electrochemistry chem 4700 chapter 4 dr. augustine ofori agyeman assistant professor of chemistry...
TRANSCRIPT
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ELECTROCHEMISTRY
CHEM 4700
CHAPTER 4
DR. AUGUSTINE OFORI AGYEMANAssistant professor of chemistryDepartment of natural sciences
Clayton state university
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CHAPTER 4
PRACTICAL CONSIDERATIONS
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- 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
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- 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
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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)
ELECTROCHEMICAL CELL
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ELECTROCHEMICAL CELL
CE WERE
N2
Opening
Teflon cap
Glass container
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- 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
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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
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- 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
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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
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- 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
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- 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
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- 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
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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
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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
REFERENCE ELECTRODES
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Silver-Silver Chloride Electrode (Ag/AgCl)
- Saturated with KCl
AgCl(s) + e- ↔ Ag(s) + Cl-
E = + 0.197 V
REFERENCE ELECTRODES
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- 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
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WORKING ELECTRODES
Chemically Inert Electrodes
- Do not participate in the reaction
ExamplesCarbonGold
PlatinumITO
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Reactive Electrodes
- Participate in the reaction
ExamplesSilver
CopperIronZinc
Mercury
WORKING ELECTRODES
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- 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
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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
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- 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
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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
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- 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)
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ROTATING DISK ELECTRODES (RDE)
Disk
Teflon insulator
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- 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)
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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)
ROTATING RING DISK ELECTRODES (RRDE)
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Disk
Teflon insulator
ROTATING RING DISK ELECTRODES (RRDE)
Ring
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- 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
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Examples
- Glassy carbon- Carbon paste- Carbon fiber- Carbon film
- Screen-printed carbon strips- Graphite epoxy
- Wax imprinted graphite- Kelgraf
CARBON ELECTRODES
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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
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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
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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
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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
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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
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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
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Diamond Electrodes
- Low sensitivity to dissolved oxygen (no surface oxide formation)
- Reproducible results
- Extreme hardness
- Small double-layer capacitance
CARBON ELECTRODES
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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
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- 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
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- 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
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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
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- 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)
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- 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
CHEMICALLY MODIFIED ECTRODES (CME)
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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)
CHEMICALLY MODIFIED ECTRODES (CME)
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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)
CHEMICALLY MODIFIED ECTRODES (CME)
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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
CHEMICALLY MODIFIED ECTRODES (CME)
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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
CHEMICALLY MODIFIED ECTRODES (CME)
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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)
CHEMICALLY MODIFIED ECTRODES (CME)
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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
CHEMICALLY MODIFIED ECTRODES (CME)
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Electrochemically Modified Electrodes
- 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
CHEMICALLY MODIFIED ECTRODES (CME)
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Electrochemically Modified Electrodes
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
CHEMICALLY MODIFIED ECTRODES (CME)
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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
CHEMICALLY MODIFIED ECTRODES (CME)
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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
CHEMICALLY MODIFIED ECTRODES (CME)
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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
CHEMICALLY MODIFIED ECTRODES (CME)
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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)
CHEMICALLY MODIFIED ECTRODES (CME)
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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
CHEMICALLY MODIFIED ECTRODES (CME)
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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
CHEMICALLY MODIFIED ECTRODES (CME)
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- 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
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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
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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
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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
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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
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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
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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
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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
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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
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COMPOSITE ELECTRODES
Examples
- Closely packed microdisks
- Integrated microband electrodes
Applications
- Sensing devices