Introduction%20to%20electrochemical%20techniques

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Introduction to electrochemical techniques
Valentin Mirceski Institute of Chemistry Faculty
of Natural Sciences and Mathematics Ss Cyril and
Methodius University, Skopje Republic of
Macedonia
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Electrochemistry basic terms

• Electrochemistry is interdisciplinary science
  dealing with the interrelation between the
  chemical and electrical phenomena.
• Chemical (redox) transformations caused by a flow
  of electric current
• Gaining electrical current due to spontaneous
  chemical transformation
• Understanding electrochemistry means
• Understanding electrode processes
• Understanding electrical properties of interfaces
• The main phenomena Charge transfer across an
  interfaces formed, most frequently, between an
  electric conductor of a
• first kind (an electrode) (electron
  conductivity) and
• second kind (electrolyte solution, i.e., a
  solution of ions) (ion conductivity)

3
Electrochemical cells
Electrolysis cell non-spontaneous redox
(electrode) reactions are driven by the power of
an external electric supply!
Galvanic cell Spontaneous redox (electrode)
reactions give raise to a current flow.
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Electrochemical cells and electrochemical
reactions

• The simples electrochemical experiment involves
  charge transfer across at least two interfaces
• Electric potential
• difference between the electric potential of the
  two electrodes (the main driving force as a
  measure for the energy available to drive
  electric charges through the electrochemical cell)

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Electric potential and current

• Electrical potential (E (V volt)) is a measure
  for the potential energy of a charge in an
  electric field
• The difference in the potential (potential
  energy) causes a charged species to move in the
  electric field (charge transfer)
• Potential of 1 V (volt) is equivalent to the
  potential energy of 1J of a charged species with
  a charge of 1 C (coulomb)
• Charge transfer in time is called electric
  current (I (A ampere)

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Electrode reactions, half reactions

• Electrode/electrolyte interface is characterized
  with a large potential difference , thus a strong
  electric filed exists at the interface!
• Electrode reactions
• Half-reactions
• The overall reaction
• Working electrode
• Reference electrode

electrodesolution Interface
O ne- ? R (electrode reaction)
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The main reference electrode Standard (normal)
hydrogen electrode (SHE) (NHE)

• Reference callomel electrode
• Hg/Hg2Cl2/KCl (saturated in water) 0.242 V vs
  SHE
• Reference silver-silver chloride electrode
• Ag/AgCl/KCl (saturated in water) 0.197 V vs SHE
• In the course of the electrochemical experiment
  the chemical composition, hence the electric
  potential, of the reference electrode remains
  constant!

Standard hydrogen electrode
Controlling the potential difference between the
working and reference electrode, one controls
actually the potential of the working electrode
only!
8
Current sign convention, standard redox
potential, Faraday law

• Reduction
• Reduction current ( )
• Oxidation
• Oxidation current ( )
• Standard redox potential E?, which is related to
  the standard Gibbs energy
• Synonyms standard electrode potential standard
  reduction potential.
• O ne- R
• n number of electrons
• F Faraday constant (96 485.3 C mol-1). Thus,
  the physical meaning of the Faraday constant is
  that one mole of a single charged species has a
  charge of 96 485.3 C e.g., one mole of electrons
  has a charge of - 96 485.3 C.
• Faraday law A charge of 96485.3 C corresponds to
  the transformation of 1 mol reactant and 1 mol
  product in a one-electron electrohemical reaction
  O e- R

DG ? -nFE ?
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I E curve polarisation curve

• (working electrode)
• 2H 2e- H2
• (reference electrode)
• Ag Br- AgBr e-
• 2Br- Br2 2e-
• AgBr e- Ag Br-
• The overall current flow must be equal at both
  electrodes, and it is dictated by the working
  electrode, which has much smaller electrode
  surface area.
• Limiting potentials dictated by electrochemical
  reactions of the supporting electrolyte at
  particular electrode.
• Electrode reactions are heterogeneous in their
  nature.
• The rate depends on the electric field, i.e., on
  the electrode potential.

10
Overpotential additional energy required than
thermodynamically predicted due to the slow
electrode kinetics
A large overpotential for hydrogen reduction at
Hg electrode
Hg
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Electroactive species in a supporting electrolyte
Polarization curve in the presence of traces of
electroactive species (Cd2)
12
Possible electrode reactions at different
electrodes
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Faradaic and nonfaradaic processes

• Faradaic processes charge transfer due to redox
  reactions (electrode reaction)-current flow
• Nonfaradaic processes no charge transfer across
  the interface adsorption, desorption, changes in
  the structure of the layer of the solution
  adjacent to the electrode formation of an
  electic double layer etc.
• Important although there is no charge transfer,
  the nonfaradaic processes cause the current to
  flow in the electrochemical cell!
• Ideally polarizable electrode no charge
  transfer, e.g. Hg in 1 M KCl in acetonitrile over
  2 V (from 0.25 to -2.1 V vs SHE.

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Electrode/electrolyte interface an electric
capacitor
15
Structure of the electrical doubly layer
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Potential profile across the double-layer

• Y dE/dx
• The intensity of the electric field is very high
  due to the potential variation over very small
  (nanometer range) distance!

17
Rate of an electrode reaction the flux
electrodesolution Interface
Flux (the rate of the heterogeneous electrode
reaction) is equal to the amount of reacted
material per unit of time per unit of electrode
surface area (mol s -1 cm-2). This chemical
rate is equal to the ratio of the electric
current, number of exchanged electrons in a unit
reaction and electrode surface area.
O ne- ? R (electrode reaction)
Electric current (I) measured at the electrode is
proportional to the rate (v) of the electrode
reaction! (q charge, t time, F Faraday
constant A electrode surface area n number
of electrons, n(O) number of moles of the
reactant O)
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Factors affecting the rate of electrode processes

• Modes of mass transfer
• Migration
• Diffusion
• Convection

• Nernst-Plank equation

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Electrode reaction controlled by the mass
transport
If the mass transfer is the slowest step of the
electrode reaction, then the electrode reaction
is termed as being electrochemically
reversible. At each potential difference (DE) of
the interface, the electrode reaction is in redox
equilibrium, which is described by the Nernst
equation
The Nernst eq. reveals that variation of the
potential difference at the interface (DE) causes
variation of the equilibrium concentrations of
the redox species (O and R). In other words,
it shifts the position of the redox equilibrium,
which is manifested as a flow of electric current
in the system.
O
O ne ? R (electrode reaction)
Electrode
DE felecc. fsol. (Potential difference (DE)
across the interface is externally controlled by
controlling the inner potential (f) of the
electrode. In simple words, one controls the
activity, i.e., concentration of electrons
participating in the electrode reaction, thus
affecting both the position of the redox
equilibrium O/R and the kinetics of the redox
transformation. Note, frequently, the potential
difference DE is designated simply as electrode
potential with a symbol E)
R
electrolyte solution
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Semi-empirical treatment of a voltammetric
experiment when the diffusion layer has a
constant thickness a steady-state mass transfer
In this experiment, the flux at the electrode
(i.e., the rate of the electrode reaction, thus
the current), depends on the diffusion rate only
(i.e., depends on the mass transfer only).
According to the First Fick law, the rate of
diffusion depends on the diffusion coefficient
(D) and the concentration gradient (dc/dx) (D
diffusion coefficient (it is the rate constant of
the diffusion (cm 2 s-1)). In addition, it is
assumed that the diffusion layer has a constant
thickness d. The flux of R species must be equal,
but opposite in sign, with the flux of O species.
R diffuses toward the electrode, while O, formed
by oxidation of R, diffuses away from the
electrode (in the opposite direction)
?
R O ne-
electrode
x
The maximal flux of R will be if cR(x 0) 0.
Thus, the corresponding current is termed
limiting current, Il
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Il (limiting current)
Typical I-E curve (voltammogram) for an
electriochemical experiment with a constant
thickness of the diffusion layer (steady state
voltammetry)
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Kinetics of a simple homogeneous chemical reaction
The rate of a common chemical reaction depends on
the concentrations of participants, and (through
the rate constant) on the temperature and
activation energy.
Symbols and abbreviations f forward b
backward net overall reaction K equilibrium
constant X - equilibrium concentration of a
species X
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Electrode kinetics
c cathodic (reductive) a anodic (oxidative) a
electron transfer coefficient (dimensionless
number between 0 and 1 most frequently the value
is 0.5) k? (cm s-1)- standard rate constant (rate
constant when the electrode potential is equal to
the standard potential of the redox couple, E?)
Rate constants depend on the potential! The
unique feature of electrochemical rate constants.
Thus, the rate of the electrode reaction can be
controlled by the potential!
Butler-Volmer equation
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Dependence of the current on the electrode
potential
Limiting current
The current increases exponentially with the
potential as predicted by the dependence of the
rate constants on the potential!
diffusion
O (at the electrode surface)
O (in the solution)
Electrode
v I/nFA
limiting current
R
Although the rate of the electrode reaction could
be very fast due to the large overpotential, the
overall rate will by limited by the supply of the
electrode surface with the electroactive material
by the mass transport, i.e. diffusion!
25
Electrochemical techniques chronoamperometry
The dependence of the potential and current on
time in the course of the chronoamperometric
experiment. The experiment is conducted at a
given fixed potential (E2), which is sufficiently
height (E2 gtgt E?) to cause complete
electrochemical (redox) transformation of the
electroactive species at the electrode surface.
As a consequence, the current is flowing in the
cell, and it is being measured as a function of
time.
R ? O ne
I
t
0
26
Description of the mathematical model referring
to a simple chronoamperometric experiment -
Cottrell equation
R ? O ne
Cottrell experiment Chronoamperometric
experiment in a homogenous solution containing
only R species, at a potential E gtgt E ?, thus
enabling complete transformation of all R species
at the electrode surface. Mass transfer is
occurring only by diffusion without any specific
adsorption phenomena on the electrode surface.
Concentration profiles. Variation of the
concentration of electroactive species with the
distance x measured from the electrode surface at
different times of the chronoamperometric
experiment. As shown above, the thickness of the
diffusion layer increases with time.
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Chronoamperometry with a double potential step

• For mechanistic purposes, i.e. to reveal the
  mechanism of the electrode reaction, the
  chronoamperometric experiment can be conducted
  with a double potential step, as shown in the
  figure below. At the potential E2, the initially
  present R species undergo electrochemical
  oxidation at the electrode surface to produce
  species O, resulting in the first branch of the
  current, presented in the right plot (i.e..
  chronoamperogram). In the second potential step,
  the potential is changed to a value E3, at which
  the reduction of previously formed species O is
  taking place, producing the second branch of the
  chronoamperogram presented on the right panel.

E2
E
I
t
t
0
E3
E1
t
0
t
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Chronocoulometry

• Chronocoulometry is equivalent method to
  chronoamperometry, the difference being in
  measuring the charge consumed in the course of
  the electrode reaction instead of the current. We
  recall, the definition of the current
• Hence, the charge is simply calculated as an
  integral of the current-time function, i.e.
• In the course of the experiment, contrary to the
  chronoamerometry, the response of the
  crhronocoulometry increases with time, as the
  amount of the material transformed at the
  electrode increases with time. By integration of
  the current, the noise effect is usually smoothed
  out and it is not so significant as in
  chronoametrometry. The contribution of the double
  layer as well as from electrode reaction of
  immobilized species can be easily separated from
  the contribution of diffusing species. Thus,
  chronocoulometry is especially valuable for
  studying surface processes, thus it is of
  particular importance in studying conducting
  polymers.

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• For a Cottrell experiment described on page 27,
  the chronocoulometric response is defined as
• The charge consumed during the experiment of
  species that diffuses toward the electrode is
  proportional to the square-root of time and the
  plot vs. t1/2 is linear with a slop from which
  some of the constants of the equation above can
  be obtained, given the knowledge of others.

• The eq. above shows that at t 0, the charge is
  0. However, in a real experiment the line Q vs
  t1/2 does not cross through the origin, as shown
  in the plot. This is due to the charge consumed
  by the double layer formation and by electrode
  transformation of species immobilized on the
  electrode surface. Thus the total charge can be
  separated in three terms
• The first term is due to electrode reaction
  controlled by the diffusion of the species,
  homogeneously distributed in the solution, the
  second term Qdl is due to formation of the double
  layer and the third is due to electrode
  transformation of adsorbed species.