introduction to electrochemistry - linköping university · lecture outline • modern...

Introduction to Electrochemistry

Mikhail VaginIFM, Linkoping University

Lecture Outline• Modern Electrochemistry as Technology• Electrochemical ‘Kitchen’ (how it looks like?)

• Equilibrium at Interfaces• Electrochemical thermodynamics• Electrochemical kinetics• Mass transport• Electric double layer

Modern Electrochemistry as Technology

(How we are affected by electrochemistry?)

• NiMH and Li-ion batteries

Modern Electrochemistry as Technology

(How we are affected by electrochemistry?)

• Fuel Cells

Modern Electrochemistry as Technology

(How we are affected by electrochemistry?)

• Sensors and biosensors

Modern Electrochemistry as Technology

(How we are affected by electrochemistry?)

• Solar cells

Modern Electrochemistry as Technology

(How we are affected by electrochemistry?)

• Corrosion protection

Electrochemical ‘kitchen’ (How it looks like?)

Electrochemical ‘kitchen’ (How it looks like?)

• Electrochemistry as a spectroscopic method

SYSTEMOF INTEREST RESPONSE

Electrochemical ‘kitchen’ (How it looks like?)

• Standard three electrode set-up

The potentiostat is a feedback operational amplifier,Which controls the potential at the working electrode

Electrochemical ‘kitchen’ (How it looks like?)

• Counter electrode• Currents flows between the

counter (auxiliary) and the working electrodes;

• Surface area larger than working electrode area so that the electron transfer at counter is not rate limiting;

• Should be inert to avoid dissolution.

Electrochemical ‘kitchen’ (How it looks like?)

• Reference electrode• The potential difference is

measuring between the reference and working electrodes;

• The reference electrode holds the fixed interfacial potential;

• Standard hydrogen electrode;

• Ag/AgCl;• Saturated calomel electrode

Hg/Hg2 Cl2 .

Electrochemical ‘kitchen’ (How it looks like?)

• Working electrode (‘business’ electrode)

Equilibrium at interfaces

Interfaces

Electron exchange

Ion exchange

Exchange of neutral species

phase phase

Interfaces• Phases in electrochemistry

– Electronic conductors• metals• semiconductors

– Ionic conductors• electrolytes• ionic liquids• membranes

– Mixed conductors (electrons and ions)• conducting polymers• redox polymers

Equilibrium at Interfaces

• Two metals in contact

electrons flowfrom

to

Metal Metal

Metal Metal ++++

----

Fermi level, i.e. energy

level with probability to find electron equal to 0.5

Contact potential difference

Equilibrium at Interfaces

• Metal and redox couple in contact

electrons flowfrom

to redox couple

The probability to find the particle at this energy

Equilibrium at Interfaces• Interfacial potential difference depends on

the concentrations (activities) of O and R and vice versa

][][ln0,

RO

nFRT

Nernst equation

Galvani potential difference

Equilibrium at Interfaces• By convention standard reference electrode is

Standard Hydrogen Electrode (SHE)

HeH

Equilibrium at Interfaces• Reference electrodes are used due to their fixed

interfacial potentiels

• Ag/AgCl, silver-silver chloride• Hg/Hg2 Cl2 , calomel electrodee• Hg/HgSO4 , mercury-mercurous sulphate

Electrochemical thermodynamics

Will a molecule react at electrode spontaneously?

Electrochemical thermodynamics (Will a molecule react at electrode spontaneously?)

• Control of electrochemical potential allows the reaction to be controlled

Changed due to changes in potential

by convention: reduction

by convention: oxidation

Electrochemical thermodynamics (Will a molecule react at electrode spontaneously?)

][][ln0

RO

nFRTEE

nFEGKRTGnFE

OR

nFRTE

E

ln][][ln

0

00

0

Nernst equation:

if

• Nernst equation relates the applied potential difference to the concentration of oxidized and reduced species in solution;

• Small changes in aplied potential can lead to large changes in concentration of oxidized and reduced species.

Electrochemical kinetics How does the molecule get to the electrode?

Electrochemical kinetics (How does the molecule get to the electrode?)

• Observed electrochemical response is dominated by rate determining step;

• If the transport to/from surface is slowest process, then the reaction is ‘mass transport limited;

• If the electron transfer is slowest, the reaction is ‘electron transfer limited ’;

• Or mixed control

Total reaction

Electrochemical kinetics Interfacial electron transfer

• Rates of electrode processes

• Rates and Current Densities

][ surfaceCkrate mol cm-2 s-1

Rate constant: cm s-1

Surface concentration:

mol cm-3

Fnratej **Current density: A cm-2

mol cm-2 s-1

Number of transferred electrons per one act Faraday’s constant

96485 C mol-1

Electrochemical kinetics• Dynamic equilibrium • If E >E0

metal solutionwith redox particles

e-

Fe3+

Fe2+

0jjj

Exchange current density

e-

Fe3+

Fe2+

jjj

metal solutionwith redox particles

)(EfJ ???

Electrochemical kinetics

100

32 FeFeCCnFkj

• The Butler Volmer equation

RTnF

RTnFjj exp)1(exp0

0EE where

Electrochemical kinetics

• Steady-state current density vs overpotential curve

cathodic overpotentials

= < 0anodic overpotentials

> 0

anodic currents

cathodic currents

RTnFjj )1(exp0

RTnFjj

exp0

increase

RTnF

RTnFjj exp)1(exp0

Butler-Volmer equation:

less than 100mV

increase

Here: exp(x)=1+x,

0nFjRTRCT

Electrochemical kinetics

RTnFjj )1(lnln 0

• Botler Volmer curve at high overpotentials

RTnFjj

0lnln

lnj0

Electrochemical kinetics• Example: j0 for hydrogen evolution due to reduction

0HeH

Metal -log10 (j0, A cm-2)

Ru 2.1

Pt 3.6

Fe 6.0

Cr 7.4

Hg 12.5

Electric double layer

Electric double layer

Diffuse layer

Helmholtz layer

ncompositiopTDL E

qC,,

tCEi

Charging current

Total currentTotal current = Faradaic current + Charging current + Ohmic drop

Applied by potentiostat due to electron

transfer;The most

informative part

We usually need to correct

How to split them?

potential or current stepscurrent spike

potential ramp

Mass transport

Mass transport• Why we do care about mass transport?

• Electrochemical reactions take place on electrodes (obviously!).

• Electrons can only tunnel a few Angstroms.• In the electrochemical cell, most reactants are not at the

electrode surface.• The rate of electron transfer (or measured current) can

be dependent on getting reactants to electrode surface.

Rate determining steps here, in this

chapter

Mass transport• Mass transport in electrochemistry

• Diffusion is a movement of molecules along a concentration gradient, from a area of high concentration to the area of low concentration

• Migration is a movement of charged species under the influence of an electric field.

• Convection is a movement of species by hydrodynamic transport (e.g. natural thermal motion and/or stirring).

but it will help us later!

Mass transport• Fick’s laws

xODJ

][

00

tODnFAi

][

1st law:

2nd law:

Quantification of movement of a species with respect to distance x from electrode with the flux J

2

2

0][][

xOD

tO How the surface concentration

changes as a function of time

Solving for planar geometry and

electrodereductionc OnFAki ][

Bulk concentration

Surface concentration

What does it mean?

Cottrell equation:

• Fe3+ solution of low concentration in background electrolyte;

• Apply 0.5 V (E1) then jump to 0 V (E2):

0.5 V0 V

• Measure the change in current with time.

Mass transport

eFeFe 32

eFeFe 32

tODnFAi

][

• Chronoamperometry experiment‘Excitation’

Response

tODnFAi

][

DOAFnt

i bulk22222 ][

1 Straight line means

diffusion controltODnFAi

][

Mass transport

eFeFe 32

• Diffusion layer

time

diffusion layeraround 500 nm Dt • If D = 10-10 m2s-1, after 1 s

is around 20m

Voltammetry (the most popular electrochemical

technique)

Voltammetry• Linear Sweep Voltammetry

• Instead of step (‘pulse’) lets sweep!

exponential increase with potential:kinetic control i

exp E

50/50

[O] depleted from the surface

Cottrell area:Diffusion controlI

1/(t)1/2

Voltammetry• Effect of scan rate

Scan rate

• Since the current is proportional to flux, and the flux is proportional to gradient of concentration between bulk and surface, it should be evident that a higher scan rate give sharper gradient and higher current.

Voltammetry

mVn

EEE cathodicpeak

anodicpeakpeaks

59

1cathodicpeak

anodicpeak

i

ica

peaki ,

• Cyclic voltammetry

If: • Fully reversible electrode reaction

• Randles-Sevcik equation:

DOFAnip ][2/3

Voltammetry• Rotating Disk Electrode • Sometimes controlled

convection is not bad!

timet

no rotation: rotation is on:

ft

0.1 0.2 0.3 0.40.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8 2400rpm

i, m

A c

m-2

E vs Ag/Ag+/CH3CN, V

0rpm

Voltammetry

xO

xOD

tO

x

][][][

2

2

0

• Rotating Disk Electrode

2nd Fick’s law:

convection

6/13/2][692.0 DOnFAi bulkL

Levich equation:

exponential increase with potential:kinetic control i

exp E

limiting current ilim f(t)area of mass transport control

Voltammetry• Microelectrodes

0

0 ][][r

OnFADt

ODnFAitotal

Cottrell steady-statecurrent

Voltammetry• Microelectrodes

Macro

MicroEnhanced diffusion at microelectrodes:•Shorter response time•Faster kinetics can be studied•Higher Signal-to-Noise ratios

(Faradaic vs charging currents)•Less ohmic drop•Insensitive to convection•Could be integrated into microsystems

Thank you!