sub.A and 6.sub.C includes respective channels
7.sub.A and 7.sub.C for supplying reactants to the anode and
cathode electrodes and for removing excess reactants and liquid and
gaseous products formed by the electrochemical reactions. Lastly,
MEA 9 is provided with gaskets 5 around the edges of the cathode
and anode electrodes for preventing leaking of fuel and oxidant to
the exterior of the assembly. Gaskets 5 are typically made of an
O-ring, a rubber sheet, or a composite sheet comprised of
elastomeric and rigid polymer materials.
[0049] Desirable characteristics of hydrophobic MPL 4.sub.C for
ensuring adequate removal of liquid product (e.g., water in the
case of DMFC cells) from the cathode electrode of MEA 9 in order to
minimize flooding during operation at low oxidant stoichiometry
ratios (flow rates) include: [0050] 1. sufficient electrical
conductivity; [0051] 2. highly hydrophobic characteristics for
water repellency; and [0052] 3. sufficient porosity for good gas
permeability.
[0053] Typically, MPL 4.sub.C is optimized for liquid product
(water) removal by use of a composite material formed of a carbon
black and PTFE, with a layer thickness of about 10 -50 .mu.m and an
average pore size between 10 and 500 nm. The carbon black (e.g.,
Vulcan XC72R) provides the composite material with electrical
conductivity and porous structure, and the PTFE provides the
composite material with highly hydrophobic characteristics.
[0054] Further improvement of the properties of the cathode gas
diffusion medium (GDM) 3.sub.C is considered desirable and
necessary for facilitating operation at low oxidant
stoichiometries, i.e., at low air flow rates, to thereby enable use
of highly concentrated fuel (methanol) in the above described
DOFC/DMFC systems.
[0055] The cathode gas diffusion medium (GDM) 3.sub.C typically
includes a backing layer and a microporous layer, with a carbon
paper or woven or non-woven cloth constituting the backing layer.
Functions of the cathode GDM 3.sub.C include: providing a path for
oxygen (O.sub.2) transport to the cathode catalyst layer 2.sub.C,
collecting current, and removing water from the cathode electrode
thereby to prevent flooding. Preferably, the carbon paper or woven
or non-woven cloth constituting the backing layer has a porosity
greater than about 70% and fluoropolymer loading in the range from
about 5 to about 30 wt. %.
[0056] As indicated above, in order to improve the performance of
DOFC's/DMFC's operating at low oxidant (air) stoichiometries, it is
necessary to improve the cathode GDM 3.sub.C. The key to obtaining
such improvement lies in the choice of material of the backing
layer and constitution of the microporous layer. Significant
factors affecting performance of fluoropolymer-based microporous
layers, e.g., poly(tetrafluoroethylene)-based (hereinafter
"PTFE-based") microporous layers, include fluoropolymer loading,
thickness, and porosity.
[0057] According to the present disclosure, a slurry for
fabricating improved microporous layers for use in cathode GDM's
can be made by mixing electrically conductive particles or
nanofibers, e.g., carbon particles such as Vulcan.RTM. XC-72R
(Vulcan.RTM. is a registered trademark of Cabot Corp., Billerica,
Mass.), isopropyl alcohol, Teflon.RTM. solution (Teflon.RTM. is a
registered trademark of the E.I. DuPont de Nemours Co.), and
deionized water. The slurry is coated onto the surface of a
suitable support material, e.g., carbon paper or woven or non-woven
cloth, etc. The amount of fluoropolymer (e.g., PTFE) loading, i.e.,
the dry weight of the fluoropolymer divided by the total dry weight
of the constituents of the microporous layer, is a significant
factor in providing a balance between several competing factors and
obtaining good performance at low oxidant stoichiometries.
Specifically, the inventors have determined that high fluoropolymer
loading of the microporous layer of the cathode GDM advantageously
increases water removal capability, thereby preventing cathode
flooding, and provides fresh surfaces for facilitating transport of
air/O.sub.2 therethrough, but disadvantageously reduces the
electrical conductivity of the electrode and increases the
potential for catalyst flooding resulting from an increased amount
of water forced back into the catalyst layer. On the other hand,
too low fluoropolymer loading results in adverse effects on the
above-enumerated cathode performance factors.
[0058] The thickness of the microporous layer of the cathode GDM
(reflecting loading of the electrically conductive particles) is
another significant factor in obtaining good electrode performance
at low oxidant stoichiometries. For example, a too thin microporous
layer does not provide adequate cathode function because a cathode
GDL with no microporous layer, or a too thin microporous layer, is
overly sensitive to liquid water accumulation; whereas, a too thick
microporous layer increases resistance to air/O.sub.2 transport
therethrough.
[0059] Accordingly, the fluoropolymer loading and electrically
conductive particle loading of the microporous layer of the cathode
GDM should be within specific ranges for achieving good electrical
performance at low oxidant stoichiometries below about 4 and as low
as about 2.5. Specifically, fluoropolymer loading (e.g., PTFE
loading) of the microporous layer of the cathode GDM should be in
the range between about 10 and about 60 wt. %, preferably within
the range of about 30 and 40 wt. % and the carbon particle loading
of the microporous layer of the cathode GDM should be between about
0.5 and about 5 mg/cm.sup.2 for obtaining (or maintaining) good
electrical performance (e.g., steady-state power density) at low
air stoichiometries .xi..sub.c in the range from about 2.5 to about
4.
[0060] Referring to FIG. 4, shown therein is a graph illustrating
the steady-state performance of DMFC's with MEA's comprising
improved cathode electrodes with improved GDM's having PTFE-based
microporous layers constituted according to the present disclosure.
As is evident from FIG. 4, the steady-state performance exhibited
by the DMFC's with improved cathode GDM's having microporous layers
with PTFE loading and carbon loading within the above-described
ranges is very good (i.e., steady-state power densities above about
70 mW/cm.sup.2) at a number of very low oxidant stoichiometeries,
i.e., when .xi..sub.c=2, 3, and 4. Significantly, steady-state
power output performance when .xi..sub.c.about.2.5 remains
comparable to performance at higher oxidant stoichiometries, and
the ability to satisfactorily use oxidant stoichiometries
.xi..sub.c not greater than about 2.5 advantageously facilitates
operation with 20 M methanol solution at a temperature of about
60.degree. C., according to FIG. 3.
[0061] While PTFE is utilized as the fluoropolymer in the
illustrated embodiment, the disclosure is not limited thereto and a
number of other fluoropolymers may be utilized according to the
principles of the present disclosure, including, for example:
tetrafluoroethylene-hexafluoropropylene co-polymer ("FEP"),
tetrafluoroethylene-alkylvinyl ether co-polymer ("PFA"),
polychlorotrifluoroethylene ("PCTFE"), tetrafluoroethylene-ethylene
co-polymer ("ETFE"), chlorotrifluoroethylene-ethylene co-polymer
("ECTFE"), and polyvinylidene fluoride ("PVDF"). Similarly, while
carbon particles are utilized as the electrically conductive
particles in the illustrated embodiment, the disclosure is not
limited thereto and other electrically conductive particles may be
utilized according to the principles of the present disclosure,
including, for example, electrically conductive graphite fluoride
particles or carbon nanofibers with appropriate F:C ratios.
[0062] In summary, the present disclosure describes improved
cathode electrodes for use in DOFC/DMFC systems which facilitate
operation at good power densities with highly concentrated fuel at
low oxidant stoichiometries .xi..sub.c. In addition, the disclosed
methodology/technology can be practiced utilizing readily available
materials.
[0063] 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 detsil in order not to unnecessarily
obscure the present disclosure.
[0064] 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 disclosed concept as expressed herein.
* * * * *