Abstract:
A moisture sensor working to measure a moisture content of a specified measurement gas is provided which consists of an electrochemical cell having a first electrode exposed to the measurement gas and a second electrode exposed to a reference gas and a controller. The electrochemical cell outputs an electrical energy arising from chemical reaction of the measurement gas with the reference gas. The controller controls at least one of voltage appearing across and current flowing through the electrochemical cell and uses one of them to measure the moisture content of the measurement gas. This enables the moisture content to be determined free from the humidify of the measurement gas.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1 Technical Field of the Invention  
           [0002]    The present invention relates generally to an improved structure of a moisture sensor designed to measure a moisture content of a specified gas electrochemically and a fuel cell system using such a moisture sensor.  
           [0003]    2 Background Art  
           [0004]    Solid polymer fuel cells usually need to be humidified in order to keep the conductivity of an electrolyte film in the fuel cell. When a moisture content of the fuel cell is small, so that the electrolyte film is dry, it will cause an inner resistance of the fuel cell to be increased, thus resulting in a decrease in output voltage. Alternatively, when the moisture content of the fuel cell is too great, it will cause electrodes of the fuel cell to be covered with water, which disturbs diffusion of reactants: oxygen and hydrogen, resulting in a decrease in output voltage.  
           [0005]    Thus, increasing the operating efficiency of the fuel cells requires fine control of a moisture content of the fuel cells precisely. For example, Japanese Patent First Publication No. 2001-236977 proposes a control system which measures the humidity of unreacted fuel cell exhaust gas using a humidity sensor and controls the amount of moisture added to the electrolyte film as a function of an output of the humidity sensor.  
           [0006]    The humidity sensor, as employed in the above system, is of a capacitance type which is slow in response. Additionally, such a capacitance type humidity sensor encounters a difficulty in measuring the quantity of moisture correctly when the humidity is more than 100%, that is, when there are waterdrops.  
         SUMMARY OF THE INVENTION  
         [0007]    It is therefore a principal object of the invention to avoid the disadvantages of the prior art.  
           [0008]    It is another object of the invention to provide a moisture sensor designed to measure a moisture content of a gas precisely free from the humidity thereof.  
           [0009]    It is a still object of the invention to provide a fuel cell system capable of monitoring and controlling the quantity of moisture contained in a fuel cell accurately.  
           [0010]    According to one aspect of the invention, there is provided a moisture sensor which works to measure a moisture content of a specified measurement gas. The moisture sensor comprises: (a) an electrochemical cell having a first electrode exposed to the measurement gas and a second electrode exposed to a reference gas, the electrochemical cell outputting an electrical energy arising from chemical reaction of the measurement gas with the reference gas; (b) a voltage detector working to measure a voltage appearing at the electrochemical cell; (c) a current detector working to measure a current produced by the electrochemical cell; and (d) a controller controlling at least one of the voltage and the current of the electrochemical cell. One of the voltage and the current is used to determine the moisture content of the measurement gas. This structure enables the moisture content of the measurement gas to be determined free from the humidify of the measurement gas.  
           [0011]    In the preferred mode of the invention, the measurement gas is one of a hydrogen gas or air. The reference gas is also one of a hydrogen gas or air.  
           [0012]    The controller may determine the moisture content of the measurement gas as a function of a value of the current produced by the electrochemical cell when the voltage of the electrochemical cell is controlled to a given voltage value. The given voltage value may be altered depending upon the concentration of the gasses.  
           [0013]    The controller may alternatively determine the moisture content of the measurement gas as a function of a value of the voltage of the electrochemical cell when the current produced by the electrochemical cell is controlled to a given current value. The given current value may be altered depending upon the concentration of the gasses.  
           [0014]    According to the second aspect of the invention, there is provided a fuel cell system which comprises: (1) a moisture sensor working to measure a moisture content of a measurement gas, the moisture sensor including (a) an electrochemical cell having a first electrode exposed to the measurement gas and a second electrode exposed to a reference gas, the electrochemical cell outputting an electrical energy arising from chemical reaction of the measurement gas with the reference gas, (b) a voltage detector working to measure a voltage appearing at the electrochemical cell, (c) a current detector working to measure a current produced by the electrochemical cell, and (d) a controller controlling at least one of the voltage and the current of the electrochemical cell, the controller determining the moisture content of the measurement gas using one of the voltage and the current and providing a signal indicative thereof; (2) a fuel cell producing an electrical energy through chemical reaction between hydrogen and oxygen; (3) a moisture controlling mechanism working to control a quantity of moisture within the fuel cell; and (4) a system controller working to determine a moisture condition within the fuel cell using the signal outputted from the moisture sensor. The system controller actuates the moisture controlling mechanism to control the quantity of moisture within the fuel cell to a desired value based on the determined moisture condition.  
           [0015]    In the preferred mode of the invention, if the measurement gas and the reference gas are identical in kind with each other, the controller works to bring a potential difference between the first and second electrodes of the moisture sensor to a given potential difference and determines that the quantity of moisture within the fuel cell is excessive when the current produced by the electrochemical cell is smaller than a first preselected current value.  
           [0016]    Alternatively, if the measurement gas and the reference gas are different in kind from each other, the controller works to bring the voltage of the electrochemical cell to a given voltage and determines that the quantity of moisture within the fuel cell is excessive when the current produced by the electrochemical cell is smaller than a first preselected current value.  
           [0017]    The controller determines that the fuel cell lacks in moisture content thereof when the current produced by the electrochemical cell is greater than a second preselected current value.  
           [0018]    If the measurement gas and the reference gas are different in kind from each other, the controller may determine that the quantity of moisture within the fuel cell is excessive when a value of the voltage of the electrochemical cell under control in which the current flowing through the electrochemical cell is kept at a given current value is smaller than a first preselected voltage value.  
           [0019]    If measurement gas and the reference gas are different in kind from each other, the controller may determine the fuel cell lacks in moisture content thereof when a value of the voltage of the electrochemical cell under control in which the current flowing through the electrochemical cell is kept at the given current value is greater than a second preselected voltage value.  
           [0020]    If the measurement gas and the reference gas are identical in kind with each other, the controller may determine that the quantity of moisture within the fuel cell is excessive when the a potential difference between the first and second electrodes of the electrochemical cell under control in which the controller controls the current flowing through the electrochemical cell to a given current value is greater than a first preselected potential difference.  
           [0021]    If the measurement gas and the reference gas are identical in kind with each other, the controller may determine that the fuel cell lacks in moisture content thereof when the potential difference between the first and second electrodes of the electrochemical cell under control in which the controller controls the current flowing through the electrochemical cell to the given current value is smaller than a second preselected potential difference.  
           [0022]    The fuel cell system may further comprise an oxygen gas drain line through which an oxygen gas discharged from an oxygen electrode of the fuel cell flows and a hydrogen gas drain line through which a hydrogen gas discharged from a hydrogen electrode of the fuel cell flows. The moisture sensor may be installed in at least one of the oxygen gas drain line and the hydrogen gas drain line to measure a moisture content of at least one of the hydrogen gas and the oxygen gas discharged from the fuel cell.  
           [0023]    The fuel cell system may further comprise an oxygen supply line through which an oxygen gas is supplied to the fuel cell and a hydrogen gas supply line through which a hydrogen gas is supplied to the fuel cell. The moisture sensor may alternatively be installed in at least one of the oxygen gas supply line and the hydrogen gas supply line to measure a moisture content of at least one of the hydrogen gas and the oxygen gas supplied to the fuel cell.  
           [0024]    The moisture sensor may alternatively be installed within the fuel cell and work to measure a moisture content of at least one of an oxygen or a hydrogen gas within the fuel cell. In this case, the electrochemical cell of the moisture sensor is formed by a portion of the fuel cell.  
           [0025]    According to the third aspect of the invention, there is provided a fuel cell system which comprises: (a) fuel cell stack including a plurality of cells each of which is made up of a pair of collection members and an electrolyte film disposed between the collection members; (b) a moisture sensor working to measure a quantity of moisture within at least one of the cells and output a signal indicative thereof; (c) a moisture controlling mechanism working to control a moisture content of the fuel cell stack; and (d) a system controller working to determine a moisture condition within the fuel cell stack using the signal outputted from the moisture sensor, the system controller actuating the moisture controlling mechanism to control the quantity of moisture within the fuel cell stack to a desired value based on the determined moisture condition. The moisture sensor includes (a) an electrochemical cell having electrodes formed by portions of the pair of collection members of the cell and the electrolyte film and (b) a resistance measuring circuit working to measure a resistance value of the electrochemical cell. The quantity of moisture within the cell is determined as a function of the resistance value of the electrochemical cell.  
           [0026]    In the preferred mode of the invention, the resistance measuring circuit is designed to apply a sinusoidal wave signal to an output signal of the electrochemical cell and change a frequency of the sinusoidal wave signal to measure an AC impedance of the electrochemical cell. The resistance measuring circuit works to determine a resistance value of the electrolyte film and a reaction-caused resistance value of the electrodes of the electrochemical cell.  
           [0027]    The system controller determines that the fuel cell stack lacks in moisture content thereof when the resistance value of the electrolyte film is greater than a first preselected resistance value.  
           [0028]    The system controller determines that the quantity of moisture within the fuel cell stack lies within an allowable range when the resistance value of the electrolyte film is smaller than the first preselected resistance value and when the reaction-caused resistance value is smaller than a second preselected resistance value.  
           [0029]    The system controller determines that the quantity of moisture within the fuel cell stack is excessive when the reaction-caused resistance value is greater than the second preselected resistance value.  
           [0030]    The fuel cell system may further comprise moisture sensors installed in some of the cells of the fuel cell stack which are identical in structure with the moisture sensor. 
       
    
    
     BRIEF DESPCRIPTION OF THE DRAWINGS  
       [0031]    The present invention will be understood more fully from the detailed description given hereinbelow and from the accompanying drawings of the preferred embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments but are for the purpose of explanation and understanding only.  
         [0032]    In the drawings:  
         [0033]    [0033]FIG. 1 is a block diagram which shows a fuel cell system according to the first embodiment of the invention;  
         [0034]    [0034]FIG. 2 is a circuit diagram which shows a moisture quantity sensor employed in the fuel cell system of FIG. 1;  
         [0035]    [0035]FIG. 3 is a graph which shows sensor characteristics of the moisture quantity sensor of FIG. 2;  
         [0036]    [0036]FIG. 4 is a graph which shows a relation between the quantity of moisture contained in a measurement gas and an output current of an electrochemical cell when the moisture quantity sensor is controlled to a constant voltage in a case where the measurement gas and reference gas are different in kind from each other;  
         [0037]    [0037]FIG. 5 is a flowchart of a moisture control program executed by the fuel cell system of FIG. 1;  
         [0038]    [0038]FIG. 6 is a graph which illustrates a relation between a moisture content of a measurement gas and a potential difference between electrodes of a moisture quantity sensor when it is placed under constant current control in a case where the measurement gas and reference gas are different in kind from each other;  
         [0039]    [0039]FIG. 7 is a graph which illustrates a relation between a moisture content of a measurement gas and an output current of an electrochemical cell of a moisture quantity sensor when it is placed under constant voltage control in a case where the measurement gas and reference gas are identical in kind with each other;  
         [0040]    [0040]FIG. 8 is a graph which illustrates a relation between a moisture content of a measurement gas and an output voltage of an electrochemical cell of a moisture quantity sensor when it is placed under constant current control in a case where the measurement gas and reference gas are identical in kind with each other;  
         [0041]    [0041]FIG. 9 is an exploded view which shows a modification of a moisture quantity sensor;  
         [0042]    [0042]FIG. 10 is a partially sectional view which shows the moisture quantity sensor of FIG. 9;  
         [0043]    [0043]FIG. 11 is a circuit diagram of the moisture quantity sensor of FIG. 9;  
         [0044]    [0044]FIG. 12 is a perspective exploded view which shows a second modification of a moisture quantity sensor;  
         [0045]    [0045]FIG. 13 is a perspective exploded view which shows a third modification of a moisture quantity sensor;  
         [0046]    [0046]FIG. 14 is a circuit diagram of the moisture quantity sensor of FIG. 13;  
         [0047]    [0047]FIG. 15 is a circuit diagram which shows an impedance analyzer used in the circuit of FIG. 14;  
         [0048]    [0048]FIG. 16 is a graph which illustrates a relation between a moisture content of a cell of a fuel cell stack and an internal resistance thereof;  
         [0049]    [0049]FIG. 17 illustrates an equivalent circuit of a cell of a fuel cell stack;  
         [0050]    [0050]FIG. 18 is a graph which illustrates the impedance of a cell, as expressed on a complex plane, when a sinusoidal current applied to the equivalent circuit of FIG. 17 is changed from high to low frequency wave;  
         [0051]    [0051]FIG. 19 is a block diagram which shows a modification of a fuel cell system; and  
         [0052]    [0052]FIG. 20 is a sectional view of a fuel cell stack in which a plurality of moisture quantity sensors are installed. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0053]    Referring to the drawings, wherein like reference numbers refer to like parts in several views, particularly to FIG. 1, there is shown a fuel cell system according to the first embodiment of the invention which consists essentially of a fuel cell stack  10 , an air supply device  22 , a hydrogen supply device  32 , humidifiers  23  and  33 , moisture quantity sensors  24  and  34 , and a controller  40 .  
         [0054]    The fuel cell stack  10  works to convert the energy produced by electrochemical reaction of oxygen and hydrogen into electric power. The fuel cell stack  10  is made up of a plurality of solid polyelectrolyte fuel cells. Each cell is made of a pair of electrodes (will also called an oxygen and a hydrogen electrode below) and an electrolyte film disposed between the electrodes. The fuel cell stack  10  is used to supply the power to an electric load  11 . The fuel cell stack  10  is supplied with hydrogen and air (oxygen) and induces electrochemical reactions thereof at the electrodes which are of the forms:  
         [0055]    Hydrogen electrode H 2 →2H + +2e −   
         [0056]    Oxygen electrode 2H + +½O 2 +2e −→H   2 O  
         [0057]    The above electrochemical reactions produce water. Additionally, humidified hydrogen and air gasses, as described later, supplied into the fuel cell stack  10  will cause condensate water to be produced therein. The moisture is, thus, produced both in a hydrogen flow path and in an air flow path within the fuel cell stack  10 .  
         [0058]    The fuel cell system also includes a voltage sensor  12  and a current sensor  13 . The voltage sensor  12  works to measure an output voltage of the fuel cell stack  10  and outputs a signal indicative thereof to the controller  40 . The current sensor  13  works to measure an output current of the fuel cell stack  10  and outputs a signal indicative thereof to the controller  40 .  
         [0059]    The fuel cell system also has an oxygen supply line  20  for supplying oxygen-contained air to oxygen electrodes (i.e., positive electrodes) of the fuel cell stack  10 , an oxygen drain line  21  for draining an oxygen exhaust gas containing oxygen not consumed in the electrochemical reactions from the fuel cell stack  10 , a hydrogen supply line  30  for supplying hydrogen gas to hydrogen electrodes (i.e., negative electrodes) of the fuel cell stack  10 , and a hydrogen drain line  31  for draining a hydrogen exhaust gas containing hydrogen not consumed in the electrochemical reactions.  
         [0060]    The air supply device  22  is located on the most upstream of the oxygen supply line  20  and may be implemented by a compressor. The hydrogen supply device  32  is located on the most upstream of the hydrogen supply line  30  and may be implemented by a reformer working to produce hydrogen through reforming reactions or a hydrogen tank having disposed therein a hydrogen storage such as a hydrogen absorbing alloy in which pure hydrogen is stored. The air supply device  22  and the hydrogen supply device  32  have regulators capable of regulating a supplied amount of air (i.e., oxygen) and hydrogen, respectively.  
         [0061]    The electrochemical reactions within the fuel cell stack  10  require the electrolyte films of the cells to be moist. This condition is established by the humidifiers  23  and  33  disposed in the oxygen supply line  20  and the hydrogen supply line  30  which work to humidify the air and hydrogen supplied to the fuel cell stack  10 . The humidifiers  23  and  33  have regulators capable of regulating the amount of moisture to be added to the air and hydrogen, respectively.  
         [0062]    The moisture quantity sensors  24  and  34  are disposed in the oxygen drain line  21  and the hydrogen drain line  31  which work to measure the quantity of moisture contained in the oxygen exhaust gas and the hydrogen exhaust gas, respectively.  
         [0063]    The fuel cell system also includes an air back-pressure regulator valve  25  and a hydrogen back-pressure regulator valve  35  disposed in the oxygen drain line  21  and the hydrogen drain line  31 , respectively. The air back-pressure regulator valve  25  works to regulate the pressure of air flowing through the oxygen supply line  20  and the fuel cell stack  10 . The regulation of the air pressure within the oxygen supply line  20  and the fuel cell stack  10  is achieved by controlling the degree of opening of the valve  25  since the air to be supplied to the fuel cell stack  10  is pressurized by the compressor  22 . Similarly, the hydrogen back-pressure regulator valve  35  works to regulate the pressure of hydrogen gas flowing through the hydrogen supply line  30  and the fuel cell stack  10 .  
         [0064]    When each of the back-pressure regulator valves  25  and  35  is moved toward a valve-open position, it will result in an increase in gas flow velocity, which causes the quantity of moisture staying within the fuel cell stack  10  to decrease. Conversely, when the back-pressure regulator valves  25  and  35  are shifted toward a valve-closed position, it will result in a rise in pressure of the hydrogen gas and increase in flow velocity thereof, which causes the quantity of moisture staying within the fuel cell stack  10  to increase. The adjustment of the gas flow velocity (i.e., the amount of moisture staying within the fuel cell stack  10 ) may also be achieved by regulating the amount of air supplied from the air supply device  22  or the amount of hydrogen gas supplied from the hydrogen supply device  32 .  
         [0065]    Specifically, the quantity of moisture within the fuel cell stack  10  may be controlled using one of the humidifiers  23  and  33 , the back-pressure regulator valves  25  and  35 , the air supply device  22 , and the hydrogen supply device  32  or a combination of some or all of them.  
         [0066]    The controller  40  receives outputs of the voltage sensor  12 , the current sensor  13 , and the moisture quantity sensors  24  and  34  and works to output control signals to the air supply device  22 , the hydrogen supply device  32 , the humidifiers  23  and  33 , and the back-pressure regulator valves  25  and  35 .  
         [0067]    The moisture quantity sensor  24 , as clearly shown in FIG. 2, consists of an electrochemical cell  240  made up of a measurement gas electrode  240   a , a reference gas electrode  240   b , and a solid electrolyte  240   c  retained between the electrodes  240   a  and  240   b , a voltage detector  241 , a current detector  242 , and a control circuit  243 . The electrodes  240   a  and  240   b  each support catalyst. The voltage detector  241  works to measure the voltage between the electrodes  240   a  and  240   b . The current detector  242  works to measure the current flowing through the electrochemical cell  240 . The control circuit  243  works to control the voltage and current of the electrochemical cell  240 .  
         [0068]    Similarly, the moisture quantity sensor  34  consists of an electrochemical cell  340  made up of a measurement gas electrode  340   a , a reference gas electrode  340   b , and a solid electrolyte  340   c  retained between the electrodes  340   a  and  340   b , a voltage detector  341 , a current detector  342 , and a control circuit  343 . The electrodes  340   a  and  340   b  each carry catalyst. The voltage detector  341  works to measure the voltage developed between the electrodes  340   a  and  340   b . The current detector  342  works to measure the current flowing through the electrochemical cell  340 . The control circuit  343  works to control the voltage and current of the electrochemical cell  340 .  
         [0069]    The measurement gas electrodes  240   a  and  340   a  of the moisture quantity sensors  24  and  34  are respectively exposed to the air and hydrogen gases emitted from the fuel cell stack  10  which are to be measured in moisture content. The reference gas electrodes  240   b  and  340   b  need to be constant in electric potential and are exposed to air or hydrogen gas used as a reference gas whose moisture content is known. In this embodiment, the measurement gasses to which the measurement gas electrodes  240   a  and  340   a  are exposed are different from the reference gasses to which the reference gas electrodes  240   b  and  340   b  are exposed. Specifically, if the air is used as the measurement gas, the hydrogen gas is used as the reference gas.  
         [0070]    When the electrochemical cell  240  is exposed at the measurement gas electrode  240   a  and the reference gas electrode  240   b  to the air and hydrogen gasses, respectively, it will generate electric potential between the electrodes  240   a  and  240   b . If the air is used as the reference gas, it may be provided from the atmosphere. Alternatively, if the hydrogen gas is used as the reference gas, it may be provided from the hydrogen supply device  32 . The same applies to the electrochemical cell  340 .  
         [0071]    [0071]FIG. 3 shows sensor characteristics of the moisture quantity sensors  24  and  34 . A solid line indicates a voltage-current relation in a case where the measurement gas contains a desired quantity of moisture. As the moisture content of the measurement gas increases, a relative concentration of oxygen or hydrogen contained in the measurement gas decreases, thus resulting in a decrease in limiting current, as illustrated by broken lines, to be outputted by the electrochemical cells  240  and  340 . Specifically, decreasing of the value of current produced by the electrochemical cells  240  and  340  as a function of an increase in moisture content of the measurement gas is established by controlling the voltage developed by the electrochemical cells  240  and  340  to a constant level.  
         [0072]    [0072]FIG. 4 is a graph which shows a relation between the quantity of moisture contained in the measurement gas and an output current of the electrochemical cells  240  and  340  when the moisture quantity sensors  24  and  34  are controlled in voltage to a constant level in a case where the measurement gas and reference gas are different in kind from each other. The graph shows that the measurement gas and the output current have a correlation which will be discussed below in detail.  
         [0073]    Keeping a potential difference between the electrodes of each of the sensors  24  and  34  constant under the constant voltage control, as described above, requires keeping a difference in gas concentration between the electrodes constant. When the moisture content of the measurement gas is increased, it will cause the concentration of oxygen or hydrogen to be decreased, thus resulting in a decrease in quantity of oxygen or hydrogen of the measurement gas consumed in adjusting the concentration of oxygen or hydrogen on the electrode to a desired value. Specifically, an increase in quantity of moisture contained in the measurement gas results in a decrease in quantity of oxygen or hydrogen of the measurement gas, thus resulting in a decrease in output current of the sensors  24  and  34 .  
         [0074]    Accordingly, a determination of the quantity of moisture within the fuel cell stack  10  may be made by placing the sensors  24  and  34  under the constant voltage control and measuring a resulting value of current produced therefrom. In practice, when the current outputted from the sensors  24  and  34  decreases below a first preselected value, the controller  40  determines that the quantity of moisture in the fuel cell stack  10  has increased over an allowable or suitabel range. When the current output exceeds a second preselected value higher in level than the first preselected value, the controller  40  determines that the fuel cell stack  10  lacks in moisture content.  
         [0075]    [0075]FIG. 5 is a flowchart of a program or sequence of logical steps executed in the controller  40 .  
         [0076]    After entering the program, the routine proceeds to step  10  wherein the value of voltage output of the fuel cell stack  10  is measured using an output of the voltage sensor  12 . The routine proceeds to step  11  wherein the value of current output of the fuel cell stack  10  is measured using an output of the current sensor  13 .  
         [0077]    The routine proceeds to step  12  wherein an output (i.e., a current value) of the moisture quantity sensor  34  installed in the hydrogen drain line  31  is monitored. The routine proceeds to step  13  wherein it is determined whether the output of the moisture quantity sensor  34  is smaller than a preselected value or not. It a YES answer is obtained meaning that the quantity of moisture on the side of the hydrogen electrodes of the fuel cell stack  10  is excessive, then the routine proceeds to a sequence of steps  14  to  17  to decrease a moisture content of the fuel cell stack  10 .  
         [0078]    Specifically, in step  14 , the humidifier  33  is controlled to decrease the amount of moisture added to the hydrogen gas supplied to the fuel cell stack  10  through the hydrogen supply line  30 . The routine proceeds to step  15  wherein the hydrogen back-pressure regulator valve  35  is opened further to decrease the hydrogen pressure, thereby increasing the flow velocity inside the fuel cell stack  10 . The routine proceeds to step  16  wherein the hydrogen supply device  32  is controlled to increase the flow velocity of the hydrogen gas supplied therefrom. The routine proceeds to step  17  wherein an over-moisture flag is raised.  
         [0079]    If a NO answer is obtained in step  13  or after step  17 , the routine proceeds to step  18  wherein an output (i.e., a current value) of the moisture quantity sensor  24  installed in the oxygen drain line  21  is monitored. The routine proceeds to step  19  wherein it is determined whether the output of the moisture quantity sensor  24  is smaller than a preselected value or not. It a YES answer is obtained meaning that the quantity of moisture on the side of the oxygen electrodes of the fuel cell stack  10  is excessive, then the routine proceeds to a sequence of steps  20  to  23  to decrease a moisture content of the fuel cell stack  10 .  
         [0080]    Specifically, in step  20 , the humidifier  23  is controlled to decrease the amount of moisture added to the air supplied to the fuel cell stack  10  through the oxygen supply line  20 . The routine proceeds to step  21  wherein the oxygen back-pressure regulator valve  25  is opened further to decrease the air pressure, thereby increasing the flow velocity of the oxygen gas in the fuel cell stack  10 . The routine proceeds to step  22  wherein the air supply device  22  is controlled to increase the flow velocity of the air supplied therefrom. The routine proceeds to step  23  wherein the over-moisture flag is raised.  
         [0081]    If a NO answer is obtained in step  19  or after  23 , the routine proceeds to step  24  wherein it is determined whether the over-moisture flag is raised or not. If a YES answer is obtained, then the routine terminates the moisture control. Alternatively, if a NO answer is obtained meaning that the over-moisture flag is not raised, then the routine proceeds to step  25  wherein a voltage output of the fuel cell stack  10  is lower than a preselected value or not. Specifically, the controller  40  stores therein a map indicating a relation between the output current and the output voltage of the fuel cell stack  10  and determines whether the voltage output, as measured in step  10 , is lower than a voltage value which corresponds in the map to the current output, as measured in step  11 , or not.  
         [0082]    If a YES answer is obtained meaning that a moisture content of the fuel cell stack  10  is not excessive and that the output of the fuel cell stack  10  is dropping, it may be concluded that the fuel cell stack  10  lacks in moisture content. The routine proceeds to step  26  wherein the humidifiers  23  and  33  are controlled to increase the quantity of moisture added to the air and the hydrogen gas, thereby increasing the moisture content of the fuel cell stack  10 . Additionally, the back-pressure regulator valves  25  and  35  may also be brought to a closed position to decrease the flow velocity of the gasses outputted from the air supply device  22  and the hydrogen supply device  32 .  
         [0083]    Alternatively, if a NO answer is obtained in step  25  meaning that the moisture content of the fuel cell stack  10  is adequate to operate the fuel cell stack  10  normally, then the routine proceeds to step  27  wherein the quantity of moisture outputted from the humidifiers  23  and  33  is adjusted to a normal quantity to keep the moisture content of the fuel cell stack  10  as it is.  
         [0084]    As apparent from the above discussion, the fuel cell system of this embodiment is designed to measure a moisture content of exhaust gasses from the fuel cell stack  10  using the moisture quantity sensors  24  and  34  made of the electrochemical cells  240  and  340  to determine a moisture condition within the fuel cell stack  10 . The use of the electrochemical cells  240  and  340  enables the moisture content of the measurement gasses to be determined regardless of the temperature thereof and also results in an improved response rate of the sensors  24  and  34  as compared with capacitance type humidity sensors.  
         [0085]    The fuel cell system of the second embodiment will be described below with reference to FIG. 6 which is different from the first embodiment in that the current flowing through each of the moisture quantity sensors  24  and  34  is controlled at a constant level.  
         [0086]    [0086]FIG. 6 illustrates a relation between a moisture content of the measurement gas and a potential difference between the electrodes of the moisture quantity sensors  23  and  34  when they are placed under the constant current control in a case where the measurement gas and the reference gas are different in kind from each other. The moisture content of the measurement gas and the potential difference have a correlation as discussed below.  
         [0087]    Usually, the quantity of hydrogen or oxygen gas consumed on the electrodes of the moisture quantity sensors  24  and  34  is constant when the current flowing through the moisture quantity sensors  24  and  34  is controlled at a constant level, but however, the concentration of hydrogen or oxygen on the measurement gas electrodes  240   a  and  340   a  decreases with an increase in moisture content of the measurement gas, thereby resulting in a change in potential difference between the measurement and reference gas electrodes. Specifically, when the moisture content of the measurement gas increases, it will result in a relative decrease in hydrogen or oxygen concentration of the measurement gas, which causes the potential difference between the measurement and reference gas electrodes to be decreased. Therefore, a determination of the quantity of moisture within the fuel cell stack  10  may be made by placing the electrochemical cells  240  and  340  of the moisture quantity sensors  24  and  34  under the constant current control and measuring the potential difference between the measurement and reference gas electrodes. In practice, when the potential difference decreases below a first preselected value, the controller  40  determines that the quantity of moisture in the fuel cell stack  10  has increased over a suitable range. When the potential difference exceeds a second preselected value higher in level than the first preselected value, the controller  40  determines that the fuel cell stack  10  lacks in moisture content. Other arrangements and operations of the fuel cell system are identical with those in the first embodiment, and explanation thereof in detail will be omitted here.  
         [0088]    The fuel cell system of the third embodiment will be described below with reference to FIGS. 7 and 8 which is different from the first embodiment in a combination of the measurement and reference gasses. Other arrangements are identical, and explanation thereof in detail will be omitted here.  
         [0089]    The measurement gas and the reference gas used in each of the moisture quantity sensors  24  and  34  are identical in kind with each other. Specifically, when the measurement gas is hydrogen gas, hydrogen gas is also used as the reference gas. Alternatively, when the measurement gas is air, air is also used as the reference gas. The control circuits  243  and  343  of the moisture quantity sensors  24  and  34  in this embodiment also work as a voltage applying circuit.  
         [0090]    In a case where the measurement gas and the reference gas are identical in kind, a difference in concentration between the measurement and reference gasses arising from a difference in moisture content thereof will result in a change in output of each of the moisture quantity sensors  24  and  34  produced by passage of hydrogen or oxygen ions through a corresponding one of the electrolytes  240   c  and  340   c . Sensor characteristics of the moisture quantity sensors  24  and  34  when the measurement and reference gases are identical in kind are the same as those of FIG. 3 as discussed in the first embodiment.  
         [0091]    In a case where air is used as the reference gas, the supply of the reference gas may be achieved, like the first embodiment, with the ambient air. In a case of hydrogen gas, the supply of the reference gas may be achieved using the hydrogen supply device  32 . Moreover, in a case where the measurement and reference gasses are made of hydrogen gas, hydrogen gas containing moisture may be supplied to the measurement gas electrode  340   a  so that hydrogen ions having passed through the electrolyte  340   c  may be pumped out by the reference gas electrode  340   b  to produce hydrogen gas as the reference gas.  
         [0092]    [0092]FIG. 7 illustrates a relation between a moisture content of the measurement gas and an output current of the electrochemical cells  240  and  340  of the moisture quantity sensors  23  and  34  when they are placed under the constant voltage control in a case where the measurement gas and the reference gas are identical in kind with each other.  
         [0093]    Keeping a potential difference between the electrodes of each of the moisture quantity sensors  24  and  34  constant under the constant voltage control requires keeping a difference in gas concentration between the electrodes constant. When the moisture content of the measurement gas is increased, it will result in a decrease in concentration of oxygen or hydrogen of the measurement gas, thus causing the quantity of oxygen or hydrogen of the measurement gas consumed in adjusting the concentration of oxygen or hydrogen on the electrode to a desired value to decrease. Specifically, an increase in quantity of moisture contained in the measurement gas results in a decrease in quantity of oxygen or hydrogen of the measurement gas, thus resulting in a decrease in output current of the sensors  24  and  34 . Accordingly, a determination of the quantity of moisture within the fuel cell stack  10  may be made by placing the sensors  24  and  34  under the constant voltage control and measuring a resulting value of current produced therefrom. In practice, when the current outputted from the sensors  24  and  34  decreases below a first preselected value, the controller  40  determines that the quantity of moisture in the fuel cell stack  10  has increased over an allowable or suitable range. When the current output exceeds a second preselected value higher in level than the first preselected value, the controller  40  determines that the fuel cell stack  10  lacks in moisture content.  
         [0094]    [0094]FIG. 8 illustrates a relation between a moisture content of the measurement gas and an output voltage of the electrochemical cells  240  and  340  of the moisture quantity sensors  23  and  34  when they are placed under the constant current control in a case where the measurement gas and the reference gas are identical in kind with each other.  
         [0095]    Usually, the quantity of hydrogen or oxygen gas consumed on the electrodes of the moisture quantity sensors  24  and  34  is constant when the current flowing through the moisture quantity sensors  24  and  34  is kept constant, but however, the concentration of hydrogen or oxygen on the measurement gas electrodes  240   a  and  340   a  decreases with an increase in moisture content of the measurement gas, thereby resulting in a change in potential difference between the measurement and reference gas electrodes. Specifically, when the moisture content of the measurement gas increases, it will result in a relative decrease in hydrogen or oxygen concentration of the measurement gas, which causes the potential difference between the measurement and reference gas electrodes to be increased. Therefore, a determination of the quantity of moisture within the fuel cell stack  10  may be made by placing the electrochemical cells  240  and  340  of the moisture quantity sensors  24  and  34  under the constant current control and measuring the potential difference developed between the measurement and reference gas electrodes. In practice, when the potential difference increases above a first preselected value, the controller  40  determines that the quantity of moisture in the fuel cell stack  10  has increased over an allowable range. When the potential difference drops below a second preselected value lower in level than the first preselected value, the controller  40  determines that the fuel cell stack  10  lacks in moisture content thereof.  
         [0096]    When the circuit of each of the moisture quantity sensors  24  and  34  is opened, the moisture quantity sensors  24  and  34  exhibit the same characteristics as those in FIG. 8. In this case, the moisture content of the fuel cell stack  10  may be determined by measuring the potential difference between the electrodes of each of the moisture quantity sensors  24  and  34  without controlling the current flowing therethrough.  
         [0097]    The fuel cell system of the fourth embodiment will be described below with reference to FIGS.  9  to  11  which is different from the first embodiment in that a moisture quantity sensor is installed within the fuel cell stack  10  instead of the moisture quantity sensors  24  and  34 . Other arrangements are identical, and explanation thereof in detail will be omitted here.  
         [0098]    [0098]FIG. 9 is an exploded view which shows one of the cells  110  of the fuel cell stack  10 . The cell  100  is made of a laminate of an electrolyte film  101 , a pair of collection plates  102  and  103 , and a pair of separators  104  and  105 . The collection plates  102  and  103  may be made of carbon cloth. Leads  106  and  107  are connected to the separators  104  and  105 , respectively.  
         [0099]    The electrolyte film  101 , the collection plates  102  and  103 , and the separators  104  and  105  have insulating layers  101   a ,  102   a ,  103   a ,  104   a , and  105   a  formed in opposed portions thereof in order to isolate portions  101   b ,  102   b ,  103   b ,  104   b , and  105   b  of the electrolyte film  101 , the collection plates  102  and  103 , and the separators  104  and  105  electrically from remaining portions thereof. The separators  104  and  105  each have gas supply grooves, not shown, formed in surfaces thereof. The insulating layers  104   a  and  105   a  formed in the separators  104  and  105  also work to isolate the portions  101   b  to  105   b  from a metal housing, not shown, used to retain the separators  104  and  105 .  
         [0100]    [0100]FIG. 10 is a partially sectional view which shows the cell  100 . FIG. 11 is a circuit diagram of the moisture quantity sensor  14  provided within the cell  100  of the fuel cell stack  10 . The portions  101   b ,  102   b ,  103   b ,  104   b , and  105   b  of the electrolyte film  101 , the collection plates  102  and  103 , and the separators  104  and  105  constitute an electrochemical cell  140  of the moisture quantity sensor  14 . Of these, the portions  102   b ,  103   b ,  104   b , and  105   b  of the collection plates  102  and  103  and the separators  104  and  105  form electrodes of the moisture quantity sensor  14 .  
         [0101]    The separators  104  and  105  have, as clearly shown in FIG. 10, gas supplying grooves  104   c ,  104   d ,  105   c , and  105   d  formed therein. The measurement and reference gasses flow through the gas supplying grooves  104   c  and  105   c  of the separators  104  and  105 , respectively. As the measurement gas, either of air and hydrogen gas (i.e., gas for power generating) is used. As the reference gas, a gas containing a know quantity of moisture is used which may be identical with or different in kind from the measurement gas. Through the gas supplying grooves  104  and  105 , the power generating gasses (i.e., the air and hydrogen gas) flow.  
         [0102]    The electrochemical cell  140  of the moisture quantity sensor  14  is, as clearly shown in FIG. 11, connected to a control circuit  143  through current and voltage detectors  142  and  141 . The control circuit  143  works to place the electrochemical cell  140  under the constant voltage or current control and measures a moisture content of the power generating gasses within the fuel cell stack  10 . The moisture quantity sensor  14  may be formed only in any one of the cells  100  of the fuel cell stack  10  which is easy to dry.  
         [0103]    The fuel cell system of the fifth embodiment will be described below with reference to FIG. 12 which is different from the fourth embodiment in that the electrochemical cell  140  of the moisture quantity sensor  14  formed independently from the cell  100  is installed in the cell  100 . Other arrangements are identical, and explanation thereof in detail will be omitted here.  
         [0104]    The cell  100  has, as clearly shown in FIG. 12, a cutout formed in a corner thereof in which the electrochemical cell  140  is disposed. The electrochemical cell  140  has insulating layers  101   a  to  105   a  interposed between a major portion of the electrochemical cell  140  and the cell  100 . Outer surfaces of the separators  104   b  and  105   b  are covered with the insulating layers  104   a  and  105   a , respectively.  
         [0105]    The electrochemical cell  140  of the moisture quantity sensor  14  has a structure similar to that of the cell  100  and is made of a laminate of the collection plates  102   b  and  103   b  disposed on opposed sides of the electrolyte film  101   b  and the separators  104   b  and  105   b . The collection plates  104   b  and  105   b  may be made of carbon cloth. The separators  104   b  and  105   b  have, as clearly shown in FIG. 12, measurement and reference gas supplying grooves formed therein, respectively. As the measurement gas, either of air and hydrogen gas (i.e., gas for power generating) is used. As the reference gas, a gas containing a know quantity of moisture is used which may be identical with or different in kind from the measurement gas.  
         [0106]    The fuel cell system of the sixth embodiment will be described below with reference to FIGS.  13  to  18  which is different from the fourth embodiment in that a moisture content of the cell  100  is measured as a function of a resistance value of the cell  100 . The same reference numbers as employed in the fourth embodiment will refer to the same parts, and explanation thereof in detail will be omitted here.  
         [0107]    The collection plates  102  and  103  have the insulating layers  102   a  and  103   a  formed therein to electrically isolate the portions  102   b  and  103   b  from major portions of the collection plates  102  and  103 , respectively. The portions  102   b  and  103   b  of the collection plates  102  and  103  form electrodes of the electrochemical cell  140 . Leads  106  and  107  are joined to the portions  102   b  and  103   b , respectively. The separators  104  and  105  have the insulating layers  104   a  and  105   a  formed therein to electrically isolate the portions  104   b  and  105   b  from major portions of the separators  104  and  105 , respectively.  
         [0108]    The moisture quantity sensor  14  is so designed as to measure an internal resistance thereof as a function of an AC impedance. The electrochemical cell  140  is, as clearly shown in FIG. 14, made up of the electrolyte film  101  and the electrodes  102   b  and  103   b  which are joined to the control circuit  143 . The voltage and current detectors  141  and  142  are installed between the electrochemical cell  140  and the control circuit  143 . The sine wave generator  144  is installed between the electrode  102   b  and the current detector  142  which works to output a sinusoidal current having a selectable frequency.  
         [0109]    The moisture quantity sensor  14  also includes an impedance measuring circuit which consists, as shown in FIG. 15, of filters  145  and  146 , FFT (Fast Fourier Transform) processors  147  and  148 , and an impedance analyzer  149 . The filters  145  and  146  are joined to the voltage detector  141  and the current detector  142 , respectively, and work to remove noise components from outputs of the voltage detector  141  and the current detector  142 . The FFT processors  147  and  148  work to calculate FFTs of the outputs of the filters  145  and  146  and output them to the impedance analyzer  149 . The impedance analyzer  149  works to determine the impedance of the cell  100  using voltage and current components derived through the FFT.  
         [0110]    [0110]FIG. 16 illustrates a relation between a moisture content of the cell  100  of the fuel cell stack  10  and an internal resistance thereof. The graph shows that the moisture content has a correlation to the internal resistance. Specifically, when the quantity of moisture in the cell  100  decreases, it will cause the moisture within the electrolyte film  101  to decrease, thereby resulting in a drop in conductivity of the electrolyte film  101 . This results in an increase in resistance of the electrolyte film  101 , which will be described later in detail. Accordingly, when the resistance of the electrolyte film  101  exceeds a first preselected value, the controller  40  may determine that the quantity of moisture in the fuel cell stack  10  lacks in moisture content. When the moisture within the cell  100  is excessive, it will result in an increase in reaction-caused resistance of the electrodes of the electrochemical cell  140 , which will be described later in detail. Accordingly, when the reaction-caused resistance of the electrodes exceeds a second preselected value, the controller  40  may determine that the moisture in the fuel cell stack  10  is excessive. When another condition is encountered, the controller  40  may determine that the quantity of moisture in the fuel cell stack  10  lies within a suitable range.  
         [0111]    A voltage drop of the cell  100  of the fuel cell stack  10  typically results from the reaction-caused resistance produced by the electrochemical reaction and the resistance of the electrolyte film  101  of the cell  100 . The reaction-caused resistance and the resistance of the electrolyte film  101  may be measured in the following manner.  
         [0112]    [0112]FIG. 17 illustrates an equivalent circuit of the cell  100  of the fuel cell stack  10 . R 1  indicates the resistance of the electrolyte film  101 . R 2  indicates the reaction-caused resistance. Application of a sinusoidal current having a given frequency to the equivalent circuit causes a change in voltage to lag behind a change in the current.  
         [0113]    [0113]FIG. 18 illustrates the impedance of the cell  100 , as expressed on a complex plane, when the sinusoidal current applied to the equivalent circuit is changed from high to low frequency wave. When the frequency of the applied sinusoidal current is infinite (ω=∞), the impedance is given by R 1 . Alternatively, when the frequency of the applied sinusoidal current is low (ω=0), the impedance is given by R 1 +R 2 . The impedance when the applied sinusoidal current is changed from high to low frequency wave changes, as clearly shown in the drawing, along a semi-circle.  
         [0114]    Specifically, the resistances R 1  and R 2  may be measured in dependently by changing the frequency of the sinusoidal current applied by the sine wave generator  144  to the electrochemical cell  140 . The resistance R 1 , as described above, indicates the resistance of the electrolyte film  101 . Thus, when the resistance R 1  exceeds a first preselected value, the controller  40  may determine that the electrolyte film  101  lacks in moisture content. When the resistance R 1  is lower than the first preselected value, the controller  40  may determine that the quantity of moisture in the electrolyte film  101  lies within the suitable range. The resistance R 2 , as described above, indicates the reaction-caused resistance of the electrodes. Thus, when the resistance R 2  exceeds a second preselected value, the controller  40  may determine that the moisture on the electrodes is excessive. When the resistance R 2  is less than the second preselected value, the controller  40  may determine that the quantity of moisture on the electrodes lies within the suitable range.  
         [0115]    The portions  102   b  and  103   b  of the collection plates  102  and  103  of the cell  100 , as already described, form the electrodes of the electrochemical cell  140  of the moisture quantity sensor  14 , so that an output current of the moisture quantity sensor  14  is much smaller than an output current of the cell  100  and hardly impinges upon output of the fuel cell stack  10 . This permits the moisture quantity sensor  14  to measure the moisture within the cell  100  during operation of the fuel cell stack  10 .  
         [0116]    While the present invention has been disclosed in terms of the preferred embodiments in order to facilitate better understanding thereof, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modifications to the shown embodiments witch can be embodied without departing from the principle of the invention as set forth in the appended claims.  
         [0117]    For instance, only one of the moisture quantity sensors  24  and  34  employed in the first embodiment may alternatively be installed in either of the oxygen drain line  21  and the hydrogen drain line  31 .  
         [0118]    The moisture quantity sensors  24  and  34  may be, unlike the first embodiment, installed in the oxygen supply line  20  and the hydrogen supply line  30 , as shown in FIG. 19. Alternatively, only one of the moisture quantity sensors  24  and  34  may be installed in either of the oxygen supply line  20  and the hydrogen supply line  30  to measure a moisture content of the air or hydrogen gas supplied to the fuel cell stack  10 . The same sensors as the moisture quantity sensors  24  and  34  may be installed both upstream and downstream of the fuel cell stack  10 .  
         [0119]    The sine wave generator  144  employed in the fuel cell system of the sixth embodiment may alternatively be designed to produce and apply a sinusoidal voltage to a DC voltage outputted by the electrochemical cell  140  of the moisture quantity sensor  14 .  
         [0120]    The internal resistance of the electrochemical cell  100  is determined by the AC impedance in the sixth embodiment, but may alternatively be determined by increasing an output current of the electrochemical cell  100  gradually and measuring a resulting output voltage thereof to determine a voltage drop.  
         [0121]    The same sensors as the moisture quantity sensor  14  constructed by the part of the cell  100  of the fuel cell stack  10  in the fourth to sixth embodiments may be installed, as clearly shown in FIG. 20, in some of the cells  100  of the fuel cell stack  10 , thereby enabling a variation in quantity of moisture within the fuel cell stack  10 .