1

Fundamentals of

Electrochemistry

2

1. Distinctive Concepts in Electrochemistry

2. Thermodynamics and Kinetics

in Electrochemistry

3. Electrode potentials

4. Ions in solution: electrolytes

5. Electrochemical kinetics

6. Electrochemical cells

3

Heterogeneous charge transfer reaction;

Electron tunneling at electrode/solution interface

Cu2+ + 2e → Cu0 (electrochemical reduction)

Fe2+ → Fe3+ + e (electrochemical oxidation)

Anode (산화전극): electrochemical oxidation

Cathode (환원전극): electrochemical reduction

Electrochemical reactions

4

Heterogeneous charge transfer (전하전달)

Cu2+ + 2e → Cu

5

1. Electrode potential is an expression of electron energy

2. Many things can happen at once

3. Electrochemical system is not homogeneous

4. Current is an expression of reaction rate

5. E and I can not be controlled simultaneously

6. Current passes through a “closed loop”

Distinctive Concepts in Electrochemistry

6

Electrode potential is an expression of electron energy

Electrode potential (전극전위)↔ Electron energy (전극 내 전자의 에너지)

7

Electrode potential is tunable by potentiostat

일정전위기

8

Electrode potential is an expression of electron energy

FeCp2+

FeCp2+ + e → FeCp2

LUMO; Lowest un-occupied molecular orbital

HOMO; highest occupied molecular orbital

9

Many things can happen at once

Mass transfer:

(물질전달)

Diffusion (확산)

Migration (이동)

Convection (대류)

Electrochemical reactions: Mass transfer + Charge transfer

Rate-determining step

10

Electrochemical system is not homogeneous

11

Electrochemical system is not homogeneous

General chemical reactions: Homogeneous

Electrochemical reactions: only within diffusion layer

12

Current is an expression of reaction rate

PSV

FeCp2+

FeCp2

R’

O’

ee

e e

Ref CtrWk

전류 (i) ∝ 반응속도

13

How to control reaction rate (current) ?

E and I can not be controlled simultaneously !!

과전압

Charge transfer limited

14

Current vs. rate-limiting step

Charge transfer limited

15

Current passes through a “closed loop” (닫힌고리)

- electrode + electrode

ic: cathodic current

ia : anodic current

16

Cathode area >> Anode area, but ic = |ia |, what happens?

Current passes through a “closed loop”

1) Cathode

2) Anode

3) Electrolyte

4) Circuit

Crevice corrosion

(틈새 부식)

17

“closed loop” in redox-flow battery

Vanadium redox-flow cell

18

Shutdown mechanism in lithium ion batteries

19

Anode Dye Electrolyte Cathode

R O

Ecell

Do/D

+

Mediator

D*

eLoad

e

e

e

e

hv

“closed loop” in dye-sensitized solar cells

염료감응 태양전지

20

Thermodynamics and Kinetics

in Electrochemistry

21

Thermodynamics vs. Kinetics

FeCp2+ Faradaic current

Non-faradaic current

Thermodynamics: 가능성

Kinetics: 속도(전류)

22

Thermodynamics vs. Kinetics

Equivalent circuit

(등가회로)

23

Ideally polarized electrodes (이상 분극전극)

24

Ideally polarized electrodes (이상 분극전극)

25

1) Always double-layer charging current with E change

2) Ideally polarized electrodes: Rct → ∞

3) Ideally non-polarizable electrodes: Rct → 0

(이상 비분극 전극)

4) EDLC; electric double-layer capacitors

(전기이중층 커패시터)

26

Electrode potentials

1. Standard electrode potential

2. Equilibrium potential

3. Formal potential

4. Mixed potential

27

1. Electrical potential is defined by difference (차이)

2. Electrical potential : charge separation at

interfaces (계면에서 전하 분리)

3. Infinitesimally small (극소량) charge separation

4. Potential difference must be defined at

equilibrium or steady-state (평형 또는 일정 상태)

Electrode Potentials (전극전위)

28

Electrode Solution

Electron Cation

r

Infinitesimally small charge separation

29

Potential difference at interfaces

Potential difference must be defined

at equilibrium (평형) or steady-state (일정 상태)

Ag+ + e (in Ag) → Ag

30

Chemical Potential (화학전위)?

Escaping tendency

(이탈 경향)

i

in

GjnPT ,,

i

(a)

31

Electrochemical Potential (전기화학전위)?

Fziii

i

( ions)

Additional energy

(b)

Q x V = energy

32

Conditions for Equilibrium

Potential-determining equilibrium

(전위결정 평형식)

Ag+ + e (in Ag) = Ag

33

Potential difference at equilibrium state

34

Inert electrode/redox pair or gas

PtFe

3+

Fe2+

X-

(a)

PtH2

2H+

X-

(b)

모든 반쪽전지 (half-cell)는 1개의 전위결정 평형식을 가짐

35

Standard electrode potential (표준 전극전위)

HCl

H2

Pt electrode

Fe2+

Fe3+

Pt wire

Salt

bridge

V

NHE (SHE)를 기준으로 반쪽전지의 전압(V) 측정

36

Standard electrode potential (E0)

V

Pt

H2

H+

Fe2+

Fe3+

X-

X-

Pt

H2/H+

Fe2+

/Fe3+

Vlj

Pt

Pt

1

2

Potential 차이는

상(phase)과 상의

계면에서 !!

37

Nernst equation

전위결정 평형식 (potential-determining equilibrium)

에서 이온들의 활동도(activity)만으로 표시

Fe3+ + e = Fe2+

38

Standard electrode potential (E0)

Standard electrode potential (E0)

Half-cell에 관여된 potential-determining equilibrium에서

모든 이온의 activity = 1인 경우 Eeq

평형상태의 값이다 (thermodynamic parameter)

모든 반쪽전지는 고유한 E0을 갖는다

NHE (normal hydrogen electrode)를 기준으로 한 250C에

서 값

Fe3+ + e = Fe2+

39

40

41

Standard electrode potential (E0)

Li+ + e = Li E0 = - 3.04 V (vs. NHE)

2H+ + 2e = H2 E0 = 0.0 V (vs. NHE)

Cl2 + 2e = 2Cl- E0 = 1.36 V (vs. NHE)

← More negative More positive →

← 산화경향 증가 환원경향 증가 →

금속의 이온화 경향:

Li> K> Ca> Na> Mg> Al> Zn> Fe> Ni> Sn> Pb> Cu> Au

← ← ← ← E0 more negative

42

Practical reference electrodes(기준전극)

E (vs. NHE)

0.2412 V

0.197 V

0.0 V

Hg/Hg2Cl2(s), KCl(sat’d)

Ag/AgCl(s), KCl(sat’d)

Pt/H2/H+ (NHE)

43

Formal potential

E0 ≈ E0’

44

Mixed potentials (혼성전위 )

Zn

Zn2+

( =1)

Zn2+

AnodeCathodeX

-

2H+

H2(g)

e ea

For a Galvanic cell

1) Cathode

2) Anode

3) Electrolyte

4) Circuit

45

Mixed potentials; Zn corrosion

2H++ 2e H2(g)ic

ia

0.0 -0.76

Mixed potential

E (-) (vs. NHE)

Zn Zn2+

+ 2e

icia

(ic = |ia|)

Zn의 용해(부식)과

수소의 발생

46

Mixed potentials; Zn corrosion

2H++2e H2(g)ic

ia

0.0 -0.76 E (-) (vs. NHE)

Zn Zn2+

+2e

pH

Mixed potentialDecrease in ic and ia

pH dependence

Eeq = Constant - 0.059 V x pH

(at 298K)

47

H2 and O2 evolution as a function of pH

48

Alkaline battery

Negative cover

Seal

Steel can

Cathode (MnO2)

Separator

Anode (Zn)

Current

collector

Electrolyte: concentrated KOH

Negative electrode; Zn powder

Positive electrode: MnO2 (EMD)

Mercury-free cells

Addition of mercury (Hg)

49

Metal - log I0

(A/cm2)

Metal - log I0

(A/cm2)

Ru 2.1 Fe 6.0

Pd 2.3 Cu 6.7

Rh 2.8 W 7.0

Pt 3.6 Cr 7.4

Co 5.2 Zn 10.5

Ni 5.2 Cd 11.0

Ag 5.4 Pb 12.2

Au 5.5 Hg 12.5

Exchange current density for H2 evolution

1.0 M H2SO4

- Large overpotential

for H2 evolution

- Large Rct

50

Ion-ion interactions

Ion-solvent interactions

Ions in solution : electrolyte

1) Activity coefficient; Debye-Hü ckel theory

2) Migration of ions

3) Molar conductivity

51

(Salt Conc.)1/2

(Salt Conc.)1/2

log

0.0

L

L̊

Activity coefficient and transport property

52

Debye-Hückel theory

Ionic atmosphere

Ionic concentration ↑; Ionic strength ↑; Debye length ↓;

More stabilized; γj↓

d-

d d-

d-d-

d-

I

1Debye length

53

Activity coefficient vs ionic strength

0.0

log

DHL

EDHL

Real

R-S

I

j

54

Li+ Na+ K+

Crystal radius a /Å0.60 0.95 1.33

Hydrated radius b/Å2.4 1.8 1.3

Hydration number2-22 2-13 1-6

Li+, Na+, K+ 이온의 결정학적 반경, 수화된 반경, 수화 수

a From x-ray diffraction analysis, b Using Stokes’ law

55

Fe3+

H

H

OH

H

O

H

H

OH

H

OO

H

H

O

H

H O

H

H

O

H

H

O

H

H

O

H

H

O

H

HO

H

H

O

H

H

O

H

H

O

H

H

O

H

H

O

HH

O

HH

O

HH

O

HH

O

HH

O

HH

O

HH

O

HH

Bulk solution

Inner

hydration

shellOuter

Hydration

shell

Ion-solvent interaction

Ion-solvent interactions

56

Ionic conductivity

A A

l

V

57

Molar conductivity

NiSO4 (weak electrolyte)

Ideal

KCl (strong electrolyte)

L

L

˚

(Salt Conc.)1/2

58

Relaxation effect and electrophoretic effect

Eexternal

Erelaxation

Ionic atmosphere

Decrease in mobility

59

1. Charge transfer kinetics

Butler-Volmer equation

Tafel plot

2. Mass transfer kinetics

Electrochemical kinetics

60

Activated complex theory

Reaction coordinate

Fre

e e

ne

rgy

∆G‡

∆G‡

Reactant Product

Activated complex

b

f

61

Activated complex theory

62

Activation energy by potential control

Reaction coordinate

Fre

e e

ne

rgy

nF(E - Eo’) nF(E - E

o’)

(1-)nF(E - Eo’)

O + ne R

∆G‡ ∆G‡

∆G‡

oc oa

oc

63

Activated complex theory

64

Butler-Volmer equation

c

ic

Eeq = - 0.11 V

E (vs. SCE)

= 2.2

ia

= 2.2 A

A

ic

ia

io

io

inet

Charge transfer rate

65

Linear relationship: i vs. E

inet

ic

ia

ic

ia

E (-)

Slope

(-)

nF i0=

RT=

1

Rct

=

inet =

66

Tafel plot

67

Tafel plot

log │inet│

- 0.10.10.2

(V)

log i0

Slope(1-)nF

=2.3RT

SlopenF

=2.3RT

- 0.2

η = a + b log inet

Tafel equation

Tafel behavior

Tafel region

68

Electrochemical cells

1. Electrolytic cells

2. Galvanic cells

69

Electrolytic cells (전해셀)

Brine (NaCl/H2O) electrolysis (소금물 전기분해 (또는 전해))

RT aOEeq = E0 + — ln ——

nF aR

Nernst equation

E0 : Standard electrode potential (표준전극전위) at 298 K

NHE: Normal hydrogen electrode

70

Electrolytic cells

1) Cathode

2) Anode

3) Electrolyte

4) Power supply

71

Electrolytic cells; charge transfer limited

72

Electrolytic cells; charge transfer limited

Eappl: 전해를 위해 두 전극 사이 걸어주어야 하는 전압

73

ic

ia

E

ηa

1.36 V

Electrode

A

ηa

Electrode

B

( - )

Electrolytic cells

Electrode selection

Rsolution

Electrode gap

Separator (분리막)

74

Galvanic cells (갈바니 셀)

2H2 + O2 → 2H2O (∆G

0

<< 0 ; spontaneous, 자발적)

Electrochemical pathway (전기화학 경로)

Anode: 2H2 → 4H+ + 4e

Cathode: O2 + 4H+ + 4e → 2H2O

Overall: 2H2 + O2 → 2H2O (∆G < 0 ; spontaneous)

75

Galvanic cells

Fuel cell (연료전지)

76

Galvanic cells

77

Galvanic cells

Ewk: 갈바니 셀의 작동 전압

78

Secondary batteries (이차전지)

79

Secondary batteries (이차전지)

80

Cathode vs. anode

Cathode: emit electrons

Anode: accept electrons

Cathode: Cu2+ + 2e → Cu0

(emit electrons)

Anode: Fe2+ → Fe3+ + e

(accept electrons)

Cathode ray tube

81

Cathode vs. anode

Hollow Cathode Lamp

Ar → Ar+ + electrons

electrons; to anode

(accept electrons)

Ar+ ; attack cathode

(accept positive charges

= emit negative charges

(electrons)