0581.5271 Electrochemistry for Engineers LECTURE 9 Lecturer: Dr. Brian Rosen Office: 128 Wolfson Office Hours: Sun 16:00.

0581.5271 Electrochemistry for Engineers

LECTURE 9

Lecturer: Dr. Brian Rosen Office: 128 Wolfson

Office Hours: Sun 16:00

Fuel Cells

Important Definitions

• Coulombs – Unit of CHARGE• Amperes – Unit of CURRENT [Coulombs per second] • Volts – Unit of POTENTIAL [Joules per Coulomb]• Watt – Unit of POWER [Joules per second]• Joule – Unit of ENERGY [Watts x seconds]

– Watt-hour (Wh) is also a unit of energy [Watts x hours]

• POWER DENSITY – Rate of Energy Transfer per unit volume or mass [kW/m3 or kW/kG]

• • ENERGY DENISTY – The amount of energy

stored in a given system [kJ/m3 or kJ/kg]

Important Definitions Pt 2

Electrical Power Production

Chemical Energy Nuclear Energy

Heat

Combustion Fission Fusion

Steam Engine IC Engine

Mechanical Energy Thermoelectric

Electrical Energy (current)

Fuel Cells

Solar and Wind Energy

Fuel Cell vs. IC Engine

When to use a Fuel Cell - Temp

H

CHcarnot T

TT

Electrochemical Systems

Electrochemical Systems

Power Density of Systems

Power Density Cont’d

Fuel Cell-Based Technologies

Direct Liquid Fuel Cells

Liquid FuelFC

electronics

Ancillaries

Air

PortableTransportation

Stationary /Back-up

Common Types of Fuel Cells

Phosphoric Acid(PAFC)

Alkaline(AFC)

Polymer Electrolyte Membrane

(PEMFC)

Direct Methanol

(DMFC)

Solid Oxide

(SOFC)

Molten Carbonate(MCFC)

Types of Fuel Cells

Polymer Electrolyte Membrane(PEMFC)

Direct Methanol(DMFC)

Solid Oxide(SOFC)

Voltage = 0.6 V

Cathode Reaction

O2 + 4H+ + 4e- 2H2O

Air

Anode

Cathode

e-

Membrane Electrode Assembly

Gas Diffusion Layer

Gas Diffusion Layer

H2

Bipolar plate

Bipolar plate

Load

H+

Anode Reaction

2H2 4H+ + 4e-

NIST

PlatinumCatalyst

Carbonblack

PEM Fuel Cells

3kW, 48Vfuelcellstore.com

A small stack of about 10 cells

NMSEA

NREL

PEM Fuel Cell Stacks

Advantages of Fuel Cells

1. Higher efficiency compared to IC engines

2. Zero emissions at the point-of-use (PEM)

3. No moving parts in the stack, so quieter

Challenges Facing Fuel

Cells

1. Cost (materials, labor, economy of scale)

2. Durability (membrane, catalyst)

3. Lack of H2 Infrastructure: H2 is difficult to produce, transport, and store

Pro’s and Cons of PEM FCs

Pro’s and Con’s Pt 2

• Advantages:– High efficiency– High energy density– Fuel flexibility– Environmental clean

• Limitations:– Fuel Crossover– Transport Issues– Water Management

(cathode flooding and anode dry-out)

Litster et al., Journal of Power Sources,130, 2004, 61

Direct Methanol Fuel Cell (DMFC)(acidic conditions)

Dramatic increase in public and private investment since 1983 as shown by the steady rise in Automotive Fuel Cell Patents

Progress Over the Years!

Reminder: Thermodynamic Potentials

QRTnFEG

SSS

GGG

HHH

rxn

reactprodrxn

reactfprodfrxn

reactfprodfrxn

ln

,,

,,

Reminder: Products-Reactants

coeffstoichtsreac

tsreac

coeffstoichproducts

productsrxn XXX .

tantan

.

Fuel Cell Efficiency

FEfuelvoltagethermoreal

E

Vactualvoltage

red

oxO

a

a

nF

RTEE log

sec

molsfuel

fuelfuel v

nFi

Etotal

Euseful

H

G

rxn

rxnthermo

Where..

Where..

concohmiccactaactOCPFuelCell EE ,,

Overpotential Losses in Fuel Cell

Total Energy Lost as Heat

nF

HEH

Note that EH is not a real potential,but instead a theoretical potentialassuming 100% thermodynamic

Efficiency. Therefore, EH will be higherthat the open circuit potential.

Power Density Curve

Activation Region

+

=

Exchange Density for H2 Oxidation

Exchange Density’s for PEM

Recall: Activation Barrier

RT

F

da

RT

F

Oxc

a

c

eCnFAki

eCnFAki

)1(

Re0

0

Current at Sacrifice of Voltage

Great catalyst

Good catalyst

Poor catalyst

Ohmic Region

Ohmic Overpotential

• j = charge flux • σ = conductivity

j = σ (dV/dx) = σ (V/x) V = i(L/Aσ ) = iR

ηohm = i(Relec + Rionic) ≈ iRionic

Rionic is on the order of 500-1000 mΩ

Charge transport between the electrodes is not a frictionless process,

therefore, there is a penalty of a loss of FC voltage

V=iR=jL/σ

xL

V

Direction of Voltage Drop

Anode (-)

Cathode (+)

Anode (-)

Cathode (+)

The voltage drop MUST be negative from the anode to the cathode in order to providethe driving force for migration towards the cathode.

Requirements for Electrolyte

• High ionic conductivity• Low electronic conductivity• High stability for oxidation and reduction• Low fuel crossover• Mechanical strength• Easily manufactured

Structure of Membranes

Dow Membrane (EW=800)

Nafion 117 (EW=1100) Nafion 105 (EW=1000) Membrane C (EW=900)

CF2

F-C-CF3

CF CF SO H2 32

CF2

CF2

SO H3

m n(CF CF)2 (CF CF )2 2 (CF CF)2(CF CF )2 2 mn

OO

O

Membranes containsphase-separated regionson the order of 5 nm in diameter

These swell with water uptake.

Sulfonic Acid (SO3- H+) Membranes

Polarization Curves for 3M MEA (hydrogen fuel)

Endurance (Durability) Test Results for Gore Primea 56 MEA at Three Current Densities (hydrogen fuel)

Laminar Flow Fuel Cells (LFFC)

DiffusionDepletion Depletion

Mass Transport Region

Mass Transport at PEM Anode

Depletion Still Occurs: Harder to Model

A Familiar Solution

Constant Flow or Stoichiometry

• Constant Flow – The flow rate of fuel is constant regardless of the required current. Typically enough fuel is provided for maximum power

• Constant Stoichiometry – The fuel is provided for 100% fuel utilization at a given current density (λ=1). λ = 1.5 means 50% extra fuel is provided

Constant Flow or Stoichiometry

Concentration Affects Nernst Voltage

.

ln

coefftricstoichiome

a

a

nF

RTEE

i

prod

prodO

i

i

treac

treacconc C

jC

nF

RT

tan*

tan)(ln

Concentration Affects Nernst Voltage

jj

j

C

jCnDF

jCjC

nFD

jC

nFDCj

L

L

L

L

*

)(

*)(

*

*

jj

j

nF

RT

L

Lconc

ln

Concentration Affects Rate of Reaction

Triple Phase Boundary High porous materials to achieve intimate contact between gas phase pores, electrically conducting catalyst and ionic conducting electrolyte

-Mechanical Strength-Electrical conductivity-Low corrosion-High porosity-Easily manufactured-High j0

A mixture of fuel and oxidant flows through a fully porous anode-electrolyte-cathode assembly. CMR cells require selective electrode catalysts (Ru/Se), the need for key enabling materials in direct methanol fuel cell development programmes.

www.cmrfuelcells.com

Compact Mixed Reactors (CMR)

Overall Analysis of Losses

Resistors in Series