Fuel Cell Fundamentals

Chapter 1. Introduction

1.1. What is a fuel cell?

Fuel Cell: ‘Factory’ that takes fuel as input and produces electricity as outputtransforms the chemical energy stored in a fuel into electrical energy

Hydrogen-oxygen (H2-O2) fuel cell

In a conventional combustion engine, fuel is burned, releasing heat

H2+1/2 O2 H2O +Heat

The energy difference between the initial and final statesReconfiguration of electronsRecovered as heat

To produce electricity:HeatMechanical EnergyElectricity(Complex and inefficient)

To produce electricity directly form the chemical reaction by harnessing the electronsThis is what fuel cell does!!

1. No bonds exists and the system has high energy

2. The system energy is lowered until the most stable bonding configuration

3. Further overlap between atoms is energetically unfavorable

1.2 A Simple Fuel Cell

H2 2H+ +2e- electrolyte 1/2O2+2H++2e- H2O

Load ( e.g. light bulb) is introduced along the path of the electronsthe flowing electrons will provide power to the load

FUEL CELL VS BATTERY

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FUEL CELL

BATTERY

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1.3. Fuel Cell Advantages

Electrochemical energy conversion devices: combine many of the advantages of both engine and batteries

Far more efficient than Combustion Engine

Lack of Moving parts: Silent

No undesirable products: NOx and SOx

1.4. Fuel Cell Disadvantages Cost: Major barrier to fuel cell implementation(economically competitive for a few highly specialized applications)

Should figure out problem related to volumetric power density

Hydrogen: difficult to store

Alternative fuels (gasoline..-high volumetric density) require reformingrequire ancillary equipments

1.5 Fuel Cell types

Five Major Types of Fuel Cells

1. Phosphoric acid fuel cell (PAFC)2. Polymer electrolyte membrane fuel cell (PEMFC) 3. Alkaline Fuel Cell (AFC) 4. Molten Carbonate Fuel Cell (MCFC) 5. Solid-oxide Fuel Cell (SOFC)

PEMFCs (thin polymer membrane)

H2 2H+ +2e-

1/2O2+2H++2e- H2O

H+ is the ionic charge carrier

SOFC (thin ceramic membrane)

H2+ O2- H2O+2e-

1/2O2+2e- O2-

O2- is the ionic charge carrier

Low temperature and high power density

High temperature (> 600 oC)

1.6. Basic Fuel Cell Operation

Conversion involves an energy transfer step

Finite rate: occur at an interface or reaction surfaceAmount of electricity scales with the amount of reaction surface area or interfacial area

Electrodes are highly porous

Anode: where electrons flow out

Cathode: Where electrons flow in

H2 2H+ +2e- HOR reaction

1/2O2+2H++2e- H2O ORR reaction

1. Reactant delivery into the fuel cell

2. Electrochemical reaction

3. Ionic conduction and electronic conduction

4. Product removal from the fuel cell

Step 1: Reactant Transport-When operated at high current, demand for reactants is voracious-Efficient delivery: Flow field plates in combination with porous electrode structures

Step 2: Electrochemical Reaction-Current is related to how fast the electrochemical reactions proceed-Obviously, sluggish reactions result in low current output-catalysts are designed to increase the speed and efficiency of electrochemical reactions

Step 3: Ionic (and Electronic) conduction-To maintain charge balance, ions and electrons should be transported easily-Ionic conduction: more difficultIons are much larger and more massive than electronsIons move via ”hopping mechanism” (less efficient)Electrolytes as thin as possible

Step 4: Product Removal-If products are not removed, eventually “strangle” the fuel cell

1.7. Fuel Cell Performance

An ideal fuel cell would supply any amount of current (sufficient fuels), while maintaining a constant voltage determined by thermodynamics

In practice, less than the ideal thermodynamically predicted voltage The more current, the lower voltage output

The current supplied by a fuel cell: proportional to the amount of fuel consumed

Maintaining a high fuel cell voltage under current load

Three irreversible loss1. Activation loss (electrochemical reaction)2. Ohmic loss (due to ionic and electronic conduction)3. Concentration loss (losses due to mass transport)

V=Ethermo-act-ohmic-conc

1.10 Fuel Cells and The Environment

“Hydrogen Economy”

At night and wind-stop, the fuel cells could be dispatched to provide on-demand power

Fossil fuels are completely removed

Homework

Chapter exercise

1.11.71.8