An introduction to fuel cells

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An introduction to fuel cells P. A. Christensen
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Winshields Crag on the Roman Wall
For nearly 2000 years, Hadrians wall has
brooded over the borderlands between Scotland and
England. In that time, kings and queens have
come and gone, empires have rose and fell, but
not a single g of coal or ml of oil has been
formed
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Genesis 1839 - 1842
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The fuel cell can trace its roots back to the
1800's. A Welsh born, Oxford educated barrister,
Sir William Robert Grove, who practiced patent
law and also studied chemistry or "natural
science" as it was known at the time.
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Grove realized that if electrolysis, using
electricity, could split water into hydrogen and
oxygen then the opposite would also be true.
Combining hydrogen and oxygen, with the correct
method, would produce electricity.
2H2O 2H2 O2 E0 1.23V
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To test his reasoning, Sir William Robert Grove
built a device that would combine hydrogen and
oxygen to produce electricity, the world's first
gas battery, later renamed the fuel cell. His
invention was a success, and Grove's work
advanced the understanding of the idea of
conservation of energy and reversibility.
Grove's drawing of one of his experimental "gas
batteries" from an 1843 letter
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Why hydrogen?
Storage method Energy density /kWh kg-1
Hydrogen 38
Gasoline 14
Lead acid battery 0.04
Flywheel, fused silica 0.09
(Methanol) (6)
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An early demise ..and resurrection 1960
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The world speed record in 1899, of 104 km h-1,
was held by an electric vehicle, the Jamais
Contente. In 1900 in the USA, there were 1681
steam-driven vehicles, 1575 electric vehicles and
only 936 driven by petrol engines. All electric
vehicles were powered by lead-acid batteries. A
fuel tank is lighter than a lead-acid battery and
can be recharged more rapidly. A tank of fuel
gives a much longer range than a fully charged
battery- current target of 300 km Still remains
elusive (battery should not exceed ca. 1/3 of
total weight of vehicle). The advent of the
self-starter (powered by a lead-acid
battery!) finally clinched the relegation of
electric vehicles to milk floats and fork-lift
trucks.
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In the late 19th and early 20th centuries, coal
was king But all attempts to make coal fuel
cells failed, and fuel cells fell out of favour
until the 1960s, due to interest from an out of
this world source!
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The Gemini capsule and fuel cell
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The fuel cell concept
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Products
Oxidant
e-
Solid, liquid or polymer electrolyte
H
Anode
Cathode
Fuel
Products
The fuel cell concept
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• The electrolyte essentially
• Separates fuel and oxidant
• Facilitates ion transport between anolyte and
  catholyte
• Prevents electrical short circuit between anode
  and cathode
• And can be liquid, solid or polymeric

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O2
e-
Pt
Pt
2H2 4H 4e-
4H 4e- O2 2H2O
H
Cathode (Pt catalyst)
Anode (Pt catalyst)
Electro lyte
H2
H2O
The simplest realisation the H2/O2 fuel cell
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Practicalities
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FlowField
Catalyst Layer
Gases Wettability Flooding/conductivity 3-phase
interface
Gas Diffusion Layer
The structure of Gas Diffusion Electrodes
(GDEs) (Porous carbon area up to 1000 m2 g-1)
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In aqueous solution H2 or O2 only soluble to ca.
1mM at 1 atm.
E l e c t r o d e
H2 or O2
Zone of high current density - 3 phase zone
Electrolyte
The three-phase zone in a gas diffusion electrode
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Carbon Pt
Gas space
Pore
Electrolyte
Carbon Pt
Reaction zone
The three-phase zone in a single pore of a gas
diffusion electrode
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Thermodynamic cell voltage
Ohmic loss
Activation overpotential loss - catalysts
Mass transport loss
A typical (H2/O2) fuel cell voltage vs current
plot
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Sources of hydrogen 5 Electrolysis (wind,
solar, wave, nuclear.) expensive
(operating cost 50p per kWh) but pure
hydrogen 95 Reforming of organics
-operating cost for H2 production 5p per
kWh, but fuel cell system more complex
and more expensive to construct.
650 850 ?C/Rh CnH2n2 nH2O ? nCO
(2n1)H2 CO H2O ? CO2 H2 (Methanol can be
reformed at 300 ?C)
Either high T or CO-tolerant anode catalysts.
H2S can also be present in reformed fuel.
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Common Fuel cells
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The most general fuel and oxidant are H2 and O2
(air). The highest temperature fuel cells (SOFC
MCFC) can use a variety of organics directly as
fuels, whilst methanol is used in the low
temperature Direct Methanol Fuel Cell.
1. Low temperature fuel cells
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• Alkaline Fuel Cells (AFC)
• The simplest realisation of the fuel cell concept
• Operates at 70 C, PTFE-bound porous carbon
  electrodes with Pt catalysts, 30 KOH electrolyte
• Runs on pure H2 pure O2
• Power generating efficiencies of up to 70, 0.3
  12 kW
• Compact.
• Small commercial units available up to 100 kW
• High power/weight ratio (hence space application)
• Produces pure water and heat
• Low thermal signature, silent, pollution-free
  exhaust
• Alkaline solution do not need noble metal
  catalysts (Siemens 1 mg cm-2 Ti-doped Raney
  Nickel/60 mg cm-2 Ag)

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The Alkaline Fuel Cell (AFD)
Gas Diffusion Electrode
Electrode support
KOH
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• AFC Problems
• Use of KOH as electrolyte and air as oxidant
  leads to fouling by precipitation of K2CO3
• High efficiencies achieved with high catalyst
  loadings
• H2O product dilutes KOH and reduces performance
  hence needs water evaporator
• CO or H2S in reformate poisons anode catalyst
• 1500 - 2500 per kW fuel cost 0.50 per kWh

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Monday 13 April 1970, 9.07 pm
200,000 miles out in space
O2 cryotank 2 explodes on Apollo 13
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The Apollo fuel cell power plant. 31 cells, 100
mA cm-2, in total 1.12 kW at 28V. 110 kg
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Solid Polymer Electrolyte (SPE) aka Polymer
Electrolyte Membrane (PEM) Fuel Cells

• Most favoured for traction (cars and buses)
  small family car (800 kg unladen weight, 80 km
  hr-1 cruising speed) needs 6 12 kW. Otherwise
  military (submarine) and space
• Runs on pure H2 air/O2
• Operating T up to ca. 90 C, PTFE-bound porous
  carbon electrodes with Pt catalysts, Solid
  Polymer (Nafion) electrolyte
• Small commercial units up to 500 W available

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• SPE Problems
• 2500 - 5000 per kW fuel cost 0.50 per kWh
• Needs water separator
• CO in fuel must be below 100 ppm

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Cathode
Anode
Hydrogen
Air (Oxygen)
H2 ? 2H 2e-
½O2 2H 2e- ? H2O
Methanol
CH3OH H2O ? 6H CO2 2e-
4 At the cathode, the electrons and positively
charged hydrogen ions combine with oxygen to form
water which flows out of the cell.
PEM
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2. Intermediate temperature fuel cells
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• Phosphoric Acid Fuel Cells (PAFC)
• The only commercially available fuel cell (gt 200
  fuel cell systems have been installed all over
  the world)
• Runs on H2, methane, natural gas air/O2
• Generate electricity at gt 40 efficiency (ca.
  85 if the steam produced is used for
  cogeneration cff ca. 35 for the utility power
  grid in the USA)
• Graphite feltlow Pt loading, concentrated
  phosphoric acid (polyphosphoric acid) electrolyte
  absorbed in SiC
• Operating temperatures 150 - 220 C
• High O2 solubility
• CO tolerant ca. 1 - 2 percent due to higher
  operating T
• Existing PAFCs have outputs up to 200 kW (11 MW
  units have been tested). Combined Heat and Power
  operation.

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• PAFC Problems
• 2000 per kW fuel cost 0.50 per kWh with
  reformer
• H2S in reformate poisons anode catalyst
• Need desulfurizer, water separator, heat
  exchanger and reformer- complex (especially wrt
  heat management) heavy system hence mainly
  stationary applications, although also buses.
• Oxidation of carbon support, agglomeration of Pt
  particles, flooding of electrodes and loss of
  acid- eg. reliability, lifetime and maintenance
  costs

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3. High temperature fuel cells
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• Molten Carbonate Fuel Cell (MCFC)
• Molten alkali metal carbonate (Li, Na, K)
  electrolyte in a cermaic tile, Ni anode and
  lithiated nickel oxide cathode
• Runs on H2, methane, natural gas air/O2
• 650 C as carbonate must be molten and conductive
• Higher overall system efficiencies combined
  cycle possibility for heat usage
• Greater flexibility in the use of available
  fuels.
• Envisaged for power production and load levelling

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• MCFC Problems
• Cost per kW not yet known, but must be brought
  down to lt 500 - 1000 per kW to match costs of
  conventional power stations fuel cost 0.50 per
  kWh with reformer
• Complexity- needs water evaporator, heat
  exchanger and reformer (but possibility of
  internal reforming-right T)
• Stability of electrodes and electrolyte matrix
  the high operating temperature, however, imposes
  limitations and constraints on choosing materials
  suitable for long lifetime operations

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• Solid Oxide Fuel Cell (SOFC)
• Solid, nonporous metal oxide electrolytes
  (stabilised ZrO2)
• 1000 C, hence internal reforming and rapid
  kinetics with nonprecious materials nickel
  anode, Sr-doped LaMnO3 cathode, ZrO2.15Y2O3
  solid electrolyte
• Produces high quality heat
• No restriction on the cell configuration.
• Power generating efficiencies of SOFCs could
  reach 60, 85 with co-generation.
• Experimental systems up to few kW

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• SOFC Problems
• Cost per kW not yet known fuel cost 0.50 per
  kWh with reformer
• Complexity- needs water evaporator, heat
  exchanger and reformer (but possibility of
  internal reforming-right T)
• Stability of electrodes and electrolyte matrix
  the high operating temperature imposes
  limitations and constraints on choosing materials
  suitable for long lifetime operations. Biggest
  problem is thermal expansion, rendering SOFC
  intolerant to repeated start-up-shut-down cycles.

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• 4. Other Fuel Cells
• Bio-
• Micro-

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5. The Direct Methanol Fuel Cell a
low temperature fuel cell
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• In the DMFC Methanol is oxidized directly at the
  anode (as opposed to H2 as in the commonly known
  Hydrogen PEMFC).

• Liquid CH3OH is preferred over vapour due to the
  simplicity of design offered existing liquid
  fuel distribution network.
• CH3OH is considered by some of have lower market
  entry barriers than H2 (eg. less explosive)

• Cell Reactions
• Anode CH3OH(l) H2O -gt 6e- 6H CO2(g)
  PtRu catalyst
• Cathode 1.5 O2(g) 6e- -gt 3H2O(l) Pt
  catalyst
• Overall CH3OH(l) 1.5 O2(g) -gt 3H2O(l)
  CO2(g) (E1.2 V, 90C)

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• DMFC problems
• Low temperature- poor kinetics at anode and
  cathode-much lower power density than H2/O2
• Needs Ru co-catalyst- Pt poisons otherwise
• Methanol cross over through membrane to cathode,
  Pt active for methanol oxidation, hence mixed
  potential

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Medium and high temperature Fuel cells have a
potentially major role in Distributed power
systems
Distributed generation commonly refers to on-site
power generation technology, which is tailored to
meeting the needs of the consumer. Combined Heat
and Power (CHP) systems are on-site generation
systems, which achieve high efficiency through
the concurrent production of electric power and
process heat (PAFC-heat houses, MCFC and SOFC
operate steam turbine). Distributed generation is
an alternative or complementary approach to
reliance on grid power. It provides another means
of meeting the nations future energy and security
needs while increasing the reliability of power
supply to the owners.
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Community Project Middlehaven
The Creation of an Energy Services Company (ESCo)
to Create a New Energy Approach to a Major
Regeneration Project
Coordinated new energy approach to whole
development including Energy saving design, Gas
Engine Combined heat and power, District heating
and Fuel Cell System balancing heat and power
requirement.
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The fuel cell stack
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The (SPE) fuel cell stack
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Heliocentris Water-cooled PEM fuel cell stack of
20 single cells.Rated output 300 W. Electric
heat output 300 W thermal. Open circuit voltage
18 V. DC rated voltage 12 V DC.
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1.5 kW H2/O2 Arbin Fuel Cell stack in Newcastle
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Facts and figures
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• For the hydrogen economy in general and
  hydrogen-powered
• vehicles in particular, the key problems remain
• How to generate hydrogen cheaply
• How to store hydrogen safely, and without a
  serious weight penalty
• How to distribute hydrogen
• Public perception

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Cost /
MEAs 6000
System 850
Bipolar plates 7400
Seals 1000
End plates/ current collectors 1200
Purchasing and assembly 2600
QA 1200
TOTAL 20000
For the low and medium temperature fuel cells,
additional problems are highlighted in the
following table opposite of costs, prepared by
the Center for Solar Energy and Hydrogen Research
in Ulm, for a 1 kW H2/O2 PEM stack.
Production of 1000 units could lower this unit
cost to ca. 3000
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• Electricity from the National Grid is sold at ca.
  0.07 per kWh
• Internal combustion engine 30 - 60 per kW
• PAFC ca. 2100 per kW
• Lead-acid battery 200 - 300 per kW
• However
• Small lithium batteries 300 per kWh

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Environmental Benefits Fuel cells are considered an excellent alternative energy resource from the environmental point of view. Fuel cells are quiet and produce negligible emissions of pollutants.Efficiency Different types of fuel cells have varied efficiencies. Depending on the type and design of fuel cells, efficiency ranges from 40 to 60. Alkaline fuel cells can even achieve power generating efficiencies of up to 70. Fuel Availability The primary fuel source for the fuel cell is hydrogen which can be obtained from natural gas, coal gas, methanol, and other fuels containing hydrocarbons.

 
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Comparision of carbon dioxide, nitrous oxides,
sulphur dioxide and noise emissions between the
four main engine types.
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