Introduction to electrochemistry lab

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   !? THINGS THAT WE ARE FAMILIAR WITH !?   
Ohms law ( and Kirchoffs) (ABC...
electrical circuits)   U I ? R
, R ? ?L / S   Faradays law (ABC...
electrolysis)   m k ? Q ,
k M / n ? F ( F ?) Ficks laws
(ABC... diffusion)   J ?
D ? dC/dx ?C/?t D ? ?2C/?x2    Electrical
properties of condensed phases conducting
electrical current (metals , semiconductors)
  Ionic compounds , properties of solutions,
ionic conductivity   Redox reactions ( np. 2Cr3
3 H2O2 10 OH- 2 CrO4 - 8H2O )    Phase
boundary electrolyte - electrode   
2
Me
transport
Me
Me
electrode
electrolyte
Charge transfer
Oxidation reduction reaction
rate Diffusion
transport rate
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Each stage can determine the overall reaction
rate 1. obligatory stages charge
transfer Transport (diffusion, convection ,
migration ) 2. other possible stages Chemical
reaction before or after (c t) Crystallisation
of new phases Adsorption at the electrode
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Reaction rate v ? cA / ?t , ( or v
kA-B cA ) ( cA - volumetric concentration
we must do something about it) In electrode
kinetics the transferred charge is a measure of
reaction rate Following Faradays law mA kF
I ?t k Q (here k electrochem
equivalent, not reaction rate constant) And back
to general reaction rate formula mA cA V or
cA surface S kF I ?t v k
I / S v ( mol s-1 m) kF (mol/C) j
, j current density (A/m2)

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CURRENT DENSITY MEASURE OF ELECTRODE PROCESS
RATE
And what makes the reaction happen at
all?? Equilibrium no products ( is anything
happening?) Deviation from equilibrium -
energy impuls needed Reaction transformation
to new equilibrium state What might be an energy
impuls?
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• energy state of a particle chemical potential
  
• µi µ o RT ( ai)
• Charged particle - electrochemical potential
  , possible responce to electrical field
• f fo RT/nF ln ( ai(n) )
• Equilibrium - equal potentials of a particle in
  two phases (electrode electrolyte)
• E E0 RT/nF ln ( aelectrode / aelectrolyte
  )

Change in concentration, temperature
ENERGY IMPULS
Overpotential applied to the electrode
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At equilibrium Redox transitions on molecular
scale Identical overall charge for oxidation and
reduction jk ja , overall current density
jk - ja 0
At overpotential ?E j jk - ja ? 0
as measure for reaction rate, so j/nF v
krr C
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Reaction rate constant - overpotential krr
ks exp a n F ?E / RT (one equilibrium two
constants anodic and cathodic) Combining v
.. And k . (To get current-overpotential
dependence) i nF S ks cutl exp(-an F ?E / RT
) cred (ßn F ?E / RT) where ks -
standard reaction rate constant a i ß
coefficients for energy barrier symmetry ?E
overpotential
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• Electrode process heterogenous, charge transfer
  at phase boundary transport
• Electrode element of electrical circuit
• Measurement two electrodes form a cell
• Circuit measurable voltage and current
• Difference in V/I response for a.c and d.c.

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Transport properties

• Structure of electrolytes, dissociation
• Movement of ionic species
• Mobility, velocity of part i vi E ui
• Conductivity ? e Ni zi ui
• Transference number

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Cell voltage or electrode potential

• Equilibrium at the electrode Nernst pot.
• Overpotential driving force for the reaction
• Current electrical measure of reaction rate
• Voltage measure of potential difference !
• For kinetics we must know the potential of a
  single electrode!

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3 electrode cell

• 3-electrode cells WE and CE - working circuit
• reactions at electrodes , current flow
• WE- RE - measuring circuit , high input
  impedance
• on RE no current flow
• Reference electrodes very precise potential,
• examples Hg/ Hg2Cl2 , Ag/ AgCl ,
• Quasi-reference W, Ta, other non-reactive
  metals
• idea stable potential, easy assembly,
• Function current-potential diffusion and
  kinetics in the
• cell must be described
  electroanalysis

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Electrolytic cell and potentiostat Electrodes
Meas.
Parameters CE - counter E - WE
potential RE - reference Ez -
applied potential WE - working I - current in
CE-WE circuit
WE
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EIS

• Electrotechnical aspect a.c.circuit
• Electrochemical aspect approximation of
  electrode process with circuit elements

Charge transfer Conductivity Resistance of layer
Resistances R Z R
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Double layer capacity Capacity of layers
Capacities C Z -j/?C
Constant phase element Admittance Y Yo
(j?)n for n0 resistance For n1 capacity
Diffusion phenomena Roughness of
surface Inhomogenity of layer
Corrosion processes (many reactions and
equilibria)
Inductance L Z j ?L
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Equivalent circuits

• Electrical model of electrode
• Connections in series and parallel
  interpretation of consecutive or simultaneous
  reactions / phenomena
• Physical sense vs. numerical possibilities

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Our lab sessions
EIS dr Regina Borkowska ( 5h basic electrode
kinetics) Voltammetry dr Regina Borkowska
5h Conducting polymers dr M. Siekierski
5h Batteries dr Marek Marcinek (5h basic cells
5h Li- cells) Transference numbers Msc Michal
Piszcz(5h diffusion coefficient 5h
transference numbers in Li systems) Ion
associations Dr Leszek Niedzicki (5h
Fuoss-Kraus formalism electrochemical
approach) Corrosion dr Andrzej Królikowski
5h Instructions and auxillary materials
download from http//pirg.ch.pw.edu.pl/