EXPERIMENT 7-Addtion

EXPERIMENT 7-Addtion

How Can We Use a Hydrogen Fuel Cell to Generate Clean Energy and Connect Chemistry to the Real World?

INTRODUCTION

A Brief Summary

This new fuel cell lab has been developed to illuminate connections between chemical energy, thermodynamics, electrolysis and a hydrogen based transportation economy. In this fuel cell lab, hydrogen (H2) and oxygen (O2) gases will be generated by electrolysis and then be put into a fuel cell to power a model car and a small cell phone vibrator. As shown in the equation below, electrolysis of water (H2O) requires an input of electrical energy to produce H2 and O2 whereas a hydrogen fuel cell converts the chemical energy stored in H2 and O2 into electrical energy. Since O2 and H2 react to form H2O, hydrogen fuel cells generate clean energy without any pollution to the environment.

In this lab, a fuel cell will be used to power small electric devices such as a model car or mini cell phone vibrator and the net energy output as well as energy efficiency of hydrogen fuel cell will be determined. You will learn how the fuel cell converts chemical energy directly into electrical energy, which can then be converted to mechanical energy to do physical work. The aim of this experiment is to use fuel cell to help you develop a tangible understanding of concepts introduced in the lecture, such as chemical and electrical energy, electrochemistry, and thermodynamics.

In your current experiment 7, only electrolysis of water in a beaker is discussed. This new addition focuses on the fuel cell which provides electric work. Before we start to discuss the experimental details, we review several basic concepts in the following to help you to understand the experiment. More information will be found in your prelab questions/answers on Mycourse website.

Electrochemistry and Hydrogen Fuel Cells

Electrochemistry, the study of interchange of chemical and electrical energy, primarily involves twoprocesses: the generation of an electric current from a chemical reaction and the opposite process, the use of an electricity to produce chemical energy.A battery is a device for generating electricity from chemical reaction; whereas an opposite device called electrolyzer can be utilized to generate chemicals from electric current such as electrolysis of water to produce O2 and H2.

A fuel cell is a type of galvanic cells (batteries) where spontaneous chemical reactions that convert chemical energy of a fuel (hydrogen, natural gas, methanol, gasoline, etc.) and an oxidant (air or oxygen) into electric energy. There are many types of fuel cells. In this lab we will focus on the study of hydrogen fuel cell that uses H2 as fuel and oxygen as an oxidant. Unlike a traditional battery that is a closed system, fuel cell operates with a continuous flow of reactants (fuel and oxygen) through the cell whereas a traditional battery eventually runs out of fuel or oxidants. In principle, a fuel cell does not run down or require recharging as long as fuel and an oxidizer are supplied while a battery’s current runs lower and lower till it can no longer supply electricity.

There are many types of fuel cells out there to choose from but the one that can demonstrate most of a fuel cell’s characteristics as well as be affordable, easy to use and safe is the PEM “reversible” fuel cell. Unlike the electrolysis of water using the electrolyte NaOH in experiment 7, A PEM fuel cell uses Proton Exchange Membrane (PEM)as electrolyte to separate the hydrogen protons and electrons to produce electricity. The “reversible” means that it serves as both an electrolyzer (for electrolysis of water to hydrogen and oxygen) and a fuel cell (that uses hydrogen and oxygen). You get the best of both worlds with this device as it mimics a rechargeable battery. However, the main difference is that the fuel is externally supplied and does not get used up in the process of generating DC electricity, and such fuel cell can continue to generate electricity as long as hydrogen and oxygen are available.

A “reversible” PEM fuel cell operates in two distinct modes – Electrolysis Mode and Fuel Cell Mode. There are also two chemical processes involved – oxidation and reduction.

Figure 1Figure 2

In the Electrolysis Mode (Figure 1) water is introduced to both sides of the fuel cell where it is electrolyzed into hydrogen at the cathode (negative) and oxygen at the anode (positive) by a small voltageless than 1.5 volts (called the water decomposition voltage). The water is oxidized into oxygen and hydrogen ions on the anode and then hydrogen ions on the anode pass through the membrane to gain electrons to form hydrogen at the cathode. Electrolysis of pure water requires excess energy in the form of over-potential to overcome various activation barriers. Without the excess energy, the electrolysis of pure water occurs very slowly if at all. This is in part due to the limited self-ionization of water because pure water has an electrical conductivity about one millionth that of seawater. Nevertheless, electrolysis can be accomplished. The following are the chemical reactions occurring during electrolysis:

Electrolysis Mode Reactions

In the Fuel Cell Mode (Figure 2) the process is reversed along with the polarities of the anode and cathode. As hydrogen flows into the fuel cell on the anode (negative), the hydrogen molecules lose electrons to form hydrogen ions on the anode and then hydrogen ions pass through the membrane to the cathode where oxygen molecules on the cathode (positive) gain electrons and combine with hydrogen ions to form water. The electrons, which cannot pass through the membrane, flow from the anode to the cathode through an external load such as a car motor or a mini cell-phone vibrator, which consumes the power generated by the fuel cell. The overall electrochemical process of a fuel cell mode is called "reverse electrolysis," or the opposite of electrolyzing water to form hydrogen and oxygen. Once again, here are the chemical reactions:

Fuel Cell Mode Reactions

Thermodynamics and Electrochemistry in Fuel Cells

(The related information can be found from Zumdahl, chapter 11.3)

Electric Work

Wel = -E q (E is fuel cell voltage in Volts, q is charge or transferredelectron quantity inColumbus),

Or Wel = -E n F (n is transferred electron quantity in moles, F is Faraday’s constant)

Remember that the electric work is done by consuming H2 fuel. For each mole consumption of H2 fuel, the transferred electron (n) is 2 moles.

Gibbs Free Energy

Based on thermodynamic laws, the maximum possible useful work obtainable from a fuel cell at constant temperature and pressure is equal to the change in Gibbs free energy of electrochemical reaction, so

ΔG rxn =Welmax = - E n F (Wel is measureed at a maxium cell potential or an open circuit potential)

This equation relates the electrochemistry with thermodynamic parameter. So for each mole consumption of H2 in fuel cell, we can determine the Gibbs free energy change by the electric work performed.

Efficiency of Fuel Cell

In hydrogen fuel cell, an overall reaction is that H2 and O2 are converted to H2O like in “combustion”. However, unlike a true combustion reaction, the energy is not transferred to heat, but to electric energy. Since the traditional method of measuring efficiency in combustion engines cannot be applied to a fuel cell, the most useful way of determining efficiency is by comparing the work done by the fuel cell with the energy released by the combustion of the fuel (the reaction enthalpy of an overall reaction):

η =| Wel /ΔH rxn|

The maximum fuel cell efficiency is ηmax =|Welmax /ΔH rxn|= |ΔG rxn /ΔH rxn|.

For combustion of each mole of H2 fuel under standard condition, the reaction enthalpy is known as

ΔH rxn= - 286 kJ/mol H2

Entropy Change

Fuel cell efficiency in providing electrical power supply is usually in the range of 35 to 55%, so difference between the theoretical energy (-ΔH rxn) and actual electrical work (Wel or -ΔG rxn) would generate heat and cause entropy increase of the fuel cell. By thermodynamic basics, we have

ΔHrxn = ΔGrxn + T ΔSrxn

Basic Concepts in Electrical Measurements

Ohm’s Law I = E / R (I is current through the conductor in Amperes, E is potential in Volts, R is the resistance of the conductor in units of ohms.)

Electrical Power P = E ×I (P is the electrical work per second, in unit of Watts)

PROCEDURE (WORK IN TEAMS)

Wear safety glasses at all times to protect eyes from injury. Although the fuel cell car set is supposed to be very safe, follow the instructions and do not use the materials for other purposes. When running the electric circuit, take care in handling the vibrator and the car motor.

Materials

Fuel Cell Car Kit / Special Equipment
Fuel cell / Circuit board containing:
Car motor attached to the / Switch
car chassis with 4 wheels / Cell phone vibrator
2 graduated cylinders (H2 and O2) / 0.1  resistor
2 inner chambers / Wires
2 short tubing with red or black caps / Multimeter
2 long tubing / 2 alligator clips
1 syringe
Battery Charger

This is a team collaboration experiment. Every member should participate and contribute equally to the testing of the fuel cell. For example, student A may set up the preparation; student B watch the change of gas volume and roughly record the time; student C control the battery charger and hydrolysis; student D take the electric measurement.

Figure 3below shows a photo of a fuel cell car with major parts labeled in order to give you an overall picture of what a fuel cell car looks like in this experiment.

1. Insert the fuel cell into the rectangular slot located on the car chassiswith “O2” “H2” symbols upside. Remove the small red cap from a short tube connected to the oxygen side of fuel cell. Use the syringe and inject about 1-2 ml of distilled water (no tap water) into the fuel cell through the short tube. Make sure that water passthrough theoxygen-side chamber in front of the screen to humidify the membrane and the whole chamber is wet with water (Figure 4). Record the starting time for humidifying the fuel cell on your notebook.
2. Fill the hydrogen and oxygen storage cylinders with distilled water up to the “0” line. Insert the inner cylinders into the outer cylinders. The inner cylinders should be filled with water without any gas bubbles at top.In case you cannot get any large bubble out by adding water into the cylinders, ask the TA for help. Also ensure that they fit firmly onto the plastic rim and the two notches (small openings) at bottom of the inner cylinders are not blocked by the inner plastic rims.
3. Firmly attach the long tubes onto the top nozzles of inner storage H2 and O2 cylinders (if they are not already attached) and the opposite ends to the lower nozzles located on the lower positions of the H2 and O2 sides of fuel cell, respectively. Make sure the tubes are connected correctly to the corresponding sides of the fuel cell.
4. Make sure that the battery charger is at “off” position. Insert the red banana plug from battery charger into the red banana socket on the fuel cell, and the black plug into the black banana socket. The colors must match. When reversed black and red wires accidentally connect to the power, you will destroy the fuel cell and you and your team members will have significant points off from your lab report grades!

Figure 3 A fuel cell car with a built-in electric circuit board for measuring voltage and current of a fuel cell.

Figure 4 Injecting water into the oxygen side of the fuel cell (on the left); connecting tubing (on the right)

1. Ask your TA to check your setup and check the time recorded in your notebook to see if you humidify your fuel cell for more than 5 minutes. Once TA approves and your fuel cell is humidified for more than 5 minutes, you can turn on the battery charger to initiate hydrolysis process. You will observe the hydrogen and oxygen gases coming from the fuel cell into the inner cylinders soon after electrolysis starts.Watch hydrogen and oxygen gases being generated and stored in the inner storage cylinders, the water levels in these inner cylinders would decrease.
2. Record the room temperature in your lab notebook while waiting for hydrolysis. When the inner chambers are completely filled with hydrogen, gas bubbles will come out from the bottom notchof the hydrogen cylinder (This is a good indication for the completion of the electrolysis since sometimes it is difficult to tell if H2 completely fill up the inner cylinder). When you first observe the H2 gas bubbles coming out from the inner cylinder, turn off the battery charger and disconnect the cables of a battery charger from the fuel cell. If it takes longer than 10 minutes to finish electrolysis, please notify TA.
3. The following step is to measure the open circuit potential to calculate the maximum work of the fuel cell. Write down the serial number of multimeter on your notebook (e.g. TE549254). Take the two probe cables from the multimeter (if a multimeter has no cables attached, you need to insert a black pin into the “COM” and red pin to “Vsockets on the multimeter); plug the black cable from “COM” into the hydrogen side banana socket and the red one from “Vinto the oxygen side of the fuel cell. Turn on the multimeter and set the scale to voltage “2V” range. Take three measurements within 20 seconds and record the voltage values up to 3 decimal points.
4. A circuit board (shown in Figure 3) with cell phone vibrator had already been mounted on the front of the car and connected to the car motor. Connect twopower plugs from the circuit board to the banana sockets on both sides of the fuel cell (color matching). Ask your TA to check your connection. To measure the voltage through the fuel cell while cell phone vibrator is running, use an alligator clip to connect between the positive probe cable from the multimeter to “V+” terminal on the circuit board and the negative probe cable to the “V-“ terminal on the board. The switch on the circuit board has a three positions: “Down” position for cell phone vibrator, “middle” for off position, and “UP” position for fuel cell car. Turn the switch on the circuit board to the “Down” position for running the cell phone vibrator. Take three voltage measurements within 20 seconds and record the values up to 3 decimal points in your lab notebook. To obtain the current during the fuel cell operation for running a mini cell phone vibrator, we measure the voltage across a fixed 0.1 resistor built in the circuit instead. The voltage of the fixed 0.1 resistor can be measured by connecting multimeter (in the same manner as the voltage measurement) to “VR+” and “VR-” on the circuit board. Change the scale to “200 mV” range in your multimeter. Measure the voltages in units of mV for three times within 20 seconds and record the values up to 1 decimal points in mV. From voltage and resistance, you can calculate the current using Ohm’s law
5. Finally, after all the measurements, it is time to play with the car! Check the volume level of the H2 and O2 gas in the inner cylinders. Turn the switch on the circuit board to the “UP” position to power the car motor and watch the hydrogen fuel cell-powered car running! Please turn the switch to the “middle” position to power off the car after 3 minutes!!! A fuel cell has a limited lift time and we need to save the time for other students. Watch the decrease in O2 and H2 gasses after running the car.
6. After finishing the experiment, turn off the switch and disconnect the cables. Put the fuel cell back into the small plastic bag and leave the short tubes connected with the fuel cell. Make sure the Ziploc bag is not broken (if necessary, obtain a new bag from the stockroom) and sealed well to keep the cell under humidified condition. Make sure that you put everything including tubes and small components back into the box. Before leaving, ask the TA to check the car kit.
7. Back to the classroom area for about 10 minutes'group discussion.Discuss andwork outoneof the following questions.The same questions are given in your lab report. Each group may submit the same answer summing up the ideas of all members for yourlab report.

a) Why efficiency (η, %) can hardly equal 100% in this fuel cell experiment?

b) List more than one differences between the electrolysis using the fuel cell and the electrolysis in Part A experiment 7.

Lab 7 Fuel Cell Lab Report Fall ‘11

How Can We Use a Hydrogen Fuel Cell to Generate Clean Energy and Connect Chemistry to the Real World?

You can type your lab report on a separate page or write by hand. Please attach the following pages to the front page of your lab report for experiment 7 Part A from the lab manual. You need to show all your work to get full credits. Your report needs to include:

Your Name______TA______Lab Section______

Group Members______

I. Describe the purpose of the experiment with a few sentences. You can find the information from the introduction, but do not copy them directly.