Nuts and Volts - June 2010

Figure 8.

Mass Transport

State.

Figure 9. Beginning and Ending Electrical Data for Five Minutes.

Efficiency Experiments

Now, let’s determine our fuel cell’s efficiencies (Energy and Faraday) by first generating 6 ml of hydrogen this time. We want to set up the Test Bed with a five ohm load (two 10 ohm resistors in parallel or equivalent). Then, we need to time the rate of hydrogen consumption with a wristwatch or your computer’s clock. Start by generating the 6 ml of hydrogen; when you’re ready, connect the fuel cell to the Test Bed and begin timing the process. We want to allow the hydrogen to be consumed over exactly five minutes or 270 seconds.

When the time is up, disconnect the fuel cell from the Test Bed and visually measure the remaining hydrogen left in the storage cylinder. During the timing, record both the starting and ending voltages and currents. Figure 9 shows the start and stop values that I measured; your data may differ. Now, let’s plug in some numbers to see how efficient our fuel cell performed.

voltage (E) and current (I) must be determined. The averages are computed between the readings at time zero and at 270 seconds. Table 1 illustrates these averages based on the data taken. Your values may be different.

Entering these values into the equations: between the theoretically calculated volume of hydrogen consumed by the load (at a certain current) and the experimentally calculated volume of hydrogen consumed. Faraday Efficiency is computed as:

η F = H2 Volume (theoretical) / H2 Volume (experienced) where … Electrical energy in Joules H2 = E (average) x I (average) x time (seconds) Electrical energy in Joules H2 = 0.539 x 0.119 x 270 = 17. 3 J H2 Volume (theoretical) = [Electrical charge in Coulombs (C)] x [H2 Volume per mol] Therefore, the volume of H2 used can be found as: Electrical Charge in Coulombs (C) = I (ave) x time (sec) = 0.119 x 270 = 32.13C H2 Vol (theoretical) = ( 32. 13 C / 193,000 C/mol) x 24,000 ml/mol = 3.99 ml Volume of H2 consumed in mL / 24,000 mL/mol = mol of H2 5 mL / 24,000 mL/mol = 0.00021 mol H2

Therefore, the Faraday Efficiency we measured is: The energy content consumed H2 is given by:

η F = H2 Volume (theoretical) / H2
Volume (experienced)
η F = 3.99 / 5.0 = 0.798 x 100%
= 79. 8

Energy Efficiency Calculations

The energy efficiency of a fuel cell is the ratio of the electricity produced by the consumed hydrogen compared with the calculated theoretical energy contained in the consumed hydrogen (expressed as a percentage). In order to compute the required values, the average Energy content of consumed H2 (theoretical) = mol H2 x 286,000J/mol Energy content of consumed H2 (theoretical) = 0.00021 x 286,000 = 59. 6 J Energy efficiency can be computed as: A value like 79.8% efficiency looks pretty good, but it can be better or worse depending on the electrical current used in the experiment. If you care to, modify the experiment with a different load resistor and repeat it.