Fuel Cells Lecture Outline

CHEE 470Fuel Cells Lecture

Outline

1. Definition

2. Principles of Operation

3. Components of a Single Fuel Cell

4. Fuel Cell Stack and System Components

5. Types of Fuel Cells

6. Fuel-Oxidant Combinations

7. Fuel Cell Performance

8. Fuel Cell Efficiencies

9. Applications

10. Direct Methanol Fuel Cells (DMFCs)

11. Direct Methanol Fuel Impurities

12. Methanol as a Direct Feed to Fuel Cells Issues

Purpose

A tidy and linear overview of fuel cell science and technology,

which leads to a discussion of the issues arising in developing an approachto providing high purity methanol as a direct fuel cell feed.

1 Definition

Galvanic cells are devices which convert the intrinsic chemical free energy of a fuel directly into direct-current electrical energy in a continuous catalytic process.

Fuel cells are steady-state reactors to which reactants are continuously supplied and from which products are continuously withdrawn

The fuel cell itself should not itself undergo change; that is, unlike the electrodes of a battery, its electrodes ideally remain invariant.

1.1 Batteries

As so-called Primary cells, batteries are unsteady-state devices containing initial fixed amounts of reactants

Secondary cells function as galvanic cells in use and are regenerated by reversing the cell reaction through the addition of electrical energy- a lead-acid automotive battery is a series-connected device

1.2 Electrolytic cells

Electrochemical cells in which chemical reactions are forced by the application of electrical energy

2 Principle of operation

An ionic mechanism produces and consumes electrons at two isolated phases of the cell the electrodes.

The electrochemical reaction occurs at the interface between the electrodes and an electrolyte the third essential phase in an electrochemical cell.

Electrocatalysis drives the power generation process.

A transfer of charge occurs by the flow of the electrons between the electrodes through an external circuit.

2.1 Electrodes are good electron conductors

An electrochemical cell may be considered to consist of two half cells- each is associated with one of the cell electrode reactions2.1.1 Anode

the electrode at which an electron-producing ionic reaction dominates

the electrode potential, anode, ascribed to this reaction is referred to as the oxidation potential2.1.2 Cathode

the electrode at which an electron-consuming ionic reaction dominates

the electrode potential, cathode, ascribed to this reaction is referred to as the reduction potential

2.1.3 Cell potential

The magnitude of the potentials in each of the half cells is quite arbitrary

It is the difference between the potentials the cell potential which drives the flow of current.

For a galvanic cell

- the cathode is positive with respect to the anode

The direction of electron flow in the external circuit is always from anode to cathode.

Both oxidation and reduction reactions occur at an electrode, even if one direction dominates when a net current is produced.

When the electrodes exist at thermodynamic (internal) equilibrium, and when the cell and the external circuit are in equilibrium, so that there is no net current flow, the electrodes are at their reversible potential, and the potential difference between them is the electromotive force (emf) of the cell.

To produce a net current flow, the electrodes are moved away from their reversible potentials- the extent to which the electrodes become polarized away from their respective reversible potentials is referred to as their overpolarization.

The cell potential when current is drawn is the cell voltage.

The cell potential when the external current is zero, but the electrodes are not in the thermodynamic equilibrium condition is referred to as the open circuit voltage.2.2 Electrolyte

Internal charge transfer by ionic conduction is needed to complete the circuit

Electrolytes need to be poor electronic conductors in order to avoid internal short-circuiting

Depending upon the electrolyte, either:protons are produced by reaction at the catalytic anode-electrolyte interface; or,oxide ions are produced by reaction at the catalytic cathode-electrolyte interface

These are transported through an ion-conducting but electronically insulating electrolyte to combine with oxide or protons at the other electrode to generate water and electric power.

The electrolyte must limit the crossover of fuel from the anode to the cathode in order to avoid creating mixed potential- that is, some electrochemical oxidation of fuel competes with oxygen reduction at the cathode, and as a consequence the cathode potential is drawn towards the equilibrium oxidation potential, reducing the cell potential difference.

2.3 Electrocatalysis

Charge moving across the electrode/electrolyte interface must overcome an activation energy barrier.

This is accomplished by the overpolarization of the electrodes.

The degree of activation polarization required is determined by the activity of the catalyst.

2.3.1 Generic requirements of hetereogeneous catalysts(e.g.,) a large number of sites

high intrinsic activity (turnover frequency)

chemical stability

cost and quality of manufacture

2.3.2 Specific requirements

porous catalytic materials with high surface areas- to ensure a large contact area between electrolyte and gas

good electrochemical contact with the membrane to ensure low resistance to the interfacial transport of charge.

high electrical conductivity- to minimize resistive losses in the layer

2.4 The H2/O2 Cell

The conduction or, carrier - ion is the hydrogen ion, produced from hydrogen at the anode and consumed to give water at the cathode.

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2.4.1 Anode:

The fuel hydrogen is catalytically oxidized into electrons and protons.

When operating on pure hydrogen, the anode stays at a potential close to the theoretical reversible potential of a hydrogen electrode of zero- indicative of the hydrogen oxidation being a kinetically facile reaction- a low activation reaction

By convention, the standard hydrogen electrode is assigned a potential of zero. The hydrogen electrode is said to have zero overpotential

2.4.2 Cathode:

Oxygen is reduced to oxide species in a series of complex reactions taking place at the catalyst layer

The oxygen reduction reaction requires the breaking of a double bond with transfer of four electrons per molecule in a complex series of reactions.

As an activated reaction, high overpotential contributes to the losses in fuel cell voltage at a given current in low-temperature fuel cellsIn high-temperature systems, oxygen reduction contributions to voltage efficiency losses are less significant, since the reaction rate increases with temperature.

2.4.3 Net Reaction:

3 Components of a single fuel cell

3.1 Electrode materials

The type of electrolyte employed strongly influences electrode material and structure requirements- (e.g.,) corrosion effects, interfacial phenomena

Carbon fibre papers or woven cloths may serve as substrate materials

3.2 Catalyst materials

3.2.1 Noble metals

Platinum affords excellent activity and stability

catalyst loading of platinum minimized - supported catalyst technology employed in gas phase kinetics adopted- (e.g.,) highly dispersed particles in specialized carbon substrates

highly susceptible to poisoning by carbon monoxide

3.2.2 Other materials

Nickel alloys is a good anode material for hydrogen- nickel oxide cathode

Specialized carbon materials and structures

solid oxide materials are employed in cells operating at high temperature (~ 1000C)

3.3 Electrolyte

Anisotropic structures- solid/solid or molten liquid /solid

3.4 Cell internal impedance

Ohmic losses occur during transport of electrons and ions (protons)

Electrolyte materials, structures and thickness chosen to balance high conductivity against low porosity

3.4.1 Geometry and resistance

The electrode substrate layer is designed to balance achieving low ohmic losses high land area in contact with the catalyst layer - against achieving effective fuel distribution through having a high open area

Thin catalyst layers in good ionic contact with thin electrolyte sections

3.5 Mass transfer effects

Macroscopic flooding of the electrode at which the product water is formed, or of the anode when a liquid fuel is used, must be avoided.

Blocking of gas access to the catalyst pores by water droplets can lead to reduced oxygen partial pressure, due to nitrogen blanketing, when air is the oxidant

Composites structures impregnated with Telfon may be used for controlled wetting of electrodes

4 Fuel cell stack and system components

Planar stack design generally favoured- mechanical stability and sealing- minimization of the contribution of stack components to ohmic losses

Tubular bundle configuration may be employed with a solid electrolyte

4.1 Membrane Exchange Assembly (MEA)

Catalysts form thin (several microns to several tens of microns) gas-porous layers on either side of a membrane electrolyte to create a compact cell core- ionomer coating of the electrodes may be employed to enhance ionic contact with the MEA- applied (for example) by hot-pressing techniques

4.2 Bipolar plates

The substrate layer may be linked to adjacent cells of a stack

High conductivity balanced against gas access

Machined flow fields with high large-scale porosity- (e.g.,) Serpentine flow channels

Gas distribution design must account for two-phase flow- at the anode when water is added for humidification of membrane electrolytes- at the cathode for product water removal

Graphite-based materials are expensive

Corrosion resistance is a concern with metallic plates

4.3 Heat management

Closed-loop circulation of fuel or antioxidant flows in excess of the stoichiometric requirements may be employed for cooling

Endothermic fuel reforming chemical reactions may provide a heat sink for removal of heat generated by the exothermic electrochemical reactions within the cell

Cooling plates may be employed between bipolar plates in high power output stacks

Cooling jackets or air cooling may be employed

The heat generated can be used in cogeneration heating- offering enhanced overall process efficiency(and the generated water may be useful)4.4 Other stack components

4.4.1 Current collector plates

4.4.2 End plates

5 Fuel Cell types

Fuel cells are classified by the nature of the electrolyte- as this defines chemical environment

The other significant feature defining fuel cell classification is the temperature of operationTable 1 Currently Developed Types of Fuel Cells

Fuel CellAcronymTemp.range (C)Anode Reaction(1)Cathode Reaction(1)

AlkalineAFC60 90

Solid

PolymerSPFC,PEMFC(270 90

Phosphoric acidPAFC~220

MoltenCarbonateMCFC~650

Solid OxideSOFC~1000

EMBED Equation.3

Notes:

(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

5.1. Sulphuric acid - Groves experiments (1842)

Two platinum electrodes were half-way submerged into a beaker of sulphuric acid

Two tubes containing hydrogen and oxygen gas, respectively, were inverted and lowered over each of the electrodes

the gases displaced the electrolyte- leaving only a thin film

flow of electrons between the electrodes indicated by galvanometer deflection

after the initial deflection, the current decreased the reaction rate could be restored by recreating the electrolyte layer- thin enough to allow gas to diffuse to the solid electrodeShell employed sulphuric acid in their experiments on Direct Methanol Fuel Cells (DMFC) in the 1960s- when they were looking for an inexpensive, less complex alternative to their demonstration hydrogen-air system

5.2. Strong alkalis (NaOH, KOH)

absorption of carbon dioxide reduces electrolyte conductivity

used in space program Apollo - with cryogenic hydrogen and oxygen- operating at 260C, with sintered nickel oxide cathodesallowed the use of electrode materials besides noble metals (platinum)

The space shuttle orbiter: PTFE-bonded low-loading platinum-based electrodes and graphite intercell separator plates.

5.3. Molten phosphoric acid

Developed to use natural gas- steam reformed to produce hydrogenHigher temperature of operation chosen to:- assist the removal of carbon monoxide produced in the reforming operation- improve heat management by meeting the heat requirements of the endothermic reforming reaction by burning excess fuel from the anode reaction

Held in some form of matrix- solidification at 45C

5.4. Molten Bases

Mixture of alkali carbonates- typically Li2CO3 and K2CO3

Sealing of the two gas compartments from each other by using nanoporous materials with tightly controlled pore size

Operate on CO as a fuel

Nickel-based anode catalyst allows direct internal processing of fuels such as natural gas

High temperature corrosion a concern5.5. Solid Oxides

Entirely solid-state design

Tube bundle design attempts to deal with sealing problems associated with the conventional planar design

Choice of materials governed by thermal stress and corrosion resistance requirements

5.6. Solid polymer electrolyte

A membrane electrolyte must form a sound electronic insulator and gas barrier, while allowing rapid proton transport

Nafion displays high dimensional stability and chemical inertness

It consists of a Polytetrafluoroethylene (PTFE) backbone with a perfluorinated vinyl polyether ether side chain- attached via oxygen atoms

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OCF2CF2SO3H

The sulphonic acid groups at the end of the side chain impart the cation exchange functionality to the polymer

The membrane relies on the presence of liquid water for effective proton conduction- this limits the temperature of operation

vehicle (H3O+) mechanism of proton transport proposed- other principal mechanisms involve reorganization of the proton environment to create uninterrupted pathways for proton migration

Proton conductivity versus fuel permeability becomes the key concern in material selection

The current high cost of Nafion is a barrier to its commercial use

6. Fuel-oxidant combinations

6.1. Oxidants

Oxygen from air for economic reasons

or as a cryogenic liquid for example, for use in space craft

6.1.1. oxidant utilization

the Utilization factor is usually in the range of 50 to 60%

6.2. Fuels

6.2.1. Hydrogen

Hydrogen fuel is generated from fuels such as natural gas, propane, methanol, or other petroleum products

In the high-temperature cells, internal steam reforming of simple hydrocarbons and alcohols for example, methane and methanol can take place by the injection of the fuel with steam.

Typically reformed gas contains approximately 80% hydrogen, 20% carbon dioxide

CO consumed directly via the internal water-gas shift process

Storage technologies: high pressure gas; cryogenic liquid; metal hydride matrix

Practical hydrogen-bearing fuels which have reactivities in low-temperature cells provided that suitable anode catalysts are used - include:hydrocarbons