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
Basic Concepts of Electrochemistry
Sotiris Sotiropoulos, Chemistry Department, Aristotle University of Thessaloniki
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
Basic Concepts of Electrochemistry
Sotiris Sotiropoulos, Chemistry Department, Aristotle University of Thessaloniki
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 ⎯→←+ −
Basic Concepts of Electrochemistry
Sotiris Sotiropoulos, Chemistry Department, Aristotle University of Thessaloniki
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)
Basic Concepts of Electrochemistry
Sotiris Sotiropoulos, Chemistry Department, Aristotle University of Thessaloniki
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)
Basic Concepts of Electrochemistry
Sotiris Sotiropoulos, Chemistry Department, Aristotle University of Thessaloniki
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 )
Basic Concepts of Electrochemistry
Sotiris Sotiropoulos, Chemistry Department, Aristotle University of Thessaloniki
• 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
Basic Concepts of Electrochemistry
Sotiris Sotiropoulos, Chemistry Department, Aristotle University of Thessaloniki
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.
Basic Concepts of Electrochemistry
Sotiris Sotiropoulos, Chemistry Department, Aristotle University of Thessaloniki
ELEMENTARY STEPS OF AN ELECTRODE PROCESS
Ox (bulk)
e-Red (surf)
Ox (surf)
Charge
Transfer
Red (bulk)Mass transfer
Mass transfer
ELECTRODE
BULK SOLUTION
Basic Concepts of Electrochemistry
Sotiris Sotiropoulos, Chemistry Department, Aristotle University of Thessaloniki
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
=
Basic Concepts of Electrochemistry
Sotiris Sotiropoulos, Chemistry Department, Aristotle University of Thessaloniki
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+
=
Basic Concepts of Electrochemistry
Sotiris Sotiropoulos, Chemistry Department, Aristotle University of Thessaloniki
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
Basic Concepts of Electrochemistry
Sotiris Sotiropoulos, Chemistry Department, Aristotle University of Thessaloniki
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
Basic Concepts of Electrochemistry
Sotiris Sotiropoulos, Chemistry Department, Aristotle University of Thessaloniki
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)
Basic Concepts of Electrochemistry
Sotiris Sotiropoulos, Chemistry Department, Aristotle University of Thessaloniki
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)
Basic Concepts of Electrochemistry
Sotiris Sotiropoulos, Chemistry Department, Aristotle University of Thessaloniki
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)
Basic Concepts of Electrochemistry
Sotiris Sotiropoulos, Chemistry Department, Aristotle University of Thessaloniki
(Heterogeneous) Charge transfer Mass transfer Heat transfer
Ion migration Molecular diffusion Convection
Convective diffusion
Forced Convection Natural Convection
TRANSFER PHENOMENAIN ELECTROCHEMICAL PROCESSES
Basic Concepts of Electrochemistry
Sotiris Sotiropoulos, Chemistry Department, Aristotle University of Thessaloniki
MASS TRANSFER EQUATIONS
υ+Ψ−−=r
r
CuCgradDgradCAdt
Nd• Mass flow:
diffusion ionic
migration
flow
• Concentration variation:
gradCgradC grad uCDdtdC 2 υ+Ψ−∇= r
Basic Concepts of Electrochemistry
Sotiris Sotiropoulos, Chemistry Department, Aristotle University of Thessaloniki
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 ==
Basic Concepts of Electrochemistry
Sotiris Sotiropoulos, Chemistry Department, Aristotle University of Thessaloniki
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 →∞→ −=
Basic Concepts of Electrochemistry
Sotiris Sotiropoulos, Chemistry Department, Aristotle University of Thessaloniki
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
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