CA2142090A1

CA2142090A1 - Fuel cell and method for moistening the electrolyte

Info

Publication number: CA2142090A1
Application number: CA002142090A
Authority: CA
Inventors: Karl Strasser
Original assignee: Karl Strasser; Siemens Aktiengesellschaft
Current assignee: Siemens AG
Priority date: 1992-08-10
Filing date: 1992-08-10
Publication date: 1994-02-17
Also published as: WO1994003937A1; US5543238A

Abstract

Abstract In the case of fuel cells having an electrolyte which conducts oxygen ions,, hydroxide ions or protons, the problem fundamentally exists of moistening the air of the combustion gases in order to prevent the electrolyte drying out or thinning and thus to prevent defective operation of the fuel cell during air operation. In this case, the design and financial cost, in particular, for moistening of the air is of concern.
In order to avoid this disadvantage, it is provided according to the invention that exhaust gas, which occurs on the cathode side, of the fuel cell is at least partially recirculated into the cathode of the fuel cell. In consequence, the water content of the electro-lyte can be set within wide limits by simple adjustment of the recirculated exhaust-gas quantity. In consequence, economical use of the PEM fuel cell becomes possible.
The invention can be used in principle for all fuel cells having an electrolyte which conducts oxygen ions, hydroxide ions or protons.

Description

flLII~. P~H~THIS AM~NrJL 1::1 92P3426P ~ F TRANSLAT10~
Fuel Cell and Method for Moi~tening the Electrolyte The invention relate~ to a ~uel cell, preferably a PEM fuel cell, and to a method for moistening th0 elea-trolyte of the fuel call.
A ~uel cell in general co~pri~e~ an elsctrically conductive current tran~fe~ plate, a cathode, an inter-mediate layer which conducts :ions, an anode and a further electrically conductive current trans~er plate, all of which are stacked on one another in the stated sequence a~ flat plates. Fuel cells of thi~ construction are alraady known, inter alia, ~rom the "Fuel Cell Handbook"
by Appleby and Foulkes, New York, 1989 and from the artidle by K.Strasser "Brenn~toffzellen fur Elektrot-raktion" [Fuel Cells $or Eleatrical Traction], VDI
Reports No.912, 1~92, Paga~ 125 to 145. Since the ~ual cell can convert chemically bonded energy directly into electrical energy, it ma~e~ it possible to con~ert ~uels ~uch ae hydrogen, natural gas, biogas, for example, into electrlcal energy with a high~r efficiency and in a more environmentally friendly manner than it is po~sible to do using the pre~iously known conventional thermal power atations whose effici~ncy is limited by the so-called Carnot'~ process.
A8 a conseguence of the above-mentionsd docu-ments, a polymer-electrolyta-membxane fuel cell (PEM fuel cell) is fa~ored in conjunction with an electrical dr~ve.
This ~uel cell type aan be operated both with technically pure ga~es and with gases and air containing CO2. The low operating t~mperature k 100 C), the high power density, the favorable long-term behavior and the lac~ o$ a corro~ive, li~uid electrolyte, for example, are ;'`''''~
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92P3426P - 2 - ~ ;
particularly advantageous for use in a vehicle. Corrosive liquid electrolyte~ are used, for example, in the acidic or alka~ine fuel cell. ~ -The water balance in the electrolyte during ;.
operation of the fuel cell~ repre~entB a particular problem in the case of the said ~uel cells. The operability of the fuel cell i8 clo~ely linked to the water content in the fuel cell and, in particular, in the electrolyte. An excessively high water content in the electrolyte lead~ to the available power from the fuel cell being reduced, as a result of the excessively high dilution of said electrolyte. An exce~sively low water content o~ the electrolyte likewise leads to the eleatri- i~
cal power from the fuel cell being reduced aa a result of `;`~
the inorea~e in the internal resi3tance. Furthermore, even in the case of the electrolyte partially drying out, gas breakdown and thu~, the formation of combu~t~ble ga~
mixtures, can occur. In the wor~t ca~e, this leads to damage or destruction of the fuel cell in the event of 20 combustion of the gas m~xture. ```-~
A relatively costly vaporizer-condenser arran- -gement has there~ore already been proposed for adjusting ` ;~
the water content of the electrolyte in an acidic or alkaline fuel cell, in the case of which arrangement at 25 lea~t one of the gases flowing into the fuel cell is used ~P
to tran~port water vapor and, for this purpo~e, i~ al~o passed over a lukewarm water surface. -~
A PEM fuel cell, which is preferably operated ~ `
with hydrogen and air, require~ a vaporizer arrangement for adjusting the water content in the membrane, which conducts protons. The dimensioning of the vaporizer arrangement must be matched to the lowest system pres~ure ` ,,''`','.
'''`''``"

0~0 P~T/DE 92/00661 ~ince the maximum volume flow~ must be moistened in thi~
case at a aonstant temperature and the large~t material exchange areas are therefore required. Whe~ such a vaporizer arrangement i8 u~ed, the relatively large physical volume, which can reach the size of the actual fuQl cell block, and the high i~vestment costs linked thereto must be accepted as di.~advantage~. When using a PEM fuel cell, these disadvantages are 80 seriou~ that they preclude use, especially mobile use, and thus wider application of such ~uel cells.
The invention is thus based on the object o specifying a low-temperature fuel cell, in particular a PEM fuel cell, and a method for moietening the electro-lyte which allow the said di~advantages to be avoided to ~uch an extent that the fuel cell, in particular the PEM
fuel cell, can be used from economical viewpoint~.
With respect to the method, this ob~ect is achieved in that exhaust gas wh~ch occurs on the cathode side of the low-temperature fuel cell is at least par-tially reciraulated into the cathode of the low-tempera-ture fuel cell, the part of the exhaust gas which is recirculated by means of an adjusting element being ~et in proportion to the power output ~rom the low-tempera-ture fuel cell.
The term low-temperature fuel cells mean~ poly-mer-electrolyte membrane fuel cells, acidic fuel cells and alkaline fuel cell8. However, the following text refers to the PEM fuel cell without in 80 doing preclu-ding the other two types mentioned above.
In consequence, a part of the water (product water) which is produced during the electrochemical reaction in the fuel cell is initially carried away with the other exhauat gas from ~he cathode of the PEM fuel cell AMENDED SHEET

; .. . .. . . ,: . ... ., . ~ . , . , ~ ... . .: . .

2al90 ~R 92 P 3426 P - 4 -and i6 then at least partially recirculated into the cathode of the PEM fuel cell, as a result of which the moisture level o~ the oxidation agent flowing into the cathode is rai~ed and improved moistening of the electro-lyte in the P~M fuel cell i8 ensur~d. The expression supply of the oxidation agenl: to the cathode o the PEM
~uel cell in this case means not only the supply of the air oxygen from the surrounding air but also, altar-natively, the supply o the technically pure oxygen.
With respect to the low-temperature fuel cell, this object is achieved according to the invention in that a recirculation line is connected to an exhaust-gas line which is connected to the low-t~mperi~ture fuel cell on the cathode side, via which recirculation line at least a part of the low-temperature fuel cell can be recirculated, the recirculation line being assigned an ad~usting element, and the recirculated part of the exhaust gas being adjusted by means of the setting element in proportion to the power output from the low-temperature fuel cell.
In consequence it is possible for a part o the water and of the heat which are extracted from the cathode of the PEM fuel cell to be fed back into the cathode. The recirculated part of the exhaust gas can be adjusted by means of the adjusting element. The power output from the fuel cell san easily be determined by current and voltage mea~urement, the material aonversion of the fuel cell also rising in proportion to the power output as the power output from the fuel cell rises. The recirculated part of the exhaust gas can be set approp~
riataly using the adjusting element. This results in a moisture level of the polymer electrolyte membrane (PEM), which is pre~erred ~or disturbance-free operation of the PEM fuel ceIl~ being obtained a~l the time.
In order to compress the recirculated part of the exhaust gaa again to the inlet air pressure on the cathode side, it i8 advantageous if the recirculation AMENDED SHEET
''' ' '' ' 09~
, ~R 92 P 3426 P - 4a -line opens via a ga~ compre~or into an air ~upply line connected on the cathode ~ide. In this case, the ga~
compre~or need co~pensate only for a relati~ely ~mall pressure difference between the cathode inlet and outlet, I , I . :

AMæND~D S~EET
,-., ~i, 142~

and compress relatively ~mall air quantities.
Two examplary PmhodimPnt~ of the invention are explained in more detail with reference to a drawing, in which~
Figure 1 ~hows a schematic illustration of a P~M ~uel cell according to the invention ha~ing a recirculation line ~or exhaust gas which occurs on the cathode side of the fuel cell, and ~igure 2 shows a detail of the opening, which is modi~
fied ~rom that in Figure 1, of the recirculation line into the air ~upply line.
The ~uel cell 2, which i~ illustrated schemati~
cally in Figure 1, compri~e~ a cooling space 4, a s2acer 6 on the cooling water side, an air gas 3pace 8, a plate on the cathode side made of carbon paper 10, a plate cathode 12, a PEM membrane 14 (commercially available, for example, under the name "Nafion 117"), a plate anode 16, a plate 18 on the anode ~ide made of carbon paper, a hydrogen gas space 20, a spacer 22 on the cool~ng water ~ids, and a cooling space 24, which are all stacked on one another ln thi~ sequence a~ flat plate~. The cooling ~pace 4 on the cathode ~ide and the cooling space 24 on the anode ~ide can be connectad to a cooling water circuit, which ~8 not illu~trated in mora detail. An air supply line 26 i~ connected to the air gas space 8 on the input sidR, and an exhau~t-gas line 28 is connected to it on the output side, the latter leading to the open air via an adjusting element 30 and a relief turbine 32. A
recirculation line 34, which opens via a gas compre~sor 36 into the air ~upply line 26, i~ connected to the ~ adju~ting el~ment 30. An air compressor 38 i~ connected ;~4;~090 .

in the air supply line 26 in ~ront of th~ opening of the recirculation line 34 into the air supply line 26, in the flow direction of the air. A part of the drive power of the air comprQs~or 36 and of the further air compressor 38 i~ applied via a connection 40, which i~ indicated only dchematically here, from the exhaust-ga3 re~ief turbine 32. The remaining drive power ~ust be provided by a motor 41, which i~ not illu~trated in more detail here.
A hydrogen supply line 42 i8 connected to the hydrogen ga~ space 20 on the input side. ThiR line 42 leads from a hydrogen ~ource 44, via a val~e 46 and a g39 moistener 48 into the hydrogen gaa space 20. On the output side, a return line 50 ~or hydrogen is connected to the h~drogen space 20, which return line 50 opens into 15 the hydrogen ~upply line 42 via a gas compres~or 52 between the air moistener 48 and the hydrogen gas space 20.
During operation of the fuel cell 2, the hydrogen gas space 20 in the exemplary embodiment is aated on by a hydrogen partial pressure of approximately 2 bar. The air ga~ space 8 has air applied to it by mean~ of the gas compre~sor 36 and the air compressor 38, the static air pre~ure being approximately 1.3-4 bar a in the exemplary embodiment. In the cathode, the air oxygen molecules are in each case catalytically converted, with the absorption of ~our electron3, into two oxygen ion~ with double negative charges. The oxygen ions are pas~ed to the boundary layer between the cathode 12 and the PEM 14. The eleatrons which are regu$red for reduction o~ the oxygen ara produced catalytically in the anode, on which two ~hydrogen molecule~ are in each case ~plit into ~our hydrogan ions and ~our eleatrons. In this ca~e, a voltage 0 ~ ~ ~
/ ~ .
92P3426P - 7 ~
U~z of approximately 0.5 - 1 V, depe~ding on the load current set, i8 applied to a contact 54, which i~ con-nected to the aarbon paper plate 10 on the cathode side, and to a contact 56, which i~ connected to the carbon paper plate 18 on the anode ~ide.
If an electrical load i~ connected between the contacts 54 and 56, the elec~trons which become free in the anode flow via an am~eter 58 and an external electri-cal load, which is not illustrated in more dekail, to the cathode 12. The fuel cell 2 then starts its correct operation and in 80 doing reache~ a ~peaific power up to approximately 700 mW/cm2 and a current den~ity o~
1000 mA/cm2. The operating t~mperature i~ in this case approximately 80 C. The hydrogen gas, which flows to the anode 16 via the hydrogen supply line 42, was previously passed through the gas moistener 48 and was moiatened there, i8 partially consumed in the fuel cell, with electrons being released and water subsQquently being formed. Since this water, also called product water, is ~ormed virtually exclusively on the boundary surface between the cathode 12 and the PEM 14, that part of the hydrogen gas which is not consumed is pas~ed into the hydrogen return line 50. The hydrogen gas, which i~
moi~tenad with the product water, is subsequently passed via the gas compressor 52 again into the hydrogen supply line 42 and, as a result of ~ts ~ubseguently being introduced into the anode 16, prevent3 the PEM 14 drying out on the boundary layer between the PEM 14 and the anode 16. The consumed part of the hydrogen gas i8 in this ca~e supplemented from the hydrogen source 44 and i8 moistened by means o the gas moistener 48. The ga3 moistensr 48 can be supplied with condensed water, which is obtain~d from the exhaust gas on the cathode ~ide, in a manner which is not illu~trated in more detail h~re.

~ 2090 , ;

The produc~ water which i~ produced on the cathode ~iide i~ r~moved with the air flow from the air gaB space 8, being introduced into the exhaust-gas line ~8 from the fuel cell 2. A ~art of the exhau~t gas i~
introduced into the recircula1:ion line 3~ by means of the adjusting elemant 30, a~ a function of the output power from the fuel cell, and i~ ~upplied from there via the gas compre~sor 36 into the air supply line 26 again. In con~equence, a part of the wat:er which i8 produced on the boundary layer between the cathode 12 and the PEM 14 during the electrochemical reaction is also re~irculated into the cathode l2, as a result o~ which the PEM 14 i~
pr~vented from drying out, and defective operation of the fuel cell 2 ia thu~ prevented. When the fuel cell 2 i8 on full load, the recirculated air quantity is approximately half the exhau~t-gas air quantity. Adequate moi~tening of I the PEM 14 on the ~ide of the cathode 12 i8 thus ~180 j~ ensured.
In the event of a required air ratio of m 2 2.5 !~i 20 and in the event of half the exhaust-air quantity being returned, the total air guantity conveyed through the air gas space is increased by approximately 20%, and thu~ the pressure drop in the air path through the fuel cell 2 and, in consequence, the power requir~ment for air compression a~ well, are al~o increased by approximately ~' 20~. The air ratio m is in thi~ ca~ie defined as the ratio of the oxygen guantity in the air to the oxygen require-ment (stoichiometric). Added to this there i8 also the power reguir~ment of the gas compressor 36 for the recirculated exhaust-gae quantity, the ga~ compre~sor 36 having to compensate ~or only a ~mall air-pres~ure difference in order to compress the recirculated exhaust gas again to the input air pressure of the air ga~ BpaCe - 8. A part of the power con~umed by the air compre~or 38 is in thi3 aase applied by means of the exhaust-ga~
relief turbi~e 32', which is driven by the remaining I~ Dt-glll3 qUll:ltity.

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Without thi3 recirculation of the exhauRt-ga3 air, a gas moi~tener, which i8 not illu~trated here, would have to be connected up~itream o~ the air compre~or 38 in order to avoid the PE~ drying out on the cathode side. In this case, the dimensioning of this so-called membrane mo~stener would have to be carried out in accordance with the lowest ~ystem pressure, that i8 to J~ say matched to the maximum po~sible air quantity. Such a m~mbrane moi~tener can be implemented technically but has a multiple of the volume of a ~tack arrangement of uel cells 2 and a relatively high production price. The disadvantage~ which are linked to tha use of a membrane ~)~ moistener would thu3 preclude the u~e of a PEM fual cell ,!, 2.
:~ 15 Figure 2 shows an alternati~e pos~ibility for introducing recirculated air into the air supply line 26 ~l and in ~o doing compensating for the pressure difference.
.~; For this purpose, an air ~et compre~30r 63 i8 installed ~,j at the opening point for the recirculation line 34 ~uch P~ 20 ~hat ita induction connecting piece 60 i~ aonnected to the recirculation line 34 and its compressed-air supply ~¦ aonneating piece 62 is connected to the air compressor 38. This results in the recirculated gas mixture being ~ucked in by the compressed air flowing to the fuel cell 2 in accordance with the instantaneous ~etting of the adjusting element 30.
The rscirculation according to the invention of exhaust ga~ which occur~ on the cathode side saves a voluminous and costly air moistener with little expendi-ture and thu~ crsates a precondition for reducing theproduction costs for fuel cells 2 on the basis ofl a P~M
r~ ~ 14 -A construction, which is only ~lightly changed ^~ from that in Figure 1, can al~o ~ave the u3e of separate air moi~teners on the cathode ~ide of the fuel cell in ~4;~

the ca~e of an alkaline or acidic fuel aell. Overall, the measures proposed according to the invention would al~o lead to an improvement in ~he overall efficiency of the .:
fuel cell in the ca~e of the~e fuel cell~. The overall efficiency o~ the exemplary ~odiment which i~ described in Figure 1 i~ over 60% in partial-load operation, for example t a load factor of 20%.

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Claims

Patent Claims

1. A method for moistening the electrolyte of a low-temperature fuel cell (2), especially of a polymer-electrolyte membrane fuel cell (PEM fuel cell), charac-terized in that the exhaust gas, which occurs on the cathode side, of the fuel cell (2) is at least partially recirculated into the cathode (12) of the fuel cell (2), the part of the exhaust gas which is recirculated by means of an adjusting element (30) being set in propor-tion to the power output from the fuel cell (2).

2. A low-temperature fuel cell for carrying out a method as claimed in claim 1, characterized in that a recirculation line (34) is connected to an exhaust-gas line (28) which is connected on the cathode side to the fuel cell (2), via which recirculation line at least a part of the exhaust gas which occurs on the cathode side can be recirculated into the cathode (12) of the fuel cell (2), the recirculation line (34) being assigned an adjusting element (30), and the part of the exhaust gas which is recirculated by means of the adjusting element (30) being adjustable in proportion to the power output from the fuel cell (2).

3. The low-temperature fuel cell as claimed in claim 2, characterized in that the recirculation line (34) opens via a gas compressor (36) into an air supply line (26) which is connected on the cathode side.

4. The low-temperature fuel cell as claimed in claim 3, characterized in that an air compressor (38) is installed in the air supply line (26), in front of the opening of the recirculation line (34) in the flow direction of the air.

5. The low-temperature fuel cell as claimed in claim 2, characterized in that a gas compressor is installed in the air supply line after the opening of the recirculation line in the flow direction of the air, and a restriction point is installed in front of the opening.

6. The low-temperature fuel cell as claimed in claim 2, characterized in that the recirculation line (34) opens into the induction connecting piece (60) of an air jet compressor (63) which can be supplied with compressed air from an air compressor (38) connected in the air supply line (26), and is connected on the cathode side to the fuel cell (2).