2012 Fuel cells.pdf

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Hilde Venvik, Institutt for kjemisk prosessteknologi.

Norges teknisk-naturvitenskapelige universitet

TKP4515 KEM – Fall 2012Katalyse i energi og miljøsammenhengFuel cells

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Fuel cell principle I

Chemical energy –> electrical energy (reverse electrolysis):• Exothermic reaction• Transport of charge in electrolyte• Transport of electrons in outer circuit • Heat of reaction can be extracted as circuit load• Carnot cycle (efficiency limitation) ommitted

– Internal cobustionen engine: high η @ high load

• Efficiency determined by transport phenomena and activation energies– high η @ low load (”idling”)

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Fuel cell principle II

Principle discovered 1838 by German scientist Christian Friedrich Schönbein Demonstrated 1839 by Welsh scientist and barrister Sir William Robert Grove Similar materials to today's phosphoric-acid fuel cell.

Grove’s sketch from Philosophical Magazine and Journal of Science 1842

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E < 1.23 V

Helge Weydahl: Unitised regenerative fuel cells - opportunities and obstacles

Fuel cell principle III (H2-PEM example)PEM: Polymer Electrolyte Membrane/

or Proton Exchange Membrane

Most common is Nafion® (DuPont)”sulfonated tetrafluoroethylene based fluoropolymer-copolymer”

Other electrolytes are emerging

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Fuel cell principle IV

2

2 2

Nernst equation: ln H O

H O

pG G RT

p p

æ ö÷ç ÷çD =D + ÷ç ÷ç ÷÷çè ø

2

2 2

ln2

H O

A H O

pRTeN p p

e eæ ö÷ç ÷ç= - ÷ç ÷ç ÷÷çè ø

1.19V2 A

GeN

eD

=- =

i.e. the maximum voltage by a single cell is given by the free energy of formation of water

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Different fuel cells and their specifications.

1 Based on the principles of PEMFC.

Fuel cell type

Operating temperatu

re [°C] [74]

Operating pressure

[bar]

Anode reaction

Cathode reaction

CO tolerance Sulphur tolerance

[ppm]

Anode catalyst

Cathode catalyst

Polymer Electrolyte/ Membrane (PEMFC)

“VEHICLES”

80-120 up to 10 H22 H+ + 2 e-

½ O2 + 2 H+

+ 2 e- H2O50 ppm very low Pt Pt

Alkaline (AFC)“THE

CLASSIC”100-250 ~ 5 H2 + 2 OH-

2 H2O + 2 e-

½ O2 + H2O + 2 e-

2 OH-

very low CO and CO2

tolerancevery low Ni-Al, Pt

or Ag Pt

Phosphoric Acid (PAFC) 150-220 up to 5 H2

2 H+ + 2 e-½ O2 + 2 H+

+ 2 e- H2O1 % 50 Pt Pt

Molten Carbonate (MCFC)

600-700 up to 5 H2 + CO32-

H2O + CO2 + 2 e-½ O2 + CO2 + 2 e- CO3

2- - 0.5 Ni NiO

Solid Oxide (SOFC)“Power

generation”

600-1000 up to 15

H2 + O2- H2O + 2 e-

CO + O2- CO2+ 2 e-

½ O2 + 2 e-

O2- - 1 Nimixed

conducting oxides

Direct Methanol Fuel Cell (DMFC)1

150 °C up to 3

CH3OH + H2O CO2 + 6 H+ +

6 e-

3/2 O2 + 6 H+ + 6 e- 3 H2O

- very low Pt-Ru Pt

Ingrid Aartun, SINTEF Rapport STF80MK F05205 ”Distributed energy supply systems with CO2 removal”

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Brenselceller - typer

Fra “Fuel Cells – Green Power” av Sharon Thomas og Marcia Zalbowitz ved Los Alamos National Laboratory, New Mexico, USA

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THE SOLID OXIDE FUEL CELL(SOFC)

Please consult the lecture «Power production…» on

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Hydrogen fuel cell (”PEM”)

Catalyst (Pt) on conductive support (C) for ”H2 oxidation” and ”O2 reduction”

Proton conducting electrolyte

Gas diffusion layerGas diffusion layer

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Requirements for the hydrogen fuel cell MEA• MEA = membrane electrode assembly• Electric conductivity of electrode materials (for

example carbon/grafite)• Highly dispersed electrocatalyst (Pt) on the electrode

(i.e. small particles)• Contact between catalyst and membran/electrolyte to

facilitate ionic transport.• Geometry that facilitates gas diffusjon of incoming

reactants to all parts of the electrode, i.e. a certain porosity

• Hydrofobicity of the electrodes to prevent flooding

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Material properties

High ionic conductivity

High electrical conductivity

Mechanical strength

Porous

Hydrophobic

Electrochemically stable


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Fuel cell stacks• Several cells are connected in

stacks to obtain higher output voltages

• ”bipolar plates” are used between the cells, that must have– Good electric contact to anode on

one side and cathode on the other– Geometries (patterns that allows for

gas flow

• Air/O2 will be fed to all cathodes, H2 to all anodes

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Processes Reducing Cell Lifetime

2e-

2H+

O2 (air)H2

2e- 2e-

H2O2

H2O

Degradation of the polymer membrane

Agglomeration of catalyst particles reducing the activity

Increased interfacial contact resistance, de-lamination Reduced conductivity of the

ionomer in the catalyst layer and membrane phases

Changes in diffusion backing properties, (hydrophobicity/hydrophilicity)

Poisoning of the catalyst materials

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Recent development: Carbon Nanofibers as Catalyst Supports

TEM image of Ni particleson oxidized herring-boneCNF prepared by incipientwetness (IW). (12.5 wt% Ni)

E. Ochoa-Fernandez, Z. Yu, M. Rønning,A. Holmen, D. Chen, submitted.

16 Selective synthesis of CNF/CNT

CNT/CNF as catalyst supports

Pt/CNF

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Recent development: Novel catalyst

Core shell is less activebut more stable than Pt-Ru alloy

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Challenges to the PEM fuel cellPEM is preferred in vehicles because of compactness (current density)

• The proton conducting membrane in the ”standard” PEM fuel cell is madeby NafionTM, that needs a certainhumidity to sustain proton transporte(essentially transport in solution)

• > The temperature in H2-PEM og DMFC must therefore be kept below100 C

• > CO will adsorb strongly to the Pt anode catalyst, if present, and blocksites for hydrogen adsorption and dissociation.

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Recent development: High Temperature PEM: PBI

Polybenzimidazole, PBI, has certain advantages

– Possible operating temperature up to 200 ºC

– High thermal resistance (Tg = 420 °C)

– Fairly high conductivity when doped with phosphoric acid

– Cheaper than Nafion– 'Solid' membrane as

opposed to phosphoric acid fuel cells

– Low reactant cross-over (important in DMFC)

0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4

Current density /Acm-2

Pow

er d

ensi

ty/ W

cm-2

1 bar, Air 2 bar, Air3 bar, Air4 bar, Air5 bar, Air1 bar, O2

Higher temperatures improve:– Electrode reaction kinetics– CO tolerance– Water management– Heat management

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Recent development: Solid Acid Fuel Cell

Hau H. DuongSuperprotonic, Inc.May 22, 2009

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Recent development: Solid Acid Fuel Cell

SUPERPROTONIC is nowSAFCell, Inc. (Pasadena, CA) and has partnership withNordic Power Systems AS(Norwegian company)

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Fuel cells – what you need to know

• The basic principles– How they work, and how they are built up– What limits the maximum voltage output– What limits the technology beyond the theoretical limitation– Typical materials, electrolytes and catalysts

(PEM and SOFC are the most important cases!)– Some proposed improvements

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Other fuel cells – what you need to learn

• This presentation mainly concerns PEM fuel cells– LOW temperature (80-250 C), proton transport– Cars and other portable technology (even mobile phone) is main

application

• Solid oxide fuel cells– High temperature (600-1000C)– Transports of O2- across a dense oxide membrane– Feed could be H2 or H2+CO (indirect reforming) or CH4 (direkt)– Main application is stationary power generation (CHP).

• Potential of very high efficiency in combination with turbine. • Potential for CO2 capture

– See ”power generation” presentation for more details on materials and catalysts

• Note: the alkaline (KOH) fuel cell is ”the classic”. Applied in space!

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Fuel cells – where are we?

The technology is not commercial, with the exception narrow market segments/niche products Material costs (precious metal catalysts, membranes (Nafion),…) Stability Engineering (humidification, cooling,...) The hen and the egg:

A fuel cell market is based on hydrogen availability.A hydrogen market requires fuel cells to be available.

The potential is high -> continued research and development No harmful emissions High efficiency, particularly at low load No noise

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The break-through of fuel cells –where and when?1. Auxiliary power in trucks and ships2. Battery replacement for PCs, mobil phones and tools3. Remote areas with poor/expensive/no electricity grid

connection – possibly combined with a special energy situation Islands, such as Utsira, Røst, … Icelandic New Energy (http://www.newenergy.is/)

4. Fleet vehicles5. Passenger cars and

motor bikes

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• A final question that you may (should) have asked yourself:

”The PEM fuel cell and the PEM electrolyzer look very similar – can they be combined in a single unit?”

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Unitised regenerative fuel cell (URFC)URFC in electrolysis mode:


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Unitised regenerative fuel cell (URFC)URFC in fuel cell mode:


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Main advantage• Reactants not a part of electrodes

• High specific energy density (~500 Wh/kg vs. ~200 Wh/kg for advanced battery systems [1])

• Feasible in applications where low weight is of major importance

[1] F. Barbir, T. Molter, L. Dalton, Int. J. Hydrogen Energy 30 (2005) 351-357


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• Telecom satellites• High-altitude unmanned

aerial vehicles• Space station• Mini-rover for planetary

exploration• Deep-space flights

Space applications

http://telecom.esa.int/


www.nasa.gov

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• Light duty armoured vehicle • Submarines• Range extension for hybrid

electric vehicle [1,2]

Terrestrial applications


[1] G.J. Suppes, Int. J. Hydrogen Energy 31 (2006) 353-360 [2] G.J. Suppes, Int. J. Hydrogen Energy 30 (2005) 113-121

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ElectrocatalystsHydrogen electrode Oxygen electrode

Fuel cell mode

H2 2 H+ + 2 e-

Hydrogen oxidation reaction

½ O2 + 2 H+ + 2 e- H2O

Oxygen reduction reaction

Electrolysis mode

2 H+ + 2 e- H2

Hydrogen evolution reaction

H2O ½ O2 + 2 H+ + 2 e-

Oxygen evolution reaction

PtRuO2/IrO2


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WettabilityHydrogen electrode Oxygen electrode

H2 2 H+ + 2 e-

Not critical

½ O2 + 2 H+ + 2 e- H2O

Hydrophobic

2 H+ + 2 e- H2

Not critical

H2O ½ O2 + 2 H+ + 2 e-

Hydrophilic


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Conclusion• A unitised regenerative fuel cell is a rechargeable gas

battery • Competitive in applications where weight is critical,

i.e., space• Challenges related to efficiency and stability are

addressed by trade-off solutions and material science


2012 Fuel cells.pdf

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