1

Brief Introduction to Fuel Cells,Hydrogen Production and Storage

Jens Oluf Jensen

Summer School on Materials for the Hydrogen Society,University of Iceland in Reykjavik, Iceland , June 26. to June 30. 2006

Materials Science Group, Department of Chemistry, Technical University of Denmark,Kemitorvet, building 207, DK-2800 Lyngby, Denmark

Outline

Electrolysis

Reforming

Microbial

Thermal

Photolysis

Production

Transmission

Storage

Intermediate Conversion

Fuel cells (electr.)

IC engines (mech.)

Burners (heat)

Energy Energy

Fuel Cells

IRD Fuel Cells A/S

Extraction of the energy

H2 +½O2 → H2O

H Oe-

H → H+ O → O2-

e-

Oxidation Reduction

2

Extraction of the energy

H2 → 2H+ + 2e-½O2 + 2e- → O2-

½O2 + 2H+ + 2e- → H2O

H+ →

(←O2-)(←OH-)

(←CO32-)

Electrolyte

Elec

trode

Negative electrodeAnode

(oxidation)

Positive electrodeCathode

(reduction)

Elec

trode

The available energy

H2(g) +½O2(g) → H2O(l)

Standard Enthalpy, ΔHºf,H2O = ΔHºcomb.,H2 = -286 kJ/molStandard Gibbs free energy, ΔGºf,H2O = ΔGºcomb.,H2 = -237 kJ/mol

Higher heating values (HHV)

H2(g) +½O2(g) → H2O(g)

Standard Enthalpy, ΔHf,H2O = ΔHcomb.,H2 = -242 kJ/molStandard Gibbs free energy, ΔGf,H2O = ΔGcomb.,H2 = -229 kJ/mol

Lower heating values (LHV)

The available energy

Maximum fuel cell efficiency:

Carnot efficiency:(any heat engine)

high

lowhigh

carnotT

TT !="

oFC

H

TG

!

!=

)("

Maximum efficiency

0,00

0,20

0,40

0,60

0,80

1,00

0 200 400 600 800 1000

Temperature (oC)

Max. E

ffic

iency

Carnot efficiencyTlow= 0, 20, 50, 100ºC

Eff. (LHV)

Eff. (HHV)

3

The cell voltage

V23.1=!"

=nF

GEMax. cell voltage: at 25ºC, 1 bar

n = no. of electrons, F = 96485 C/mol (Faradays constant)ΔG, dim: energy/mol = potential·charge/mol

0

50

100

150

200

250

300

0 200 400 600 800 1000

Temperature (oC)

-_G

, -_

H (

kJ/m

ol)

0

0,2

0,4

0,6

0,8

1

1,2

1,4

Cell

voltage (

V)

The theoretical cell voltage

ΔH

ΔGVoltage

Voltage losses

Polarization of electrodes (kinetic or activation losses)At the catalysts

Ohmic losses (IR losses)Mainly in the electrolyte

Mass transport lossesIn the electrode pores

0

0,2

0,4

0,6

0,8

1

1,2

0 0,5 1 1,5 2

Current density (A/cm2)

Voltage (

V)

The polarization curve

Polarization curve

Ideal voltage (25°C)

4

0

0,2

0,4

0,6

0,8

1

1,2

0 0,5 1 1,5 2

Current density (A/cm2)

Voltage (

V)

The polarization curve

Ohmic part

Chargetransfer

part

Mass transportpart

Cross over

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.5 1 1.5 2

Current density (A/cm2)V

oltage (

V)

0

10

20

30

40

50

60

70

80

90

100

Co

nvers

ion

eff

icie

ncy (

%)

The heat evolved

ΔH(T) HHV (LHV could have been chosen )

Electricalenergy

Heat evolved

Types of fuel cells

Type Abrev. Temp. Electrolyte Low temperature systemsAlkaline FC AFC 60-80°C aq. KOHPolymer FC PEMFC 60- 80°C Polymer Direct methanol FC DMFC 60- 80°C PolymerPhosphoric acid FC PAFC 200°C H3PO4

High temperature systemsMolten carbonate FC MCFC 650°C Molten saltSolid oxide FC SOFC 700-1000°C Ceramic

Types of fuel cells

Larminie and Dicks. Fuel Cell Systems Explained 2. ed. Wiley 2003

5

Hydrogen Production

Electrolysis Reforming4% 96%

Reforming

Steam reforming (SR)

(methane)CH4 + H2O → CO + 3H2

Highly endothermic ⇒ Use high temperatureΔN(gas) = 2 ⇒ use low pressure

Water gas shift (WGS)

CO + H2O → CO2 + H2

Exothermic ⇒ use low temperature

Reforming techniques

Key issue: how to provide the heat for reforming

1. Burn some fuel1. Primary fuel (e.g.methane, methanol)2. Anode off gas from fuel cell

2. Waste heat from elsewhere1. From fuel cell2. From elsewhere

Reforming techniques

1. Steam reforming (SR) CH4 + H2O → 3H2 + COHeat from external burner

2. Partial oxidation (POX)CH4 + ½O2 → CO + 2H2 (exothermic)1200-1500°C without catalyst(sulphur tolerant)Alternatively catalytic POX (CPO) at lover temp.

3. Autothermal reforming (ATR)A combination of 1 and 2 (heat balanced)CH4 + aO2 + bH2O → cCO + dH2

6

Reforming system

CO clean-up for PEM fuel cells

After 200ºC shift: say 0.2-0.5 % COA PEMFC tolerates 20-50 ppm CO

1. Preferential oxidation (PROX)(a small fraction of air in fuel)2. Methanation (back-conversion of CO to methane)3. Membrane filtration (Pd-Ag membrane)4. Pressure swing adsorption techniques (PSA)

Steam reforming system

Electrolysis

sparknotes.com

H2O → H2 + ½O2

The opposite of a fuel cell

IHT

Alkaline electrolyzer

Electrolyte: KOH (or similar)

Separator: asbestos, NiO

Cathode catalyst: Ni, Pt

Anode catalyst: Metal oxides like NiCo2O4, La1-xSrxCoO3 or Pt, Ru

Container: Ni plated Steel

7

0

50

100

150

200

250

300

0 200 400 600 800 1000

Temperature (oC)

_G

, _

H (

kJ/m

ol)

0

0,2

0,4

0,6

0,8

1

1,2

1,4

Cell

voltage (

V)

The theoretical cell voltage

ΔH

ΔGVoltage

El.

Heat

The thermo-neutral potential

H2O(l) → H2(g) + ½O2(g)

),(48.1 waterliquidHHVVnF

HEtn =

!=

),(25.1 vapourwaterLHVVnF

HEtn =

!=

Endothermic

Types of electrolyzers

SOEC (700-1000ºC)SOFCSolid Oxide

(MCEC)MCFCM. Carbonate

(PAEC)PAFCPhosph. acid

PEMEC (60-150ºC)PEMFCPolymer

AEC (60-150ºC)AFCAlkaline

Electrolyzer cellsFuel cells

Electrolysis

8

Electrolysis PEM and Solid oxide electrolyzers

PEMEC, Proton Energy Systems Inc HOGEN GC 600 (Lab gasses)

(Risø)

SOEC, Risø DTUUnder development

Hydrogen Storage Ways of storing hydrogen

Physical storage

Easy access

Chemical storage

DecompositionReforming

Liquid20 K

Chemicalcompounds,

Organics,NH3

Metalhydrides

Adsorbedlarge surface

AbsorbedMetal

hydrides

Compressed200-700 bar

9

Compression

Advantages200-300 bar well-known technology

700-800 bar possible in composite tanks

DisadvantagesBulky

Energy to compression (~10%)Safety

Compression, density

0

10

20

30

40

50

60

70

80

0 200 400 600 800 1000

Pressure (bar)

De

ns

ity

(k

g/m

3) Liquid H2

Idealgas

Real gas

Composite tanks

From Dynetek

10

Composite tanks

Wrapping a composite tank

From Quantum Technologies

Liquid H2

AdvantagesDenser that compressed gasKeeps cold by evaporation

DisadvantagesExpensive

Energy to cooling andcondensation (30-40%)

Self dischargeSafety

AdvantagesPossibly denser than compressed H2

DisadvantagesEnergy to cooling and condensation

Capacity not as high as liquid H2Safety

High surface adsorption High surface adsorption

Carbon materials: Active carbon (2000 m2/g)NanotubesGraphite ??

Zeolites:

Other high surface: Metal organics

H:C 1:1 ⇒ 8.3 wt%

11

High surface adsorption

Physisorption on large surfaces

Most efficient at low temperature like 77 K (liquid N2)(an intermediate between compression and liquefaction)

Con

cent

ratio

n

PressureTem

perature

Capacities:

~ 4 wt% at 77 K~ 1 wt% at ambient temp.(depends on pressure)

Hear Hirsher !!

Interstitial Metal Hydride Formation

H2 gas

Host metal

Hydridephase

Solid solution

Adsorption/Dissociation

Plateau pressures

Plateau pressures

LaNi5 + 3H2

↑↓

LaNi5H6

periodic table, AB5

Magnesium

A metals(high affinity)

B metals(low affinity)

12

Well known hydrides

Room temperature

LaNi5H6 1.4 w% 2 bar (AB5)

MmNi3.6Co0.7Mn0.4Al0.3Hx 1.0 w% <1 bar (AB5)

CaNi5H6 1.8 w% 0.5 bar (AB5)

TiFeH2 1.9 w% 3 bar (AB)

300°C

Mg2NiH4 3.6 w% 1 bar (A2B)

MgH2 7.6 w% 1 bar

periodic table, AB5

Magnesium

Complex hydrides

Center ionMHx

–yCounter ion

M+z

Formula w%NaAlH4 6.9LiAlH4 9.5NaBH4 9.6LiBH4 15.5Mg2NiH4 3.6

periodic table, AB5

Magnesium

Potential elements(no interstitial hydrides)

Higher capacity, lighter hosts

Li3N + 2H2 ⇒ Li2NH + LiH + H2 ⇔ LiNH2 + 2LiH

Total 11.5 wt% stored (2. step rev.: 7%)

Chen et al. Nature 420: 302-304 (2002)