Saline Fuel Cell
Science at Home
Four strings with crocodiles.
4.5 V Battery.
Two platinum strings.
Sodium chloride (NaCl).
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1. Pour a teaspoon of sodium chloride in a glass with distilled water and mixture the solution.
2. Dive partially the platinum electrodes in the solution and link them to the voltmeter terminals. (you can verify that the voltmeter doesn't detects any potential, even in the most sensible range)
3. Connect the battery to the circuit. (you can verify a vigorous production of gases in the electrodes)
4. Change the voltmeter scale to 0-20 VDC and measures the differential potential between the two electrodes. (to have a positive potential link the voltmeter black string to the platinum electrode that is linked to battery [-] pole and link the voltmeter red string to the electrode is linked to the battery [+] pole. You can verify that the measured differential potential is almost the one of the battery)
5. Take out the battery of the circuit. (don't move the glass. The gas bubbles in the surface of the electrodes will be our fuel)
6. Measure the differential potential between the electrodes. (you can verify that even without battery, the system continues to produce energy, 1.2 V)
7. Change the voltmeter scale progressively to smaller measurement ranges. (you can verify that the differential potential is decreasing with time)
Link again the battery to the circuit and repeat the previous procedure. (you can verify that you renewed the gas bubbles in the electrodes surface)
The present experiment can be explained using the same electrochemistry basic principles. The electricity production in the saline fuel cell is due to the electrolysis reverse reaction of a sodium chloride aqueous solution (Na+Cl- ions).
In the first part of the present experiment you have accomplished the electrolysis of NaCl. When you linked the battery to the circuit, it supplied electrons to the platinum electrode linked to the pole [-] and received electron from the electrode linked to the pole [+].
In this case (electrolysis), the cathode is the platinum electrode linked to the [-] battery pole. The anode is the electrode linked to the [+] battery pole.
In the cathode, the electrons supplied by the battery react with the Na+ ions and water, forming gaseous (H2) and sodium hydroxide (NaOH). H2 is freed to atmosphere and NaOH to the aqueous solution, increasing the pH of it. In the anode, the chloride ions (Cl-) in the solution free electrons and produce chlorine gas (Cl2, which is freed to atmosphere.
The differential potential is practically the same as the supplied by the battery due to the presence of sodium chloride ions in water (strong electrolyte).
When you take out the battery of the circuit it happens the reverse reaction. In other words, the electrode that was cathode in the electrolysis becomes anode and the anode becomes cathode. This fact happens due to the catalytic capacity of platinum, increasing the reverse reaction speed to the electrolysis.
The gas hydrogen in the electrode surface reacts producing electrons and protons (H+). The electrons flow in the electric circuit to the cathode and the protons are freed to the aqueous solution. In the other electrode, molecular chlorine reacts with the electrons produced in the anode and water, forming chloride ions again (Cl-).
The initial differential potential of the fuel cell is close to 1,4 V. It decreases with time because the amount of chlorine and hydrogen decreases, being consumed in the oxidation and reduction reactions. However, if these reactants were fed continuously to the electrode, the home-made fuel cell would be able to produce energy continuously.
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