Electrochemical reaction
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electrochemistry
electrochem. reaction mechanism
electrode potential
Faradays law
electrode reaction kinetics
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Electrochemistry in industry
Chlor-Alkali
galvano industry
production of other inorganic./organic compounds
recycling
wastes treatment and reprocessing
dezinfection / toxic wastes elemination
contaminated soils treatment
electric energy sources (battery, fuel cells, ...)
electroanalytical methods
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Advantages
versatile
direct oxidation or reduction
mediated oxidation or reduction
simple construction
scalable
selective
electrode potential
material of electrode (overpotentials)
separation of electrode chambers
ecological
electron - „clean reactant“
recycling of raw materials
waste treatment
Disadvantages
expensive
mass transfer
Faradays law + price of electric energy
electrode materials
heterogeneous process - concentrations limited by
mass transfer
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1
Basic terms
Electrochemical reactions - the transfer of electrons
between the electrode surface and molecules in the
electrolyte.
reactant - electron
rate of electron flow – electric current
oxidation/reduction power - potential (voltage)
amount of electrons – electrical charge
Separation of electrode reactions in reactor enable to proceed processes
hardly realizable (or unrealizable) by other way.
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Basic terms I
Electrode – (from technological point of view) piece of
electrically conductive matter (of suitable shape) where
electrode reaction take place (on its surface)
Anode – electrode with oxidative reaction
Cathode – electrode with reductive reaction
Electrolyte – ion conductive medium. Mainly solutions with
dissociated molecules (ions).
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Basic terms II
Electrochemical reactors:
Electrolyser - to do electrolysis i.e. redox reaction driven by
external voltage on the anode and cathode
Galvanic cell – batteries, accumulators
Elchem. generators - fuel cells
} power sources
Separator – permeable barrier for anodic and cathodic
chamber separation. Membrane or diaphragm.
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2
Process size
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F. C.Walsh, Pure Appl. Chem., Vol. 73, No. 12, pp. 1819–1837, 2001.
Electrode reaction
reaction with electron as a reactant or product
a A + n e- = m M
Simultaneously on the second electrode another reaction occurs to
enable charge flow.
b B = z Z + n etotal reaction in the system is the sum of both electrode reactions
aA + b B = mM + zZ
! Electrode reactions occurs at electrodes separated by electrolyte.
One electrode is called halfcell
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Electrochemical reaction
Bagdad Battery 2000 BC
9
3
Mechanism of electrode reaction
reaction mechanism consists from several steps
transport to the electrode
adsorption on the el. surface
reaction on electrode
products desorption from he surface
transport to the bulk
more than one electron reaction – less probable – usually sequence
of individual steps
NO3- + 3 H2O + 5 e- = 1/2 N2+ 6 OH-3
-2
-1
0
+1
+2
+3
+4
+5
-3.1
N 3+0 .4 2
-1. 0 5
+ 0 .1
NH3
+0 . 73
N 2H 4
-3 .0 4
+0 .1 5
+ 0.9 4
N H 2O H
N2
+0 .7 6
N 2O
-1 . 1 6
+0 .0 1
+0 .4 6
+ 0 .8 7
N O 2-
NO
+ 1.5 2
-0 .8 6
N 2O 4
N O 3-
+ 0. 1 8
N 2 O 2 --0 . 7 6
-0. 1 4
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a lk a lic k é p ro st ř e d í
Polarisation curve
Polarization curve - dependence of current on voltage refers to
the course of events on the electrode in the system
Pt v 0.5M H2SO4,
2.0E-05
1.0E-05
0.0E+00
0.0
I [A]
Reversible reaction
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Polarografický záznam
-1.0E-05
-2.0E-05
-3.0E-05
-4.0E-05
E [V]
Pt electrode in H2SO4
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Electrode potential
Galvani potential – the work done in moving a unit positive charge
from infinity to that point
Usually difference of potentials
Standard (reference) electrode – electrode of known potential
restive to the potential shift (calomel, mercurosulpahate, Ag/AgCl)
For solutions in protic solvents, the universal reference electrode
for which, under standard conditions, the standard electrode
potential (H+ / H2) is zero at all temperatures. :
SHE
2 H + ( aq ) + 2 e − ⇔ H 2 ( g )
Activity of each compounds in SHE must be = 1
(p(H2)= 101,3 kPa, a(H+)=1)
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Fundamental relationships I
Current density – j = I/A
El. charge –
Faraday’s law –
m is the mass of the substance produced at the electrode (in grams),
Q is the total electric charge that passed through the solution (in coulombs),
q is the electron charge = 1.602 x 10-19 coulombs per electron,
n is the valence number of the substance as an ion in solution (electrons per ion),
F is Faraday's constant,
M is the molar mass of the substance (in grams per mole), and
NA is Avogadro's number = 6.022 x 1023 ions per mole.
I is current
t is time (in seconds), A is electrode area
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Fundamental relationships II
Electrode reaction potential – ∆rG= - n F Er
relation between el. potential and Gibbs energy
Nernst equation –
E = Er −
νp
RT a p .....
ln νr
nF
ar ....
electrode potential calculation
Tafel equation –
"a" and "b" are characteristic constants of the electrode system
kinetic factor of electrochemical reaction
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Faradays law
m is the mass of the substance produced at the electrode (in grams),
Q is the total electric charge that passed through the solution (in coulombs),
q is the electron charge = 1.602 x 10-19 coulombs per electron,
n is the valence number of the substance as an ion in solution (electrons per ion),
F is Faraday's constant,
M is the molar mass of the substance (in grams per mole), and
NA is Avogadro's number = 6.022 x 1023 ions per mole.
I is current
t is time (in seconds), A is electrode area
Task: calculate volume and weight of produced Cl2 during 1h in the case of total current
10 kA, pressure 101,3 kPa and temp. 250C. Current efficiency is 95 %.
Mr (Cl2) = 70,9 g/mol
Q = I t = 10 000 * 3600 = 3,6 107 C
Q(Cl2) = 3,6 107 * 0,95 = 3,42 107 C
n(Cl2) = 3,42 107/(96485 * 2) = 177,23 mol
V = n R T / p = 177,23 * 8,314 * 298 / 101,3 103 = 4,3346 m3
m = n*M = 177,23 * 70,9 = 12565,5 g = 12,5655 kg
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Nernst equation
Electrode potential calc.
E = Ero −
νp
RT a p .....
ln νr
nF
ar ....
in tables potentials in the reduction direction
Task: calculate potential of hydrogen electrode in HCl at conditions pH=1,
pressure H2 99,8 kPa and temp 250C.
reaction:
+
−
2 H ( aq ) + 2 e ⇔ H 2 ( g )
E = E0 −
a(H+)
=
10-pH
=
10-1 =
0,1
aH 2
RT
ln
2 F (aH + )2
a(H2) = p(H2)/fstd= 99,8/101,3 = 0,985
E = 0 – (8,314* 298)/(2*96500) * ln(0,985/0,12) = - 0,0589 V
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Nernst equation II
Cell potential
for cell potential the difference between electrodes (cathode and anode) is
calculated.
in praxis both electrodes are calculated in the reduction form and the finall cell
potential ic calculated as:
Ecelk = Ekat – Eand
Uelz = - Ecelk
Task: calculate theoretical cell potential for HCl electrolysis in the system from
previous task
Cl2(g) + 2 e- = 2 Cl- Eo = 1.358 V
E(H+/H2) = - 0,0589 V
E(Cl2/Cl-) = Eo – RT/nF ln(a(Cl-)2/a(Cl2))
E (Cl2/Cl-) = 1,358 – (8,314* 298)/(2*96500) * ln(0,12/0,985) = 1.417 V
Ecelk = Ekat – Eand = - 0,0589 - 1.417 = - 1.476 V → Uelz = - Ecelk = 1.476 V
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Counter electrode reaction
Electric energy demand is very high with respect to the requested
charges/currents (100 kA). Each small decrease of potential causes high energy
savings.
Productions o Cl2 by HCl electrolysis. Replacemnet of cathode
hydrogen production – for 1 t Cl2 is necessary 1000 kWh beside
1700 kWh.
Eo(H+/H2)= 0,000V; Eo (O2/H2O)= 1,229V; Eo (Cl2/Cl-)= 1.358V
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http://research.bayer.com/edition_16/16_Electrolysis.pdfx
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Galvanoindustry
Proper selection of galvanic bath composition it is possible to deposit
simultaneously more metals (bronzes, brass)
Cu2+ + 2e = Cu Eo = 0,337 V
Sn2+ + 2e = Sn Eo = - 0,140 V
Zn2+ + 2e = Zn Eo = - 0,736 V
Sn(OH)62– + 2 e- = HSnO2– + 3 OH– + H2O
HSnO2–
+ H2O + 2
e-
= Sn + 3
OH–
Eo =
Eo = – 0.93 V
– 0.909 V
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Kinetics of electrode reaction
Rate of electrochemcial reaction is depenedent on various
parameters
electrode material, electrolyte composition, temperature etc.
Overvoltage – difference between potential at OCP and potential at
current density
η = E( j ) − E r
30000
20000
Exponential dependence of current
(reaction speed) on overvoltage
j/Am
-2
10000
0
Er
-10000
Anode – positive sign
-20000
-30000
-0.4
-0.2
0.0
E / V vs. Er
0.2
0.4
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Overvoltage
Various types:
activation – rate determining step – electron transfer between
electrolyte and electrode
concentration (difusion) – mass transfer to the electrode surface
other overvoltagesreaction,…
sorption,
preceding
reaction,
surface
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Kinetics of electrochemical reaction
overpotential – irreversibility of reaction- activation
- concentration
Butler-Volmer equation:
j is the anodic or cathodic current density;
α = charge transfer barrier or symmetry coefficient for the anodic or cathodic reaction. α values are
typically close to 0.5;
ηact = Eapplied - Eeq, i.e. positive for anodic polarization and negative for cathodic polarization;
n = number of participating electrons;
R = gas constant;
T = absolute temperature;
F = Faraday = 96485 C mol-1
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Tafel equation
η = a + b·log j
or for anode
Kinetics of electrode reaction
Task: Calculate potential of anode for chlorine production, j = 60 A/dm2 and
temp. = 70 ºC.
Activity coef. for Cl- and Cl2 are (a(Cl-) = 1, a(Cl2) = 1). Concentration of
chlorides c(Cl-) = 1mol/L.
Reaction operates at pressure 760 mm Hg.
1) On graphite - Tafel plot for 70 ºC
j = [A/cm2]
ηCl2 = 0,6 + 0,12 log j
2) on ATA – Tafel plot
η Cl2 = 0,2 + 0,06 log j
E
0
Cl 2
Cl −
= 1,3595 V
dE
= - 0,389 mV/K
dT
j = [A/cm2]
při 25 ºC
pH2O = 233 mm Hg
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Kinetics of electrode reaction
Cl2 + 2e- = 2Cl-
Electrode reaction
Electrode potential from Nernst eq.
r
E = E0 −
a2 −
RT
⋅ ln Cl
nF
aCl2
E10 = E20 −
E10 = 1,3595 − 0,389 ⋅10 −3 (70 − 25) = 1,3595 − 0,0175 = 1,342 V
aCl 2 =
r
E = 1,342 −
p − pH 2O
p0
⋅γ
a Cl 2 =
dE
(T1 − T2 )
dT
760 − 233
= 0,693
760
8,314 ⋅ (273,15 + 70)
1
⋅ ln
= 1,342 − 5,4 ⋅ 10− 3 = 1,337 V
2 ⋅ 96487
0,693
60 A/dm2 = 0,6 A/cm2
Graphite overvoltage
η Cl = 0,6 + 0,12 log 0,6 = 0,6 − 0,027 = 0,573 V
2
E = 1,337 + 0,573 = 1,91V
E = r E + ηCl2
η Cl = 0,2 + 0,06 log 0,6 = 0,2 − 0,0008 = 0,1992 V
ATA overvoltage
2
E = 1,337 + 0,199 = 1,536 V
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Concentration overvoltage
transport of ion from bulk to the electrode surface
J=D
c0 − cs
δN
cτ
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c0
3
j/ Am
-2
0
-3
-6
-9
-12
-0.3
cs
-0.2
-0.1
0.0
η/V
0.1
0.2
0.3
δN
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Importance of overvoltage – galvanotechnics
Additives for increased concentration overvolatges – smooth
surface
Polarization curve for the potentiostatic deposition of copper.
(a) Overpotential; 200 mV, deposition time: 6 hours;
(b) Overpotential: 300 mV, deposition time: 5 hours;
(c) Overpotential: 700 mV, deposition time: 2 min.
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Overvoltage fo Cl2 production
NaCl electrolysis
High overvoltage for O2 enable production opf Cl2
Eo (O2/H2O)= 1,229 V
Eo (Cl2/Cl-)= 1.358 V
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Amalgam/mercury electrolysis
Power sources overvoltage
bateries, accumulators, fuel cells
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