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LABORATORY NUMBER 11

ENERGY CONVERSION EFFICIENCIES IN AN ELECTROLYZER/FUEL CELL

Acknowledgement: This laboratory experiment is based upon materials developed by the Schatz Energy Research Center at Humboldt State University, as part of the H2E3 project supported by

the U.S. Department of Energy. 1.0 INTRODUCTION An electrolyzer is an electrochemical device that uses electrical energy to split water (H2O) into hydrogen gas (H2) and oxygen gas (O2). A fuel cell is another electrochemical device that does the opposite – it combines hydrogen and oxygen to produce water and electrical energy. Like all energy conversion devices, both the electrolyzer and fuel cell have an efficiency less than 100%, where the efficiency is defined as the useful energy output divided by the required energy input. Part of the energy input for each device is lost as heat during the energy conversion process. This heat is “wasted” since it is simply lost to the environment; thus, it constitutes the inefficiency of each device. An electrolyzer and a fuel cell can be combined with a renewable electricity source, hydrogen storage, and an electrical load to create a complete, stand-alone energy generation system with minimal environmental impact. Fuel cells and electrolyzers are not always found together. Many companies are developing fuel-cell powered vehicles. The hydrogen used in these vehicles may come from an electrolyzer, but it can also be derived by processing any hydrocarbon, such as fossil fuels or biomass. Electrolyzers are used in many industrial settings to produce hydrogen, which may be used for a number of purposes, including food processing or petroleum refining. In this lab, you will operate a bench-scale electrolyzer/fuel cell system. You will measure the efficiency of each step in the process of generating hydrogen with the electrolyzer and using it in the fuel cell to power an electrical load. The objectives of this lab are to: (1) explore the relationship between energy and power and learn to make measurements and calculations of energy and power; (2) gain understanding of energy efficiency and the Second Law of Thermodynamics; and (3) learn about hydrogen energy, fuel cells, and systems for generating and storing hydrogen fuel. 2.0 THE EXPERIMENT 2.1 EXPERIMENTAL MATERIALS AND APPARATUS The following equipment and materials are required for this laboratory experiment:

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For each lab group Fuel cell Electrolyzer Small fan for load DC power supply Two multimeters (or alternatively, measurements via data acquisition PC) Gas tubing and valves Stop watch Safety glasses (one for each student)

In the laboratory room

Room thermometer Barometer

The fuel cell and electrolyzer apparatus is shown in Figure 11.1, and consists of the following labeled components Main Components 1. electrolyzer hydrogen cathode 2. electrolyzer oxygen anode 3. hydrogen-side water columns 4. hydrogen supply tubing 5. U-tube electrolyzer 6. oxygen supply tubing

7. oxygen-side water column 8. hydrogen storage columns 9. fuel cell 10. oxygen storage column 11. fuel cell hydrogen anode 12. fuel cell oxygen cathode

Valves A. hydrogen storage purge valve B. hydrogen supply valve C. oxygen supply valve

D. oxygen storage purge valve E. fuel cell hydrogen purge valve F. fuel cell oxygen purge valve

An energy flow digram for this laboratory is provided in Figure 11.2. 2.2 TEST PROCEDURE Read this procedure and the below Required Results section carefully before beginning your experiment. This will help you to understand the purpose of the lab activity. Important: The liquid in the electrolyzer is a caustic base (potassium hydroxide) which has a pH above 12. There is a small chance or leaks of minor spills during the lab. Do not touch any visible liquid! If liquid gets into your eyes it can cause blindness and on the skin it will result in irritation. You must wear your safety glasses during this lab activity. Note also that the H2 gas produced during this experiment is highly flammable. Although only small amounts of gas are produced, make sure there are no flames or ignition sources in the area.

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2.1 SETUP 1. Make sure all tubes on your apparatus are connected as shown in Figure 11.1.

Make sure all valves are closed (with the valve lever perpendicular to the tubing). Record the room temperature and the room pressure. You will use these values to perform ideal gas calculations.

2. The water columns and gas storage columns should be partially filled with deionized

water. The glass electrolyzer U-tube should be partially filled with a 4M potassium hydroxide (KOH) solution. If you do not see liquid in the columns and U-tube, please consult with your instructor.

Figure 11.1 - Hydrogen fuel cell / electrolyzer experimental apparatus (see text for description of labeled componenets)

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3. Note how the electrodes are labeled on the electrolyzer and fuel cell in the photo.

Remember that the anode on a device is where current (i.e., positive charge or electron “holes”) enters. Since the electrolyzer receives current (from the power supply) and the fuel cell delivers current (to the load), the cathode and anode are in opposite positions on these two devices. Another way to describe this difference: in oxidation-reduction (redox) reactions that take place in an electrochemical device, oxidation always occurs at the anode, and reduction at the cathode.

2.2 MAKING H2 AND O2 GAS VIA ELECTROLYSIS 4. First you will operate the electrolyzer. To measure electrical power consumed by

the electrolyzer, connect one multimeter across the electrolyzer terminals to read voltage and a second multimeter in a series circuit with the electrolyzer and power supply to read current. See Figure 11.3 for terminal polarities. Set the voltage multimeter to read in a range of 0.1 Volts and the current multimeter to read in the range of 10 Amps (or the closest available setting). If directed to do so by your instructor, you may also use the data acquisition computer to measure electrical power consumption.

5. Open both storage purge valves (levers in line with the tubing). Plug the DC power

supply into an AC electrical outlet. Observe the production of gas bubbles in the electrolyzer. In which column are gas bubbles being generated faster – hydrogen or oxygen? Why? For your first run, generate and vent the gases for about 60 seconds (i.e., operate the electrolyzer with the storage purge valves open). Venting clears air out of the tubing, so that when the fuel cell operates, it will receive pure hydrogen and oxygen from the electrolyzer’s gas storage columns.

Figure 11.2 - Energy flow diagram.

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6. Close both storage purge valves (levers perpendicular to the tubing) and unplug the

DC power supply. Record the initial conditions at time 0:00 (milliliters of hydrogen to the nearest 1 ml, voltage and current), or begin the computer data acquisition procedure.

7. Plug in the DC power supply. Record the milliliters of H2 produced, voltage and

current at regular time intervals (if recording data by hand, time intervals of 30 seconds or 1 minute are appropriate). Watch gases accumulate in the columns and displace the water.

8. Generate at least 25 ml of gas. Unplug the DC power supply and record gas level. 9. Let the unit sit for 15-20 seconds while watching the gas levels. If the gas levels

change, this indicates you have a gas leak. Ask your instructor for assistance in eliminating the gas leak.

10. Repeat the electrolysis process at least two more times (after completing the

subsequent fuel cell trial in steps 11-17, below). Record the initial and final levels of H2 each time. Generate at least 25 ml of H2 for each run. You should not vent off the gas between runs.

Figure 11.3 - Polarity diagrams for electrolyzer and power supply.

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2.3 PERFORMING FUEL CELL LOAD TRIALS 11. Rearrange the two multimeters to measure voltage and current produced by the fuel

cell. For now, leave the series circuit that includes the fan motor open, by leaving one of the alligator clips unconnected. If directed to do so by your instructor, you may also use the data acquisition computer to measure electrical power. Refer to Figure 11.4 for fuel cell and fan motor terminal polarities.

12. Open the fuel cell gas supply valves. For the initial trial, air in the fuel cell must be

purged. Open and immediately close the fuel cell purge valve on the hydrogen side, then repeat on the oxygen side. If you purge for too long, you will lose all the stored gases in the columns. What happens to the fuel cell voltage? Why? If the fuel cell voltage is greater than or equal to about 0.7 Volts, then the fuel cell is ready to operate. If not, purge again until the voltage reaches 0.7 Volts. If the voltage does not reach 0.7 Volts, the fuel cell may be flooded.

13. Close the fuel cell supply valves. Record the initial volume of stored H2 and fuel cell

voltage before operating the load. 14. Hold the fan motor so the propeller can spin freely. If using the data acquisition

computer, start the data acquisition program. Complete the series circuit that includes the fuel cell, fan, and ammeter. Open the fuel cell supply valves. The fan should begin running. If collecting data manually, record every 30 seconds or 1 minute the time, volume of H2, and fuel cell voltage. Operate the fuel cell until you have consumed at least 25 ml of H2. If the voltage begins to drop quickly during the run, you will need to abort the run, purge both sides of the fuel cell, re-generate gases as needed (see Section 2.2), and start your load trial over again.

Figure 11.4 - Polarity diagrams for fuel cell and fan motor.

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15. Conduct at least two more trials (which will require completing steps 4-10 again). Use at least 25 ml of H2 for each run.

16. Close the fuel cell supply valves. Note: If you leave the supply valves open and

continue to consume gases, the fuel cell can create a vacuum and pull water into the line, which can flood the fuel cell and cause it to malfunction.

17. When you are finished, unplug the electrolyzer power supply from the wall and

disconnect the electrical leads from the voltmeter, the fuel cell, and the electrolyzer.

Important: Before leaving your lab station, release any stored hydrogen and oxygen from the storage columns and ensure that all the supply valves on your team’s kit are closed.

3.0 REQUIRED RESULTS I. Calculate the efficiency of the electrolyzer.

a. Calculate the amount of electrical energy used to produce hydrogen. Note that energy is the integral of power over time. If you recorded data by hand, you can calculate the electrical power (P=IV) used by the electrolyzer during each time period. By knowing the time over which this power was drawn, you can determine the energy used during this time period. Adding up the energy used during each time period will give the total energy used to produce the hydrogen. Alternatively, you can calculate the average power draw during the experiment and multiply this by the full time interval.

b. Calculate the amount of chemical potential energy created. Use the ideal gas

equation (PV = nRT) to calculate the moles of hydrogen produced during electrolysis. Use your observed values of ambient temperature (in absolute units), ambient pressure, and volume, along with the appropriate universal gas constant, R, for your chosen unit system. (Note: Use the ambient room pressure for P, ignoring the slight pressure elevation caused by the weight of the water column on the hydrogen gas.)

The Gibbs Free Energy (a concept you may not have studied yet) represents the work required to cause a non-spontaneous process to occur. This term is commonly used to calculate the efficiency of electrolysis and fuel cell reactions. At an assumed temperature of 25°C, the ∆G for the water electrolysis reaction:

H2O H2 + ½ O2 Eq. 1 is +237 kJ/mole – i.e., 237 kJ is required to produce each mole of H2 gas. Using

∆G, calculate the amount of chemical energy stored in the hydrogen (Echem) during electrolysis. Use this value and the electrical energy consumption from I(a) above to determine electrolyzer efficiency.

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II. Calculate the efficiency of the fuel cell.

a. Calculate the amount of chemical energy used. Knowing the amount of hydrogen used to power the load, use the ideal gas equation and ∆G = -237 kJ/mole for the fuel cell reaction (the reverse of Eq. 1) to calculate the amount of chemical energy used by the fuel cell.

b. Calculate the amount of electrical energy produced by the fuel cell. Using your fuel cell current and voltage data, calculate the electrical power produced by the fuel cell. Compare this to the chemical energy used to determine the efficiency of the fuel cell.

4.0 DISCUSSION Submit your results in a form requested by the instructor. You should consider the following when writing your discussion. What is the average efficiency of your fuel cell? What is the average efficiency of

your electrolyzer? What is the “wire to wire” efficiency of this energy storage system, from electricity in to electricity out? Are you surprised by the value(s) you obtained?

Research charge/discharge cycle efficiency for a battery and compare this with the electrolyzer/fuel cell system. With this in mind, what arguments might there be for choosing a fuel cell vehicle over a battery electric vehicle?