Introduction to electrochemistry - Basics of all techniques -

1
Introduction to electrochemistry- Basics of all
techniques -
2
Electrochemistry is the study of phenomena at
electrode-solution interfaces
3
Two quite different aspects of the field of
electrochemistry
4
An introduction to redox equilibria and electrode
potentials
The more negative the value, the stronger
reducing agent the metal is. The more positive
the value, the stronger oxidising agent the metal
ion is.
5
Redox Potentials for non-metal and other systems

• Chlorine gas is the strongest oxidising agent (E
  1.36 V).
• A solution containing dichromate(VI) ions in
  acid is almost as strong an oxidising agent (E
  1.33 V).
• Iron(III) ions are the weakest of the three new
  ones (E 0.77 V).
• None of these three are as strong an oxidising
  agent as Au3 ions (E 1.50 V).

6
Looking at this from an equilibrium point of view
Suppose you have a piece of magnesium in a beaker
of water. There will be some tendency for the
magnesium atoms to shed electrons and go into
solution as magnesium ions. The electrons will be
left behind on the magnesium.
7
A dynamic equilibrium will be established when
the rate at which ions are leaving the surface is
exactly equal to the rate at which they are
joining it again.
8
At that point there will be a constant negative
charge on the magnesium, and a constant number of
magnesium ions present in the solution around it.
9
Copper is less reactive and so forms its ions
less readily. Any ions which do break away are
more likely to reclaim their electrons and stick
back on to the metal again. You will still reach
an equilibrium position, but there will be less
charge on the metal, and fewer ions in solution.
10
standard hydrogen electrode
As the hydrogen gas flows over the porous
platinum, an equilibrium is set up between
hydrogen molecules and hydrogen ions in solution.
The reaction is catalysed by the platinum.
11
The standard hydrogen electrode is attached to
the electrode system you are investigating - for
example, a piece of magnesium in a solution
containing magnesium ions.
12
Magnesium has a much greater tendency to form its
ions than hydrogen does. The position of the
magnesium equilibrium will be well to the left of
that of the hydrogen equilibrium. That means that
there will be a much greater build-up of
electrons on the piece of magnesium than on the
platinum.
13
What if you replace the magnesium half cell by a
copper one?
14
standard electrode potentials

• The standard electrode potential of a metal /
  metal ion combination is the electro-motive force
  (emf) measured when that metal / metal ion
  electrode is coupled to a hydrogen electrode
  under standard conditions.

15
In the copper case
16
The two equilibria which are set up in the half
cells are
17
Obviously, the voltmeter will show that the zinc
is the negative electrode, and copper is the
(relatively) positive one.
18
(No Transcript)
19
(a) Galvanic and (b) electrolytic cells
20
Introduction
Scope of electrochemistry
Introduction

• Investigation of chemical phenomena associated
  with a charge transfer reaction
• To assure electroneutrality two (or more)
  half-reactions take place in opposite directions
  (oxidation/reduction)
• If the sum of free energy changes at both
  electrodes is negative electrical energy is
  released ? battery
• If it is positive, external electrical energy has
  to be supplied to oblige electrode reactions ?
  electrolysis

21
Reactions and electrodes

• The overall chemical reaction taking place in a
  cell is made up of two independent
  half-reactions, which describe the real chemical
  changes at the two electrodes.
• Most of the time one is interested in only one of
  these reactions, and the electrode at which it
  occurs is called the working (or indicator)
  electrode, coupled with an electrode that
  approaches an ideal nonpolarizable electrode of
  known potential, called the reference electrode.
  In experiments, the current is passed between the
  working electrode and an auxiliary(or counter)
  electrode.
• Three electrodes are frequently placed in three
  compartments separated by a sintered-glass disk.

22
Reference electrode

• A reference electrode is used in measuring the
  working electrode potential of an electrochemical
  cell.
• The reference electrode acts as a reference point
  for the redox couple. 
• A Luggin capillary is often used to position the
  sensing point of a reference electrode to a
  desired point in a cell.

23
Reference electrode

• ? The potential of the working electrode is
  monitored relative to a separate reference
  electrode, positioned with its tip near the
  working electrode.
• ? The internationally accepted primary reference
  is the standard hydrogen electrode (SHE) or
  normal hydrogen electrode (NHE), which is
• Pt/H2(a1)/H(a1,aqueous)
• ?By far the most common reference is the
  saturated calomel electrode (SCE) and the
  Silver/Silver Chloride (Ag/AgCl) electrodes. SCE
  is
• Hg/Hg2Cl2/KCl (satd in water). Its
  potential is 0.242 V vs. NHE.

24
The device minimizes any iR drop in the
electrolyte associated with the passage of
current in an electrochemical cell.
25
working electrode

• A fixed potential difference is applied
  between the working electrode and the reference
  electrode. This potential drives the
  electrochemical reaction at the working
  electrode's surface.  The current produced from
  the electrochemical reaction at the working
  electrode is balanced by a current flowing in the
  opposite direction at the counter electrode.

26
Materials of working electrode

• A wide variety of working electrodes are now
  available. Originally the carbon paste electrode
  was developed but this was soon replaced by more
  "convenient" and stable carbon-based working
  electrodes including those made from glassy
  carbon, pyrolytic carbon and porous graphite.  
  Metals such as platinum, gold, silver, nickel,
  mercury, gold-amalgam and a variety of alloys are
  now also commonly used as working electrode
  materials.

27
Choice of working electrode

• Carbon paste electrodes cannot be used with
  mobile phases containing high amounts of organic
  modifier because the electrode will dissolve
  unless a polymeric binder is used.
• The optimal working electrode choice is dependent
  upon many factors, including the usable applied
  potential range, involvement of the electrode in
  the redox reaction, and kinetics of the electron
  transfer reaction. 

28
Three-electrode cell and notation for the
different electrodes
29
Potential window

• A working electrode will only function within a
  defined potential window. For example,
  electrolysis of many compounds will readily occur
  on a glassy carbon working electrode up to
  approximately 1300mV vs. a silver/silver chloride
  reference electrode.
• The applied potential to the working electrode is
  dependent upon both the working electrode
  material and the pH of the mobile phase.

30
The potential window of various working
electrodes under acidic and basic conditions.
31
Kinetics of the Electron Transfer Reaction

• Electron-transfer reactions can be either
  kinetically fast or slow. For a fast reaction
  most of an analyte will react at the working
  electrode's surface. For slow reactions not all
  of the analyte reaching the working electrode's
  surface will have time to react.  To drive the
  electrolytic reaction at a faster rate a much
  higher potential or "overpotential" must be
  used.  It is usually found that organic species
  react more favorably on one particular working
  electrode material than another.

32
Factors affecting electrode reaction rate

• In general, the electrode reaction rate is
  governed by rates of processes such as
• Mass transfer (e.g., from the bulk solution to
  the electrode surface).
• (2) Electron transfer at the electrode surface.
• (3)Chemical reactions preceding or following the
  electron transfer.
• (4)Other surface reactions.
• ? The magnitude of this current is often limited
  by the inherent sluggishness of one or more
  reactions called rate-determining steps.

33
Conditions for electrochemical experiments

• Reproducible experimental conditions must be
  given
• Interfering side effects must be avoided as
• Migration effects
• High solution resistance
• -these effects can be minimised by adding an
  inert supporting electrolyte (around 1 mol/L)
• Undefined or large diffusion layer
• A complete study of the electrode process
  requires the measurement of kinetic as well as
  thermodynamic parameters.

34
Faradaic and nonfaradaic processes

• Charges (e.g., electrons) are transferred across
  the electrode-solution interface and causes
  oxidation or reduction to occur. Since these
  reactions are governed by Faradays law, they are
  called faradaic processes.
• Under some conditions, processes such as
  adsorption and desorption can occur, and the
  structure of the electrode-solution interface can
  change with changing potential or solution
  composition, these processes are called
  nonfaradaic processes.

35
Capacitance and charge of an electrode

• The behavior of the electrode-solution interface
  is analogous to that of a capacitor. When a
  potential is applied across a capacitor, charge
  will accumulate on its electrode plates.
• At a given potential there will exist a charge on
  the metal electrode, qM, and a charge in the
  solution, qs. At all times, qM-qs.
• At a given potential the electrode-solution
  interface is characterized by a double-layer
  capacitance, Cd, typically in the range of 10 to
  40µF/cm2.

36
(No Transcript)
37
The nature of electrode reactions

• Electrode reactions are heterogeneous and take
  place in the interfacial region between electrode
  and solution ? diffusion layer
• The charge separation at each electrode is
  represented by a capacitance
• the difficulty of charge transfer by a resistance
• The electrode can act as (1) a source of
  electrons (cathode) ? reduction ,(2) a sink of
  electrons transferred from species in solution
  (anode) ? oxidation
• The amount of electrons transferred is related to
  the current flowing between the two electrodes

38
Thermodynamics and kinetics

• Thermodynamics and kinetics
• The potential at which a reduction or oxidation
  takes place (measured relative to the normal
  hydrogen electrode) is given by the Nernst
  equation
• ?i stoichiometric numbers positive for reduced
  species, negative for oxidised species
• E0 standard electrode potential
• ci concentration (ai has to be applied if
  activity coefficient is not 1)

E E0 (RT/nF) ??i ln ci
39
Thermodynamics and kinetics

• ?The concentration of species at the electrode
  interface depends on its mass transport
  coefficient kd
• ?The rate of the electrode reaction is expressed
  by the standard rate constant kf0 which is the
  rate when E E0
• reversible reaction ? kf0 gtgt kd
• irreversible reversible reaction ? kf0 ltlt kd,
• an overpotential ? has to be applied
  additionally to overcome this kinetic barrier
• ?A behaviour in between these extremes is called
  quasireversible reaction

40
Reference

• A.J. Bard, L.R. Faulkner, Electrochemical
  MethodsFundamentals and Applications. New York
  John Wiley Sons,1980.