08-17-07, 01:08 PM #1
Construction of a simple alcohol fuel cell
Here is some information on a simple fuel cell that I built a while ago. It might be useful as a demonstration project for younger students, or just as a fun toy. Anyway, I hope someone here finds it interesting.
This cell is powered by common alcohols and atmospheric oxygen. Commercial fuel cell PEMs are made of complex organic polymers that are specifically designed to allow easy migration of protons while restricting the movement of the fuels. This cell will use a polypropylene tube filled with sulfuric acid as a PEM. Although this will not be as efficient as a commercial PEM, it should be sufficient to allow the migration of protons from the anode to the cathode while preventing significant mixing of the fuels. The oxidation-reduction reaction will be catalyzed by platinum, which will be electroplated onto the cell’s electrodes.
Methanol is oxidized into carbon dioxide and water via the half reactions:
CH3OH + H2O -> CO2 +6H +6e; E=0.016V/SHE
O2 +4H +4e -> 2H2O; E=1.229V/SHE
The overall reaction is:
CH3OH + (3/2)O2 -> CO2 + 2H2O
This predicts a maximum potential of 1.21 V, but in reality the voltage will be reduced by incomplete oxidation of the methanol to form formic acid or formaldehyde instead of CO2. Larger alcohols such as ethanol or isopropanol will be oxidized into carboxylic acids.
Inspiration for the construction of the cell was taken from Zerbinati’s article A Direct Methanol Fuel Cell in Journal of Chemical Education’s issue 79-9. The cell consisted of 2 10ml syringes, the bottoms of which were connected by a polypropylene tube. Platinum coated electrodes were placed in each syringe. The syringes were filled with about 15 ml of 1M H2SO4 to act as an electrolytic bridge and to facilitate the oxidation of the alcohol. A small amount of alcohol was placed in one syringe, which became the anode. The platinum surface of the electrodes catalyzes the oxidation of the alcohol. Protons, which are produced by the oxidation of the alcohol, migrate through he polypropylene tube to the cathode. Here they react with dissolved oxygen to produce water. The reaction continues until one of the fuels is depleted. Generally oxygen will be depleted first, as great quantities of it are not usually dissolved in water.
Plating the electrodes with platinum was the most complex part of assembly. The electrodes were made of 0.25 mm 80/20 NiCr wire that was wrapped into a spiral spring shape approximately 30 mm in diameter. Each electrode was 25 cm long, giving each a surface area of about 2 cm^2. The NiCr wire was electrocleaned in 1M H2SO4 for 15 seconds with a 9V battery and a 47 Ohm resistor. A graphite rod was removed from a #2 PaperMate pencil to serve as the cathode. The wires were washed with deinozed water after cleaning and stored in a beaker of deionized water under paraffin while waiting to be electroplated.
The electroplating solution consisted of 0.3g H2PtCl6*6H20 (hydrogen hexa-chloroplatinate hydrate) dissolved in 30 ml of 1M HCl. The H2PtCl6 was extremely hygroscopic; hast is advised when working with it in the open air.
Each wire was electroplated in the platinum solution for 30 minutes at 9V with a 470 Ohm resistor. A graphite rod was used as the anode. After 30 minutes the electrodes had a noticeable platinum coating. The finished electrodes were stored under deionized water when not in use.
Tests were conducted to determine how the current and voltage output of the cell changed with different fuels. The current vs. time measurements were taken by shorting the cell with a 470 ohm resistor and measuring the current over several minutes with a stopwatch. The stability of the cell was measured by recording the potential of the cell over time without current flowing.
The cell often behaved strangely, sometimes displaying large fluxuations in voltage for no apparent reason. It seems likely that much of its odd behavior was caused by varying alcohol diffusion patterns around the electrodes. It is also likely that the polypropylene tubing did not act as a perfect PEM, which could have allowed gradual migration of the fuel from one cell to another to occur.
Methanol proved to be the most stable of the fuels, generating a steady voltage of around 0.4. The current was around 83 mA. Ethanol produced a higher voltage and greater overall power output, although it was not as constant as methanol. Isopropanol initially produced the greatest voltage and power output, but it quickly fell to a level below methanol. The current and voltage became relatively stable for each fuel after several minutes. This allows us to estimate the short-term stable power output for each fuel.
Although this design for an alcohol fuel cell was relatively easy to create, it was difficult to gather reliable data on the effects of various fuels. The main problem was controlling the rate of diffusion of the alcohol over the anode. Wildly different voltages could be produced (from 0.8V to less than 0.1V) for each fuel by varying the extent to which the alcohols were dispersed into the water. Unfortunately the fuel cannot be mixed ‘thoroughly’ because this results in alcohol migrating through the polypropylene tube and disrupting the reaction. Eventually a method was developed in which the fuel would be carefully added to the top of the anode, then mixed by a singe injection of 10 ml of air into the bottom of the anode syringe. I believe that this produced a relatively constant amount of mixing, but it is difficult to be certain. The difficulty in controlling the rate of alcohol diffusion makes this design inappropriate for actually powering things, but it is still useful for demonstration purposes.
Check out Zerbinati, O. J. Chem Ed. 2003, 79-9, page. 829-831 for instructions on how Zerbinati built a similar cell.
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