POWDER TECHNOLOGY COURSE PROJECT

“AN INTRODUCTION TO

CATALYST SYNTHESIS

TECHNIQUES” WITH AN EMPHASIS ON PEM FUEL CELL

CATALYSTS

Supervisor: Dr. Rezvanpour

Student: Ali Hashemi

Catalyst Synthesis Techniques

Particle Size and Shape Control

Physical Characterization of Electrocatalysts

Catalyst Contamination in PEM Fuel Cells

Fuel cell …

◦ is an electrochemical device that continuously and

directly converts the chemical energy of externally

supplied fuel and oxidant to electrical energy

Five most common technologies are:

◦ Polymer electrolyte membrane fuel cells (PEM fuel cells

or PEMFCs)

◦ Alkaline fuel cells (AFCs)

◦ Phosphoric acid fuel cells (PAFCs)

◦ Molten carbonate fuel cells (MCFCs) and

◦ Solid oxide fuel cells (SOFCs)

Basic Reactions

Anode Reaction

H22H++2e

Cathode Reaction

1/2O2+2H++2eH2O

Catalysis Synthesis Methods

Low-temperature Chemical

Precipitation

Supported and unsupported catalysts …

can be made via addition of a reducing agent to a

platinum-salt solution

Bimetallic catalysts …

can be made by the co-precipitation of a solution of

two metal precursor salts

To avoid chloride contamination

One route is to use carbonyl precursors that decompose

at low temperatures

Colloidal

Same as chemical precipitation except

It involves the added benefit of a capping agent

… that allows for size control of the catalysts and

prevents agglomeration of the catalyst particles

The experimental procedure is as simple as …

combining the metal source, a reducing agent, and a

capping agent together and mixing

Sol-gel(Chemical Solution Deposition)

Fabrication of materials (typically a metal oxide) starting from a chemical solution (or sol) that acts as the precursor for an integrated network (or gel) of either discrete particles or network polymers

which undergo various forms of hydrolysis and polycondensation reactions.

The precursor sol can be either deposited on a substrate to form a film (e.g., by dip coating or spin coating), cast into a suitable container with the desired shape (e.g., to obtain monolithic ceramics, glasses, fibers, membranes, aerogels), or used to synthesize powders (e.g., microspheres, nanospheres)

Sol-Gel Technologies and Their

Products

In acid-catalyzed sols, the

interparticle forces have

sufficient strength to cause

considerable aggregation

and/or flocculation prior to

their growth.

In base-catalyzed sols, the

particles may grow to

sufficient size to become

colloids, which are affected

both by sedimentation and

forces of gravity

Incorporation of metallic nanocatalysts:

Adding prefabricated particles into the sol-gel

mixture

Addition of metal salts during gel formation or after

the mesoporous structure has formed

A significant drawback!

catalytic nanoparticles may be buried within the

structure rather than near the pores

Sol-Gel Process:

1. Formation of a liquid solution of suspended

particles (a sol)

2. Aging

◦ to allow fine-tuning of the gel properties

3. Drying

◦ to remove the solvent from the gel

4. Calcification

◦ to permanently change the physical and chemical

properties of the solid

Impregnation

High-surface-area carbon black can be

impregnated with catalyst precursors by mixing the

two in an aqueous solution

Following the impregnation step, a reduction step is

required to reduce the catalyst precursor to its

metallic state

The most common platinum precursors used for impregnation are chloride salts

To avoid chlorine poisoning…

Metal sulfite salts, metal carbonyl complexes, and metal nitrate salts have been used instead

Metal carbonyl complexes :

can easily be made by direct oxidation of the metal chloride salt with carbon monoxide

An external reducing agent is not required, as nanoparticles can then be formed by thermal decomposition of the metal carbonyl complexes

Microemulsions Schematic of a reversed micelle formed using Na+-AOT as

surfactant ion

Microemulsions are clear, stable, isotropic liquid

mixtures of oil, water and surfactant, frequently in

combination with a cosurfactant.

The aqueous phase may contain salt(s) and/or other

ingredients, and the "oil" may actually be a complex

mixture of different hydrocarbons and olefins.

In order to obtain the catalyst nanoparticles,

Metal salt is reduced by adding a reducing agent into the

microemulsion system

Another approach is to

Mix the microemulsion system that contains a reducing agent

with a microemulsion system that contains the metal salt

Once the nanocatalysts are formed …

They can be deposited onto a support

By adding a solvent like tetrahydrofuran (THF) in conjunction

with the support powder to the microemulsion

The solvent destabilizes the microemulsion by competing with

the surfactant to adsorb onto the particles, and in the

destabilized system the particles will adsorb onto the

support.

It is likely that …

Formation of catalysts via the microemulsion process

proceeds in two steps:

1. Nucleation of the metal catalyst inside the droplet

2. Aggregation of multiple nuclei via collision and

coalescence of droplets to form the final

nanocatalysts

Electrochemical Deposition

Occurs at the interface of an electronically

conductive substrate and an electrolyte solution

containing the salt of the metal to be deposited

Five stages to electrochemical

deposition of metals …

1. Transport of metal ions in solution to the electrode

surface

2. Electron transfer

3. Formation of metal ad-atoms via adsorption

4. Nucleation and growth, two- or three-dimensional,

of metal particles

5. Growth of the three dimensional bulk metal phase

2D or 3D Nuclei Growth?

Binding energy of the metal ad-atom to the

substrate Vs Binding energy of the metal ad-atom

to itself

Spray Pyrolysis

A process in which a thin film is deposited by

spraying a solution on a heated surface, where the

constituents react to form a chemical compound

Spray Pyrolysis Process:

An aqueous solution containing the metal precursor

is atomized into a carrier gas that is passed through

a furnace.

Second, the atomized precursor solution deposits

onto a substrate, where it reacts and forms the final

product

Advantages compared to other metal-forming

techniques …

Very easy to dope films or form alloys in any proportion by manipulating the spray solution

Neither high-purity targets and substrates nor vacuum set-ups are required

Deposition rates and therefore film thickness can easily be controlled by controlling the spray parameters

Moderate operation temperatures (100–500 °C) allow for deposition on temperature-sensitive substrates

Low energy consumption

Relatively limited environmental impact since aqueous precursor solutions can be used

The process is scalable, with production rates as high as 1.1 kg/h

Vapor Deposition

Chemical Vapor Deposition (CVD)

In a typical CVD process, the wafer (substrate) is

exposed to one or more volatile precursors, which react

and/or decompose on the substrate surface to produce

the desired deposit.

Types of chemical vapor deposition

Classified by operating pressure:

Atmospheric pressure CVD

Low-pressure CVD (LPCVD): Reduced pressures tend to

reduce unwanted gas-phase reactions and improve film

uniformity across the wafer

Ultrahigh vacuum CVD (UHVCVD)

Metal alloys can be fabricated by CVD

Using a single-source precursor (that remains coordinated in the

vapor phase) allows for precise control of the ratio of the two

metals

Physical Vapor Deposition (PVD)

A variety of methods to deposit thin films by the

condensation of a vaporized form of the material onto

various surfaces

Four essential components

Vacuum

A source to supply the material, called a target

A substrate on which the film is deposited

An energy supply to transport the material from the source

to the substrate.

High-energy Ball Milling

Is a mechanical alloying

process and a method

of

Grinding and mixing

materials in the absence

or presence of a liquid.

Lead antimony grinding

media with aluminum

powder

Particle Size and Shape Control

Why?

Increase in the ratio of surface atoms to bulk atoms

Activity for many reactions shows a maximum at a

particular particle size

Importance of crystal phases for catalytic reactions

Mechanism for Size Control Using

Colloidal Synthesis Methods

Size range of interest in practical fuel cell catalysis

1 to 5 nm range

In the case of the reduction of O2 (ORR), Pt

particles in the 3.5 to 4 nm size range are believed

to be the most active catalysts, while for bi-metallic

Pt/Ru catalysts the optimal size range appears to

be in the range of 2.5 nm

Pt-sols Made Using Organic Stabilizers

The stabilizer adsorbs on the surface of the Pt nuclei

and prevents them from agglomerating

Control particle size via

Concentration of the metal precursor salts

Stabilizing agent,

Synthesis temperature.

Modified Polyol Methods

Polyol Method:

Synthesis of high surface area catalysts in ethylene glycol

Ethylene glycol acts as solvent and reducing agent.

Modified Polyol Method: A straightforward,

“inexpensive” catalyst synthesis method of low toxicity

Size of the Pt and Ru colloids can be controlled by the

addition of H2O to the synthesis solution.

A large amount of glycolic acid (D) is formed which acts as

a stabilizer for the Pt/Ru colloids

Size Control Using Electrochemical

Methods

May not be as attractive for the preparation of

large-scale catalysts as, for example, chemical

methods.

Advantages:

Stabilizers and/or capping agents are not required

Particle size can be controlled by varying the length

and the amplitude of the potential pulse

Attention!

Progressive Vs Instantaneous Nucleation

Assuming that all the nuclei grow at the same rate,

instantaneous nucleation results in mono-sized particles and

is preferable in fabricating nanocatalysts of a specific size

A shift from progressive to instantaneous nucleation by:

Increasing deposition overpotential

or

Increasing the electrolyte conductivity (using chloride, sulfate,

and perchlorate anions)

Shape Control

Final shape of the nanoparticles is affected by:

Reducing agent

Ratio of capping agent to platinum source

Identity of the capping agent

Extent of particle growth

Physical Characterization of

Electrocatalysts

Analysis of Composition and Phase of Catalyst

X-ray Diffraction (XRD) and Electron Diffraction (ED)

X-ray Fluorescence (XRF), X-ray Emission (XRE), and

Proton-induced X-ray Emission (PIXE)

Measurement of Physical Surface Area

and Electrochemical Active Surface Area

BET Method and Physical Surface Area

Basically uses the surface of adsorbed gas molecules as a

ruler

Electrochemical Hydrogen Adsorption/Desorption

Is based on the formation of a hydrogen monolayer

electrochemically adsorbed on the catalyst’s surface

Morphology of Catalysts and Their

Active Components

Scanning Electron Microscopy (SEM)

Uses a beam of electrons to scan the surface of a sample and build a

three-dimensional image of the specimen

Signals can include:

Secondary electrons (electrons from the sample itself),

Backscattered electrons (beam electrons from the filament

that bounce off the nuclei of atoms in the sample),

X-rays, Light, heat, and even transmitted electrons (beam

electrons that pass through the sample)

Transmission Electron Microscopy(TEM)

Builds an image by way of differential contrast

Those electrons that pass through the sample go on to form

the image, while those that are stopped or deflected by

dense atoms in the specimen are subtracted from the image

TEM images of microwave-synthesized PtRu nanoparticles supported on

different carbon samples: (a) Vulcan XC72 carbon; (b) carbon nanotubes

Electrochemical Methods for Catalyst

Activity Evaluation

Cyclic Voltammetry

Basic Principles

Refers to cycling the potential between chosen low and high points and recording the current in the potential cycling region

The highest anodic (or cathodic) current is reached when the potential reaches a value at which all the reduced (or oxidized) form of the electrochemically active species at the electrode surface is consumed

Applications

Features of a Pt Electrode in Acidic Electrolyte

Catalyst Activity Analysis

Catalyst Contamination in PEM Fuel

Cells

Kinetic losses

◦ Poisoning of both anode and cathode catalyst sites or a decrease in the catalyst activity

Ohmic losses

◦ Increase in the resistance of membrane and ionomer, caused by alteration of the proton transportation path

Mass transfer losses

◦ Changes in structure and in the ratio between the hydrophobicity and hydrophilicity of CLs, GDLs, and the PEM

Anode Catalyst Layer

Contamination

Strategies for mitigating CO Poisoning

Using CO-tolerant PtRu alloy catalyst

Introducing a small amount of oxidant(for example, air

or oxygen) into the fuel stream

Operating the PEMFC at elevated temperatures (>

120 °C)

Impacts of Carbon Dioxide

In situ production of CO from CO2 on the platinum

surface through either

Reverse water-gas shift reaction or

Electrochemical reduction of CO2

A common reformate gas that contains about 25%

CO2 approximately 20–100 ppm CO in equilibrium

concentrations

Effect of CO2 concentration on PEM

fuel cell performance

Impacts of Hydrogen Sulfide (H2S)

H2S also strongly adsorbs on the Pt catalyst,

competing for the active sites with hydrogen

adsorption and hydrogen oxidation

Comparison of the effects of CO and

H2S on PEMFC performance

Effect of current density on performance degradation during

exposure to 20 ppm H2S in the H2 stream.

Effect of temperature on cell performance deterioration during

exposure to (a) 10 ppm and (b) 20 ppm H2S/H2 at 0.5 A cm–2

Impacts of Ammonium (NH3)

Ammonium (NH3) is present in the hydrogen-rich

fuel stream, either due to

Use of NH3 as the hydrogen carrier

Reforming process involves homogeneous pre-

combustion with air

Fuel itself contains nitrogen-containing species

NH3 in the fuel stream of a PEMFC, even at the level

of a few ppm, can cause significant cell

performance loss

Degradation of cell performance mainly

by increasing membrane conductivity through NH3

reacting with protons in both the bulk membrane and

the ionomer in the catalyst layer

NH3 (gas) NH3 (membrane)

NH3 (membrane) + H+ NH4

+

Cathode Catalyst Layer

Contamination

SOx Contamination

In the presence of SOX …

pH inside the MEA is decreased

resulting in free acids in the MEA and causing potential

shifts.

SOx can also adsorb on the Pt surface

competing with oxygen adsorption and leading to

performance degradation

Constant-current discharging curve of the PEMFC during running

with 1 ppm SO2/air for 100 hours at 70 °C. Current density: 0.5

A cm2

Recoverability of Fuel Cell Performance

After SOx Contamination

After the contamination source is cut off …

Setting cell voltage at open circuit voltage (OCV)

Operating the fuel cell with pure air

CV scanning

Pt Alloy and CO-tolerant Catalysts

CO is adsorbed strongly and irreversibly at the active

sites on a pure Pt surface, mostly through “Bridge-

Bonding”

Poisoning Mechanism

Blocking H2 adsorption

CO lowers the reactivity of the remaining uncovered

sites through dipole interactions and electron capture

Pt Alloy and CO-tolerant Catalysts

Alloying Pt with a second element can enhance the catalytic ability of the primary element

Bifunctional effects

◦ Second component provides one of the necessary reactants

ligand (electronic) effects

◦ Promoter alters the electronic properties of the catalytically active metal to affect the adsorption /desorption of the reactants/intermediates/poisons;

Ensemble (morphological) effects

◦ Dilution of the active component, Pt, with the catalytically inert metal changes the distribution of the active sites, thereby opening different reaction pathways

Figure 8: CO coverage on various surfaces of alloy electrodes,

under steady H2 oxidation conditions

Supported Pt Catalysts

Carbon Black

◦ Relatively higher stability than unsupported catalysts

In terms of agglomeration

◦ Porosity of carbon black

assures gas diffusion to the active sites

◦ Good electric conductivity of the carbon support

Allows electron transfer from catalytic sites to the conductive

carbon electrodes

◦ Small dimensions of catalyst particles

Maximize the contact area between catalyst and reagents.

Further Supporting Materials

Nanostructured carbon such as carbon nanotubes

Ultra-thin nanostructured (NS) film system

Carbon aerogels and carbon cryogel

The Mere Reference!

• Jiujun Zhang, PEM Fuel Cell Electrocatalysts and Catalyst

Layers (Fundamentals and Applications)

Any Questions?

Thank You!