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 water as the only product. 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. A fuel cell converts chemical energy directly into electrical energy whereas in a conventional power plant the chemical energy is converted into heat energy then to mechanical energy and finally into electrical energy.
HOW DOES IT WORK?
A fuel cell (Figure 1) consists of two platinum coated carbon electrodes bonded to a Proton Exchange Membrane (PEM).
Figure 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 2.
Figure 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.
The membrane electrode assembly consists of two porous carbon-cloth electrodes bonded to each side a polymer electrolyte membrane. This material, the electrolyte, allows the conduction of hydrogen protons from one side of the membrane through to the other. At the same time it prevents oxygen molecules from flowing in the reverse direction.
On the electrodes are nano-sized particles of platinum. The platinum acts as a catalyst for the redox reaction to take place. Initially the hydrogen molecules are chemically adsorbed onto the platinum surface, forming hydrogen-platinum bonds. Platinum is unique in that it has the ideal bonding strength to both break the hydrogen molecule bond, to form the hydrogen-platinum bonds, while being able to release the hydrogen, allowing the redox reaction to proceed.
A fuel cell therefore supplies a current, much the same way as a battery. However, unlike a battery a fuel cell can provide electrical power indefinitely provided hydrogen and oxygen are supplied.
The reactions occurring in the cell are:
Oxidation reaction at the anode:
2 H2(g) ® 4 H+(aq) + 4 e- E° = 0.00 V eqn (1)
Reduction reaction at the cathode:
O2(g) + 4 H+(aq) + 4 e- ® 2 H2O() E° = 1.23 V eqn (2)
Hydrogen fuel is fed to one electrode where the reaction results in it losing electrons. The electrons travel through the external circuit while the protons, 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() E° = 1.23 V eqn (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 theoretical maximum amount of non-expansion work (e.g. electrical energy) available is the free energy change for the cell, DG (DG = -nFE). The maximum energy released as heat when a fuel is burned at constant pressure and temperature is the enthalpy change, DH. One of the measures used to evaluate the potential viability of a fuel cell system is the efficiency value, e=DG/DH.
WHERE IS THE SOURCE OF 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() ® 2 H2(g) + O2(g) E° = - 1.23 V eqn (4)
generates the gases that are necessary for the fuel cell reaction (eqn (3)). Note that equation (4) is the reverse of equation (3). Thus, the theoretical decomposition voltage of 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 and the actual decomposition voltage is called overpotential or overvoltage. 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 minimum voltage 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.
THE METHANOL FUEL CELL
The methanol fuel cell uses methanol as fuel. Figure 3 shows a schematic diagram of the methanol fuel cell.
Figure 3 - 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.
At the anode methanol is supplied. At the cathode, oxygen from air is fed in.
Oxidation reaction at the anode:
CH3OH() + H2O() ® CO2(g) + 6 H+ (g) + 6 e- eqn (5)
Reduction reaction at the cathode:
O2(g) + 4 H+(g) + 4 e- ® 2 H2O() eqn (6)
The overall cell reaction is
CH3OH() + 1.5 O2(g) ® CO2(g) + 2 H2O() E° = 1.21 V eqn (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.
Part A - The Electrolyzer
Purpose:
1. Determine the current and voltage relationship in the electrolyzer.
2. Determine the minimum voltage for the electrolysis of water.
Procedure:
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 1 mL mark. Be careful to not get water into the flexible tubes leaving the electrolyzer.
3. Connect the electrolyzer, power supply and load measurement box (Set to “short circuit”).
4. Slowly turn the voltage of the power supply up to 1.3 volts and record both current and voltage readings on the Load Measurement Box.
5. The power supply current and voltage knobs are very sensitive. Try to adjust the voltage and current knobs so that you can obtain a reading at either 0.1 volt increments or 0.05 A increments whichever occurs first beginning at 1.3 volts.
DO NOT LET THE CURRENT EXCEED 0.5 A, IT WILL DAMAGE THE ELECTROLYZER.
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.
Part B - Faraday Efficiency of the Electrolyzer
Purpose:
1. Determine the 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:
ŋ = Volume (H2)exp / Volume (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 96,485 coulombs (Faraday’s constant, F). If the temperature and pressure are measured then, the volume of 1 mole H2 (g) at can be calculated from the ideal gas law.
The theoretical volume, Volume (H2) theoretical, produced:
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%). The Faraday efficiency can be less than one for the following reasons:
1. Competing simultaneous electrochemical reactions in the fuel cell, which supply fewer electrons for the same volume of hydrogen consumed.
2. Chemical reactions between hydrogen and oxygen at the catalysts.
3. Hydrogen and oxygen recombination or diffusion by leakage through the electrolyte membrane.
A low efficiency would be a great disadvantage, since it would not only shorten the service life of the electrolyzer, but also necessitate a higher energy input.
Procedure:
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. Record the initial volume of H2.
4. To measure the efficiency of the electrolyzer, select two currents from Part A. Select one voltage with a small current (~ 0.20 A) and a second with higher current ( ~ 0.45 A).
5. Set your power supply to the selected current flowing to the electrolyzer. Note: It is best to operate the power supply in constant current mode. Record the voltage and current.
6. Close the tube on the hydrogen side with a tubing stopper and start your timer.
7. Record the volume of hydrogen gas produced in about 180 seconds. Record the actually time used.
8. Remove the tubing stopper on the hydrogen to let the water refill the storage cylinder.
9. Close the tube on the hydrogen side with a tubing stopper.
10. Repeat steps 6 and 7 for the second current selected.
11. Record the room temperature and pressure.
Part C - The Characteristic Curve of the Hydrogen Fuel Cell
Purpose:
1. Determine the voltage profile ("Voltage vs. Current") of the hydrogen fuel cell.
2. Determine the power curve of the hydrogen fuel cell.
Figure 4 - A schematic diagram of a fuel cell.
Figure 4 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.
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).
Procedure:
1. Connect the electrolyzer to the fuel cell with the patch cords provided. Connect the red terminal of the electrolyzer to the red terminal of the fuel cell.
2. Connect the fuel cell to the voltmeter and the ammeter to measure both voltage and current produced by the fuel cell at the same time. (Connect the red terminal to the red terminal and the black terminal to the black terminal.)
3. Set the power supply to deliver 0.45 A at a constant current.
4. Fill the gas storage cylinders of the electrolyzer with distilled water to the 1 ml mark.
5. 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.
6. Purge the system for 5 minutes.
7. Measure the open circuit voltage of the fuel cell.
8. Record the voltage and current of the cell when the cell is subjected to different loads:
200 Ω, 100 Ω, 50 Ω, 10 Ω, 5 Ω, 3 Ω, and 1 Ω.
5. Determine the power output for the different loads, for the lamp and the motor.
Part D - The Efficiency of the Hydrogen Fuel Cell
Purpose:
1. Determine the Faraday efficiency of the hydrogen fuel cell.
The Faraday efficiency, ŋ, is determined from the volume of hydrogen theoretically need to produce the measured power divided by the volume of hydrogen actually used:
ŋ = Volume (H2)theoretical / Volume (H2)exp
where the Volume (H2) theoretical is calculated as in Part B.
2. Determine the energy efficiency of the fuel cell.
The theoretical maximum electrical energy available in any electrochemical cell is the free energy change for the cell, -DG (DG = -nFE). The maximum heat energy released when a fuel is burned is the enthalpy change, -DH. This allows one to calculate the energy efficiency in two ways. It can be expressed in terms of the electrical energy obtained from the fuel cell compared to energy available from the combustion of the consumed hydrogen, ecell=VIt/DH. The other is what is the best efficiency that is possible for a given fuel cell system (set of redox reactions), etheoretical = DG/DH
Procedure:
1. Connect the electrolyzer and fuel cell as in Part C.
2. Set the load resistor to “open”.