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steady-state Galvanic reactors to which reactants are continuously supplied and from which products are continuously withdrawn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?)