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Fuel Cells

Nathan Nguyen, Feifei Peng, Melissa Puga, Edward Trujillo  |  Created: February 28, 2012  |  Last Modified: December 6, 2012

Summary:

IMPORTANT!!! Under no circumstances should an unsupervised minor perform the procedures described herein. All the following described experiments and methods should be supervised by an adult who is completely familiar with and takes full responsibility for all possible hazards.

General Information Essential Questions Bibliography Materials and Methods

Background for Teachers Videos Intended Learning Outcomes

Instructional Procedures Optional Activities & Extensions Assessment Plan

General Information:
Main Curriculum Tie:	Chemistry, Electrochemistry, Physics
Additional Curriculum Ties:	Algebra, Thermodynamics, Fluid Dynamics
Career Connections:	Alternative Energy systems will require expertise in a number of different disciplines and chemical engineers are ideally suited to work in these fields. Various other engineering disciplines are involved in fuel cell applications, such as mechanical engineering, electrical engineering, environmental engineering and material science engineering
Mean Time Frame:	Setup: 5 minutes Demonstration: 5 to 15 minutes Discussion: 10 minutes
Group Size:	Medium
Student Prior Knowledge:	Chemistry, Stoichiometry, Gas properties, Electricity

 

Essential Questions

• What are the components in a fuel cell?
• What fuels can be used in a fuel cell?
• What are the electrochemical reactions that are occurring within a fuel cell?
• How much power can a single fuel cell produce?
• What are the essential components of a fuel cell?

 

Bibliography

1. Hoogers, Gregor (editor), Fuel Cell Technology Handbook, CRC Press, New York, 2003.
2. Thomas, Sharon and Marcia Xalbowitz, Fuel Cells – Green Power, Los Alamos National Laboratory, Los Alamos http://www.lanl.gov/orgs/mpa/mpa11/Green%20Power.pdf

 

Materials & Methods

• PEM fuel cell
• Distilled water
• Absorbent cloths

 

Background for teachers

The Fuel Cell basically converts chemical energy into electrical energy using a catalyst and a renewable fuel such as hydrogen. While there are several different types of fuel cells, the one that will be discussed here is the Polymer-Electrolyte-Membrane or Proton-Exchange-Membrane (PEM) fuel cell operated at relatively low temperatures. This module will explain how a hydrogen fuel cell works.

The main components of a PEM fuel cell are the electrodes (anode and cathode) and the membrane separating the two. The membrane is made of nafion which is similar to teflon, but has a sulfonic acid compound attached to the end of the carbon chain.

This diagram shows the structure of nafion.

The electrodes are made up of porous carbon which allows the diffusion of gases through them. There are also nano-sized particles of Platinum embedded within the porous carbon which serve as catalyst particles. On one side of the fuel cell is the anode, where hydrogen gas diffuses through the electrode and is decomposed into hydrogen atoms and electrons.

2H₂ → 4H⁺ + 4e^-

(1)

The positively-charged hydrogen atoms diffuse through the membrane but the electrons cannot and thus the electrons must travel through another route - the circuit. Once the positively charged hydrogen atoms reach the other side of the membrane, they encounter the cathode. Within the cathode are oxygen atoms, which are diffusing to the membrane from air circulating outside the cathode, and electrons coming from the anode through the electrical circuit. These components combine to form water.

4H⁺ + 4e^- + O₂ → 2H₂O

(2)

Thus the only products are electrons (electricity) and water. There is a lot of science and engineering involved in the design of the electrodes and the membrane and the objective of current research is to make these products more efficient and more economical.

This diagram shows the process of a fuel cell.

The theory behind the electrolysis of water and the fuel cell is also explained in several YouTube videos found in the section below and from the references listed in the bibliography above.

The energy conversion of a fuel cell is simply

Chemical engery of fuel = Electric energy + Heat energy

(3)

The ideal conditions of a fuel cell using H2 gas should provide 1.16 volts at I = 0, 80 C and 1 atm pressure. A great way to measure the efficiency of a fuel cell is the ratio between the actual and the ideal fuel cell outputs.

η = ^{Actual Voltage} ⁄ _{Ideal Voltage}

(4)

Where η = Efficiency

The remaining energy will translate as heat. To see how efficiency is affected, a characteristic performance curve is used. For a fuel cell, the performance is measured by how much DC voltage is being delivered as a function of current density. Current density is the total current divided by the area of the membrane.

To measure the power output of a fuel cell, take the product of the current drawn and the terminal voltage.

P = I V

(5)

Power is the rate at which energy is being transferred.

P = ^ΔE ⁄ _{Δ t}

(6)

When working with a fuel cell, it is important to note the mass and the volume of the system. The mass of the cell can be related to the power produced by a cell by taking the ratio of the two in order to obtain the specific power. The ratio between the power produced and the volume of the fuel cell results in current density. To optimize the system, a high specific power and current density is necessary, so the volume and the weight must be minimized.

In order to predict the maximum voltage in a fuel cell process, the energy differences between the initial and final state must be evaluated. These evaluations rely on thermodynamic functions. For a fuel cell, Gibbs free energy will be used. To determine what the maximum cell voltage is, we will use the equation

ΔE = ^-ΔG ⁄ _(n*F)

(7)

Where ΔE is the cell voltage, ΔG is Gibbs free energy change for the reaction, n is the number of moles of electrons involved in the reaction per mole of H₂, and F is Faraday's constant which is 96,487 coulombs (joules ⁄ volt), the charge transferred per mole of electron.

Assuming standard temperature and pressure (STP) the Gibbs free energy can be obtained by

ΔG = ΔH - T*ΔS

(8)

Where ΔH is -285 kJ and ΔS is -163.2 ^kJ ⁄ _K

Where ΔG will be -237.2 kJ

Taking our ΔG we just obtained and assuming our system to be a hydrogen/air fuel cell at STP, the cell voltage results in

ΔE = 1.23 V

Again, the ideal conditions include STP with pure oxygen and dry gases. Under normal circumstances, the fuel cell is exposed to changing temperatures and pressures. An additional correction must be made for the humidified air and hydrogen.

To calculate the power due to heat, simply take the difference between the total power generated and the electrical power.

P_Heat = P_Total - P_Electrical

(9)

P_Total = V_Ideal * I_Cell

(10)

P_Electrical = V_Actual * I_Cell

(11)

 

Videos

• YouTube Videos:
• http://www.youtube.com/user/UnvUtahChenEng

 

Intended Learning Outcomes

• Understand how a PEM fuel cell works and the components that go into its design.
• Develop an understanding of the chemical nature of materials and how those material can be designed to propagate ions and electrons to generate electricity.
• Develop an understanding of catalytic reactions and electrochemistry.
• Understand the principles of electricity-current, voltage, amperes, resistances, open circuits.

 

Instructional Procedures

Dismantling
• Untighten the four nuts and remove the four recessed hexagon-head screws holding the cell together.
• Dismantle the cell. You are left with two housing plates and the proton-conductive membrane.
• Carefully remove the electrical terminals from the housing plates together with the membrane.
• Carefully remove the membrane. The electrodes remain attached to the perforated plates.
• Screw the fittings out of the housing plates.

 

Optional Activities & Extensions

• Allow students to use a fuel cell in a system (See Fuel Cell Car module).
• Connect a hydrogen generator to the fuel cell in order to produce a current.
• Measure current and voltage as a function of time.

 

Assessment Plan

Possible questions to be used for assessment:

Select the check box next to the questions you wish to use; then hit the submit button at the bottom of the page to create your worksheet.

Select all

Q1. What is an electrochemical reaction?

- A1.

An electrochemical reaction is a chemical reaction that transfers electrons through a system. The system consists of an anode, cathode and salt bridge.

 

Q2. What does PEM stand for and what is the main purpose of a PEM?

- A2.

Proton Exchange Membrane or Polymer Electrolyte Membrane. A PEM is a semipermeable membrane designed to allow protons to conduct while being impermeable to gases.

 

Q3. Which side will the H+ ions reside in a electrochemical system?

- A3.

The H+ ions migrate through the proton-conductive membrane to the cathode side.There, the electrons travel from the anode to the cathode side.

 

Q4. From the efficiency curve for a single fuel cell unit, how does the efficiency change based off of the current density and why?

- A4.

The voltage of a single fuel cell decreases as the current density increases because of equation (5).

P = I * V

In order for the power output of a fuel cell while increasing the current, the voltage must decrease in order for the equation to stay true. Efficiency is proportional to voltage, therefore resulting in a decrease in efficiency.

 

Q5. Assume a 200 cm² fuel cell is operating under the conditions of 80°C with a pressure of 1 atm. This fuel cell is running at 0.9 V while generating a .7 A/cm² of current. Estimate the heat generated by this cell.

- A5.

Use Equation (9), (10) and (11) to calculate the heat lost.

P_Heat = P_Total - P_Electrical

Where:

P_Heat = (V_Ideal * I_Cell) - (V_Cell * I_Cell)

P_Heat = (V_Ideal - V_Cell) * I_Cell

V_Ideal = 1.16 V

V_Cell = 0.9 V

I_Cell = (Area)*(Current Density) = (200 cm²)*(0.7 A/cm²) = 140 A

P_Heat = (1.16 V - 0.9V) * 140 A

P_Heat = 0.26 V * 140 coulombs/sec. * 60 seconds/min.

P_Heat = 2184 J/min.

 

    

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If you have a question regarding this teaching module or any other,
please feel free to contact Professor Butterfield, tony.butterfield@utah.edu.