electrochemistry chem 4700 chapter 4 dr. augustine ofori agyeman assistant professor of chemistry...

ELECTROCHEMISTRY

CHEM 4700

CHAPTER 4

DR. AUGUSTINE OFORI AGYEMANAssistant professor of chemistryDepartment of natural sciences

Clayton state university

CHAPTER 4

PRACTICAL CONSIDERATIONS

- Electrochemical cell with a three-electrode systemWorking Electrode (WE)Reference Electrode (RE)

Counter/Auxiliary Electrode (CE/AE)

- Potentiostat (Voltammetric Analyzer)

- Plotter

- Other components may be required depending on the type of experiment

BASIC REQUIREMENTS OFCONTROLLED POTENTIAL EXPERIMENTS

- Covered glass container of 5 – 50 mL volume

- Contains three electrodes (WE, RE, CE) immersed in the sample solution

- Electrodes are inserted through holes in the cell cover

- N2 gas used as deoxygenated gas

ELECTROCHEMICAL CELL

Working Electrode (WE)- Electrode at which the reaction of interest occurs

(Pt, Au, Ag, C)

Reference Electrode (RE)- Provides a stable and reproducible potential

- Independent of the sample composition(Ag/AgCl, SCE)

Counter/Auxiliary Electrode (CE/AE)- Current-carrying electrode made of inert conducting metal

(Pt wire, Graphite rod)



CE WERE

N2

Opening

Teflon cap

Glass container

- Medium for electrochemical measurements

- Contains a supporting electrolyte

- Choice of solvent depends on the solubility and the redox activity of the analyte

Solvent Properties- Electrical conductivity

- Electrochemical activity - Chemical reactivity

SOLVENTS

Additional Properties Considered- Should not react with analyte or product

- Should not undergo electrochemical reactions over a wide range of potential

Examples - Water (the most common)

- Acetonitrile- Dimethylsulfoxide (DMSO)

- Methanol- Dimethylformamide (DMF)

SOLVENTS

- Inert

- Decrease the resistance of the solution

- Eliminate electromigration effects

- Maintain a constant ionic strength

- Concentration range in usually 0.1 M – 1.0 M

- Should be in large excess of analyte concentration

SUPPORTING ELECTROLYTE

Examples For Aqueous Media- Inorganic salts (NaCl, KCl, KNO3)

- Mineral acids (HCl, H2SO4)

Examples For Organic Media- Tertaalkylammonium salts

Buffer Systems- Used when pH control is essential

- Phosphate, citrate, acetate

SUPPORTING ELECTROLYTE

- Purging with an inert gas for about 10 minutes

- Nitrogen gas is usually used

- Purging is done just before voltammetric measurements

Other Methods- Formation of peroxides followed by reduction of peroxides

- Reduction by addition of sodium sulfite or ascorbic acid

- Use of electrochemical or chemical scrubbers (zinc)

OXYGEN REMOVAL

- Consists of two circuits

- A polarizing circuit that applies the potential to the cell

- A measuring circuit that monitors the cell current

The Applied Potential (Eapp)Eapp = EWE – ERE – iR

iR = ohmic potential drop

INSTRUMENTATION

- RE is placed as close as possible to WE to minimize potential drop caused by the cell resistance (iR)

- Flow cannot occur through RE hence the need for CE to complete the current path

- Current flows through solution between WE and CE

- Voltage is measured between WE and RE

INSTRUMENTATION

REFERENCE ELECTRODES

- Provides known and constant potential

Examples- Saturated Calomel electrode (SCE)

- Silver-silver chloride electrode (Ag/AgCl)

- Mercury/Mercurous sulfate reference electrode

- Alkaline/Mercurous oxide reference electrode

Saturated Calomel electrode (SCE)

- Saturated with KCl

- Different KCl concentrations can be used (0.1 M KCl is least temperature sensitive but saturated

KCl solution is easier to make and maintain)

1/2Hg2Cl2(s) + e- ↔ Hg(l) + Cl- E = + 0.241 V

- The reference is not 0.000 V (SHE) but 0.241 V (SCE)

- Stored in KCl solution when not in use


Silver-Silver Chloride Electrode (Ag/AgCl)

- Saturated with KCl

AgCl(s) + e- ↔ Ag(s) + Cl-

E = + 0.197 V


- Should possess high signal-to-noise ratio characteristic

- Should be reproducible

Selection Depends on Two Main Factors- The redox behavior of the target analyte

- Background current over the potential region required

Other Factors IncludePotential window, electrical conductivity, surface reproducibility,

mechanical properties, cost, availability, toxicity

WORKING ELECTRODES

WORKING ELECTRODES

Chemically Inert Electrodes

- Do not participate in the reaction

ExamplesCarbonGold

PlatinumITO

Reactive Electrodes

- Participate in the reaction

ExamplesSilver

CopperIronZinc

Mercury

WORKING ELECTRODES

- Extended cathodic potential window (due to its high hydrogen overvoltage)

- Highly reproducible (minimized effect of impurities)

- Readily renewable

- Smooth surface

Disadvantages- Limited anodic potential range (due to the oxidation of Hg)

- Toxicity

MERCURY ELECTRODES

Types of Mercury Electrodes

Dropping Mercury Electrode (DME)- Used in polarography and electrocapillary studies

Hanging Mercury Drop Electrode (HMDE)- For stripping analysis and cyclic voltammetry

Mercury Film Electrode (MFE)- For stripping analysis and flow amperometry

- Thin layer of Hg covering a conducting or inert support - Support is usually glassy carbon or iridium

Solid Amalgam Electrode

MERCURY ELECTRODES

- Have extended anodic potential windows

- Better for monitoring oxidizable compounds than Hg electrodes

- Requires polishing to obtain reproducible results

- May be stationary or rotating (planar disk)

- Consists of a short cylindrical rod of the material tightly embedded in an insulating material

SOLID ELECTRODES

Insulating Material - Teflon

- Kel-F [polychlorotrifluoroethylene (PCTFE)]

- Sealing between rod and insulating material is essential to avoid solution creeping and subsequent

background response

Disk solid electrodes are employed in flow analysis

ExamplesCarbon, Platinum, Silver, Gold, Nickel, Copper

SOLID ELECTRODES

- Vertically mounted in a shaft of controllable speed

- Rotated with a constant angular velocity (ω) about an axis perpendicular to the plain of disk surface

- Thickness of diffusion layer is independent of diameter of disk

- Provides efficient and reproducible mass transport

- High sensitivity and precision

ROTATING DISK ELECTRODES (RDE)

ROTATING DISK ELECTRODES (RDE)

Disk

Teflon insulator

- Addition of a concentric ring separated by a small insulating gap

- For elucidating various electrode mechanisms

- For detection of short lived intermediate species

- Species generated at the disk are detected at the ring

ROTATING RING DISK ELECTRODES (RRDE)

Ring : Disk Current Ratio (N)

N = - iR/iD

- Fraction of species generated at the disk that are detected at the ring

- Currents are in opposite directions hence the negative sign

- Collection current (iR) is proportional to generation current (iD)


Disk

Teflon insulator


Ring

- Solid electrodes based on carbon- Broad potential window- Low background current- Rich surface chemistry

- Low cost- Chemically inert

- Suitable for sensing and detection applications

However- Have slower electron transfer rates than metal electrodes

CARBON ELECTRODES

Examples

- Glassy carbon- Carbon paste- Carbon fiber- Carbon film

- Screen-printed carbon strips- Graphite epoxy

- Wax imprinted graphite- Kelgraf

CARBON ELECTRODES

Glassy Carbon Electrodes

- Vitreous (shiny and nonporous)- Good mechanical and electrical properties

- Wide potential window- Solvent resistance (chemically inert)

- Reproducible- Surface pretreatment (polishing) is essential

A Special TypeReticulated Vitreous Carbon (RVC, sponge-like or network) for

flow analysis and spectroelectrochemistry

CARBON ELECTRODES

Carbon Paste Electrode

- Graphite powder mixed with various water-immiscible nonconducting organic binders

- Surface is easily renewable and modified

- Low cost

- Very low background current contributions

- Exact behavior is not fully understood

CARBON ELECTRODES

Carbon Paste Electrode

Examples of Pasting Liquids- Mineral oil (Nujol)

- Paraffin oil- Silicone grease

- Bromonaphthalene

- Decrease in pasting liquid increases electron transfer rates

- Decrease in pasting liquid increases background current contributions

CARBON ELECTRODES

Carbon Fiber Electrode

- Made up of fibers of about 5 – 20 μm diameter

- Fibers are mounted at the tip of a glass capillary with epoxy adhesive

- Contamination of the carbon surface with epoxy should be avoided

- Attractive for anodic measurements

CARBON ELECTRODES

Carbon Fiber Electrode

- For microenvironments and detection of neurotransmitter release

Three CategoriesLow-modulus, Medium-modulus, and High-modulus

High-modulus - Well-ordered graphite-like

- Low porosity- Most suitable for electrochemical studies

CARBON ELECTRODES

Diamond Electrodes

- Boron doped diamond (BDD) film electrodes provide very low resistivity (< 0.01 Ω∙cm)

- Wide potential window (~ 3 V)

- Low and stable background currents

- Negligible adsorption of organic compounds

- Good reactivity requiring little or no pretreatment

CARBON ELECTRODES

Diamond Electrodes

- Low sensitivity to dissolved oxygen (no surface oxide formation)

- Reproducible results

- Extreme hardness

- Small double-layer capacitance

CARBON ELECTRODES

Diamond Electrodes

- Provides good results under extreme conditions such as:

very high anodic potential

surfactant-rich media

polarization in acidic media

power ultrasound

CARBON ELECTRODES

- Platinum and gold are the most widely used

- Copper, nickel, silver are other examples

- Offer favorable electron transfer kinetics

- Large anodic potential range

- Low cathodic potential window (-0.2 to -0.5 V, depends on pH)

- High background currents associated with the formation of surface oxide or adsorbed hydrogen layers

METAL ELECTRODES

- The problem of surface oxide formation is less severe in nonaqueous media

Comparison Between Pt and Au Electrodes- Gold electrodes are more inert

- Pt is more prone to surface oxide film formation

- Gold is preferred for stripping measurements of trace metals

- Gold is preferred as substrate for self assembled monolayers (SAM)

METAL ELECTRODES

Alloy Electrodes

- Used for addressing adsorption or corrosion effects of one their components

- Used for fuel cell applications

- Corrosion and heat resistant

ExamplesPlatinum-Tin, Nickel-Ruthenium, Platinum-Ruthenium

Ti-Zr-V-Cr-Ni, Tin-Lithium, Ruthenium-Cobalt

METAL ELECTRODES

- Produced by placing a reagent on electrode surface to alter the surface

- Basis of new analytical applications and different sensing devices

- Accelerates electron transfer reactions

- Enhanced selectivity and sensitivity

- Differential accumulation

- Stability of devices

CHEMICALLY MODIFIED ECTRODES (CME)

- Protection from corrosion

- Controlled and manipulated reactivity at interface

- For fuel cells

- Most common is polymer modified electrodes

Examples - Nafion cation exchanger

- Polyvinylferrocene- Polypyrrole

- Clay


Self Assembled Monolayers (SAM)

- Spontaneously adsorbed monolayers on electrode surface

- SAM film is formed by immersing electrode in a solution containing the species of interest (usually overnight)

- One end of species has special affinity for the electrode surface

- SAM film is well organized and stable

ApplicationsBiosensors, electron transfer rate determination (e.g. of proteins)


Self Assembled Monolayers (SAM)

Examples- Alkanethiols on gold surfaces

- Alkyl siloxane (R2SiO) on metal oxide surfaces (SiO2)- Chlorosilane

Factors Influencing Packing and OrderChain length, End group, Solvent, Immersion time,

substrate morphology, coassembled monolayers (mixtures)


Carbon-Nanotube-Modified Electrodes (CNT)

- Two types

Single-Wall Carbon-Nanotubes (SWCNT)- Cylindrical nanostructure formed by rolling up a

single graphite sheet into a tube

Multi-Wall Carbon-Nanotubes (MWCNT)- Multiple rolled layers (concentric tubes) of SWCNT

- Enhanced electrochemical activity- For amperometric biosensors


Sol-gel Encapsulation of Reactive Species

- Encapsulation of species within sol-gel films

- Formed by hydrolysis of alkoxide precursor [Si(OCH3)4]

- Followed by condensation

- A porous glass-like material forms

- Rigid and stable porous network


Sol-gel Encapsulation of Reactive Species

- Other composites have been formed by dispersing carbon or gold powders into sol-gel mixtures

Encapsulation The condition of being enclosed (as in a capsule)


Electrochemically Modified Electrodes

- Modification by attachment of electron transfer mediators on the electrode surface

- Catalyzes slow electron transfer kinetics

- The mediator facilitates the charge transfer between an analyte and the electrode

- Current density is increased and overvoltage is lowered

- Improved sensitivity and selectivity



- The electron transfer takes place between the electrode and the mediator (M) but not directly between the electrode

and the analyte (A)

Mox + ne- → Mred

Mred + Aox → Mox + Ared

- The active form of the mediator is electrochemically regenerated

- The process is electron shuttling



Applications- Electrocatalytic reactions in sensing and

energy-related applications

- Widely used in fuel cells for catalyzing the oxidation of methanol or the reduction of oxygen


Preconcentrating Electrodes

- CMEs are preconcentrated with species to react or bind target analytes

- Analyte is preferentially partitioned from sample into preconcentrating surface layer (nonelectrolytic step)

- Analyte is subsequently reduced or oxidized during a potential scan

- Used for chemical sensing


Preconcentrating Electrodes

Different Processes- Electrostatic binding (alkanethiol or functionalized films)

- Coordination reactions- Hydrophobic partition into a lipid coating

- Covalent reactions (ligand centers to polymer backbones)- Peptide binding

Advantages- Strong and selective binding

- Prevention of saturation- Surface regeneration


Permselective Coatings

- Provides very high selectivity and stability to electrochemical devices

- Unwanted constituents are excluded from the surface

- Transport of target analyte is not hindered

- Offers in-situ separation step

- Signals from undesired electroactive species are minimized


Permselective Coatings

Different Mechanisms- Use of size exclusion polymer films

(cellulose acetate, polyphenol)

- Charge exclusion coatings (Nafion, thioctic acid, clay)

- Hydrophobic barriers (lipids, alkanethiols)

- Mixed (composite) control (cellulose acetate + Nafion)


Conducting Polymers

- Negatively charged polymer backbones are usually used(polypyrole, polyaniline, polythiophene)

- Able to reversibly switch between positively charged conductive state and a neutral state

- Able to incorporate and expel anions (doping ions) from and to surrounding solution upon oxidation or reduction

- Offers controllable change in electrical conductivity


Conducting Polymers

Applications- Batteries- Fuel cells

- Corrosion protection- Chemical sensing

- Controlled release of chemicals- Preconcentration/stripping of trace metals

- For design of molecularly imprinted polymer (MIP)-based sensors


- Electrodes with diameter ≤ 25 μm

Advantages- Measurement of local concentration profiles

- Exploration of microscopic domains- For microenvironments

- Microflow detection systems- Analysis of microliter samples

- High resolution spatial characterization of surfaces- Monitoring stimulated release of neurotransmitters (dopamine)

MICROELECTRODES

Properties

Total Currents- Very small total current

- Ability to work in highly resistive (large iR) solutions

- Measurements can be made with little or no electrolyte

- Two electrode cell systems may be used

- Use of electrolyte-free organic media which extends potential window (acetonitrile vs Ag reference electrode: 4 V)

MICROELECTRODES

Properties

Double layer- Greatly reduced double layer capacitance

- High scan rates allowed in voltammetric experiments (>106 V/s)

- Able to probe kinetics of very fast electron transfer and coupling chemical reactions

MICROELECTRODES

Properties

Mass Transport- Enhanced rate of mass transport of electroactive species

- Decrease in electrode size increases rate of mass transport to and from the electrode

- Decrease in electrode size increases current density

- Negligible convective transport contribution

- Steady-state current and excellent signal-to-background x’tics

MICROELECTRODES

Solvents- Low dielectric solvents (benzene or toluene), frozen acetonitrile

gaseous and solid phases, oil based lubricants, ionically conductive polymers, milk

Materials- Fine metal wires or thin metal films (Pt, Au, Ir), carbon fibers

Applications - Studies of short chain alkanes are made possible

- Stripping voltammetry of trace metals

MICROELECTRODES

Diffusion at Microelectrodes

- Total diffusion limited current is composed of both the planar flux and the radial flux

itotal = iplanar + iradial

- The same electrode can exhibit peak-shaped or sigmoidal voltammograms depending on the scan rate

MICROELECTRODES

Diffusion at Microelectrodes

At Low Scan Rates- Diffusion layer thickness exceeds the size of the electrode

- Long electrolysis time- Current approaches steady-state

- Sigmoidal voltammograms are observed

At High Scan Rate- Short electrolysis time

- Planar diffusion dominates- Peak shaped voltammograms are observed

MICROELECTRODES

Configurations

- Electrode dimension is significantly smaller than the diffusion layer at the electrode surface

- Microdisk (circular conductor embedded in an insulating plane)Microring

MicrocylinderMicrohemisphere

Microband

- Cylinder and band electrodes yield larger currents hence provide more easily measurements

MICROELECTRODES

COMPOSITE ELECTRODES

- Surface consists of uniform/ordered (array) or random (ensemble) dispersion of a conductor region within a continuous

insulating matrix

- Total current is the sum of currents at individual sites(if diffusion layers do not overlap)

- Diffusion layers overlap with time and behave as if the entire geometric area is active

- System changes from isolated to merged diffusion with time

COMPOSITE ELECTRODES

Examples

- Closely packed microdisks

- Integrated microband electrodes

Applications

- Sensing devices

electrochemistry chem 4700 chapter 4 dr. augustine ofori agyeman assistant professor of chemistry...

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