Fuel Cells: Powering Progress in the 21st Century

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Fuel Cells Powering Progress in the 21st Century

• Sossina M. HaileMaterials Science

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Outline

• What is a fuel cell
• Technology Status
• H2/air fuel cell
• H2 storage
• On-board H2 generation
• Direct Methanol
• Enabling Advances

Daimler-Chrysler Necar 4, 1998 50 kW fuel cell,
400 km range
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Principle of Operation

• Major Components
• anode fuel oxidation
• cathode oxidant reduction
• electrolyte ion transport
• Example
• H2 ? 2H 2e-
• ½O2 2H 2e- ? H2O

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Fuel Cell Performance

• H2 ½ O2 ? H2O
• DGo(90C) -226.1 kJ/mole
• Eo DGo/nF 1.17 V ? OCV
• voltage losses
• fuel cross-over
• reaction kinetics
• electrolyte resistance
• mass flow limitations
• power IV
• peak efficiency at low I
• peak power at mid I

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Why Fuel Cells?
U.S. CO2 emissions by sector source DOE
atmospheric CO2 1800 280 ppm 2000 350 ppm 2030
560 ppm

• higher fuel efficiency required to reduce CO2
  emissions

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Energy Efficiency

• high efficiency
• size independent
• low power applications
• automotive
• electronics
• remote stationary
• remote residential
• power industry deregulation

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Why Fuel Cells?
Federal and California Emissions Standards

• fuel cells
• easily
• meet new
• ZEV goals

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Fuel Cell Types
By-products H2O, CO2
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Mobile Applications

• close to ambient temperature operation
• polymer electrolyte
• alkali electrolyte
• phosphoric acid electrolyte
• avoid corrosive liquids
• polymer electrolyte (Nafion)
• fuel choices
• H2 ? H2/O2 fuel cell
• CH3OH ? Direct Methanol Fuel Cell

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Proton Conducting Polymers

• Nafion (Dupont)
• teflon-like backbone
• (CF2)n ? chemical stability
• sulfonic acid groups
• (CFO CF2)nSO3- ? high conductivity
• saturate with H2O
• inverse micelle structure
• maintain high humidity during operation

1 nm
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PEM H2/O2 Fuel Cell

• Membrane
• keeps gases separate
• transports H
• Pt catalyst (0.2 mg/cm2)
• ½ H2 ? H 2e-
• ½ O2 2H 2e- ? H2O
• Contacts gas electrolyte
• Porous graphite
• transports e-
• permeable to gases

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PEM H2/O2 Fuel Cell Stack

• Single cell 0.6V, 0.8 A/cm2
• 50-75 kW for automobile
• multiple cells for ? power
• connect in series V nVo
• connect in parallel I nIo

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PEM Fuel Cell Systems

• Hydrogen source
• Stored on-board
• Delivery system
• Generated on-board
• Public acceptance

• Sensitive MEA
• Water management
• Thermal management
• Catalyst poisoning

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Hydrogen as a Fuel

• Safety
• H2 generation
• reformation of natural gas
• controlled CO2 production
• electrolysis of H2O
• H2 delivery
• tanks
• pipelines
• compressors

• Energy density
• Fuel Wh/g kWh/L
• Hydrogen 33 ???
• Diesel Fuel 13.2 21.1
• Gasoline 8.4 12.6
• Methanol 6.2 5.0
• Energy requirements
• 350 mile range
• gas 20 mpg ? 18 gal (60L)
• H2 0.6 m/kWh ? 18kg (40 lbs)

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On-Board Hydrogen Storage

• Pressurized Tanks

• Metal Hydrides/Carbon

? fuel efficiency 4-5? 18 ? 3.6kg
today 1.7 wt gt 1060 kg (2,300 lbs) target 12
wt gt 150 kg (330 lbs)
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On-board Reformers
Methanol Methane (NG) CnHm
(gasoline)
CH3OH H2O ? CO2 3H2 endothermic
250C, Cu-Zn catalyst
CH4 H2O ? CO 3H2 endothermic
900C, Ni catalyst
CnHm n/2O2 ? nCO
m/2H2 exothermic 1200C, flame
0.5 2 CO
Syngas
Syngas hydrocarbons
CO clean-up
CO H2O ? CO2 H2 exothermic (1)
350C, Ni catalyst (2) 200C, Cu-Zn catalyst
0.5 2 CO
? 20 ppm CO
PEM fuel cell
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Fuel Cell Catalysts
CO sensitivity

• Quantity of Pt required
• operate at 1 A/cm2, 0.65V
• ? 0.65 W/cm2
• require 50-75kW power
• ? 105 cm2
• Pt loading of 0.2mg/cm2
• ? 20g Pt / vehicle

S. Gottesfeld, LANL
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Fuel Cell Realities

• Fuel Cell
• Simple!!

• Fuel Cell System
• Complex!!