Liquid fuel cell

A liquid fuel cell having a circulation system for a fuel mixture comprising fuel and water, which comprises a first tank containing water or a water-rich fuel mixture comprising water and fuel, a second tank containing fuel or a fuel-rich mixture comprising water and fuel, a first detector for detecting the liquid level of the fuel mixture in the circulation system, a second detector for detecting a fuel concentration of the fuel mixture in the circulation system, or an output from the fuel cell, or a load current of the fuel cell, a valve means for controlling flow of the water or the water-rich mixture in the first tank to the circulation system in accordance with the output from the first detector, and a valve means for controlling flow of the fuel or the fuel-rich mixture in the second tank to the circulation system in accordance with the output from the second detector, can be stably and efficiently operated for a prolonged time in spite of different consumption rates of fuel and water even if the load current or operating temperature of the fuel cell or the temperature or humidity of the atmosphere is changed.

BACKGROUND OF THE INVENTION 
This invention relates to a liquid fuel cell, and particularly to a liquid 
fuel cell capable of stable operation for a prolonged time under 
controlled supply of fuel and water. 
Generally, fuel cells using a liquid fuel are classified into an acid type 
and an alkali type, and methanol, formalin, hydrazine, etc. are used as 
fuel. The working principle of such fuel cells will be briefly described, 
referring to FIG. 1, where numeral 1 shows a fuel cell and symbols + and - 
show terminals for outputting electricity. The fuel cell 1 comprises a 
fuel electrode 2-1, an oxidizing agent electrode 2-2 counterposed to the 
fuel electrode 2-1 (the oxidizing agent electrode can be called "oxygen 
electrode" when oxygen is used as an oxidizing agent, or "air electrode" 
when air is used as an oxidizing agent), an electrolyte chamber provided 
between the oxidizing agent electrode 2-2 and the fuel electrode 2-1, a 
fuel chamber 4 provided adjacent to the fuel electrode 2-1, and an 
oxidizing agent chamber 5 provided adjacent to the oxidizing agent 
electrode 2-2. In FIG. 1, numeral 6 shows the fuel (which may contain 
water), or a mixture of fuel and electrolyte and also shows its flow 
direction, and numeral 7 likewise shows the oxidizing agent and also shows 
its flow direction. 
The fuel cell as structured above works as follows. When the fuel 6 is 
supplied to the fuel chamber 4 and when the oxidizing agent 7 is supplied 
to the oxidizing chamber 5, the fuel 6 permeates into the fuel electrode 
2-1 to generate electrons through the electrochemical reaction. When a 
load is given to the external circuit, a direct current can be obtained. 
In this case, a product 81 is formed in the fuel chamber 4. The product is 
a carbon dioxide gas or carbonate when the fuel is methanol, formic acid 
or formalin, and nitrogen when the fuel is hydrazine. When the supply of 
fuel 6 of a circulating type, the product contains excess fuel or 
electrolyte, and it is necessary to separate and vent the gaseous product 
from the circulation system. 
On the other hand, when the oxidizing agent 7 is supplied to the oxidizing 
agent chamber 5, the oxidizing agent 7 permeates and diffuses into the 
oxidizing agent electrode 2-2 to consume electrons through the 
electrochemical reaction. When the electrolyte is of an acid type, a 
product 82 is formed. The product is mainly water and contains excess air. 
When the electrolyte is of a base type, water is formed at the fuel 
electrode 2-1. 
When an aqueous solution of electrolyte such as sulfuric acid or potassium 
hydroxide is used in the electrolyte chamber 3 in the unit fuel cell 1 
structured as in FIG. 1, the aqueous solution leaks from the electrolyte 
chamber 3 and thoroughly permeates also into the electrodes, and a good 
cell performance can be obtained. However, the aqueous solution of 
electrolyte also leaks into the fuel chamber 4 in this case, and thus it 
is necessary to supply fuel mixture containing the aqueous solution of 
electrolyte prepared in advance as anolyte. To this end, the fuel chamber 
4 is provided with a circulation system for supplying the fuel mixture to 
the fuel chamber 4 by a pump 9 and a system for supplying the fuel from a 
fuel tank 10 through a valve 11 to the circulation system, as shown in 
FIG. 2. 
It has been also proposed to use an aqueous solution of polymeric 
electrolyte in the electrolyte chamber 3 in place of the acid electrolyte, 
and provide the fuel chamber with a circulation system for a fuel mixture 
of fuel and water adjusted to a most suitable concentration for the 
operation as in FIG. 2, and also with a system for supplying the fuel from 
the fuel tank 10 to the circulation system. 
As shown in FIG. 2, the product gas 811 is vented from the circulation 
system after the passage through the fuel chamber 4, and the remaining 
mixture 812 is recycled to the fuel chamber. 
According to the conventional fuel supply system as described above, a fuel 
mixture in a constant mixing ratio prepared in advance is supplied to the 
circulating system from the single fuel tank 10 shown in FIG. 2. However, 
it has been found that the consumption ratios of fuel and water in the 
circulation mixture 6 containing the fuel are not always constant, and 
depend on changes in load, changes in temperature of fuel cell during the 
operation, even though the load is constant, or changes in flow rate and 
temprature or humidity of the air supplied as the oxidizing agent. 
In a fuel cell using a liquid fuel, the fuel supply system contains two 
essential components, i.e. fuel and water, and further may contain an 
electrolyte. In the most cases, these three components, i.e. fuel, water 
and electrolyte are usually contained in the fuel supply system. Among 
these three components, it is the fuel and water that are consumed. 
Usually, it is not necessary to take consumption of electrolyte into 
consideration. Consumption rate of fuel differs from that of water, 
because firstly water is always consumed at one electrode, whereas at 
another electrode water is always formed as a result of the electromotive 
reaction of a fuel cell, and formation of water at the fuel electrode or 
the oxidizing agent electrode, depends on the acidity or the alkalinity of 
electrolyte. That is, in the case of an acidic electrolyte, water is 
formed at the oxidizing agent electrode and consumed at the fuel 
electrode, whereas in the case of an alkaline electrolyte, the formation 
and consumption of water are reversed. In that case, one mole or two moles 
of water is principally formed with one mole of fuel throughout the 
reaction, depending on the species of fuel. Since the consumption and 
formation of water take place at the different electrodes, water actually 
tends to migrate through the electrolyte chamber to keep a water balance. 
Even in view of this tendency, water is short at one electrode and in 
excess at another electrode, owing to much dissipation of water and 
difficulty to keep the water balance well throughout the electrolyte 
chamber. 
Secondly, the excess or shortage of water due to water imbalance in the 
water migration between the electrodes largely depends on the operating 
temperature and the load current. 
Thirdly, the excess fuel that is not converted to the electric current at 
the fuel electrode migrates through the electrolyte chamber and permeates 
into the oxidizing agent electrode to occasion direct oxidation of the 
fuel, or water migrates as hydronium ions when the electrolyte ions 
migrate in the electrolyte chamber in the case of an acidic electrolyte. 
These phenomena also depend on the load current and operating temperature 
of a fuel cell. Furthermore, the amount of water carried by the oxidizing 
agent, for example, air by evaporation at the oxidizing agent electrode 
side depends on the feed rate, temperature and humidity of the oxidizing 
agent. 
The consumption rate of fuel differs from that of water on the grounds as 
described above, and thus the supply of a mixture of fuel and water only 
in a constant mixing ratio from a single tank to the fuel circulation 
system as shown in FIG. 2 can only meet a change in the amount of only one 
component among the two components, i.e. fuel and water, in the fuel 
circulation system including the fuel chamber. That is, adequate control 
over the fuel and water cannot be made, and stable and prolonged operation 
of a fuel cell is quite impossible to conduct. That is, the fuel in the 
fuel circulation system may be so concentrated that the heat is much 
generated or the current output is lowered, or the supply of fuel fails to 
catch up with the consumption, so that the fuel becomes short in the fuel 
circulation system. 
In a fuel cell using a liquid fuel, the cell voltage V shows a flat peak in 
a certain range of concentration C.sub.m of fuel 6 when the current is 
constant. At a lower fuel concentration C.sub.m, the fuel becomes short 
and the cell voltage is lowered, whereas at a higher fuel concentration 
C.sub.m, the excess fuel that fails to take part in the reaction at the 
fuel electrode 2-1 migrates through the electrolyte chamber 3 and 
permeates into the oxidizing agent electrode 2-2 to occasion direct 
combustion of fuel. As a result, the potential on the oxidizing agent 
electrode 2-2 is lowered with generation of heat, and consequently the 
cell voltage is lowered. When the fuel concentration is too high or too 
low (e.g. less than C.sub.m1 or more than C.sub.m2 in FIG. 3), the ratio 
of the necessary amount of electrical energy-converted fuel to the amount 
of consumed fuel will be lowered, and thus the fuel ultization efficiency 
is considerably lowered. Thus, it is very important to select an 
appropriate fuel concentration. 
An appropriate range of the fuel concentration, i.e. the range of fuel 
concentration, C.sub.m1 to C.sub.m2, shown in FIG. 3, has been so far 
experimentally studied by many researchers. For example, in the case of an 
acidic electrolyte type fuel cell using methanol as fuel, it is disclosed 
in 24th Cell Panel Discussion Lectures No. 2B02, page 254 that the 
concentration C.sub.m1 is 0.5 moles/l and the concentration C.sub.m2 is 2 
moles/l at the current density of 64 mA/cm.sup.2. Japanese Patent 
Application Kokai (Laid-open) No. 56-118273 discloses that the 
concentration C.sub.m2 is about 5% by weight (about 1.6 moles/l). 
On the other hand, even in a liquid fuel cell using hydrazine as fuel, 
Japanese Patent Publication No. 48-31300 discloses that stable operation 
is possible at 1.5% by weight (0.5 moles/l), and if the concentration is 
less than 1.5% by weight, the voltage is lowered and the temperature is 
increased. 
It is seen from the foregoing that the fuel concentration range for stable 
operation is about 0.3 moles/l as C.sub.m1 and about 2 moles/l as 
C.sub.m2. 
Thus, the fuel concentration is very important in the fuel cell, and a more 
accurate apparatus for detecting or measuring the fuel concentration is 
still required. 
A liquid fuel cell provided with an apparatus for detecting a fuel 
concentration now in practical use is shown in FIG. 4, where the same 
members as in FIG. 1 and FIG. 2 are indicated with the same reference 
numerals. 
An oxidizing agent 7 is supplied to an oxidizing agent chamber 5 by a 
blower 111, and discharged as a residual gas 82. On the other hand, a fuel 
supply system includes a system for circulating a mixture of fuel and an 
electrolyte solution (the mixture may be called "anolyte") by a pump 9 and 
a system for supplying an appropriate amount of fuel to an anolyte tank 20 
provided in the circulation system from a fuel tank 10 through a valve 17. 
The circulation system is open to the outside at an appropriate position 
to discharge the product gas 811. 
The fuel is supplied by opening the valve 17, and the opening or closure or 
control of the valve 17 is made by an apparatus 13 for detecting a fuel 
concentration provided in the anolyte tank 20 and a valve controller 171. 
The apparatus 13 for detecting a fuel concentration comprises an anode 
electrode 517 (which will be hereinafter referred to merely as "anode"), a 
cathode electrode 518 counterposed to the anode (the cathode electrode 
will be hereinafter referred to merely as "cathode"), a power source 519, 
and a detector 520. The anode 517 comprises a platinum plate 517a and a 
membrane 517b tightly laid on the platinum plate 517a by pressing. 
With such a structure as described above, when a DC voltage of e.g. 0.85 V 
is applied to between the anode 517 and the cathode 518, the quantity of 
electric current changes proportionally to the methanol concentration in 
the anolyte. Thus, it is possible to determine the concentration of 
methanol as fuel in a very simple structure. 
However, the concentration of methanol can be indeed determined by the 
apparatus with such a structure as described above, but its detection 
sensitivity is not better, as given below. 
Relationship between the fuel concentration and detected electric current 
is shown in FIG. 12, where curve a shows those determined by an apparatus 
for detecting a fuel concentration using an anode with the membrane as 
shown in FIG. 5. The electric current changes with concentration C.sub.m 
but the change in electric current is small. That is, the detection 
sensitivity is poor. 
Furthermore, the adhesion between the platinum plate 517a and the membrane 
517b (FIG. 5) is often inadequate, and the anolyte tends to stay 
therebetween, deteriorating the response to changes in the methanol 
concentration. When a platinum-based catalyst layer is laid on the 
platinum plate 517a in place of the membrane 517b, much detected current 
can be obtained as shown by curve b in FIG. 12, but there is no change in 
the detected current in the practical range (about 0.3-about 2 moles/l) 
and such a structure cannot be used as a sensor. 
Cyclic voltammetry using a reference electrode and an apparatus for 
detecting a fuel concentration by means of a small fuel cell as disclosed 
in Japanese Patent Application Kokai (Laid-open) No. 56-118273 are also 
available as another apparatus for detecting a fuel concentration. In the 
case of the cyclic voltammetry, a reference electrode is required in 
addition to the detecting electrodes, and also a function generator and 
other devices are required, complicating the detecting system and 
deteriorating the reliability, the most important task of the sensor. 
In the case of the apparatus using a small fuel cell, not only the 
apparatus is dipped in the anolyte tank, but also an additional air supply 
system is required, and there is a difficulty in reduction in the 
apparatus size as well as in the reliability. 
In the case of using methanol or formalin as fuel rather than using 
hydrazine as fuel, the detected power output changes in a complicated 
manner even according to the cyclic voltammetry, and the determination is 
sometimes difficult to make. 
There is other procedure for supplying a fuel when an integrated load 
current becomes constant, since the fuel concentration is proportional to 
the load current, but when the load is greatly changed or the operation of 
fuel cell is subject to repetitions of discontinuation, the fuel 
concentration will be greatly deviated and cannot be practically 
determined. A gas concentration sensor based on semi-conductors requires 
much time until it is settled for the measurement, and thus the response 
becomes poor. 
Thus, a liquid fuel cell with a reliable apparatus for detecting a fuel 
concentration in a simple structure is in keen demand. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a liquid fuel with an 
improved supply of fuel and water to a fuel circulation system which can 
operate continuously and stably for a prolonged time in spite of 
differences in the consumption rates of fuel and water. 
Another object of the present invention is to provide a liquid fuel cell 
with an apparatus for detecting a fuel concentration with a high 
reliability and a high sensitivity in a simple structure. 
The present invention provides a liquid fuel cell having a circulation 
system for a fuel mixture comprising fuel and water, which comprises a 
first tank containing water or a water-rich fuel mixture comprising water 
and fuel, a second tank containing fuel or a fuel-rich mixture comprising 
water and fuel, a first detector for detecting the liquid level of the 
fuel mixture in the circulation system, a second detector for detecting a 
fuel concentration of the fuel mixture in the circulation system, or an 
output from the fuel cell, or a load current of the fuel cell, a valve 
means for controlling flow of the water or the water-rich mixture in the 
first tank to the circulation system in accordance with the output from 
the first detector, and a valve means for controlling flow of the fuel or 
the fuel-rich mixture in the second tank to the circulation system in 
accordance with the output from the second detector. 
According to the present invention, an apparatus for detecting a fuel 
concentration by electrochemical reaction, comprising an anode electrode 
provided with a fuel-controlling layer for controlling permeation of fuel 
through a catalyst layer, a cathode electrode, a power source and a 
detector, the anode electrode and the cathode electrode being dipped in 
the fuel mixture and a voltage being applied to the electrodes is used as 
a second detector in the present liquid fuel cell. 
Fuel cannot be too concentrated in the fuel circulation system, because at 
a higher fuel concentration, excess fuel is liable to permeate into the 
oxidizing agent electrode from the fuel electrode through the electrolyte 
chamber, and undergo direct oxidation, i.e. direct consumption, 
considerably lowering the fuel utilization efficiency. Usually the fuel 
concentration in the fuel circulation system is about 0.3 to about 2 
moles/l, and the absolute amount of the fuel in the fuel circulation 
system is small. 
Thus, in the present invention a fuel concentration sensor is used to 
detect the fuel concentration in the fuel circulation system to supply the 
fuel, or an output voltage or output current of the fuel cell is detected 
because the output voltage or current is reduced as the fuel concentration 
is lowered. When the detected value becomes lower than the standard 
concentration, a signal to open the valve to the fuel tank is emitted to 
supply the fuel to the fuel circulation system. 
A considerably large amount of water is present in the fuel circulation 
system, and thus it is preferable to supply the water to the fuel 
circulation system to checking whether a predetermined amount of water is 
retained in the fuel circulation system satisfactorily or not. To this 
end, a liquid level sensor is provided in the fuel circulation system of 
the fuel cell to detect whether the liquid level becomes lower than the 
standard level or not. When the liquid level is detected lower than the 
standard level, a signal to open the valve to the water tank is emitted to 
supply the water to the fuel circulation system. 
In the present invention, two tanks, i.e. fuel tank and water tank, are 
provided, and only fuel is stored in the fuel tank and only water in the 
water tank. However, it is more preferable and more advantageous for the 
operation of the fuel cell to distribute the necessary amounts of fuel and 
water to the individual tanks, that is, to store mixtures of fuel and 
water in the individual tanks. When only fuel is supplied to the fuel 
circulation system from the fuel tank, higher fuel concentration is 
locally and transiently developed in the fuel circulation system owing to 
the restricted circulation rate, unpreferably lowering the fuel 
utilization efficiency transiently. This problem can be solved by storing 
a fuel-rich mixture of fuel and water in the fuel tank. Preferable molar 
ratio of water to fuel in the fuel-rich mixture is 5-0:1, where zero means 
only fuel. It is preferable to select a ratio approximating to the ratio 
of consumption rate of water to that of fuel on average during the 
operation of the fuel cell. 
When only water is supplied to the fuel circulation system from the water 
tank, lower fuel concentration is likewise locally and transiently 
developed in the fuel circulation system owing to the restricted 
circulation rate, and the fuel decomes short locally, unpreferably 
lowering the performance of the fuel cell. The problem can be solved by 
storing a water-rich mixture of fuel and water in the water tank. 
Preferable molar ratio of fuel to water in the water-rich mixture is 1 to 
0:1, where zero means only water. It is preferable to select a ratio 
approximating to the fuel concentration in the fuel circulation system in 
the fuel cell.

PREFERRED EMBODIMENTS OF THE INVENTION 
One embodiment of the present invention will be described, referring to 
FIG. 6, where a liquid fuel cell using methanol as fuel and sulfuric acid 
as an electrolyte is illustrated. Electrodes 2 (fuel electrode 2-1 and 
oxidizing agent electrode 2-2) are each made from a porous carbon plate as 
a substrate and a catalyst of platinum-based substance supported on carbon 
powders, the catalyst being deposited on the substrate. Through a fuel 
chamber 4, a liquid mixture of methanol and dilute sulfuric acid, which, 
of course, contains water, is circulated as an anolyte by a pump 9. A 
carbon dioxide gas is generated at the fuel electrode 2-1 as a product gas 
811. 
Air is supplied to an oxidizing agent chamber 5 as an oxidizing agent 7, 
and the exhaust gas 82 contains formed water at the same time. 
A liquid level sensor 12 is provided on the liquid level corresponding to 
the liquid level threshold value near the upper end of the electrodes 2 in 
the fuel circulation system. When the liquid level is lowered, the sensor 
12 works to emit a signal to open the valve 111 and supply the necessary 
amount of water from the water tank 101 to the fuel circulation system. 
A methanol concentration sensor 13 based on the electrochemical reaction is 
provided in the fuel circulation system and set to the methanol 
concentration of 1 mole/l. When the methanol concentration in the fuel 
circulation system becomes lower than the set value, the sensor 13 works 
to emit a signal to open the valve 112 and supply the necessary amount of 
the fuel from the fuel tank 102 to the fuel circulation system. The valves 
may be pumps. 
In a liquid fuel cell with the structure of FIG. 6 and with a power output 
of 12 V and 100 W, the circulation rate of the anolyte in the fuel 
circulation system is set to 700 cc/min., and about 30 cc of water is 
supplied to the fuel circulation system from the water tank 101 with one 
opening of the valve 111 by the signal from the liquid level sensor 12 
when the liquid level is lowered in the fuel circulation system. About 10 
cc of fuel is supplied to the fuel circulation tank from the fuel tank 102 
with one opening f the valve 112 by the signal from the methanol 
concentration sensor 13 when the fuel concentration becomes lower than 1 
mole/l. 
The fuel concentration during the operation of liquid fuel cell is not 
necessarily 1 mole/l, and operation at a higher fuel concentration is 
possible, if the load current is relatively large, whereas the operation 
at a lower fuel concentration is also possible, if the load current is 
relatively small. 
To set a fuel concentration, the set electric current must be changed, 
because the electric current is a function of fuel concentration according 
to the constant voltage system when the electrochemical reaction is 
utilized. 
As described above, a liquid fuel cell with two tanks, i.e. a fuel tank 
containing only fuel and a water tank containing only water can be 
operated stably against fluctuations in load current, operating 
temperature or atmosphere. 
Another embodiment of the present invention will be described below, 
referring to FIG. 7, where, when the liquid level is detected lower by the 
liquid level sensor in the same liquid fuel cell as in FIG. 6, a 
water-rich fuel mixture is supplied from the water tank 101 in place of 
only water. That is, since the fuel concentration in the fuel circulation 
system is 1 mole/l, the water-rich fuel mixture in the water tank 101 is 
made to have a methanol concentration of 1 mole/l. That is, the molar 
ratio of methanol to water is about 0.02. 
In place of measuring the fuel concentration in the fuel circulation 
system, such a phenomenon that the output voltage is lowered as the fuel 
concentration is decreased can be also utilized. To this end, a detector 
15 to check an output voltage level is provided as shown in FIG. 7, and 
when a decrease in the output voltage level is detected, the valve 112 to 
the fuel tank 102 is opened with a signal from the detector 15 to supply 
the fuel to the fuel circulation system. In that case, a fuel-rich mixture 
of fuel and water is supplied from the fuel tank 102 in place of fuel only 
to suppress local and transient increase in the fuel concentration in the 
fuel circulation system. Molar ratio of water to methanol in the fuel-rich 
mixture in the fuel tank 102 is 2. In this case, total volume of the water 
and the fuel in both tanks is the same as in the embodiment of FIG. 6. 
In this embodiment, both tanks 101 and 102 contain fuel mixtures, and local 
and transient unbalance of fuel concentration in the fuel circulation 
system can be largely improved, and thus the circulation rate by pump 9 
through the fuel circulation system can be much reduced, and a good fuel 
cell performance can be obtained even at the reduced circulation rate of 
200 cc/min. 
Further embodiment of the present invention will be shown in FIG. 8, where 
only differences from the embodiment of FIG. 7 are that a signal for 
supplying a fuel-rich mixture from the fuel tank 102 to the fuel 
circulation system is emitted in accordance with a decrease in the load 
current of a liquid fuel cell. A detector 16 is connected to two end 
points of a resistor 18 at the fuel electrode 2-1 and the valve is opened 
with a signal from the detector 16, and further that a portion or all of 
water contained in the exhaust gas 82 from the oxidizing agent chamber 5 
is recovered in a trap 17 and returned to the tank 101. By the provision 
of the water recovery trap, the capacity of water tank 101 can be reduced. 
In the foregoing embodiments, liquid fuel cells using methanol as fuel and 
an acidic electrolyte have been described, but the present invention is 
readily applicable also to an alkaline type liquid fuel cell using 
methanol as fuel, and other liquid fuel cells using hydrazine, 
formaldehyde, etc. as fuel by providing the fuel cell with two tanks and 
selecting fuel-water ratios of fuel mixtures in the tanks, as described 
above. 
When a apparatus for detecting a fuel concentration according to the 
following embodiments is used in the present liquid fuel cell, the effects 
of the present liquid fuel can be further improved as described below. 
In FIG. 9, an apparatus 516 for detecting a fuel concentration according to 
one embodiment of the present invention is schematically given, which 
comprises an anode 517, a cathode 518, a power source 519 and a detector 
520, as in the prior art, but the anode 517 has a fuel-controlling layer 
517b' through a catalyst layer 521 in the present invention. The 
fuel-controlling layer 517b' is prepared from a carbon fiber paper treated 
with a suspension of fine polytetrafluoroethylene particles by baking to 
give a controlled permeation and a strong water repellency to the paper. 
The fuel permeation can be adjusted to, for example, about 
7.times.10.sup.-6 moles/cm.sup.2 .multidot.min.multidot.mole/l by the 
treatment. A platinum-based catalyst layer 521 is provided on one side of 
the layer 517b' by kneading the catalyst with the same suspension of fine 
polytetrafluoroethylene particles as used above and applying the mixture 
to the one side of the layer 517b', followed by baking, thereby bonding 
the catalyst layer to the fuel-controlling layer. Then, the resulting 
integrated layers are tightly laid on an anode plate 517a made from, for 
example, tantalum to contact the catalyst layer with the anode plate 517a. 
It is preferable to fix the anode 517 to a frame serving also as a support 
for the anode so that the fuel can permeate from the fuel-controlling 
layer side. 
That is, resin coats or pad plates of bakelite or glass are laid on all 
other sides than the fuel-controlling layer by an adhesive resin to form a 
seal layer (not shown in the drawings), thereby preventing all the other 
sides from direct contact with the anolyte. 
In a practical test of the apparatus of FIG. 9 under such conditions that 
the electrode area is 4 cm.sup.2, the voltage is 0.9 volts, the fuel 
permeation through the fuel-controlling layer 517b' is 1.times.10.sup.-6 
to 2.times.10.sup.-5 mole/cm.sup.2 .multidot.min.multidot.mole/l) and a 
fuel concentration is 0 to 1.5 moles/l, the detected current has a good 
linearity and a good sensitivity, shown by curve C in FIG. 12. That is, in 
the apparatus of FIG. 9, the catalyst layer 521 is provided between the 
anode 517a and the fuel-controlling layer 517b', and no liquid stagnation 
occurs therebetween, improving the permeation of the liquid, detection 
sensitivity and detection response. 
The fuel-controlling layer 517b' for use in the present invention is not 
only a fibrous carbon paper but can be also a porous carbon sheet, or can 
be an electroconductive porous material such as sintered metal. In that 
case, the fuel-controlling layer must have only a function to control the 
permeation of fuel, and thus an insulating sintered ceramics or organic 
porous materials can be also used. To provide the catalyst layer on the 
fuel-controlling layer, various other techniques such as coating, 
deposition, electrophoresis, CVD, etc. can be also used. 
In FIG. 10, another embodiment of the present invention is shown, where the 
fuel-controlling layer is used double. That is, a second fuel-controlling 
layer 517c is provided on the fuel-controlling layer 517b' at the 
cathode-facing side, where the second fuel-controlling layer 517c is 
prepared from a kneaded mixture of carbon powders or graphite fluoride 
powders with a suspension of fine polytetrafluoroethylene particles having 
a water repellency and an adhesiveness by applying the kneaded mixture to 
the surface of fuel-controlling layer 517b', followed by baking to 
integrate these two layers. Cathode 518 is prepared from a cathode plate 
518a other than a platinum plate and a catalyst layer 518b laid on the 
cathode plate by deposition or by electrophoresis, and no special material 
is required for the cathode plate 518a. That is, a cathode with a good 
detection sensitivity can be obtained at a low cost. 
In FIG. 11, other embodiment of the present invention is shown, where the 
cathode is improved by preparing a cathode 518 by laying a catalyst layer 
518b on an electroconductive, porous material 518c and tightly laying the 
integrated porous material 518c and catalyst layer 518b on a cathode plate 
518a. As an electroconductive porous material, carbon fiber paper or 
electroconductive polymer, sintered metal, etc. can be used to ensure the 
tight adhesion between the cathode plate 518a and the catalyst layer 518b. 
According to the present invention, a liquid fuel cell can be stably and 
efficiently operated for a prolonged time in spite of different 
consumption rates of fuel and water even if the load current or operating 
temperature of the fuel cell or the temperature or humidity of the 
atmosphere is changed. 
Further, according to the present invention, an anode electrode having a 
fuel-controlling layer deposited thereon through a catalyst layer is used 
in the present apparatus for detecting a fuel concentration, and thus no 
liquid fuel stagnation occurs between the anode electrode and the 
fuel-controlling layer, improving the permeation of liquid fuel and 
activation of the reaction between the electrodes as well as improving the 
detection sensitivity and response and thus the reliability of the 
apparatus.