Electrochemistry
Basic terms
Fundamental relationships
Water electrolysis
Chlor-alkali industry
Chorine and Caustic soda applications
Reactor types
- mercury
- diaphragm
- membrane
environmental impact
1
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
2
Electrochemical processes scale
F. C.Walsh, Pure Appl. Chem., Vol. 73, No. 12, pp. 1819–1837, 2001.
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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.
4
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
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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Kinetics of electrochemical reaction
overpotential – irreversibility of reaction- activation
- concentration
Butler-Volmer equation:
j is the anodic or cathodic current density;
b = charge transfer barrier or symmetry coefficient for the anodic or cathodic reaction. b 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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3
Tafel equation
or for anode
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Reactor shape and material
Arrangement of reactor must reflect needs of desired application
industrial electrolysis – high currents, space utilization
growing / dissolving electrodes – continuous or frequent electrode
treatment, accessibility
low concentrations – mass transfer, specific surface area,..
electroanalysis – low electrolyte volume
Basic parts of electrochem. reactor:
current connection
anode
electrolyte (+ separator)
cathode
current connection
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Electrodes
material: durability, overvoltage, electrolyte composition
shape: stirring, active surface
Shape and durability categories:
1. Dimension changing electrodes during process – dissolution or
formation
2. Dimension stable electrodes (inert electrodes) – constant shape
during process
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4
Dimension changing electrodes
Growing electrodes
material deposition – galvanic metal deposition
production of MnO2
Dissolving electrodes
anodic dissolution – metal rafination, electrochemical
polishing/machining, electrocoagulation
lead accumulator
Pb(s) + HSO4-1 <===> PbSO4 + H+1 + 2e-1
PbO2(s) + HSO4-1 + 3H+1 + 2e-1 <====> PbSO4(s) + 2 H2O
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Inert electrodes
Application
gas evolution Cl2, H2, O2,
oxidation/reduction org. comp.
production of ClO3-, ClO4fuel cells
Ideal inert electrodes
doesn’t exist (mechanical damage, corrosion, material fatigue,
surface blocking,....) - usually described as dimension stable
anodes (DSA) (MMO)
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Electrode material
Requests
high electronic conductivity, mechanical stability, easy formation to
desired shape, valuable price, electrocalytic activity
(overvoltage), inert in electrolyte
Durability
electrode replacement – process shut down – economic losses
desired longest operation period
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5
Inert electrode materials
Cathode: reduction conditions – wide range of materials most
metals (Pt, Ir Pd, Ni, Fe, SS, Hg). Non metallic materials –
graphite, carbides, borides, diamond electrodes, ceramics
Anode: oxidative environment – limiting for most metals - Pt
metals. Non metallic materials – graphite, diamond electrodes.
Most common - ATA electrodes (Ti + oxides Ir or Ru)
ATA electrode
diamond electrodes
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Separators
Separators
Separators divide cathodic and anodic chamber, protection towards
electrolytes and reaction products (gases) mixing
Diaphragm – inert non-conductive porous barrier (azbestos, PVC,
PE, PTFE,...) only limited separation
Membrane – ion selective barrier permeable only for charged ions
of one polarity
NafionR
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Elec
Electrolyte
trolyte
Ion conductive media – 2nd order electric conductor – higher
temperature leads to higher conductivity
Solution – dissociated ions in solvent (water)
Molten salt – ions mixture
Highest conductivity – minimal ohmic losses.
Supporting electrolyte – ions increasing electrolyte conductivity but
don’t participate in electrode reaction (KOH in alkaline water elz.)
potential profile in electrolyser
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6
Electrode connection
Industrial electrolysers are equipped with many electrodes following
size and performance of electrolyser.
Arrangement
monopolar
- each electrode connected individually
bipolar
- only side electrodes are connected
monopolar and bipolar arrangement
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Recommended literature
• Ullmann's Encyclopedia of Industrial Chemistry
Published by Wiley-VCH Verlag GmbH & Co. KGaA
• Perry's Chemical Engineers' Handbook, by Robert H. Perry
and Don W. Green McGraw-Hill Inc.
• best available techniques – BAT, reference documents BREF
http://eippcb.jrc.es/
• Industrial Electrochemistry, by D. Pletcher and F.C. Walsh,
Springer
• VSCHT –library :http://www.chemtk.cz/en/
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Water electrolysis
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7
Production of H2 and O2 by water electrolysis
H2 production:
steam reformate
partial oxidation
electrolytical processes (chlor/alkali production)
water electrolysis
water electrolysis – source of highest purity H2, however for high price.
•
Cheap electric power necessary for application in industrial scale (e.g.
Norway).
•
Currently widely used as local H2 generators or on site generators for fine
technologies requesting high purity H2
•
Significant application increase with so called hydrogen economy.
In comparison to other technologies water electrolysis can directly produce high
pressure gas up to 30 Bar.
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Production of H2 and O2 by water electrolysis
Technologies
Alkaline process – electrolysis in KOH solution
PEM electrolyser – membrane electrolysis (acidic pH)
Solid oxide membrane – high temperature
with respect to H2 pressure:
low pressure
high pressure up to 30 Bar
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Norsk Hydro alkaline low pressure electrolyser
Alkaline
Alkaline process
Electrolyte:
25-30% KOH
70-80oC
Cathode: steel
Anode: Ni or nickel plated
Diaphragm: asbestos, polymeric, ceramics or composites
Gases output pressure 1-30 Bar
Electrode reactions:
Cathode:
2H2O + 2e- → H2 + 2OH-
Anode:
2OH- → 1/2O2 + H2O + 2e-
Electrolyser: bipolar, filter-press configuration
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8
PEM process
Reversal process in comparison to PEM fuel cell
Electrolyte:
perfluorinated membrane NafionR
Cathode: Pt/C
Anode: Pt-IrO2, RuO2 on suitable support (carbon?]
Electrode reactions:
Temperature 80oC
Anode:
2H2O → 4H+ + O2 + 4e-
Cathode:
4H+ + 4e- → 2H2
Electrolyser: bipolar, filter-press (stack) configuration
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PEM reactor
support Pt
Electrochemical reactions proceed on catalyst,
three – phase contact must be ensured
Nafion
e-
Catalyst based on platinum metals
- loading
- catalyst stability
- support stability
H+
H2
Components:
endplates
gas diffusion electrodes
membrane
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perfluorinated membranes
[(
copolymer of tetrafluoroethylene
and perfluorvinylethersulphate
CF2
CF2 )x CF2CF
O
]y
( CF2CFO)n (CF2 )p SO3H
CF3
Structural parameters Supplier
(and monomer content) trademark
n =1, x =5 –13.5, p =2
DuPont
Nafion® 120
Nafion® 117
Nafion® 115
Nafion® 112
n =0–1, p =1 –5
Asahi Glass
Flemion® T
Flemion® S
Flemion® R
n =0, p =2–5, x =1.5–14 Asahi Chemicals
Aciplex® S
n =0, p =2, x =3.6– 10
Dow Chemical
Dow®
Solvay
Hyflon® Ion
Equivalent weight
(IEC, meq g-1)
Thickness
(µ
µm)
1200 (0.83)
1100 (0.91)
1100 (0.91)
1100 (0.91)
260
175
125
80
1000 (1.00)
1000 (1.00)
1000 (1.00)
120
80
50
1000–1200 (0.83– 1.00)
25– 100
800 (1.25)
900 (1.11)
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9
PEM process – critical parts
Catalyst
- membrane – catalytic layer interface
- nanoparticles - high surface
- stability
Nafion membrane
- impurities influence
- membrane degradation accelerated by peroxide and ozone formed
Gas distribution and current connection, bipolar plates (Ti)
- Anodic passivation (voltage losses)
- H2 penetration to cathode – hydrogen brittleness
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Electrolyzis comparison
Oronzio De Nora (Italy)
1800
j [mA/cm2]
Nors Hydro (Norway)
Electrolyzer Corp. (Canada)
1600
Teledyne Energy System (USA)
BBC Ag (Switzerland)
Lurgi (Zdanski-Lonza) (Germany)
1400
ABB (Switzerland)
Proton OnSite (USA)
1200
VSCHT
1000
800
600
400
200
0
1,5
1,75
2
E [V]
2,25
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Future hydrogen economy – longtime
process
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