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[技术] 微型燃料电池实验手册

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发表于 2009-6-12 05:18:30 | 显示全部楼层 |阅读模式
Themini fuel cell can simply be used to demonstrate that it works, and to show a fuel cell's simplicity and such like things. However, the Mini Fuel Cell has also been designed for other more challenging experiments. Five suggestions are given in the "Manual of Experiments". This manual of experiments is presented in electronic form below.
Teachers and lecturers are permitted and encouraged to copy the instructions presented here and to use them in their own worksheets and or laboratory instructions. You will almost certainly need to modify them somewhat for your own use, this is much easier when they are supplied in this electronic form. For this reason this Manual of Experiments is available exclusively in this electronic form.
With your mini fuel cell you should have received a short manual, which gives instructions for the basic care and operation of the cell, and also explains its chemistry. The experiment instructions below sometimes refer to these instructions.
 楼主| 发表于 2009-6-12 05:20:14 | 显示全部楼层
Fuel cells - electricity         from without burning:

A series of short experiments or demonstrations that compare fuel cell operation with the ordinary burning of fuel. The aim is a thorough understanding of what is going on in a fuel cell. Most rely on the fact that the cathode is at the end of a tube, into which can be put (for example) oxygen, carbon dioxide, or water.


A series of experiments comparing fuel cell operation with ordinary burning.
Introduction
The purpose of these demonstrations or experiments is to compare what is going on in the fuel cell with ordinary burning. The point to get over is that the fuel is chemically reacting with oxygen in the air, but that electricity is being produced instead of heat and light.
These experiments can be done by the pupils, if a suitable oxygen supply is available. Alternatively they can be done as a teacher demonstration.
The experiment/demonstrations are very short, and even with the pupils doing their own experiments, and then writing them up, all should be finished within 35 minutes.
The current produced during this experiment will be strongly dependent on the resistance of the meter used. However, in all cases the proportional change in current will be much the same whatever meter is used. An important feature of this fuel cell is that no harm will be done if the cell is "short circuited" by an ammeter, even if the "short circuit" is maintained for weeks on end.
Apart from the electrolyte, and fuel, the other apparatus and materials needed are:-
liquid fuel anode
air cathode
ammeter (0-1 A)
2 connecting wires
water
a supply of oxygen
a supply of carbon dioxide (optional)
Instructions
Take care, this experiment involves the use of methanol (which is poisonous), and KOH solution (which is corrosive).
1) Fill the liquid fuel anode up to the fill line KOH electrolyte (~1 M) BUT DO NOT PUT ANY FUEL IN.
2) Connect the ammeter (0-1A) to the cell, and record the current. (It should be zero.)
3) Add about 10 cm3 of methanol to the cell. Observe the current. (It should rise to about 0.3A). Conclusion. The current is caused by some reaction involving the methanol.
4) Cut off the air supply to the upper electrode (the cathode) by pouring a little water into the air cathode, as shown in the diagram below. Record the effect on the current. (It should fall, after a few seconds, to just above zero.) Conclusion. The reaction causing the current also involves air, just like burning.
exp1.gif

Note. The current does not fall completely to zero because of the dissolved oxygen in the electrolyte and the water. Note also that the current does not fall instantly because of the electric charge stored in the pores of the electrodes.
5) Pour the water out of the cathode holder, taking care to do this over the sink, so that any drips of electolyte from the cathode do no damage.
6) Get the cell working again. Direct some oxygen into the space above the cathode. Being denser than air, the oxygen will stay in the tube above the electrode. Observe the effect on the current. It should increase noticeably. Conclusion. The reaction causing the current is between the methanol and the oxygen in the air.
7) Discuss with the students the comparisons between this reaction and the burning of methanol. There are similarities, but electricity is produced instead of heat and flame. If necessary repeat one or two of the dramatic "burning things in oxygen " type demonstrations.
8) The comparison with burning could be reinforced by "extinguishing" the fuel cell with CO2. Direct CO2 gently into the space above the cathode. The current will be observed to fall to zero, though it will rise again when the CO2 disperses.
fcgas.gif
 楼主| 发表于 2009-6-12 05:22:26 | 显示全部楼层
Investigating the         Effect of Temperature on Reaction Rate:

The current from a fuel cell       IS the reaction rate of the fuel! Because the cell is small and compact       it can be heated, or put under pressure, and an ammeter connected to it       gives a very immediate and quantitative indication of reaction rate.


Introduction
It is a simple matter to show qualitatively that most chemical reactions can be made to proceed faster if the reactants are heated. There are also applications of this in every day life, for example the faster growth of plants in hotter climates. The advantage of this experiment is that it allows students to measure the rate of reaction at different temperatures, because the current produced by the cell is the rate of reaction - the faster the reaction goes, the more electrons are produced. The student can then thus quantitatively study the effect of temperature on reaction rate.
The current produced by the fuel cell, which is the rate of reaction, is measured by connecting an ammeter directly to it. The current produced will be strongly dependent on the resistance of the meter used. However, in all cases the proportional change in current will be much the same whatever meter is used. An important feature of this fuel cell is that no harm will be done if the cell is "short circuited" by an ammeter, even if the "short circuit" is maintained for weeks on end.
This experiment works best if temperatures above and below room temperature can be used, so it is important to ensure a supply of cooled electrolyte before the experiment is done. This can be done by putting a bottle of 1M KOH in a fridge the day before. Each student needs 75ml, so a 1 litre bottle should be sufficient.
The cell should be heated by placing it in a large beaker or dish containing a little hot or warm water. The cell should be disconnected from the meter while this is being done, so that spillages are less likely while the cell is being moved about.
Apparatus
The apparatus needed by each student is:-
Fuel cell (liquid fuel) anode, cathode
ammeter, 0-1A, 2 connecting wires
0-100 oC thermometer
dish or large beaker containing hot water
Instructions
Take care, this experiment involves the use of methanol (which is poisonous), and KOH solution (which is corrosive).
1) Draw a table for the results, with two headings, "Temperature" and "Current".
2) Fill the fuel cell anode up to the fill line with chilled KOH solution. Add about 10 ml of methanol. Place the cathode into the cell. Gently shake or swirl to mix the fuel and clear any air bubbles.
3) Connect the ammeter. Place the thermometer in the cell through the holes provided.
4) Allow about 15 seconds for the meter to reach a steady value, and then record the temperature and the current.
5) Disconnect the meter, and gently heat the cell by placing it in a dish or large beaker of warm water. Wait till the temperature has risen by about 5 0C. Do not attempt to take temperature readings at round number temperatures - the heating method is too simple to allow this to be done accurately.
exp2.jpg

6) Reconnect the ammeter and wait about 15 seconds for the meter to reach a steady value. Record the values of the temperature and the current.
7) Repeat steps 4 and 5 until the temperature has reached about 45 oC. DO NOT HEAT ABOVE 50 oC. You may need to renew the warm water supply in the dish or large beaker to reach 45 oC.
8) Plot a graph of reaction rate (current ) against temperature.
Likely Results
The current produced depends very strongly on the type of ammeter used, but typically you should expect about 200 mA at 5 oC, rising to about 500 mA at 45 oC. The graph will probably be quite linear, rather than the exponential that the Arrthenius equation would predict, but only one of the reactants is being heated.
 楼主| 发表于 2009-6-12 05:24:12 | 显示全部楼层
Measuring Oxygen Consumption:
One of the best features of the electro-chem-technic cells is that their design allows the consumption of oxygen to be measured. The rate at which oxygen is consumed can be compared with the electric current. The results can be used to:-
Notice that the oxygen consumption is proportional to current, and use this to reinforce an understanding of how fuel cells work.
Relate the volume of gas consumed to the charge, and using known formulas such as PV=nRT (the universal gas law), verify the chemistry of the cell, i.e. 4 electrons released per oxygen molecule.
Assume the chemistry of the cell given in the manual is correct, and use the same calculations of charge and gas volume to find the space taken up by one oxygen molecule at NTP.
Introduction
One of the best features of the Electro-Chem-Technic cells is that their design allows the consumption of oxygen to be measured. The rate at which oxygen is consumed can be compared with the electric current. The results can be used to:-
Notice that the oxygen consuption is proportional to current, and use this to reinforce an understanding of how fuel cells work.
Relate the volume of gas consumed to the charge, and using known formulas such as PV=nRT (the universal gas law), verify the chemistry of the cell, i.e. 4 electrons released per oxygen molecule.
Assume the chemistry of the cell given in the manual is correct, and use the same calculations of charge and gas volume to find the space taken up by one oxygen molecule at NTP.
The apparatus is as shown below:-
oxcon.gif

The assembly of bung, adaptor, flexible tube and graduated tube can by purchased. It is called the "oxygen use measurement kit". However, you could easily make it yourself. The graduated tube is made from a plastic 10ml pipette, obtainable from all laboratory equipment suppliers.
The basic procedure for an experiment is as follows:-
1. Start up a mini fuel cell in the normal way.
2. Connect it to a variable resistor load, in the range 0 - 20 ohms, in series with an ammeter.
3. Set up the oxygen use measurement kit as in the diagram above. The graduated tube should be held steady with a clamp.
4. Adjust the variable resistor till a suitable current is flowing, e.g. 40 mA. Anything in the range 20 - 100 mA is suitable.
5. Observe that the water level very slowly rises up the graduated tube. When the water level inside the tube has risen to being just above the level outside start a timer.
6. Time how long it takes for the level to change by 1.0 ml (1 cm3). While this is happening, keep adjusting the load resistor so that the current is constant. (With a current of 50mA, this time will be about 5 minutes.)
7. Disconnect the load. Remove the graduated tube from the water so that it empties, and then replace.
8. Repeat instructions 4 - 7 at different currents as necessary and as time allows. The results can then be processed in a way suitable for the level of students performing the experiment. For some it will be enough to draw a graph of current vs. rate of oxygen use. More advanced students should be able to use values of R, Avagadro's number, the charge on one electron, the Universal Gas Constant and the temperature and pressure at the time of the experiment, together with PV = nRT to find the number of electrons released by each oxygen molecule. (See the "likely results" section below.)
Precautions
There are some precautions that need to be taken to get good results with this experiment:-
The liquid level inside the graduated tube must always be above that of the water outside - as in the diagram. In other words the pressure inside must be a little less than air pressure. If it is a little higher then air tends to diffuse through the porous air cathode, and the gas use readings are too high.
Don't be tempted to use a narrow tube for the water to go up. If you do then capillary action effects complicate things so much that you will get useless results.
Likely Results
Using the method descirbed above, a likely results is:-
Using a fuel cell, the load resistor was adjusted so that the current was 40 mA.
The water level slowly rose up the graduated tube till the level inside was the same as that outside. The timer was then started.
After seven minutes the level inside the tube had risen by 1.0 cm3.
Atmospheric Pressure was 100 kPa, and the temperature was 21C. These measurements can be used to find the number of electrons passing round the circuit for each molecule of oxygen, in order to better understand the chemistry of a fuel cell. This is done as follows:-
1. Use PV = nRT to find the number of moles of oxygen gas used.
P = 100 kPa = 1.0 x 105Pa.
V = 1 cm3, being the amount the water level rose, = 1.0 x 10-6 m3.
T = 21 C = 294 K
R = 8.31.
So, n = PV/RT = (105 x 10-6)/(8.31 x 294) = 4.1 x 10-5 moles.
Now we find the number of molecules of oxygen by multiplying this by Avagado's number, giving 4.1 x 10-5 x 6.02 x 1023 = 2.5 x 1019.
Next we find the number of electrons that flowed round the circuit in the 7 minutes.
First we find the electrical charge in Coulombs.
Charge = Current x Time = 40mA x 7 x 60 = 16.8 Coulombs
The charge on one electron = 1.6 x 10-19 Coulombs.
So the number of electrons = 16.8 / 1.6 x 10-19 = 1.05 x 1020
We now find the number of electrons per molecule by dividing the number of electrons that flowed round the circuit by the number of oxygen molecules used.
The number of electrons per oxygen molecule = 1.05 x 1020/2.5 x 1019 = 4.2
The "accepted" answer is 4, as explained in Section 3 of the fuel cell instruction manual.
This should be repeated for different currents and times, as time allows.
 楼主| 发表于 2009-6-12 05:25:29 | 显示全部楼层
Investigating         the Rate Determining Step in a Multi-step Reaction:

The reaction of methanol in the       fuel cell involves a multistep reaction. However the intermediate products,       methanal (formaldehyde) and methanoic (formic) acid, are quite readily available       The cell can be fuelled with these, starting the reaction at different points       in the chain. Since the current produced can give the rate of reaction,       this allows a quantitative study of an interesting multi-step reaction.       Full guidance given.


Introduction
The chemical reaction in the methanol fuel cell makes possible a very interesting study of a multi-step reaction. The speed of the various steps can be measured, and the rate determining step deduced. Section 3.2 of the Mini Fuel Cell Kit instruction manual must be read before attempting this experiment - or reading these instructions for that matter!!
The basis of the experiment is that the methanol is oxidised in three stages:-
First to methanal (formaldehyde), releasing 2 electrons.
Then to methanoic acid (formic acid), releasing 2 electrons.
And finally to carbon dioxide, releasing another 2 electrons. The interesting, and quite rare, thing about this multi-step reaction is that in the fuel cell it can it can be started at any point. The two intermediates, methanal and methanoic acid, are fairly easily obtainable, and the fuel cell can be run using them.
Furthermore, it is not difficult to relate the current from the cell to the reaction rate.
This is done as follows:-
The charge on one electron is 1.6 x 10-19 Coulombs.
So, a current of one amp is the same as 1/(1.6 x 10-19)= 6.25 x 1018 electrons passing per second.
Thus, if an electro-chemical reaction releases 1 electron each reaction, then the number of reactions is clearly also 6.25 x 1018 per second, if the current is 1 amp.
By similar logic, if an electro-chemical reaction releases 2 electrons, then the number of reactions per second = 1/(2 x 1.6 x 10-19) = 3.1 x 1018. The oxidation of methanoic (formic) acid in a fuel cell is such a reaction.
Similarly, for a reaction, such as the oxidation of methanal (formaldehyde), which releases 4 electrons, the number of reactions per second = 1/(4 x 1.6 x 10-19) = 1.6 x 1018 for each amp.
Finally, for a reaction, such as the complete oxidation of methanol in a fuel cell, which releases 6 electrons, the number of reactions per second = 1/(6 x 1.6 x 10-19), which is 1.04 x 1018 reactions per second at one amp. The three numbers in bold above are multiplied by the current (in AMPS) to give the reaction rate, in reactions per second, for methanoic acid, methanal and methanol respectively.
Instructions The procedure for the experiment is quite simple, but care must be taken, since methanol and methanal are both poisonous, and methanoic acid and the KOH electrolyte are corrosive.
Another problem is that when using methanoic acid, great care must be taken to make sure that the fuel/electrolyte mixture remains alkali. For this reason only 1ml of this fuel should be used. To keep the experiment valid, only 1ml of the other fuel should be used also. This should not present any problems.
The basic procedure is as follows:-
Put ~75ml of KOH solution into the cell.
Add 1 ml of methanol.
Short circuit the cell with an ammeter, wait about 1 minute, and record the current. Note that the current will never completely stabilise.
Multiply the current (in AMPS) by 1.04 x 1018 to give the reaction rate.
Repeat the first three steps for methanal, multiplying the current (in AMPS) by 1.6 x 1018 to get the reaction rate.
Repeat for methanoic acid, this time multiplying the current in AMPS by 3.1 x 1018 to get the reaction rate.
You should find that the reaction rate when using methanol is markedly lower than the other two fuels, showing that the step methanol ---> methanal is the rate determining step. Spectroscopic studies of methanol fuel cells, specially designed so this can be done, confirm this to be the case, with very low concentrations of methanal and methanoic acid present in a working cell, indicating that these substances are quickly oxidised.
 楼主| 发表于 2009-6-12 05:26:51 | 显示全部楼层
Measuring the Power         of a Fuel Cell

A Physics experiment. The fuel cell has quite a high internal resistance, and so lends itself well to maximum power transfer experiments.


Introduction
The fuel cell is an ideal source of electrical power for experiments on power measurement. Unlike other electrical power sources it can be run at full power indefinitely, without causing any damage. No fuses will blow, no overheating, and no expensive dry cells will be consumed. Furthermore its power is very stable at all electrical loads.
The idea of this experiment is to put a variable resistance load on the cell, and measure the voltage and current. The resistance can be varied from infinity to short circuit, and a very good power/voltage curve obtained.
This experiment cannot be done at reasonable cost with any other equipment commonly available in schools. It is an experiment that gives a very good balance between equipment handling, observation and processing of results.
Each group of students will require:-

Ammeter (0 - 1A)
Voltmeter (0 - 1V)
Variable resistor (0 - 25 ohms)
Connecting wires
Air cathode
Fuel cell anode
KOH solution and methanol OR NaCl solution

Instructions
1) Set up the fuel cell in the normal way.
2) Connect the voltmeter to cell, and measure the no load voltage.
3) Connect the ammeter, resistor and voltmeter as in the circuit diagram. Use the variable resistor to adjust the voltage to be 0.55 volts. Measure the current.
exp6.gif

4) Repeat step 3 at voltages of 0.50, 0.45, 0.40, 0.35,....etc. Keep going to as low a voltage as possible. The value of the lowest voltage will depend upon the resistance of the ammeter.
5) Calculate the power from voltage x current. Plot a graph of power against voltage.
Likely Results
Using the fuel cell, and methanol fuel, the power will be a maximum at between 0.2 and 0.3 volts. The maximum power will depend strongly on the temperature of the cell and the concentration of the KOH solution, but should be between 50 and 100 mW.
发表于 2011-4-23 21:24:29 | 显示全部楼层
发表于 2011-4-24 11:20:33 | 显示全部楼层
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