Basic Concepts of Electrochemistry · Basic Concepts of Electrochemistry Sotiris Sotiropoulos, Chemistry Department, Aristotle University of Thessaloniki ELECTROCHEMICAL CELL Power

Basic Concepts of Electrochemistry

Sotiris Sotiropoulos, Chemistry Department, Aristotle University of Thessaloniki

ELECTROCHEMISTRY

Electricity-driven Chemistry

or

Chemistry-driven Electricity

Electricity: charge flow

(electrons, holes, ions)

Chemistry (redox): reduction = electron uptake

oxidation = electron loss



ELECTROCHEMICAL CELL

Power Source

ANODE

GALVANIC CELL

ELECTROLYTIC CELL

CATHODE

Porous Diaphragm or Membrane

⊕

e- ⊕

Red1

e-

Ox1 Red2

e-Ox2

Anions, X-

Cations, M+

Electrolyte MX

Electrical Load



Anode: the site of oxidations

(Positive electrode of an electrolytic cell

negative electrode of a galvanic cell)

−+⎯→← e nOxdRe 11

Cathode: the site of reductions

(Negative electrode of an electrolytic cell

positive electrode of a galvanic cell)

22 dRee nOx ⎯→←+ −



ELECTROCHEMICAL APPLICATIONS

(Conversion of Chemical to Electrical Energy)

•Batteries(for electronic devices, automotion etc)

•Fuel Cells(for automotion, power stations etc)

•Electroanalysis(potentiometric electronalytical techniques and sensors, e.g. ion selective electrodes, gas sensors etc)



ELECTROCHEMICAL APPLICATIONS

(Conversion of Electrical to Chemical Energy)•Electrolysis

(e.g. chloralkali industry, hydrogen production)•Electrosynthesis

(e.g. adiponitrile production→ Νylon 66)•Electroplating and Metal Processing

(e.g. decorative metal plating, elctrochemical machining)•Cathodic corrosion protection of metals and metal composites

(e.g. bridge and ship protection)•Waste treatment

(e.g. metal ion removal and recovery, organics oxidation etc)•Electroanalysis and Elecrochemical Sensors

(e.g. determination of heavy metals, organic contaminants and biological compounds; glucose, oxygen, ethanol sensors)



PARAMETERS OF AN ELECTROCHEMICAL PROCESS

• Cell or electrode potential: E

• Current or current density: Ι or i=Ι/Α

• Concentration of electroactive species

in the bulk (homogeneous) solution: Cb

• time: t

i=f(E, Cb, t) or E=g(i, Cb, t )



• Cell potential : cellACcell IREEE −−=

• Electrode potential (cathode/anode):AeqAA

CeqCC

)E(E

)E(E

η+=

η−=

• Equilibrium electrode potentials:

)])C/()Cln[()nF/RT(E)E(

)])C/()Cln[()nF/RT(E)E(

sdResOx0AeqA

sdResOx0CeqC

22

11

+=

+=

• Overpotentials of cathode and anode reactions: )i(g )i(f

A

C=η

=η



GENERAL STEPS OF AN ELECTRODE PROCESS

• Mass transfer of reactants/products to/from the electrode.

• Charge transfer (heterogeneous electron or hole exchange) at the electrode surface.

• Surface reactions (e.g. adsorption, phase transitions etc).

• Homogeneous chemical reactions in the bulk solution.



ELEMENTARY STEPS OF AN ELECTRODE PROCESS

Ox (bulk)

e-Red (surf)

Ox (surf)

Charge

Transfer

Red (bulk)Mass transfer

Mass transfer

ELECTRODE

BULK SOLUTION



CURRENT DENSITY-ELECTRODE REACTION RATE

rate reaction dtdN

AnF

dtdQ

A1i ∝==

• km = mass transfer coefficient

= f(diffusion/flow rate and cell geometry )

)C,k,k(fi em=where:

• ke = charge transfer coefficient

= f(electrode reaction, electrode material, electrode potential)

)RT

EEnFexp(kk

0eq

se−α

=



CURRENT DENSITY-ELECTRODE REACTION RATE

Multi-step reaction: the slowest step determines the rate of the overall reaction (rate determining step, rds)

• slow charge transfer + small overpotential ⇒

⇒ (electrode) kinetic controlebme knFCikk =⇒pp

• fast charge transfer + high overpotential ⇒

mbme knFCikk =⇒ff

mass transfer

control⇒

me

b

k1

k1nFCi+

=



CURRENT-POTENTIAL CURVES

C

u

r

r

e

n

t

D

e

n

s

i

t

y

Electrode potential, E

Mass transfer control

iL=f(km)

independent of E

Steady state Mass transfer control

kinetic control Non-steady state

iMass transfer control mbL knFCi =

Limiting current



THERMODYNAMICS AND KINETICS OF ELECTRODE REACTIONS

←→+= jji

cathodic

anodic

Equilibrium: j=0, E=Eeq Anodic process: j>0, E>Eeq Cathodic process: j<0, E<Eeq



ELECTROCHEMICAL CELL AT EQUILIBRIUM• Total current: 0i =

• Equilibrium potential of cell :

21

21nFRT

ACcell ]s)OxC[(]s)dReC[(]s)dReC[(]s)OxC[(

ln])0eqE()0

eqE[()eqE( +−=

Ox2 + Red1 ↔ Red2 + Ox1

• Equilibrium potential of cathode:

2

2nFRT

CC ]s)dReC[(]s)OxC[(

ln)0eqE()eqE( +=

Ox2 + ne- ↔ Red2

(Nernst potential)

• Equilibrium potential of anode : Ox1 + ne- ↔ Red1

1

1nFRT

AA ]s)dReC[(]s)OxC[(

ln)0eqE()eqE( += (Nernst potential)



ELECTROCHEMICAL CELL AT EQUILIBRIUM

Free-Gibbs Energy(of the overall reaction Ox2 + Red1 ↔ Red2 + Ox1

in the electrochemical cell):

celleq)E(nFG −=Δ

• (Eeq)cell > 0 ⇒ ΔG < 0

⇒ spontaneous process (galvanic cell)

• (Eeq)cell < 0 ⇒ ΔG > 0

⇒ non spontaneous process (electrolytic cell)



KINETICS

OF AN IRREVERSIBLE-SLOW ELECTRODE REACTION

• slow charge transfer:

eb

meknFCi

kk=

⇒pp

• kinetically controlled current:

ηη αα−αα−=

⇒−=

RTFnc

RTFna

eieii

jji

00

)C()A(rs

Butler-Volmer equation

(η=Ε-Εeq)



(Heterogeneous) Charge transfer Mass transfer Heat transfer

Ion migration Molecular diffusion Convection

Convective diffusion

Forced Convection Natural Convection

TRANSFER PHENOMENAIN ELECTROCHEMICAL PROCESSES



MASS TRANSFER EQUATIONS

υ+Ψ−−=r

r

CuCgradDgradCAdt

Nd• Mass flow:

diffusion ionic

migration

flow

• Concentration variation:

gradCgradC grad uCDdtdC 2 υ+Ψ−∇= r



Linear semi-infinite diffusion to a planar electrodein a stationary solution

Diffusion to a planar electrodePlane parallel to the electrode Elementary volume

Fick’s 1st Law Fick’s 2nd Law

Flux

0x)dxdC(nFi ==



Nernst diffusion layer model

All mass transfer modes and corresponding concentration profiles

can be replaced

by linear diffusion through the stagnant layer of an equivalent linear profile.

δ

−= →∞→ )s(0x)b(x CC

nFDi

δ= Dmktrue profile

linear profileCb

)CC(nFki )s(0x)b(xm →∞→ −=



Non-steady state and steady state mass transfer

Diffusion to a planar electrode from a stagnant solution⇒

δ and i variation with time ⇒

non-steady state

Distance from electrode , x

Bulk solution concentration, Cb

increasing t,increasing δ,

decreasing km,decreasing i

δ

electroderotation solution flow

δ

solution

membrane

solution or gas

anode cathodesolution Microelectrode

(<50 μm)insulator

electrode

thin layer cell

δ

δ

Diffusion barrier of constant δ⇒ i constant with time ⇒

steady state

Documents

Basic Concepts of Electrochemistry · Basic Concepts of Electrochemistry Sotiris Sotiropoulos, Chemistry Department, Aristotle University of Thessaloniki ELECTROCHEMICAL CELL Power