Fuel Cell Technology

Summer School in Energy and Environmental Catalysis

University of Limerick, July 2005

Energy – Mostly from Fossil Fuels

Significant proportion of Energy for Electricity (flow of electrons)

Fossil fuels electricity via combustion, generating steam, turning of turbines, etc.

Electricity from chemicals, i.e. convert energy generated during a chemical reaction (e.g. combustion) directly into electric energy.

Separate chemical reaction into two reactions, one generating (pushing) electrons and one consuming (sucking) electrons, flow of electrons between two reactions usable electricity

Dry Cell Batteries

Anode reaction:

Zn Zn2+ + 2e-

Cathode reaction:

2NH4+ + 2MnO2 + 2e- Mn2O3 + 2NH3 + H2O

Overall reaction:

Zn+2NH4Cl+2MnO2 ZnCl2+Mn2O3+2NH3+H2O

Battery runs down once Zn is corroded away

Lead Storage Battery

Anode reaction:

Pb + HSO4- PbSO4 + H+ + 2e-

Cathode reaction:

PbO2 + HSO4- + 3H+ + 2e- H2O + PbSO4

Overall reaction:

Pb + PbO2 +2H2SO4 2PbSO4 + 2H2O

•PbSO4 adheres to both anode and cathode and is converted into Pb (on anode) and PbO2 (on cathode) by forcing current the reverse direction (via the alternator)

•“Health” of battery measured by measuring density of electrolyte (changes as H2SO4 is consumed)

•Batteries fail if PbSO4 is shaken from electrodes (Pb or PbO2 cannot be regenerated).

Rechargable Battery:Anode reaction: Cathode reaction:

Cd + 2OH- Cd(OH)2 + 2e- NiO2 + 2H2O + 2e- Ni(OH)2 + 2OH-

Overall reaction:

Cd + NiO2 + 2H2O Cd(OH)2 + Ni(OH)2

Products adhere to electrodes and reactants can be regenerated by forcing current in the reverse direction

FUEL CELLS

Grove in 1839

“an electrochemical device which converts the free-energy change of an electrochemical reaction into electrical energy”.

Fuel + Oxidant Products + Energy

Hydrogen (or CH4 or CH3OH) and O2 (from air)

Like a battery it produces electricity using chemicals.

H2O2

- +e- e-

H2O2

e- e-

H2O + electricity H2 + O2 H2 + O2 H2O + electricity

Electrolysis Fuel Cell

BURN FUEL

THERMAL ENERGY

Electric Energy

KINETIC ENERGY

Conventional System

Fuel Cell System

Heat

HeatHeat

FUEL

Cathode Reduction / Anode Oxidation (CROA)

Operating Temperature

Fuel Applications (Present / Potential)

Alkali 50-250 °C H2 Used in space vehicles (Apollo, Shuttle)

Phosphoric

Acid ~220 °C H2 / CH4 Medium scale CHP systems (200

kW)

Molten Carbonate

~600 °C H2 / CH4 / CO Medium to large scale CHP systems. 1-2 MW

Solid Polymer

(PEM) 50 – 100 °C H2 / CH3OH Potential for transport.

Solid Oxide

(YSZ) 500-1000 °C H2 / CH4 / CO All sizes of CHP systems, 2kW - MW

Potentially very useful

Types Of Fuel Cell

e-

electrolyte

AlkaliH2 + 2OH-2H2O +2e

OH- O2 +2H2O + 4e 4OH-

Phosphoric AcidH22H+ + 2e

  H+   O2 + 4H+ + 4e 2H2O

Molten CarbonateH2 + CO3

2-CO2 +2e  CO3

2-   O2 + 2CO2 + 4e 2CO32-

Solid PolymerH22H+ + 2e

  H+   O2 + 4H+ + 4e 2H2O

Solid OxideH2 + O2-H2O + 2e

  O2-   O2 + 4e 2O2-

Anode Reactions Cathode Reactions

Chemical reactions in different fuel cells

Notes: Cathode Reduction of O2 (reaction of electrons), Anode Oxidation of Fuel (generation of

electrons).

ELECTROLYTES – solutions, molten salts and solid polymers / oxides

OH-

Anode / Oxidation

Cathode / Reduction

ELECTROLYTE = KOH (aq)

Charge Carrier = OH-

O2 +2H2O + 4e 4OH-H2 + 2OH-2H2O +2e

Alkali Fuel Cell

e- e-

H+

Anode / Oxidation

Cathode / Reduction

ELECTROLYTE = H3PO4 (aq)

Charge Carrier = H+

O2 +4H+ + 4e 2H2OH2 2H+ +2e

Phosphoric Acid Fuel Cell

e- e-

CO32-

Anode / Oxidation

Cathode / Reduction

ELECTROLYTE = Na2CO3(l)Charge Carrier = CO3

2-

O2 + 2CO2 + 4e 2CO32-H2 + CO3

2-CO2 +2e

Molten Carbonate Fuel Cell

e- e-

H+

Anode / Oxidation

Cathode / Reduction

ELECTROLYTE = “Plastic” MembraneCharge Carrier = H+

O2 +4H+ + 4e 2H2OH2 2H+ +2e

Proton Exchange Membrane Fuel Cell

e- e-

Same reactions / charge carriers as H3PO4 – different operating conditions

O2-

Anode / Oxidation

Cathode / Reduction

ELECTROLYTE = YSZ (same as sensor in TWC technology)

Charge Carrier = O2-

O2 + 4e 2O2-H2 + O2-H2O +2e

Solid Oxide Fuel Cell

e- e-

Medium term - Potential Uses for Fuel Cells

•Combined Heat and Power Plants – for apartment blocks. More efficient than electricity generating stations, Quieter than gas or diesel turbines, Inherent Reliability.

•Transport – such as cars / buses etc. Zero Emission Vehicles.

•Mobile / Portable power sources – e.g. instead of batteries for mobile phones / PCs / radio communications / military applications. A cartridge containing methanol would be used which would be equivalent to immediate battery recharging

• High Efficiency• Not load dependent• Zero emissions• No moving parts

FUEL CELL

INTERNAL COMBUSTION

• Low Efficiency• very load dependent• NOx, CO, HC,

particulates• Several moving parts

• Expensive• Low power density• H2 as fuel• Not well developed

• Cheap• High power density• Developed fuel infrastructure• Reliable

Processes in a PEM fuel Cell

• H2 2H+ + 2e-Oxidation on anode

• H+ travels through membrane to anode

• e- travels through circuit to anode (doing work)

• O2 reacts with H+ and e- to form H2O (and heat)

PEM Animations

Fuel Cell Stack using PEM fuel cells

Allows protons to move through the membrane – must be moist to operate

ANODE CATALYST (H2 2H+ + 2e-)

Pt/Ru/WO3/SnO2 on Carbon electrode.

Ru decreases CO chemisorption / SnO2 and WO3 enhance CO oxidation

CATHODE CATALYST (O2 + 4H+ + 4e- 2H2O)Pt/ Carbon with Co / Cr and Ni – all of which are incorporated into the FCC lattice of the Pt. This aids in the adsorption and dissociation of O2 (mechanism unclear).

Both electrodes are prepared from an aqueous slurry of the metals of interest (as salts) followed by drying, reduction and heat treatment

Electrodes must be porous in order to allow gas molecules (and H2O) through to the electrolyte

Fuel for powering fuel cells

CH3OH would be desirable Easily synthesised, lots of chemical energy released during

“combustion”LiquidCan be used but is limited

Direct Methanol Fuel Cell

CH3OH + H2O

CO2

Anode reaction: CH3OH + H2O CO2 + 6H+ +6e-

H+

H+

H+

H+

e- e- e- e- e- e-

O2

H2O

Cathode reaction;1.5 O2 + 6H+ + 6e- 3H2O

Problems 1 anodes not very active / stable

2 Methanol diffuses through electrolyte (short circuiting

the cell)

Overall Reaction CH3OH + 1.5 O2 CO2 + 2 H2O

Fuel for powering Fuel Cells

H2 is the fuel of choice since it is easily activated, produces only H2O as a by-product and does not harm the anode.

Hydrogen•Is abundant on earth

•Can be produced from fossil fuels (CxHy) or from Water (H2O)

•However it is a gas and therefore has very low energy density at STP

1 Litre Petrol 9,100 Wh

1 Litre H2 2.8 Wh

Storing H2 on Board

•Store in carbon nanotubes – tubes made from C60 units can reversibly store large amounts of H2 lighter than the metal hydrides but still too heavy

•Store as a Metal Hydride – Ti2Ni-H2..5, FeTi-H2 etc. which can be dehydrogenated as needed and regenerated when used – these are very heavy – since the empty “tank” will be full of dehydrogenated metal

•Liquefy – 20 K, 2 Bar – again size / bulk + expensive refrigeration etc. – extra cost

•no H2 economy / infrastructure

PHYSICAL STORAGE compress 200 - 600 bar – Size issues, safety issues, weight issues extra cost

CHEMICAL STORAGE

Far higher energy density if stored as another chemical and then “reformed” to H2 on board – Hydrocarbons / methanol / ammonia. Infrastructure is already in place

Reforming of Hydrocarbons (or Methanol) to H2

(A) Steam reforming:

CxHy + H2O H2 + CO + CO2

An endothermic process

(B) Partial Oxidation

xs CxHy + O2 CO + CO2 + H2

An exothermic process

(C) Auto thermal reforming – a combination of both approaches which is self-sustaining.

Followed in Both cases by water Gas shift to

(a) remove CO and (b) generate more H2

PROBLEMS

(a) as well as H2 significant amounts of CO are formed (2%) and these poison the anode – resulting in far decreased performance and eventually the fuel cell stops working

(b) the fuel must be free of sulphur as this poisons BOTH catalysts (reforming catalyst AND electrocatalysts)

Effect of CO in the reformate on anode performance

CO irreversibly adsorbs on the catalytic Pt Particles

Pt + CO Pt-COads.

Time / min

0 50 100 150 200 250

0.8

0.7

0.6

0.5

Cell

Po

tenti

al

/ [V

]

10 ppm CO

40 ppm CO

100 ppm CO

Time / min

0 50 100 150 200 250

0.8

0.7

0.6

0.5

Cell

Po

tenti

al

/ [V

]

10 ppm CO

40 ppm CO

100 ppm CO

0 10 20 30 40 50

Time / min

Cell

Po

tenti

al

/ [V

]

CO

co

ncentr

ati

on /

p

pm

Methods for dealing with CO in the reformate

(A)Improve the tolerance of the anode

This is done in 2 ways

• Alloy the Pt component with Ru. This reduces the CO chemisorption strength and therefore the CO Coverage

• Add an Oxide Component to promote the electro-oxidation of CO (SnO2 or WO3). This needs a slight air bleed and is not ideal.

(B) Selectively removing the CO from the Reformate

This can be done in 4 ways

• Selective oxidation – through adding O2

• Selective methanation using H2 present in the reformate

• Permeable membrane that allows H2 through but not CO

• Pressure swing adsorption

Needs High Pressure

CO selective oxidation

• Generate a catalyst that will

CO+O2 CO2 but not 2H2 + O2 2H2O

• Au/Fe2O3/TiO2

CO adsorbs selectively on Au

Oads delivered from Fe2O3 rather than from gas

mechanism prohibits H2H2O

e-

Air / O2H2 / H2O

N2, O2, H2OH2 / H2O

Cathode ExhaustAnode Exhaust

Anode Cathode

Need to generate H2

REFORMER

e-

Air / O2H2 / H2O

N2, O2, H2OH2 / H2O

Cathode ExhaustAnode Exhaust

Anode Cathode

Air

CH3OH

H2O

H2, H2O, CO2, N2,

CO

Needs to be removed

Excess H2 should be used (burn it to generate heat and add extra “power” to reformer)

REFORMER

e-

Air / O2

H2 / H2O

N2, O2, H2OH2 / H2O

Cathode ExhaustAnode Exhaust

Anode Cathode

Air

CH3OH

H2O

CO Clean Up

Air Bleed (for electro-oxidation)

H2, H2O, CO2, N2, CO

Anode Exhaust Burner

Start Up CH3OH combustion

REFORMER

e-

Air / O2

H2 / H2O

N2, O2, H2OH2 / H2O

Cathode ExhaustAnode Exhaust

Anode Cathode

Air

CH3OH

H2O

CO Clean Up

H2, H2O, CO2, N2, CO

Anode Exhaust Burner

REFORMER

e-

Air / O2

H2 / H2O

N2, O2, H2OH2 / H2O

Cathode ExhaustAnode Exhaust

Anode Cathode

Air

CH3OH

H2O

CO Clean Up

H2, H2O, CO2, N2, CO

CATALYSTS

To Battery, Car, CHP etc.