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2 Structure of electrified interface. 1. The electrical double layer 2. The Gibbs adsorption isotherm 3. Electrocapillary equation 4. Electrosorption phenomena 5. Electrical model of the interface. 2.1 The electrical double layer. Historical milestones - PowerPoint PPT Presentation
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2 Structure of electrified interface
1. The electrical double layer
2. The Gibbs adsorption isotherm
3. Electrocapillary equation
4. Electrosorption phenomena
5. Electrical model of the interface
2.1 The electrical double layer
Historical milestones- The concept electrical double layer Quincke – 1862- Concept of two parallel layers of opposite charges Helmholtz 1879 and Stern 1924- Concept of diffuse layer Gouy 1910; Chapman 1913- Modern model Grahame 1947
Presently accepted model of the electrical double layer
2.2 Gibbs adsorption isotherm
a
s
b
Definitions
G – total Gibbs function of the system
Ga,Gb,Gs - Gibbs functions of phases a,b,s
Gibbs function of the surface phase s:
Gs = G – { Ga + Gb }
Gibbs Model of the interface
Con
cent
ratio
n
Distance
Surface excess
Hypothetical surface
The amount of species j in the surface phase:
njs = nj – { nj
a + njb}
Gibbs surface excess Gj
Gj = njs/A
A – surface area
Gibbs adsorption isotherm
Change in G brought about by changes in T,p, A and nj
dG=-SdT + Vdp + gdA + Smjdnj
– surface energy – work needed to create a unit area by cleavage
jinpTj
j nG
=,,
m - chemical potential
dGa =-SadT + Vadp + + Smjdnja
dGb =-SbdT + Vbdp + + Smjdnjb
and
dGs = dG – {dGa + dGb}= SsdT + gdA + + Smjdnjs
npTAG
,,
=g
Derivation of the Gibbs adsorption isotherm
dGs = -SsdT + gdA + + Smjdnjs
Integrate this expression at costant T and p
Gs = Ag + Smjnjs
Differentiate Gs
dGs = Adg + gdA + Snjsdmj + Smjdnj
s
The first and the last equations are valid if:
Adg + Snjsdmj = 0 or
dg = - Gjdmj
Gibbs model of the interface - Summary
2.3 The electrocapillary equation
Cu’ Ag AgCl KCl, H2O,L Hg Cu’’
sM = F(GHg - Ge)+
Lippmann equation
Differential capacity of the interface
2
2
dEd
dEdC M gs
==
Capacity of the diffuse layer
Thickness of the diffuse layer
2.4 Electrosorption phenomena
2.5 Electrical properties of the interface
In the most simple case – ideally polarizable electrode the electrochemical cell can be represented by a simple RC circuit
Implication – electrochemical cell has a time constant that imposes restriction on investigations of fast electrode process
Time needed for the potential across the interface to reachThe applied value :Ec - potential across the interfaceE - potential applied from an external generator
dCqIRE +=
} dtCRtCRECidt
CqE
t
dudu
d
t
dc ===
00
/exp/
} duc CRtE /exp1 =
Time constant of the cell
t = RuCd
=
duduc CR
tCREE exp1
Typical values Ru=50W; C=2mF gives t=100ms
Current flowing in the absence of a redox reaction – nonfaradaic current
In the presence of a redox reaction – faradaic impedance is connected in parallel
to the double layer capacitance. The scheme of the cell is:
The overall current flowing through the cell is :
i = if + inf
Only the faradaic current –if contains analytical or kinetic information