Intro to PEM Fuel Cells
What is a Fuel Cell?
A fuel cell is an energy conversion device that reacts a
fuel and oxygen to produce electricity. The most
common fuel is hydrogen.
e-
- +
e-
Is a Fuel Cell a Type of Battery?
No. Though both produce electricity, they
operate in different ways. The reactants that a
battery converts into electricity are stored within
itself (e.g. battery acid) while the reactants for a
fuel cell (e.g. H2, O2) are supplied externally.
Batteries and Fuel Cells
A battery’s reactants
are self-contained
-
+
A fuel cell’s reactants are
supplied externally
H2 in
H2 out
-
+
Air in
Air out
Fuel Cell Applications
Each type of fuel cell is particularly suited to certain applications:
• PEM: most versatile, used for portable power, transportation,
and stationary power
• SOFC: primarily used for stationary power, in development for
transportation (e.g. semi trucks)
• MCFC: power plants
• AFC: power and water production for space vehicles (e.g. Apollo
and Space Shuttle spacecrafts), in development for more general
use due to breakthroughs in alkaline media
• PAFC: stationary power, power plants
Why Use a Fuel Cell?
A fuel cell provides very clean energy virtually emissions-free. In
addition to electricity, it produces water and heat.
Because the fuel cell’s reactants are supplied externally, there is
no charge or discharge period as with a battery. Additionally,
very high energy density can be achieved because the fuel cell
design is not dependent on reactant storage.
(Energy density = lifetime between recharges)
A fuel cell can be used repeatedly - there is no package to throw
away.
Why are People interested in Fuel
Cells: DMFCs vs. batteries….
Energy Density
Li-ion batt: 150-200 Wh/kg
20 W, 2 day fuel cell system: easily
exceed 1000 Wh/kg TODAY
Many Types of Fuel Cells
The main fuel cell technologies today are:
• PEM (polymer electrolyte membrane fuel cell)
• SOFC (solid oxide fuel cell)
• MCFC (molten carbonate fuel cell)
• AFC (alkaline fuel cell)
• PAFC (phosphoric acid fuel cell)
How are They Different?
All fuel cells react a fuel and oxygen to produce electricity, but
differ in the medium or “electrolyte” in which these reactions
occur.
The nature of the electrolyte determines all of the important
characteristics of the fuel cell such as its operating
temperature, materials of construction and the variety of fuels
with which it can be used.
• PEM (proton-conducting polymer)
• SOFC (oxide ion-conducting ceramic)
• MCFC (molten carbonate salt in a ceramic matrix)
• AFC (aqueous potassium hydroxide in a matrix)
• PAFC (phosphoric acid in a matrix)
Why Use so Many Types?
Each type of fuel cell has particular advantages and disadvantages
• PEM: solid construction, low temperature, sensitive to impurities,
can only be used with hydrogen or methanol.
• SOFC: can be used with many fuels, doesn’t require precious metal
catalysts, solid, rugged, very high temperature, expensive
materials.
• MCFC: can be used with many fuels, efficient, doesn’t require
precious metal catalysts, high temperature, very corrosive
electrolyte.
• AFC: most efficient medium for oxygen reaction - high
performance, doesn’t require precious metal catalysts, sensitive to
carbon dioxide, caustic medium.
• PAFC (phosphoric acid fuel cell): same electrochemical reactions
as PEM, but not as sensitive, very corrosive.
What Types of Fuel are Used?
Typical fuel cells run on hydrogen and oxygen, but are
“fuel flexible” because many types of fuel (e.g.
methane, gasoline) can be reformed to make
hydrogen or be used in its place (e.g. methanol in PEM
fuel cells).
Pure oxygen is rarely used except for special
applications. Air is used instead, and is supplied from
a pressurized gas cylinder or from the room or
outside air via diffusion or a device such as a blower.
About Hydrogen
Hydrogen can be produced in a number of ways, either
as a direct byproduct of a reaction or by desorption
from a material. In the latter case, the material also
acts as a means of storage:
• Byproduct: steam reforming of fossil fuels,
anaerobic oxidation of bacteria, reaction of
chemical hydrides and water
• Desorption: metal hydrides, carbon nanotubes
• Storage: gas cylinders, tanks, bladders, metal
hydrides, carbon nanotubes
PEM FC - The Whole Picture of a
Single Cell
EERE
How Does a PEM FC Work?
Anode: 2H2(g) -----> 4H+(aq) + 4eCathode: O2(g) + 4H+(aq) + 4e- ----> 2H2O(l)
Anode (Negative)
Hydrogen reacts with the Pt
catalyst on the PEM
to form protons and
release electrons
H2
H+ + eH+
The protons travel
across the PEM
PEM
Cathode (Positive)
O2 + H+ + e-
H2O
The protons combine
with O2 and electrons
to form water
This reaction is also
catalyzed by Pt
Direct Methanol PEM FC
Anode: CH3OH(aq) + H2O(l) -----> CO2(g) + 6H+(aq) + 6eCathode: 3/2O2(g) + 6H+(aq) + 6e- ----> 3H2O(l)
Anode (Negative)
Methanol reacts with
the Pt/Ru catalyst on
the PEM to form
protons and
release electrons
CH3OH
H+ + e-
The protons travel
across the PEM
H+
PEM
Cathode (Positive)
O2 + H+ + eH2O
The protons combine
with O2 and electrons
to form water
This reaction is
catalyzed by Pt
Current Flow in a Fuel Cell
e-
-
+
e-
So what’s happening during operation?
We’re feeding in gases (say, hydrogen and
air) at some flow rate
• Fuel or oxygen utilization {= 1/stoichiometric ratio (usually
called ‘stoich’)}:
ratio of the moles used per unit time (related to current density
by some conversion factors) to the incoming flow rate
Gases may be humidified (esp. for single
cells)
So what’s happening during operation? II
We’re generating current and product water (that
we may have to get rid of!!!) at cathode
We’re generating heat (that we may have to get rid
of!!!)
Fuel converted to protons (go through membranes)
and electrons (go through external circuit) at
anode
•
Possibly also produce gas (e.g. DMFC)
What Does a PEM FC Look Like?
Anode Backing
Anode Flowfield
Cathode Backing
CCM
Cathode Flowfield
Fuel Cell Components
CCM (Catalyst Coated Membrane): Proton-conducting
membrane plus 2 electrodes
GDL (Gas Diffusion Layer): Carbon cloth or paper with
carbon particle filler and Teflon
Bipolar plate: Graphite, carbon composite or metal with
machined or stamped ‘flow field’
Gaskets and seals: seals around edge of structure
Fuel Cell Power
•A single fuel cell doesn’t produce enough power for most
applications; single cells are “stacked” together to meet
power needs.
•The voltage of the application will determine the number
of cells in a stack; the size of the plates will affect the
overall power output
•Fuel cells stacks are part of systems with other parts to
deliver gases, manage electrical output etc.
•Fuel cell systems can operate in the W - MW range.
A Portable PEM FC Stack
LANL
Major System Components
Heat to
customer
(optional)
Exhaust
Blower
Air
Water
treatment
Water
Fuel
Warm exhaust
Heat exchanger
DC power
Air
Fuel cell stacks
Controls
Steam
Steam
generator
Fuel/air
module
Fuel
Power
inverter
Fuel processor
Power
module
Electronics
module
AC power
DMFC Stacks & Systems
CO2 Exhaust
Port
Fluidics Module:
Feedstock Res
H2O Res
Filters
Heater
Mixing Tube, etc.
Air Exhaust
Port
LANL DMFC
STACK
LANL MeOH Sensor
Air, Feedstock,
H2O and MeOH
Pumps Under
Stack
Flex Circuit Sensor Board
DMFC-BB
Mother Board
Electronics
Condenser
and Fan
DMFC-BB
Support Module
Electronics
Aluminum
Brassboard
BRASS-BOARD SYSTEM
PACKAGED SYSTEM
Gasoline to Electricity for Autos
The DOE/OAAT-PNGV Program
H 2O
Gasoline
Ethanol
Methanol
Nat. Gas
10%
CO
AUTOTHERMAL
REFOR MER
2,000 ppm
CO
WATER GAS SHIFT
REACTOR
<100 ppm
CO
PREFERENTIAL
OXIDIZER
H2
FUEL CELL
STACK
O2
Gaskets and Seals
Critical component!
Adhesive is good if available
Frames may be combined with gaskets
Very important that this component be gas tight
and not leach anything…
May require high temperature stability
Should be a good electrical insulator
Small ‘Battery’ - Fuel Cell
1 W Air-Breather
QuickTime™ and a
Photo - JPEG decompressor
are needed to see this picture.
QuickTime™ and a
Photo - JPEG decompressor
are needed to see this picture.
Fuel Cells for Personal Electronics (“Micro” FCs)
• Substantial interest in < 2 W systems.
• Higher current densities, but higher A/V ratios.
• Maximizing active area is key.
– Different designs than the larger stacks.
Efficient Fuel Cell Systems:
1.5 kW Adiabatic Stack
Where Can You Buy a Fuel Cell Today?
Bad news: Not too many places to buy a
fuel cell…..
The Good News: Lots of Opportunity!
What Can You Operate with a Fuel Cell?
Small-scale systems: Portable Power
What Can You Operate with a Fuel Cell?
Large-scale systems: Stationary Power and Transportation