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Practical details and

electrochemical techniques

Common techniques

• Cyclic Voltammetry

• Bulk Electrolysis

• Chronoamperometry, chronocoulometry

• Square wave, staircase voltammetry

• Differential pulse voltammetry

• Electrochemical Quartz Crystal

Microbalance

• Impedance spectroscopy

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Cyclic voltammetry (CV)

• The most widely used electrochemical

technique

• Simple to perform and extremely

informative

• Principles

• Practical considerations

• Examples

Edc = Einitial + ν t

Initial potential

Switching potential

Switching potentialA = B + e-

Sweep rate

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What does this actually represent?

Take the oxidation of species R to O

Point A: Only R is present in solution : still below the redox potential of solution species

Increase the electrode potential towards the redox potential : R is converted to O.

As R is converted to O a concentration gradient is setup at the electrode

Point B: R is instantaneously converted to O

After point B the current is dependent on the rate of mass transfer to the electrode surface

What does this actually represent?

Take the oxidation of species R to O

Point C: Reverse the scan directionR is still being converted to O

Decrease the electrode potential towards the redox potential

Species O which is present near the electrode surface is reduced back to R

Point D: a maximum is reached as in the forward sweep

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Classic example 1 electron oxidation process

Reversible process

The peak potential separation (Epa - Epc) is equal to 57 mV The peak current ratio (ipa/ipc) is equal to 1 for all scan ratesThe peak current increases linearly as a function of the square root of vThe peak current is proportional to concentration

-1.2 -0.9 -0.6 -0.3 0.0 0.3 0.6 0.9

-60

-40

-20

0

20

40

I [µ

A]

E [V] vs Ag ref

Analysis

Oxidation of ferrocene

Reduction of cobaltocenium

Fe is in a 2+ oxidation state

Both one electron processes – what looks strange here?

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-1.2 -0.9 -0.6 -0.3 0.0 0.3 0.6 0.9

-60

-40

-20

0

20

40

I [µ

A]

E [V] vs Ag ref

0.0 0.1 0.2 0.3 0.4 0.5 0.6

0.000000

0.000005

0.000010

0.000015

0.000020

0.000025

0.000030

i p [A

]

sweep rate [V s-1]1/2

ferrocene

Data1B

ip = 2.69 x 105 n3/2 A D1/2 v1/2 C

Randles Sevcik Equation

n : no of electrons D : diffusion coefficient C : concentrationA : electrode area v : sweep rate

Electrochemical cell

CE REF WE

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How does it really look?

Reference Electrode

Working Electrode

Counter Electrode

RMIT University 12

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Practical detailsNearly every experiment requires the presence of a supporting electrolyte – minimises solution resistance

For CV experiments we use a 3 electrode setup

WE : working electrode : process of interest occursTypically Pt, Au, carbon, ITO, boron doped diamond

CE : counter electrode : Pt wire/coil/mesh, graphite rod

REF : Reference electrode Dependent on solvent system

A potential is applied between WE and REF while current is recorded between WE and CE.

Therefore a stable REF electrode is essential

Reference electrodes

Reference electrode is an electrode which has a stable and well-known electrode potential

Therefore to form a basis for comparison with all other electrode reactions, Hydrogen's standard electrode potential (E0) is declared to be zero at all temperatures.

Potentials of any other electrode is compared with that of the SHE at the same temperature.

Common reference electrodes : Ag/AgCl, saturated calomel electrode (SCE)

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Reference electrodes

Ag/AgCl (3M NaCl) is one of the most commonly used

Based on

AgCl(s) + e- = Ag(s) + Cl-(aq)

Ideal non-polarizable electrode

E°′ = 0.220 V vs SHEUnit activity at standard conditions

For Ag/AgCl (3M KCl)E = 0.196 V

Cell Design

– Electrodes (Working, Reference, Auxiliary)

• material

• geometry (available theory?)

• size

• location

– Quiescence- no adventitious stirring caused by

• Source of vibration - fumehoods

• gas flow through or over solution

• density gradients (electrochemically induced)

• temperature gradients

– Temperature Control

– Integrity (“air” tight; vacuum tight)

•

Solvent

Supporting Electrolyte (excess assumed)

Choose analyte concentration

selection and purification;

maximize relevant electro-

chemical window.

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Develop a protocol

Find a suitable solvent for your analyte

Find a suitable supporting electrolyte (SE)

Run a background - SE + whatever (e.g., buffer, ligand,

acid, base…..) with no analyte present

Run a simple CV with the analyte

Chosen a value of ν – typically 50 or 100 mVs-1

• Change voltage ranges within the voltage window

for the system

• See what happens when ncycles = 2, 3, 4………50

• Run CVs over a range of v consistent with

working electrode size & geometry

• Change the concentration of analyte

• Look at T-dependence

•Re-evaluate requirements and consider

– optimizing/modifying cell/electrodes

– using different solvent, SE, etc.

– variations addressing specific interests

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Some examples

Cyclic voltammetry is very powerful in determining reaction mechanisms

Seen previously a one electron transfer reaction

The peak potential separation (Epa - Epc) is equal to 57 mV

What if more than 1 electron is transferred

The peak potential separation (Epa - Epc) is equal to 57/n mV

However much more information can be gathered

Some examples

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Oxidation product unstableConsumed chemically to an electrochemically inactive species

⇒ Less oxidised product available for reduction

Example of an EC mechanism

EC mechanism

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EC mechanism

IncreaseSweep rate

ECE mechanism

IncreaseSweep rate

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ECE mechanism

Many mechanisms• Identify some “basic” mechanisms

E A + e = B

EE A + e = B; B + e = C;

EC A1 + e = B1; B1 = B2

EC’ A + e = B; B + P = A + Q

EC2 A + e = B; 2B = B2

CE Y = A; A + e = B

ECE A1 + e = B1; B1 = B2; B2 + e = C2;

• Use DigiSim or a simulator of choice to explore the behavior of selected basic mechanisms.

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Several Electrode Types

Macrodisk electrodes : typically 1 – 3 mm in diameter

Rotating disk electrodes : as above but rotated

Rotating ring disk electrodes

Microelectrodes : typically < 100 µm in diameter

Microelectrode arrays

Mercury drop electrode

Several ElectrodeTypes

Stationary Macrodisk Rotating Macrodisk Electrode

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Rotating Disk Electrode (RDE)

Levich Equation

iL = 0.62nFACD2/3νννν-1/6ωωωω1/2

ω= 2Πf : angular rotation rate of electrode (rad s-1)

ν= kinematic viscosity (cm2 s-1)viscosity (g cm-1 s-1 ) / density (g cm-3)

This equation applies if the current is limited by diffusion and not

electron transfer

Microelectrodes

a

0.0 0.1 0.2 0.3 0.4 0.5 0.6-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

ibulk

= 4nFDca

25 µµµµm Pt UME

1 mM ferrocene methanol + 0.1 M KNO3

10 mV s-1

I [n

A]

E [V] vs Ag/AgCl

10

Hemispherical diffusion of mediator to the microelectrode

Limiting current in bulk solution

Ibulk = 4nFDcan: no of electrons

F : Faraday’s constant (C mol-1)

D : diffusion coefficient (cm2 s-1)

C : concentration (mol cm-3)

A: radius of electrode (cm)

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Microelectrode Uses

Scanning electrochemical microscopy (SECM)

Dark regions ⇒ highly doped/conducting

SEM image Electrochemical map

Chronoamperometry

• In this technique the potential of the working electrode is stepped, and the resulting current from faradic processes occurring at the electrode (caused by the potential step) is monitored as a function of time.

• Remember the case in cyclic voltammetry for a reversible cyclic voltammogram

A = B + e-

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Chronoamperometry

• Stationary electrode

• Solution must be stationary and unstirred = mass transport by diffusion

• Constant potential

• Measure current vs time (t)

Theory

Assume A = B + e-

- Both A and B are soluble

- Reversible reaction (electrochemically)

- Potential (E) set so oxidation or reduction goes to completion at the electrode surface

E

t (time)0

E1

E2

Other processes occurring at the electrode can perturb the response from Cottrellian behaviour

Capacitive Current – charging current is exponential in nature

However, only influences the beginning of the transient as the capacitive current decreases more rapidly than Faradaic current so at longer times the ratio IFar/Icap is very large

Occurrence of a coupled chemical

reactions e.g. A → B + e- followed by

2B = C where C is electroactive and can also be oxidised

C → B + e-

These can affects the shape of the current-time curve and is a good first step in identifying coupled reactions

Chronoamperometry

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-0.5 0.0 0.5 1.0

-0.6

-0.4

-0.2

0.0

0.2

Cu

rren

t (x

10

-4 A

mp

s)

Potential (V)

Using chronoamperometry

• We can model electrodeposition process to determine mechanism of

growth, we do this by holding the potential at a certain value for a

specified period of time

-0.5 0.0 0.5 1.0

0.0

2.0

4.0

Curr

ent (x

10

-4 A

mp

s)

Potential (V)

Peak maximum shifts to earlier times

Can analyse i–t profile to elucidate mechanism

Hills and Scharifker model

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What do these mechanisms of growth mean for electrodeposition?

Progressive nucleation and growth

Increase deposition time

1.3 mM AgClO4 in CH3CN + 0.1 M LiClO4

Note: Still silver deposition but now in organic solvent – the growth is affected!

Analysis shows instantaneous nucleation

30 s

50 s

90 s

What do these mechanisms of growth mean for electrodeposition?

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The Randles-Sevcik equation can be used to determine the diffusion coefficient of a species in solution. Use this equation to determine the diffusion coefficient of a redox active species, with the transfer of one electron, from the information provided below.

Electrode area: 0.1963 cm2

Faraday’s constant: 96,485 C mol-1

Universal gas constant: 8.314 J mol-1 K-1

Concentration: 2.55 mM

For the following electrochemical reaction, B → C + e-, a rotating disk

electrode study was carried out to determine the diffusion co-efficient of B.

Illustrated here is a plot of the limiting current versus ω1/2.

What is the name of the equation used to plot this data?

From this plot calculate the diffusion co-efficient of B. The electrode used

had an area of 0.1963 cm2, [B] = 2.55 x 10-6 mol cm-3, kinematic viscosity of

solution = 0.00916 cm2 s-1 and Faraday’s constant = 96,485 C mol-1.