ELECTROCHEMISTRY TOOLS:

Basic concepts

MSC. JAQUELINE RUIZ MALUTA

http://gmeme.iqsc.usp.br/

1UNIVERSITY OF SÃO PAULO / SÃO CARLOS CHEMISTRY INSTITUTE

GROUP OF ELECTROCHEMISTRY MATERIALS AND ELECTROANALYTICAL METHODS

WHAT IS ELECTROCHEMISTRY?

Interrelation of electrical and chemical effects.

Study of chemical changes caused by the passage of an

electric current

Types of research

The production of electrical energy by chemical reactions

Phenomena: Corrosion, electrophoresis

Devices: electrochromic displays, electroanalytical sensors,

batteries, and fuel cells

Technologies: the electroplating of metals and the large-scale

production of aluminum and chlorine

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PRODUCTION OF CHLORINE

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WHICH TYPE OF INFORMATION

DO YOU HAVE?

Thermodynamic data about a reaction;

Analyze a solution for trace amounts of metal ions or organic species;

The design of a new power source;

Electro-synthesis of some product;

Electro-degradation of some molecule;

REQUIRES AN UNDERSTANDING OF THE FUNDAMENTAL PRINCIPLES OF ELECTRODE REACTIONS AND THE ELECTRICAL PROPERTIES OF

ELECTRODE-SOLUTION INTERFACES.

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MY FIELD OF RESEARCH

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ELECTROANALYSIS Electrochemistry in analytical context

Useful for quantitative purposes, based on measurements of the

peak current

The current is proportional to the concentration;

Environmental monitoring, industrial quality control, or biomedical

analysis.

Advantages

high sensitivity (~10-9 mol L-1), selectivity toward electroactive species, a

wide linear range, portable and low-cost instrumentation.

PORTABLE AND LOW-COST

INSTRUMENTATION Home testing of blood glucose

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Electrochemical test strips have

electrodes where a precise

voltage is applied and a current

proportional to the glucose in the

blood is measured as a result of

the electrochemical reaction on

the test strip.

http://www.docstoc.com/docs/69965634/Accu-Chek-Advantage-Electrochemistry-for-Diabetes-Management

SELECTIVITY TOWARD

ELECTROACTIVE SPECIES

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8

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WHAT DO YOU NEED?

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Voltammetric analyser

Potentiostatic circuitry and a voltage ramp generator

Cell:

Covered beaker of 5–50 mL volume;

Three electrodes:

working, reference, auxiliary/counter;

Electrolyte:

conductive sample solution

Analyte:

electroative species

Plotter

ELECTROCHEMICAL CELL

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Electrolysis Cell UV-vis Cell

Temperature control

Flow cell

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WORK ELECTRODE

The working electrode is the electrode at which the investigated

processes occur.

Electrode materials:

solid metals (e.g.,Pt, Au),

liquid metals(Hg, amalgams),

carbon

and semiconductors (indium-tin oxide).

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COUNTER ELECTRODE

The current flows between the working and the counter

electrode.

Material: usually platinum or titanium

The area of the counter electrode should be larger than that of

the working electrode.

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REFERENCE ELECTRODE

The reference electrode is keep at a constant potential.

It is used to control the potential of a working electrode

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Saturade calomelane

electrode

Ag/AgCl electrode

ELECTROLYTE

Electrolyte phase charge is carried by the movement of ions;

Ionic species ( H+, Na+, Cl-) in either water or a nonaqueous solvent

Electrolyte is usually added to the test solution to ensure sufficient

conductivity.

Migration mass transport influence is eliminated

Unstirring There is no convection mass transport

Just diffusion mass transport

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POTENTIAL WINDOW

The available potential window is determined by the currents of

reduction/oxidation of the supporting electrolyte/solvent

We should avoid entering potentials where these processes visibly

occur

The products generated at the potential limits may interfere with the

system under investigation and may affect the electrode surface.

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Too positive:H2O(l) ½ O2 + 2 H+ + 2 e-

Too negative:2 H2O(l) + 2 e-

2 H2 + OH-

POTENTIAL WINDOW

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Surfaces are partially oxidized;

Thin layers of oxides are formed at gold and platinum;

Functional groups (–C=O; –OH) are attached to the carbon materials.

The voltammograms of solid phases are specific fingerprints, even when the assignment of signals is not obvious.

Martínez-Huitle, J. Brazilian Chem Society, 2008, 19: 150-156. Maluta, J.P., J Solid State Electrochem. 2014, 18:2497–2504

VARIABLES AFFECTING THE RATE

OF AN ELECTRODE REACTION

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EXEMPLE: pH INFLUENCE

NO detection in

phosphate buffer solution

pH influence, all the

others parameters are

the same.

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0,60 0,65 0,70 0,75 0,80 0,85 0,90 0,95 1,00

0,00

4,50x10-4

9,00x10-4

1,35x10-3

1,80x10-3

I / m

A c

m-2

E / V (vs Ag/AgCl)

pH 2

pH 3

pH 4

pH 5

pH 6

pH 7

pH 8

WHAT IS HAPPENING?

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*O Oxidized specie

*R Reduced specie

WHAT IS HAPPENING?

Oxidation reaction

R O + ne-

The e- formed will be received by an electrode

Reduction reaction

O + ne- R

The electrode is the source of the electrons

Redox changes current passage

The amout of material electrochemical oxidized or reduced isproportional to the current passage

REDOX

REACTIONS

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TECHNIQUES

POTENTIOMETRY: difference in electrode potentials is measured

COULOMETRY: the cell's current is measured over time

VOLTAMMETRY: the cell's current is measured while actively altering

the cell's potential.

CYCLIC VOLTAMMETRY

SQUARE WAVE VOLTAMMETRY

DIFFERENCIAL PULSE VOLTAMMETRY

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THE ANALYTICAL SENSITIVITYCAN BE IMPROVED

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(A) Cyclic voltammograms obtained at the OMC electrode with, 0.5, 1, 1.5 mM CySH (from top to bottom) in the PBS pH 7.16

(B) Current–time response curves of the OMC electrode in PBS pH 7.16 with successive addition of 0.1 mM CySH

CYCLIC-VOLTAMMETRY

Cyclic voltammetry is often the first experiment performed in an

electroanalytical study:

Qualitative information about electrochemical reactions

Thermodynamics of redox processes

Kinetics of heterogeneous electron transfer reactions

Location of redox potentials

Effect of media on the redox process

The occurrence of chemical reactions, that precede/succeed the

redox process

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CYCLIC-VOLTAMMETRY

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29[Fe(CN)6]

4-[Fe(CN)6]

3- + e-

[Fe(CN)6]3- + e- [Fe(CN)6]

4-

ELECTROCHEMICALLY

REVERSIBILITY

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a) Reversible

Ep = independent of scan rate

IPA / IPC = 1

IP vs √scan rate = linear

Current is controlled by mass transport

Described by Nerst Equation

b) Quasi-reversible:

Depend of both electron transfer and mass transport

c) Irreversible

The electron transfer is slow

31EFFECTS DUE TO CAPACITANCE

AND RESISTANCE

a) Reversible cyclic voltammogram

b) With the effect of capacity

c) With additional resistance

COMPLICATIONS

The electrode reaction is rarely simple;

The product is either insoluble, or partly adsorbed at the electrode

surface.

Coupled chemical reactions can happen

The results should be carefully analyzed

In situ UV-vis

Scan rate test

Change the window potential

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SCAN RATE TEST

Run experiments in which you selectively vary the potential scan rate;

Plot the peak current versus the square root of the scan rate;

For a reversible system, the peak height will increase linearly with the square root of the scan rate. Also, de current is controlled by diffusion transport

The slope of the resulting line will be proportional to the diffusion coefficient (Randles-Sevcik equation)

Ip = 269 n2/3 AD1/2 v1/2 C

• ip= peak height (amp)

• n = number of electrons

• A = area (cm2)

• D = diffusion coefficient (cm2/sec)

• v = scan rate (V/sec)

• C = concentration of solution (M)

• F = Faraday constant

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SCAN RATE TEST

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D. Zheng et al. / Journal of Electroanalytical Chemistry 625 (2009) 82–87

The current linearly increases with

scan rate (not √scan rate)

suggesting a adsorption-controlled

process

NOT controlled by diffusion transport

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SOME MECHANISMS INVOLVING COUPLED CHEMICAL REACTIONS

COUPLED REACTIONS

The product R is chemically

‘removed’ from the surface

Smaller reverse peak

In the extreme case, the chemical

reaction may be so fast that all of R

will be converted, and no reverse

peak will be observed.

In faster scan rates, we can see the

reverse peak

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-0,6 0,0 0,6

-0,2

0,0

i / m

A

E / V (vs Ag/AgCl)

1

2

3

4

5

N+

OO-

+ 4 e- +

N

OHH

4 H+

nitrobenzene N-phenylhydroxylamine

N

OHH

N

O

+ 2 e- + 2 H+

N-phenylhydroxylamine nitrosobenzene

N

O

+ 2 e- + 2 H+

N

OHH

nitrosobenzene N-phenylhydroxylamine

PROGRESSIVE ADSORPTIVE

ACCUMULATION

Repetitive CV for riboflavin in a

sodium hydroxide solution.

A gradual increase of the

cathodic and anodic peak

currents

indicating progressive

adsorptive accumulation at

the surface.

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INCREASE OF ANALYTE

CONCENTRATION

Run experiments in which you selectively

vary the analyte concentration;

Plot concentration versus peak current

Check the concentration where the graphic

is linear

The equation can be used to determine an

unknown solution concentration

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Maluta, J.P., J Solid State Electrochem (2014) 18:2497–2504

MODIFIED ELECTRODES

Methods based on attaching a certain compound, or a specific

chemical group, to the surface of electrode

by adsorption, by chemical reaction or by formation of a polymer film…

Electrocatalytic modified electrodes contain attached electron-

transfer mediators

They accelerate electrode reactions

The catalyst is readily regenerated by the fast and reversible electrode

reaction

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WHAT COMPOUNDS AND

MATERIALS CAN BE STUDIED?

Highly insoluble in the electrolyte solution used;

Possess electroactivity;

the ability to be either oxidized or reduced in the accessible potential

window of the experiment.

Three different kinds of compounds:

not electroactive,

irreversibly destroyed in the electrochemical reactions,

can be reversibly reduced and oxidized.

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SAMPLE PREPARATION

Physical casting in a base electrode:

Prepare a solution in a volatile solvent (Usually alcohols or

DMF);

Put a drop and let it dry;

Nafion® or glutamin can be use together as binder;

The compound should be stable under ambient

conditions.

Impurities, which may be electrochemically active,

should not be introduced by this procedure.

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SAMPLE PREPARATION

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Chen, J. Adv. Mater. 2012, 24, 4569–4573Bojorge, N., & Alhadeff, E. Graphite-Composites Alternatives for

Electrochemical Biosensor.

CARBON ELECTRODES

Carbon electrodes: graphite, glassy carbon, graphite powder with liquid or solid binders, carbon fibers, carbon nanotubes, boron-doped diamond,

titanium carbide, ordered mesoporous carbon, graphene

The glassy carbon:

Synthesis: Very slow carbonization in inert atmosphere at 300◦C to 1200◦C.

Inside a pre-modeled polymeric resin body

With the base polished to a mirror

Carbon nanotubes:

Synthesis: arc discharge, laser ablation, or chemical vapor deposition

They appear either as multi-walled, or as single-walled.

Boron-doped Diamond

Synthesis: Low-pressure chemical vapor deposition or high pressure and high temperature

Wide window potential

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ELECTRODE PRETREATMENT

A solid electrode requires very careful pretreatment.

The best cleaning method has to be chosen from applying either

inorganic or organic solvents.

If necessary, the electrode surface should be clean and polished on

a very wet pad to mirror gloss.

Using abrasive powders (such as diamond and alumina)

Often, the solid electrode needs an electrochemical

activation/regeneration.

Cycling the potential in an appropriate range and in an appropriate

solution.

Modified solid cannot be treated by polishing.

The only way is the electrochemical treatment

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ELECTRODE PRETREATMENT

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http://www.basinc.com/mans/LC_epsilon/Maintenance/Working/working.html

REMEMBER...

Before carrying out voltammetric experiments with analytes:

Check the available potential window

Check if there are no peaks of unwanted impurities in that range

After use, the working electrode should be thoroughly rinsed and

dried

Pay much attention to the reproducibility of results!

Each set of experiments should start and end with a blank

voltammogram in order to verify the cleanliness of the electrode itself

Pay attention in all variables which may affecting the electrode reaction

The starting potential should be carefully set to a value where no

reaction is expected.

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REFERENCES

Scholz, F. (2009). Electroanalytical methods: guide to experiments and

applications. Springer.

Wang, J. (2006). Analytical electrochemistry. John Wiley & Sons.

Bard, A. J., & Faulkner, L. R. (1980). Electrochemical methods: fundamentals

and applications. New York: Wiley.

Monk, Paul M.S., Fundamentals of Electroanalytical Chemistry (2002). John

Wiley & Sons.

http://chemwiki.ucdavis.edu/Analytical_Chemistry/Analytical_Chemistry_2.0

/11_Electrochemical_Methods/11D_Voltammetric_Methods

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