Transcript here

An introduction to fuel cells
P. A. Christensen
Winshields Crag on the Roman Wall
“For nearly 2000 years, Hadrian’s 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”
Genesis 1839 - 1842
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.
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
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
Why hydrogen?
Storage method
Hydrogen
Energy density
/kWh kg-1
38
Gasoline
14
Lead acid battery
0.04
Flywheel, fused silica 0.09
(Methanol)
(6)
An early demise…
…..and resurrection 1960
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.
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 1960’s, due to interest from
an out of this world source!
The Gemini capsule and fuel cell
The fuel cell concept
Products
Oxidant
e-
H+
Cathode
Anode
Solid, liquid or
polymer electrolyte
Fuel
Products
The fuel cell concept
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
O2
+
4e-
H2
lyte
H+
Electro
4H+
Cathode (Pt catalyst)
2H2
Pt
Anode (Pt catalyst)
e-
4H+
+
4e-
+ O2
H2O
The simplest realisation – the H2/O2 fuel cell
Pt
2H2O
Practicalities
FlowField
Catalyst Layer
Gas
Diffusion
Layer
Gases:
Wettability
Flooding/conductivity
3-phase interface
The structure of Gas Diffusion Electrodes (GDE’s)
(Porous carbon area up to 1000 m2 g-1)
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
Carbon + Pt
Electrolyte
Pore
Gas space
Carbon + Pt
Reaction zone
The three-phase zone in a single pore of a gas diffusion electrode
1.2
Thermodynamic cell voltage
Cell Voltage
1.0
Ohmic loss
0.8
0.6
Activation overpotential
loss - catalysts
Mass transport
loss
0.4
0.2
0.0
0.0
0.2
0.4
0.6
0.8
1.0
Cell current /A
A typical (H2/O2) fuel cell voltage vs current plot
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:
CnH2n+2 + nH2O  nCO + (2n+1)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.
Common Fuel cells
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
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)
The Alkaline Fuel Cell (AFD)
Gas Diffusion
Electrode
Electrode support
KOH
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
Monday 13 April 1970, 9.07 pm
200,000 miles out in space
O2 cryotank 2 explodes on Apollo 13
The Apollo fuel
cell power plant.
31 cells, 100 mA cm-2, in
total 1.12 kW at 28V.
110 kg
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
SPE Problems
• £2500 - £5000 per kW; fuel cost £0.50 per kWh
• Needs water separator
• CO in fuel must be below 100 ppm
2
1
At the anode, a platinum or non- platinum
catalyst causes the hydrogen to split into
positive hydrogen ions (protons) and
negatively charged electron.
Hydrogen is channelled through field flow
plates to the anode on one side of the fuel cell,
while oxygen from the air is channelled to the
cathode on the other side of the cell
Cathode
Anode
Hydrogen
Air
(Oxygen)
H2 → 2H+ + 2e-
½O2 + 2H+ + 2e- → H2O
Methanol
CH3OH + H2O →
6H+ + CO2 + 2e-
3
The Polymer Electrolyte
Membrane (PEM) allows only
the positively charged ions to
pass through it to the cathode.
The negatively charged electron
must travel along an external
circuit to the cathode, creating
an electrical current.
PEM
4 At the cathode, the electrons
and
positively
charged
hydrogen ions combine with
oxygen to form water which
flows out of the cell.
2. Intermediate temperature fuel cells
Phosphoric Acid Fuel Cells (PAFC)
• The only commercially available fuel cell (> 200 fuel cell
systems have been installed all over the world)
• Runs on H2, methane, natural gas + air/O2
• Generate electricity at > 40% efficiency (ca. 85% if the steam
produced is used for cogeneration; cff ca. 35% for the utility
power grid in the USA)
• Graphite felt+low 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.
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
3. High temperature fuel cells
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
MCFC Problems
• Cost per kW not yet known, but must be brought down to
< £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
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, ZrO2.15%Y2O3 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
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.
4. Other Fuel Cells
•
Bio•
Micro-
5. The Direct Methanol Fuel Cell – a low
temperature fuel cell
 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 -> 6e- + 6H+ + CO2(g) PtRu
catalyst
Cathode: 1.5 O2(g) + 6e- -> 3H2O(l) Pt catalyst
Overall: CH3OH(l) + 1.5 O2(g) -> 3H2O(l) + CO2(g) (E°=1.2
V, 90°C)
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
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 onsite 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.
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.
The fuel cell stack
The (SPE) fuel cell stack
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.
1.5 kW H2/O2 Arbin Fuel Cell stack in Newcastle
Facts and figures
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
Cost /€
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.
MEAs
System
Bipolar plates
6000
850
7400
Seals
End plates/
current
collectors
Purchasing and
assembly
QA
1000
1200
TOTAL
2600
1200
20000
Production of 1000 units could lower this unit cost to ca. €3000
• 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
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.
Comparision of carbon dioxide, nitrous oxides, sulphur dioxide and
noise emissions between the four main engine types.
Silicon Chip Online http://www.siliconchip.com.au/cms/A_30527/article.html