Note: Descriptions are shown in the official language in which they were submitted.
1~44;~29
This invention relates to fuel cells wherein
reactant or product gas is conducted to or from the cells.
The invention relates more particularly to thermal control
and fuel processing in such cells.
In the design of fuel cells and like electrical
energy producing devices involving reactant or product gas
undergoing electrochemical reaction (process gas), thermal
control is a dominant parameter. The electrochemical reactions
in such devices are invariably accompanied by heat generation
or heat absorption because of entropy changes accompanying
the reation and irreversibilities caused by diffusion and
activation overpotentials and ohmic resistance. In the
accomodation of thermal control, the art has looked to
various techniques, none of which are entirely satisfactory.
The thermal control technique seemingly most
desirable takes advantage of the sensible heat of the process
gas itself as a vehicle for thermal control. Thus, if
removal of heat from the cell is desired, the incoming
process gas may be supplied to the cell at a temperature
lower than the cell operating temperature such that exiting
gas removes heat simply by increase in temperature
thereof in passage through the cell. In this technique, one
adjusts the process gas flow level above the flow level
required for production of preselected measure of electrical
energy, such additional process gas serving the heat removal
function. Disadvantages attending this practice include
undesirable pressure drops based on the increased process
gas flow, auxiliary power penalty and loss of electrolyte
through vaporization or entrainment. By auxiliary power is
meant the power requirements of apparatus accessory to the
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fuel cell proper, e.g., gas pumps, pressurizing systems and
the like. As respects electrolyte loss, all process gas in
this gas sensible heat technique is in communication with the
cell electrolyte in its passage through the cell and, where
substantial additional gas is required for thermal control, a
very high electrolyte loss due to saturating of the gas with
electrolyte vapor is observed in electrolyte gas resulting in
quite high electrolyte loss.
In a second thermal control technique, the art has
looked to limiting the temperature gradients inside fuel cells
by employment of a bipolar plate having an extended fin dis-
posed outside the cell proper, as shown in u.S. patent No.
3,623,913 to Adlhart et al. While this technique provides a
somewhat more uniform cell temperature, high gas flow passing
directly through the cell can result in high electrolyte loss
and increased auxiliary power.
A third thermal control technique relies on the
sensible heat of a dielectric liquid. Such sensible-like
liquid approach requires much lower auxiliary power as com-
20 pared to the gaseous heat transfer medium, but requires a
separate heat transfer loop and an electrically isolated mani-
folding system. To avoid shunt currents between stacked cells,
dielectric fluids such as fluorocarbon or silicon-based oils
have ~een traditionally used as the heat transfer media. Be-
cause the catalyst material may by poisoned severely by even
a trace amount of these dielectric fluids, a small leak from
the heat transfer loop may be fatal to the cell. Also, the
dielectric liquids are flammable and have toxic reaction
products.
In a fourth technique for thermal control, the art
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has relied on the latent heat o~ liquids. Latent heat liquids
(U.S. patents Nos. 3,498,844 and 3,507,702 to Sanderson; U.S.
patent No. 3,761,316 to Stedman; and U.S. patent No. 3,969,145
to Grevstad et al.) can provide at heat transfer at nearly
uniform temperature, although there may be some temperature
gradients in the stacking direction if the heat transfer plate
is placed between a group of cells. The auxiliary power re-
quirements are expected to be extremely low. Suitable di-
electric fluids having boiling points in the range of cell
operating temperature can be used, but the disadvantages of
the sensible-heat liquid approach apply here also. To over-
come these disadvantages, non-dielectric media, such as water,
can be used. If water is used, a suitable quality steam can be
generated for use in other parts of the plant. External heat
exchange also is expected to be efficient because of high heat
transfer coefficients. Unfortunately, the use of a non-dielectric
liquid necessitates elaborate corrosion protection schemes
~U.S. patent No. 3,969,145 to Grevstand et al.; U.S. patent No.
3,923,546 to Katz et al.; U.S. patent No. 3,940,285 to Nickols
et al.) and/or the use of an extremely low conductivity liquid
During operation, the conductivity may increase, so means to
restore the low conductivity may also be required. If the
cooling loop is under pressure, good seals are necessary. If
a leak develops during the life of the stack because of pin-
holes caused by corrosion or deterioration of seals, it could
paralyze the entire system. Because of the corrosion protec-
tion requirements and intricate manifolding, the cost of the
heat transfer subsystem operating on dielectric coolant could
be substantial.
In a first U.S. patent No. 4,192,906 of July 9,
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1979, commonly-assigne~ herewith,
a fundamentally different approach to thermal control
of fuel cells is set forth which provides for supplementing
the flow of process gas through an electrochemical cell, in
measure required for thermal control by sensible heat of
process gas, in manner both avoiding electrolyte loss and
pressure drop increase across the cell. In implementing this
process gas sensible-heat technique, the invention of such
commonly-assigned application introduces, in addition to the
customary process gas passage in communication with the cell
electrolyte, a process gas passage in the cell which is
isolated from the cell electrolyte and in thermal communication
with a heat-generating surface of the cell. Such electrolyte-
communicative and electrolyte-isolated passages are commonly
manifolded to a pressurized supply of process gas~ The flow
levels in the respective passages are set individually by
passage parameters to provide both for desired level electrical
energy cell output and desired heat removal.
In a second U.S. patent No. 4,169,917 of October
2, 1979, also commonly-assigned herewith,
electrochemical cell structure is set forth for im-
plementing the thermal control technique of said first U.S.
patent herein such electrolyte-isolated passages
are so arranged as to have gas-confining walls contiguous with
the electrode served with process gas by the electrolyte-commur
cative passages. Integral sheet material is preferably corruge
to define channels opening into the electrode and successive
alternate channels closed from the electrode by the sheet
material.
Apart from the foregoing thermal control techniques,
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applicants herein have considered the desirability of so-called
"reforming" of hydrocarbon content of process gas. Fuel cell
gas streams frequently contain methane and other hydrocarbons.
The heat value, and hence electrical energy producing potential
of methane is about three to four times greater than that of
hydrogen. Since methance itself is relatively electrochemically
inactive, it is very desirable to reform methane to form hydro-
gen and carbon monoxide in accordance with the reaction: CH4
+ H20 3H + CO. The hydrogen and carbon monoxide
can then participate in the fuel cell reaction either directly
or by further water-gas shift. An incentive for carrying out
such reforming reaction in a fuel cell is that the reaction is
endothermic and would serve to offset heat generated in fuel
cell operation due to inherent irreversibility. Accordingly,
internal reforming of fuel can reduce the load on the fuel cell
cooling system. Instroduction of a reforming catalyst in the
path of reactive process gas would serve to realize the fore-
going advantages. However, since the reforming reac'ion is
endothermic, it creates cold spots for the electrolyte vapor to
condense and, in turn, catalyst activity in promoting reforming
would be substantially reduced.
It is an object of the present invention to provide
for efficient use of hydrocarbon reforming in the thermal control
of fuel cells.
It is a further object of the inven~ion to provide for
in situ hydrocarbon reforming in fuel cells in manner preventing
catalyst deactivation by condensation of fuel cell electrolyte
vapor.
In attaining the foregoing and other objects, the in-
vention provides thermal control in an electrochemical cell
229
jointly through sensible heat of process gas and hydrocarbonreforming by conducting process gas through a passage formed
in or juxtaposition with the cell which is isolated from the
cell electrolyte and which includes catalyst promotive of re-
forming process gas hydrocarbon content. Other customary
passage in communication with the electrolyte is provided in
the cell and supplied with process gas for reaction purposes.
Output gas from both passages is cooled prior to recirculation
through the cell, with the gas exiting from the reforming pas-
sage being subjected to treatment removing substance therefrom(e.g. condensing the carbonates vapor) which retards promotive
reforming activity of the catalyst.
In series cell application, the invention conveys the
products of hydrocarbon reforming in a prior cell to a subseq-
uent cell for entry thereof into process gas reaction producing
electrical energy.
The foregoing and other objects and features of the
invention will be further understood from the following detailed
discussion thereof and from the drawings wherein like reference
numerals identify like parts throughout.
Fig. 1 is a sectional drawing of an explanatory em-
bodiment of an electrochemical cell in accordance with the
invention, as seen along plane I-I of Fig. 2.
Fig. 2 is a plan elevation of the Fig. 1 fuel cell,
shown together with accessory process gas supply and treatment
apparatus.
Fig. 3 is a sectional view of the Fig. 1 fuel cell,
as seen along plane iII_III of Fig. 1.
Fig. 4 is perspective illustrations of fuel cell
stacks in accordance with the invention.
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Fig. 5, sheet 2, is a sectional drawing of a further
explanatory embodiment of an electrochemical cell in accordance
with the invention, as seen from plane VI-VI of Fig. 6.
Fig. 6, sheet 2, is a side elevation of the Fig. 5 cell.
Fig. 7 is a perspective showing of the separator plate
employed in the cell of Figs. 5 and 6.
Figs. 8 and 9 are perspective showings of bipolar
separator plates for practicing the invention.
Fig. 10 is a sectional drawing of a fuel cell stack
in accordance with the invention.
Fig. 11 (a)-(d) are schematic showings of other em-
bodiments of separator plates for practicing the invention.
In Figs. l and 3, fuel cell 10 includes anode and
cathode electrodes 12 and 14, of customary gas diffusion type,
and electrolyte matrix or layer 16 therebetween. Separator
plates 18 and 20 are shown in the explanatory Fig. 1 single cell
embodiment as being of unipolar character, defining channel pas-
sages 18a, for supplying fuel/process gas to anode electrode 1
and passaqes 20a, for supplying oxidant/process gas to cathode
electrode 14. Based on the gas diffusion character of electrodes
12 and 14, passages 18a and 20a constitute elctrolyte-communi-
cative passages.
In accordance with the invention, thermal control plate
22, having reforming catalyst layers or packings 23, is stacked
on separator plate 18. Plate 22 includes conduit passage 22a
extending in like direction, i.e., across the plane of Fig. 1,
with passages 18a and is commonly connected therewith by input
anode gas manifold ~6 and output anode gas manifold 28.
Thermal control plate 24, constructed as in the first
mentioned U.S. patent 4,192,906 above, includes conduit passage 24a
5,,, ,,~
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not containing catalyst and extending in like direction, i.e.,
into the plane of Fig. l, with passages 20a and is commonly con-
nected therewith by input cathode gas manifold 30 (Fig. 2) and
output cathode gas manifold 32. Since separator plates 18 and
20 are essentially gas-impermeable, thermal control plate pas-
sages 22a and 24a constitute electrolyte-isolated passages. Thus,
process gases, i.e., fuel gas supplied from manifold 26 and
oxidant gas supplied from manifold 30, present in passages 22a
and 24a can be conducted through the fuel cell to serve thermal
control purposes and, in the case of reforming passage 22a, with-
out contributing to el!ectrolyte loss and without resulting in
electrolyte blockage due to condensation of electrolyte vapor on
cold spots resulting from the endothermic reforming reaction in
the fuel gas passage. To the contrary, process gases conducted
through channels 18a and 20a give rise to exit gas unavoidably
partially or fully saturated with electrolyte vapor. If catalyst
is applied in channels 18a, reforming cold spots can result
therein, as above discussed.
As is noted below fuel cells may employ thermal control
plates for one or the other of the process gases. Where desired,
exit admixing of process gas conducted through electrolyte-
communicative and electrlyte-isolated passages may be dispens~d
with in favor of common manifolding solely of input process gas
supplied to such diverse character passages. Also, as discussed
below, the present invention contemplates the introduction of
electrolyte-isolated, catalyst-containing process gas passages,
commonly input manifolded with a process gas supply, individually
per plural cells in a stack of fuel cells.
Referring again to Fig. 2, input anode gas manifold
26 is supplied through feed conduit 34, which is in turn fed
from pressurized input anode gaq supply 36. Process gas from
supply 36 may be admixed with, and thus supplemented by, process
gas theretofore conducted through the fuel cell. For this
purpose, output gas fxom manifold 28 is conducted through conduit
38 to unit 40, which serves both heat exchange and the removal
of catalyst-contaminating substances, and thence to a mixing
valve in supply 36. By operation of valve 42, gas may be funneled
to purge conduit 44, as desired. To remove heat from gas cond-
ucted through conduit 38 prior to recirculation, as is typical,
unit 40 is of heat reducing type whereby gas supplied from
unit 40 to supply 36 is of temperature lower than the cell oper-
ating temperature.
For thermal treatment, purging and recirculation of
cathode process gas, counterpart components include feed conduit
46, pressurized input cathode gase supply 48, input gas conduit
50, purge valve 52, purge conduit 54 and unit 46, which corres-
ponds to unit 40 in terms of cooling the process gas.
In implementation of methods of the invention, process
gas flow is established at a level or levls, as respects elec-
trolyte-communicative passages 18a and/or 20a, tc attain pre-
determined electrical energy to be produced by the electrochem-
ical cell. Even assuming reversibility of electrochemical rea-
ctions in fuel cells, a recognized minimum amount of heat is
liberated. Also, as alluded to above, irreversibility in fuel
cells, resultant from activation, concentration and ohmic over-
potentials, results in additional heat generation. Typically,
in fuel cells, about fifty per cent of input enthalpy shows up
as heat and the remainder as such predetermined electrical energy.
The heat energy may be ascribed as about one-fifth reversible
heat and four-fifths heat due to irreversibility.
1~44229
With process gas flow in passages 18a and 20a set
in accordance with such predetermined desired electrical
energy cell output, process gas flow in electrolyte-isolated
passages, 22a and/or 24a, and catalyst content of passages
22a are now set to obtain a predetermined operating temperature
range for the electrochemical cell. No completely analytical
procedure applies, since input and exit orifice geometry,
conduit sknin friction, conduit length, manifold geometries
and catalyst packing demand empirical test. The practice of
achieving desired flows in the respective passages may include
the placement of fixed or variably-settable constrictions in
either or both passages.
Referring to Fig. 4, a preferred embodiment of cell
stack 56 is shown without assiciated electrical output connect-
ions and encasements. Electrolyte layers and gas diffusion anodes
and cathodes are identified jointly as cell assemblies 58a-58j.
The top separator plate 60 is of unipolar type habing electrolyte-
communicative channel passages 60a, as in the case of separator
plate 18 of Fig. 1, and overlies the anode of top cell assembly
58a. Separator plate 62 is of biopolar type, defining electro-
lyte-communicative channel passages 62a, which underlie the
cathode of top cell assembly 58a, and 62b which overlie the anode
of second cell assembly 58b. Bipolar plates 64,66 and 68 separ-
ate cell assemblies 58b, 58c and 58d, with plate 68 gas passages
68b overlying the anode of cell assembly 58e. Separator plate
70 is of unipolar type, having passages 70a underlying the
cathode of cell assembly 58e. A sub-stack of five fuel cells
is thus provided. Thermal control plate 72 is diposed beneath
such sub-stack with its catalyst-containing conduit passage 72a
in communication with heat-generating surface of the sub-stack
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namely, the undersuface of separator plate 70. A like sub-stack
of five fuel cells, inclusive of cell assemblies 58f-~8j, is
disposed beneath plate 72. Unipolar separator plates 74 and 76
are endwise of the sub-stack and bipolar separator plates 78, 80
and 82 are intermediate the sub-stack. Thermal control plate 84
is arranged with its catalyst-containing conduit passage 84a
in communication with the under-surface of separator plate 76.
Input anode and cathode gas manifolds 86 and 88 are
shown schematically and separated from stack 56. Based on the
inclusion of thermal control plates 70 and 84 with anode gas con-
duit passages 72a and 84a, manifold 86 supplies process gas com-
monly to and through electrolyte-communicative and electrolyte-
isolated, catalyst-containing passages. Cathode oxidant flow
from manifold 88 is limited to electorlyte-communlcative passages
in this showing. In the illustrated arrangement, one electrolyte-
isolated, catalyst-containing passage is associated with each sub-
stack of five fuel cells. Where a thermal control plate is
located between sub-stacks, as in the case of plate 72, it will
serve to cool both such sub-stacks. Other assignment of thermal
control plates per fuel cells may be made as desired. Stiffening
elements 73 may be introduced in plate 72, as shown in Fig. 4,
to strengthen the stack and increase heat transfer surface area.
Such members are desirably electrically conductive to further
enhance electrical current passage through plate 72.
The thermal control method and arrangement of the in-
vention will be seen to provide several important benefits. Heat
transfer is accomplished through sensible heat of process gas
and hydrocarbon reforming by using an additional flow of process
gas without requiring any separate manifolding system, as is
necessary in case of liquid heat transfer medium. Possibility
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of corrosion by shunt currents and any harmful effects by leakage
are completely eliminated. The system reliability is, therefore
much greater than that for liquid heat transfer media. The elec-
trolyte losses by carry-over or vaporization to the process gases
are minimized because only a limited amount of process gases
contact the electrolyte. Process gases passing through the
thermal control plates do not contact the electrolyte, so vapor
losses due to flow of heat transfer gases are absent. Electro-
lyte blockage is averted since all catalyst-promoted reforming
takes place in an electrolyte-isolated environment. The thermal
control plates can serve as stiffening members, providing addi-
tional strength to the stack assembly. Further, if it is re-
quired to replace some defective cells during operation, a group
of cells between two thermal control plates can easily be removed
and new cells can be replaced.
The invention is particularly adapted for use in molten
carbonate fuel cells wherein the process gas used also in thermal
control is air/carbon dioxide cathode gas mixture and~or hydrogen-
rich anode gas mixture containing hydrocarbons and water. Where
the hydrocarbon content is methane, a suitable steam-reforming
catalyst is nickel or nickel based. A commercially available ver-
sion of such catalyst is *Girdler G-56 and is provided in pellet
form for packing in fixed bed type reactors. Suitable nickel
catalyst for this purpose and method for making the same is fur-
ther set forth in U.S. patent No. 3,488,266, in which hydrocarbon
reforming is carried out in heat exchange relationship to a fuel
cell, however in electrolyte-communicative environment.
Various changes in the methods of operation and in the
illustrated systems of Figs. 1-4 may be introduced. By way of
example, one may elect to supplement process gas furnished by
*Trademark
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supply 36 and/or supply 48 (Fig. l) solely with process gas con-
ducted through electrolyte-isolated passages, rather than the des-
cribed admixture of gases conducted through both electrolyte-
communicative and electrolyte-isolated passages. To implement
this variation, cell output gases are not manifolded but, rather,
are separately issued with the issuance of the electrolyte-iso-
lated passage being placed in communication with the input mani-
fold serving both types of passages.
In cascade arrangement of cells constructed as above
discussed, the reforming passage gas exiting the first of the
series of cells may be supplied to the reaction passage of the
second cell. In turn, the reforming passage gas exiting such
second cell may be furnished to the reaction passage of the third
cell, etc. Gas exiting the reaction passage of such first cell
may be admixed with gas exiting the reforming passage, as shown
in Figs. 1-3, or may be separately conducted to the reforming pas-
sage of the second cell, etc. Fresh fuel supplied to the first
cell may be introduced in any passage in subsequent cells. This
method of cascading has the advantage of using product water from
previous cells to enhance the steam-reforming of the hydrocarbons.
This is particulary important when the entire system is pressuri-
zed and the resulting equilibrium favors the formation of hydro-
carbons, particularly methane. Another advantage of such cascad-
ing is to maintain a higher partial pressure of hydrogen in the
fuel cell, thereby allowing more reversible operation.
Referring to Figs. 5 and 6, fuel cell llO includes
anode and cathode electrodes 114 and 112, of gas diffusion type,
and electrolyte matrix or layer 116 therebetween. Separator
plate 118 is of design having channel passages 118a, for supply-
ing process gas to cathode electrode 112. Separator plate 120 is
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constructed to implement this alternate embodiment of the inven-
tion and, as shown in explanatory Fig. 5 single cell embodiment
is of unipolar character, defining channel passages 120a, for
supplying fuel gas to anode electrode 114. sased on the gas
diffusion character of electrodes 112 and 114, passages 118a and
120a constitute electrolyte communicative passages.
Passages 120b of separator plate 120 are in flow iso-
lation with respect to anode electrode 114, the boundary walls
120c, 120d and 120e of the passages being essentially impermeable
to gas and having catalyst coating 121. Wall 120d is contguous
with electrode 114. Plate 120f is juxtaposed with passages 120b
to close the same. Accordingly, passages 120b are in flow isola-
tion with respect to electrolyte 116, and process gas supplied
to passages 120b can be conducted through the fuel cell to serve
thermal control purposes, by hydrocarbon reforming and sensible
heat, without contributing to electrlyte loss or blockage. To the
contrary, process gases conducted through passages 118a and 120a
give rise to exit gas unavoidably partially saturated with elec-
trolyte vapor. In the illustrated embodiment, passages 120a and
120b are alternately successive in a common plate location as one
progresses across that surface of electrode 114 which is contig-
uous with the crests 120d of electrolyte-isolated catalyst-con-
taining passage 120b.
In the systems of Figs. 1-4, unipolar separator plates,
such as plate 18 are used adjacent each of the cell electrodes
of a fuel cell. Supplemental process gas to be conducted through
electrolyte-isolated passages for thermal control is fed through
conduits of further plates which are spaced from the electrodes
by the unipolar separator plates. Such conduit-defining further
plates are employed in one cell in a succession of cells forming
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a stack. Since heat removal thus is affected by endothermic re-
forming reaction and sensible heat of supplemental process gas
at somewhat spaced sites, the possibility exists for thermal
gradients to be present in substantial measure. Such disadvantage
is overcome in the embodiments of Figs. 5-11, wherein thermal
gradients are reduced since heat removal may be accomplished, as
desired, from heat-generating surface of each cell.
The unipolar embodiment of separator plate 120 is read-
ily formed by the use of integral sheet material and corrugation
of same to form channels defining the respectively diverse pas-
sages. While the channels are shown as symmetric in Figs. 5-7,
they can be preselected to have different cross-sectional areas
in accordance with the ratio of flows therethrough needed to
achieve intended heat removal and electrical energy output. The
practice of achieving desired flows in the respective passages
may include variation of siae and geometry of the flow passages
and/or the placement of fixed or variably-settable constrictions
in either or both passages. As is shown by way of example in Fig.
7, a partial end wall 120g may be formed in channel 120a, or block
type obstacle 122 may be included therein.
Referring to Figs. 8 and 9, bipolar plates are shown for
implementing the invention. In Fig. 8, bipolar plate 124 includes
a corrugated sheet member 126 disposed atop a plate 128 which
defines channel passages 128a for process gas. Member 126 has
passages 126a (electrolyte-communicative) and 126b (electrolyte-
isolated and catalyst-containing).
In bipolar plate 130 of Fig. 9, backing plate 132 sup-
ports corrugated sheet members 134 and 136 and closes the elec-
trolyte-isolated passages 134b and 136b thereof. Crisscross
30 electrolyte-communicative passages 134a and 136a serve electrodes
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juxtaposed therewith (not shown) with process gases. Such Fig. 8
plate is shown in stack usage in the fuel cell stack of Fig. 10.
As the hydrocarbon content of the process gas increases,
the in situ reforming predominates the thermal balance in the sys-
tem and the benefits of higher thermal efficiency of system oper-
ation accrue.
The invention will be recognized as providing a highly
efficient vehicle for reforming of process gas, separate and apart
from thermal control.
As will be appreciated, various changes may be intro-
duced in the foregoing embodiments without departing from the in-
vention. Thus, passage geometry may be varied extensively, as
is shown by corrugated sheet members 138-144 illustrated sche-
matically in Figs. 11 (a)-(d). The particularly disclosed embodi-
ments and practices are thus intended in an illustrative and not
in a limiting sense. The true spirit and scope of the invention
is set forth in the following claims9
