Fuel Cell Technology
John Jechura – jjechura@mines.edu
Updated: January 4, 2015
Energy Markets Are Interconnected
https://publicaffairs.llnl.gov/news/energy/energy.html
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Topics
• Basics & types of fuel cells
• Fuel cells for transportation
• Hydrogen for the fuel cells
• Efficiencies
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Fuel Cell Principals
Chemistry of Fuel Cell (with H+ transfer)
• Anode side:
 2H2  4 H+ 2 e‐
• Cathode side:
 O2 + 4 H+ 2 e‐  2 H2O
• Overall:
 2H2 + O2  2 H2O
Fuel cell provides a direct current flow of electrons
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Types of Fuel Cells
• Alkaline fuel cell (AFC)
 One of the oldest designs • U.S. space program used them since the 1960s to make power & drinking water
 Very susceptible to contamination, requires pure hydrogen & oxygen
 Very expensive, unlikely to be commercialized
http://americanhistory.si.edu/fuelcells/basics.htm
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Types of Fuel Cells
• Solid oxide fuel cell (SOFC)
 Operates at very high temperatures – 700 to 1,000C
• High temperature makes reliability a problem when cycling on and off repeatedly
• Very stable when in continuous use
 High temperature can produce steam to generate more electricity – improves overall efficiency of the system  Best suited for large‐scale stationary power generators
http://americanhistory.si.edu/fuelcells/basics.htm
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Types of Fuel Cells
• Molten‐carbonate fuel cell (MCFC)
 Also best suited for large stationary power generators
• Operate at 600C & can generate steam  Lower operating temperature means they don't need such exotic materials
• Design little less expensive
http://americanhistory.si.edu/fuelcells/basics.htm
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Types of Fuel Cells
• Polymer exchange membrane fuel cell (PEMFC)
 DOE focusing on PEMFC as most likely candidate for transportation  High power density & relatively low operating temperature (60 to 80C)
• Doesn't take long for the fuel cell to warm up & begin generating electricity http://americanhistory.si.edu/fuelcells/basics.htm
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Types of Fuel Cells
• Phosphoric‐acid fuel cell (PAFC)
 Operates at higher temperature than PEMFCs, longer warm‐up time
 Potential for use in small stationary power‐generation systems but unsuitable for use in cars
• Direct‐methanol fuel cell (DMFC)
 Comparable to PEMFC (operating temperature) but not as efficient
 Requires relatively large amount of platinum to act as a catalyst – makes these fuel cells expensive 10
PEMFC: Polymer Exchange Membrane Fuel Cell
• Anode
 Conducts the electrons that are freed from the hydrogen molecules
 Has channels etched into it that disperse the hydrogen gas equally over the surface of the catalyst
• Cathode
 Has channels etched into it that distribute the oxygen to the surface of the catalyst
 Conducts the electrons back from the external circuit to the catalyst – recombine with the hydrogen ions & oxygen to form water
• Electrolyte is proton exchange membrane
 Only conducts positively charged ions & blocks electrons
 Membrane must be hydrated in order to function & remain stable
• Limits how low a temperature the fuel cell can operate
• Catalyst facilitates reaction of oxygen & hydrogen
 Usually made of platinum nanoparticles very thinly coated onto carbon paper or cloth
http://auto.howstuffworks.com/fuel‐efficiency/alternative‐fuels/fuel‐cell2.htm
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Possible Fuel Cell Vehicle
http://www.fueleconomy.gov/feg/fuelcell.shtml
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Large Scale Hydrogen Production
• Steam Reforming
 CH4 + H2O  CO + 3∙H2
 Highly endothermic
• Partial Oxidation
 2 CH4 + O2  2 CO + 4 H2
 Highly exothermic
 If solid feedstock, one possible gasification reaction
• Autothermal Reforming
 Combines both steam reforming and partial oxidation to achieve an energy‐neutral process
 Often uses oxygen rather than air
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Real Process – Steam Methane Reforming & Water Shift
Reforming
Reactor
Natural Gas
High Temperature
Shift Reactor
Low Temperature
Shift Reactor
Hydrogen
Purification
Methanation
Reactor
Flue Gas
Steam
Fuel Gas
Hydrogen
CO2
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Reforming. Endothermic catalytic reaction, typically 20‐30 atm & 800‐880°C (1470‐
1615°F) outlet.
CH4 + H2O  CO + 3 H2
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Shift conversion. Exothermic fixed‐bed catalytic reaction, possibly in two steps. CO + H2O  CO2 + H2
HTS: 345‐370°C (650 – 700F)
LTS: 230°C (450F)
• Gas Purification. Absorb CO2 (amine) or separate into pure H2 stream (PSA or membrane).
• Methanation. Convert residual CO & CO2 back to methane. Exothermic fixed‐bed catalytic reactions at 370‐425°C (700 – 800F).
CO + 3 H2  CH4 + H2O
CO2 + 4 H2  CH4 + 2 H2O
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On‐Board Fuel Reforming
• Liquid fuel would avoid having heavy high‐pressure gas containers
 Gasoline, alcohols (methanol, ethanol, …)
• Reforming of fuel produces CO2 emissions
 Will not qualify as zero emissions vehicles (ZEVs) under California's emissions laws
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Current Problems with Reformers Supplying Fuel Cells
• Reforming reaction takes place at high temperatures – slow to start up & requires costly high temperature materials
• Sulfur compounds in the fuel poison certain catalysts
 Research into sulfur‐tolerant catalysts
• Low temperature polymer fuel cell membranes can be poisoned by CO produced by the reactor
 PEMFC need complex CO‐removal systems
 SOFC & MCFC operate at higher temperatures & do not have this problem
• Efficiency of process 70% ‐ 85% (LHV basis)
• Catalyst in low temperature fuel cells is based on platinum & is very expensive
 Typical automotive fuel cell stack (100kW) contains 20‐30 g of platinum metal –
currently ~$1700 per troy oz ($60 per g)
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Overall Efficiencies
• Gasoline internal combustion – 20 – 25%
• Battery powered vehicle
 65% of electricity in
• Batter efficiency – 90% • Charging efficiency – 90% • Motor/inverter – 80%
 Efficiency of power generation?
• Combustion based 40% – 26% overall
• Hydro electric based – “free” electricity? • Fuel cells with pure hydrogen
 Potentially 80% efficient
 Overall efficiency with 80% efficient motor/inverter – 64%
• Fuel cells with reformed fuel
 Including reformer efficiency – 45 to 51%
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