electroanalytical techniques (chronoamperometry ... · abc’s of electrochemistry july 12, 2011...

Electroanalytical Techniques

(chronoamperometry/

chronocoulometry)

ABC’s of Electrochemistry

July 12, 2011

Vedasri Vedharathinam

Center for Electrochemical Engineering

Research

Department of Chemical and Biomolecular

Engineering

Electrochemical

Engineering

Research Lab, Ohio

Overview

1. Electrochemistry basics

2. Electrochemical techniques

– Sweep method

• Cyclic voltammetry

– Step method (current or potential)

• Chronoamperometry

• Double step chronoamperometry

• Chronocoulometry

• Double step chronocoulometry

Basic concepts in electrochemistry

• Chemistry that deals with chemical reactions in a metal (e-

conductor) – solution (ion conductor) interface.

• Involves e- transfer between the elctrode and the elecrolyte or

species in solution.

• This transfer creates a current, the magnitude of which can

give us clues about the active species.

• Electrochemistry is based on Electron transfer reactions:

oxidation-reduction (redox) reactions.

• These reactions result in the generation of an electric current

(electricity) or caused by the application of an electric current.

– Chemical rection driven by an external voltage – electrolysis

– Chemical reaction producing voltage - battery

Applications

• Batteries

• Fuel cells

• Electrolysis

• Corrosion

• Industrial production of chemicals such as Cl2,

NaOH, F2 and Al

• Biological redox reactions

• Redox reactions employed in biological sensing

• Amperometric sensors

Terminology

• OXIDATION—loss of electron(s) by a species;

increase in oxidation number;

Fe2+ Fe3+ + e-

• REDUCTION—gain of electron(s); decrease in

oxidation number; decrease in oxygen;

Fe3+ + e- Fe2+

• OXIDIZING AGENT—electron acceptor; the

reagent is reduced

• REDUCING AGENT—electron donor; the reagent

is oxidized.

When a piece of metal is placed in a solution containing ions,

there is a charge separation across the boundary between the

metal and the solution. This sets up a potential , which cannot

be measured directly but requires a second half cell.

Electrochemical behavior of an electrode in

solution

Electrochemical cell

2 – electrode cell 3 – electrode cell

• Interested in only one of the reactions, and

the electrode at which it occurs is called the

working (or indicator) electrode, coupled with

an electrode that approaches an ideal

nonpolarizable electrode of known potential,

called the reference electrode.

• Current is passed between the WE and CE

• Consistent, reliable and accurate.

• Used where measurement of the whole cell

voltage is significant (e.g. batteries, fuel cells,

super caps).

• where the counter electrode potential can be

expected not to drift over the course of the

experiment. E.g. systems with very low

currents and/or relatively short timescales

and which also have a well poised counter,

e.g. a micro working electrode and a much

larger silver counter electrode.

2-electrode vs. 3-electrode systems

• Requires a precise control of the

potential at the electrode.

• Three electrodes:

– Working electrode (WE),

– Counter electrode (CE)

– Reference electrode (RE).

• No current through RE ideally.

• RE is used to provide precise

control of potential at the WE, and

the current from WE to CE is

measured.

Three electrode cell

Faradiac process: Electron transfer causes oxidation and

reduction to occur. This process is governed by Faraday’s law.

Faradaic process

Electrochemical

Engineering

Research Lab, Ohio

Mass transport due to diffusion

In order to react a species at an electrode it needs to be

transported from bulk to surface.

Three principal mechanisms:

• Diffusion is the movement of molecules along a concentration

gradient, from an area of high concentration to an area of low

concentration.

• Migration is the transport of a charged species under the

influence of an electric field.

• Convection is the transport of species by hydrodynamic

transport (e.g. natural thermal motion and/or stirring).

Mass transport in electrochemistry

Current flow at Electrode Surface

The current that flows from a surface electrochemical

reaction can be defined as (using the example of reduction

of O):][ electroderedc OnFAki

F = 96485 Cmol-1. The amount of charge in C transferred for 1 mole of

reactant.

To understand an electrochemical reaction it is necessary to have a feeling

for the concentration of the reactant [O] as a function of distance from

electrode and with respect to time as a reaction progresses.

Mass transport in electrochemistry

Diffusion

Fick’s first law quantifies the movement of a species (under diffusion

control) with respect to distance x from an electrode with the flux, J.

ODJ oo

2 ][][

More important is to understand how surface concentration changes as

function of time:

1st law

2nd law

Diffusion limited electrode reaction & Fick’s law

Solving Fick’s second law (for planar electrode boundary conditions), and then substituting gives the

Cotrell equation:

[O] is now the bulk concentration of O.

electredc OnFAki

DOnFAi

Concentration verses distance

above the electrode before voltage stepConcentration verses distance

above the electrode just after pulse

ODJ oo

i ∝ J

Fe3+ + e- → Fe2+

(reduction)

Current behavior with time

Electrochemical techniques -

Voltammetry

Electrochemical techniques

Voltammetry

Voltammetry: measurement of current (I)

as a function of applied potential (E) over

a time. Under condition with polarization

(η). Negligible consumption of analyte.

– Amperometry: measure I at a fixed E

– Potentiometry: measure E when I 0,

no polarization

– Coulometry: measure C, polarization is

compensated, all analyte is consumed.

Commonly uses three electrodes

- Working electrode

- Auxillary electrode

- Reference electrode

Why use voltammetry?

• Handles high salt concentrations better than

chromatographic instrumentation

• Can differentiate between ionic species

Example: Ni2+ Ni3+

• Extremely low detection limits – high sensitivity

• Can detect a wide range of species

Types of Voltammetry

1. Sweep methods

• Cyclic voltammetry

• Linear sweep voltammetry

• Rotating disk electrode

2. Step and pulse methods

• Step voltammetrya. Chronoamperometry

b. Chronocoulometry

c. Chronopotentiometry

• Pulse voltammetrya. Normal pulse voltammetry

b. Differential pulse voltammetry

c. Square wave voltammetry

What can be learnt from voltammetry?

• Mechanism of electrode reaction.

• Concentration of oxidative or reductive

species: useful for making a sensor.

• Determination of Diffusion coefficent ofelectroactive species, D.

• Rate constant.

• Type of reaction mechanism

Electrochemical

Engineering

Research Lab, Ohio

Cyclic Voltammetry

Electrochemical techniques – Sweep method

Cyclic voltammetry

Applied waveform

Resulting voltammogram

Electrochemical techniques – Sweep method

Cyclic voltammetry

For a reversible electrochemical reaction the CV recorded has certain

well defined characteristics.

I. The voltage separation between the current peaks is

I. The positions of peak voltage do not alter as a function of voltage

scan rate

II. The ratio of the peak currents is equal to one

Ia / ic = 1

I. The peak currents are proportional to the square root of the scan

Electrochemical

Engineering

Research Lab, Ohio

a. Chronoamperometry

b. Double step chrono amperometry

c. Chronocoulometry

d. Double step chronocoulometry

Step methods

Electrochemical techniques – Step methodW

Chronoamperometry

Chronopotentiometry

Chronocoulometry

Chronoamperometry

Electrochemical techniques – Step method

Voltage applied to cell begins at V1 where no reaction occurs

and is stepped up to V2 causing electrode process to begin

and a current spike results.

Stable R species

Chronoamperometry

Faradaic current under diffusion controlled conditions is related directly to the

concentration gradient, ∂Ci / ∂x, evaluated at x = 0. Thus, as the slope of the

concentration profile for Ox decreases with time following the potential step, so will the

observed current.

Current drops off with time according to the Cottrell

equation since material must diffuse to the

electrode surface in order to react.

Chronoamperometry

i ∝ 1 / √t

DOnFAi bulk

• Perform a potential step measurement.

• Ignore current before potential step.

• Linearise Cottrell equation

Plot 1 / i2 vs tSlope = /n2F2A2[O]2D

• Slope will give the value of ―D”

DOnFAi bulk

i bulk

22222 ][

Chronoamperometry - estimation of diffusion co-efficient

t-1/2, s-1/2

Cottrell plot

• If the diffusion coefficient of an electroactive species is known, or

has been calculated, the diffusion layer thickness can be estimated

using this equation:

• The diffusion layer extends into the bulk solution more and more

slowly after application of a potential step. Hence for a molecule with

a diffusion coefficient of 1 x 10-10 m2s-1, the diffusion layer thickness

is around 20 mm after 1 second.

• The fraction of molecules oxidised or reduced can also be estimated

by calculating the volume of a hemispherical diffusion layer around a

circular electrode as a fraction of the total solution.

Chronoamperometry - estimation of diffusion layer thickness

Double potential step Chronoamperometry

To study the chemical reactions which follow electron transfer

E: O + e R

C: R X

E: X + e P

Unstable R species

FORWARD STEP

REVERSE STEP

Oxidised species

Reduced species

FORWARD STEP REVERSE STEP

t = 400ms

t = 200ms

t = 300ms

J. Phys. Chem., 1965, 69 (1), pp 30–40

Theoretical working curves for double

potential chronoamperometry applied to EC

mechanism

Kinetic plot for double potential

chronoamperometry

Slope = rate constant

– Measurement of surface area

– Measurement of diffusion co-efficient

– Determination of heterogeneous rate constant

– Determination of diffusion layer thickness

– Evaluation of ECE mechanisms

Chronoamperometry - Applications

DOnFAi bulk

Measuring instantaneous currents is not easy.

CADnti

2/12/1

tCADnQ

Chronocoulometry

Q t( ) =2nFADO

* t1/2

p 1/2+Qc

Q t( ) = FnACbD

èçö

dt + ic dtòò

Qc – response in the absence of reactant

(i.e only supporting electrolyte)

Linear plot with Qc intercept, slope

proportional to concentration of

reactantP

Cottrell current

For O + n e- R, plot Q vs. t1/2

If plot linear, the reaction is Diffusion Controlled

Chronocoulometry

Q t( ) =2nFADO

* t1/2

p 1/2+Qc

Charge due to cottrell current

Interfacial capacitance charge

Chronocoulometry

What if redox species (O) is adsorbed on electrode surface?

Q t( ) =2nFADO

* t1/2

p 1/2+Qc +Qads

QadsQDL (blank - only S.E)

OF AnQads

O the quantity of adsorbed reactants

Charge flowing into the interfacial

capacitance when the electrode

potential is stepped from E1 to E2

Extra charge produced by the

adsorbed reactant

Double potential step Chronocoulometry

• Measuring Qc is not a problem –

adsorbed species produces little or no

change in the value of Qc

• But, adsorbed species produces

significant Qc values, so that the

evaluated Qc in the absence of reactant

(blank) do not apply.

SOLUTION

Double potential chronocoulometry

Forward

reverse

Qt<t =Qc + nFAGO +2nFADO

* t1/2

Qt>t =Qc +2nFADO

p 1/2t 1/2 + t -t( )

1/2- t1/2( )

Qads + Qc

adsorbed!

For adsorption:Qf vs. t

sec1/2

Get Qads by subtraction.

Thank you !

Questions????

electroanalytical techniques (chronoamperometry ... · abc’s of electrochemistry july 12, 2011...

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