Introduction to Fuel Cells Hocking College Nelsonville Ohio July 24, 2007.

Introduction to Fuel CellsIntroduction to Fuel Cells

Hocking CollegeHocking College

Nelsonville OhioNelsonville Ohio

July 24, 2007July 24, 2007

Fuel Cells Generally ContainFuel Cells Generally Contain

AnodeAnodeAnode CatalystAnode CatalystCathodeCathodeCathode CatalystCathode CatalystGas Diffusion LayerGas Diffusion LayerElectrolyteElectrolyteA load to complete the circuitA load to complete the circuitContinuous feed of fuelContinuous feed of fuelContinuous feed of air or oxygenContinuous feed of air or oxygen

Types of Fuel CellsTypes of Fuel Cells

Fuel Cells are generally named by the Fuel Cells are generally named by the electrolyte in the cell.electrolyte in the cell.

An exception to this is the methanol fuel An exception to this is the methanol fuel cell, which is called the Direct Methanol cell, which is called the Direct Methanol Fuel Cell (DMFC).Fuel Cell (DMFC).

Types of fuel cellsTypes of fuel cells

PEM (polymer electrolyte membrane fuel PEM (polymer electrolyte membrane fuel cell)cell)

DMFC (polymer electrolyte membrane fuel DMFC (polymer electrolyte membrane fuel cell)cell)

SOFC (solid oxide fuel cell)SOFC (solid oxide fuel cell)

MCFC (molten carbonate fuel cell)MCFC (molten carbonate fuel cell)

AFC (alkaline fuel cell) AFC (alkaline fuel cell)

PAFC (phosphoric acid fuel cell)PAFC (phosphoric acid fuel cell)

A Fuel Cell Converts Chemical A Fuel Cell Converts Chemical Energy to Electrical EnergyEnergy to Electrical Energy

Fuel

Hydrogen

Air or Oxygen

Load

Anode CathodeElectrolyte

e-

Catalyst Catalyst

H+

Gas Diffusion LayerGas Diffusion Layer

The Electrical Energy is in the form The Electrical Energy is in the form of DC voltageof DC voltage

The voltage is usually between 0.5 and 1.0 The voltage is usually between 0.5 and 1.0 volts DC (direct current).volts DC (direct current).

Several fuel cells are connected together Several fuel cells are connected together in series to obtain higher voltages 25—100 in series to obtain higher voltages 25—100 volts.volts.

These fuel cells connected in series are These fuel cells connected in series are called stacks.called stacks.

The fuel cell stacks still give DC voltage.The fuel cell stacks still give DC voltage.

An Inverter is used to change the An Inverter is used to change the DC voltage to AC voltage DC voltage to AC voltage

(alternating current).(alternating current).

Direct Current (DC) has a voltage that is Direct Current (DC) has a voltage that is always in one direction.always in one direction.DC voltage is the kind of voltage that DC voltage is the kind of voltage that comes from a battery.comes from a battery.Alternating Current (AC) has a voltage that Alternating Current (AC) has a voltage that varies from positive to negative 60 varies from positive to negative 60 times/second (50 times/second in Europe).times/second (50 times/second in Europe).Normal household current is AC.Normal household current is AC.

Fuel CellType

Electrolyte Anode Gas Cathode Gas Temperature Efficiency

Proton Exchange Membrane(PEM)

solid polymer membrane hydrogen pure or atmospheric oxygen75°C

(180°F)35–60%

Alkaline(AFC)

potassium hydroxide hydrogenpure

oxygenbelow80°C

50–70%

Direct Methanol(DMFC)

solid polymer membrane methanol solution in wateratmospheric

oxygen75°C

(180°F)35–40%

Phosphoric Acid(PAFC)

Phosphoric Acid hydrogenatmospheric

oxygen210°C

(400°F)35–50%

Molten Carbonate(MCFC)

Alkali-Carbonates

hydrogen,methane

atmosphericoxygen

650°C(1200°F)

40–55%

Solid Oxide(SOFC)

Ceramic Oxidehydrogen,methane

atmosphericoxygen

800–1000°C(1500–1800°F)

45–60%

Each type of fuel cell is particularly Each type of fuel cell is particularly suited to certain applications:suited to certain applications:

PEM: most versatile, used for portable power, PEM: most versatile, used for portable power, transportation, and stationary powertransportation, and stationary powerDMFC: used for portable powerDMFC: used for portable powerSOFC: primarily used for stationary power, in SOFC: primarily used for stationary power, in development for transportation (e.g. semi trucks)development for transportation (e.g. semi trucks)MCFC: power plants MCFC: power plants AFC: power and water production for space AFC: power and water production for space vehicles (e.g. Apollo and Space Shuttle vehicles (e.g. Apollo and Space Shuttle spacecrafts), in development for more general spacecrafts), in development for more general use use PAFC: stationary power, power plantsPAFC: stationary power, power plants

Each type of fuel cell has particular Each type of fuel cell has particular advantages and disadvantagesadvantages and disadvantages

PEM & DMFC: solid construction, low temperature, sensitive to PEM & DMFC: solid construction, low temperature, sensitive to impurities, can only be used with hydrogen or methanol. impurities, can only be used with hydrogen or methanol. SOFC: can be used with many fuels, doesn’t require precious metal SOFC: can be used with many fuels, doesn’t require precious metal catalysts, solid, rugged, very high temperature, expensive materials. catalysts, solid, rugged, very high temperature, expensive materials. MCFC: can be used with many fuels, efficient, doesn’t require MCFC: can be used with many fuels, efficient, doesn’t require precious metal catalysts, high temperature, very corrosive precious metal catalysts, high temperature, very corrosive electrolyte. electrolyte. AFC: most efficient medium for oxygen reaction - high performance, AFC: most efficient medium for oxygen reaction - high performance, doesn’t require precious metal catalysts, sensitive to carbon dioxide, doesn’t require precious metal catalysts, sensitive to carbon dioxide, caustic medium. caustic medium. PAFC (phosphoric acid fuel cell): same electrochemical reactions as PAFC (phosphoric acid fuel cell): same electrochemical reactions as PEM, but not as sensitive to impurities in the fuel, very corrosive.PEM, but not as sensitive to impurities in the fuel, very corrosive.

AdvantagesAdvantages

PEM & DMFC: solid construction, low PEM & DMFC: solid construction, low temperaturetemperature

SOFC: can be used with many fuels, SOFC: can be used with many fuels, doesn’t require precious metal catalysts, doesn’t require precious metal catalysts, solid, rugged, very high temperature (high solid, rugged, very high temperature (high enough for power generation)enough for power generation)

AdvantagesAdvantages

MCFC: can be used with many fuels, efficient, MCFC: can be used with many fuels, efficient, doesn’t require precious metal catalysts, high doesn’t require precious metal catalysts, high temperature (high enough for power generation)temperature (high enough for power generation)

AFC: most efficient medium for oxygen reaction AFC: most efficient medium for oxygen reaction - high performance, doesn’t require precious - high performance, doesn’t require precious metal catalystsmetal catalysts

PAFC (phosphoric acid fuel cell): same PAFC (phosphoric acid fuel cell): same electrochemical reactions as PEM, but not as electrochemical reactions as PEM, but not as sensitive to impurities in the fuelsensitive to impurities in the fuel

DisadvantagesDisadvantages

PEM & DMFC: low temperature (low grade PEM & DMFC: low temperature (low grade energy), must have pure hydrogen.energy), must have pure hydrogen.SOFC: very high temperature, expensive SOFC: very high temperature, expensive materials. materials. MCFC: liquid, high temperature , very corrosive MCFC: liquid, high temperature , very corrosive electrolyteelectrolyteAFC: liquid, sensitive to carbon dioxide, caustic AFC: liquid, sensitive to carbon dioxide, caustic medium, low temperature (low grade heat)medium, low temperature (low grade heat)PAFC: liquid, very corrosive, low temperature, PAFC: liquid, very corrosive, low temperature, (low grade heat)(low grade heat)

Typical fuel cells run on Typical fuel cells run on hydrogen and oxygenhydrogen and oxygen

Methane, methanol, and other Methane, methanol, and other hydrogen containing compounds can hydrogen containing compounds can be reformed to make hydrogen.be reformed to make hydrogen.

Air is usually used instead of pure Air is usually used instead of pure oxygen.oxygen.

PEM Fuel CellPEM Fuel Cell

Anode Flowfield (hydrogen)

Cathode Flowfield (oxygen)

Anode Backing Cathode Backing

PEM

Pt Catalyst Pt Catalyst

Source: Thampan, PEM Fuel Cell as a Membrane Reactor

PEM or DMFC Fuel Cell PEM or DMFC Fuel Cell ComponentsComponents

CCM (Catalyst Coated Membrane): Proton-conducting CCM (Catalyst Coated Membrane): Proton-conducting membrane plus 2 electrodes membrane plus 2 electrodes

GDL (Gas Diffusion Layer): Carbon cloth or paper with GDL (Gas Diffusion Layer): Carbon cloth or paper with carbon particle filler and Tefloncarbon particle filler and Teflon

Anode and Cathode plate: Graphite, carbon composite Anode and Cathode plate: Graphite, carbon composite or metal with machined or stamped ‘flow field’or metal with machined or stamped ‘flow field’

Gaskets and seals: seals around edge of structureGaskets and seals: seals around edge of structure

SOFC ComponetsSOFC Componets

Anode:Anode: Ni/YSZ CermetNi/YSZ Cermet

Electrolyte:Electrolyte: Yttrium Stabilized Zirconia Yttrium Stabilized Zirconia (YSZ)(YSZ)

Cathode: Lanthanum Strontium Cathode: Lanthanum Strontium Manganite (LSM)Manganite (LSM)

MCFC ComponentsMCFC Components

Anode is a highly porous sintered nickel Anode is a highly porous sintered nickel powder, alloyed with chromium powder, alloyed with chromium

Electrolyte is a liquid in a lithium—Electrolyte is a liquid in a lithium—Aluminum Oxide matrixAluminum Oxide matrix

Cathode is a porous nickel oxide material Cathode is a porous nickel oxide material doped with lithium. doped with lithium.

AFC ComponentsAFC Components

Electrodes: Electrodes: >>porous (and catalyzed) graphite electrodes porous (and catalyzed) graphite electrodes >>semi-permeable, Teflon coated carbonsemi-permeable, Teflon coated carbon material material >>heavily catalyzed as compared with other heavily catalyzed as compared with other types of fuel cells types of fuel cells

Electrolyte: potassium hydroxide (KOH) solution Electrolyte: potassium hydroxide (KOH) solution retained in a porous stabilized matrix retained in a porous stabilized matrix

PAFC ComponentsPAFC Components

Electrodes: porous carbon containing Electrodes: porous carbon containing Pt or its alloys as catalystsPt or its alloys as catalysts

Electrolyte: liquid phosphoric acid in Electrolyte: liquid phosphoric acid in Teflon-bonded silicon carbide matrixTeflon-bonded silicon carbide matrix