Module 2

Fuel Cells and the Hydrogen Economy

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Sustainable Energy Workshop for Science and Technology Teachers (SEWFSTT)

Fuel Cells

Discussion: Prior Exposure with Fuel Cells

(outside of the automotive industry)

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2

Fuel Cells come in all shapes and sizes

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8

4

3

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Basic Definition of a Fuel CellElectrochemical Energy Conversion Device

Different Kinds of Fuel Cells

Energy Conversion: PEM Fuel Cell

Reduction-Oxidation Rxn (redox)

eHH 442 2

OHeHO 22 244

ProductOxidant e

eProductReductant

energyOHOH 222 22

Anode Half-Reaction

Cathode Half-Reaction

Hydrogen and oxygen are combined in a non-combustion process

Electricity, heat and water are produced

Electric Potential Developed• Redox potential is a measure of a substance’s electronegativity

(affinity for electrons)• “Downhill” in Energy Diagram -- Free energy• Don’t have oxidation reaction without reduction reaction

present at same time (matched set)• Nernst Equation

• Calculation of electric potential in “non-ideal” circumstances

eHH 442 2

OHeHO 22 244

Oxidation Half-Reaction

Reduction Half-Reaction

SHEVE 00

SHEVE 229.10

Proton Exchange Membrane Fuel Cell(Polymer Electrolyte Membrane)

Between the reduction and Between the reduction and oxidation stages, the oxidation stages, the electrons are electrons are routedrouted through through a circuita circuit

eHH 442 2

Hydrogen ions (protons) Hydrogen ions (protons) permeate through the permeate through the electrolyte membraneelectrolyte membrane

Reduction reactionReduction reaction

Oxidation reaction facilitated Oxidation reaction facilitated by a catalyst by a catalyst - typically Pt - typically Pt ($$$)($$$)

OHeHO 22 244

1.23 V

Power Produced – Watts/m2

• Activation Loss– potential difference above

the equilibrium value required to produce a current (depends on activation energy of the reaction)

– energy is lost as heat• Ohmic Loss

– voltage drop due to resistance of the cell components and interconnects

• Mass Transport Loss– depletion of reactants at

catalyst sites under high loads

PEM (Polymer Electrolyte Membrane)

• Polymers such as polyphenylenes, Nafion are used

• Water is a crucial participant in the process

•absorption of water increases the proton conductivity

•membrane is confined – not free to swell – pushes electrodes

Platinum needs to be placed to maximize surface area

Needs to be encased in engineered components

• Thickness of the membrane and catalyst in the PEM can vary …

• Example: catalyst layers containing about 0.15 milligrams (mg) Pt/cm2

• thickness of the catalyst layer is close to 10 micrometers

•yields a MEA with a total thickness of about 200μm (or 0.2 mm or 20 sheets of paper)

•generates more than half an ampere of current per cm2 at a voltage of 0.7 volts

PEM (Polymer Electrolyte Membrane)

Design Goals: Limited Overview• Deliver Hydrogen• Deliver Oxygen

• Chemical reaction – what can influence rate of reaction

• Water Management• Maintain hydration levels• Remove water by-product

• Efficient path for electrons to ‘migrate’ to electrodes

• Thermal management

Parts of a Fuel Cell

Bipolar Plates

• Serpentine channels for hydrogen and oxygen to flow through device

• Acts as a current collector – electrons enter and exit cell through the plate

Anode

• Conducts electrons away from catalyst to external circuit

• Channels to supply H2 evenly to the surface of the catalyst

Cathode

• Channels to supply O2 evenly to the surface of the catalyst

• Conducts electrons back to catalyst for recombining

Parts of a PEM Fuel CellMembrane Electrode Assembly

• Anode

• Cathode

• PEM (Polymer Electrolyte Membrane)

• conducts only positively charged ions

• blocks electrons and other substances

• Catalyst

• thin coat of platinum powder applied to carbon paper or cloth

•maximizes surface area

• Backing Layers• porous carbon cloth conducts electrons

away from catalyst to external circuit• allows right amount of water vapor to

enter/exit• too much blocks the pores• membrane needs to be humidified

Schematic of Fuel Cell Operation

energyOHOH 222 2

1

1.2 V = theoretical maximum voltage generated by this reaction

Typical output = 0.7V – 0.9V ….. (1 W per cm2)

• Anode

Schematic of Fuel Cell Operation

Electron is stripped from Hydrogen as it makes contact with Pt catalyst which is embedded in a carbon nanoparticle

Electron conducted away through circuit

Hydrogen nucleus (proton) passes through PEM membrane to cathode

Hydrogen gas is circulated through ‘serpentine’ channels

Hydrogen from channels passes through porous medium

(gas diffusion backing)

Activity: PEM Fuel Cell Car (Pairs)• Outline:

– Produce hydrogen and oxygen via electrolysis

– Use stored H2 and O2 to generate electricity and drive motor

Educational Objectives for this Activity:

• Recognize H2 and O2 as portable fuels: same role as gasoline in an IC engine

• Recognize that a separate process is required to produce hydrogen

• Observation of the relationship between the volumes of displaced water in the hydrogen and oxygen tanks (and relationship to redox equations)

• Recognize that the hydrogen and oxygen produced came from initial injection of water

• Discussion of extension activities

Parts Identification• Battery components

– Battery pack– 2 AA Batteries– Connection Cable

Add batteries to battery pack

Two cylinders + 2 cup-like caps w/ long hoses attached

Incredibly important!

• Chassis

• Fuel Cell– Identify Hydrogen and Oxygen side

• H2 and O2 Storage Tanks

… but wait … there’s more …

Parts Identification• Hydration Components

– Syringe– Two short, narrow tubes with black and red caps– Short length of wide tubing– 90mL of distilled water + cup

Very important - needs to be distilled water (NOT

Purified water)

------ Why?

If you do not ALSO have a short length of wide tubing, you’re OK – just remove the black plug and use the narrow tube

Hydrate Fuel Cell

• Fill the syringe with distilled water and (gently) inject a small amount in to the LOWER nozzle on the HYDROGEN side

You will see the water fill in the fuel cell – you can go all the way until the water pours out the top

nozzle.

• Remove the syringe and insert the tube with the black cap in the LOWER nozzle

GENTLY

Hydrate Fuel Cell

• Fill the syringe with distilled water and (gently) inject a small amount in to the LOWER nozzle on the OXYGEN side You will see the water fill in the fuel cell – you can

go all the way until the water pours out the top nozzle.

• Remove the syringe and insert the tube (red cap) in the LOWER nozzle

O2

GENTLY

Prepare to Generate Hydrogen and Oxygen• Insert the cup-like caps in to

the Hydrogen and Oxygen tanks

• Fill the tanks to the zero (0) mL mark with distilled water– Suggestion: Use the syringe (each will take about 30mL)

• Align the notch in the cap with the gap in the tank

– we want to allow trapped air to escape when we fill the tanks with water

0 mL

Prepare to Generate Hydrogen and Oxygen

Connect the tank hoses to the upper nozzles on their respective sides (i.e. Hydrogen tank to Hydrogen nozzle)

Don’t forget to connect the Oxygen side too!

Prepare to Generate Hydrogen and Oxygen

Connect battery pack to connector

Connect banana plugs to fuel cell (black to black, red to red)

Make sure battery pack is turned off

Don’t turn it on yet …..

Double -Check

Double check connections

Turn on the battery pack and observe the production of H2 and O2

(Black-to-Black, Red-to-Red)

Disconnect Battery Pack

You now have a full ‘gas tank’ and a flow-through battery

Need a DC motor and wheels to drive a car

Transfer Assembly to Car and Connect Motor• As a unit,

transfer the gas tanks and the fuel cell to the car chassis

• Connect the banana plugs from the motor to the fuel cell (black-to-black) to begin operation

Be careful moving the tanks – a leak at this stage means you are “out of gas”!

Reflection

• Amount of hydrogen and oxygen produced during electrolysis

• Source for all this power – the original fuel?

• Moving the gas tanks to the car – Production of H2? Where?

Activity: Construct a PEM Fuel Cell

• A small, single cell, PEM fuel cell can easily be constructed

• Source of hydrogen needed• Chemical reaction• Fuel cell production + storage

• Kits: • Helpful hint: accordion

Activity

Comments• Fuel cells need to be hydrated in

order to run properly, if a fuel cell has been sitting un-used for a long time it may need a soaking rest to re-hydrate. Try placing it in a plastic bag with a wet towel for a few hours

Educational Objectives – Curricular Connections

Discussion: Available Animations

Energy Chain for Fuel Cells

Something is missing here ….

Are there associated societal issues associated with fuel cells (power generation &/or propulsion?)

Energy Chain for Fuel Cells

?!

Multiple Instructional Levels• Exposure/Exploration

• Process understanding• Chemical Reaction

– Rate of reaction: dependence on pressure, temperature, etc.

• Load Impact on Cell Efficiency

• High voltage vs. low voltage applications

• Activation energy

Activity: Fuel Cell Stack

• Exploration of a pressurized fuel cell stack

Different From a Battery?Redox (Oxidation-Reduction Reaction)

Baghdad Battery – 250 BC

Lead-Acid Batteries• e.g. car batteries, deep-cycle batteries

Energy-to-weight ratio very low

Energy-to-volume ratio: low

But ….Power-to-Weight ratio: LARGE

RECHARGABLE

)(2)(4)(24)(2

)(4)(24)(

224:

2:

lsaqs

saqs

OHPbSOeHSOPbOCathode

ePbSOSOPbAnode

(2V per cell)

Similarities and Differences

Similarities

• Chemical potential energy converted in to Electric potential energy

• Cellular structure

• Redox reactions

Differences

• Passage of H2 and O2 thru vs. storage of chemicals in battery

• Flow battery

Cellular Structure of Fuel Cells• Batteries in series

• Fuel cells are essentially flow-through batteries• Challenge is getting H2 and O2 uniformly to all of the cells

Stationary Power Facility: Stacks

5 PC 25TM Fuel Cells sited in Anchorage, Alaska (International Fuel Cells, LLC)

200-kW + 900,000 BTU heat

PAFC (Phosphoric Acid Fuel Cells)

5 kW Fuel Cell System, Manufactured by PlugPower, Installed at a USDOD Facility

Parts of a PEM Fuel Cell System

• Propulsion System

Automotive Application

Volkswagen’s HTFC

The Hydrogen Economy

• Hydrogen as a storage medium for energyProblem: Hydrogen does not occur naturally in nature as H2

The Hydrogen Economy• Infrastructure

http://www.hydrogen.energy.gov/systems_integration.html

How does one go about developing a production, delivery and use system for an energy storage medium that is only in its infancy

Hydrogen Production and Delivery

Currently: Steam Reforming of Natural Gas

• Biological Water Splitting

• Photoelectrochemical Water Splitting

• Reforming of Biomass and Wastes

• Solar Thermal Water Splitting

• Renewable Electrolysis

Advantage: Hydrogen is storage medium – Production from a variety of sources

Community Adoption – Priming the Pump

Hydrogen Storage

• Pressurized Steel and Composite Tanks

• Hydrogen can cause metals to become brittle (not good!)

• Metal Hydride

• H2 is locked in another chemical

• Chemical reaction releases that metal

• Micropore Storage

• Buckyballs & nanoscale methods

Hydrogen is not an ‘energy dense’ fuel (need lots to go anywhere)

Metal Decorated Nanostructures

The Hydrogen Safety Movie

• It’s not what you may think

Discussion: Synthesis of Fuel Cell

Challenges for PEM Fuel Cells• Platinum: reduction of amt of material used = reduced cost

– Wikipedia: 2002 cost was $1,000 per kW

• Water management– Too little --- membrane dries up

– Too much --- pores blocked, efficiency drops

• Steady Fuel Supply– Controlling amount of incoming gas + pressure

• Poisoning of the anode by carbon monoxide• Temperature control

This technology is coming out of its infancy …..