Chapter 3 ppt

Chapter 3: Interfacial Elctrochemistry
AMU, CNS, DEP’T OF CHEMISTRY May, 2018

Introduction
 An interface is a surface forming a common boundary
among two different phases of matter, such as:
an insoluble solid and a liquid,
two immiscible liquids,
a liquid and an insoluble gas or
a liquid and vacuum
 The importance of the interface depends on the type of
system.
 The bigger the quotient area/volume, the more effect the
surface phenomena will have.
 Interfaces can be flat or curved. For example oil droplets
in a salad dressing are spherical but the interface between1

Cont’d…
Surfaces and interfaces define a boundary between a
material and its surrounding environment and
influence interactions with that environment.
As a consequence, these surface atoms with changed
atomic and electronic structures exhibit high chemical
reactivity.
This property makes surfaces and interfaces a
favored medium for chemical and biological
processes in nature and in technological applications.
Most of the events in electrochemistry take place at
an interface and that is why the interfacial
electrochemistry constitutes the major part of
electrochemical science. 2

Cont’d…
 Relevant interface here are:
Metal - liquid interface
Metal – solid electrolyte
Semiconductor electrolyte interface and
The interface between two immiscible
electrolyte solutions
 These interfaces are chargeable, that is when the
external potential is applied charge separation of
positive and negative charges on the two sides of the
contact occurs.
 Such an interface can accumulate energy and be
characterized by electric capacitance within the
3

Cont’d…
 Properties of the interface play a crucial role in
electrochemical energy conversion, electrolysis,
electro catalysis and electrochemical devices.
 Generally, Interfacial Electrochemistry is the
phenomena that take place at the electrified
interface (metal| electrolyte, electronic| ionic
conductor) to study:
Potential differences across interface
charge densities
dipole moments
electrical currents of interface
conversion of energy
conversion of chemical species and
transformation of energy 4

Potential differences across
interfaces
 Distribution of potential near charged interface is crucial
in electrode kinetics, since the potential drops plays a role
of a variable driving force of reaction.
 The interface between two dissimilar interfaces is
electrified. Almost all surfaces carry an excess electric
charge.
 Hence when two dissimilar phases come into contact,
charge separation occurs in the interfacial region which
results in the generation of an interfacial potential
difference or electric field.
5

Cont’d…
Consider a metal electrode in contact with an aqueous
solution containing salt (e.g KCl (aq))
 The solution contains solvated charged ions and
solvent dipoles.
 Forces experienced by ions and solvent molecules in
bulk of solution are isotropic:
 There is no net alignment of solvent dipoles, and
positive and negative ions are equally distributed
throughout any volume element of the solution.
 Electro neutrality operates in bulk solution region
very far from electrode surface.
 In this region forces experienced by ions and solvent
dipoles are no longer isotropic and homogeneous. 6

Cont’d…
 New solvent structure, different from that of the
bulk, develops because of the phase boundary.
 Electro-neutrality breaks down on the solution side
of the interface.
 There will be a net orientation of solvent dipoles and
a net excess charge in any volume element of the
solution adjacent to the electrode surface.
 The solution side of the interface becomes
electrified.
 Once the solution side of the interface becomes
electrified (acquires a net or excess charge), an
electric field will operate across the phase boundary.
 Since the metallic phase contains charged particles,
the latter will respond to this E field. 7

Cont’d…
 The free electrons will move away from or move
towards the interface depending on the direction of
the E field.
 Thus a net charge will be induced on the metal,
which will be equal in magnitude and opposite in
sign to that on the solution side of the phase
boundary.
 Thus charge separation occurs across the M/S
interface, and this gives rise to an interfacial
potential difference.
 Typically the potential difference is 1.0 V. However
the spatial dimensions of the interface region are
very small, typically 1 nm thick. Thus the electric8

Cont’d…
 A more complicated example is provided by the
contact of two electronic conductors, metals or
semiconductors.
 At equilibrium the electrochemical potential of
electrons must be equal in the bulk phases.
 The potential difference between them is determined
by the bulk properties of the media in contact, i.e.,
by the difference between the chemical potential of
electrons.
 Where e is elementary charge
 Generally, Electrode processes (reactions) take place
at the surface of the electrode, and produce a slight9
1 21
2
m mm
m e e
e
 


 

Cont’d…
 The result is an interfacial potential difference which
can materially affect the rate and direction of the
reaction.
 Much of the importance of electrochemistry lies in
the ways that these potential differences can be
related to the thermodynamics and kinetics of
electrode reactions.
 In particular, manipulation of the interfacial
potential difference affords an important way of
exerting external control on an electrode reaction.
10

Electrical Double layer
 When a metal M is immersed in a suitable electrolyte,
initially its atoms oxidize at a relatively high rate and
subsequently the oxidation (dissolution) process gradually
ceases and eventually stops due to a negative charge
build-up on the metal surface.
 Thus, the dynamic equilibrium is attained, provided that
atoms are removed from their lattice sites to ionize as
cations (M+) into the electrolyte forming a negatively
charge metal surface.
 The polar (H2O) water molecules and the hydrogen
cations (H+) are in electrolyte, they are attracted by the
negatively charged metal surface to form an ionic
structure (layer). 11

Cont’d…
 This ionic structure is known as electrical double
layer (EDL).
 This layer is referred to as the Helmholtz layer which
behaves like charged capacitor. or
 The region in which the influence of the charge is
appreciable is known as Electrical Double Layer
(EDL).
12

Cont’d…
 This model indicates that some negatively charged
ions adsorbed on the metal electrode surface and
polar water covers the rest of this surface, forming
protective layer.
 The positively charged hydrogen is in contact with
negatively charged metal surface.
 Thus one can conclude that:
1. The inner Helmholtz plane (IHP): is an ionic
layer that consists of adsorbed dipole H2O
molecules. The majority of the anions do not
penetrate this layer. The inner potential on the
boundary of this ionic plane is
2. The outer Helmholtz plane (OHP) : consists of
a plane of adsorbed ions due to electrostatic forces in13
1
2

Cont’d…
3. The diffuse layer (DL) : is a thick layer located in
a region of diffusely ions in contact with the OHP
and the bulk of solution at potential range
 The electric double layer consists of two regions: an
inner region including adsorbed ions and a diffuse
region in which ions are distributed according to the
influence of electric forces and random thermal
motion.
 The electric potential ψ0 at a flat surface and ψ at
distance x from the surface in the electrolyte
solution. The potential decreases exponentially with
distance from the charged surface:
14
1 3diffuse   
0 exp( )kx  

Cont’d…
 The interfacial potential within the electrolyte is
defined as:
Or
Where K = rate constant = C(x)/Co
C(x) = concentration (activity) at x>0
Co = concentration at x = 0
 The metal surface has lattice of +ve and electron of
–ve charge
 The following models used to explain their behavior
a) Helmholtz – Perrin model
b) Gouy – Chapman diffuse charge model 15
( ) ln[ ]
RT
x K
zF
  
( )
( ) ln[ ]
o
RT C x
x
zF C
  

Cont’d…
A) Helmholtz – Perrin Model: according to
Helmholtz - Perrin model:-
i) Charge on the metal draws out by randomly
distributed ions in solution which also make a
counter layer of opposite charge
ii) The interface has two sheets of charge one on
electrode and other in solution.
iii) A double layer formed which behave like all plate
condenser
iv) Charge magnitude both sheets is equal but opposite
in sign
v) The potential drop between the two layers of charge
is linear. 16

Cont’d…
 Now according to electrostatic theory of capacitor
for the double layer, the potential difference across a
condenser of unit area is give by:
or
17
( ) m
o
d
E
  ( ) m
o
d
E
   

Cont’d…
Where d = is the distance between the plates
E = the dielectric constant of the medium
between the plates
εo = the permittivity of free space (8.85×10–12
F/m)
= charge density
 But the capacitance is:
at constant composition
 This model is satisfactory if E and d remain
constant. The experimental results reveal that
electro capillary curves are asymmetric and not18
m
( )
( )
m oE
C
d
 


 


Cont’d…
B. Gouy – Chapman Diffuse Model
i) Contrary to Helmholtz – Perrin model ions liberate
from sheath to electrode
ii) Ions undergo thermal agitation with solution
particles
iii) Both electric forces of electrode along with thermal
forces influence the behavior of ions in electrode
vicinity.
iv) The excess charge density on OHP is not equivalent
to that on the metal but less
v) Excess charge density in solution decreases with
distance from the electrode like ionic atmosphere
and become zero where thermal forces equalize the
electrode force. 19

Cont’d…
vii) This model has field in planar symmetry as
electrode behave like an infinite plane.
 This theory of diffuse double layer is analogous to
that of long range ion –ion interaction, so the
electric field or potential gradient at distance x from
the electrode is given as:
20
1
2
8
( ) sinh
2
o x
o
kTC zed
dx E kT


 

Cont’d…
Where Co = concentration of species in the bulk of
solution
= outer potential difference b/n point x from
electrode and the bulk of solution.
 According to Gauss’s law the charge is given by:
 To find the total diffused charge density Gaussian
has been extended from x = 0 (near electrode) to x
= (in bulk solution)
 After integration and rearranging equations, the
differential capacitance
x
o
d
q E
dx



2 2
0
.
2
( ) [ ] [cosh( ]
2
m d o m
cons comp
m
q E z e C ze
C
kT kT
  
 
 
    
 
21

Cont’d…
At 25 oC, 1/k = 30.4 Ao (1A0 = 10-10 m)
 This function gives inverse parabola, so C at
interface is not constant and is function of potential
across interface, which is supported experimentally.
Weakness: the reason for the failure of this model is
constancy of E (dielectric constant) between
electrode and bulk of solution.
22

Cont’d…
C) Stern Model
i) This is the combination of Helmholtz – Perrin and
Gouy – Chapman model.
ii) Point charge approximation is ruled out and divides
solution in to two parts.
where qS = the solution charge
qH = Helmholtz – Perrin charge
qG = Gouy – Chapman charge
23
S H Gq q q 

Cont’d…
iii) Potential drop occur due to separation of charge
iv) It include two potential drop i.e.,
v) Potential drop curve with distance satisfy the
behavior of both models
Linear variation from x = 0 to position of OHP (x
= 0) region
Exponential variation from OHP (x = 0) to bulk
region ( x= )
 The separation of charges and potential regions
produce a separation of differential capacities.
( ) ( )m bulk m H H bulk         
24

Cont’d…
 On differentiating potential difference across
interface wrt charge on the metal we have:
 But
 Therefore
where C = total charge capacity, CH = Helmholtz –
Perrin capacity and CG = Gouy – Chapman capacity
25
( ) ( ) ( )m bulk m H H bulk
m m m
     
  
  
    
  
m
m
C





1 1 1
H GC C C
 

Cont’d…
26
Special Case:
a) When a concentration of ions is very large CG
increase with Co increase and CH remain constant.
1 1
G HC C
 for all practical
purposes
HC C

Cont’d…
 This suggests that most of the solution charge
confined in region close to Helmholtz - Perrin plane
and very small scatter diffusely in solution.
b. In dilute solution or when concentration of ions is in
solution is very low:
 Then Thus
 It implies that the solution charge scattered under
the combined effect of electrical and thermal forces
and electrolyte – electrode interface.
 Weakness: This model fails when ion like Cl-, Br-, I-
adsorbs on the electrode.
27
1 1
G HC C
 GC C

Thermodynamics of Electrified
Interfaces
 There are two thermodynamic approaches for treating
interfaces. The oldest was put forward by Willard Gibbs,
and the more recent by Edward Guggenheim.
 Consider two phases α and β separated by an interphase σ
. Even though the boundaries α| σ and σ| β are arbitrary,
the total volume V remains the sum of the volumes of the
three phases
28
V V V V  
  

Cont’d…
 In the same way, for all the extensive variables X of
the system (ni, U, S, etc.), we will have:
 At equilibrium, all the intensive variables (T, p, µi,
etc.) must be equal in the three phases.
Consequently, we can write the variations in internal
energy of each phase as:
29
X X X X  
  
i i
i
dU TdS PdV dn   
   
i i
i
dU TdS PdV dA dn   
     
i i
i
dU TdS PdV dn   
   

Cont’d…
 The internal energy being a homogeneous first
order state function, we can apply Euler’s theorem
of integration with constant intensive variables and
write:
 Which in this instance reads:
30
( )k j j
j j
U
U X
X





i i
i
U TS PV n   
   
i i
i
U TS PV A n   
     
i i
i
U TS PV n   
   
By differentiating these equations and subtracting
respectively

Cont’d…
 The equations defined for the phases α and β are the
Gibbs - Duhem equations for these phases, which
show that we cannot independently vary the
temperature, the pressure and the composition of a
phase.
 The Gibbs - Duhem equation for the interphase is
called the Gibbs adsorption equation.
31
0i i
i
S dT V dP n d  
  
0i i
i
S dT V dP Ad n d  
    
0i i
i
S dT V dP n d  
  
i i
i
Ad S dT V dP n d  
     

Cont’d…
This equation provides a thermodynamic definition of:
 But this is not a practical definition since Uσ
depends on the arbitrary boundaries.
 There are various types of electrified interfaces
between conducting phases that can be metallic,
semi-conductors, ionic crystals (e.g. AgI), molten
salts, electrolyte solutions etc.
32
, ,( ) iS V n
U
A






Electrochemical Kinetics
 As we have noted in the first section it is possible to
transfer electrons between an electrode and a chemical
species in solution.
 This process is called electrolysis and results in a reactant
undergoing an oxidation or reduction reaction.
 Unlike equilibrium measurements recorded using a two
electrode, two compartment cell and electrolysis results in
the flow of current around an electrical circuit.
 The current can be controlled by a numbers of factors: the
two most common are:
The rate of electron transfer between the metal and
species in solution
The transport of material to and from the electrode
interface. 33

Cont’d…
Kinetics of Electron transfer
 The current flowing in either the reductive or
oxidative steps can be predicted using the following
expressions model for the influence of electrode on
the rate of electron transfer.
 For simplicity we will consider a single electron
transfer reaction between two species (O) and (R).
 The current flowing in either the reductive or
oxidative steps can be predicted using the following
expressions.
34
( ) ( ) ( )
red
ox
k
k
O s e m R s  
[ ]c red oi FAk O 
[ ]a ox oi FAk R

Cont’d…
 For the reduction reaction the current (ic) is related
to:
 the electrode area (A),
 the surface concentration of the reactant [O]o,
the rate constant for the electron transfer (kRed
or kOx) and
 Faraday's constant (F).
 A similar expression is valid for the oxidation, row
the current is labeled (ia), with the surface
concentration that of the species R.
 Note that by definition the reductive current is
negative and the oxidative positive, the difference in
sign simply tells us that current flow in opposite35

Cont’d…
 To establish how the rate constants kOx and kRed are
influenced by the applied voltage we will use
transition state theory from chemical kinetics.
 You will recall that in this theory the reaction is
considered to proceed via an energy barrier. The
summit of this barrier is referred to as the
transition state.
36

Cont’d…
 The rate of reaction for a chemical process
 is predicted by an equation of the form
 Where the term in the exponential is the free energy
change in taking the reactant from its initial value to
the transition state divided by the temperature and
gas constant.
 This free energy plot is also qualitatively valid for
electrode reactions.
37
k
A B
exp( )
G
k Z
RT


( ) ( ) ( )
red
ox
k
k
O s e m R s  

Cont’d…
 Where the free energy plot below corresponds to the
thermodynamic response at a single fixed voltage.
 Using this picture the activation free energy for the
reduction and oxidation reactions are:
and
38
o
redG G G 
   R
oxG G G 
  

Cont’d…
 and so the corresponding reaction rates are given
by:
and
 if we now plot a series of these free energy profiles
as a function of voltage it is apparent that the plots
alter as a function of the voltage
39
exp( )red
red
G
k Z
RT


 exp( )ox
ox
G
k Z
RT




Cont’d…
 At voltage V1 the formation of the species O is
thermodynamically favoured.
 However as we move through the voltages to V4 the
formation of R becomes the thermodynamically
favoured product.
 In order to formulate a model we will assume that
the effect of voltage on the free energy change will
follow a linear relationship.
 Using this linear relationship the activation free
energies for reduction and oxidation will vary as a
function of the applied voltage (V) as follows
and 40
red rednovoltageG G FV 
    (1 )ox oxnovoltageG G FV 
   

Cont’d…
 The parameter α is called the transfer coefficient and
typically is found to have a value of 0.5.
 Physically it provides an insight into the way the
transition state is influenced by the voltage.
 A value of one half means that the transition state
behaves mid way between the reactants and
products response to applied voltage.
 The free energy on the right hand side of both of the
above equations can be considered as the chemical
component of the activation free energy change, i.e.,
it is only dependent upon the chemical species and
not the applied voltage.
41

Cont’d…
 We can now substitute the activation free energy
terms above into the expressions for the oxidation
and reduction rate constant, which gives:
 These results show us that rate constants for the
electron transfer steps are proportional to the
exponential of the applied voltage.
 So the rate of electrolysis can be changed simply by
varying the applied voltage.
42
exp{( )( )}
rednovoltage
red
G FV
k Z
RT RT


 

1 )
exp{( )( )}
oxnovoltage
ox
G FV
k Z
RT RT


 


Cont’d…
 In conclusion we have seen that the rate of electron
transfer can be influenced by the applied voltage and
it is found experimentally that this behaviour can be
quantified well using the simple model presented
above.
 However the kinetics of the electron transfer is not
the only process which can control the electrolysis
reaction.
43

Cont’d…
The effect of mass transfer
 The effect of mass transfer on the kinetics of the
electrode is studied as follows:
 Let us consider a reduction reaction as:
 As an electron transferred from one electrode to
another, the electrical work done is where e is
electronic charge and is potential difference
between electrodes.
 Gibbs free energy of activation is changed from
to
if the transition state corresponds to Ox being very
44
ReOx e d
 
e  
G
 G F 
  

Cont’d…
 If ,more work will be done to bring Ox to its
transition state which results an increase in Gibbs
free energy of activation.
 Contrary to it, if transition state corresponds to Ox
being far from electrode i.e., close to OHP of double
layer, then is independent of .
 But actually, situation is somewhere in midway of
the two. Hence GFE of activation for reduction as
 Now let us consider an oxidation reaction
 Here Red give up electron and result that extra work
needed to reach the transition state is zero, if this
state lies close to the electrode. 45
0 
G

G F 
  
Red Ox e
 

Cont’d…
 If it lies away from the electrode is close to OHP of
double layer, the work need is - F so that to
 At equilibrium and net current is zero and
equilibrium current densities are equal. If potential
difference differs from its equilibrium value by over
potential value such that
 The two current densities are:
and 46
G
 (1 )G F 
   
(1 )
( )
{ [ ]exp( )} { [ ]exp( )}
F F
RT RT
a c
G Gi Fk red e Fk Ox e
RT RT
      
 
  
a ci i i 
eqm   
eqm   
(1 )
,
F
RT
a a eqmi i e
 
 ,
F
RT
c c eqmi i e



Cont’d…
 Since both are equal current
densities, then:
This is called Butler – Volmer Equation
Special case of Butler – Volmer Equation
I) When η is very small and positive such that
47
, ,a eqm c eqmi i
(1 )
[ ]
F F
RT RT
oi i e e
   
 
1F
RT
 
o
Fi i
RT

( )
o
RT i
F i
 

Cont’d…
 if η is +ve the current is anodic and if η is –ve,
current is cathodic.
II) When η is very large and +ve
or (x)
If η is very large but –ve
or (y)
Equation (x) and (y) are called Tafel equations
48
(1 ) F
RT
oi i e
 

(1 )
ln ln o
F
i i
RT
 
 
F
RT
oi i e

 
ln( ) ln o
F
i i
RT

  

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