experiment 10 - british columbia institute of...

EXPERIMENT 10 – FUEL CELL

OBJECTIVE

To study the hydrogen fuel cell

Part A - The Electrolyzer

1. Determine the current and voltage relationship in the electrolyzer.2. Determine the minimum voltage for the electrolysis of water.

Part B - Faraday Efficiency of the Electrolyzer

Determine the Faraday efficiency of the electrolyzer.

Part C - The Characteristic Curve of the Hydrogen Fuel Cell

1. Determine the voltage profile ("Voltage vs. Current") of the hydrogen fuel cell.2. Determine the power curve of the hydrogen fuel cell.

THEORY

What is a fuel cell?

A fuel cell is a device that converts chemical energy into electrical energy. It works on the same principle as a battery but is continually fed with fuel. One of the simplest fuel cells involves the reaction between H2 (g) and O2 (g) to produce the only product, water. Fuel cells usually have no moving parts, they are silent, and need little maintenance.

Fuel cell technology is receiving attention to address the depletion of natural resources and global environmental concerns such as global warming and the greenhouse effect. Fuel cells also promise greater operating efficiency with lower emissions over conventional power sources used today.

How does it work?

A fuel cell (Figure 10-1) consists of two platinum coated carbon electrodes bonded to a Proton Exchange Membrane (PEM).

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Figure 10-1 - The schematic diagram of a fuel cell.

The membrane is made from a solid proton-conducting polymer, which allows the proton to pass through the material. In the late 1980s DuPont introduced a membrane material NafionTM PEM. This chemical structure of a NafionTM PEM is shown in Figure 10-2.

Figure 10-2 - The chemical structure of a NafionTM PEM.

This electrolyte membrane was a material like Teflon. Sulphonic acid groups (-SO3H) are attached to the carbon chain of the polymer to allow protons to pass through the material. In order to achieve the maximum efficiency of the membrane, it needs to be fully hydrated (humidified) during operation. This means PEM must operate at temperatures below 100oC.

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The carbon electrodes are treated with platinum and are hot-press bonded to the polymer membrane, so that the membrane extends partially into the porous electrodes. This ensures maximum surface area. The Platinum acts as a catalyst facilitating the reactions at the two carbon electrodes.

The reactions occurring in the cell are:

Oxidation reaction at the anode:

2H2 (g) 4H+ (aq) + 4 e- rxn (10-1)

Reduction reaction at the cathode:

O2 (g) + 4 H+ (aq) + 4 e- 2 H2O (l) rxn (10-2)

Hydrogen fuel is fed to one electrode where the reaction results in it losing electrons. The electrons travel to the external circuit while the proton, or Hydrogen ion, drifts through the polymer membrane (i.e. electrolyte). At the cathode oxygen is reduced to water.

The overall cell reaction

2 H2 (g) + O2 (g) 2 H2O (l) E = 1.23 V rxn (10-3)

A single hydrogen fuel cell has a maximum theoretical voltage of 1.23 V. In practice, because of internal resistance and inefficient diffusion of the gases, the voltage obtained is 0.6 - 0.9 V. By stacking the cells, connected in series, voltages of about 200 V can be attained. The maximum current that can be drawn from the fuel cell depends upon the surface area of the electrodes.

The electrolyzer - generation of the hydrogen fuel

Hydrogen for a fuel cell may be generated by electrolysis of water. When water is decomposed, the ratio of hydrogen gas to oxygen gas produced is 2:1. The decomposition reaction

2 H2O (l) 2 H2 (g) + O2 (g) E = - 1.23 V rxn (10-4)

generates the gases that are necessary for the fuel cell reaction (rxn (10-3)). Note that reaction (10-4) is the reverse of reaction (10-3). This means that the theoretical voltage that one needs to supply from an external source (i.e. from a power supply) to decompose water is 1.23 V. In practice, the external voltage applied to split water always exceeds 1.23 V. The difference between the theoretical decomposition voltage (i.e. 1.23 V) and the actual decomposition voltage (i.e. usually a voltage greater than 1.23 V) is called overpotential or overvoltage. For example, if 2.10 V is the actual voltage that is required to cause water to decompose such

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that hydrogen gas and oxygen gas are observed, then the overpotential would be be 2.10 V-1.23 V = 0.87 V. The overpotential is a function of the electrode material, the electrode surfaces, the type and concentration of the electrolyte, the current density and the temperature. An overpotential is needed to overcome interactions at the electrode surface and are particularly common when gases are involved. In Part A, we will determine the overpotential required for the electrolysis of water.

Recent work on hydrogen fuel storage has focused on cartridges of metal hydrides or carbon nanofibres. The nanofibres are built up from graphite platelets arranged so that hydrogen can adsorb on the edges and between the platelets. Hydrogen can be released at room temperature by a reduction of pressure, or the material can be warmed up.

Faraday Efficiency of the Electrolyzer

The Faraday efficiency, ŋ, is determined from the volume of hydrogen found by experiment and the volume of hydrogen calculated from theory:

η=Volume of H2 (exp erimental )

Volume of H2( theoretical)

The Faraday efficiency should be close to 1 (i.e. 100%). The number of moles of electrons involved in the decomposition of water is 2 moles of electrons/mole of H2O. One mole of electrons has a charge equal to 96500 coulombs. At 20oC, the molar volume of H2 (g) is 24000 mL.

The theoretical volume, Volume (H2) theoretical, produced:

Volumeof H2( theoretical )= current( amp)⋅time(s )⋅24000(mL)2 molese−⋅96500 coulombs

mole s of e−

The Faraday efficiency of the electrolyzer shows how much of the electric charge is converted in the desired reaction. In commercial electrolyzers, the Faraday efficiency must be close to 1 (i.e. 100%). A Faraday efficiency much smaller than one would mean that secondary reactions were taking place in the system (i.e. corrosion). This would be a great disadvantage, since it would not only shorten the service life of the electrolyzer, but also necessitate a higher energy input.

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The Characteristic Curve of the Hydrogen Fuel Cell

Figure 10-3 shows a schematic diagram of the fuel cell. In order to understand the characteristic curve of a fuel cell, recall the characteristic curve of the electrolyzer (see Part A). The processes in the fuel cell are the reverse of those that take place in electrolysis.

Figure 10-3 - A schematic diagram of a fuel cell

The theoretical voltage of the fuel cell is 1.23 V. In practice, a lower voltage is observed. The difference in voltage is very much dependent on the volume and purity of the input gases. The more current is drawn from the fuel cell, the lower the voltage becomes.

In practice, efforts are made to draw as much current as possible from the fuel cell (i.e. maximum output). However, the efficiency of the fuel cell declines at high current values. So, the task is to find an optimum operating point (i.e. high efficiency, high output).

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The methanol fuel cell

The methanol fuel cell uses methanol as fuel. Figure 10-4 shows a schematic diagram of the methanol fuel cell.

Figure 10-4 - The methanol fuel cell

The major difference is that both electrodes are made of precious metal such as platinum or ruthenium. At these metal electrodes catalyzed chemical reactions take place. The metals themselves are not subject to reaction.

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At the anode methanol is supplied. At the cathode, oxygen from air is fed in.

Oxidation reaction at the anode:

CH3OH (l) + H2O (l) CO2 (g) + 6H+ (aq) + 6 e- eqn (10-5)

Reduction reaction at the cathode:

1.5 O2 (g) + 6 H+ (aq) + 6e- 3 H2O (l) eqn (10-6)

The overall cell reaction is

CH3OH (l) + 1.5 O2 (g) CO2 (g) + 2 H2O (l) E = 1.21 V eqn (10-7)

The theoretical voltage of a methanol fuel cell is 1.21 V. In practice, depending on the current load, the voltage is somewhere between 0.6 and 0.2 V. The electrode material, the internal resistance, the temperature, as well as the amount of methanol at the anode and the amount of oxygen from the air at the cathode, all influence the magnitude of the current.

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PROCEDURE Part A - The Electrolyzer

1. Turn on the power supply and make sure the voltage and current knobs are turned down. (i.e. - turn both knobs counterclockwise.).

2. Fill the gas storage cylinders of the electrolyzer with distilled water to the 0 mL mark. 3. Set the resistor dial on the load box to 'SHORT CIRCUIT'.4. Follow Figure 10-5 and connect the power supply, load box and electrolyzer in series with

the patch cords provided.

Figure 10-5 - Circuitry to determine the decomposition voltage of water

Power supply connections: (a) Connect the red terminal of the power supply to the red terminal of the electrolyzer. Connect the black terminal of the power supply to the black terminal of the load box's ammeter. Electrolyzer connections:(b) Connect the black terminal of the electrolyzer to the red terminal of the load box’s ammeter. Connect the red terminal of the electrolyzer to the red terminal of the load box’s voltmeter. Connect the black terminal of the electrolyzer to the black terminal of the load box’s voltmeter.

Set the voltage knob to limit the current to 0.45 A. Slowly turn the current knob of the power supply. The power supply current and voltage knobs are very sensitive. Adjust the power supply to obtain, roughly, the following current readings:

0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.08, 0.10,

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0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45 A

5. At each of the current values, record both current and voltage readings of the power supply using the current and voltage displays on the load box.

DO NOT LET THE CURRENT EXCEED 0.5 A, IT WILL DAMAGE THE ELECTROLYZER.

7. Do not disconnect your connections.

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1. Use the same set up as in Part A.2. Fill the gas storage cylinders of the electrolyzer with distilled water to the 1 mL mark.3. Attach the reservoir tubes to the gas storage cylinders by wetting the rubber adapter with a

small amount of distilled water. Use a gentle twisting motion to secure the rubber adapter in place.

4. Close the tube on the hydrogen side with a tubing stopper.5. To measure the efficiency of the electrolyzer, select two voltages from Part A. Select one

voltage with a small current (i.e. around 0.20 A). Select another voltage with a high current (i.e. around 0.45 A; do not exceed 0.50 A). Ask your lab instructor to verify your selections.

6. Set your power supply to the selected current and record the voltage and current flowing to the electrolyzer. Start your timer and record the volume of hydrogen gas produced in approximately 180 seconds.

7. Repeat step 6 for the second voltage selected.

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1. Set the load box resistor dial to ‘OPEN’. 2. Fill the gas storage cylinders of the electrolyzer with distilled water to the 1 mL mark.3. Connect a rubber tubing from to the H2 side of the electrolyzer to the H2 side of the fuel

cell. Ensure that there is no water trapped inside the rubber tubing.4. Connect a rubber tubing from to the O2 side of the electrolyzer to the O2 side of the fuel

cell. Ensure that there is no water trapped inside the rubber tubing.5. Follow the diagram below to connect the power supply, electrolyzer, fuel cell, and load

box.

Circuitry for connecting the fuel cell to various loads

Power supply to the electrolyzer connections: (a) Connect the red terminal of the power supply to the red terminal of the electrolyzer. Connect the black terminal of the power supply to the black terminal of the electrolyzer. Load box to fuel cell connections:(b) Connect the red terminal of the ammeter to the red terminal of the fuel cell. Connect the black terminal of the ammeter to the black terminal of the fuel cell.(c) Connect the red terminal of the voltmeter to the red terminal of the fuel cell. Connect the black terminal of the voltmeter to the black terminal of the fuel cell.Ground the power supply to the fuel cell's ground:(d) Connect the black terminal of the power supply to the black terminal of the fuel cell.

6. Set the current to 0.45 A and purge the fuel cell with H2 and O2 for several minutes.7. Dial the load box resistor knob to ‘OPEN’ and measure the voltage of the fuel cell. This is

the fuel cell's open circuit voltage. Record this in two places on the data sheet.8. Dial the resister knob on the load box to the various loads given below. Record the voltage

and current of the cell when the cell is subjected to following loads:

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200 , 100 , 50 , 10 , 5 , 3 , and 1 .

Take these voltage and current measurements fairly quickly.9. Record the voltage and current of the fuel cell when the cell is used to light up the lamp.10. Record the voltage and current of the fuel cell when the cell is used to power the motor.11. Determine the power output for the different loads, for the lamp and the motor.

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DATA


Current (amp) Voltage (volts) Current (amp) Voltage (volts)

Observations

1. The voltage at which bubbles start to form is _____________________________.

2. The H2 side of the electrolyzer forms ________________ bubbles than the O2 side. (more or less)


Time (sec)

Voltage (V)

Current (Amp)

Vfinal (mL)

Vinitial (mL)

Volume of H2 (g)

produced(mL)

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1. Measure and record the open circuit voltage of the fuel cell: _____________________

2. Record the voltage, current of the fuel cell at different loads.

Load () Voltage (V) Current (amp) Power (watt) = voltage current

OPEN

200

100

50

10

5

3

1

LAMP

MOTOR

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TREATMENT OF DATA

Questions


1. The "Current Vs. Voltage" curve shows that a current only starts to flow at a certain voltage and then it rises as the applied voltage across the electrolyzer is increased. Plot the graph of "Current Vs Voltage" and indicate on the graph the value of the voltage at which a current starts to flow.

2. At what voltage do you start to notice bubbles of Hydrogen and Oxygen gases?

3. Which side forms more gas bubbles? Why?

4. What is the theoretical decomposition voltage of water?

5. Determine the overpotential of the electrolyzer.

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1. Calculate the Faraday efficiency, ŋ, of the electrolyzer for each of the two voltages.

2. Based on your results, comment on the voltage that should be chosen to operate the electrolyzer to decompose water.


1. Plot the graph "Voltage versus Current" for the fuel cell. Indicate the maximum voltage obtainable from the cell on the graph. Place the lamp and motor on the graph.

2. Plot the graph "Power versus Current" for the fuel cell. Place the lamp and motor on the curve on the graph. Determine the maximum power of the fuel cell. Comment on the lamp and motor in terms of the maximum power of the fuel cell.

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experiment 10 - british columbia institute of...

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