A Discussion of Fuel Cells
with particular reference to Direct Methanol Fuel Cells (DMFC’s)
Outline
Fuel Cell Definition
• Principle of operation
• Components: cell, stack, system
• Types
• Fuel-oxidant combinations
• Performance
• Efficiencies
Applications
Direct Methanol Fuel Cell (DMFC)
• Effect of Methanol impurities on cell performance
Issue: methanol as a high-purity cost-effective “direct” fuel cell feed
- specifications versus current commercial standards
- “benchmark” a distillation-based purification technology
Principle of Fuel Cell Operation
Consider a fuel cell reaction in which the fuel-oxidant combination
is hydrogen (H2) and oxygen (O2) - the reversal of water electrolysis
– in a solid polymer membrane-partitioned cell
H 2 g  
1
O2  g   H 2 Ol 
2
R
Key factors governing the operation
of a fuel cell
• Electrodes
Anode
Cathode
H2→ 2e-+ 2H+
+ 2e- → H2O
2H+ + ½O2
Proton flow
• Cell potentials
• Electrolyte
• Electrocatalysis
• Electrical charge transfer
Membrane
Fuel cells are steady-state Galvanic reactors to which reactants are
continuously supplied and from which products are continuously withdrawn
electrons
Ohmic losses occur during transport of electrons and ions
Flow field plate and gas porous anode substrate
Bipolarity: the substrate layer may be linked to adjacent cells
Electrolyte: materials, structures and thickness balance high conductivity against low porosity
Thin gas porous catalyst layer
- good ionic contact with the electrolyte is essential
Stack components
Key design concerns:
• Mass transfer effects
• Heat management
Fuel Cell Components
• Bipolar plates
• Membrane Exchange Assembly
(MEA)
• Current collector plates
• End plates
Types of Fuel Cells defined by:
a) electrolyte, as this defines chemical environment; and, b) by temperature of operation
Fuel Cell
Acronym
Temp.
range
(°C)
Alkaline
AFC
60 – 90
Solid
Polymer
SPFC,
PEMFC(2
70 – 90
Phosphoric
acid
PAFC
~220
Molten
Carbonate
MCFC
~650
Solid Oxide
SOFC
~1000
Anode Reaction
(1)
Cathode Reaction
(1)


H 2  2O H  2H 2O  2e


H 2  2H  2e
1 O  2H   2e   H O
2
2 2
1 O  2H   2e   H O
2
2 2


H 2  2H  2e
2
H 2  CO  H 2O  CO2 2e
3
H2 O
2
 H 2 O  2e
1 O  H O  2e   2O H
2
2 2

1 O  CO  2e   CO2
2
3
2 2
1 O  2e   O2
2 2
(1) The charge carrier in the case of each of the fuel cell types is shown in bold letters.
(2) Proton Exchange Membrane Fuel Cell
Fuel – Oxidant Combinations
Oxidant: Oxygen from air for economic reasons
Fuels
Hydrogen:
• generated from fuels such as natural gas, propane, methanol, petrochemicals
- typically reformed gas contains approximately 80% hydrogen, 20% CO2
• in high temperature cells, internal steam reforming of (for example) methane and
methanol can take place by the injection of the fuel with steam
• storage technologies: gas cylinders; cryogenic liquid, metal hydride matrix
• “renewable” hydrogen from water electrolysis
• the demand for hydrogen purity decreases with increasing operating temperature
Methanol:
• reforming takes place at 250°C
• “direct” feed to the cell in water mixture
Fuel Cell Performance
Energy generation by electrochemical reaction: dWe = - Vdq = - V[nΓdε]
Reversible potential - maximum cell potential:
for hydrogen oxidation
o
rev = ΔG /nΓ
o
E rev = 1.23 v
E
o
the equilibrium oxidation and reduction rates of reaction at the electrode
defines the exchange current density
– a strong measure of the facility of the overall electrochemistry
E0mf
slope reflects ohmic resistance
Characteristic
Performance Curve
kinetic effects
E0mf - V
Vc
= overpotential
Voltage
mass transfer effects
Current
Overpotential = f(T, exchange current density)
Heat generation = f(overpotential)
Fuel Cell performance
A high performance cell:
1 Acm-2 at 1 Volt potential (1 Wcm-2 power density)
Fuel Cell
Temp.
°C
Pressure
atm
(kPa)
Current
density
A/cm2
Voltage
V
Alkaline
70
1 (101)
0.2
0.8
Phosphoric
acid
190
1 (101)
0.324
0.62
Phosphoric
acid
205
8 (808)
0.216
0.73
Molten
carbonate
650
1 (101)
0.16
0.78
Solid oxide
1000
1 (101)
.2
0.66
Power Generating Fuel Cell Efficiency
• efficiency at a given current density:
E = 0.675V
• H2/O2 cell: theoretical maximum thermodynamic efficiency:
Eth = 83%
• at an open-cell voltage of 1 Volt (let us say), the max. electrochemical efficiency is 80%
corresponding to an open-circuit fuel-cell efficiency of approximately 65%
Chemical Energy of the Fuels
Heat
Engine:
Thermal Energy
Conversion
Electrochemical reaction
Electrical Energy
Conversion
Mechanical Energy
Conversion
The theoretical maximum thermodynamic efficiency of a heat engine is:
Ecarnot = 1 – TL/TH
The Carnot cycle must draw its energy from a heat source at 1480°K in order to
match the theoretical maximum thermal efficiency of the H2/O2 fuel cell
Currently Developed Types of Fuel Cells
- after Gregor Hoogers, (ed.,) Fuel Cell Technology Handbook, CRC Press, 2002
Fuel Cell
Fuel
Electrolyte
Electric
Efficiency
(system) (%)
Alkaline
Pure H2
35 – 50% KOH
35 - 55
Proton
Exchange
Membrane
Pure H2
(e.g.,)
NAFION®
35 - 45
Phosphoric acid
Methanol
Pure H2
Concentrated
phosphoric acid
40
> 50
Molten
carbonate
H2, CO, CH4,
other
hydrocarbons
Lithium and
potassium
carbonate
Solid oxide
H2, CO, CH4,
other
hydrocarbons
Yttriumstabilized
zirconium
dioxide
CHP: combined heat and power generation
Power Range
and
Application
< 5 kW
military,
space
5 –250 kW
portable, CHP,
transportation
200 kW
CHP
More Power
for less Fuel
> 50
200 kW-MW
CHP, gridindependent
power
2 kW – MW
CHP, gridindependent
power
Applications
Smart Fuel Cell A25-0
www.smartfuelcell.com
• Portable market:
recreation, remote industrial
• 25W @ ~12 V
• 1.5 L Methanol/ KWh
• 2.5 L plastic container
Siemens-Westinghouse
Stationary Power Generation Unit
Direct Methanol Fuel Cell (DMFC)
Anode:
dilute methanol/water feed
CO2 rejection
Potential benefits
• Liquid fuel
- high energy density/unit volume
• Current distribution network
• No need for fuel reforming
Technological Limitations
• Poor electrode kinetics
- anode andcathode
• Mass transport effects
- CO2 and water rejection
Pt-based catalyst system
PEM membrane
• Methanol crossover
Methanol Purity Requirements
Published allowable impurity limits in commodity methanol not directly applicable
ASTM
Fuel Cell (ppm)
carbon monoxide
wt %, max
0.0001
1
methane
wt %, max
0.005
50
acetone + aldehydes
wt %, max
acetone
wt %, max
0.001
10
ethanol
wt %, max
0.01
100
acidity
wt %, max
0.003
water
wt %, max
0.01
0.003
2.0
• CO as an inert adsorbate on Pt surface
- at 10 ppm reduces H2/PEMFC cell voltage by 50% at 0.5 Acm-2
• CO2 effect is modest compared with CO
• ethanol and aldehydes are electrochemical fuels
Methanol as a Direct Feed to Fuel Cells - Issues
•
•
•
•
What is the commercial value of ultra-pure liquid methanol in direct methanol
electro-oxidation?
Can the ultra-pure methanol be produced at commodity prices
- without necessarily having the benefit of economy of scale
- using distillation as the primary purification technology?
This project serves to establish an important technological and economic
“benchmark”:
- the “distillation + recycle” case
What is the relationship between purity and energy requirement?
Is there a need and opportunity to make some of the energy versus buying all
of the requirement?
(Are there special storage requirements for ultra-pure methanol?)