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Methanol-air fuel cell with hydrophilic air electrodes
Energy Conversion.
Vol, 17. pp. 67 72. Pergamon Press, 1977. Printed in Great Britain
M E T H A N O L - A I R F U E L CELL WITH H Y D R O P H I L I C AIR ELECTRODES C, L. SYLWAN Department of Chemical Technology, The Royal Institute of Technology, S-10044 Stockholm. Sweden (Received 23 May 1977)
Abstract--A methanol-air fuel cell system with alkaline electrolyte and hydrophilic air electrodes was built and tested. Heat and fuel losses as well as auxiliary system power demand have been measured. Information is gained about attainable temperatures and voltage efficiencies as a function of the number of cells delivering a given electrical output. The effects of some stack imperfections are discussed. Methanol air fuel cell Traction fuel cell Fuel cell system
INTRODUCTION
Hydrophilic air electrodes
trodes and circulating electrolyte have been described. Systems using acid electrolyte have the advantage of spontaneous carbon dioxide removal but at the same time the acid electrolyte gives rise to difficulties concerning corrosion and air electrode performance. However Cathro and Weeks obtained 2275 hours service time with an acid methanol-air system [11]. An acid system has also been tested by ESSO [12]. Alkaline systems exhibit less severe material problems and allow the use of carbon- and silver catalysts. which have good selectivity in favor of oxygen reduction. On the other hand, carbonate build up in the electrolyte necessitates its occasional change or regeneration or use of an invariant electrolyte. A sucessful methanol-oxygen fuel cell with Cs2CO 3 as invariant electrolyte is reported by Cairns and Bartosik [13]. Figure 1 is a summary of some design principles concerning direct methanol-air fuel cell stacks. Principle Nos. 2, 7 and 8 have been realized using alkaline systems. Our methanol-air fuel cell was designed for a net effect (150 W) and size suitable for an electric wheel chair with a 12 V electric system. The hydrophilic air electrodes were attached to frames not allowing supply of air by free convection.
During 1970-72, the Swedish metal-air battery project [1] produced battery prototypes for an electric van. The air electrodes consisted of pressed and sintered nickel powder with silver as catalyst. Since these electrodes were hydrophific, a differential pressure of ca. 20 kPa was needed. The aim of the present work was to identify problems related to the use of these electrodes in methanol-air fuel cell stack building. Most methanol electrodes were made by applying small amounts of platinum metals into porous nickel matrices, similar to those used on the air side [2]. The electrolyte consisted of potassium hydroxide solution mixed with methanol. Some circumstances make methanol an interesting fuel: It can be produced from different raw materials such as oil, coal, brown coal, natural gas, peat, oil shale, urban wastes and wood. The reactivity in fuel cells is high, although some scientists prefer to convert the methanol to hydrogen gas, feeding hydrogen fuel cells [3-6]. The hydrogen-air fuel cell has a greater power density, but the converter and the hydrogen purification device make those fuel cells more complex than the direct methanol-air battery. In the direct cell, the methanol is often mixed with the electrolyte. The most simple systems use respirating air electrodes and non-circulating electrolyte. Such systems can be made highly versatile and thus suitable for long unattended duty [7, 8]. Power density, however, is rather low. A system intermediate between the direct and indirect is described by Kordesch and Koehler [9]. Methanol is fed into the interior of the anode, where it is decomposed catalytically, forming hydrogen ions. An interesting system is indicated by Warszawski et aL [10]. This system uses bipolar electrodes based on a thin metal sheet sepai'ating the electrolyte-methanol mixture from electrolyte containing finely divided air bubbles. A variety of systems including gas diffusion elec-
CELL STACK
The principal design of the fuel cell is shown in Fig. 2. The over all cell reaction CH3OH + 3/2 0 2 + 2 O H - - - - } C O 2- + 3 H 2 0 gives a theoretical cell voltage of 1.35V at pH = 16116]. Practical open voltage values for single cells have amounted to 0.7 to 0.8 V. Figure 3 shows polarisation and efficiency values for a 200 cmz unit. Using the above figures, the 150W/12V demand plus a loss of 1-2 A to the auxiliary system leads. in case of series connection, to 0.30-0.35 V cell voltage and a voltage-efficiency of ca. 25°/~,, if service temperature can be maintained at 70°C. The stack will then consist of about 30 cells, 400 cm 2 each. 67
68
SYLWAN:
METHANOL AIR FUEL CELL Stockdesign
Air electrode A i r electrodes with pump or fon
type
Circulating electrolyte
Non.circulating elect rol yte 2
I
Respiroting oir electrodes Non circulating
Circulating
electrolyte
3
Air electrodes with commonair ond electrolyte space
electrolyte
4
+
Electrolyte with Electrolyte finely divided air bearing separator I0
H Hyd rophilic Separator with knobs or ribbons
in contact with ,air electrode. ~ir is between
(nabs or ribbons see below
5
7
6
+
8
+
Hydrophobic
[ r], [8] t
R
M
[14]
[,o]
Non-circulating electrolyte. All of the electrolyte is situated inside the cells, leading to a big electrode distance. Electrolyte resistance will become acceptable only at low power densities. System including electrolyte pump and manifold. Precautions avoiding leak currents and trapping of gas must be taken.
Special system providing frequent changes of the electrolyte bearing ribbed or knobbed separators.
Special equipment for providing the cells with electrolyte containing finely divided air, as well as for separating oxygen exhausted air from the electrolyte leaving the cells.
Air fan or pump with manifolding system securing even distribution of air to all cells. Special arrangements for separating leaking electrolyte and/or condensed water. Vacuum-pump when differential pressure is requested. Rises methanol vaporisation. Power consumption sensitive to air leakages. No pressure on the intake air requires more vacuum pump power.
Air electrodes which are able to work under conditions of negative differential pressure. Fig. 1.
[15]
SYLWAN: METHANOL-AIR FUEL CELL
69
that reason, a 5-stage centrifugal fan, type ETRI 601 SAG 01, was chosen. It possesses 5 series connected impellers (width 4.2 mm and diameter 106 mm each). Fan performance according to Fig. 4, permits the requested air flow/pressure data with a power consumption of only 8 W.
c.,,.t.ck
Electrolyte pump Air exit
Fig. 2. Schematic drawing of the fuel cell stack with auxiliary system. 100 0 100
Z ua
The pump was an "EHEIM" with magnetic clutch and impeller-diameter 45 mm. The impeller and housing material was Hostalen P P N 1075. Together with a 5 W PM-motor this pump produced a flow rate of 20 ml/sec, which proved enough to secure electrolyte distribution to all cells in a 30-cell stack. Vacuum Pump
CC
200
#
300 400
> --~ 500
N~ethanO~
uJ
O~L 600 700 800
J 10
210
I 30
I 40 0.5
0A Z ua
0.3
S Q. ~2 ua O
0
This pump creates the differential pressure between the air and the electrolyte compartments, by percolating air out from the latter. Compared to compression of the intake air, this is the energetically cheaper method, provided the air electrodes and other separating elements are sufficiently leak proof. According to earlier experience, an air electrode couple leaks approximately 1/6 ml/sec, which gives 5 ml/sec for our projected stack. In order to establish the power consumption of the needed vacuum pump, it was considered convenient to build one using a glass syringe. Special care was spent avoiding unnecessary dead space (Fig. 5). The result was a power consumption of about 2 W maintaining a differential pressure of 24kPa and the required flow rate. Summary
D.1 O>
I
10
I
I
20 30 CURRENT mA/c m2
/0
Fig. 3. Polarisation curves, output and voltage efficiency for a set of fuel cell electrodes in a cell. The electrode area in the cell is 400 cm z of each polarity. All electrodes are pressed and sintered nickel. Air electrode catalyst: 15 mg/cm 2 silver. Methanol electrode catalyst: 1 mg/cm 2 Pt and 0.55 mg/cm 2 Pd.
• The power consumption of the auxiliary system thus sums up to 15 W, which is within the assumption above. Figure 6 shows the complete system. The stack and the electrolyte compartment are surrounded by a 30 mm thick heat insulating layer of polyurethane foam.
'
'
AUXILIARY SYSTEM Fan
The stoichiometric air consumption in a cell producing 15 A is 0.0045 1/sec. Experiments show 4 to be a convenient enlargement factor for converting the stoichiometrical consumption to practical need in single cells of the type used here. Assuming sufficiently good air distribution over the cell stack, 30 cells will need ca. 0.551. air/sec, which, as data for our single cells showed, leads to a pressure drop of ca. 4 cm of water. Conventional centrifugal fans achieve this performance consuming more than 20 W, whereby the air flow will be much smaller than the design flow. For
="%. I 1
\
2
3
FLOW RATE I/s
Fig. 4. Aerodynamic curves for the 5-stage centrifugal fan ETRI 5 x 106 x 4.2.
70
SYLWAN: METHANOL AIR FUEL CELL 150
125
~:
loo
50
25
I
00
L
I
I
~
10 20 30 40 50 °C ABOVE AMBIENT TEMP=21°C
60
Fig. 7. Heat losses through casing at different temperatures.
Fig. 5. The vacuum pump. RESULTS, ACTUAL FUEL CELL STACK Heat losses
Heat leaves the system partly through the casing, partly by the outgoing air streams containing water and methanol steam. The heat losses through the casing were determined by heating the fuel cell, except methanol, electrically to different steady state temperatures (Fig. 7).
A fuel cell stack which is leak proof as expected is only subject to a negligible cooling by the air stream through the vacuum pump. On the other hand, the air stream from the fan causes a substantial loss of energy by passing the cell stack. This was determined by rotameter measurements and by cooling the outgoing air with liquid nitrogen, causing water and methanol to condense completely, enabling analysis by gas chromatography. The outgoing air was saturated with water vapor, while the methanol content was only half of that corresponding to equilibrium. In other words, the air electrodes suppress the passage of methanol in favor of water. Using the above measured values, a calculation of the possible steady state temperatures of the fuel cell system has been made as shown in Fig. 8. Also shown are the voltage efficiencies as a function of the number of cells and the temperature. (Losses through the vacuum pump as well as direct oxidation of methanol
Fig. 6. The fuel cell system except vacuum pump and heat insulating cover.
SYLWAN:
METHANOL-AIR FUEL CELL
Cooled Stack
25
30
35
40 45 50 55 N U M B E R OF C E L L S FOR 150 w
5
60
65
70
Fig. 8. Influence of number of cells and temperature on heat balance and voltage efficiency when stack output is 150W. Continuous lines represent some voltage efficiencies, while dashed lines represent points of heat balance for three different cases: 1/1 represents the actual case described here but with the assumption that air leakage into electrolyte room and self-oxidation of methanol on the air electrodes are zero. 1/2 and 1/4 represent cases where the heat losses by the exiting air is reduced to half resp. to one quarter by, for instance, heat exchange. Apart from this, points above or below a dashed line can only be reached by heating resp. cooling the system by external means. Cell performance is according to Fig. 3.
on the cathodes assumed zero.) The difficulties involved in reaching higher voltage efficiencies under the present heat-loss conditions are evident.
Methanol losses by outgoing air streams If the cell stack is fairly leak proof, the only methanol loss of importance is via the fan air. However, these losses are substantial. At 67°C and 2 M KOH--3 M M e O H initially, the mean methanol losses amounted to 11.5 mg/sec accounting for a chemical energy loss of not less than 280 W. C E L L STACK D E S I G N F E A T U R E S The design of the stack is, in this case, to a great extent determined by the air electrode pairs and their plastic frames. The air electrodes are 0.6 mm thick double layer plates of pressed and sintered nickel. The nickel current collectors are welded to the electrodes, and the electrodes are melted to the plastic frames (Penton) which, put together, form the cell stack with incorporated air and electrolyte channels [1]. All the cells are connected in series. Actual performance data of the fuel cell stack reached ca. 60% of the original design data, mainly due to four different types of errors. A. Electrolyte leakage from inside the electrolyte compartments. This error has been relatively easy to compensate for by turning on the speed of the electro-
71
lyte pump. Also some sealing could be done from the outside of the stack using a plastic enamel. B. Air leakage from the air compartments. These leakages, which occurred preferentially where the air electrodes are melted against the plastic frames, have a disastrous effect on efficiency. Methanol losses will ris ~, substantially and so will the cooling of the system. Even the power consumption of the vacuum pump will go up and unless it has sufficient capacity the differential pressure will drop, leading to increased air electrode polarisation. A leakage of 0.5 1/sec about triples the methanol losses in the case where the vacuum pump has the capacity. C. Short circuiting inside cells. Some short circuits have appeared. The causes of these errors have been faulty location of separators or the existence of metal particles at the bottom of the electrolyte or air compartments. Total shorting of a cell results in a current drain of ca. 40 A which represents a loss of 54 W or 2 mg MeOH/sec more than with the normal loss of 7 W/cell. D. Deactivation of electrodes. Strong deactivation leads to hydrogen evolution in case of an air electrode and to oxygen evolution in case of a methanol electrode provided the stack is under load. Deactivation of a complete cell results in a negative potential of more than one volt at 15 A, thus neutralizing ca. 3 good cells, according to Fig. 4. It is evident that a serious risk for defects of the types B, C and/or D will prevent the stack from delivering power output during any reasonable time span. Large efforts have to be devoted to stack development in close connection with electrode specifications. The risk for errors of the B-type, for instance, could be lowered essentially by introducing air electrodes for low or even negative differential pressures. Such electrodes of the hydrophobic type are now under development in our institute. The losses of methanol by the exiting air-stream could be lowered by lowering temperature, if future electrode performances permit this step.
REFERENCES [1] O. Lindstr6m, Power Sources vol. 5, p. 283. Academic Press, New York (1975). [2] C.L. Sylwan, Methanol fuel cell electrodes consisting of platinized nickel matrices, Energy Conversion 15, 137 (1975). I3] O. Bloch and M. Prigent, Evolution des recherches sur les piles h m6thanol, Entropie 14, 52 (1967). [4] Y. Breelle, Le g~n6rateur 61ectrique m6thanol air, Entropie 33, 21 (1970). [5] H. B6hm and K. Maass, Methanol/air acidic fuel cell system, 9th Intersociety Energy Conversion Engineering Conference proceedings, San Francisco. p. 836 (1975). 1'6] A. Winsel, Verfahrenstechnik in Brennstoffzellen, Chem. Ing. Tech. 48, 103 (1976). 1'7] H. G. Plust, Alkohol-Luft-Brennstoffzellen als Langzeit Energiequellen, Brown Boveri Mitteil. 53, 5 (1966).
72
SYLWAN:
METHANOL-AIR FUEL CELL
[8] J. Perry, Methanol-air battery. Proc. 26th Power Sources Symposium, Chicago, 29-30 April, 1-2 May, p. 171 (1974). [9] K. V. Kordesch and J. O. Koehler. Swedish patent No. 206127. [10] B. Warszawski, B. Verger and J.-C. Dumas, Alsthom fuel cells for marine and submarine applications, M T S J. 5, 28 (1971). [11] K.J. Cathro and C. H. Weeks, Acid fuel cell batteries using soluble fuels--II. Methanol air system, Energy Conversion 11, 143 (1971). [12] E. H. Okrent and B. L. Tarmy, Heat and water balancing of the methanol-air fuel cell, Energy con-
[13]
[14] [15] [16]
version systems. Chem. Engng Progr. Syrup. Ser. 63, 74 (1967). E. J. Cairns and D. C. Bartosik, A methanol fuel cell with an invariant alkaline electrolyte. J. electrochem. Soc. 111, 1205 (1964). M. Osumi, M. Ikeyama, H. Manabe, T. Iwaki, Y. Kobayashi and M. Fukuda, 10 W Methanol-air fuel cell power source, Nat. Teeh. rpt 16, 281 (1970). O. Lindstr6m, Swedish patent appliance No 75070041-7. M. Prigent, Oxydation 61ectrochimique du m~thanol dans les piles h comhustibles~ Rev. I2 1nstitut Francais Pdtrole 19, 1 (1964).
Vol, 17. pp. 67 72. Pergamon Press, 1977. Printed in Great Britain
M E T H A N O L - A I R F U E L CELL WITH H Y D R O P H I L I C AIR ELECTRODES C, L. SYLWAN Department of Chemical Technology, The Royal Institute of Technology, S-10044 Stockholm. Sweden (Received 23 May 1977)
Abstract--A methanol-air fuel cell system with alkaline electrolyte and hydrophilic air electrodes was built and tested. Heat and fuel losses as well as auxiliary system power demand have been measured. Information is gained about attainable temperatures and voltage efficiencies as a function of the number of cells delivering a given electrical output. The effects of some stack imperfections are discussed. Methanol air fuel cell Traction fuel cell Fuel cell system
INTRODUCTION
Hydrophilic air electrodes
trodes and circulating electrolyte have been described. Systems using acid electrolyte have the advantage of spontaneous carbon dioxide removal but at the same time the acid electrolyte gives rise to difficulties concerning corrosion and air electrode performance. However Cathro and Weeks obtained 2275 hours service time with an acid methanol-air system [11]. An acid system has also been tested by ESSO [12]. Alkaline systems exhibit less severe material problems and allow the use of carbon- and silver catalysts. which have good selectivity in favor of oxygen reduction. On the other hand, carbonate build up in the electrolyte necessitates its occasional change or regeneration or use of an invariant electrolyte. A sucessful methanol-oxygen fuel cell with Cs2CO 3 as invariant electrolyte is reported by Cairns and Bartosik [13]. Figure 1 is a summary of some design principles concerning direct methanol-air fuel cell stacks. Principle Nos. 2, 7 and 8 have been realized using alkaline systems. Our methanol-air fuel cell was designed for a net effect (150 W) and size suitable for an electric wheel chair with a 12 V electric system. The hydrophilic air electrodes were attached to frames not allowing supply of air by free convection.
During 1970-72, the Swedish metal-air battery project [1] produced battery prototypes for an electric van. The air electrodes consisted of pressed and sintered nickel powder with silver as catalyst. Since these electrodes were hydrophific, a differential pressure of ca. 20 kPa was needed. The aim of the present work was to identify problems related to the use of these electrodes in methanol-air fuel cell stack building. Most methanol electrodes were made by applying small amounts of platinum metals into porous nickel matrices, similar to those used on the air side [2]. The electrolyte consisted of potassium hydroxide solution mixed with methanol. Some circumstances make methanol an interesting fuel: It can be produced from different raw materials such as oil, coal, brown coal, natural gas, peat, oil shale, urban wastes and wood. The reactivity in fuel cells is high, although some scientists prefer to convert the methanol to hydrogen gas, feeding hydrogen fuel cells [3-6]. The hydrogen-air fuel cell has a greater power density, but the converter and the hydrogen purification device make those fuel cells more complex than the direct methanol-air battery. In the direct cell, the methanol is often mixed with the electrolyte. The most simple systems use respirating air electrodes and non-circulating electrolyte. Such systems can be made highly versatile and thus suitable for long unattended duty [7, 8]. Power density, however, is rather low. A system intermediate between the direct and indirect is described by Kordesch and Koehler [9]. Methanol is fed into the interior of the anode, where it is decomposed catalytically, forming hydrogen ions. An interesting system is indicated by Warszawski et aL [10]. This system uses bipolar electrodes based on a thin metal sheet sepai'ating the electrolyte-methanol mixture from electrolyte containing finely divided air bubbles. A variety of systems including gas diffusion elec-
CELL STACK
The principal design of the fuel cell is shown in Fig. 2. The over all cell reaction CH3OH + 3/2 0 2 + 2 O H - - - - } C O 2- + 3 H 2 0 gives a theoretical cell voltage of 1.35V at pH = 16116]. Practical open voltage values for single cells have amounted to 0.7 to 0.8 V. Figure 3 shows polarisation and efficiency values for a 200 cmz unit. Using the above figures, the 150W/12V demand plus a loss of 1-2 A to the auxiliary system leads. in case of series connection, to 0.30-0.35 V cell voltage and a voltage-efficiency of ca. 25°/~,, if service temperature can be maintained at 70°C. The stack will then consist of about 30 cells, 400 cm 2 each. 67
68
SYLWAN:
METHANOL AIR FUEL CELL Stockdesign
Air electrode A i r electrodes with pump or fon
type
Circulating electrolyte
Non.circulating elect rol yte 2
I
Respiroting oir electrodes Non circulating
Circulating
electrolyte
3
Air electrodes with commonair ond electrolyte space
electrolyte
4
+
Electrolyte with Electrolyte finely divided air bearing separator I0
H Hyd rophilic Separator with knobs or ribbons
in contact with ,air electrode. ~ir is between
(nabs or ribbons see below
5
7
6
+
8
+
Hydrophobic
[ r], [8] t
R
M
[14]
[,o]
Non-circulating electrolyte. All of the electrolyte is situated inside the cells, leading to a big electrode distance. Electrolyte resistance will become acceptable only at low power densities. System including electrolyte pump and manifold. Precautions avoiding leak currents and trapping of gas must be taken.
Special system providing frequent changes of the electrolyte bearing ribbed or knobbed separators.
Special equipment for providing the cells with electrolyte containing finely divided air, as well as for separating oxygen exhausted air from the electrolyte leaving the cells.
Air fan or pump with manifolding system securing even distribution of air to all cells. Special arrangements for separating leaking electrolyte and/or condensed water. Vacuum-pump when differential pressure is requested. Rises methanol vaporisation. Power consumption sensitive to air leakages. No pressure on the intake air requires more vacuum pump power.
Air electrodes which are able to work under conditions of negative differential pressure. Fig. 1.
[15]
SYLWAN: METHANOL-AIR FUEL CELL
69
that reason, a 5-stage centrifugal fan, type ETRI 601 SAG 01, was chosen. It possesses 5 series connected impellers (width 4.2 mm and diameter 106 mm each). Fan performance according to Fig. 4, permits the requested air flow/pressure data with a power consumption of only 8 W.
c.,,.t.ck
Electrolyte pump Air exit
Fig. 2. Schematic drawing of the fuel cell stack with auxiliary system. 100 0 100
Z ua
The pump was an "EHEIM" with magnetic clutch and impeller-diameter 45 mm. The impeller and housing material was Hostalen P P N 1075. Together with a 5 W PM-motor this pump produced a flow rate of 20 ml/sec, which proved enough to secure electrolyte distribution to all cells in a 30-cell stack. Vacuum Pump
CC
200
#
300 400
> --~ 500
N~ethanO~
uJ
O~L 600 700 800
J 10
210
I 30
I 40 0.5
0A Z ua
0.3
S Q. ~2 ua O
0
This pump creates the differential pressure between the air and the electrolyte compartments, by percolating air out from the latter. Compared to compression of the intake air, this is the energetically cheaper method, provided the air electrodes and other separating elements are sufficiently leak proof. According to earlier experience, an air electrode couple leaks approximately 1/6 ml/sec, which gives 5 ml/sec for our projected stack. In order to establish the power consumption of the needed vacuum pump, it was considered convenient to build one using a glass syringe. Special care was spent avoiding unnecessary dead space (Fig. 5). The result was a power consumption of about 2 W maintaining a differential pressure of 24kPa and the required flow rate. Summary
D.1 O>
I
10
I
I
20 30 CURRENT mA/c m2
/0
Fig. 3. Polarisation curves, output and voltage efficiency for a set of fuel cell electrodes in a cell. The electrode area in the cell is 400 cm z of each polarity. All electrodes are pressed and sintered nickel. Air electrode catalyst: 15 mg/cm 2 silver. Methanol electrode catalyst: 1 mg/cm 2 Pt and 0.55 mg/cm 2 Pd.
• The power consumption of the auxiliary system thus sums up to 15 W, which is within the assumption above. Figure 6 shows the complete system. The stack and the electrolyte compartment are surrounded by a 30 mm thick heat insulating layer of polyurethane foam.
'
'
AUXILIARY SYSTEM Fan
The stoichiometric air consumption in a cell producing 15 A is 0.0045 1/sec. Experiments show 4 to be a convenient enlargement factor for converting the stoichiometrical consumption to practical need in single cells of the type used here. Assuming sufficiently good air distribution over the cell stack, 30 cells will need ca. 0.551. air/sec, which, as data for our single cells showed, leads to a pressure drop of ca. 4 cm of water. Conventional centrifugal fans achieve this performance consuming more than 20 W, whereby the air flow will be much smaller than the design flow. For
="%. I 1
\
2
3
FLOW RATE I/s
Fig. 4. Aerodynamic curves for the 5-stage centrifugal fan ETRI 5 x 106 x 4.2.
70
SYLWAN: METHANOL AIR FUEL CELL 150
125
~:
loo
50
25
I
00
L
I
I
~
10 20 30 40 50 °C ABOVE AMBIENT TEMP=21°C
60
Fig. 7. Heat losses through casing at different temperatures.
Fig. 5. The vacuum pump. RESULTS, ACTUAL FUEL CELL STACK Heat losses
Heat leaves the system partly through the casing, partly by the outgoing air streams containing water and methanol steam. The heat losses through the casing were determined by heating the fuel cell, except methanol, electrically to different steady state temperatures (Fig. 7).
A fuel cell stack which is leak proof as expected is only subject to a negligible cooling by the air stream through the vacuum pump. On the other hand, the air stream from the fan causes a substantial loss of energy by passing the cell stack. This was determined by rotameter measurements and by cooling the outgoing air with liquid nitrogen, causing water and methanol to condense completely, enabling analysis by gas chromatography. The outgoing air was saturated with water vapor, while the methanol content was only half of that corresponding to equilibrium. In other words, the air electrodes suppress the passage of methanol in favor of water. Using the above measured values, a calculation of the possible steady state temperatures of the fuel cell system has been made as shown in Fig. 8. Also shown are the voltage efficiencies as a function of the number of cells and the temperature. (Losses through the vacuum pump as well as direct oxidation of methanol
Fig. 6. The fuel cell system except vacuum pump and heat insulating cover.
SYLWAN:
METHANOL-AIR FUEL CELL
Cooled Stack
25
30
35
40 45 50 55 N U M B E R OF C E L L S FOR 150 w
5
60
65
70
Fig. 8. Influence of number of cells and temperature on heat balance and voltage efficiency when stack output is 150W. Continuous lines represent some voltage efficiencies, while dashed lines represent points of heat balance for three different cases: 1/1 represents the actual case described here but with the assumption that air leakage into electrolyte room and self-oxidation of methanol on the air electrodes are zero. 1/2 and 1/4 represent cases where the heat losses by the exiting air is reduced to half resp. to one quarter by, for instance, heat exchange. Apart from this, points above or below a dashed line can only be reached by heating resp. cooling the system by external means. Cell performance is according to Fig. 3.
on the cathodes assumed zero.) The difficulties involved in reaching higher voltage efficiencies under the present heat-loss conditions are evident.
Methanol losses by outgoing air streams If the cell stack is fairly leak proof, the only methanol loss of importance is via the fan air. However, these losses are substantial. At 67°C and 2 M KOH--3 M M e O H initially, the mean methanol losses amounted to 11.5 mg/sec accounting for a chemical energy loss of not less than 280 W. C E L L STACK D E S I G N F E A T U R E S The design of the stack is, in this case, to a great extent determined by the air electrode pairs and their plastic frames. The air electrodes are 0.6 mm thick double layer plates of pressed and sintered nickel. The nickel current collectors are welded to the electrodes, and the electrodes are melted to the plastic frames (Penton) which, put together, form the cell stack with incorporated air and electrolyte channels [1]. All the cells are connected in series. Actual performance data of the fuel cell stack reached ca. 60% of the original design data, mainly due to four different types of errors. A. Electrolyte leakage from inside the electrolyte compartments. This error has been relatively easy to compensate for by turning on the speed of the electro-
71
lyte pump. Also some sealing could be done from the outside of the stack using a plastic enamel. B. Air leakage from the air compartments. These leakages, which occurred preferentially where the air electrodes are melted against the plastic frames, have a disastrous effect on efficiency. Methanol losses will ris ~, substantially and so will the cooling of the system. Even the power consumption of the vacuum pump will go up and unless it has sufficient capacity the differential pressure will drop, leading to increased air electrode polarisation. A leakage of 0.5 1/sec about triples the methanol losses in the case where the vacuum pump has the capacity. C. Short circuiting inside cells. Some short circuits have appeared. The causes of these errors have been faulty location of separators or the existence of metal particles at the bottom of the electrolyte or air compartments. Total shorting of a cell results in a current drain of ca. 40 A which represents a loss of 54 W or 2 mg MeOH/sec more than with the normal loss of 7 W/cell. D. Deactivation of electrodes. Strong deactivation leads to hydrogen evolution in case of an air electrode and to oxygen evolution in case of a methanol electrode provided the stack is under load. Deactivation of a complete cell results in a negative potential of more than one volt at 15 A, thus neutralizing ca. 3 good cells, according to Fig. 4. It is evident that a serious risk for defects of the types B, C and/or D will prevent the stack from delivering power output during any reasonable time span. Large efforts have to be devoted to stack development in close connection with electrode specifications. The risk for errors of the B-type, for instance, could be lowered essentially by introducing air electrodes for low or even negative differential pressures. Such electrodes of the hydrophobic type are now under development in our institute. The losses of methanol by the exiting air-stream could be lowered by lowering temperature, if future electrode performances permit this step.
REFERENCES [1] O. Lindstr6m, Power Sources vol. 5, p. 283. Academic Press, New York (1975). [2] C.L. Sylwan, Methanol fuel cell electrodes consisting of platinized nickel matrices, Energy Conversion 15, 137 (1975). I3] O. Bloch and M. Prigent, Evolution des recherches sur les piles h m6thanol, Entropie 14, 52 (1967). [4] Y. Breelle, Le g~n6rateur 61ectrique m6thanol air, Entropie 33, 21 (1970). [5] H. B6hm and K. Maass, Methanol/air acidic fuel cell system, 9th Intersociety Energy Conversion Engineering Conference proceedings, San Francisco. p. 836 (1975). 1'6] A. Winsel, Verfahrenstechnik in Brennstoffzellen, Chem. Ing. Tech. 48, 103 (1976). 1'7] H. G. Plust, Alkohol-Luft-Brennstoffzellen als Langzeit Energiequellen, Brown Boveri Mitteil. 53, 5 (1966).
72
SYLWAN:
METHANOL-AIR FUEL CELL
[8] J. Perry, Methanol-air battery. Proc. 26th Power Sources Symposium, Chicago, 29-30 April, 1-2 May, p. 171 (1974). [9] K. V. Kordesch and J. O. Koehler. Swedish patent No. 206127. [10] B. Warszawski, B. Verger and J.-C. Dumas, Alsthom fuel cells for marine and submarine applications, M T S J. 5, 28 (1971). [11] K.J. Cathro and C. H. Weeks, Acid fuel cell batteries using soluble fuels--II. Methanol air system, Energy Conversion 11, 143 (1971). [12] E. H. Okrent and B. L. Tarmy, Heat and water balancing of the methanol-air fuel cell, Energy con-
[13]
[14] [15] [16]
version systems. Chem. Engng Progr. Syrup. Ser. 63, 74 (1967). E. J. Cairns and D. C. Bartosik, A methanol fuel cell with an invariant alkaline electrolyte. J. electrochem. Soc. 111, 1205 (1964). M. Osumi, M. Ikeyama, H. Manabe, T. Iwaki, Y. Kobayashi and M. Fukuda, 10 W Methanol-air fuel cell power source, Nat. Teeh. rpt 16, 281 (1970). O. Lindstr6m, Swedish patent appliance No 75070041-7. M. Prigent, Oxydation 61ectrochimique du m~thanol dans les piles h comhustibles~ Rev. I2 1nstitut Francais Pdtrole 19, 1 (1964).