A Discussion of Fuel Cellswith 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
Issue: methanol as a high-purity cost-effective “direct” fuel cell feed- specifications versus current commercial standards- “benchmark” a distillation-based purification technology
Direct Methanol Fuel Cell (DMFC)• Effect of Methanol impurities on cell performance
R
H2→ 2e-+ 2H+ + 2e- → H2O 2H+ + ½O2
Anode Cathode
Proton flow
Membrane
Principle of Fuel Cell Operation
Consider a fuel cell reaction in which the fuel-oxidant combinationis hydrogen (H2) and oxygen (O2) - the reversal of water electrolysis
– in a solid polymer membrane-partitioned cell
lOHgOgH 222 2
1
• Electrodes
• Cell potentials
• Electrolyte
• Electrocatalysis
• Electrical charge transfer
Key factors governing the operation of a fuel cell
Fuel cells are steady-state Galvanic reactors to which reactants are continuously supplied and from which products are continuously withdrawn
Fuel Cell Components
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
electronsOhmic losses occur during transport of electrons and ions
Stack components
• Bipolar plates
• Membrane Exchange Assembly (MEA)
• Current collector plates
• End plates
Key design concerns:
• Mass transfer effects
• Heat management
eOHH 22222 OH OH22222
1 eOHO
eH 222 H OHeO 222221 H
eH 222 HOHeO 22222
1 H
eCOOHH 2222-2
3CO
23COeCOO 2222
1
eOHH 2222O 2OeO 222
1
Fuel Cell AcronymTemp.range (°C)
Anode Reaction(1)
Cathode Reaction(1)
Alkaline AFC 60 – 90
SolidPolymer
SPFC,PEMFC(2
70 – 90
Phosphoric acid
PAFC ~220
MoltenCarbonate
MCFC ~650
Solid Oxide SOFC ~1000
Types of Fuel Cells defined by: a) electrolyte, as this defines chemical environment; and, b) by temperature of operation
(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: Eo
rev = ΔGo/nΓ
for hydrogen oxidation Eo
rev = 1.23 v
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
Overpotential = f(T, exchange current density)
Heat generation = f(overpotential)
E0mf
Voltage
Current
Vc E0mf - V
Characteristic Performance Curve
kinetic effects
slope reflects ohmic resistance
mass transfer effects
= overpotential
Fuel Cell Temp. °C
Pressureatm(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
Fuel Cell performance
A high performance cell:1 Acm-2 at 1 Volt potential (1 Wcm-2 power density)
Thermal EnergyConversion
Mechanical EnergyConversion
Chemical Energy of the Fuels Electrical Energy Conversion
Electrochemical reaction
Heat Engine:
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%
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
Fuel Cell Fuel ElectrolyteElectricEfficiency(system) (%)
Power Range andApplication
Alkaline Pure H2 35 – 50% KOH 35 - 55< 5 kWmilitary,space
Proton ExchangeMembrane
Pure H2
Methanol(e.g.,) NAFION®
35 - 45 5 –250 kWportable, CHP, transportation
Phosphoric acid Pure H2
Concentrated phosphoric acid
40200 kWCHP
Molten carbonate
H2, CO, CH4,
other hydrocarbons
Lithium and potassium carbonate
> 50200 kW-MWCHP, grid-independent power
Solid oxide H2, CO, CH4,
other hydrocarbons
Yttrium-stabilized zirconium dioxide
> 502 kW – MWCHP, grid-independent power
Currently Developed Types of Fuel Cells- after Gregor Hoogers, (ed.,) Fuel Cell Technology Handbook, CRC Press, 2002
CHP: combined heat and power generation
More Power
for less Fuel
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)
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
• Methanol crossover
Anode:
dilute methanol/water feedCO2 rejection
Pt-based catalyst system
PEM membrane
carbon monoxide wt %, max 0.0001 1
methane wt %, max 0.005 50
acetone + aldehydes wt %, max
acetone wt %, max 0.003 0.001 10
ethanol wt %, max 0.01 100
acidity wt %, max 0.003
water wt %, max 0.01 2.0
Methanol Purity Requirements
ASTM Fuel Cell (ppm)
Published allowable impurity limits in commodity methanol not directly applicable
• 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?)