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2012 Fuel cells.pdf
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Transcript of 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
Helge Weydahl: Unitised regenerative fuel cells - opportunities and obstacles
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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:
Helge Weydahl: Unitised regenerative fuel cells - opportunities and obstacles
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Unitised regenerative fuel cell (URFC)URFC in fuel cell mode:
Helge Weydahl: Unitised regenerative fuel cells - opportunities and obstacles
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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
Helge Weydahl: Unitised regenerative fuel cells - opportunities and obstacles
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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/
Helge Weydahl: Unitised regenerative fuel cells - opportunities and obstacles
www.nasa.gov
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• Light duty armoured vehicle • Submarines• Range extension for hybrid
electric vehicle [1,2]
Terrestrial applications
Helge Weydahl: Unitised regenerative fuel cells - opportunities and obstacles
[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
Helge Weydahl: Unitised regenerative fuel cells - opportunities and obstacles
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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
Helge Weydahl: Unitised regenerative fuel cells - opportunities and obstacles
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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
Helge Weydahl: Unitised regenerative fuel cells - opportunities and obstacles